Corrosion of Ni-Zn Electrodeposited Alloy*

Transcription

1 Corrosion of Ni-Zn Electrodeposited Alloy* By David W. SII TARI,* * Masaru SAGI YAMA* * * and Tomihiro HARA* * * Synopsis The corrosion behavior of unpainted Ni-Zn electrodeposited alloy was investigated. Localized corrosion in the form of cracks were observed on samples containing more than 8 % Ni. Corrosion resistance in the Salt Spray Test was significantly affected by the size and spacing of the cracks, and the maximum in red rust resistance near 13 % Ni appears to be related to the pattern of thin, closely-spaced cracks observed in this composition range. The development of red rust initiates with the formation of cracks or pits, leading to exposure of the underlying steel. As corrosion proceeds, Ni content in the coating lay:~r increases relative to that of the Zn. This process would be expected to lead to a decrease in galvanic protection effect of the coating, eventually leading to the formation of red rust. As to the origin of the cracks, they were not observed before corrosion except on high Ni content samples, and composition nonuniformities were ruled out as the cause of the localized corrosion in crack patterns. High stress of a tensile nature were observed in the coating layer after plating. The fine crack structure of the Ni-Zn coating, caused by the stress in corrosion, may provide protection to the substrate by spreading out the anodic reaction and thus preventing severe local corrosion. I. Introduction Galvanized steel sheets have proven to be quite effective for preventing automobile corrosion by road deicing salts. Recently, alloy coatings such as Ni-Zn have been shown to have corrosion resistance superior to that of pure Zn, as well as improved welding and painting properties.)-3} However, the mechanism of alloy corrosion is yet poorly understood. So that coatings having optimal corrosion resistance can be developed, an understanding of the mechanism is desirable. In the present study, the corrosion behavior of unpainted Ni-Zn alloy that was electrodeposited at high current densities was investigated. In particular, the reasons for the superior corrosion resistance that is exhibited in the composition range % Ni were studied. During' the very early stages of corrosion we observed localized corrosion of a cracking nature, a phenomenon that has not been previously reported. Studies on the surface morphology, electrochemical behavior, chemical composition, and internal stress of the coatings are described in this report. exposed to 5 % NaCI in anodic polarization, beaker immersion, and Salt Spray Tests. For all of the samples, regardless of producing conditions or test method, localized corrosion in the form of pits or cracks was observed. In order to determine the cause and nature of the observed localized corrosion, the following methods were used to investigate the properties of the coated layer. Scanning Electron (SEM) and optical microscopy were employed to observe the morphology of the coatings before and after corrosion. Surface distributions of Ni and Zn were measured with X-ray microanalysis (XMA) and Auger Spectroscopy. The cracking nature of the corrosion attack led to an investigation of the internal stress in the electrodeposited layer. The bending of a cathode plated on one side, in the form of a Spiral Contractometer (Yamamoto Mekkishikenki Co. ), was used as a measure of the internal stress. Plating baths used for the stress measurements were approximately 3 l in volume, with airstirring from the bottom. Other plating conditions are shown in Table 2. As compared to the current densities used in the flow cell, a relatively low current density of 10 A/dm2 was used in the air-stirred cell because the agitation conditions did not allow high diffusion rates to be obtained. III. Results and Discussion 1. Salt Spray Test Salt Spray Test (SST) results for samples in the composition range 6'-2O % Ni are shown in Fig. 1. Samples in the range l O-.15 % Ni showed the highest red rust resistance, in agreement with other investiga- Table 1. Typical flow cell plating conditions. ph=2, Temperature=50 C, Flow velocity= l m/s, wt% Ni in coating II. Experimental Procedure Samples in the composition range 6'25 wt% Ni were prepared in a laboratory flow cell, using a variety of sulfate bath compositions and current densities. Typical plating conditions are indicated in Table 1. Some samples plated in a full-scale production line were also tested. The corrosion behavior was observed for samples * ** *** Received March 15, ISIJ Visiting Researcher, National Steel Corporation, Kawasaki-ku, Kawasaki 210. Technical Research Center, Nippon Kokan K.K Weirton, W. Va 26062, U.S.A.; Technical Research Center, Nippon Kokan K.K.,., Minamiwatarida-cho, Kawasaki-ku, Kawasaki 210. Technical Report ( 959)

2 9 (960) Transactions ISIJ, Vol. 23, 1983 Table 2. Stress measurement plating conditions. ph =2, Temperature = 50 C, Air stirring from bottom, Current density : 10 A/dm2 for all experiments Fig. 1. Red rust resistance in Salt Spray Test. tions. The sharp maximum in corrosion resistance is rather unusual, and one of the main objectives of this investigation was to develop an understanding of the reasons for this strong dependence of corrosion resistance on Ni concentration. 2. Localized Nature of Corrosion4> At first, corrosion rate measurements by standard polarization techniques (linear and Tafel methods) were attempted. However, these techniques are based on the assumption of uniform corrosion over the whole specimen surface, a condition that did not obtain for the samples tested. Localized corrosion initiated at an early stage of the corrosion process, as can be seen in Photo. 1. The samples shown were immersed in stirred, air-saturated 5 % NaCI for the purpose of performing Tafel polarization measurements. But after only 20 min of immersion, and before polarization, the coating on the 6 % sample was corroded to the extent that bare steel was exposed. Also within a period of 20 min, the 11 % sample developed large cracks, and both the 14 and 25 % samples showed thinner, more closely-spaced cracks than the 11 % specimen. Samples exposed to SST also developed similar forms of localized attack. After only 2 hr of exposure, followed by water rinsing and gentle wiping to remove corrosion products, the 6 % Ni sample showed large areas of dissolved metal and the 11 % sample displayed rather large cracks in the surface (see Photo. 2). Many thin, closely-spaced cracks were seen on the 14 % specimen. Large cracks and areas of coating detachment were observed on the 25 % sample. For Ni-Zn alloys produced under the conditions described herein, it is clear that corrosion performance Photo. 1. SEM micrographs after 20 min immersion aerated 5 io NaC1. in stirred, will be strongly affected by localized corrosion. Methods of corrosion rate measurement based on the assumption of uniform corrosion were thus abandoned in order to further study the localized nature of the process. 3. Polarization in Deaerated 5 % NaCI In order to determine the effect of Ni composition on the electrochemical behavior of Ni-Zn coatings, anodic polarization measurements were conducted in N2-saturated 5 % NaCI. The polarization curves shown in Fig. 2 are quite complicated in shape and are not yet completely understood, but some correla-

3 Transactions ISIJ, Vol. 23, 1983 (961) tion between the structure of the corroded surfaces and the polarization behavior can be made. For the 11 % and 25 % Ni samples, the change in slope of the polarization curves at more noble potentials (approx. '-'950 and N 650 mv, respectively) can be attributed to the outbreak of localized corrosion. The polarization curve for the 25 % specimen showed a drastic decrease in slope, and the surface after polarization (right-hand side of Fig. 2) showed both large cracks and coating loss. Correspondingly, the red rust resistance in SST was poor. Results of a similar nature were observed for the 11 % sample. The potential at which the slope of the polarization curve changed is not a so-called " pitting potential " because cracks Photo. 2. SEM micrographs after exposure to Salt Spray 2 hr Test. Fig. 2. Anod is polarization (2 mv/s) in N2-saturated 5 NaCI. Technical Report

4 (962) Transactions ISIJ, Vol. 23, 1983 were also produced when the sample was potentiostatically held at more active potentials (e.g., mv for 11 % sample). The change in slope is apparently related to the exposure of bare steel, as seen in the SEM micrographs. For the 14 % sample, which showed the best SST performance, the slope of the polarization curve did not decrease, but actually showed a slight increase near mv. The surface morphology, with many thin, closely-spaced cracks, would clearly be expected to show the best red rust resistance of the samples shown here. The Salt Spray Test is very sensitive to breaks in the coating layer and it appears that the localized corrosion attack shown in Photos. 1 and 2 and Fig. 2 can account for the observed dependence of SST performance on Ni concentration. That is, the large cracks and coating losses observed in both the high and low Ni composition ranges would result in poor red rust resistance, when compared to the thin cracks observed on the 14 % specimen. Certainly there are other important factors, such as electrochemical reaction kinetics and corrosion product properties, that influence the overall corrosion behavior of Ni-Zn coatings. For example, pure Zn does not develop cracks, but it shows the poorest corrosion resistance. Unfortunately, corrosion rate measurements of Ni/Zn alloy are difficult to interpret because of the complicating factors of localized corrosion and corrosion product precipitation. However, protection provided by corrosion products would not seem to be able to explain the variation in red rust resistance seen at the same Ni composition, (Fig. 1). Because the exposure of bare steel at a very early stage of corrosion would be expected to exert a strong influence on subsequent corrosion mechanisms, the early stages of the corrosion process were given the most attention in this study. 4. Nature of Cracking Regarding the origin of the cracks, a number of possible causes were investigated. The question naturally arises as to whether they were present in the coatings before exposure to the NaCI solutions. As will be seen in later photographs, there were no cracks observed before corrosion except on the 25 % Ni specimen. Thus, other likely explanations as to the appearance of the cracks were investigated. A possible reason for the appearance of the cracks would be localized corrosion caused by a nonuniform distribution of Ni and Zn in the coating. To check this possibility, XMA was used to measure the surface distribution of Ni and Zn for samples exposed to stirred 5 % NaCI as shown in Photos. 3 to 5. Different samples, having the same composition, were used for the " before " and " after " measurements. The short immersion time (<20 min) and stirred solution precluded deposition of visible corrosion products. Photograph 3 shows results for the 6 % sample. There were no pits or cracks visible before corrosion, and there were no nonuniformities in the distributions of Ni and Zn, except for the region of steel exposed after corrosion. Results for the 11 % sample shown in Photo. 4 also showed similar characteristics : no surface cracks or composition nonuniformities before exposure. After corrosion, bare steel is apparently exposed at the bottom of the cracks. As shown in Photo. 5, the 25 % sample had a rough surface con- Photo. 3. XMA results for 6 % Ni exposed to stirred 5 % NaGI.

5 Transactions ISIJ, Vol. 23, 1983 (963) taming some small cracks before corrosion. In addition, the distributions of Ni and Zn were nonuniform. It is evident that the corrosion cracks developed in the regions of lower Zn (higher Ni) concentration. The 14 % sample shown in Photo. 6 was anodically polarized in deaerated 5 % NaCI before XMA analysis, conditions slightly different than for the above samples. The upper portion of the sample shown in Photo. 6 was exposed to the NaCI electrolyte while the lower portion was covered with masking tape. Hence, surface conditions both before and after corrosion are shown. Surprisingly, the XMA analysis did not detect a change in composition between the corroded and uncorroded portions. It would be expected that the electrochemically more active metal, Zn in this case, would preferentially dissolve and thereby form a layer enriched in Ni after corrosion. Approximately 10 % of the coating was dissolved, as Photo. 4. XMA results for 11 % Ni exposed to stirred 5 % NaC1. Photo. 5. XMA results for 25 % N i exposed to stirred 5 % NaC1.

6 0 ( 964 ) Transactions ISTJ, Vol. 23, 1983 measured with a coulombmeter. Perhaps the depth of analysis of the XMA was too great to detect a change in composition occurring only at the uppermost surface. When the surface was analyzed by Auger Spectroscopy, which is very sensitive to the surface layer, a slight increase in the Ni content in the corroded region was observed, as shown in Photo. 7. The same specimen as was shown in Photo. 6 was used for the Auger measurements. The Auger Image and Line analyses (after 1 min sputtering) for Ni indicate that the corroded region on the right-hand side of the photographs is relatively enriched in Ni, as compared to the uncorroded region on the left. Results of Shibuya et al.1 also indicated increases in Ni at the surface. The Line Analysis results indicated a change in Ni composition on the order of a few percent (the zero baseline is not shown). One important effect of an increase in the Ni concentration will be a reduction in the galvanic protection provided by the coating to exposed steel. 5. Internal Stress Results of internal stress measurements with the Spiral Contractometer are shown in Fig. 3 for the composition range 8'-35 wt% Ni. The plating conditions shown in Table 2 were used, and stress values were measured 10 min after removal from the plating bath. Stress measurements at the high Ni concentrations (>25 %) were difficult to reproduce, the stress tending to be reduced significantly as both the bath ph and composition changed during bath usage. The stress was tensile and showed a sharp increase for more than 10 % Ni in the deposit, decreasing again at higher Ni concentrations. During the time that the plating current was applied, the stress was initially compressive and gradually became tensile as shown in Fig. 4. When the current was switched off, in all cases the stress rapidly approached its maximum tensile value, requiring less than 1 minute to become quite stable. In the Ni composition range corresponding to the eta and gamma phases, Kurachi et al.,5~ also reported compressive stresses during plating and changes in the direction of tensile stress after switching off the plating current. Their results differ from those in the present investigation in that the stresses remained in the compressive region. The relationships among internal stress, crack structure, and corrosion behavior are very complex and no fundamental explanations are possible at this time. However, by the use of some simple corrosion concepts, some correlation of observed behavior can be made. The fine crack structure of the Ni-Zn coating may spread out the anodic reaction and thus prevent severe local attack.6~ The crack structure variation with Ni content seen in previous photographs may arise according to the concept that anodic attack is concentrated in regions of high stress.7> The larger number of cracks and high stresses observed in the composition range above 10 % Ni would be in agreement with such a concept. The compressive stress and the stress reversal observed during plating show some similarity to the Photo. 6. XMA results for 14 % Ni anodically polarized in deaerted 5 o/ NaCI.

7 Transactions IS IJ, Vol. 23, 1983 ( 965) Fig. 3. Internal stress after plating. Fig. 4. Internal stress during plating. Photo. 7. Auger results for same sample different orientation and magni as in Photo. 6 fication). (note behavior of Cr.s~ The compressive stress during the initial stage of the Cr plating was attributed to hydrogen inclusion on the substrate.a> Mechanical properties of the coating would be expected to be significantly affected by the reversal of stress. The structure of cracks in Ni-Zn described above also shows a striking resemblance in appearance to those which arise under certain conditions of Cr plating.a~ The important effect of plating conditions on the crack structure and corrosion resistance can be seen by comparison of samples of the same composition produced under different conditions. Shown in Photo. 8 are samples from the laboratory and from a commercial-scale line. The 8.7 % laboratory sample showed very large cracks and performed poorly in SST. The 8.8 and 9.9 % samples from the production line showed very small cracks and excellent red rust resistance. It is difficult to define precisely the effect of plating conditions on corrosion behavior because of the great number of experimental variables involved. The relationship between crack structure and alloy phase composition is not clear, as all of the samples shown in Photo. 8 were a mixture of eta and gamma phases. The production samples show a crack pattern similar to that of the gamma-phase alloys (approx. 14 % Ni) produced in the laboratory. Another factor that may be of importance is the surface texture of the underlying substrate. Dennis8~ reported that the substrate texture was the most significant factor affecting the crack structure on Cr. Because of the practical importance of the factors described in this paragraph, further study is warranted.

8 (966) Transactions Is", Vol. 23, 1983 in the Salt Spray Test was found to be significantly affected by the size and spacing of the cracks, and the maximum in red rust resistance near 13 % Ni appears to be related to the pattern of thin, closely-spaced cracks observed in this composition range. The development of red rust initiates with the formation of cracks or pits, leading to exposure of the underlying steel. As corrosion proceeds, there is evidence that the Ni content in the coating layer increases relative to that of the Zn. This process would be expected to lead to a decrease in the galvanic protection effect of the coating, eventually leading to the formation of red rust. As to the origin of the cracks, they were not observed before corrosion (except on high Ni content samples), and composition nonuniformities were ruled out as the cause of the localized corrosion in crack patterns. Internal stress measurements indicated that there may be some similarity with the behavior of microcracked Cr. During plating, a reversal of stress from compressive to tensile was measured, and high stresses of a tensile nature were observed after plating. Crack geometries were very similar in appearance to some cases of microcracked Cr. Of practical importance is the observation that the crack patterns (and thus corrosion resistance) are significantly affected by plating conditions. This suggests that further studies which clearly define the optimal relation between plating conditions and crack structure may lead to further improvement in the corrosion performance of Ni-Zn alloy. Photo. 8. SEM micrographs after anodic polarization: tory and production-line samples. labora- Iv. Conclusions An investigation into the corrosion behavior of unpainted Ni-Zn electrodeposited alloy revealed that localized corrosion in the form of cracks occurred on samples containing more than 8 % Ni. Performance REFERENCES 1) A. Shibuya, T. Kurimoto, K. Korekawa and K. Noji: Tetsu-to-Hagane, 66 (1980), ) A. Shibuya, T. Kurimoto and K. Noji: " Corrosion Resistance of Ni-Zn Alloy Plated Steel Sheet ", Proceedings of Interfinish '80, Metal Finishing Soc. Japan, Kyoto, October, 1980, ) R. Noumi, H. Nagasaki, Y. Foboh and A. Shibuya: SAE Meeting, Soc. Automotive Engineers Inc., Detroit, 1982, Paper No ) D. W. Siitari, M. Sagiyama and T. Hara: Tetsu-to-Hagane, 68 (1982), ) M. Kurachi and K. Fujiwara : Trans. JIM, 11 (1970), 44. 6) H. Tsuji and M. Kamiya: Proceedings of Metal Finishing Soc. Japan, No. 66, Metal Finishing Soc. Japan, (1982), ) M. Clarke: Corros. Sci.,10 (1970), ) J. K. Dennis: Trans. Inst. Metal Finish., 43 (1965), 84.