The Cathodic Reduction of AISI 304 Stainless Steel ...

The Cathodic Reduction of AISI 304 Stainless Steel Thermal Oxides in Chromic Acid Based Electrolyte and Associated Mechanisms

By

Anthony P. Smith Department of Materials Loughborough University

Project Final Report Module:14MPD110

2015

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Acknowledgements A huge thank you to my tutors Dr. Rebecca L. Higginson and Dr Geoffrey D. Wilcox for keeping me on track, providing me with invaluable knowledge, and humouring me in my individual ways throughout this project. I would like to extend my considerable gratification to Emma Murrel and Daniel Lake, both of whom have been wonderful peers to work alongside, providing principals and expertise which I would not have been able to finish this project without. Thank you,

Anthony Smith

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Contents Chapter 1 : Abstract and Introduction ......................................................................... 4 1.1 Abstract ............................................................................................................ 4 1.2 Introduction ....................................................................................................... 5 Chapter 2 : Literature Review ..................................................................................... 7 2.1 Colouration Mechanism .................................................................................... 7 2.2 Chemical Oxidation to form Oxide films ............................................................ 8 2.3 Cathodic Hardening Process .......................................................................... 13 2.4 Thermal forming of coloured oxides................................................................ 14 2.5 Measuring Colours.......................................................................................... 17 2.6 Hull Cell .......................................................................................................... 20 Chapter 3 : Experimental Procedures ...................................................................... 21 3.1 Sample Mounting and Optical Microscope Investigation ................................ 21 3.2 Thermal Oxidising Procedures........................................................................ 22 3.3 Colour Measuring Procedure .......................................................................... 23 3.4 MEF3 Analysis Procedure .............................................................................. 23 3.5 Cathodic Electrochemical “Hardening” Procedure .......................................... 23 3.6 FEGSEM and Dual Beam SEM Procedure ..................................................... 25 3.7 Hull Cell Procedure ......................................................................................... 26 3.8 XPS Procedure ............................................................................................... 27 Chapter 4 Results..................................................................................................... 28 4.1 Un-oxidised, Untreated Stainless Steel Sample Optical Micrographs............. 28 4.2 Colour Measurements .................................................................................... 30 4.3 Optical Micrographs of Thermally and Electrochemically Treated Stainless Steel ..................................................................................................................... 33 4.4 Field Emission Gun Scanning Electron Microscope (FEGSEM) Images ........ 34 4.4.1 Untreated sample ..................................................................................... 34 4.4.2 Thermally Treated Samples ..................................................................... 35 4.4.3 Thermally and Electrochemically Treated Samples ................................. 36 4.4.4 Thermally and Electrochemically Treated TT8m Series Samples ............ 37 4.4.5 Thermally and Electrochemically Treated Hull Cell Samples ................... 40 4.5 Hull Cell Sample Code and Photographs ....................................................... 41 4.6 Chemical Analysis Results ............................................................................. 42

3 4.6.1 Untreated and Thermally Treated Samples.............................................. 42 4.6.2 Thermally and Electrochemically Treated Samples ................................. 43 4.6.3 Hull Cell Samples ..................................................................................... 45 4.6.4 Thermally and Electrochemically Treated TT8m Series Samples ............ 45 Chapter 5 Discussion ............................................................................................... 48 5.1 AISI 304 Stainless Steel Samples .................................................................. 48 5.1.1 AISI 304 Stainless Steel Samples Processing Route ............................... 48 5.1.2 Surface Microscopic Imaging (FEGSEM) and Surface Chemistry (XPS) . 48 5.2 Producing Coloured Stainless Steel Using Thermal Techniques .................... 49 5.2.1 XPS and FEGSEM Analysis of the Thermally Formed Oxides................. 49 5.2.2 La*b* Measurement of Thermally Treated Sample Oxide Layer Colours . 52 5.3 Electrochemical Treatment ............................................................................. 53 5.3.1 La*b* Results and Colour Reversion of First Electrochemically Treated Samples ............................................................................................................ 53 5.3.2 Electrochemical Mechanisms Responsible for Colour Reversion ............ 54 5.4 Hull Cell Experiment ....................................................................................... 57 5.4.1 Reasoning Behind Using Hull Cell Experiments....................................... 57 5.4.2 Hull Cell Experimentation ......................................................................... 57 5.4.3 Visual Observation, La*b* and Optical Microscopy Results of the Hull Cell Experiments ...................................................................................................... 58 5.4.4 XPS and FEGSEM Analysis of Hull Cell Sample TT10 ET120s21°C0.1A 59 5.5 Further Investigation into the Reduction Mechanisms and Conditions ........... 61 5.5.1 Further Electrochemical Experimentation on Coupon Samples ............... 61 5.5.2 La*b* Analysis of Electrochemically Reduced Coupons ........................... 62 5.5.3 XPS and FEGSEM Analysis of Electrochemically Reduced Coupons ..... 62 Chapter 6 Conclusion ............................................................................................... 67 Chapter 7 Future Works ........................................................................................... 70 Bibliography ............................................................................................................. 72 Appendix .................................................................................................................. 76

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Chapter 1 : Abstract and Introduction 1.1 Abstract This report analyses how a cathodic electrochemical process, designed for the hardening of chemically formed interference films, consisting of chromic acid, with small amounts of phosphoric acid and Fumetrol 21 (a mist suppressant), may alter the surface composition of thermally grown oxide interference films on AISI 304 stainless steel, and thus the colour of the surface oxides. Thermally formed oxides were generated on small coupons of dimensions 20mm, by 20mm, by 2 mm, and large samples of dimensions 101mm by 75mm by 2 mm, in laboratory atmosphere at 600°C at times of 4, 5, 8 and 10 minutes. These thermal oxides resulted in gold (4 and 5 minutes), red-purple (8 minutes) and blue-purple (10 minutes) coloured oxides on the sample surface. It was discovered by literature and using a combination of X-ray Photoelectron Spectroscopy (XPS) and Field Emission Gun Scanning Electron Microscopy (FEGSEM) techniques, that these colours were a result of an interference film consisting of a chromium oxide film with iron oxide nodules. The cathodic electrochemical process was found to be a reduction reaction at the cathode, which reduced iron oxide to iron metal, and the aqueous hexavalent chromium ions from the chromic acid based electrolyte to chromium oxide which was resultantly deposited on the surface of the samples. This reduction resulted in the colour of the oxide film to be effectively removed, resulting in a very pale gold colour. The larger samples were used in Hull Cell experiments in order to determine the conditions over which the electrochemical reduction took place, to better investigate and identify the reduction mechanisms occurring in the electrochemical process. The reaction was found to be highly time and current density dependent. It was also found that the reduction of aqueous hexavalent chromium to chromium oxide was energetically favourable to the reduction of solid iron oxide to iron metal. Conditions were identified where chromium oxide was deposited, with no iron oxide reduction, resulting in a highly vivid blue coloured surface oxide. Finally the reduction experiments solidified theories about the thermally treated oxide structure, and the reduction mechanism was analysed at different times throughout the reduction

5 process. Using XPS, FEGSEM and CIELAB techniques it was found that the reduction of iron oxide occurs from the inner oxide layers, outwards, and that the chromium oxide appears to be deposited fairly evenly, plugging gaps between iron oxide nodules. These reduction mechanisms result in the destruction of the interference film responsible for generating the colour of the oxide, thus changing the colour to appear more like the substrate AISI 304 stainless steel, whilst retaining an oxide film on the surface of the stainless steel.

1.2 Introduction The process of colouring AISI 304 stainless steel to achieve aesthetically pleasing surface coatings has been around since 1927 (1). These coloured stainless steels have been used generally in the architectural industry as aesthetically pleasing panelling on buildings and even on indoor surfaces due to the coatings impressive lustre post colouration. The colour generated on the surface of the stainless steel is due to an extremely thin oxide film, which in the past has been chemically formed on the surface of AISI 304 stainless steel (stainless steel). The process of electrochemically oxidising the samples, involves the use of chromic acids which dissociate and form hexavalent chromium ions in solution (Cr VI or 𝐶𝑟 6+ ). On April 18th 2013 the European Union (EU) announced that an EU regulation known as REACh, which is responsible for the registration evaluation authorisation and restriction of chemicals within the European Union, placed hexavalent chromium in annex XIV. The REACh Annex XIV is a list of banned substances, and as of 21st September 2017 Hexavalent Chromium will be banned from use in industry without a REACh authorisation (2). The reason Hexavalent and Trivalent Chromium were added to annex XIV is due to the chemicals highly carcinogenic and mutagenic properties (2). Since the announcement of the addition of Cr VI to Annex XIV, there have been several movements to find various replacement processes or chemicals where Cr VI is traditionally used. One such area of inquiry is the production of coloured oxides on stainless steels. The reason this is such an important line of research is because the current methods of industrial manufacture of these coatings relies heavily on chromic acid. Processes such as chemical oxidising, electrochemical hardening, square wave current pulse, triangular current scan, (1,3–6) are now under scrutiny due to

6 their use of chromic acid. It has been found that a similar range of colours can be generated using a thermal treatment technique which relies on the thermal oxidation of the stainless steel between 1-10 minutes (7). This report looks into the amalgamation of two of these processes, the thermal oxidation and electrochemical “hardening” of the oxide using a cathodic process in the presence of chromic acid. This project looks at how a chemical hardening technique using chromic acid, could potentially be applied a thermal oxide. This project will investigate any visual and microscopic changes that may occur, along with the surface chemistry of the films. The chromic acid used in this project is to assess wither or not the mechanisms can be defined and manipulated using a more environmentally friendly route, once the mechanisms have been established for this process i.e. a route without the use of chromic could be used in the future for manufacturing the lustrous and aesthetically pleasing colours by oxidation.

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Chapter 2 : Literature Review In this chapter of the report various aspects of coloured AISI 304 stainless steel (stainless steel) will be investigated, from different processing routes for forming the coloured oxides as outlined in the introduction, to the nature of the oxide film, and the mechanism which causes the coloured appearance of the stainless steel.

2.1 Colouration Mechanism The method of forming these oxide films can be separated into two distinctly different processes; chemically and thermally. The way in which these films generate their colour is remarkably similar. A number of papers produced on the oxide film formed by chemical processes denote it as being a highly porous oxide, stating that the porosity of which is responsible for its coloured nature. According to Naylor (8) the chemically grown coloured oxide films are interference films, meaning that incident light is reflected off the surface of the oxide and from the surface of the substrate metal . These reflected light waves interact, cancel out certain visible light wavelengths and result in the perception of colour. Due to the lights direct interaction with the metal substrate, the material retains the lustrous appearance of the metal. This phenomenon is known as an interference film. Naylor (8) also demonstrated that the time the stainless steel is exposed to the colouring solution results in a thickening of the oxide film, thus changing the optical interference, and therefore the colour of the film. The oxide thickness is controlled by the height of small nodules which grow under the chemical oxidation conditions, an illustration of which can be seen in Figure 2.1. These nodules are responsible for the interference film discussed, which results in the appearance of different colours.

8 Chromic and Iron oxide film nodules Extremely thin oxide thickness (order of nanometres)

AISI 304 stainless steel substrate

Figure 2.1 - This Figure 2.illustrates the physical appearance of the oxide nodules on the substrate

surface, both thermally and chemically formed.

Higginson (7) has postulated that the coloured oxides formed when stainless steel is thermally oxidised are also forming interference films, and are formed in extremely similar ways. Instead of the oxide forming in a chemical solution (which tends to be a chromic acid based solution), it is formed due to a thermal reaction of the substrate metal constituent elements with the oxygen in the air at high temperatures. In these high temperature conditions a nodular structure still forms. The nodules formed seem to be of similar size and density for both thermal and chemical processes, but with slightly different physical chemistry. The chemically formed oxide nodules seem to be hydrated chromium oxide (𝐶𝑟2 𝑂3 . 𝐻2 𝑂) on the outer layers, with a spinel structure of hydrated chromic and iron oxides with a small amount of nickel oxide in the lower layers, ((𝐶𝑟𝐹𝑒)2 𝑂3 . 𝑥𝐻2 𝑂) closest to the substrate, before any hardening process which dehydrates the oxides (9). The thermally formed oxide consists of a chromium-iron oxide spinel structured layer ((𝐶𝑟𝐹𝑒)2 𝑂3) with iron oxide – haematite structure (𝐹𝑒2 𝑂3) nodules (7).

2.2 Chemical Oxidation to form Oxide films Evans’ (1,9,10) chemical oxidation process has two distinct stages; 1. Colouring: Immersion of the steel in order to grow the coloured oxide on the surface of the stainless steel, in a hot chromic and sulphuric acid bath of composition and temperature seen in Table 2.1. 2. Hardening: Cathodic treatment of the chemically oxidised sample, which hardens the thermal oxide and gives it better wear resistance. This process is also in a chromic and sulphuric acid bath, with a slightly different composition and temperature to the colouring bath shown in Table 2.2. The cathodic process is used because the oxide formed using the colouring process alone is extremely soft and has poor mechanical properties, especially regarding

9 wear and scratch resistance. The hardening process used has excellent throwing power, resulting in a well hardened, uniformly thick chemical oxide on the surface of the metal. The coloured oxide demonstrates thermal resistance up to 200°C and in a boiling water solution (1). There have been a range of different processes by which the chemical formation of coloured oxides has been achieved such as; Clegg and Greening (11), James, Smith and Tottle (12), Evans, James and Smith (13), and James, Smith and Smith (14), all of which using chromic acid based baths in order to form the coloured oxides, and various other environmental enhancements such as AC or DC current, resulting in some processes being more successful and or efficient than others, but each with their individual issues of small ranges of colours obtained, undesirable colours obtained or and/or poor oxide wear resistance. The most successful process seems to be the one outlined above using chemical and hardening processes (1), the chemical compositions of the chemical baths used are outlined in Tables 2.1 and 2.2.

Table 2.1 – Composition of the colouring bath in the chemical oxide formation process (1)

Chromium Trioxide - CrO3

250g/l (2.5 Moles)

Sulphuric Acid - H2SO4

490g/l (5 Moles)

Temperature

70°C

Table 2.2 – Composition of the hardening bath in the chemical oxide formation process (1)

Chromium Trioxide - CrO3

250g/l (2.5 Moles)

Sulphuric Acid - H2SO4

2.5g/l (0.0025 Moles)

Temperature

40°C

Current Density

2.5 A/dm2

Time

15 minutes

10 Generally in order to produce different colours using a chemical oxidation process, the immersion time in the colouring stage is altered, and blue, gold, mauve, and green colours can be attained. The colour achieved in the colouring process has good uniformity, unless a heavily cold worked region is present on the sample surface which can result in slight colour variation (1). In order to increase the rate of oxide film thickness growth, the concentration of either sulphuric acid, or chromium trioxide, or the temperature of the colouring bath may be increased. This increased rate of oxide thickness growth can make it more difficult to control the oxide film colour shade (9). On examining T. E. Evans et al’s colouring method (9), it is noted that when the samples are in the colouring bath, the potential across the stainless steel increases by almost 30µA, which coincides with a small weight loss of the sample and the increase in the thickness of the oxide film. As previously mentioned; this increasing of the thickness of the oxide directly corresponds to an increased thickness of the

Figure 2.2 - Optical reflectance spectra of specimen 1-4, from

Figure 2.2 - Optical reflectance spectra of specimen 5-9, from

Evans et al (9). This demonstrates how the resulting colours are generated by an interference film. Here lower wavelength regions are being absorbed due to interference resulting in a yellow-gold colouration of the film.

Evans et al (9). This again demonstrates how the resulting colours are generated by an interference film. Here selective wavelengths are being absorbed resulting in a transition of colours from green through to blue, then purple/ violet.

11 interference film. As the levels of interference changes, new reflected spectra form, as shown in Figures 2.2 and 2.3, resulting in the appearance of different colours. In Figures 2.2 and 2.3, specimen 1 is immersed in the colouring bath for a short period of time (250s) and 9 for a long period (2070s), with 2-8 immersion times progressively increasing. These spectra demonstrate how increasing the immersion time of the sample in the chemical bath results in the appearance of different colours due to the absorption of different wavelengths of light (9). These coloured oxide films grown are assumed to be of extremely similar chemical composition, due to the colour being perceived due to the interference nature of the oxide film, rather than specific absorption of certain wavelengths of light. Table 2.3 shows the chemical composition of the films (9). The oxide film was measured to be 20-30% porous, which depended upon the crystallographic orientation of the substrate at the surface. It appeared that the spinel structure was quite uniform, with pore size and distribution only fluctuating by a small amount (9).

Table 2.3 – Composition of T. E. Evans’ chemically grown coloured oxide films(9)

Chromium (%wt.)

Iron (%wt.)

Nickel (%wt.)

19.6 - 21.3

11.5 - 11.7

2.1 – 6.3

The oxide film is formed by cathodic-anodic reactions. The cathodic and anodic sites on the substrate surface are induced by the highly oxidising nature of the sulphuric and chromic acid solution by the following reactions: 𝑀 → 𝑀𝑛+ + 𝑛𝑒 −

Equation 2.1

Where M can be 𝐹𝑒, 𝐶𝑟 or 𝑁𝑖 and 𝑀𝑛+ could be 𝐹𝑒 3+ , 𝐶𝑟 3+ or 𝑁𝑖 2+ causing reduction of chromic acid to produce more 𝐶𝑟 3+ ions as below: 𝐶𝑟2 07 2− − 14𝐻 + + 6𝑒 − → 2𝐶𝑟 3+ + 7𝐻2 𝑂

Equation 2.2

12 In this reaction it can be seen that 14 𝐻 + ions are used up in the reaction forming water, thus reducing the pH of the solution in the volume surrounding the reaction. It is this dilution of the acidity, which results in conditions promoting the growth of the oxide spinels which develop by the following hydrolysis reaction: 𝑥𝐶𝑟 3+ + 𝑦𝑀𝑛+ + 𝑧𝐻2 𝑂 = 𝐶𝑟𝑥 𝑀𝑦 𝑂4 (𝑧 − 4)𝐻2 𝑂 + 8𝐻 + 3𝑥 + 𝑛𝑦 = 8

Equation 2.3

This reaction results in a predominantly chromium based oxide, with other metal oxide compounds as denoted by 𝑀𝑦 in the given electrolysis reaction, tending to be iron and small amounts of nickel, as briefly outlined in the colouration mechanism section of this chapter(9). The growth of the film is instantaneous with the dissolution of the stainless steel substrate implying that the porous film offers little to no barrier against the acidic dissolution of the substrate, proven by the appearance of the 2030% porosity of the oxide film formed. This means that this behaviour is likely explained by anodic dissolution of the substrate at the base of the pore with a cathodic reaction occurring at the top of the film nodules (9). A model of this process can be seen in Figure 2.4.

𝐻2 𝑂

Figure 2.3 - This Figure 2.demonstrates the formation of

cathodic and anodic sites resulting in the deposition and growth of chromic oxides, the growth of other metal oxides and the dissolution of substrate metal. From Evans et al (9).

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2.3 Cathodic Hardening Process The so called hardening process is specifically designed to harden the soft hydrated oxides formed in the chemical oxidation process which can be whipped away with a thumb post chemical treatment (10), but also to limit the extent of finger marks and staining occurring on the surface of the coloured oxide. The hardening process has been reported to have a small impact upon the shade of the oxide film when hardening most colours, whilst retaining its lustrous and reflective surface properties (9). The hardening stage vastly increases the wear and scratch resistance of the material. Two tests were carried out to verify this: A 500g loaded pencil eraser which indicated a vast increase in the wear resistance of the coloured oxide layer. A hardened steel needle was dragged over the surface of the coloured oxide film, under a range of weights, before and after the hardening stage. Although these tests are crude and quite unorthodox, it was found that the load required to scratch the surface coating increased by over an order of magnitude post hardening, thus serving their purpose well (9). A large advantage of doing the hardening process, other than the increased number of the achievable colours, is the formability of the steel post colouring. The oxide formed does not loose colour during many forming processes, such as deep drawing or stretch forming, but may need to be protected against scratching from machinery, for which a thin polyethylene sheet is adequate. The colour intensity is retained almost to the point of material fracture. If the chemically coloured oxide is left unhardened it is extremely soft and is not able to retain its colour during any form of useful engineering processing (9). The corrosion resistance of the coloured films have been tested in the hardened and non-hardened conditions under long-term industrial and marine exposure. Both hardened and non-hardened films maintain colour without degradation, and film integrity remains unaffected. The chemically oxidised films exhibit corrosion properties similar to that of the stainless steel substrate, and the hardening process appears to have little effect on the corrosion properties of the coloured oxide film in either of the mentioned environments, but when tested under BS 4601:1970 conditions, the hardened coloured panels show less rust than unhardened coloured

14 panels and uncoloured panels, these results were also true for a pitting corrosion test carried out in a ferric chloride solution at room temperature for 1 hour (1). These properties make it an extremely attractive material for the use in outdoor architecture form a mechanical and corrosion resistance standpoint. This chemically coloured stainless steel has been used as a cladding material in a number of applications globally (15). Under extended periods of dry heat (200°C for 3 weeks) the coloration changes in the early stages, but no further degradation takes place. The film therefore is not suited to higher temperatures, as a new oxide growth occurs resulting in colour change (9). There is very little said about the mechanisms driving the hardening process in any literature found containing notes on this process.

2.4 Thermal forming of coloured oxides The thermal colouring of stainless steels is quite a simple process and relies upon the stainless steel to react with the surrounding atmosphere at high temperatures in order to form an oxide interference film as outlined in Section 2.1 of this chapter. The following section will evaluate different methods of thermally forming these oxide layers. The thermal forming of coloured oxides is also known as “heat tinting” throughout industry (15). The process is largely dependent on atmospheric composition, temperature of environment, and time exposed to the environment. The temperature range over which oxide formation on AISI 304 stainless steel has been tested is between 300°C to 1150°C (7,16–19), over many separate studies. These studies have also been carried out over a range of times, from 4 minutes per sample, through to 6 hours and over, per sample. A range of atmospheric compositions such as pure 𝑂2, laboratory air, air + 𝐻2 𝑂 etc. and at varying pressures. Almost all of the papers looked at referenced to the colour of the oxides generated, with a few of them referencing to interference films. It is likely that the colour generated on the surface of the samples post thermal treatment is due to an interference film for every noted mention of the colouration of the stainless steel oxides.

15 The surface preparation of the sample is incredibly important, as this can have a large impact upon the surface composition of the samples, resulting in a variation of the nucleation sites for the possible oxides (chromium, ferrous or nickel) (3). This surface treatment can also have a large impact upon the adhesion of the first oxide layer to the surface of the stainless steel substrate, where the cleanliness of the surface is of high priority (20). One such method of generating coloured oxides thermally has been established by Higginson (7). A range of colours can be achieved by placing clean, bright annealed AISI 304 stainless steel in a box furnace in laboratory air environment at 650°C, for times from 1 through to 10 minutes. The colours achieved by this process were in the gold, red, purple and blue. Higginson uses spectrophotometer techniques to convert the colours into qualitative data using L, a*, b* and L, c, h methods to allow the reader to get a fuller understanding of the colours, and to allow them to actually reproduce the colours on Adobe Photoshop software using the L, a*, b* values, rather than relying on subjective descriptions of the colours (7). These colour measurement techniques will be evaluated later in this chapter. The thermally formed oxide film consists of a chromium-iron oxide spinel, where the chemical composition of the top of the oxide nodule was iron oxide of 𝐹𝑒2 𝑂3 structure, as outlined in Section 2.1and the base of the nodules seem to be a combination of chromium and iron oxides of 𝐴𝐵2 𝑂4 where A and B can 𝐹𝑒, 𝐶𝑟, 𝑁𝑖 or 𝑀𝑛 but predominantly 𝐹𝑒 or 𝐶𝑟. The radius of these oxide nodules tends to be less than approximately 500𝑛𝑚 (7).

Figure 2.4 - This Figure 2.represents the inhomogeneity of the oxide nodules (the white circles/ dots seen in

the above two images) grown on an AISI 304 stainless steel sample at 650°C for 7 minutes. From Higginson et al (7).

16 As the oxidising time increased, so did the size and density of coverage of the oxide nodules formed on the substrate AISI 304 stainless steel resulting in a change in the interference film characteristics, thus changing the colour (7). The oxide nodules are not evenly spread over the surface of the metal, resulting in an inhomogeneous oxide growth on the surface of the material depicted in Figure 2.5. As the oxide film developed, the homogeneity of the oxide increased, the nodule size and density of coverage increased, and the depth of oxide increased, resulting in the change of colours due to the oxide films interference nature. The colours progressed from gold, to red, to purple, to blue with increasing thermal treatment time up to 10 minutes (7). It has been shown in the past that the growth of an oxide results in a gain of mass on the surface of the material, over an extended period of time rather than a loss of mass (16) which results a certain thickness of oxide film formed (16). This indicates that the films form stable passive films rather than flaky oxide films. These oxide films are extremely thin; less than 500 nm thick when formed over a small period of time (less than 10 minutes) in order to establish interesting and aesthetically pleasing colours (7). When the stainless steel is kept under high temperatures for extended periods of time dull blacks, browns and greys are formed (16–18). The films are characterised as slow forming films due to the small amount of oxide grown over extended periods of time. The oxide film initially forms at a relatively high rate of about 0.36𝜇𝑔 𝑐𝑚−2 𝑝𝑒𝑟 𝑚𝑖𝑛𝑢𝑡𝑒 for the first 5-10 minutes then slows down and levels out at a much lower rate of about 6.5𝑛𝑔 𝑐𝑚−2 𝑝𝑒𝑟 𝑚𝑖𝑛𝑢𝑡𝑒 after approximately 30 minutes. At these lower times (less than 10 minutes) the rate of reaction follows a linear rate law, at the higher times (more than 10 minutes) they follow a parabolic rate law (16). Due to the porous nature of the film it does not offer much protective properties regarding corrosion or heat resistance and therefore the properties of the steel remain unchanged (16). When AISI 304 Stainless steel is oxidised for a long period of time (50 hours) mixed oxides are observed containing chromium-iron oxides (𝐶𝑟, 𝐹𝑒)2 𝑂3, with iron oxide on the outer surface of the oxide layer. At this length of time the oxide composition consists mainly of chromia with some iron oxide on the outer most surface. A safe

17 assumption is that these oxides do not exhibit aesthetically pleasing colours as Huntz et al (20) do not note its colour, and the colours of oxide films formed at 6 hours were generally dull browns and black according to Gulbransen (16). Due to AISI 304 stainless steel’s austenitic (FCC) atomic structure, it has a low chemical diffusion coefficient. Low diffusion coefficient means that the chromium composition in the stainless steel is insufficient to support the growth of a chromia protective film, which is why the formation of chromia spinel nodules occur. This is also why there is a formation of iron oxide as well as the chromium oxide despite chromium being the easiest oxidising alloy element (21,22), the diffusivity of the stainless steel cannot support further growth even when some chromium oxide is formed (20). The larger iron oxide fraction in the surface oxide is due to the much larger proportion of iron in the alloy in accordance with the Wagner equation (19) which tells us that if an alloy has a limited chemical diffusion coefficient, there is a minimum required atomic fraction for any oxidisable element, dependent upon its oxidisablity. As the atomic fraction of chromium in the stainless steel does not reach this required level a protective chromium oxide layer is not formed, which is why we end up with a porous oxide film which has little effect upon the corrosion resistance of the material (20). It appears that the oxide film is more dense at the grain boundaries at high oxidation temperatures (850-950°C) (20) which may also be the case at 600-650°C.

2.5 Measuring Colours Colour can be an extremely hard quality to quantify. Different descriptions have often caused issues throughout engineering in the past, including with the description of coloured oxides on stainless steels. Many of the papers which talk about the colours of chemically or thermally developed oxides describe the colours as being “gold”, “green”, “mauve”, “purple”, “blue”, “brown”, “golden-blue”, “pink”, “straw”, “straw-pink”, and the list of descriptions goes on. These are very subjective descriptions and are qualitative descriptions rather than the desired quantitive data, open for interpretation (1,3,4,6,16–18,20,23). Higginson (7) has identified a method for removing the confusion and ambiguity of these qualitative descriptions by giving numerical representations of the colours

18 using La*b* and Lch values. These values are used to quantify these colours by attributing numerical values to L, a* and b* variables for one technique, and L, c and h for another. The first system for the numerical measurement of colour was developed by Munsell in in 1905 and was called the Munsell Colour System (24). Munsell (24) separated colours into 3 categories, Value (lightness), Chroma and Hue. Hue is defined as how we would naturally describe colours as red, blue green etc. What Munsell (24) did with Chroma is separate it into 10 definite colours which were red, yellow-red, yellow, yellow-green, green, blue-green, blue, blue-purple, purple, purple-red. An example of a hue wheel has been included in Figure 2.6. Value or “lightness” gives a description of the shade or tint of a colour. Munsell (24) defines shade as being a darker colour, or nearer black, and tint as being a lighter colour or, nearer white. Figure 2.7 gives a pictorial representation of lightness for multiple hues. Chroma is defined as how vivid the colour is, and is seen as a gradient from the most vivid hue, through to grey. Chroma is sometimes understood as being the “saturation” of the colour. Figure 2.8 offers a pictorial representation of Chroma, more specifically the Chroma of red (24– 26).

Figure 2.6 A representation of a Hue Wheel, demonstrating the 4 key constituents of the 10 basic hue colours. From X-Rite (28)

Figure 2.6 A 3D illustration on how value or lightness appears. From X-Rite (28)

Figure 2.6 A 2D representation of the Chroma gradient of a red hue (28)

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Figure 2.7 A pictorial representation of the La*b* opponent system for colour representation. From Berns (25)

Modern colour measurement systems are based upon The Munsell Colour System. Two modern colour measurement systems are the CIELCH or CIELAB methods. There are other methods of colour measurement, which are outside the scope of this report. CIELCH (Lch) uses a polar co-ordinate system to define colours using lightness, chroma and hue descriptors (27). This is a very difficult system to visualise, but is generally accepted as being an accurate way of reproducing perceived colour. CIELAB (La*b*) uses a Cartesian co-ordinate system in order to describe its colours (28). CIELAB is an opponent type system which uses Cartesian co-ordinates to describe colour or the lack thereof. CIELAB uses the co-ordinates of L, a* and b* in order to describe its colours, where L is a measure of lightness in the same way as defined by Munsell (24), a* represents redness or greenness whilst b* represents blueness or yellowness. The higher the L value the lighter the colour, the higher the a* value the more red the colour, thus the lower the a* value the more green the colour. The higher the b* value the more yellow the colour, so the lower the b* value the more blue the colour (25). Figure 2.9 gives a good pictorial representation of how the La*b* system represents colour. Although La*b* measurements are generally accepted to have less accurate representation of perceived colour, it is much easier for the reader to visualise how the altering of La*b* values correspond to the changing colour of an object. La*b* values can also be used to regenerate colours with easy by using readily available software such as Adobe Photoshop CS6, as previously mentioned in Section 2.4. This system was developed by Hunter(29), along with algorithms for the assignment of numerical values to the La*b* variables, a system which is now used widely in colour measurement systems such as spectrophotometers.

20

2.6 Hull Cell A Hull Cell is an electroplating device which allows the user to test a range of current densities upon one cathode in one experiment simultaneously (30) The range of current densities tested in each test covers an order of magnitude. This equipment itself is very low cost, and allows the user to run cheap experiments which allow the examination of a range of current densities in one test, resulting in saving a great deal of time compared to if a scientist followed the traditional trial and error method for testing a range of current densities (27). As far as electrochemical devices go this is an incredibly useful one as this will generate an electrodeposit which will be of varying quality due to the varied current density on the surface of the sample which allows the electroplater to find the most effective current density for a specific plating process (28). A Hull Cell works on the same principals as any electrochemical deposition system; relying on a cathode, anode and electrolyte to conduct any experiment. A Hull Cell has set dimensions and is designed to be run with a set volume of electrolyte which is demonstrated by Figure 2.9 (27). These set dimensions allow the user to use a Hull Cell Ruler (an example of which can be seen in Figure 2.10) in order to establish the current density along different sections of the cathode, allowing the user to examine the electroplated surface quality and attain the corresponding current density which resulted in the quality of electrodeposit demonstrated in a given segment of test sample (29).

Figure 2.9 Dimensions of a 267ml Hull Cell. From Smirnov (30)

Figure 2.9 Image of a Hull Cell Ruler, and Hull Cell sample. The numbers on the right hand side above “Hull Cell Scale” indicate the current density above that width of sample in 𝒂𝒎𝒑𝒔/𝒇𝒕𝟐. This can be converted to 𝒎𝑨/𝒄𝒎𝟐. From Allardyce (31)

21

Chapter 3 : Experimental Procedures Two different sizes of AISI 304 stainless steel samples were used. The smaller samples or “coupons” had dimensions of 20mm x 20mm x 2mm squares with a small 3mm diameter hole in a corner of the coupon for hanging in electrochemical treatment baths. The total surface area of the coupons were approximately 370mm2, giving a high surface area over which to form a continuous oxide film. The larger samples had dimensions of 101mm x 75mm x 2mm flat rectangles. The total surface area of the sample was approximately 7575mm2.

3.1 Sample Mounting and Optical Microscope Investigation The first experiment conducted was to establish the manufacturing route of the AISI 304 stainless steel samples, in order to gather more information on the samples, to gain a better understanding of how the oxides form and develop on the material surface during thermal processing. This information will also allow the differentiation of how the material processed in this particular way might behave differently to the same material processed slightly differently. In order to observe the microstructure of the AISI 304 stainless steel base metal, a coupon was cut into 4 sections, in order to allow optical microscopic examination of the 3 different planes within the material as shown in Figure 3.1.

Figure 3.1 - This Figure illustrates the planes of the coupon examined under an optical microscope as described in Section 3.1.

22 Once the coupon had been cut into 4, 3 sections were used to view the 3 planes within the material and then mounted into Bakelite to make the samples easier to handle and polish due to their small size. Once mounted the samples polished to 1𝜇𝑚 then observed at 200× magnification. The images captured of the microstructure at this magnification can be observed in Chapter 4 Section 4.1, Figure 4.2.

3.2 Thermal Oxidising Procedures To test how the hardening bath affected the stainless steel coloured thermal oxide films, first the oxide films were formed. The oxides were formed by The following process: First removing the protective film and then the surface cleaned with cotton wool soaked in methanol, followed by a thorough rinsing in methanol in order to remove all adhesive, grease, and any other inclusions. Once rinsed in methanol the sample was dried using a hot air gun, ensuring not to leave any sort of drying pattern on the surface as these drying patterns resulted in unwanted coloured patterns post heat treatment as seen in Figure 3.2. Once the sample was clean it was quickly placed on a ceramic sample stand, in the oven, at 600°C using oven gloves, safety glasses and long tongs in order to ensure personal safety during this procedure. The samples were oxidised for 4, 8 and 10 minutes in order to generate the thermal oxide colours required. The colours generated were gold, red-purple and blue-purple respectively.

Figure 3.2 – This figure shows a coupon heat treated at 600°C for 10 minutes which has two obvious colour defects on the front left and right corners, due to methanol drying patterns on the front right and left corners.

23

When removing the samples from the oven, care was taken to avoid scratching the surface with the long tongs, or dropping the samples to promote uniform surfaces. Once removed the samples were placed directly on a cinder block where cooled in laboratory air atmosphere. Once the samples were cooled to touch they were stored in a box and covered to reduce the risk of contamination/ surface damage.

3.3 Colour Measuring Procedure An X-rite Colour Spectrophotometer was used to quantify the colours generated on the sample surfaces. The spectrophotometer was calibrated before each batch of colour measurements in order to increase the precision of the results. The spectrophotometer was then used to gain L, a* and b* values by CIELAB as explained in Chapter 2, Section 2.5. L is a measure of the “lightness” of the colour where a higher L value is a lighter colour. a* is a measure of the colour component from red to green where a more positive a* value represents a more red hue and a more negative a* value a more green hue. b* is also a measure of the colour component, measuring yellow to blue where a more positive b* value represents a more yellow hue, and a more negative b* value represents a more blue hue. For the coupons, one La*b* reading was taken. For the larger samples generated for Hull Cell experimentation, 3 values were taken to represent how the colour of the oxide is effected by the current densities being examined. The sectioning of the Hull Cell samples for the 3 La*b* readings is discussed in Section 3.7 of this Chapter.

3.4 MEF3 Analysis Procedure An MEF3 Optical Microscope was used to produce colour images of the microstructure of the samples at low magnifications (x200). The sample surfaces were kept as clean as possible before examination using the MEF3 microscope, but no special sample preparation was done before examination.

3.5 Cathodic Electrochemical “Hardening” Procedure Table 3.1 represents the chemical composition of the electrochemical “hardening” bath. Due to the toxic, carcinogenic, mutagenic and corrosive nature of the

24 constituent compounds (especially Chromium Trioxide), the process of setting up the electrochemical bath was done in a fume cupboard to inhibit inhalation of toxic chemicals. Care was also taken with the handling of chemicals and those not being used were kept covered in the fume cupboard or in relevant storage areas to reduce the risk of spilling or contamination. The addition of Fumetrol 21 was to suppress any toxic, carcinogenic hexavalent chrome ions (𝐶𝑟 6+ ) from the chromium trioxide making it into the surrounding atmosphere as it is a mist suppressant with the expressed property of reducing 𝐶𝑟 6+ release into atmosphere (33). Before the beaker was filled with the chemical components, a magnetic stirrer was placed in the beaker to induce forced convection of the system for mixing. The electrochemical bath was prepared by adding chromium trioxide flakes to a beaker, which was then topped up to the relevant volume using deionised water. Finally phosphoric acid and Fumetrol 21 were added to the bath or “electrolyte”. Table 3.4 – Cathodic hardening bath solution chemical constituents

Constituents

Volume

Chromium Trioxide (𝐶𝑟𝑂3 )

250𝑔/𝑙

Phosphoric Acid (𝐻3 𝑃𝑂4 )

1.5𝑚𝑙

Fumetrol 21

4.1𝑚𝑙

Deionised Water (𝐻2 𝑂)

To make solution up to 1𝑙

Once the solution was prepared a lead anode was established by submerging strips of lead into the solution, which were then anodised by an external power supply. A brass rod was placed over the electrolyte filled beaker, from which the coupons could be hung and then made to be the cathode of the electrochemical system. Once the anode and cathode were set up and the power supply is correctly connected to establish an anode and cathode, the solution could be heated using a hot plate and thermometer (making sure the thermometer was not touching any surface equipment surface) to maintain the desired temperature. Whilst heating, the solution was stirred using the magnetic stirrer. Temperatures used were 50°C, 40°C, and room temperature (22°C - 21°C). No heating was required for the room temperature experiments. An illustration of the hardening bath set up can be seen in Figure 3.3.

25

Figure 3.3 – An illustration of the physical set up of the cathodic hardening bath used to electrochemically treat the thermally oxidised coupons.

When the temperature was established the correct current was set up to generate the correct current density over the surface of the coupons. The magnetic stirrer was then stopped, the coupon immersed in the electrolyte, the current turned on and the timer started. Whilst the experiment was in progress the temperature was closely studied to minimise fluctuation but, due to the lack of thermocouples, could not always be reliably controlled. The samples were electrochemically treated for set times. The times examined for were 15, 20, 25, 30, 60 120, 450, 600 and 750 seconds. Once the time had elapsed, the current was turned off, the sample removed from the solution and rinsed with deionised water into a waste chromic acid beaker, which was then disposed of in a controlled manor in accordance with chromic acid MSDS (34). Once the samples were dry they were stored in a box and covered in order to reduce the risk of contamination/ surface damage.

3.6 FEGSEM and Dual Beam SEM Procedure Sample topographical imaging was done using a LEO 1530VP Field Emission Gun Scanning Electron Microscope (FEGSEM) and a Nova 600 NanoLab Dual Beam SEM-FIB. The samples were examined using the in-lens detectors on both of the microscopes in order to obtain high magnification, high detailed images. An added benefit of using in-lens detectors is that it results in very clear pictures of the nodules due to charging as explained in Chapter 5 Section 5.2.1.

26 As there were two different SEMs used, the magnifications were modified in order to give very similar fields of view resulting in consistent image width throughout testing and in the results section of this report in Chapter 4. SEM images were taken of the untreated AISI 304 stainless steel samples, the thermally oxidised samples and the thermally oxidised, electrochemically treated samples.

3.7 Hull Cell Procedure AISI 304 stainless steel samples of dimensions 101mm by 75mm by 2mm were thermally oxidised as outlined in Section 3.2 of this chapter. Before the electrochemical process took place the samples were labelled with segments 1, 2, and 3 as in Figure 3.4 in order to laterally segment the large samples when analysing La*b* values. Segment 1 was always of the highest current density. The Hull cell procedure was designed to work upon the same principals as the cathodic hardening procedure but to examine a range of current densities at once, as discusses in Chapter 2 Section 2.6. As this procedure is highly sensitive to chemical composition and the sample size is considerable, after only 5 tests the electrolyte used was disposed of as waste and a fresh electrolyte solution prepared and used. Batches of 600ml of electrolyte solution were made using the compositions in Table 3.2 using careful measurements. Table 3.5 – Hull Cell electrolyte solution chemical constituents for 600𝒎𝒍

Constituents

Volume

Chromium Trioxide (𝐶𝑟𝑂3 )

150𝑔

Phosphoric Acid (𝐻3 𝑃𝑂4 )

0.9𝑚𝑙

Fumetrol 21

2.46𝑚𝑙

Deionised Water (𝐻2 𝑂)

To make solution up to 600𝑚𝑙

27 A standard Hull Cell was used, with dimensions conforming to the sketch seen in Figure 3.5. The cell was initially set up dry by placing the anode and cathode material in the correct areas of the cell, as labelled in Figure 3.5. The electrodes were then secured in order to maintain constant distances between the electrodes. A volume of 267𝑚𝑙 of electrolyte solution was then poured in to the cell. The cell was then placed inside a large beaker lined with absorbent paper in order to obstruct any solution expelled from the cell during the reaction time. The cell was then electrified and the timer started. After the allotted time period the cell was de-electrified and the cell removed from the flask. The sample was then removed from the cell and rinsed with deionised water then dried in cold air before being stored for examination. The lead anode was then removed and rinsed with deionised water. After 5 Hull Cell Experiments, the used electrolyte solution was decanted from the cell which was then rinsed thoroughly with deionised water.

3.8 XPS Procedure Surface chemical analysis and depth profiling chemical analysis were carried out using a Thermoscientific K-Alpha X-ray Photoelectron Spectroscopy using a monochromated 𝐾𝛼 Aluminium X-ray source. This process was carried out on the untreated AISI 304 stainless steel samples, thermally oxidised samples and thermally oxidised, electrochemically treated samples in order to gain information on the chemical composition of the oxide films and upper atomic levels of bulk substrate.

Figure 3.4 – An image of a hull cell sample post treatment with the segments labelled and clearly separated

Figure 3.5 – Illustration of the physical set up for the Hull Cell apparatus. The anode is an inert Lead anode, the cathode is a thermally oxidised stainless steel sample. All dimensions are accurate

28

Chapter 4 Results The results are presented in six sections; un-oxidised, untreated stainless steel sample optical micrographs, colour measurements, optical micrographs of thermally and electrochemically treated stainless steels, field emission gun scanning electron microscope (FEGSEM) images, Hull Cell sample code and photographs, and chemical analysis results. These sections are then brought together and considered in the discussion section. Each sample has been labelled with a code that outlines the samples thermal and chemical treatments. An example of one of these codes is as follows: TT8m ET15s22°C10mA. TT stands for Thermal Treatment at 650°C which is followed by the thermal treatment time (𝑥m = 𝑥 minutes). ET stands for Electrochemical Treatment which is followed by the treatment time (𝑥s = 𝑥 seconds), treatment temperature (𝑥°C), and current density (𝑥mA = 𝑥 𝑚𝐴𝑐𝑚−2 ). The coding is therefore to be viewed in the following format: Thermal Treatment: 𝑥 minutes, Electrochemical Treatment: 𝑥 seconds, 𝑥°C, 𝑥 𝒎𝑨𝑐𝑚−2

If we take our example: TT8m ET15s22°C10mA we can deduce that this sample was thermally treated at 650°C for 8 minutes, and electrochemically treated for 15 seconds at 22°C under a current density of 10 𝑚𝐴𝑐𝑚−2

4.1 Un-oxidised, Untreated Stainless Steel Sample Optical Micrographs The chemical composition of these samples, i.e. the samples substrate is shown in Table 4.1, which has been obtained by using XPS procedures as denoted in Chapter 3, Section 3.8. Only the key elements have been shown, as every peak of the trace picked up by XPS survey scan of the bulk stainless steel material can be accounted for by key element peaks. In Figure 4.1 the vertical lines are binding energies for the key element, which are shown to account for every peak shown. There seems to be no manganese in this particular batch of AISI 304 stainless steel. Table ‎4.1 – Chemical Composition of the AISI 304 Stainless Steel Samples

29

Figure 4.1 Shows an XPS survey of sample TT0 ET0s0°C0mA showing the bulk composition of the metal. This batch of metal is used throughout this report and is representative of the bulk metal composition for every sample.

Figure 4.2 shows non-oxidised, untreated, AISI 304 stainless steel polished to 1 micron finish and etched in Shaftmeisters reagent as described in Chapter 3 Section 3.1. Twinning can be seen throughout the micrographs, and directional alignment of grains can also be seen in image c following the red arrow. An explanation, and diagram, of x and y plane configuration of the test coupon can be found in Chapter 3 Figure 3.1. Each image is taken at 200 x magnification.

a

b

c

Figure 4.2 These images show the microstructure of a TT0m ET0s0°C0mA sample. Image a. is a micrograph of the top surface, b the inner x plane, and the inner y plane.

30

4.2 Colour Measurements La*b* values were taken for each sample and coupon, before and after electrochemical treatment and are listed in Tables 4.2-4.4. The complete table of La*b* values for all small coupons tested, before and after electrochemical treatment can be found in Appendix Table 1. Table 4.2 gives the La*b* values for the coupons of key interest to this report, both before and after electrochemical treatment. It also shows the thermal and electrochemical treatment conditions for each coupon. Post electrochemical treatment the La*b* values for samples TT4m ET0s0°C0mA and TT8m ET0s0°C0mA are very similar. Table 4.3 is a table of La*b* values for Hull Cell samples before electrochemical testing, at points 1, 2 and 3. These can be directly cross-referenced to any samples oxidation data in the same table. Table 4.4 shows the same Hull Cell samples post electrochemical treatment with electrochemical treatment conditions. “Sample Number” relates to their sample number throughout testing, and in Appendix Table 1.

31

Sample Numer

0 5 10 11 20 9 18

44 43 23 29 25 24

Thermal Treatment Electrochemical Treatment PhotoSpectroscopy Examination After Electrochemical Treatment Oxidising Oxidising Temperature Current Density Before Electrochemical Treatment Time (mins) Time (s) a* b* L a* b* Time (mins) Time (s) (°C) (mA/cm^2) L 0 0 0 0 0 0 83.57 0.13 5.70 4 240 0 0 0 0 57.01 9.30 21.05 4 240 0 0 0 0 56.63 9.61 25.03 8 480 0 0 0 0 46.50 8.99 9.67 8 480 0 0 0 0 42.08 6.27 0.04 4 240 3.5 210 40 20 55.42 9.94 19.51 63.55 2.45 16.77 8 480 7.5 450 40 10 42.97 9.43 4.83 57.53 2.37 12.20 4 240 0.25 15 22 20 68.31 5.16 25.24 67.39 5.28 26.03 4 240 0.5 30 22 20 64.35 6.83 25.28 59.65 3.52 26.70 8 480 0.25 15 22 10 42.75 8.11 2.73 41.63 7.78 1.36 8 480 0.33 20 22 10 43.53 8.24 3.22 44.75 5.60 13.06 8 480 0.42 25 22 10 44.53 8.31 5.27 52.08 3.26 16.84 8 480 0.5 30 22 10 45.86 8.61 6.93 54.87 2.76 15.67

Table 4.2 Table of values for the coupons containing thermal treatment, electrochemical treatment and La*b* data for 20mm by 20mm by 2mm coupons

Sample Code TT0m ET0s0°C0mA TT4m ET0s0°C0mA TT4m ET0s0°C0mA TT8m ET0s0°C0mA TT8m ET0s0°C0mA TT4m ET210s40°C20mA TT8m ET450s40°C10mA TT4m ET15s22°C20mA TT4m ET30s22°C20mA TT8m ET15s22°C10mA TT8m ET20s22°C10mA TT8m ET25s22°C10mA TT8m ET30s22°C10mA

32

Sample Code TT5m ET0s0°C0A TT5m ET0s0°C0A TT5m ET0s0°C0A TT5m ET0s0°C0A TT5m ET0s0°C0A TT10m ET0s0°C0A TT10m ET0s0°C0A TT10m ET0s0°C0A TT10m ET0s0°C0A TT0m ET0s0°C0A TT12m ET0s0°C0A TT10m ET0s0°C0A TT10m ET0s0°C0A TT10m ET0s0°C0A TT10m ET0s0°C0A 5

5

5

5

5

600

600

300

300

300

300

300

Thermal Treatment Oxidising Oxidising Time (mins) Time (s)

10 600

720

10

600

10

12 600

-

10 600

600

10 600

10

10

-

10

L 65.76 64.81 59.93 61.4 62.82 38.13 38.43 38.59 37.34 39.4 49.53 41.33 41.3 40.06

a

Point 1 5.97 6.54 8.74 8.34 7.42 4.92 6.64 4.6 5.46 3.28 11.23 9.91 12.08 9.76

b 28.69 27.92 28.35 28.43 17.92 -4.1 -2.15 -3.78 -5.11 -4.18 37.06 5.75 9.51 4.82

L

68.29 64.57 63.02 63.2 65.94 37.9 38.19 38.05 36.98 38.63 53.49 43.73 44.13 39.74

a

4.44 6.92 7.46 7.68 6.33 5.25 7.45 5.13 5.29 2.38 9.21 11.36 12.34 10.21

26.36 28.08 28.59 28.81 27.76 -4.2 -3.01 -4.4 -5.31 -5.54 38.1 11.5 13.46 4.57

Before Electrochemical Testing Point 2 b L

Table 4.3 Table of La*b* values for the Hull Cell samples before electrochemical testing

Sample L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L12 L13 L14 L15

Thermal Treatment Electrochemical Treatment Sample Code Oxidising Oxidising Temperature Applied Time (mins) Time (s) Time (mins) Time (s) (°C) Current (A) L TT5m ET0s0°C0A 5 300 TT5m ET60s21°C0.5A 5 300 1 60 21 0.5 TT5m ET0s0°C0A 5 300 TT5m ET60s21°C0.4A 5 300 1 60 21 0.4 TT5m ET60s21°C0.3A 5 300 1 60 21 0.3 TT10m ET240s21°C1A 10 600 4 240 21 1 TT10m ET120s21°C1A 10 600 2 120 21 1 TT10m ET60s21°C1A 10 600 1 60 21 1 TT10m ET30s21°C1A 10 600 0.5 30 21 1 TT0m ET0s0°C0A TT12m ET20s21°C0.2A 12 720 0.33 20 21 0.2 TT10m ET0s0°C0A 10 600 TT10m ET60s21°C1A 10 600 1 60 21 0.4 TT10m ET15s21°C0.2A 10 600 0.25 15 21 0.2 TT10m ET120s21°C0.1A 10 600 2 120 21 0.1

Table 4.4 Table of La*b* values for the Hull Cell samples after electrochemical testing

Sample L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L12 L13 L14 L15

a

a

Point 3

b

6.02 8.98 7.93 8.58 7.57 4.63 6.61 5.24 3.34 1.97 11.7 11.22 11.33 7.89

1.93 2.14 2.1 2.09 2.74 2.31 3.2 3.37 3.06 4.58 8.23

Point 1

65.49 59.42 62.42 60.95 63.09 37.57 38.29 38.04 37.49 39.52 47.42 43.33 43.11 39.48

68.39 66.85 68.15 53.79 52.47 53.37 49.63 43.32 55.92 52.57 39.2

b

L

69.83 66.09 70.92 52.04 53.07 52.48 49.28 42.01 57.69 53.9 37.87

a

1.75 2.5 1.59 2.81 3.08 2.76 3.24 3.65 3.01 4.56 8.22

17.37 22.19 18.36 13.45 15.31 13.31 14.04 10.27 17.26 21.45 -0.71

After Electrochemical Testing Point 2 b L

28.67 28.95 27.98 28.25 28.36 -5.27 -2.33 -3.84 -6.27 -4.74 33.29 10.73 11.09 1.16

18.01 18.13 22.29 9.94 13.45 11.43 13.92 10.71 16.5 21.56 2.2

65 65.3 65.09 49.29 51.67 49.73 46.75 38.67 65.63 45.22 37.68

a

Point 3

2.47 2.45 4.23 3.04 3.23 3.08 3.08 2.6 3.17 7.87 5.44

b

21.28 21.35 28.82 12.81 14.93 13.4 11.85 1.84 17.99 19.8 -3.45

33

4.3 Optical Micrographs of Thermally and Electrochemically Treated Stainless Steel Optical micrographs were taken of the thermally coloured oxide surfaces. These images show that the coloured oxides are made up of microscopic areas of different colours as shown in Figures 4.3-4.6. Figure 4.3 shows that TT4m ET0s0°C0mA coupons consists of a mainly golden coloured microstructure with small areas of red microstructure. Figure 4.4 shows that TT8 ET0s0°C0mA coupons consist of gold, red and blue microstructures amounting to a red-purple appearance. Figure 4.5 demonstrates that Hull Cell sample TT10m ET120s21c0.1A surface was made up of red, blue and gold microstructures, which again amounted to a red-purple appearance. Sample TT10m ET120s21°C0.1A consisted of blue and gold microstructure with small areas of red. This microstructure is heavily dominated by blue, which gives it its blue appearance.

Figure 4.3 Optical micrograph of sample TT4m ET0s0°C0mA

Figure 4.4 Optical micrograph of sample TT8 ET0s0°C0mA

Figure 4.5 Optical micrograph of non-electrochemically treated sample TT10m ET0s0°C0mA

Figure 4.6 Optical micrograph of electrochemically treated sample TT10m ET120s21°C0.1A

34

4.4 Field Emission Gun Scanning Electron Microscope (FEGSEM) Images In this section a “FEGSEM image“ refers to a micrograph taken, using a FEGSEM as per the procedure outlined in Chapter 3 Section 3.6. Figures 4.7 to 4.17 are FEGSEM images of relevant unoxidised, oxidised, electrochemically untreated and electrochemically treated samples. 4.4.1 Untreated sample Figure 4.7 shows a FEGSEM image of an untreated AISI 304 stainless steel sample. There appears to be porosity seen as small black dots on the sample surface, grain boundaries which appear as cracks and evidence of crystallographic orientation seen as waves on the sample surface. There are also a small number of white dots on the sample surface.

Figure 4.7 TT0m ET0s0°C0mA - FEGSEM image of the bright annealed AISI 304 stainless steel.

35 4.4.2 Thermally Treated Samples Figures 4.8 and 4.9 show thermally treated, electrochemically untreated samples TT4m ET0s0°C0mA and TT8m ET0s0°C0mA. A high density of small white dots can be seen on the surface of these samples. There is also a slightly more roughened looking wavy appearance on both samples. The white dots seen in Figures 4.8 and 4.9 are of a much higher density, and are of larger size than the white dots on sample TT0m ET0s0°C0mA in Figure 4.7. The density of the white dots are higher and more extensive for sample TT8m ET0s0°C0mA than for TT4m ET0s0°C0mA. a

b

Figure 4.8 TT4m ET0s0°C0mA – FEGSEM images of a stainless steel sample oxidised at 600°C for 4 minutes (240 seconds). These micrographs were taken before any electrochemical treatment took place.

a

b

Figure 4.9 TT8m ET0s0°C0mA - FEGSEM images of a stainless steel sample oxidised at 600°C for 8 minutes (480 seconds). Image b has higher magnification than image a. These micrographs were taken before any electrochemical treatment took place.

36 4.4.3 Thermally and Electrochemically Treated Samples Figure 4.10 and 4.11 show electrochemically treated samples TT4m ET210s40°C20mA and TT8m ET450s40°C10mA. The white dots can again be seen on both samples but at a much lower density. The coverage of the white dots is also greatly decreased when compared to samples TT4m ET0s0°C0mA and TT8m ET0s0°C0mA in Figures 4.8 and 4.9. There is much greater coverage of the wavy topography which now appears in swathes, indicating the deposition of new matter on the sample surfaces in Figures 4.10 and 4.11 when compared to Figures 4.8 and 4.9 respectively. a

b

Figure 4.10 TT4m ET210s40°C20mA - FEGSEM images of a stainless steel coupon oxidised at 600°C for 240 seconds then electrochemically treated for 210 seconds at 40°C with a current density of 20𝒎𝑨𝒄𝒎−𝟐

a

b


37 4.4.4 Thermally and Electrochemically Treated TT8m Series Samples Figures 4.12 to 4.15 show FEGSEM images of samples TT8m ET15s22°C10mA through to TT8m ET30s22°C10mA, which are clearly labelled. Interestingly the density of the white dots decreases as the time of electrochemical treatment increases. There is also an increased appearance of the wavy topography, indicating that as the electrochemical treatment time increases more the deposited matter is deposited. Both of these phenomenon can be seen most clearly in the higher magnification micrographs in each figure (Figure 4.12b, 4.13b, 4.14b and 4.15d). Figure 4.15 contains images from different areas of sample TT8m ET30s22°C10mA. The images are of the sides of the sample, the middle of the sample and the transition space between the two. This images show that the density of the white dots is far higher at the centre of the sample than at the sides. There appears to be a higher white dot density in the middle of the sample than at the sides. Please see the following two pages for these figures.

38

a

b


a

b


a

b


39 a

b

c

d

e

Figure 4.15 TT8m ET30s22°C10mA - FEGSEM images of a stainless steel coupon oxidised at 600°C for 480 seconds then electrochemically treated for 30 seconds at 22°C with a current density of 10𝒎𝑨𝒄𝒎−𝟐 . Images a and b are taken at the middle of the steel coupon, images c and d at the edge of the coupon and image e shows the transition between the differing densities of the white dots

40 4.4.5 Thermally and Electrochemically Treated Hull Cell Samples Figures 4.16 and 4.17 show FEGSEM images for Hull Cell sample TT10m ET120s21°C0.1A before and after electrochemical treatment. There are areas of the sample images especially 4.16a and 4.17a which show areas of little to no white dots, this is due to sample preparation before thermal treatment (similar to the drying pattern explained in Chapter 3, Section 3.2, Figure 3.2) and has no effect upon electrochemical treatments or coloured appearance. There is no observable change in the density of the white dots, but an increase in porosity seen at the sample surface post treatment, seen as the small black dots encircled in Figure 4.17a. There is inconclusive evidence of an increase in the wavy topography indicative of the deposited material. Finally the white dots appear to be less bright in Figure 4.17b compared to Figure 4.16b due to the grey appearance of the centre of the dots. a

b

Figure 4.16 Hull Cell sample TT10m ET120s21°C0.1A before treatment

a

b

Figure 4.17 Hull Cell sample TT10m ET120s21°C0.1A after treatment

41

4.5 Hull Cell Sample Code and Photographs The coding for Hull Cell samples is slightly different to the coding used by the smaller coupons. The last digit (𝑥𝑚𝐴) denoting the current density for the coupons, has been replaced by an applied current (𝑥𝐴) for the Hull Cell samples. The code for Hull Cell samples is as follows: Thermal Treatment: 𝑥 minutes, Electrochemical Treatment: 𝑥 seconds, 𝑥°C, 𝑥 𝑨𝑚𝑝𝑠 An example of this code is TT10m ET15s21°C0.2A denoting that this sample has been thermally treated for 10 minutes, and electrochemically treated for 15 seconds at 21°C with an applied current of 0.2 Amps. This section contains photographs of Hull Cell samples. The brightness and contrast of these photographs have been altered to increase the clarity of the important factors of the photographs explained later in this section. These samples can be seen to be split into segments 1, 2 and 3 for reasons explained in Chapter 3 Section 3.7. The segments are labelled at the top of the samples in black marker. Figure 4.18 is split into two areas of different colours; gold and very pale gold. The golden area of the sample is thermally treated and electrochemically untreated, whereas the very pale gold area has been electrochemically treated in a Hull Cell as discussed in Chapter 3 Section 3.7. Figure 4.19 is also split into two clear area colours; red-purple and a dirty gold colour. The red-purple section is thermally treated and electrochemically untreated. The dirty gold colour has been thermally treated and electrochemically treated in a Hull Cell, again as discussed in Chapter 3 Section 3.7. Both figures show small gradients of colour on the far right of segment 3 in the electrochemically treated areas.

Figure 4.18 A photograph of Hull Cell sample TT5m ET60s21°C0.3A.

Figure 4.19 A photograph of Hull Cell sample TT10m ET15s21°C0.2A.

42

4.6 Chemical Analysis Results The following section contains graphical representations of depth profile chemical analysis obtained using XPS techniques as outlined in Chapter 3 Section 3.8. Identified in each graph are the following elements and compounds; iron, iron oxide, chromium, chromium oxide, nickel and oxygen. Carbon traces were removed to produce easier to read results. Throughout the XPS results the oxygen trace never reaches 0 even when etching in the bulk metal substrate; this is due to the re-passivation of the base metal due to an incomplete vacuum in the XPS chamber. 4.6.1 Untreated and Thermally Treated Samples Figure 4.20 shows direct comparisons of the atomic % vs etch times for the oxide films of samples TT0m ET0s0°C0mA, TT4m ET0s0°C0mA and TT8m ET0s0°C0mA. Sample TT0m ET0s0°C0mA has a very thin oxide film denoted by the iron and chromium oxide compositions at the lowest etch times. Sample TT4m ET0s0°C0mA reveals that after 4 minutes of oxidation, the iron and chromium oxide compositions have greatly increased and the depth of the oxide has also increased. Sample TT8m ET0s0°C0mA demonstrates very little change in the composition of the oxide film from sample TT4m ET0s0°C0mA, but the depth of both oxides greatly increased when compared to sample TT0m ET0s0°C0mA.

Figure 4.20 - The two key chemical constituents of the oxide films; chromium and iron oxides for samples TT0m ET0s0°C0mA, TT4m ET0s0°C0mA and TT8m ET0s0°C0mA shown as atomic % against etch time.

43 4.6.2 Thermally and Electrochemically Treated Samples Figures 4.21 and 4.22 are of XPS chemical composition traces for samples TT4m ET0s0°C0mA and TT4m ET210s40°C20mA respectively. Interestingly it seems that the iron oxide atomic % has greatly decreased which co-insides in the formation of iron metal in the oxide layers i.e. at etch times lower than 100 seconds, post electrochemical treatment for sample TT4m ET210s40°C20mA. The chromium oxide trace also changes appearing to show much more chromium oxide on the outer layer of the oxide, i.e. less than 100 seconds post electrochemical treatment for sample TT4m ET210s40°C20mA.

Figure 4.21 - This graph shows the XPS chemical composition traces for sample TT4m ET0s0°C0mA


Figure 4.23 and 4.24 show XPS chemical composition traces for samples TT8m ET0s0°C0mA and TT8m ET450s40°C10mA respectively. The iron oxide atomic % on sample TT8m ET450s40°C10mA also seems to have been decreased under electrochemical treatment which again co-insides with the formation of iron metal on the oxide surface i.e. at etch times less than 200 seconds. There evidence of deposition of chromium oxide onto the outermost surface, i.e. at etch times less than 200 seconds. It seems that there appears to be greater deposition of chromium oxide on sample TT8m ET450s40°C10mA than TT4m ET210s40°C20mA as the newly deposited chromium oxide appears to be seen at higher etch times in Figure 4.24 than on Figure 4.22, corresponding to a thicker deposition depth.

44



Figure 4.25 shows the same TT8m ET450s40°C10mA sample as in Figure 4.24 but this XPS trace has been made using a much slower etch with a similar scan speed in order to gain many more chemical composition scans per mm to give more detailed chemical traces, resulting in much smother curves. There appears to have been a very small amount of chromium metal deposition on the outer surface of the oxide which is more easily observed in the more detailed scan in Figure 2.25

Figure ‎4.25 - This graph shows more detailed XPS chemical composition traces for sample TT8m ET450s40°C10mA

45 4.6.3 Hull Cell Samples Figure 4.26 compares the key chemical composition traces of interest for sample TT10m ET120s21°C0.1A. The chromium oxide traces seem to be slightly different at the outer oxide layer, i.e. at etch times lower than 100 seconds post electrochemical treatment. The iron metal and oxide traces seem to be largely unchanged.

Figure 4.26 - This Graph compares the iron metal, iron oxide and chromium oxide traces for Hull Cell sample TT10m ET120s21°C0.1A before and after electrochemical treatment.

4.6.4 Thermally and Electrochemically Treated TT8m Series Samples This section of the report deals with the TT8m series of coupons which were electrochemically treated for increasing periods of time between 15 and 30 seconds. The following graphs demonstrate how each important chemical compound composition; iron oxide, iron metal, chromium oxide and chromium metal, alters with electrochemical treatment time. These graphs also compare the same chemical composition traces to samples TT8m ET0s0°C0mA and TT8m ET450s40°C10mA to directly compare how the oxides have changed from before electrochemical treatment, to how the chemical compositions change after extended electrochemical treatment at elevated temperature. Figure 4.27 demonstrates how iron metal compositions change with electrochemical treatment times for the TT8m series of thermally treated samples. It can be seen that a peak appears at the low etch times and this peak size increases with electrochemical treatment time. It can also be seen that the traces migrate left on the graph indicative that the oxide thickness is decreasing with increased electrochemical treatment time, up to 30 seconds. Finally, the gradients of the traces in stage 2 increases with increased electrochemical treatment time.

46 Figure 4.28 demonstrates how iron oxide composition changes with electrochemical treatment time. It is seen that the iron oxide depth decreases as the electrochemical treatment time increases by the decreasing etch time at which the iron oxide atomic % gets to 0 as the electrochemical treatment time increases. The peak iron oxide atomic % is also seen to decrease as electrochemical treatment time increases. The outer lay Sample TT8m ET0s0°C0mA seems to have an abnormally thin oxide film as it doesn’t appear to fit the trend seen with the rest of the samples and can be explained due to experimental error at the thermal oxidation stage as this was from a different batch to the other coupons used.

Figure 4.27 - This graph shows the iron metal XPS chemical composition traces for samples TT8m ET0s0°C0mA, TT8m ET15s22°C10mA, TT8m ET20s22°C10mA, TT8m ET25s22°C10mA, TT8m ET30s22°C10mA and TT8m ET450s40°C10mA

Figure 4.28 - This graph shows the iron oxide XPS chemical composition traces for samples TT8m ET0s0°C0mA, TT8m ET15s22°C10mA, TT8m ET20s22°C10mA, TT8m ET25s22°C10mA, TT8m ET30s22°C10mA and TT8m ET450s40°C10mA

Figure 4.29 demonstrates how chromium metal traces change with electrochemical treatment time. There is a small spike in atomic % at the lower etch times, corresponding to each samples relative inner oxide levels. Figure 4.30 demonstrates how chromium oxide traces change with electrochemical treatment time. Initially there is a very small amount of chromium oxide at the lowest etch times, i.e. at the outer most layer of the oxide (etch time less than 50 seconds), but this greatly increases as electrochemical treatment time increases. The depth of the oxide decreases as electrochemical treatment time increases as the etch time at which the chromium oxide atomic % value decreases to 0. The chromium oxide trace for sample TT8m ET0s0°C0mA has the same issue with fitting the trend of oxide

47 thickness as the iron oxide does, supporting the case that this is a comparatively shallower oxide compared to the other TT8m ET0s0°C0mA samples.

Figure 4.29 - This graph shows the chromium metal XPS chemical composition traces for samples TT8m ET0s0°C0mA, TT8m ET15s22°C10mA, TT8m ET20s22°C10mA, TT8m ET25s22°C10mA, TT8m ET30s22°C10mA and TT8m ET450s40°C10mA

Figure 4.30 - This graph shows the chromium oxide XPS chemical composition traces for samples TT8m ET0s0°C0mA, TT8m ET15s22°C10mA, TT8m ET20s22°C10mA, TT8m ET25s22°C10mA, TT8m ET30s22°C10mA and TT8m ET450s40°C10mA

48

Chapter 5 Discussion This chapter looks at the results gathered for the AISI 304 stainless steel samples, thermally treated samples and electrochemically treated samples and attempts to explain the phenomenon observed and looks at the mechanisms involved in the thermal and electrochemical treatment stages.

5.1 AISI 304 Stainless Steel Samples The AISI 304 stainless steel was first examined on its own, before any thermal or electrochemical treatment. The stainless steel processing route was characterised using optical microscopic techniques, and an understanding of the chemistry of the bulk sample and natural oxides for the stainless steel samples was found using XPS as denoted in Chapter 3 Section 3.8. 5.1.1 AISI 304 Stainless Steel Samples Processing Route As seen in the Chapter 4 Section 4.1, twinning is observed in each image in Figure 4.2 along with grain orientation in 4.2c. The microstructure observed gives an indication of how the samples were processed. The aligned grains are indicative that the samples were rolled when being processed into sheet form. The twins seen are likely to be annealing twins as the AISI 304 stainless steel is austenitic and therefore has an FCC structure. Twinning in an FCC structure is far more likely to be annealing twins rather than mechanical twinning, suggesting these stainless steel samples have been annealed (35). This information coupled with its smooth and shiny surface finish suggests the stainless steel had been bright annealed (36), before being covered by a protective Polyethylene sheet to retain the high standard surface finish. 5.1.2Surface Microscopic Imaging (FEGSEM) and Surface Chemistry (XPS) Figure 4.7 shows a surface topology with grain boundaries and what appears to be crystallographic orientation seen as a wavy topology. There are a significant number of small pores with some surface cracking, due to the rolling process, with large equiaxed grains as a result of the annealing process (35). There are a very small number of white dots also present; these are oxide nodules (7). The reasoning behind identifying these white dots as oxide nodules is discussed in Section 5.2.1 of this chapter. These nodules are again, as a result of the annealing process.

49 The XPS traces in Figure 4.20 shows an extremely thin oxide film already on the surface of sample TT0m ET0s0°C0mA, as you would expect from an austenitic stainless steel such as AISI 304 (37). This thin oxide film consists almost solely of chromium oxide and iron oxide. Figure 4.20 gives a graphical representation of the chemical composition of the key elements of the oxide films found using XPS which are iron oxide, and chromium oxide. The thickness of this oxide film has been controlled during the annealing process by being bright annealed, therefore having been annealed in a non-oxygen containing atmosphere to prevent any formation of extra oxide on the stainless steel surface.

5.2 Producing Coloured Stainless Steel Using Thermal Techniques The next step of the experimental side of this project was to generate the thermally coloured oxides on the surface of stainless steel samples, as per the experimental procedure in Chapter 3 Section 3.2, before electrochemically treating them. This thermal treatment results in the growth of the oxide layer. The process used for the growth of these oxide films is very similar to Higginson’s process (7), except the oxidising temperature used in this project for every sample was 600°C rather than Higginson’s 650°C. 5.2.1 XPS and FEGSEM Analysis of the Thermally Formed Oxides After 4 minutes of oxidation at 600°C, the iron and chromium oxide compositions have greatly increased from 7.1 and 5.4 atomic %, respectively for sample TT0m ET0s0°C0mA, to 25.2 and 10.7 atomic % respectively for sample TT4m ET0s0°C0mA, seen in Figure 4.20. The depth of the oxide increases which is obvious when looking at the iron oxide and chromium oxide traces in Figure 4.20 as the oxide atomic % values decrease to 0 at higher etch times as thermal treatment time increases indicating the increase in oxide thickness. The micrographs of sample TT4m ET0s0°C0mA in Figure 4.8 show many small white dots on the sample surface. The observed white dots are due to the charging of these small areas, more than the surrounding material (38). The fact these small areas are charging more than the surrounding material indicates that the path for charge movement is inhibited more at these points than the surrounding surface, signifying that the dots are actually oxide nodules which stand above the surrounding surface. These nodules create a longer path through insulating oxide ceramic material to the

50 conducting substrate underneath, which the charge can’t pass through, causing the nodule to become charged by the FEGSEM’s electron gun (39). This observation is supported by Higginson’s findings (40). The wavy appearance of the samples crystallographic orientation is much harder to see on sample TT4m ET0s0°C0mA in Figure 4.8 than on sample TT0m ET0s0°C0mA in 4.7. It appears increasingly roughened by the oxide growth. The reduced observation of the wavy crystallographic structure of the substrate below also indicates there is a layer of oxide that forms obstructing this from view. In order to satisfy the described observations a cross sectional structure of the oxide layer has been theorised and illustrated in Figure 5.1. This oxide structure holds true for all of the thermally treated samples as the same observations can be seen but to a larger extent on sample TT8m ET0s0°C0mA in Figure 4.9 due to a thicker oxide. It is therefore theorised that there are nodules formed which stand above an uneven oxide film. It is unknown at this stage what the oxide composition consists of either in the nodules or on the film. Oxide Nodules Oxide Layer Stainless Steel Substrate

Figure 5.1 This figure shows attempts to theorise about the cross sectional structure of the AISI 304 stainless steel oxide film structure after high temperature oxidation. The oxide consists of oxide nodules surrounded by an oxide layer on top of the stainless steel substrate.

After oxidation at 600°C for 8 minutes, sample TT8m ET0s0°C0mA’s iron and chromium oxide composition atomic % peaks have changed very little from TT4m ET0s0°C0mA oxide compositions atomic % peaks. The difference of 1.02 and 0.99 atomic% for iron oxide and chromium oxide (Figure 4.20) can be accounted for by variation in the atomic % of each compound throughout the uneven oxide surface and equipment sensitivity, meaning that the composition of the oxide has not changed significantly. The thickness of both the chromium and iron oxides, however, is shown to increase. The elongation of the tails of the TT8m ET0s0°C0mA chromium oxide and iron oxide traces by comparison with the same traces for

51 sample TT4m ET0s0°C0mA in Figure 4.20 indicate that the oxide is thicker and runs deeper into the substrate, causing the oxide atomic % to decrease to 0 at higher etch times. This also creates a larger gradient between oxide and metal substrate. The nodule density is observed to increase, which is demonstrated by Figure 4.9 when compared to sample TT4m 0s0°C0mA in Figure 4.8. The structure of the thermally formed oxide films have been theorised and proven by Higginson, Saeki and Huntz (7,17,23). This oxide film consists of an inner layer of chromium oxide, and an outer layer of chromium oxide with a spinel structure of iron and chromium oxides. The spinel structure indicates that the structure of the iron oxide and chromium oxide become interlinked forming interlinked FCC unit cells (41). All three sample’s oxide films in Figure 4.20 form the same structure; an innermost layer of chromium oxide with an outer layer of iron oxide. Evidence of this structure is found in Figure 4.21 for sample TT4m ET0s0°C0mA and Figure 4.23 for sample TT8m ET0s0°C0mA, As the XPS traces show that the iron oxide sits on top of the chromium oxide, and that there are atomic layers where both oxides exist, indicating a spinel layer. A revised illustration of the theorised structure of the oxide is shown in Figure 5.2. Iron Oxide Chromium Oxide Stainless Steel Substrate

Figure 5.2 This figure shows attempts to theorise about the cross sectional structure of the AISI 304 stainless steel oxide film structure after high temperature oxidation. The oxide consists of iron oxide nodules surrounded by an oxide layer of iron oxide and chromium oxide, with a spinel layer represented where the red and blue lines overlap, on top of the stainless steel substrate.

The structure of the thermally formed coloured oxides is very different to the oxide structure generated by the chemical formation of coloured oxides. The chemically formed oxides have iron oxide as the inner oxide layer and chromium oxide as the outer layer (1,5,9,10). This difference in the composition of the oxide is likely responsible for the inability of the thermally formed oxides to generate certain colours, such as green, which can be developed by chemical oxidation processes (1,5,9,10). The colours formed are due to the combined effect of the interference film

52 and absorption of certain wavelengths of light due to the chemical composition of outer layers of oxide. The combination of fact that the oxide film formed acts as an interference film as discussed in Chapter 2 Section 2.1, and that absorption and transmission of certain wavelengths of light will occur as the light interacts with the surfaces of the different oxides with different chemical compositions, which results in the production of the coloured appearance of the sample surface. This coloured appearance is dependent on the chemical composition and crystallographic orientation of the oxides involved, and the distance between the two oxide layers (25). As the nodule density and oxide thickness increases this has an impact upon the interference film which changes the colour of the oxide (25). The colours formed on the sample surfaces have been recorded using La*b* values as outlined in Chapter 3 Section 3.3. 5.2.2 La*b* Measurement of Thermally Treated Sample Oxide Layer Colours The La*b* values for the most important samples gathered in this report can be seen in Table 4.2. Samples 10 and 11 best represent sample types TT4m ET0s0°C0mA and TT8m ET0s0°C0mA respectively. The two samples appear quite different to the naked eye, as sample TT4m ET0s0°C0mA has a lustrous gold appearance, and sample TT8m ET0s0°C0mA has a lustrous purple-red appearance. These colours are can be generated from the recorded La*b* values using Adobe Photoshop CS6 as described in Chapter 2 Section 2.5, and can be viewed in Figure 5.3 images b and c respectively. The difference in L values reveals that the TT4m ET0s0°C0mA is a brighter colour as it has an L value of 56.63 compared to TT8m ET0s0°C0mA of 46.50 (25). This difference can be explained by the shallower oxide depth, and lower density of oxide nodules resulting in less absorption of light waves by the oxide, resulting in more light waves reflected off the bright substrate surface. This oxide would also offer less interference due to the limited oxide thickness. The results show very limited difference in a* values indicating the samples retain their shade of red between the two oxidation times (25). This retention of the red colour can be seen in Figures 4.3 and 4.4 as the surface microstructure demonstrates retained areas of red amongst blue areas in Figure 4.4. This retention of microstructure colouration is also mirrored by Higginson (7). Finally there is a substantial difference in b* values between the two oxidation times. Sample TT4m ET0s0°C0mA’s b* value of 25.03 is much higher than TT8m ET0s0°C0mA’s of 9.67. This decrease in b*

53 value is indicative that the sample colour becomes more blue (25). This is demonstrated by the difference in surface microstructural colour seen in Figures 4.3 and 4.4 with the appearance of blue areas on the microstructure in Figure 4.4, which represents a TT8m ET0s0°C0mA sample.

a

b

c

d

e

Figure 5.3 - Here are colours generated from La*b* values using Adobe Photoshop CS6. Image a represents sample TT0m ET0s0°C0mA, image b represents sample TT4m ET0s0°C0mA, image c represents sample TT4m ET0s0°C0mA, image d represents sample TT4m ET210s40°C20mA, and image e represents sample TT8m ET450s40°C10mA

5.3 Electrochemical Treatment The main objective of this report has been to investigate how a cathodic hardening process using a chromic acid based electrochemical bath could affect a thermally formed coloured oxide on the surface of AISI 304 stainless steel. This part of the report is aimed at discussing how the electrochemical treatment of the thermally formed surface oxides changes the chemical composition and physical properties of the oxide film. The mechanisms involved in this electrochemical treatment are also discussed. 5.3.1 La*b* Results and Colour Reversion of First Electrochemically Treated Samples It was found after 7-12.5 minutes of electrochemical treatment at temperatures in the range of 30°C-50°C, with a current density range of 10𝑚𝐴/𝑐𝑚2 -50 𝑚𝐴/𝑐𝑚2 in an electrochemical bath of the composition given in Chapter 3 Section 3.5 Table 3.1, that the samples were reverting to a very similar colour, which appeared to indicate that the interference film was being destroyed, even though there was still a surface oxide present. The reason for this thought process is that the colour of the oxide films post electrochemical treatment appears to be similar to the colour of a sample thermally treated for 1-2 minutes where only a very limited interference film exists. Figure 5.3 images d and e demonstrates the colour generated for two good examples of the colour generated on the sample surface post electrochemical treatment. These images have been generated using La*b* values for samples

54 TT4m ET210s40°C20mA, and TT8m ET450s40°C10mA. These colours are much paler than the colour generated on sample TT4m ET0s0°C0mA (Figure 5.3b), and are very different to the red-purple colour of sample TT8m ET0s0°C0mA. The La*b* values for these two samples, although initially very different due to their surface microscopy developed at different thermal oxidation times, appear to convene upon very similar values post electrochemical treatment as seen in Table 4.2. There are many examples of samples which were tested within this range of variables, all resulting in very similar La*b* values which can be seen in the Appendix Table 7.1. The range of values which the electrochemically treated coupons seem to convene to are shown in Table 5.1. For a* and b* values, the first range shown in for TT8m series of electrochemically treated samples, and the second range is for TT4m series of electrochemically treated samples. Table ‎5.1 The range of La*b* values seen for electrochemically treated samples. This table excludes some anomalous results from Table 7.1 in Chapter 7.

La*b* Variable

Range of Values

L

44-61

a*

2-3 & 7-9

b*

12-15 & 24-25

5.3.2 Electrochemical Mechanisms Responsible for Colour Reversion The oxide chemical composition is seen to change quite drastically after electrochemical treatment of a TT4m ET0s0°C0mA sample. Sample TT4m ET210s40°C20mA is a prime example of this. When the sample is electrochemically treated iron oxide is electrochemically reduced to iron metal and the chromic acid from the electrochemical bath which exists as hexavalent chromium ions (𝐶𝑟 6+ ) is reduced to chromium oxide which is deposited onto the sample surface, due to making the sample the cathode. Evidence can be seen for these reactions when comparing Figure 4.22 (XPS traces for sample TT4m ET0s0°C0mA) with Figure 4.21 (XPS traces for ET210s40°C20mA). This electrochemical reduction of the iron oxide results in the formation of iron metal on the outer surface of the oxide layer following Equation 5.1(42,43). The reduction of 𝐶𝑟 6+ to chromium oxide forms as a deposit on the outer surface of the oxide, seen as the increasing atomic % of chromium oxide at

55 low etch times (on the outer surface of the oxide layer) following Equation 5.2. This is clarified by the fact that this 𝐶𝑟2 𝑂3 deposit appears on top of the iron oxide reduction indicating it is being deposited from outside of the sample, indicating the 𝐶𝑟2 𝑂3 deposit is from the 𝐶𝑟 6+ in the chromic acid based electrolyte. The 𝐶𝑟 6+ could also be reduced further to chromium metal as per Equation 5.3 but this only happens in very rare circumstances as mentioned later in this section.

𝐹𝑒2 𝑂3 + 6𝑒 − + 6𝐻 + → 2𝐹𝑒 + 3𝐻2 𝑂

Equation ‎5.1

2𝐶𝑟 6+ + 6𝑒 − + 3𝐻2 𝑂 → 𝐶𝑟2 𝑂3 + 6𝐻 +

Equation ‎5.2

𝐶𝑟2 𝑂3 + 6𝑒 − + 6𝐻 + → 2𝐶𝑟 + 3𝐻2 𝑂

Equation ‎5.3

The same electrochemical reduction observations are made when comparing sample TT8m ET450s40°C10mA to TT8m ET0s0°C0mA in Figures 4.24 and 4.23 respectively. It seems that although the samples have been thermally treated for a longer time period, the identical electrochemical reductions occur as the composition of the oxide film is the same between TT4m and TT8m series samples. The iron metal peak is much higher in Figure 4.24 compared to 4.22 due to sample TT8m ET450s40°C10mA’s increased time of reduction compared to sample TT4m ET210s40°C20mA (450 seconds compared to 210), combined with the increased depth of oxide due to the longer thermal oxidation period (8 minutes compared to 4 minutes). When looking at low etch times for iron metal and iron oxide traces in Figure 4.22 and 4.24 there is evidence of a small amount of iron oxide on top of the iron metal. This can be explained due to the oxidation of the iron in atmosphere post treatment (44). Figure 4.25 is a collection of more highly detailed XPS traces for sample TT8m ET450s40°C10mA obtained by XPS procedures with a much slower etch rate which enables the XPS to scan more atomic layers, promoting more detailed atomic % traces to be formed. With the increased clarity of atomic % traces, Figure 4.25

56 reveals a very small amount of chromium metal in the oxide layer. This is likely due to localised areas of higher current density at the oxide surface, resulting in the further reduction of 𝐶𝑟 6+ to chromium metal rather than chromium oxide as in Equation 5.4. Analysing the micrographs taken for samples TT4m ET210s40°C20mA and TT8m ET450s40°C10mA which can be seen in Figures 4.10 and 4.11, and comparing them with samples TT4m ET0s0°C0mA and TT8m ET0s0°C0mA, in Figures 4.8 and 4.9 respectively, show that the nodule density is greatly decreased post electrochemical treatment. This observation is particularly obvious in the lower magnification images in each of the figures. In Figure 4.10a it appears there are almost no nodules left on the surface of sample TT4m ET210s40°C20mA, only a small number of small nodules can be seen in Figure 4.10b. The decrease in oxide nodule density for samples TT4m ET210s40°C20mA and TT8m ET450s40°C10mA co-insides with the conversion of iron oxide to iron metal for both samples TT4m ET210s40°C20mA and TT8m ET450s40°C10mA as discussed earlier in this section. This co-incidence of these two phenomenon implies that the oxide nodules are largely iron oxide as the electrochemical reduction of the iron oxide to iron metal would allow charge to dissipate, thus diminishing the charging effect on the nodules, stopping them appearing as white dots on the FEGSEM micrographs. As this is based on only two results it will require further investigation to clarify. As the chemical composition of the nodules alone cannot be measured due to their extremely small size (less than 500nm according to Higginson(7)) more electrochemical testing is carried out to test this observation further. It also appears in these Figures that swathes of new material have been deposited of a wavy topology. The observation of the deposited material in the same figures co-inside with evidence of chromium oxide deposition as previously described in this section, leading to the hypothesis that these swathes of new oxide are likely to be electrodeposited chromium oxide. This theory explains why on the XPS traces the chromium oxide appears at lower etch times in Figure 4.22 and 4.24 by comparison with 4.22 and 4.24 respectively, as it is now closer to the outer most layer of the oxide. Again this is only based upon very limited findings and will require further investigations for clarification.

57

5.4 Hull Cell Experiment Having found that the electrochemical bath reduces the oxide on the stainless steel surface, Hull Cell experiments were designed to find out the exact conditions under which this reduction takes place. 5.4.1 Reasoning Behind Using Hull Cell Experiments Using the initial findings from samples TT4m ET210s40°C20mA and TT8m ET450s40°C10mA and other samples which can be seen in Appendix Table 7.1 the decision was made to conduct Hull Cell type experiments to test multiple current density conditions at once in order to find the exact temperatures, current densities and time period over which the reduction of the stainless steel oxide, as outlined in Section 5.3.2, takes place. As previously introduced in Chapter 2 in Section 2.6, the Hull Cell is a piece of equipment used almost solely by electroplaters in order to find optimum current densities for good quality electroplating. As the Hull Cell simply works upon standard electrochemical principals (31) there is little reason opposing the use of Hull Cell experiments in order to obtain an idea of how current density effects the reduction of the thermally treated stainless steel oxide film, and to identify the current density conditions over which the reduction occurs. In theory varying time and temperature would give a gradient of colour over certain current densities, identifying the required variables over which the reduction was occurring when the reaction was stopped. 5.4.2 Hull Cell Experimentation Prior to starting this experiment it was understood that the low electrical conductivity of the oxide film would impede the transfer of charge through the cathode, thus the apparent current density measured using a Hull Cell Ruler (as discussed in Chapter 2 Section 2.6) would not be 100% accurate as models explaining the current distribution across a thermally oxidised stainless steel surface do not exist (32,45). What this means is that the current densities must be taken as guides rather than definitive values. Table 4.3 and 4.4 are the La*b* values before and after Hull Cell testing respectively, along with the thermal and electrochemical treatment data. Initially it was found that the thermally treated stainless steel Hull Cell samples experiencing full colour reversion over the whole surface of the electrochemically

58 treated area. After careful examination of the reaction it was seen that hydrogen evolution (31) occurred on the surface of the cathode which could be seen as bubbles forming on the surface of the sample at the cathode. This reaction seemed to be strongest within the first 15 seconds of reaction for the TT10m series of samples, and the first 60 seconds of reaction for the TT5m series of samples. Once the effervescence had begun, only a very short period of time was needed before turning off the current, stopping the reaction before the whole face of the Hull Cell sample was completely reduced resulting in the complete colour reversion over the whole Hull Cell sample face. In order to keep this rapidly occurring reaction as “slow” as possible, only room temperature was investigated rather than increasing the temperature, allowing greater control over how the reaction progressed, making it easier to identify the reduction conditions. As altering temperature can have an impact on the potentials across the cell, the controlled room temperature of the cell allowed better understanding of the potentials across the cell (45). 5.4.3 Visual Observation, La*b* and Optical Microscopy Results of the Hull Cell Experiments Three results were of particular interest; samples TT5m ET60s21°C0.3A, TT10m ET15s21°C0.2A, and TT10m ET120s21°C0.1A. Only these samples appeared to generate a gradient of colour in segment 3 (as discussed in Chapter 3 Section 3.7) of the samples i.e. the area of lowest current density. Photographs of samples TT5m ET60s21°C0.3A and TT10m ET15s21°C0.2A show the gradients of colour in segment 3 in Figures 4.18 and 4.19. This appearance of a gradient gave the indication that after the tested period of time, at the corresponding current densities to the segment of sample over which the gradient is seen (calculated using the Hull Cell Ruler), the reduction of the thermally formed oxides was starting to occur/ were occurring at the time at which the experiment was stopped. The fact that the colours had not yet fully reverted, as described in Section 5.3.2 of this chapter, meant that the reduction reactions had not completely destroyed the interference film. Using the found current density, temperature and time conditions would allow investigation into how the reduction reaction propagates over time. Table 5.2 shows the range of current densities of interest for the controlled electrochemical reduction of each sample.

59 Table ‎5.2 Current densities of interest for Hull Cell samples TT5m ET60s21°C0.3A, TT10m ET15s21°C0.2A, and TT10m ET120s21°C0.1A determined by Hull Cell Experiments using a Hull Cell Ruler (see Figure 2.11 in Chapter 2)

Sample

Current Density (mA/cm^2)

Highest Current Density (mA/cm^2)

TT5m ET60s21°C0.3A

0.15-0.6

15

TT10m ET15s21°C0.2A

0.1-0.6

10

TT10m ET120s21°C0.1A

0.05-0.2

5

Samples TT5m ET60s21°C0.3A, and TT10 ET15s21°C0.2A appeared to have been reduced in the exact same way that the coupons described in Section 5.3.2, such as samples TT4m ET210s40°C20mA and TT8m ET450s40°C10mA, judging by the appearance of the colour and La*b* values of segments 1 and 2 as seen in Figures 4.18 and 4.19, and Table 4.4. These samples had the appearance of a fully reverted coloured oxide in segments 1 and 2. Most importantly, the familiar La*b* values in segments 1 and 2 for samples TT5m ET60s21°C0.3A, and TT10 ET15s21°C0.2A (seen in Table 4.4), are similar to the La*b* values post electrochemical reduction for samples TT4m ET210s30°C20mA and TT8m ET450s40°C10mA in Table 4.2. Sample TT10m ET120s21°C0.1A does not show evidence of being reduced in the same way. The appearance of this sample remained relatively unchanged at segments 1 and 2, but the colour of segment 3 appears far more blue which is clarified by the substantial drop in b* value from 1.16 post thermal treatment to -3.45 post electrochemical treatment in segment 3 (25). This change of colour can be seen at a microscopic level; Figure 4.5 shows a micrograph of segment 3 before treatment, and Figure 4.6 shows a micrograph of segment 3 post treatment. Figure 4.6 demonstrates a substantially larger proportion of blue coloured microstructure compared to Figure 4.5. The blue achieved here was the most vivid blue seen in this project, and the colour retains its lustrous nature. 5.4.4 XPS and FEGSEM Analysis of Hull Cell Sample TT10 ET120s21°C0.1A The XPS traces of sample TT10 ET120s21°C0.1A at segment 3 of the Hull Cell sample, both before and after treatment, can be seen in Figure 4.26. Figure 4.26 reveals the deposition of chromium oxide from the 𝐶𝑟 6+ in the electrolyte, onto the outer layers of the sample oxide film. On examining the oxide trace there seems to be no reduction of iron oxide to iron metal as there is no sign of any iron metal formation in the oxide film. The small differences in iron metal traces before and after

60 electrochemical treatment can be attributed to the uneven oxide surface, and inaccuracies with the XPS equipment (46). Variation in oxide thickness can be attributed to the naturally uneven surface due to nodular growth (40), chemical composition fluctuations in the bulk metal, and/or the damaging of oxide in sample storage, movement and preparation. The deposition of chromium oxide without the reduction of iron oxide reveals that it is energetically favourable (requires less activation energy) to reduce aqueous hexavalent chromium to chromium oxide than to reduce solid iron oxide to iron metal On examining the FEGSEM micrographs for sample TT10 ET120s21°C0.1A in Figures 4.16 and 4.17, no noticeable decrease in nodule density is seen post treatment. The only changes seen are a change in the brightness of the nodules in Figure 4.17b compared to 4.16b, and an increase in oxide film porosity. This change in brightness of the oxide nodules demonstrates a lowering in the charging of the nodules by the FEGSEM electron beam (39). This reduced charging effect could be due to a reduction of the height of the nodules, a natural decrease in the oxide thickness over this area of the sample, the deposition of a more conductive layer on top of the sample or even simply a change in the brightness or contrast for this particular image. As there is no way to test which explanation is responsible for the darkening of the oxide nodules in Figure 4.17 in the timeframe allowed for this report, this result remains inconclusive. The increase in porosity is likely due to the deposited chromium oxide being a hydrated oxide, which has subsequently dried under atmospheric conditions or been removed by the FEGSEM vacuum conditions, resulting in a cracked and pot marked topology. The fact that the iron oxide and iron metal XPS traces don’t change post treatment (Figure 4.26), and neither does the nodule density (Figure 4.17 compared to 4.16), again suggest that the nodules consist largely of iron oxide which remains unreduced by the electrochemical treatment. The XPS traces display the deposition of chromium oxide over the entire surface of the sample, meaning on top of nodules and filling in the gaps in-between the nodules. The most marked increase appears to be on the outer most layers of the oxide film. This means there is chromium oxide on top of the iron oxide nodules, which when combined with the deposition of chromium oxide between the oxide nodules still alters the depth of the interference film, whilst also changing the chemical composition of the outer most layer of the oxide film.

61 This change in depth and chemical composition of the interference film will results in the surface colour to be changed to a highly vivid blue. In summary, The Hull Cell experiments revealed that the current density has a significant effect upon the rate of reduction at the cathode, as was expected (16,18,20,23) and a range of potential current densities were identified. Careful control of current density can cause reduction of 𝐶𝑟 6+ to chromium oxide, whilst leaving the iron oxide unreduced. It was also found that the reduction reaction of both aqueous hexavalent chromium to solid chromium oxide and solid iron oxide to solid iron metal occurs extremely rapidly and within the time window of 15-60 seconds, depending on the thermal treatment time, revealing that this reaction in highly time dependent due to the ability to observe the effervescence off the surface of the samples as they gassed hydrogen under cathodic conditions.

5.5 Further Investigation into the Reduction Mechanisms and Conditions In order to take a closer look at the mechanism for this reduction process, the conditions measured in the Hull Cell experiments were recreated in an electrochemical bath with the layout outlined in Chapter 3 Section 3.5, on small coupons, at different times to attempt to capture the reduction process at different times as it developed. 5.5.1 Further Electrochemical Experimentation on Coupon Samples It was quickly found that the “current densities of interest” seen in Table 5.1, were not applicable for use with the smaller coupons in a larger electrochemical bath. This is likely due to the difference in oxide thickness between the TT5m series and TT4m series of samples, and the TT10m and TT8m series of samples but due to the time constraints of this project could not be investigated further. Unfortunately there was not enough time to make new Hull Cell samples in the TT4m and TT8m series’, or to repeat the experiment to gather current density data for the TT8m series. The highest current densities seen on each of the Hull Cell experiments (as seen in Table 5.2) were used as a benchmark to replicate the reduction processes on the smaller coupons, to reasonable success. Unfortunately the conditions found for the reduction of Hull Cell samples TT5m ET60s21°C0.3A and TT10m ET120s21°C0.1A could not be replicated due to time restraints on this project, however the necessary

62 a

b

c

d

e

f

Figure 5.4 - Colours of samples TT8m ET0s0°C0mA, TT8m ET15s22°C10mA, TT8m ET20s22°C10mA, TT8m ET25s22°C10mA, TT8m ET30s22°C10mA, and TT8m ET450s40°C10mA generated from La*b* values using Photoshop CS6

conditions for the reduction of the TT8m sample series were found, and the progression of the reduction reactions on the TT8m sample series is captured by samples TT8m ET15s22°C10mA, TT8m ET20s22°C10mA, TT8m ET25s22°C10mA, and TT8m ET30s22°C10mA. These four samples illustrate the gradual change of colour of a TT8m series sample, with time under cathodic reduction between 15 and 30 seconds. 5.5.2 La*b* Analysis of Electrochemically Reduced Coupons La*b* values were taken for each of the TT8m ET15s22°C10mA, TT8m ET20s22°C10mA, TT8m ET25s22°C10mA, and TT8m ET30s22°C10mA samples before and after electrochemical treatment and can be seen in Table 4.2. The L values gradually increases indicating that the colour is getting continuously lighter, the a* values are gradually decreasing indicating that the colour is getting less red, and finally the b* values gradually increase indicating the samples to be getting gradually less blue, all with the increasing reduction time. This reduction in both the red and blue colours is again indicative of the reversion of colour, and therefore the breakdown of the interference film (25), as theorised in Section 5.3.1 of this chapter. In order to illustrate the change of colour the La*b* values for samples TT8m ET0s0°C0mA, TT8m ET15s22°C10mA, TT8m ET20s22°C10mA, TT8m ET25s22°C10mA, TT8m ET30s22°C10mA, and TT8m ET450s40°C10mA were generated using La*b* values measured, on Photoshop CS6, in the same way as in Section 5.2.2. These colours are shown in Figure 5.4. 5.5.3 XPS and FEGSEM Analysis of Electrochemically Reduced Coupons On analysing the XPS traces for samples TT8m ET15s22°C10mA, TT8m ET20s22°C10mA, TT8m ET25s22°C10mA, and TT8m ET30s22°C10mA it was prudent to group chemical compounds and to compare them against one another to demonstrate how each chemical compound changes. These samples were compared to sample TT8m ET0s0°C0mA to show how the chemistry of the oxide films developed with reduction time from its original composition before

63 electrochemical reduction. Sample TT8m ET450s40°C10mA was included in the comparison to demonstrate what would happen after elongated periods of reduction. These graphs can be seen in Chapter 4, Figures 4.27-4.30. During the electrochemical reduction, iron metal is again seen to form in the oxide film, which can be seen in Figure 4.27. The atomic % of iron metal is seen to increase in the oxide film (in etch times less than ~200 seconds) with increasing reduction time. Combining Figures 4.27 and 4.28 demonstrates that the reduction seems to occur across the whole oxide film. Although the iron metal appears to show that the oxide thickness is reducing due to the gradual shift of the iron metal traces to the left of Figure 4.27, and that there seems to be a large deposit of iron metal in the outer layers of the iron oxide, the shift of the iron metal to the left of the graph doesn’t match up with the iron oxide thickness decrease. On closer examination of Figure 4.28, it appears that the reduction of iron oxide atomic % at higher etch times, i.e. the inner layers of the oxide, occurs at higher rate compared to the reduction of atomic % at lower etch times, i.e. the outer oxide, especially at lower electrochemical treatment times such as on sample TT8m ET15s22°C10mA. This can be seen in Figure 4.27 by the steepening of the iron metal gradient in the indicated “stage 2” section of the graph, signifying more iron metal at inner oxide depths. Figure 4.28 also demonstrates the substantial decrease in atomic % of iron oxide at the inner oxide levels, especially between samples TT8m ET20s22°C10mA and TT8m ET25s22°C10mA i.e. in the early stages of reduction. The occurrence of the apparent higher rate of reduction at the inner oxide layers is due to the insulating properties of the oxide not allowing sufficient electron charge transfer from the substrate through the material and to the outer layers, resulting in a lower effective potential on the outer layers of the oxides. This lower effective potential may not supply the required energy to complete the reduction of the iron oxide. As the iron oxide at the inner layers of the oxide becomes reduced to iron metal, this allows better transfer of charge to outer layers of the oxide through the conducting iron metal. This increase in conductivity increases the effective potential on the outer layers of the oxide, enabling the outer layers of the iron oxide to be heavily reduced on the samples which have been reduced for extended periods of time. Evidence of this is observed as the large spike in iron metal atomic % at low etch times for

64 samples TT8m ET30s22°C10mA and especially TT8m ET450s40°C10mA in Figure 4.27, at low etch levels. The reduction in iron oxide atomic % through samples TT8m ET15s22°C10mA to TT8m ET8m ET30s22°C10mA, again co-insides with the reduction of nodule density over the samples. Figures 4.12 - 4.15 show FEGSEM images of samples TT8m ET15s22°C10mA through to TT8m ET30s22°C10mA. Throughout this sequence of figures it can be clearly seen that the oxide nodule density decreases as the time of reduction increases. This is a repetition of the relationship between oxide nodule density and iron oxide outlined in Section 5.3.2 from the original electrochemically reduced samples. This gradual decrease of nodule density, co-insiding with the gradual decrease in iron oxide atomic % through samples TT8m ET15s22°C10mA to TT8m ET8m ET30s22°C10mA gives significant evidence to prove that the nodules formed in the thermal treatment stage consist largely of iron oxide. The oxide thickness is again shown to decrease when looking at Figure 4.30 at the chromium oxide XPS chemical composition traces, except for sample TT8m ET450s40°C10mA. The tails of the traces seem to reduce in length as the reduction time increases i.e. the chromium oxide atomic % decreases to 0 at lower etch times, indicating the gradual decrease in oxide penetration into the substrate. The gradual increase in chromium oxide atomic % at the oxide surface proves that chromium oxide is being deposited all over the sample’s surface, resulting in a higher density of chromium oxide on the outer surface of the oxide film coming from the reduction of aqueous hexavalent chromium from the electrolyte. This deposition of chromium oxide results in the “filling in” of the gaps between the iron oxide nodules as they are reduced, thus giving evidence for the destruction of the interference film. Although chromium oxide is also deposited on top of the iron oxide nodules, as they reduce in size the effect of this has less of an impact than the chromium oxide filling in the gaps between the nodules. Further evidence of the destruction of the interference film can be seen in Figures 4.12 through to 4.15, and Figure 4.11. Figures 4.12 to 4.15, as already discussed in in this section, show a gradual reduction in the nodule density. Interestingly, sample TT8m ET30s22°C10mA only appears to conform to the pattern in nodule density decrease (with increasing electrochemical reduction time) of the previous samples

65 on the edge of the coupon. This could be explained by a slower cooling time post thermal treatment for this particular sample due to experimental error, which allowed the oxide in the centre of the sample to develop further, thus requiring a longer time to reduce at the centre of the sample. It could also be due to the electrochemical reduction starting at the outer rim of the samples, and working in. Unfortunately due to time restraints no retest or more testing could be done to clarify which reason is responsible for this unexpected phenomenon. Figures 4.12b, 4.13b, 4.14b and 4.15d also show the gradual increase in the appearance of the wavy topology. This gradual increase in the wavy topology co-insides with the gradual increase in the surface composition of chromium oxide. This co-incidence is further evidence for this wavy structure actually being the deposited chromium oxide, as theorised in Section 5.3.2. As the interference film is responsible for the appearance of colour, which has been identified as being destroyed by the electrochemical reduction process, the colour is effectively progressively diminished. This diminishing of the colour of the oxide can be measured by the increasing L, decreasing a* and increasing b* values with increased reduction time. Figure 5.5 shows theorised cross sections of the AISI 304 stainless steel before and after a period of chemical treatment greater than ~1 minute. It shows the iron oxide reduced to iron metal, and how chromium oxide is deposited as an even layer on the outermost surface of the oxide layer. There is overlap between many of the layers such as the oxide and the substrate. This is to represent a gradient as it is highly unlikely for there to be a definitive layer between any compounds.

66 a

b

Iron Oxide Chromium Oxide Stainless Steel Substrate

Iron Oxide Nodules Iron Oxide Chromium Oxide Layer Iron Metal from Iron Oxide Reduction Stainless Steel Substrate

Figure 5.5 Illustrations of cross sections of a TT8m series oxide layer before (a) and after (b) electrochemical reduction.

In summary, in this section was proven that the oxides formed after thermal treatment consist of iron oxide nodules which protrude from an oxide layer consisting of largely chromium oxide due to the observations made during the electrochemical reduction of the TT8m series of thermally formed oxide layers. During the electrochemical reduction the iron oxide is reduced to iron metal, a process that occurs from the inner most layer of the oxide layer, out towards the outer layers of the oxide. The outer layers are enabled to be reduced by the increased conductivity in the oxide layer due to the formation of the conductive iron metal allowing charge transfer to the outer layers of the oxide. Aqueous hexavalent chromium ions are reduced to chromium oxide simultaneously with the iron oxide reduction. This chromium oxide is deposited on the outer surface of the oxide. The combination of the two reduction processes results in the destruction of the interference film, and alters the chemistry of the outer layers of the oxide which results in the colour reversion phenomenon observed on the samples electrochemically reduced for periods of time over 30 seconds, under current densities above 10𝑚𝐴/𝑐𝑚2.

67

Chapter 6 Conclusion Using optical microscopy the AISI 304 stainless steel samples were identified as been processed by rolling then bright annealed. After periods of 4 and 8 minutes of heat treatment at 600°C a thicker oxide film is formed which consists of an outer layer of iron oxide, an inner layer of chromium oxide, and a spinel structure between the two oxide layers. The oxide films formed also have a gradient from the substrate metal rather than existing as two distinct layers of oxide and metal substrate. The iron oxide forms into nodule structures, which stand above a layer of porous oxide, which consists predominantly of chromium oxide. This difference in oxide heights causes an interference film to form. This interference film combined with the reflection and absorption of certain light wavelengths when interacting with the different oxide compositions, is responsible for the colour of the thermal oxide. As the time of thermal treatment increases from 4 minutes to 8 minutes the composition of the oxide film formed doesn’t change, but the depth of oxide increases. This increase in oxide depth alters the interference film, which causes the colour to change from gold to red-purple. When the thermally treated samples were electrochemically treated within the following range of conditions; 30°C-50°C, with a current density range of 10𝑚𝐴/𝑐𝑚2 50 𝑚𝐴/𝑐𝑚2 for 7-12.5 minutes, the colour of the oxide seemed to revert to very similar pale gold colour. This colour looked like a colour you would expect to see on a sample which had been thermally treated for 1-2 minutes at 600°C. This result gave the impression that the interference film was being destroyed by the electrochemical treatment. On consulting FEGSEM micrographs of the sample surfaces post electrochemical treatment it was found that the nodule densities decreased, and that there seemed to also be a deposition of an oxide, which had a wavy topography. On analysing the XPS traces for the post electrochemically treated samples it was found the electrochemical process was reducing the iron oxide to iron metal, and that the aqueous hexavalent chrome ions from the electrolyte was being reduced to chromium oxide. This chromium oxide was being deposited onto the oxide surface. Hull Cell experiments were conducted in order to identify the electrochemical conditions over which the reduction occurred on both the TT4m and TT8m series

68 samples. These experiments revealed that the reduction reaction occurs within a time range between 15-60 seconds for both iron oxide to iron metal, and aqueous hexavalent chrome ions to chromium oxide. The applied required for the reduction reaction to occur were very low, implying the requirement of low current densities for the reduction reaction. It was also identified that the temperature should be kept at room temperature to attempt to slow the reaction down as much as possible due to the rapid nature of the reaction. Finally it was discovered that with close control of the current densities used, the reduction of iron oxide to iron metal could be stopped, whilst continuing to deposit chromium oxide from reduced aqueous hexavalent chrome ions, demonstrating that the reduction of aqueous hexavalent chrome ions is energetically favourable to the reduction of solid iron oxide. It was quickly discovered that the current densities identified by the Hull Cell experiments did not match up with the conditions required for reduction on the smaller coupons. This was postulated to be largely due to the difference in thickness of the Hull Cell samples and the coupon samples. The correct conditions for the electrochemical reduction of the TT8m series of samples were found and multiple samples made, demonstrating the electrochemical reduction process at different times ranging from 15 seconds to 30. The chemical composition of the oxide films were directly compared and it was found that the iron oxide was reduced from the inner layers of the oxide first, to iron metal. Aqueous hexavalent chrome ions from the electrolyte were being reduced to chromium oxide and deposited onto the surface of the cathodic samples, which was filling in the gaps between the nodules. Both of these reductions were occurring straight away, evidence of which could be seen on all of the samples, including the sample tested for only 15 seconds. It was observed that the reduction of iron oxide nodules to iron metal and the deposition of chromium oxide resulted in the nodule height decreasing, and the gaps between the remaining nodules being partially filled in. The combined effect of these two processes resulted in the destruction of the interference films and the altering of the surface chemistry, which ultimately altered the coloured appearance of the surface oxide, reverting the colour to a pale gold after 30 seconds of electrochemical reduction.

69 As a final summary, it was identified that the thermal treatment of these samples results in the formation of oxide nodules, the chemical composition of which is largely iron oxide. It also results in the formation of a very thin layer of chromium oxide. The formation of this oxide layer results in an interference film to be formed. The electrochemical treatment process where the thermally treated samples are made cathodic, is an electrochemical reduction process. The electrochemical reduction mechanism of the thermally treated samples was identified as a combination of the reduction of the iron oxide nodules, and the filling in of the gaps between the nodules with the deposited chromium oxide. These two reductions constituted to the destruction of the oxide interference film responsible for the appearance of colour on the AISI 304 stainless steel samples, which gave the appearance of removing the colour on the stainless steel.

70

Chapter 7 Future Works Further looking into the reduction conditions for multiple thermal treatment times, in order to further analyse the reduction mechanism for the different oxide thicknesses, and to discover if the mechanism changes with oxide thickness. Re-testing the TT8m ET30s22°C10mA sample to see if the areas of different nodule density occur at the middle and sides of the sample again, or if this occurred due to experimental error. Further experimentation to try and replicate the conditions, which resulted in the formation of the vivid blue on the TT10m ET120s21°C0.1A Hull Cell sample on a smaller TT8m series coupon. If this process can be replicated, it should be held for an extended period to see how the colour changes. An attempt should be made to apply this process of depositing chromium oxide without reducing iron oxide to a gold sample to investigate how the colour changes on the thinner oxide. Observe how the topography changes in each of these tests in order to try and identify how the topographical appearance of the sample changes with the deposition of the chromium oxide alone, to conclusively identify the “wavy topography” as being evidence of chromium oxide deposition. Investigate how and why oxide thickness has an effect upon the current density calculations from Hull Cell experiments in order to gain a further understanding of the Hull Cell experiment principals and to explore further applications of Hull Cell experimentation. Attempt to find a method for capturing high magnification, cross sectional images of the oxides in order to obtain a definitive mechanism for the deposition of the chromium oxide (i.e. is it deposited and or formed as a film or as a platelet structure). Any images found should be used to calculate the thicknesses of the formed oxides in order to give thickness values for the oxide films rather than relying on etch time. This will enable an easier and fuller understanding of the behaviour of the formed oxides for the reader. XRD analysis of the thermally, and electrochemically treated samples to attempt to identify the crystallographic structures for the different oxide films rather than relying on works done previously on similar projects such as oxides formed at higher

71 temperatures over longer time periods. This will promote further understanding of the chemistries involved in the electrochemical reduction process.

72

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Appendix Table 9.1 - Small sample La*b* values before and after electrochemical treatment with oxidation and electrochemical treatment times. Sample number 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

Thermal Treatment Oxidising Time Oxidising (mins) Time (s) 0 0 4 240 4 240 4 240 4 240 4 240 4 240 4 240 4 240 4 240 4 240 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 8 480 10 600 10 600 10 600 10 600 10 600 10 600 10 600 10 600 10 600 10 600 4 240 4 240 4 240 4 240 4 240

Time (mins) 0 7 9 11 13 7.5 7.5 5 2.5 3.5 0 10 12.5 7.5 7.5 7.5 7.5 7.5 7.5 0.25 2 0.25 0.5 0.42 2 2 2 0.33 5 0.25 1 0.5 0.25 -

Electrochemical Treatment PhotoSpectroscopy Examination Temperature Current Density Before Electrochemical Treatment After Electrochemical Treatment L a* b* L a* b* (°C) (mA/cm^2) 0 0 0 83.57 0.13 5.70 420 RT 30 59.85 8.82 24.56 58.35 9.28 26.06 540 RT 30 59.00 8.90 25.46 59.80 8.83 25.41 660 RT 30 61.28 8.10 24.72 58.12 8.86 26.44 780 RT 30 59.89 8.56 23.37 59.24 9.27 24.84 450 40 10 57.01 9.30 21.05 56.07 7.84 26.88 450 40 20 58.96 9.01 23.24 66.81 2.09 15.35 300 40 20 60.79 8.77 23.95 67.91 1.95 15.50 150 40 20 54.70 10.05 20.45 54.20 10.28 20.26 210 40 20 55.42 9.94 19.51 63.55 2.45 16.77 0 0 0 56.63 9.61 25.03 57.75 9.64 25.02 600 50 50 46.50 8.99 9.67 56.76 3.13 17.90 750 50 50 46.81 11.23 11.22 57.61 2.91 15.51 450 50 50 45.25 11.40 8.99 60.32 2.51 14.21 450 40 50 46.29 9.86 8.95 60.38 2.54 15.21 450 40 40.675 43.42 10.46 4.15 58.05 2.53 13.37 450 40 30 45.05 9.45 7.75 56.69 2.86 15.30 450 40 20 42.73 10.58 4.20 57.33 2.43 12.75 450 40 10 42.97 9.43 4.83 57.53 2.37 12.20 42.13 7.67 2.21 42.26 7.77 2.23 42.08 6.27 0.04 41.76 6.22 0.17 15 22 0.25 44.39 8.19 3.79 43.76 8.06 3.36 120 22 10 43.32 8.99 6.27 53.85 2.74 14.86 15 22 10 42.75 8.11 2.73 41.63 7.78 1.36 30 22 10 45.86 8.61 6.93 54.87 2.76 15.67 25 22 10 44.53 8.31 5.27 52.08 3.26 16.84 120 22 0.25 46.02 8.71 7.48 45.50 8.71 6.97 120 22 5 43.65 7.88 3.90 52.51 2.66 14.27 120 22 5 45.06 8.81 6.05 44.48 8.86 5.62 20 22 10 43.53 8.24 3.22 44.75 5.60 13.06 44.70 8.95 6.22 45.21 8.53 5.62 44.07 7.72 3.72 43.31 8.43 2.15 43.09 7.05 1.13 300 22 5 41.92 7.07 -1.00 40.60 7.01 0.31 43.12 8.29 2.59 43.87 7.90 4.05 44.30 7.04 2.53 42.81 8.17 0.47 42.68 6.96 1.92 15 15 63.38 7.24 25.15 61.30 7.57 26.44 60 15 65.52 6.23 25.89 63.81 6.67 27.32 30 20 64.35 6.83 25.28 59.65 3.52 26.70 15 20 68.31 5.16 25.24 67.39 5.28 26.03 50.99 9.53 15.64 -

Time (s)