Salient features of initial stages of aluminum crevice corrosion in a dilute chloride solution have been expressed with mathematical equations in order to relate corrosion behavior to transport and electrochemical phenomena. The features included in the model equations are metal dissolution, metal‐ion hydrolysis, oxygen reduction, diffusion along the crevice, and ohmic resistance effects. The system of nonlinear differential equations was solved with a digital computer. Computations based on experiments described in the first part of this series were compared with experimental data including pH, potential, and current distributions. Theoretical predictions of the time at which breakdown occurred were found to be about one‐half of the initiation time observed experimentally. The calculations gave trends in system behavior which correctly tracked observations upon variation of crevice gap and solution conductivity, but not crevice depth. A critically important but highly uncertain feature of the model was the mechanism of passivity breakdown of aluminum in dilute chloride solution. Improved understanding of the breakdown mechanism would enhance predictive capabilities of the model.

The mechanism of passivity breakdown within crevices was investigated for aluminum in 0.05M NaC1 and for iron in dilute mixtures of NaC1, Na2SO4, and NaNO2. The pH, electrode potential, and current distribution were monitored continuously in segmented crevices which simulated realistic corrosion conditions. Variation in the crevice geometry was found to have a significant effect on the current distribution behavior during initiation of crevice corrosion. Breakdown of passivity on aluminum typically took place within a matter of hours and was accompanied by hydrogen bubble evolution within the crevice at a pH near 4.1. The behavior for iron was qualitatively different than that for aluminum, indicating that the mechanism of passivity breakdown is different for the two metals.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-03-24 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-03-24 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-03-24 to IP

A theoretical model for predicting the current and potential distribution within a prototypic pit was used to estimate conditions under which cathodic reaction occurs in the occluded region. The model is developed for a long, narrow occluded cell where the anode is localized in the tip region. Cathodic reaction (hydrogen reduction) on the side walls and external surface is predicted by assuming metal-ion hydrolysis, diffusion of hydrogen ions, Ohm's law, cathodic reduction via Tafel kinetics, and constriction of current lines entering the occluded region. Results indicate that cathodic reaction within occluded regions is favored for deep narrow regions in solutions of low hydrogen ion concentration. Dimensionless groupings of system parameters are used to scan behavior over wide ranges of parameter space and to compile conditions under which appreciable cathodic reaction can be anticipated within occluded cells, * Electrochemical Society Active Member. ** Electrochem.'cal Society Student Member.

SynopsisThe 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. IntroductionGalvanized 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 10-15 % 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.

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