Quantitative study of exfoliation corrosion: Exfoliation of slices in humidity technique

A new technique, exfoliation of slices in humidity (ESH), was developed for the determination of exfoliation corrosion (EFC) susceptibility and quantification of EFC kinetics. Two AA7178 plates taken from the wingskin of a retired KC135 airplane were used as test samples. Slices of the plate were pretreated by potentiostatic polarization in chloride solution to develop localized corrosion sites. Subsequent exposure to high humidity after pretreatment of properly oriented and unconstrained samples resulted in the development of EFC at the edges of the slices. The EFC kinetics were determined by measuring the width of the central unattacked region of the samples. The ESH results were representative of the different EFC behavior of the two plates during outdoor exposure. These results show the capability of the ESH test to discriminate between plates of varying susceptibility and to determine EFC rates quantitatively. The different susceptibility of the two plates to EFC was attributed to differences in microstructure and grain boundary chemistry.