The Influence of NaNO3 on the Atmospheric Corrosion of Zinc

Journal of The Electrochemical Society, 150 共12兲 B583-B588 共2003兲

B583

0013-4651/2003/150共12兲/B583/6/$7.00 © The Electrochemical Society, Inc.

The Influence of NaNO3 on the Atmospheric Corrosion of Zinc Rakel Lindstro¨m,a,z Lars Gunnar Johansson,a,* and Jan-Erik Svenssonb,* a

Department of Inorganic Chemistry, Go¨teborg University, Go¨teborg, Sweden Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 Go¨teborg, Sweden

b

The influence of NaNO3 salt deposits on the atmospheric corrosion of zinc in humid air has been studied. Comparisons are made with the effects of NaCl and Na2 SO4 . Also the combined effect of NaNO3 with NaCl or Na2 SO4 was investigated. The salts were applied to zinc samples prior to exposure. The samples were exposed to purified humid air with careful control of relative humidity 共95%兲, temperature 共22.0°C兲, and flow conditions. The CO2 concentration was 350 ppm and the exposure time was 4 weeks. Mass gain and metal loss results are reported. The corrosion products were analyzed by gravimetry, ion chromatography, and X-ray diffraction. Results show that the corrosion rate in the presence of NaNO3 is only a third of that registered in the presence of NaCl and Na2 SO4 . Further, a slight inhibitive effect of NaNO3 was found in the presence of NaCl or Na2 SO4 . The decreased corrosion rate in the presence of NaNO3 was attributed to the reduction of nitrate to nitrite at the zinc surface. Nitrite is suggested to act as a corrosion inhibitor toward zinc. © 2003 The Electrochemical Society. 关DOI: 10.1149/1.1627348兴 All rights reserved. Manuscript submitted April 2, 2003; revised manuscript received June 25, 2003. Available electronically November 5, 2003.

This work is a continuation of an earlier paper on the influence of a number of chloride and sulfate salts on the atmospheric corrosion of zinc.1 The previous study showed the overwhelming importance of the cation on the salt-induced atmospheric corrosion of zinc. Sodium sulfate and sodium chloride are much more corrosive compared to the corresponding salts of Zn2⫹, Mg2⫹, and NH⫹ 4 . The surface electrolyte formed by soluble salts supports the electrochemical corrosion of the metal. In contrast to the other cations studied, sodium does not form insoluble hydroxy salts and sodium salts therefore form electrolytes that are stable at the high pH values occurring at the cathodic sites. Further, it was shown that the corrosion rate of zinc is related to the amount of sodium ion present on the surface rather than to the amount of chloride or sulfate. These results prompted us to study the influence of NaNO3 on the atmospheric corrosion of zinc. Due to the emissions of NOx from combustion and traffic, the atmospheric deposition of nitrate is considerable. In a study performed in California 共1986兲, NO⫺ 3 contributed to as much as 20% of the total mass 共87.4 ␮g/m3兲 of the tropospheric fine particles.2 Considering the importance of nitrate deposition, investigations of the influence of nitrate salts on atmospheric corrosion are scarce. In a preliminary study of the salt-induced atmospheric corrosion of zinc, samples treated with NaNO3 , Na2 SO4 , and NaCl were exposed in a closed container at 95% relative humidity 共RH兲. It was found that when the differently treated samples were exposed in the same container, the corrosion of NaCl-treated zinc was significantly less compared to when NaCl-treated samples were exposed by themselves. After such a ‘‘mixed’’ exposure, significant amounts of nitrite were detected on the NaCl-treated samples. This indicates that HNO2 (g) had evaporated from the samples treated with NaNO3 and deposited on the NaCl-treated samples. With this indication of corrosion inhibition in mind, we decided to study the influence of NaNO3 and combinations of NaNO3 and Na2 SO4 or NaNO3 on the atmospheric corrosion of zinc. This work addresses the effects of individual pollutants on the corrosion chemistry and the corrosion process of the zinc surface. To this end, all other parameters 共e.g., RH and temperature兲 are kept constant. Systematic studies of the relative importance of different pollutants can contribute toward understanding the atmospheric corrosion behavior of zinc and for developing improved procedures for corrosion testing.

* Electrochemical Society Active Member. z

E-mail: [email protected]

Experimental Zinc sheet of electrolytic grade 共99.9% purity兲 was used in all experiments. The samples had a geometrical area of 20 cm2 (30 ⫻ 30 ⫻ 1 mm). Before exposure the samples where polished in ethanol on SiC paper, up to 1000 mesh. The samples were ultrasonically cleaned in ethanol, dried in air, and stored in a desiccator over silica gel. Salt was added by spraying a mixture of water and ethanol saturated with the specific salt on the samples. Thereafter the samples were dried in a cold air stream. Care was taken to avoid droplet formation on the samples during spraying. The distribution of salt on the surface after spraying was quite even. The amount of salt was determined gravimetrically. The salt treated samples were stored over night before the start of the exposure. Duplicate samples were exposed. The amount of NaNO3 added was between 0.2 to 2.5 ␮mol/cm2. The experimental setup is described in Fig. 1. The exposure system is entirely made of glass and Teflon. There are eight parallel exposure chambers through which the gas is sequentially distributed, the whole gas flow passing through each chamber in turn for 15 s. The gas flow was 1000 mL/min in all experiments, corresponding to a net average gas velocity of 7 mm/s when a chamber is open. Each sample is suspended by a nylon string in the middle of the chamber. Only one sample is exposed in each chamber. The chambers have an inner diameter of 55 mm and a volume of 0.4 L and are immersed in a water tank held at constant temperature 22.0 ⫾ 0.03°C. The temperature in the room is kept at 25°C to avoid condensation in the parts of the system outside the water tank. RH is regulated by mixing known amounts of dry air and air which is saturated with water vapor at the exposure temperature. RH was 95% in all exposures and was controlled with an accuracy of ⫾0.3%. Carbon dioxide was added from a tube. The concentration of CO2 was 350 ppm, corresponding to ambient air. In some selected exposures, a gas trap containing 0.01 M NaOH共aq兲 was placed downstream to each chamber in order to collect HNO2 (g) that was vaporized from the samples. The exposures lasted 4 weeks. Crystalline corrosion products were identified by X-ray diffraction 共XRD兲 using a Siemens D5000 powder diffractometer equipped with a grazing incidence beam attachment fitted with a Go¨bel mirror. The amount of soluble anions on the samples after exposure and the nitrite collected in the gas trap were determined by ion chromatography 共IC兲. A Dionex 1000 was used equipped with a Ionpac AD4-SC Analytic Column. The eluent was 1.8 mM Na2 CO3 / 1.7 mM NaHCO3 and the flow rate was 2 mL/min. The concentration of ammonium was measured using a Skalar instrument. The ammonium ions are converted to gaseous ammonia. The ammonia is then reacted with phenol, forming a colored indo-phenol complex

B584


Figure 1. The experimental setup for the corrosion exposures: 共1兲 dried purified air; 共2兲 flow control; 共3兲 humidifier; 共4兲 vessels for permeation tubes 共not used in this study兲; 共5兲 CO2 inlet; 共6兲 gas mixer; 共7兲 eight exposure chambers with one sample in each; 共8兲 wash bottles with 0.1 M NaOH共aq兲 used as gas traps; 共9兲 eight-channel solenoid valve; and 共10兲 water tank at constant temperature.

that is determined spectrophotometrically. The morphology of the corroded samples was studied by optical and environmental scanning electron microscopy 共ESEM; Electroscan 2020兲. An energydispersive X-ray detector 共EDX; Link ISIS兲 was connected to the microscope. Results Atmospheric corrosion in the presence of NaNO3.—Figure 2a and b shows the wet mass gains and the corrosion rate data for different amounts of NaNO3 added. The large wet mass gain in the beginning of the exposure partly reflects the uptake of water by NaNO3 and the formation of an aqueous electrolyte on the metal surface. The deliquescence point of NaNO3 is reported to be 80% RH at 22°C.3 Throughout the exposure, the samples appeared wet. This is in accordance with the large difference between the wet mass gain 共a兲 and the dry mass gain 共b兲, especially when large amounts of salt are applied. The mass gain and the metal loss vs. the amount of salt added are plotted in Fig. 3. For comparison, corresponding results for NaCl-

Figure 2. 共a, left兲 Wet mass gain and 共b, right兲 corrosion rate data for different amounts of applied NaNO3 : 共䊏兲 22.2, 共⽧兲 55, 共䉱兲 105.5, and 共䊉兲 204.5 ␮g/cm2. The exposure time was 4 weeks, RH 95%, temperature was 22.0°C, and CO2 concentration 350 ppm.

Figure 3. Dry mass gain and metal loss as a function of the amount of sodium ion applied: 共䉱兲 Na2 SO4 , 共䊏兲 NaCl, and 共䊉兲 NaNO3 . The exposure time was 4 weeks, RH 95%, temperature 22.0°C, and CO2 concentration 350 ppm.

and Na2 SO4 -treated samples have been included as reported in a previous paper.1 Notably, the metal loss in the presence of NaNO3 is only a third of that registered in the presence of an equivalent amount of sodium ions in the form of NaCl and Na2 SO4 . In the range investigated, the metal loss and the mass gain increase linearly with the amount of salt added. Compared to zinc treated with Na2 SO4 and NaCl, samples treated with NaNO3 appeared darker, the corrosion attack was more even, and fewer corrosion product aggregates formed 共Fig. 4a-c兲. Figure 5 shows a representative ESEM/EDX micrograph of the corroded surface after 4 weeks exposure. The corrosion products are seen to be unevenly distributed on the micrometer scale, corrosion product islands taking up a large part of the surface. Sodium is enriched in areas that are relatively unaffected by corrosion 共compare the Zn mapping兲. XRD showed that zinc hydroxy carbonate 关 Zn4 (OH) 6 CO3 • H2 O兴 was the dominating crystalline corrosion product. In addition, there were indications for unreacted NaNO3 and for NH4 NO3 . The corrosion product ratio 共metal loss ⫹ mass gain兲/metal loss兲 is about 1.7 in all exposures with NaNO3 . This corresponds to the ratio calculated for pure zinc hydroxy carbonate, supporting the predominance of this phase in the corrosion product. Zinc hydroxy carbonate was also the main corrosion product in the presence of NaCl and Na2 SO4 . However, in the latter cases zinc hydroxy chloride and zinc hydroxy sulfate also formed. After exposure, the samples were leached in water 共pH 7兲. The IC analysis of the leachings and the analysis for ammonium are presented in Fig. 6, showing that nitrite formed on all samples. Traces of ammonium 共corresponding to less than 2% of the added nitrate兲 were also found. The fraction of leachable ions on the samples after exposure 共in comparison to the amount of nitrate applied兲 increases with the amount of salt added. In the case with the largest NaNO3 addition, the amount of nitrate found after exposure corresponded to about 60% of the amount applied, while nitrite accounted for about 20%. One sample was leached directly after exposure without being dried. Corrosion effect of salt mixtures; NaNO3⫹NaCl and NaNO3 ⫹ Na2SO4.—In order to investigate the possible inhibitive effect of nitrate on zinc corrosion, exposures were performed with mixtures


B585

Figure 5. ESEM image and EDX maps of zinc exposed in the presence of NaNO3 . The amount of sodium ion applied was 1.25 ␮mol/cm2 . The exposure time was 4 weeks, RH 95%, temperature 22.0°C, and CO2 concentration 350 ppm. 共Increased brightness signifies high concentrations兲.

Na2 SO4 is slowed down slightly by NaNO3 . However, the corrosion rate in the presence of the salt mixtures is faster compared to the samples treated with only NaNO3 . Zinc samples exposed in the presence of salt mixtures had a similar appearance as samples treated with only NaNO3 . The main difference is that in the case of samples treated with salt mixtures, small amounts of solid corrosion products were visible at the end of the exposure 共Fig. 8a and b兲. However, the solid corrosion products identified by XRD were similar to those found on samples treated with only NaCl and Na2 SO4 . 1 Thus, simonkolleite together with zinc hydroxy carbonate were found on the samples treated with NaCl ⫹ NaNO3 , whereas zinc hydroxy sulfate and zinc hydroxy carbonate were found on the samples treated with Na2 SO4 ⫹ NaNO3 . In contrast to the case where only NaNO3 was present,

Figure 4. Photographic image of zinc exposed in the presence of 共a兲 NaCl, 共b兲 Na2 SO4 , and 共c兲 NaNO3 at 22°C. The amount of sodium ion applied was the same 共1.25 ␮mol/cm2 兲. The exposure time was 4 weeks, RH 95%.

containing equal amounts of NaNO3 and Na2 SO4 or NaCl with respect to the sodium ion. After the first week, all samples appeared wet. With increasing exposure time, the samples treated with the salt mixtures slowly dried up. However, the samples dried more slowly than samples treated with pure NaCl or Na2 SO4 . Figure 7 shows that the atmospheric corrosion of zinc in the presence of NaCl and

Figure 6. The amounts of leachable nitrate and nitrite analyzed by IC on the samples treated with NaNO3 after 4 weeks exposure at 95% RH, 22.0°C, and 350 ppm CO2 plotted as a function of applied nitrate. The open features are the fraction of total amount applied nitrate.

B586


Figure 7. The metal loss 共negative values兲 in ␮mol/cm2 or in mg/cm2 and the amount of applied salt 共positive values兲 in respect to the sodium ion: 共1兲 NaCl, 共2兲 NaCl ⫹ NaNO3 , 共3兲 Na2 SO4 , 共4兲 Na2 SO4 ⫹ NaNO3 , and 共5兲 and 共6兲 two levels of NaNO3 . The exposure time was 4 weeks, RH 95%, temperature 22.0°C, and CO2 concentration 350 ppm.

Figure 9. The amounts of leachable anions as analyzed by IC on the samples treated with 共1兲 NaNO3, 共3兲 NaCl, or 共5兲 Na2 SO4 or 共2 and 4兲 combinations of the salts after 4 weeks exposure at 95% RH, 22.0°C, and 350 ppm CO2 .

NaNO3 and NH4 NO3 were not identified by XRD. Also in this case, the corrosion product ratio corresponds to the value calculated for zinc hydroxy carbonate 关 Zn4 (OH) 6 CO3 •H2 O兴 . Figure 9 shows the amount of water-soluble ions on the sample and the amount of nitrite in the gas trap solution 关designated HNO2 (g)] after exposure. It may be noted that less than 60% of the nitrogen added as sodium nitrate was present on the samples in ionic form after exposure. Nitrite had formed on all samples treated with NaNO3 . The fraction of nitrite is significantly greater on the samples treated with salt mixtures 共2 and 4兲 than on samples treated with only NaNO3 . Further, in all cases where NaNO3 was present, nitrite but no nitrate was found in the gas trap solution. No ammonium was detected on the samples treated with salt mixtures. However, the gas trap solution contained traces of ammonia 共nanomoles兲. The fraction of water-leachable chloride and sulfate increased considerably when NaNO3 was present. Discussion

Figure 8. Photographic image of zinc exposed in the presence of 共a兲 NaCl ⫹ NaNO3 and 共b兲 Na2 SO4 ⫹ NaNO3 at 22°C. The exposure time was 4 weeks, RH 95%, and the CO2 concentration ambient 共350 ppm兲.

The influence of the amount of salt on corrosion.—Figure 3 shows that zinc corrosion in the presence of NaNO3 is only a third of that registered in the presence of corresponding amounts of NaCl and Na2 SO4 . This is somewhat surprising considering that NaNO3 creates a surface electrolyte under our experimental conditions, just as in the cases of sodium sulfate and sodium chloride. The association of sodium ions with unreacted zinc 共see Fig. 5兲 confirms the electrochemical nature of corrosion attack in the presence of NaNO3 . It was reported previously1 that the atmospheric corrosion of zinc in the presence of soluble chlorides and sulfates results in the formation of solid zinc hydroxy-chloride and -sulfate, respectively. These corrosion products are sparingly soluble and are frequently reported in field studies.4 Several workers have suggested that the precipitation of hydroxy chlorides and hydroxy sulfates inhibits corrosion, either by physically blocking anodic or cathodic sites or because of decreasing surface conductivity. As zinc does not form insoluble nitrate-containing corrosion products,5 the relatively slow corrosion of zinc in the presence of NaNO3 must be explained by other factors. Nitrate differs from chloride and sulfate in its ability to act as an oxidant toward metallic zinc in aqueous solution. It is argued that this is the ultimate cause for the anomalously low corrosion rate in


B587

The formation of NO is supported by the results reported by Svensson,10 showing by on-line gas analysis that NO共g兲 is produced when zinc is exposed to sub-parts per million concentrations of HNO2 (g) in humid air. However, the presence of NO共g兲 was not analyzed in the present study as the alkaline gas trap solution only collects acid gases such as HNO2 (g) and does not absorb NO共g兲. Based on the evidence related it is suggested that the nitrogen ‘‘lost’’ from the samples has evaporated in the form of NO共g兲 or NH3 (g). However, the formation of N2 O and N2 cannot be ruled out. Nitrate in the surface electrolyte migrates toward the anodes. Consequently, the reduction of nitrate to nitrite is likely to take place at the anodic sites where zinc is dissolved ⫺ Zn共 s兲 ⫹ NO⫺ 3 ⫹ H2 O → Zn共 OH 兲 2 共 s 兲 ⫹ NO2

关3兴

The nitrite generated in this way is a much more active oxidant than nitrate and is reduced further ⫺ Zn共 s兲 ⫹ 2NO⫺ 2 ⫹ 2H2 O → Zn共 OH 兲 2 共 s 兲 ⫹ 2NO共 g 兲 ⫹ 2OH 关4兴

Figure 10. Stability diagram for nitrate, based on equilibria at 25°C, and tot关NO⫺ 3 兴 ⫽ 100 mM. The stability regions for zinc metal and zinc hydroxycarbonate, (tot关Zn2⫹兴 ⫽ 100 mM) at atmospheric CO2 pressure 共350 ppm兲 are included in the diagram as indicatd by the shaded areas. The dashed lines correspond to the oxidation and reduction of water.

the presence of nitrate. Figure 10 shows an E-pH diagram6 for the nitrogen system in aqueous solution. (N2 is not considered as its formation is very slow under the present circumstances.7兲 The stability regions for zinc and zinc hydroxy carbonate are also indicated. As seen, the reduction of nitrate to nitrite/nitrous acid and ammonia/ ammonium is thermodynamically allowed in the presence of metallic zinc. In the absence of other salts, the fraction of nitrite found on the samples after exposure corresponds to up to 20% of the NaNO3 added. The presence of nitrite in the alkaline gas trap solution indicates that HNO2 (g) has vaporized from the surface. The relevant equilibrium constants (pK a HNO2 ⫽ 3.3, K H ⫽ 49 M/atm8兲 imply that appreciable HNO2 vaporization can occur even at neutral pH. The evidence for crystalline NH4 NO3 and the ammonium ions found on the samples by IC shows that the reduction of nitrate has proceeded all the way down to N共-III兲. The reduction of nitrate to ammonium by zinc is well known.9 At high pH, ammonium forms 7 ammonia (pK a NH⫹ 4 ⫽ 9.26 兲 that is lost to the gas phase NH3 共 g兲 NH3 共 aq兲

log K H ⫽ 1.77

关1兴

The detection of ammonium ions in the gas trap proves that nitrogen has left the samples in the form of ammonia. 共The ammonia was not determined quantitatively because the trap solution had a relatively high pH in order to catch HNO2 .) Figures 6 and 9 show that more than 20% of the nitrogen added to the samples in the form of nitrate before exposure is neither found ⫺ ⫹ on the samples (NO⫺ 3 ,NO2 ,NH4 ) nor in the gas trap 关 HNO2 (g) and NH3 (g)] after the experiment. Obviously, the nitrogen lost from the system must have escaped in gaseous form and we have to consider the different possibilities. The absence of nitrate in the gas trap solution shows that NO2 (g) is not a major reaction product. This is because NO2 (g) disproportionates to nitrite and nitrate in aqueous solution.8 Nitrite and nitrous acid are easily reduced to NO共g兲9 HNO2 ⫹ H⫹ ⫹ e⫺ → NO共 g兲 ⫹ H2 O

E o ⫽ ⫹1.00 V 关2兴

These reactions can be expected to slow down the zinc dissolution process, either due to the formation of zinc oxide or zinc hydroxide films or by the precipitation of zinc hydroxy carbonate in the presence of CO2 . The dark appearance of the metal surface after exposure is in accordance with results reported by James et al.11 showing that zinc becomes black in the presence of KNO3 (aq). The dark color was attributed to the presence of submicrometer metallic zinc particles embedded in the hydroxide film. Nitrite is reported to act as an anodic corrosion inhibitor toward certain transition metals, notably iron, by adsorbing on the surface and providing a barrier film.12 However, it is unclear whether this happens in the case of zinc. Reportedly, nitrite is not an efficient inhibitor for zinc.13 To summarize, it is suggested that the comparatively low corrosivity of NaNO3 toward zinc in humid air is due to a pH increase at the anodes caused by the reduction of nitrogen species. Corrosion effect of mixtures of NaNO3 with Na2SO4 or NaCl.—According to Fig. 7, the corrosion of zinc in the presence of NaCl or Na2 SO4 is slightly depressed by NaNO3 . This is in accordance with a report by Mikhailovskii and Sokolova,14 showing that the corrosion rate in the presence of sulfate was decreased by adding nitrate. The low corrosion rates are notable as it is generally considered that increased amounts of ions increase the surface conductivity and therefore the corrosion rates. It is argued that the reduction of nitrate is the reason for the inhibition also in this case. The fraction of soluble chloride and sulfate is greater on samples treated with mixtures of salts compared to samples treated with only NaCl or Na2 SO4 . In the presence of high concentrations of chloride and sulfate, zinc hydroxy chloride and zinc hydroxy sulfate are stable in a narrow pH range close to neutral.1 It is argued that the increased pH on the surface due to nitrate reduction decreases the stability of these zinc hydroxy salts. The IC results show that in comparison to the pure nitrate runs, the presence of sodium chloride or sulfate results in the formation of more nitrite. It is argued that the anodes are more active in the presence of chloride or sulfate, giving rise to an enhanced reduction of nitrate into nitrite. Conclusions In spite of the fact that nitrogen-containing corrosion products have not been reported from the field, this study shows that NaNO3 influences the atmospheric corrosion of zinc. Corrosion in the presence of NaNO3 at high RH is only a third of that registered in the presence of equal amounts of NaCl or Na2 SO4 . That is remarkable, as NO⫺ 3 does not form insoluble zinc salts that can obstruct the corrosion process. Instead, it is suggested that the unexpectedly weak corrosivity of NaNO3 is connected to the reduction of nitrate

B588


by zinc. The IC analysis shows that nitrate is reduced to nitrite and even to ammonium on the zinc surface. The reduction reactions increase pH, resulting in the blocking of the anodic dissolution sites as the passivity region of zinc is reached. Nitrate is an important component of the aerosols that deposit on surfaces exposed to ambient air. This study shows that NaNO3 is slightly inhibitive toward the atmospheric corrosion of zinc induced by NaCl or Na2 SO4 . Go¨teborg University assisted in meeting the publication costs of this article.

References 1. R. Lindstro¨m, J.-E. Svensson, and L. G. Johansson, J. Electrochem. Soc., 149, B57 共2002兲. 2. J. H. Seinfeld and S. N. Pandis, in Atmospheric Chemistry and Physics, From Air Pollution to Climate Change, Wiley Interscience, New York 共1998兲. 3. L. Greenspan, J. Res. Natl. Bur. Stand., 81, 89 共1977兲.

4. X. G. Zhang, in Corrosion and Electrochemistry of Zinc, Plenum Press, New York 共1996兲. 5. T. E. Graedel, J. Electrochem. Soc., 136, 193C 共1989兲. 6. I. Puigdomenech, Technical Report, TRIKA-OOK-3010, Royal Institute of Technology, Department of Inorganic Chemistry, Stockholm 共1983兲. 7. B. J. Finlayson-Pitts and J. N. Pitts, in Atmospheric Chemistry: Fundamentals and Experimental Techniques, John Wiley and Sons, New York 共1986兲. 8. S. E. Schwartz and W. H. White, in Advances in Environmental Science and Engineering, J. R. Pfafflin and E. N. Ziegler, Editors, Gordon and Breach Science Publishers, New York 共1981兲. 9. D. F. Shriver, P. W. Atkins, and C. H. Langford, in Inorganic Chemistry, Oxford University Press, Oxford 共1994兲. 10. J.-E. Svensson, Doctoral Thesis, Goteborg University, Goteborg, Sweden 共1995兲. 11. W. J. James, M. E. Straumanis, and J. W. Johnson, Corrosion, 23, 15 共1967兲. 12. Z. Szklarska-Smialowska, in Passivity of Metals, p.443, R. P. Frankenthal and J. Kruger, Editors, The Electrochemical Society Inc., Princeton, NJ 共1978兲. 13. L. L. Shrier, R. A. Jarman, and G. T. Burstein, in Corrosion, 3rd ed., ButterworthHeinemann, Oxford 共1994兲. 14. Y. N. Mikhailovskii and T. I. Sokolova, Zash. Met., 24, 9 共1988兲.