Photocathodic Protection of 316L Stainless Steel by ... - Springer Link

ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2013, Vol. 49, No. 1, pp. 109–112. © Pleiades Publishing, Ltd., 2013.

PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION

Photocathodic Protection of 316L Stainless Steel by Coating of Anatase Nanoparticles1 N. Baratia, M. A. Faghihi Sania, and H. Ghasemib a

Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, P.O. Box 113659466, Tehran, Iran b Department of Mechanical Engineering, University of Toronto, 5 King’s College Road, Toronto, M5S 3G8, Canada email: [email protected], [email protected] Received February 6, 2012

Abstract—Uniform nanostructure anatase films were coated on 316L stainless steel by the solgel dip coating method. Sols with different values of pH were applied. The corrosion protective behavior of coated samples was investigated by electrochemical measurements in 0.5 molar NaCl solutions on samples placed under UV illumination and dark condition using Tafel curves. It was found that in addition to acting as a physical barrier, anatase thin films are more protective under UV illumination due to photocathodic protection. Neutral sols give better protection due to formation of more uniform and less defective coatings. DOI: 10.1134/S2070205113010036 1

1. INTRODUCTION L Stainless steel is one of the most commonly used metals in corrosive environments due to proper resis tance to general corrosion. However, due to auto passi vation properties, they undergo local corrosion in chlo ride ion containing medias [1]. In order to protect met als from corrosion, organic coatings are a widely used option in industry. Ceramics, especially transition metal oxides, are another attractive coating materials because of their proper chemical stability [2, 3]. TiO2 as a ntype semiconductor is one of these protective coatings [4]. The applications of TiO2 coating for cathodic protec tion of metals under ultraviolet (UV) illumination and the related mechanisms have been reported [4–6]. Under illumination of light having a wavelength equal to the band gap of TiO2 (~3.2 eV), electronhole pairs are created in the TiO2 layer. The holes migrate to the electrolyte, reacting with the H2O molecules, forming O2 and H+ according to reaction (1). 1/2O2 + 2H. (1) H2O + 2h The electrons are transferred to the underlying metal substrate to lower its potential or react with protons or H2O to form H2 or OH–, according to reactions (2) and (3). 2H+ + 2e– H2, (2) – – 1/2O2 + 2e + H2O 2OH . (3) This effect of TiO2 semiconductor coating, similar to the cathodic protection of metals by sacrificial anodes such as zinc, is called photocathodic protection. Never theless, unlike sacrificial anodes in corrosion systems, TiO2 act as a nonsacrificial photo anode. In TiO2 case, 1

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the anodic reaction is not decomposition of TiO2, but the oxidation of water and/or adsorbed organic species by photogenerated holes [7, 8]. In the current work, anatase nanostructure films were prepared on 316L stainless steel substrates by sol gel method. The electrochemical characteristics of these films were investigated in 0.5 molar NaCl solu tions under UV illumination and in dark conditions. The influence of pH of the sol and uniformity of the coating on electrochemical characteristics and photo cathodic protection of 316L stainless steel samples were studied. 2. MATERIAL AND METHODS Similar to procedure reported by N. Barati, et al. [9], TiO2 stable sol was prepared from TetraηButyle ortho Titanat (TBT) as a precursor. Ethanol as solvent and Ethyl Aceto Acetate (EAcAc) as catalyst, with specially defined ratio [9], were added to the precursor at room temperature. To investigate pH effect of the sol on elec trochemical characteristics, sols with different pH val ues were prepared by adding different amount of HCl. The 316L stainless steel samples (20 × 15 × 5 mm3) were grounded with no. 801500 emery papers, polished with 0.3 and 0.05 μm alumina powder suspension, and cleaned by ethanol and acetone. TiO2 thin films were coated on samples by dip coating method with with drawal speed of 3 cm/min. After drying at 150°C for 30 min, the samples were treated at 400°C for 1h with increasing temperature rate of 5°C/min to form a nanostructure film of anatase phase [9]. Scanning Elec tron Microscopy (SEM) was used to evaluate the coat ing homogeneity. The measurement of polarization curves were carried out in a threeelectrode cell in

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3. RESULTS AND DISCUSSION

10 µm Fig. 1. SEM image of the uniform nanostructure anatase film formed at 400°C for 1 h.

0.4 0.2 E, V

0 –0.2 –0.4

1 2

–0.6 –0.8 –14

–12

–10

–8 –6 log I, A cm–2

–4

–2

0

Fig. 2. Tafel curves of bare 316L stainless steel (S.S) and ana tase coated 316L stainless steel in 0.5 mol L–1 NaCl solution under UV illumination.

0.5 mol L–1 NaCl solution (pH 4.5), using EG&G Electrochemical measurement system. The coated samples were used as working electrodes; the test cell included a bare 316L Stainless Steel electrode and a sat urated calomel reference electrode (SCE). The Tafel curves were measured in the range of ±400 mV at the open circuit potential at the rate of 0.167 mV/s. The experiments were started after 15 min immersion of samples in NaCl solution. To investigate effect of UV illumination on the cathodic protection, three coated samples were placed under UV illumination after heat treatment, and for comparison, three coated samples were placed in a dark place for one month. The polar ization tests were also carried out for coated samples formed by sols with different values of pH.

As reported by N. Barati, et al. [9], the formed phase after calcination at 400°C is anatase phase with average crystallite size of 8.5 nm. Anatase phase has much higher photo catalytic properties than other TiO2 iso morphic phases [10–12]. Figure 1 shows the SEM mor phology of anatase film formed on 316L stainless steel calcined at 400°C for 1 h. It indicates the formation of a uniform anatase thin film. Figure 2 compares Tafel polarization curves of UV illuminated bare 316L stain less steel and the nanostructure coated sample. The electrochemical parameters obtained from Tafel curves through the analytical program of auto lab system are given in Table 1. From anodic current densities (Icorr) and corrosion rate, one finds that the tendency of cor rosion in the coated samples is much lower than the bare samples. Anatase thin films have enhanced corro sion protection by forming a physical barrier that effec tively separated the anode from the cathode electrically. In both samples, the illumination of UV light was the same. Figure 3 compares Tafel polarization curves of coated samples under UV illumination and under dark condition. Electrochemical parameters of the tests are also given in Table 2. It is obvious that corrosion the rate of 316L stainless steel coated samples under UV illumi nation is lower than the same samples under dark con dition. Under UV illumination, corrosion takes place at higher potential. By looking at the anodic current den sities (Icorr) and corrosion potentials (Ecorr) in Table 2, one finds that the UV illuminated samples has lower tendency to corrosion in 0.5 mol L–1 NaCl solution. The anatase coating does not provide protection under dark condition, because of the charge recombination. However, UV illumination has important effect on cor rosion protection. In UV illuminated samples, the cor rosion protection is due to two different phenomena, namely physical barrier effect and photo catalytic prop erty of anatase coating. UV illumination causes transfer of photogenerated electrons in conduction band of anatase to the 316L stainless steel, resulting in a poten tial shift of metal substrate to the corrosion immunity region. In the other words, an adscititious photo gener ated cathodic current inputs to the interface of the coat ing and stainless steel, and total current of the system increases. Therefore, the resistance of the anatase film decreases and the rate of electrochemical reaction at the interface of metal and coating increases. At the same time, the rate of mass transfer trends comparatively slowly. On the other hand, the photo generated holes

Table 1. Electrochemical parameters of bare 316L stainless steel (S.S) and anatase coated 316L stainless steel samples derived from Fig. 2 Sample

Preparation Condition

Bare S.S Coates S.S

UV illumination UV illumination

Ecorr, mV –250 –48

Icorr, A/cm2

Corrosion rate, mpy

320 × 10–9 1.58 × 10–9

1.37 0.685 × 10–3

Epas, mV –200 –10

PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 49 No. 1 2013

PHOTOCATHODIC PROTECTION OF 316L STAINLESS STEEL 0.3

0.8

0.2

0.6

0.1

0.4 0.2

–0.1

E, V

E, V

0 –0.2 –0.3

0 –0.2 –0.4

–0.4 –0.5 –0.6 –14

111

–12

–10

–8 –6 log I, A cm–2

–4

1 2 3

–0.6

1 2 –2

0

–0.8 –15

–10

–5 log I, A cm–2

0

Fig. 3. Tafel curves of anatase coated 316L stainless steels under UV illumination (1) and under dark condition (2) in 0.5 mol L–1 NaCl solution.

Fig. 4. Tafel curves of anatase coated 316L stainless steels in 0.5 mol L–1 NaCl solution formed by sol with pH of 7 (1), 4.5 (2), and 2.3 (3).

migrate to the interface of coating/electrolyte and react with the H2O molecules as a hole scavenger, forming O2 and H* in NaCl solution. The results of photo electro chemical measurement show that under UV illumina tion, the excess electron on the Fermi level of metals may provide an effect of cathodic protection. However, due to the charge recombination problem under dark condition, protection cannot be maintained.

desorption of hydroxide ions or protons on the Ti(OH)4 as the precursor and on the anatase TiO2 as the final product [13]. At higher pH values, condensation pro ceeds along apical directions, leading to skewed chains of the anatase structure [14]. Particle size and corrosion resistance may be related to each other. Lower crystal lite size and therefore higher surface area of coating, prepared in acidic sol, leads to an increase in photo cat alytic property of the coating. Therefore, it can be expected that the corrosion resistance improves by reduction of sol’s pH. However, as demonstrated in Fig. 4, decrease in pH value of the sol and crystallite size, increases the corrosion of the samples. In fact, there is another governing parameter. Smaller anatase crystallites give an increase in grain boundaries, which have higher density of defects. These defects encourage diffusion of corrosive ions and therefore inhibit corro sion protection of the coating.

Figure 4 shows pH effect of initial sol on corrosion behavior of the coated samples. Electrochemical parameters obtained from Tafel curves in Fig. 4 are given in Table 3. Electrochemical parameters shows that the coatings prepared with neutral sol have better corrosion resistance. Decrease in pH of the sol, results in cathodic shift of Ecorr and consequently occurrence of corrosion at lower potential. It is shown that crystallite size of the coatings prepared with the neutral sol is larger than the acidic sol [9]. This phenomenon is attributed to the rate of growth and nucleation. The mechanism of this phenomenon is mainly governed by adsorption and

Table 2. Electrochemical parameters of anatase coated 316L stainless steel samples derived from Fig. 3 Sample


Ecorr, mV

Icorr, A/cm2

Corrosion rate, mpy

Epas, mV

Coated S.S Coated S.S

UV illumination UV illumination

–48 –86.53

1.58 × 10–9 79.4 × 10–9

0.685 × 10–3 0.13

–10 –

Table 3. Electrochemical parameters of anatase coated 316L stainless steel samples derived from Fig. 4 Sample


pH of Sol

Coated S.S Coated S.S Coated S.S

UV illumination UV illumination UV illumination

7 4.5 2.3

Ecorr, mV –48 –110 –270

Icorr, A/cm2 1.58 × 10–9 0.403 × 10–9 0.794 × 10–9


Corrosion rate, mpy 0.685 × 10–3 0.175 × 10–3 0.344 × 10–3

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4. CONCLUSIONS Nanostructure anatase photocathode films are coated on 316L stainless steel substrates by solgel method. SEM images show the uniformity of the coat ings. Electrochemical measurements in 0.5 molar NaCl solutions demonstrate that protection behavior of ana tase coating is due to serving as a physical barrier and under UV illumination as a photo catalytic inhibitor as well. Under UV illumination, the induced photogen erated electrons result in a potential shift of metal sub strate to the corrosion immunity region. Decrease in pH of sol gives a cathodic shift in corrosion potential. Although this decrease provides smaller crystallites and consequently better photocathodic properties, it intro duces more defect sites on the surface. REFERENCES 1. SzklarskaSmiaowska, Z., Pitting Corrosion of Metals, Houston: National Association of Corrosion Engineers, 1986, p. 201. 2. Masalski, J., Glaszek, J., Zabrzeski, J., et al., J. Thin Solid Films, 1999, vol. 349, p. 186.

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