induced stress corrosion cracking

Probing propensity of grade 2205 duplex stainless steel towards atmospheric chlorideinduced stress corrosion cracking ¨ rnek D. L. Engelberg* and C. O The propensity of grade 2205 duplex stainless steel towards atmospheric chloride-induced stress corrosion cracking at 50uC has been investigated. Electron backscatter diffraction has been used to characterise as received and 750uC heat-treated microstructures. Screening tests in chloridecontaining aqueous environments were employed to investigate the corrosion behaviour of both microstructures. These tests indicated significantly increased corrosion rates when exposed to HCl or FeCl3-containing environments. Stress corrosion cracking tests with atmospheric exposures for up to 12 months showed selective dissolution of the ferrite, accompanied by stress corrosion microcracks in the austenite. This work demonstrates that grade 2205 duplex stainless steel microstructure may be rendered susceptible to stress corrosion cracking under atmospheric exposure conditions at 50uC. Keywords: Duplex stainless steel, Microstructure, Electron backscatter diffraction, Stress corrosion cracking, Localised corrosion, Selective corrosion

This paper is part of a special issue on ‘Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems’

Introduction The UK’s intermediate level radioactive waste (ILW) is stored in type 316L and 304L austenitic stainless steel containers and drums, which have so far shown excellent resistance in service to atmospheric chloride-induced stress corrosion cracking (AISCC).1,2 The disposal scenario for ILW considers above ground interim storage for several decades, followed by an extended storage period in a geological disposal facility before the facility vaults are backfilled and sealed. However, since no geological disposal facility is yet available and the timescale for implementing the latter is becoming more-andmore uncertain, an extended period of interim storage is currently anticipated. It is therefore key to underpin existing knowledge of the behaviour of stainless steels under extended storage periods, in particular whether anticipated environmental conditions lie within the safe operating envelope to ensure the long term integrity of ILW packages. Atmospheric chloride-induced stress corrosion cracking of types 304 and 316 austenitic stainless steel has been reported at temperatures far below 60uC with exposure to MgCl2, CaCl2, or artificial seawater salts.3–6 Though, it has also been recognised that exposure conditions for simulating AISCC frequently relies on far too aggressive, non-representative environments.4,7 In parallel, to have stress corrosion cracking (SCC) resistant material options available it is important to test higher alloyed materials Corrosion & Protection Centre, School of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK *Corresponding author, email [email protected]

ß 2014 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 5 March 2014; accepted 10 June 2014 DOI 10.1179/1743278214Y.0000000205

for their propensity towards AISCC, such as super austenitic stainless steels (e.g. grade 904)3 or duplex/ super-duplex stainless steels (e.g. grades 2205, 2304, 2507).3,8 Most duplex grades are considered more SCC resistant compared to their austenitic counterparts, making this material attractive as a potential replacement option for more durable ILW storage containers.3,9,10 The work reported in this paper is part of a larger research project to explore the propensity of grade 2205 duplex stainless steel microstructure towards AISCC. Long term material behaviour with exposure periods of up to 12 months is reported in this paper.

Experimental methods The material used in this study was an as received, mill annealed grade 2205 duplex stainless steel (UNS32205) plate with a composition (in wt-%) of 22?4Cr, 5?8Ni, 3?2Mo, 1?5Mn, 0?4Si, 0?016C, 0?18N and Fe (bal.) according to accompanying manufacturer notes. Rectangular specimens were cut from the as received plate and the microstructure metallographically characterised. A second set of samples was heat treated at 750uC for 300 min followed by a water quench to simulate the effect of second phase precipitation on microstructure behaviour. Metallographic assessment was performed on 0?06 mm colloidal–silica (OP–S) fine polished sample surfaces. Electron backscatter diffraction (EBSD) was carried out to identify and quantify crystallographic phases using a FEI Quanta 650 scanning electron microscope interfaced with a HKL Nordlys EBSD detector and Oxford Instruments Aztec Version 2.2 data acquisition software. Data post processing was undertaken with

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¨ rnek Engelberg and O

Probing propensity of grade 2205 duplex stainless steel

1 Direct tension AISCC test rig with miniature tensile sample and six deposited salt droplets (inset image). The appearance of a typical droplet of MgCl2 (top) and MgCl2 with FeCl3 (bottom) is shown on the right

Oxford Instruments Channel 5 software and the inhouse software programme Vmap. The high angle grain boundary threshold was set to mis-orientations §15u. Corrosion screening tests were conducted by exposing small coupon samples (10610610 mm cubes) to aqueous chloride-containing solutions. Immersion tests were conducted to obtain information about the corrosion propensity of both microstructures by exposing samples in glass beakers for 49 days at 50uC. Aqueous solutions of (i) 1M hydrochloric acid (HCl), (ii) 0?5M and (iii) 1M MgCl2, and (iv) 0?167M FeCl3 were investigated. The corrosion behaviour of all coupon samples was determined by weight loss and postexposure examination of the coupon surfaces using optical and scanning electron microscopy imaging. Atmospheric chloride-induced stress corrosion cracking tests were carried out by depositing droplets of various concentrations of aqueous MgCl2, FeCl3, or mixtures of MgCl2: FeCl3 solutions onto the surfaces of microtensile samples. Droplets with chloride deposition densities from 0?2 to 2000 mg cm22 and droplet volumes between 0?5 and 2?5 mL were investigated. All samples were then exposed to controlled environmental conditions at 50uC and 30% RH in a Binder KBF 240 climate chamber. A number of AISCC tests were set-up with as received, 10 to 40% cold rolled, and at 750uC heat-treated samples. Some of the exposed sample surfaces were stained in a 40 wt-% KOH solution to provide contrast for image analysis and correlation, and to identify which crystallographic phase was more susceptible to corrosion. Different levels of strain were applied to these samples via the direct tension set-up shown in Fig. 1. During the 12 months exposure all samples were removed from the climatic chamber at regular intervals and the surface imaged with a stereomicroscope, to obtain information about the earliest onset of corrosion.

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Fluctuations in RH with values of up to 60% were observed during the last 2 weeks of exposure, associated with equipment problems, and all long term tests were therefore terminated after 12 months. Post AISCC exposure all samples were rinsed in hot water, the extent of corrosion/cracks imaged, and the sample then thoroughly cleaned in 10 wt-% citric acid at 80uC for up to 2 h, to remove remaining salt deposits and corrosion products. This paper reports the first preliminary results of these AISCC exposures with a more comprehensive investigation of all samples currently under way.

Results and discussion The as received and 750uC heat-treated microstructures are shown in Fig. 2. Electron backscatter diffraction analysis of the as received microstructure indicates that only austenite (c) and ferrite (a) were present with an area ratio of 57 : 43. No second phase precipitates were observed in this microstructure (EBSD step size of 70 nm). Large area image analysis of KOH etched samples gave a similar ratio, and supported observations that the as received microstructure was free of second phase precipitate.11 The as received microstructure can be described as large austenitic islands embedded in a ferritic matrix, where ferrite has a continuous structure (Fig. 2a). A mean grain size of 7–8 mm in both the ferrite and the austenite phase was obtained by EBSD analysis, with however maximum grain dimensions measured of up to 35–45 mm. The 750uC heat treated microstructure is shown in Fig. 2b, containing 47% austenite, 36% ferrite, 14–15% sigma (s), and y2% chi (x) phase. High resolution EBSD analysis also indicated the presence of y0?5% chromiumnitrides in the microstructure. According to phase transformation kinetics in the 750uC temperature

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2 Phase maps (EBSD) of a as received and b 750uC heat-treated microstructures, with ferrite (a) in red, austenite (c) in blue, sigma phase (s) in turquoise, chi phase (x) in lilac, and Cr2N in yellow, with superimposed interface and grain boundaries in light grey (scale bars are 20 mm)

regime, part of the ferrite transforms via the eutectoid reaction: aRszxzCr2N.12–14 Secondary austenite (c2) may also form at this heat treatment temperature, which could however not be differentiated by EBSD from the pre-existing austenite in the microstructure. M23C6 type chromium carbides are also expected to have formed during the annealing process.13 Interestingly, the overall fraction of austenite was expected be the same or even larger than in the as received microstructure, but was found to be nominally y10% lower. The reason for this is not clear, but may be related to the small sampling size for EBSD analysis in conjunction with microstructure anisotropy.15

Corrosion screening in aqueous environment The weight loss of samples immersed in various aqueous environments is summarised in Fig. 3. Samples exposed to 0?5M and 1M MgCl2 (IM05, IM06) indicated similar weight loss with corrosion rates of 1–2 mm/year if a density of 7?8 g cm23 is assumed. Figure 4a shows images of the as received microstructures after exposure to 0?5M MgCl2 showing that pitting corrosion with lacy

3 Weight loss results of as received (IM01, 05, 06, 07) and 750uC heat-treated (IM08) samples after immersion in different aqueous environments for 49 days at 50uC

cover formation had occurred. The pit nucleation seemed to be associated with the interface and ferrite, with however pit growth occurring within both phases. Interestingly, exposure of 750uC heat-treated microstructure (IM08) to 0?5M MgCl2 gave similar weight loss results to the as received material, indicating that only localised corrosion had occurred in this sample. This was confirmed by inspecting the surface of this sample in the microscope. In contrast exposure of the as received microstructure to 1M HCl and 0?167M FeCl3 resulted in corrosion rates 10 and 100 times larger. The application of HCl was chosen to provide information about the effect of pH on localised corrosion in chloride-bearing environment. The reduced nickel content of duplex stainless steel compared to their austenitic counterparts is deleterious when exposed to reducing conditions, such as HClcontaining solutions.16 The effect of FeCl3 on the significantly increased corrosion rate is in parallel related to the high oxidising potential of this solution, promoting pitting corrosion in this environment compared to MgCl2 exposure (e.g. ASTM G48).17 Figure 4b shows the surface of the as received sample after exposure to FeCl3 containing solution, indicating severe pitting corrosion accompanied by widespread selective dissolution of the ferrite. These tests were carried out at 50uC, which is far above typical threshold pitting temperatures reported for FeCl3 solution exposure, e.g. for 6% FeCl3 15–30uC.16 It is also not clear whether selective dissolution or microgalvanic coupling between the ferrite and austenite is responsible for the observed corrosion attack. Interestingly, similar behaviour of a selective corrosion pathway by dissolving the ferrite whilst the austenite remains mostly unaffected was observed during MgCl2 induced-atmospheric corrosion tests of duplex stainless steel grades 2205 and 2507.8 Selective corrosion pathways are affected by local microstructure susceptibility (e.g. strain and/or chemical composition). This has been demonstrated, for example, by investigating the relationship between critical pitting


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4 Images of surface of as received microstructure after 49 days of immersion in a 0?5M MgCl2 (IM05) and b in 0?167M FeCl3 (IM07)

temperature and associated microstructure attack of heat-treated grade 2205 duplex stainless steel.18

Atmospheric-induced stress corrosion cracking testing at 50uC The surface after AISCC testing of an as received microtensile sample which was dosed with a 0?5 mL droplet containing 0?2M aqueous chloride solution, resulting in a chloride deposition density of 390 mg cm22 is shown in Fig. 5. The droplet contained mixed MgCl2 : FeCl3 salts with a mol ratio of 1 : 1. The sample was tensile loaded before exposure to 3% strain and regular observations of the surface indicated corrosion occurring after less than 63 days, which was apparent by corrosion products present within the salt droplet. After 12 months of exposure and termination of the test small stress corrosion cracks were observed (Fig. 5b). These microcracks seemed to be associated with local corrosion sites in the austenite, and the presence of almost parallel cracks indicated the possible involvement of slip planes in the local nucleation processes. Chloride-induced SCC is typically driven by a tri-axial stress state in the presence of

a tensile principal stress, whereas the presence of a compressive principal stress may retard cracks.19,20 The observed microcracks in Fig. 5b are small in length, with observed crack lengths in the austenite varying between hundreds of nanometres and 15 mm. Chloride-induced SCC of duplex stainless steel has been reported in a range of environments, often associated with boiling CaCl2 or MgCl2 solution at temperatures in excess of 100uC.10 Stress corrosion cracking was also shown to occur in grades 2205 and 2507 below the yield stress at temperatures of 70 and 80uC using a seawater drop evaporation test methods.21 Potentio-static polarisation tests on grade 2205 duplex stainless steel in acidified 26 wt-% NaCl solution indicated the presence of discrete potential regions for SCC and hydrogen embrittlement. Stress corrosion cracking was observed close to the open circuit potential and under anodic polarisation simulating oxidising environments, whereas cathodic polarisation induced hydrogen embrittlement.22 Atmospheric chlorideinduced stress corrosion cracking on U-bend samples of grade 2205 was also observed under 30% MgCl2 droplets at 40uC and 35% RH.6

5 Images (SEM) of a local attack on as received sample after 1 year of exposure at 50uC, and b higher resolution image of local corrosion sites with microstress corrosion cracks in austenite. The sample was strained to 3%, with a 0?5 mL droplet of 390 mg cm22 MgCl2 with FeCl3 deposited on the surface

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Interestingly, the conditions applied in our paper to produce AISCC were far more benign with exposure to only 50uC and low-to-medium chloride deposition densities. The addition of FeCl3 promoted more oxidising exposure conditions, inline with observations of SCC under anodic polarisation.22 The full set of exposed samples is currently being assessed and further AISCC results will be reported in another publication.

Conclusions Two grade 2205 microstructures were metallographically characterised, with the as received microstructure containing austenite and ferrite only, whereas the 750uC heat-treated material also had sigma, chi, and chromium nitride second phases. Immersion tests in 1M HCl and 0?167M FeCl3 indicated an increased weight loss compared to exposure to MgCl2 containing environments, highlighting the effect of pH and ferric ions as corrosion promoters. After heat treatment at 750uC a similar corrosion rate in MgCl2 was observed as in the as received condition. Atmospheric-induced stress corrosion cracking testing at 50uC produced selective corrosion of the ferrite, with the presence of stress corrosion crack nucleation sites in the austenite observed after one year of exposure.

Acknowledgements The authors acknowledge EPSRC (EP/I036397/1) and NDA (NPO004411A-EPS02) for financial support. D. L. Engelberg acknowledges valuable discussions with Dr Cristiano Padovani (NDA) and Dr Alison Davenport (University of Birmingham). The authors are grateful for the kind provision of Grade 2205 Duplex Stainless Steel plate by Rolled Alloys.

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