Passive films, surface structure and stress corrosion and crevice corrosion susceptibility

Abstract

A feasibility study was made to determine if qualitative ellipsometry can be used together with electrochemical pH and potential measurements as a technique for the study of metal substrates protected by transparent organic coatings. The objective of the work was to gain a deeper understanding of the mechanisms governing the corrosion protective actions of paints on metal surfaces.Computer modeling and experiments with collodion coatings on iron substrates indicated that changes in the ellipsometric parameter A could, for the most part, be safely interpreted as thickness alterations in the substrate oxide film.Experiments with the Fe-collodion system in dilute chloride solutions exhibited three sequential stages of activity, two of which could be interpreted in terms of corrosion mechanisms using the optical and electrochemical measurements. Chromate ion as a corrosion inhibitor in coatings was also studied using this technique, and was shown to have significant effects upon the development of the sub-coating processes. The reflectance infrared spectra of the collodion before and after the extended immersion in the chloride medium were essentially identical.The observed decrease in A is probably due to the development of a liquid film under the coating and is consistent with the computer modeling results. 2)It is possible to develop reasonable and consistent (consistency between optical, pH and potential measurements) interpretations of the processes occurring in the second and third stages but not for the first.3)It is possible to interpret the effect of the addition of inhibitory chemicals on the processes observed and to consider them with respect to measurements of repassivation kinetics., Repassivation kinetic studies of iron in simulated sub-coating i 4 environments using the triboell ipsometric technique [3] are in progress and will be the subject of a future publication.Thus, while much remains to be done to refine both the experimental and interpretive approaches (for example, the paper by Theeten, et a]_ [6] suggest the possibility that variation of X may help), this feasibility study hasshown that the application of qualitative ellipsometry to the study of the corrosion processes under organic coatings shows considerable promise.-7 -ACKNOWLEDGEMENTS

The Intersection of Design, Manufacturing, and Surface Engineering

“…193 The corrosion protective ability of paints on metal surfaces has been predicted using models which take into account changes in the ellipsometric parameter related to thickness alterations in the substrate oxide film. 194 Durability of coatings has been predicted by Cr solubility in Fe during a two-phase Cr coating process, showing good agreement with experiment. 195 Optical and topographic measurements have recently been combined in computer models to provide realistically rendered graphic images representing color and gloss standards for development of paint formulations.…”