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Pits with coloured halos formed on 1Cr18Ni9Ti stainless steel surface after ennoblement in seawater
Materials Science and Engineering C 29 (2009) 851–855
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Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m s e c
Pits with coloured halos formed on 1Cr18Ni9Ti stainless steel surface after ennoblement in seawater Wei Wang a,⁎, Xia Zhang a, Jia Wang a,b a b
Ocean University of China, College of Chemistry and Chemical Engineering, Qingdao, 266100, China State Key Laboratory for Corrosion and Protection, Shenyang, 110016, China
a r t i c l e
i n f o
Article history: Received 13 November 2007 Received in revised form 8 July 2008 Accepted 26 July 2008 Available online 5 August 2008 Keywords: Haloed pits Pitting corrosion Stainless steel Ennoblement Seawater
a b s t r a c t Micropits surrounded by coloured halos were observed under incident-light microscope on 1Cr18Ni9Ti stainless steel surface after “ennoblement” in seawater. Ennoblement has been attributed to biofilm formation on stainless steel surface in seawater. In this study, the environment in biofilm for ennoblement was simulated by adding H2O2 into seawater at concentrations that were reported to detect in marine biofilm [N. Washizu, Y. Katada, T. Kodama, Corros. Sci.46 (2004)1291.]. H2O2 increased the passivity of stainless steel in seawater, but this passivation was not uniform. The probability of pitting corrosion was increased after ennoblement. Equal thickness interference on the deposition film around the pits was believed to be the reason for the coloured fringes. It is conceivable that haloed pits on the stainless steel surface are the characteristic morphological indications for pitting corrosion formed under ennoblement condition in seawater. Examinations on the microbial and structural effects of the biofilm were not included in this study. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ennoblement is an interesting phenomenon for stainless steel immersed in seawater. If localized corrosion does not occur in natural seawater, open circuit potential (OCP) of stainless steel will shift to noble direction slowly and the value can approach several hundred millivolts [1]. Several chemical changes at the metal/biofilm interface due to the metabolism of the bacterial colonies, including the production of pH gradients, dissolved oxygen and the enzymes, are believed to be the reason for ennoblement [2]. A recent review by I. B. Beech discussed the impacts of biomineralization processs, enzymes and extracellular polymeric substances (EPS) on electrochemical reactions at the biofilm–metal interface [3]. The hydrogen peroxide (H2O2) generated by microorganisms in natural biofilms can shift the OCP of stainless steel to a nobler direction, and the value of ennoblement is related to season and H2O2 level [4]. An increase in OCP by about 500 mV has been observed after microbial colonization on passive metals [5]. Theoretically, this potential should approach the critical pitting potential of stainless steel. Therefore, the risk of pitting and crevice corrosion onset on stainless steel surfaces can be increased due to ennoblement [6]. Although there are many studies focusing on the mechanisms of ennoblement, few were reported on the characterization of the localized corrosion on stainless steel after ennoblement in seawater. Manganese oxidizing bacteria (MOB) as an ennoblement-causing bacterium in fresh water have been extensively studied for its ⁎ Corresponding author. Tel./fax: +86 532 66782510. E-mail address: [email protected] (W. Wang). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.07.034
involvement in pits initiation which has aspect ratios and shapes closely resembling the aspect ratio and shape of the bacteria [7]. This is the only literature we can find which reported distinct corrosion morphology on stainless steel after ennoblement. We report here for the first time an observation of coloured halos formed around pits on the 1Cr18Ni9Ti stainless steel surface after ennoblement in seawater. The chemical environment in biofilm was simulated by chemically adding H2O2 into seawater according to the concentrations reported in a literature [4]. The evaluation of the biological and biofilm structure effects was not included in this study. A possible mechanism of the formation of the pitting corrosion with coloured halos was proposed based on the results. 2. Materials and methods Natural seawater used in this study was taken from the seashore of Qingdao, China. The raw seawater from the inlet was pumped into a setting tank to remove the large particles. The chemical properties of the seawater are: salinity, 33.4 to 35.5‰; pH value: 7.9 to 8.1; dissolved oxygen: 5–7 mg/L. The 1Cr18Ni9Ti stainless steel (a stainless steel used in China) used in the study has the following composition: C ≤ 0.12, Si ≤ 1.00, Mn ≤ 2.00, P ≤ 0.035, S ≤ 0.030, Ni 8.00–11.00, Cr17.00–19.00, balance Fe. The electrodes used for the electrochemical test were cylindrical (diameter 1 cm) and were masked by epoxy resin except for the working surfaces. Their surfaces were polished with metallurgical paper up to No.1000 grade, rinsed with distilled water followed by acetone, and stored in a desiccator until using. The electrochemical instrument used to record the potential changes was PARStat 2263. All
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corrosion potentials mentioned in this paper are expressed on the SCE scale; the counter electrode was platinum plate. Seawater with H2O2 was prepared by adding a certain amount of concentrated H2O2 (30% in the weight percentage) to seawater to bring the H2O2 concentration to a desired value (from 5 to 30 ppm, according to what was reported in [4]). The changes of potential after the H2O2 addition were recorded. Morphological observations of the corrosion on electrodes were made under Leika (MZ16) and Pixera (100 C) microscopes. These two types of microscope differ essentially in light source mode: the former with outside light source and the latter with incident-light source (resembles an epifluorescence microscope but with white light source). Although there were differences in colours in the photographs due to being taken from different angles of light incidence, it does not affect the data analysis. 3. Results and discussion 3.1. Relationship between H2O2 and the OCP of stainless steel After the addition of different concentrations of H2O2 into the seawater, the OCP of stainless steel shifted to noble direction rapidly. In some cases this change was smooth, as shown in Fig. 1 (a). In other cases it was jagged [Fig. 1(b)]. Generally speaking, the shift of the OCP of stainless steel towards the noble direction suggests an increase of stainless steel passivity. As a common oxidant and a chemical passivating agent, H2O2 may cause the modification of the nature of the surface oxides layers, and the enrichment in oxide abundance at the stainless steel surface may also contribute to ennoblement [8]. It is thought that the distinct
Fig. 2. OCP evolution of stainless steel during localized corrosion occurrence after ennoblement in seawater.
difference between chemical passivation and electrochemical passivation is different ways to polarize metal [9]. It is hypothesized that the ennoblement process may be equivalent to applying anodic polarization to stainless steel to some degree. The development of the thickness of passive film on stainless steel was believed to be linear to applied potential [10]. According to the latest published literature[11], the oxygen concentration in the 316L stainless steel passive film and the molar concentration ratio of oxidized species FeOX/CrOX ratio increased, and growth of nanoparticles (presumed as ferric oxide/ hydroxide) on stainless steel surface was observed after the addition of H2O2 in synthetic fresh water(NaCl 0.46 mmol L− 1). Therefore, passivity of stainless steel strengthened after addition of H2O2 into seawater despite the very low concentration. In seawater, the presence of chloride adsorbed on the surface or incorporated in the passive film can be detrimental to film stability and lead to pit initiation. Potential negative shift is usually accompanied with the initiation of pit on passive film. When oxidant exists in solution, the potential oscillation between activation and passivation is observed, indicating that activation and passivation changes can cycle in short time [9]. This explains the potential fluctuation during the ennoblement in Fig. 1(b). The huge fluctuation means that severe corrosion occurred on stainless steel but repassivated quickly. Due to the differences between passive film of each stainless steel sample, detrimental effect of Cl− on some samples was less than others, therefore less potential fluctuation during potential evolution was observed after H2O2 addition, as shown in Fig. 1(a). Thus in seawater, after addition of H2O2 there were two processes on stainless steel surface: one detrimental, the other increasing overall passivity. This made passivity of stainless steel not uniform although the whole electrode surface passivity is increased and the potential shifted to noble direction. A recent literature [11] suggested that, after H2O2 addition, particles with larger size and irregular shape were formed on stainless steel, provoking a roughness higher than that in fresh water(with no H2O2). Some of these particles exceeded 10 nm in height. It is also possible that the complex microstructure of stainless steel which could be composed of non-metallic inclusions, intermetallic particles and defects such as cracks and dislocation on passive film may also increase the heterogeneous passivity distribution. Therefore, all of above factors leads to heterogeneousness of passive film development during ennoblement of stainless steel in seawater. 3.2. Morphology of localized corrosion after ennoblement
Fig. 1. Relationship between H2O2 and ennoblement of stainless steel in seawater.
Fig. 2 shows the changes of stainless steel OCP during pitting corrosion occurred after ennoblement (H2O2, 20 ppm). OCP decreased
W. Wang et al. / Materials Science and Engineering C 29 (2009) 851–855
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Fig. 3. (a) Morphology of localized corrosion in center of stainless steel electrode (Leika 200×); (b) small concentric coloured halos around a micropit was observed in (a) under Pixera microscope; (c) amplified pit in (a), a blurred coloured ring can also be observed around the pit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
steeply in about transient range of 40 s and leveled off thereafter indicating that the localized corrosion had occurred. The electrode was taken out and a localized corrosion spot became visible on the electrode surface. The electrode was examined under the microscope after being rinsed by distilled water and dried by hair drier. Irregular coloured fringes can be observed around pit as shown in Fig. 3(a). Fig. 3(c) is the magnification of the pit in Fig. 3(a) and a blurred coloured halo can also be seen around the amplified pit. There was little rust on and around the pit (Fig. 3(c)) in small area. This may relate to anodic and cathodic potential field characteristic distribution around pit during localized corrosion occurrence. Fig. 3(a) and (c) was obtained by a Leika microscope. An interesting morphological pattern of pitting corrosion with coloured halos was found under Pixera microscope, as shown in Fig. 3 (b). Halos around the micropit in Fig. 3(b) are more round and obviously concentric than that in Fig. 3(a) and (c). These halos also showed different colours. The real area of coloured halos is bigger than that in Fig. 3(b) (for limited visual field of microscope, only coloured halos around pit were taken) and coloured halos in large area are shown in Fig. 4. Fig. 4 shows that coloured fringes such as green and pink appear repeatedly and the width of pink gradually increases with ring radius. Light yellow and blue can also be observed. Because it can only be observed under the Pixrea microscope (incident-light source), the location of pit with coloured halos in Fig. 3
(b) was determined by video produced in experiment (with the Pixera microscope). Lots of corrosion product can be found on pits and surrounding area during auto-catalytic process in pitting corrosion. Because little rust was observed in this experiment, and considering the temporal patterns shown in Fig. 2, we conclude that little auto-catalytic process occurred. The corroded passive film must re-passivate quickly in seawater. This localized corrosion and repassivation process may be controlled by metal ions diffusion, migration and hydrolysis in seawater. In this experiment, small pitting and crevice corrosion with coloured fringes also occurred in seawater under other H2O2 concentrations, but they were less regular than that shown in Fig. 3(b). The pattern in Fig. 3(b) is “perfect” for pitting corrosion due to ennoblement in seawater. We luckily observed this fringe pattern in Fig. 3(b) and this morphological pattern of pitting corrosion may vividly reflect a mechanism of localized corrosion after ennoblement. 3.3. Mechanism of pit formation with coloured halos around after ennoblement Although H2O2 increased passivity and thickness of passive film on stainless steel, the film thickness varies in seawater because influence from detrimental Cl−, roughness and heterogeneousness increasing of passive film. Passive film thinning area on stainless steel normally has low potential. Therefore, potential differences exist between different
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W. Wang et al. / Materials Science and Engineering C 29 (2009) 851–855
Fig. 4. Photographs of coloured halos around and in vicinity of a micropit on stainless steel, (a) light gray can be observed on ordinary surface of stainless steel; (b) outmost coloured halos around the micropit; (c) photograph near the micropit; (d) the micropit with coloured halos around. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
micro-zones on the same electrode. Faint current flows between “micro-anodes” at thin film zone and cathode around them. For high resistance of passive film, anodic current density is low at the beginning of ennoblement. During the ennoblement, potential difference and anodic current density gradually increased. When potential difference reached a certain threshold value and anodic current exceeded passivate current density, passive film broke down and localized corrosion occurred quickly. This process resembles local galvanic corrosion to some degree. We also believe that there was a small difference of 0.2–0.5 nm in the film thickness on stainless steel surface [12] at the first pit initiated. Fe2+, Cr3+, Ni2+ diffused from pits will react with OH− in seawater or produced by cathodic reaction around pits (these metal ions may also react with other anions in seawater). Fe( ) could be oxidized to Fe( ) by O2 or H2O2 produced by cathodic reaction [13,14]. Because of high potential distribution around the pit, the deposition film may be oxidized and have some passive characters. This resulted in a layer of complicated
deposition film around the pit. The film deposited on the stainless steel surface firmly and could only be removed with metallurgical paper. Passive film of stainless steel is colourless and different film thickness leads to different colours due to the interference of light reflected at the oxide film/air and steel/oxide film interfaces [15–17]. We reason that the deposition film may have some characteristics of passive film. For ion diffusion reason, deposition film around pit gradually becomes thinner for outwards and the section of this film is just like a wedge. When the light is at normal incidence equal thickness interference will occur on the wedge film, as shown in Fig. 5 (where n is index of refractive, θ is the angle of wedge, d is the thickness at light reflect point on top and bottom surface of wedge). Points on lines parallel with line where up and bottom side of wedge are in contact have the same thickness. Then the series of interference bands are parallel with line of the wedge. Interchanged bright and dark fringes appear if monochromatic light reflect from two sides of wedge film. Every bright band on wedge
Fig. 5. Interference between two light waves reflected from the two sides of a wedge. The angle θ has been exaggerated for clarity.
W. Wang et al. / Materials Science and Engineering C 29 (2009) 851–855
film becomes coloured fringes when white light is used, and the sequence of colours in each fringe is from blue to red along film thickness increasing direction. For this reason, a series of concentric coloured halos appeared on the round wedge deposition film around pit, as shown in Fig. 3(b). The colour sequence of each interference fringe is from red to blue along the film thickness decreasing direction (the colour sequence in this test is not clear as fringes on oil film or soap bubble, but some colours, e.g. pink and green appear repeatedly suggesting that interference is going on.). The colour sequence occurred repeatedly but irregularly in Fig. 3(a) due to the variations of film thickness. From the theory of interference we also know that for monochromatic incident light the interference space between bright or dark fringes on wedge film is proportional to 1 / θ. The smaller the θ, the wider the fringe space [18]. No space was observed between coloured fringes in this test. Because the deposition film could not be an ideal wedge and the up surface was not a plane. Then this deduction could partially explain that why the width of coloured fringes were not even and some colour fringe became wider for outwards, such as pink fringe. This can also explain why the colour of outmost fringes is always light blue. Because electrodes in this experiment were face vertically held, the upper side of the morphology of pitting corrosion in Fig. 3(a) is semicircle and obviously controlled by ions diffusion; the “tail” of the morphology is irregular and controlled by gravity and coarseness of stainless steel surface. Because just outside the bigger pit, the concentration of cations is high, therefore, the coloured halos in Fig. 3(c) are hardly influenced by surface coarseness. The farther distance from the pit, the more irregular the morphology of fringes are. If pit is very small and concentration of cations diffused from pit is low, then the pattern of deposition film obviously controlled by diffusion and the morphology of coloured halos is nearly normal round, as shown in Fig. 3(b). No coloured halos or fringes were observed around pits on stainless steel after anodic polarization in seawater. The mouth surface surrounding the pit is clean and no deposition is found. In the polarization test, anodic current density rapidly increased when pitting corrosion occurred under anodic polarization. Ion current density increased greatly and cations dissolved from pits electromigrated to counter electrode direction quickly. Then hydroxide and other products can hardly be adsorbed on stainless surface.
occurs in short time on stainless steel during passivity strengthening process. But for excellent corrosion-resistant stainless steel, the probability of this process taking place in natural environment is very low. This unique morphology has not been reported in ennoblement study. From the point of view of localized corrosion, the present results shed a light on the morphology of pit corrosion. This morphology could give us a better understanding of pitting corrosion during ennoblement. Examination of the morphology of localized corrosion on other stainless steel samples after ennoblement is ongoing. 4. Conclusions 1. Risk of localized corrosion on 1Cr18Ni9Ti stainless steel was increased for ennoblement after addition of H2O2 into seawater; 2. Micropits with unique coloured halos formed on 1Cr18Ni9Ti stainless steel surface after ennoblement in seawater. Interference on deposition film around micropit can account for the coloured fringes. 3. The mechanism for distinctive localized corrosion on stainless steel after ennoblement in seawater is different from localized corrosion formed under anodic polarization. Acknowledgments The authors would like to thank The National Natural Science Foundation (NSFC) of China for the financial support. No. 40506019. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
3.4. Under what condition could micropits with coloured halos appear again
[14] [15]
Several conditions are required for the occurrence of micropits surrounded by unique coloured fringes on stainless steel surface. Heterogeneous passivity distribution and ennoblement of passive film could provide the main driving force for pitting corrosion in this test. Small “galvanic corrosion” and passive film breakdown in short time during ennoblement are the reasons. We predict that coloured halos/ fringes around pit will appear again if passive film breaks and corrosion
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[16] [17] [18]
S.C. Dexter, G.Y. Gao, Corrosion 14 (1988) 717. V. Scotto, M.E. Lai, Corros. Sci. 40 (1998) 1007. I.B. Beech, J. Sunner, Curr. Opin. Biotechnol. 15 (2004) 181. N. Washizu, Y. Katada, T. Kodama, Corros. Sci. 46 (2004) 1291. K. Mattila, L. Carpen, T. Hakkarainen, M.S. Salkinoja-Salonen, Int. Biodeterior. Biodegrad. 40 (1997) 1. A. Bergel, D. Féron, A. Mollica, Electrochem. Commun. 7 (2005) 900. M. Geisera, R. Avcib, Z. Lewandowski, Int. Biodeterior. Biodegrad. 49 (2002) 235. W.H. Dickinson, Z. Lewandowski, Biodegradation 9 (1998) 11. Q.X. Zha (Ed.), An Introduction to Kinetics of Electrode Process, 3rd ed., Science Press, Beijing, 2002, p. 322, (Chinese). C.-O.A. Olsson, D. Landolt, Electrochim. Acta 48 (2003) 1093. J. Landoulsi, M.J. Genet, C. Richard, K. El Kirat, P.G. Rouxhet, S. Pulvin, J. Colloid Interface. Sci. 320 (2008) 508. K. Sugimoto, S. Matsuda, Y. Ogiwara, K. Kitamura, J. Electrochem. Soc. 132 (1985) 1791. N. Le Bozec, C. Compère, M.L'. Her, A. Laouenan, D. Costa, P. Marcus, Corros. Sci. 43 (2001) 765. Y. González-García, G.T. Burstein, S. González, R.M. Souto, Electrochem. Commun. 6 (2004) 637. E. Kikuti, N. Bocchi, J.L. Pastol, M.G. Ferreira, M.F. Montemor, M. da Cunha Belo, A.M. Simões, Corros. Sci. 49 (2007) 2303. R. Conrrado, N. Bocchi, R.C. Rocha-Filho, S.R. Biaggio, Electrochim. Acta 48 (2003) 2417. K. Ogura, W. Lou, M. Nakayama, Electrochim. Acta 41 (1996) 2849. J.J. Mao, M. Gu (Eds.), University Physics(Chinese), Volume Two, Higher Education Press, Beijing, 2006, p. 101.
Contents lists available at ScienceDirect
Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m s e c
Pits with coloured halos formed on 1Cr18Ni9Ti stainless steel surface after ennoblement in seawater Wei Wang a,⁎, Xia Zhang a, Jia Wang a,b a b
Ocean University of China, College of Chemistry and Chemical Engineering, Qingdao, 266100, China State Key Laboratory for Corrosion and Protection, Shenyang, 110016, China
a r t i c l e
i n f o
Article history: Received 13 November 2007 Received in revised form 8 July 2008 Accepted 26 July 2008 Available online 5 August 2008 Keywords: Haloed pits Pitting corrosion Stainless steel Ennoblement Seawater
a b s t r a c t Micropits surrounded by coloured halos were observed under incident-light microscope on 1Cr18Ni9Ti stainless steel surface after “ennoblement” in seawater. Ennoblement has been attributed to biofilm formation on stainless steel surface in seawater. In this study, the environment in biofilm for ennoblement was simulated by adding H2O2 into seawater at concentrations that were reported to detect in marine biofilm [N. Washizu, Y. Katada, T. Kodama, Corros. Sci.46 (2004)1291.]. H2O2 increased the passivity of stainless steel in seawater, but this passivation was not uniform. The probability of pitting corrosion was increased after ennoblement. Equal thickness interference on the deposition film around the pits was believed to be the reason for the coloured fringes. It is conceivable that haloed pits on the stainless steel surface are the characteristic morphological indications for pitting corrosion formed under ennoblement condition in seawater. Examinations on the microbial and structural effects of the biofilm were not included in this study. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Ennoblement is an interesting phenomenon for stainless steel immersed in seawater. If localized corrosion does not occur in natural seawater, open circuit potential (OCP) of stainless steel will shift to noble direction slowly and the value can approach several hundred millivolts [1]. Several chemical changes at the metal/biofilm interface due to the metabolism of the bacterial colonies, including the production of pH gradients, dissolved oxygen and the enzymes, are believed to be the reason for ennoblement [2]. A recent review by I. B. Beech discussed the impacts of biomineralization processs, enzymes and extracellular polymeric substances (EPS) on electrochemical reactions at the biofilm–metal interface [3]. The hydrogen peroxide (H2O2) generated by microorganisms in natural biofilms can shift the OCP of stainless steel to a nobler direction, and the value of ennoblement is related to season and H2O2 level [4]. An increase in OCP by about 500 mV has been observed after microbial colonization on passive metals [5]. Theoretically, this potential should approach the critical pitting potential of stainless steel. Therefore, the risk of pitting and crevice corrosion onset on stainless steel surfaces can be increased due to ennoblement [6]. Although there are many studies focusing on the mechanisms of ennoblement, few were reported on the characterization of the localized corrosion on stainless steel after ennoblement in seawater. Manganese oxidizing bacteria (MOB) as an ennoblement-causing bacterium in fresh water have been extensively studied for its ⁎ Corresponding author. Tel./fax: +86 532 66782510. E-mail address: [email protected] (W. Wang). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.07.034
involvement in pits initiation which has aspect ratios and shapes closely resembling the aspect ratio and shape of the bacteria [7]. This is the only literature we can find which reported distinct corrosion morphology on stainless steel after ennoblement. We report here for the first time an observation of coloured halos formed around pits on the 1Cr18Ni9Ti stainless steel surface after ennoblement in seawater. The chemical environment in biofilm was simulated by chemically adding H2O2 into seawater according to the concentrations reported in a literature [4]. The evaluation of the biological and biofilm structure effects was not included in this study. A possible mechanism of the formation of the pitting corrosion with coloured halos was proposed based on the results. 2. Materials and methods Natural seawater used in this study was taken from the seashore of Qingdao, China. The raw seawater from the inlet was pumped into a setting tank to remove the large particles. The chemical properties of the seawater are: salinity, 33.4 to 35.5‰; pH value: 7.9 to 8.1; dissolved oxygen: 5–7 mg/L. The 1Cr18Ni9Ti stainless steel (a stainless steel used in China) used in the study has the following composition: C ≤ 0.12, Si ≤ 1.00, Mn ≤ 2.00, P ≤ 0.035, S ≤ 0.030, Ni 8.00–11.00, Cr17.00–19.00, balance Fe. The electrodes used for the electrochemical test were cylindrical (diameter 1 cm) and were masked by epoxy resin except for the working surfaces. Their surfaces were polished with metallurgical paper up to No.1000 grade, rinsed with distilled water followed by acetone, and stored in a desiccator until using. The electrochemical instrument used to record the potential changes was PARStat 2263. All
852
W. Wang et al. / Materials Science and Engineering C 29 (2009) 851–855
corrosion potentials mentioned in this paper are expressed on the SCE scale; the counter electrode was platinum plate. Seawater with H2O2 was prepared by adding a certain amount of concentrated H2O2 (30% in the weight percentage) to seawater to bring the H2O2 concentration to a desired value (from 5 to 30 ppm, according to what was reported in [4]). The changes of potential after the H2O2 addition were recorded. Morphological observations of the corrosion on electrodes were made under Leika (MZ16) and Pixera (100 C) microscopes. These two types of microscope differ essentially in light source mode: the former with outside light source and the latter with incident-light source (resembles an epifluorescence microscope but with white light source). Although there were differences in colours in the photographs due to being taken from different angles of light incidence, it does not affect the data analysis. 3. Results and discussion 3.1. Relationship between H2O2 and the OCP of stainless steel After the addition of different concentrations of H2O2 into the seawater, the OCP of stainless steel shifted to noble direction rapidly. In some cases this change was smooth, as shown in Fig. 1 (a). In other cases it was jagged [Fig. 1(b)]. Generally speaking, the shift of the OCP of stainless steel towards the noble direction suggests an increase of stainless steel passivity. As a common oxidant and a chemical passivating agent, H2O2 may cause the modification of the nature of the surface oxides layers, and the enrichment in oxide abundance at the stainless steel surface may also contribute to ennoblement [8]. It is thought that the distinct
Fig. 2. OCP evolution of stainless steel during localized corrosion occurrence after ennoblement in seawater.
difference between chemical passivation and electrochemical passivation is different ways to polarize metal [9]. It is hypothesized that the ennoblement process may be equivalent to applying anodic polarization to stainless steel to some degree. The development of the thickness of passive film on stainless steel was believed to be linear to applied potential [10]. According to the latest published literature[11], the oxygen concentration in the 316L stainless steel passive film and the molar concentration ratio of oxidized species FeOX/CrOX ratio increased, and growth of nanoparticles (presumed as ferric oxide/ hydroxide) on stainless steel surface was observed after the addition of H2O2 in synthetic fresh water(NaCl 0.46 mmol L− 1). Therefore, passivity of stainless steel strengthened after addition of H2O2 into seawater despite the very low concentration. In seawater, the presence of chloride adsorbed on the surface or incorporated in the passive film can be detrimental to film stability and lead to pit initiation. Potential negative shift is usually accompanied with the initiation of pit on passive film. When oxidant exists in solution, the potential oscillation between activation and passivation is observed, indicating that activation and passivation changes can cycle in short time [9]. This explains the potential fluctuation during the ennoblement in Fig. 1(b). The huge fluctuation means that severe corrosion occurred on stainless steel but repassivated quickly. Due to the differences between passive film of each stainless steel sample, detrimental effect of Cl− on some samples was less than others, therefore less potential fluctuation during potential evolution was observed after H2O2 addition, as shown in Fig. 1(a). Thus in seawater, after addition of H2O2 there were two processes on stainless steel surface: one detrimental, the other increasing overall passivity. This made passivity of stainless steel not uniform although the whole electrode surface passivity is increased and the potential shifted to noble direction. A recent literature [11] suggested that, after H2O2 addition, particles with larger size and irregular shape were formed on stainless steel, provoking a roughness higher than that in fresh water(with no H2O2). Some of these particles exceeded 10 nm in height. It is also possible that the complex microstructure of stainless steel which could be composed of non-metallic inclusions, intermetallic particles and defects such as cracks and dislocation on passive film may also increase the heterogeneous passivity distribution. Therefore, all of above factors leads to heterogeneousness of passive film development during ennoblement of stainless steel in seawater. 3.2. Morphology of localized corrosion after ennoblement
Fig. 1. Relationship between H2O2 and ennoblement of stainless steel in seawater.
Fig. 2 shows the changes of stainless steel OCP during pitting corrosion occurred after ennoblement (H2O2, 20 ppm). OCP decreased
W. Wang et al. / Materials Science and Engineering C 29 (2009) 851–855
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Fig. 3. (a) Morphology of localized corrosion in center of stainless steel electrode (Leika 200×); (b) small concentric coloured halos around a micropit was observed in (a) under Pixera microscope; (c) amplified pit in (a), a blurred coloured ring can also be observed around the pit. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
steeply in about transient range of 40 s and leveled off thereafter indicating that the localized corrosion had occurred. The electrode was taken out and a localized corrosion spot became visible on the electrode surface. The electrode was examined under the microscope after being rinsed by distilled water and dried by hair drier. Irregular coloured fringes can be observed around pit as shown in Fig. 3(a). Fig. 3(c) is the magnification of the pit in Fig. 3(a) and a blurred coloured halo can also be seen around the amplified pit. There was little rust on and around the pit (Fig. 3(c)) in small area. This may relate to anodic and cathodic potential field characteristic distribution around pit during localized corrosion occurrence. Fig. 3(a) and (c) was obtained by a Leika microscope. An interesting morphological pattern of pitting corrosion with coloured halos was found under Pixera microscope, as shown in Fig. 3 (b). Halos around the micropit in Fig. 3(b) are more round and obviously concentric than that in Fig. 3(a) and (c). These halos also showed different colours. The real area of coloured halos is bigger than that in Fig. 3(b) (for limited visual field of microscope, only coloured halos around pit were taken) and coloured halos in large area are shown in Fig. 4. Fig. 4 shows that coloured fringes such as green and pink appear repeatedly and the width of pink gradually increases with ring radius. Light yellow and blue can also be observed. Because it can only be observed under the Pixrea microscope (incident-light source), the location of pit with coloured halos in Fig. 3
(b) was determined by video produced in experiment (with the Pixera microscope). Lots of corrosion product can be found on pits and surrounding area during auto-catalytic process in pitting corrosion. Because little rust was observed in this experiment, and considering the temporal patterns shown in Fig. 2, we conclude that little auto-catalytic process occurred. The corroded passive film must re-passivate quickly in seawater. This localized corrosion and repassivation process may be controlled by metal ions diffusion, migration and hydrolysis in seawater. In this experiment, small pitting and crevice corrosion with coloured fringes also occurred in seawater under other H2O2 concentrations, but they were less regular than that shown in Fig. 3(b). The pattern in Fig. 3(b) is “perfect” for pitting corrosion due to ennoblement in seawater. We luckily observed this fringe pattern in Fig. 3(b) and this morphological pattern of pitting corrosion may vividly reflect a mechanism of localized corrosion after ennoblement. 3.3. Mechanism of pit formation with coloured halos around after ennoblement Although H2O2 increased passivity and thickness of passive film on stainless steel, the film thickness varies in seawater because influence from detrimental Cl−, roughness and heterogeneousness increasing of passive film. Passive film thinning area on stainless steel normally has low potential. Therefore, potential differences exist between different
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Fig. 4. Photographs of coloured halos around and in vicinity of a micropit on stainless steel, (a) light gray can be observed on ordinary surface of stainless steel; (b) outmost coloured halos around the micropit; (c) photograph near the micropit; (d) the micropit with coloured halos around. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
micro-zones on the same electrode. Faint current flows between “micro-anodes” at thin film zone and cathode around them. For high resistance of passive film, anodic current density is low at the beginning of ennoblement. During the ennoblement, potential difference and anodic current density gradually increased. When potential difference reached a certain threshold value and anodic current exceeded passivate current density, passive film broke down and localized corrosion occurred quickly. This process resembles local galvanic corrosion to some degree. We also believe that there was a small difference of 0.2–0.5 nm in the film thickness on stainless steel surface [12] at the first pit initiated. Fe2+, Cr3+, Ni2+ diffused from pits will react with OH− in seawater or produced by cathodic reaction around pits (these metal ions may also react with other anions in seawater). Fe( ) could be oxidized to Fe( ) by O2 or H2O2 produced by cathodic reaction [13,14]. Because of high potential distribution around the pit, the deposition film may be oxidized and have some passive characters. This resulted in a layer of complicated
deposition film around the pit. The film deposited on the stainless steel surface firmly and could only be removed with metallurgical paper. Passive film of stainless steel is colourless and different film thickness leads to different colours due to the interference of light reflected at the oxide film/air and steel/oxide film interfaces [15–17]. We reason that the deposition film may have some characteristics of passive film. For ion diffusion reason, deposition film around pit gradually becomes thinner for outwards and the section of this film is just like a wedge. When the light is at normal incidence equal thickness interference will occur on the wedge film, as shown in Fig. 5 (where n is index of refractive, θ is the angle of wedge, d is the thickness at light reflect point on top and bottom surface of wedge). Points on lines parallel with line where up and bottom side of wedge are in contact have the same thickness. Then the series of interference bands are parallel with line of the wedge. Interchanged bright and dark fringes appear if monochromatic light reflect from two sides of wedge film. Every bright band on wedge
Fig. 5. Interference between two light waves reflected from the two sides of a wedge. The angle θ has been exaggerated for clarity.
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film becomes coloured fringes when white light is used, and the sequence of colours in each fringe is from blue to red along film thickness increasing direction. For this reason, a series of concentric coloured halos appeared on the round wedge deposition film around pit, as shown in Fig. 3(b). The colour sequence of each interference fringe is from red to blue along the film thickness decreasing direction (the colour sequence in this test is not clear as fringes on oil film or soap bubble, but some colours, e.g. pink and green appear repeatedly suggesting that interference is going on.). The colour sequence occurred repeatedly but irregularly in Fig. 3(a) due to the variations of film thickness. From the theory of interference we also know that for monochromatic incident light the interference space between bright or dark fringes on wedge film is proportional to 1 / θ. The smaller the θ, the wider the fringe space [18]. No space was observed between coloured fringes in this test. Because the deposition film could not be an ideal wedge and the up surface was not a plane. Then this deduction could partially explain that why the width of coloured fringes were not even and some colour fringe became wider for outwards, such as pink fringe. This can also explain why the colour of outmost fringes is always light blue. Because electrodes in this experiment were face vertically held, the upper side of the morphology of pitting corrosion in Fig. 3(a) is semicircle and obviously controlled by ions diffusion; the “tail” of the morphology is irregular and controlled by gravity and coarseness of stainless steel surface. Because just outside the bigger pit, the concentration of cations is high, therefore, the coloured halos in Fig. 3(c) are hardly influenced by surface coarseness. The farther distance from the pit, the more irregular the morphology of fringes are. If pit is very small and concentration of cations diffused from pit is low, then the pattern of deposition film obviously controlled by diffusion and the morphology of coloured halos is nearly normal round, as shown in Fig. 3(b). No coloured halos or fringes were observed around pits on stainless steel after anodic polarization in seawater. The mouth surface surrounding the pit is clean and no deposition is found. In the polarization test, anodic current density rapidly increased when pitting corrosion occurred under anodic polarization. Ion current density increased greatly and cations dissolved from pits electromigrated to counter electrode direction quickly. Then hydroxide and other products can hardly be adsorbed on stainless surface.
occurs in short time on stainless steel during passivity strengthening process. But for excellent corrosion-resistant stainless steel, the probability of this process taking place in natural environment is very low. This unique morphology has not been reported in ennoblement study. From the point of view of localized corrosion, the present results shed a light on the morphology of pit corrosion. This morphology could give us a better understanding of pitting corrosion during ennoblement. Examination of the morphology of localized corrosion on other stainless steel samples after ennoblement is ongoing. 4. Conclusions 1. Risk of localized corrosion on 1Cr18Ni9Ti stainless steel was increased for ennoblement after addition of H2O2 into seawater; 2. Micropits with unique coloured halos formed on 1Cr18Ni9Ti stainless steel surface after ennoblement in seawater. Interference on deposition film around micropit can account for the coloured fringes. 3. The mechanism for distinctive localized corrosion on stainless steel after ennoblement in seawater is different from localized corrosion formed under anodic polarization. Acknowledgments The authors would like to thank The National Natural Science Foundation (NSFC) of China for the financial support. No. 40506019. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
3.4. Under what condition could micropits with coloured halos appear again
[14] [15]
Several conditions are required for the occurrence of micropits surrounded by unique coloured fringes on stainless steel surface. Heterogeneous passivity distribution and ennoblement of passive film could provide the main driving force for pitting corrosion in this test. Small “galvanic corrosion” and passive film breakdown in short time during ennoblement are the reasons. We predict that coloured halos/ fringes around pit will appear again if passive film breaks and corrosion
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[16] [17] [18]
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