In-situ Observation of Cosmetic Corrosion Behavior under Atmospheric Environment

Abstract

It is well known that the types of automotive corrosion can be divided into perforation, and cosmetic corrosion. Although a lot of studies concerning the mechanism of perforation corrosion has been conducted so far, there are very few studies for the mechanism of cosmetic corrosion. In our previous work, the authors have found out that the initial corrosion behavior consists of 3 steps by in-situ observations under a cyclic corrosion test. In the present work, in-situ observation under the exposure test at Okinawa was performed for the painted sample with scratch in order to examine the cosmetic corrosion behavior under the actual environment. It was found that the corrosion started at the timing of salt depositing around the scribed part from the air containing salts. After that, the initial corrosion behavior under the exposure test consisted of 3 steps, as same as the one under a cyclic corrosion test. In the first step, black rust of Fe₃O₄ generated at the scribe part, and in the second step, under-film corrosion progressed from the scribed part at which the rust generated. In the third step, the tip of the under-film corrosion displayed swelling behavior. The each step behavior was also discussed by combining in-situ observation results with the analysis of environmental factors such as relative humidity, or with the EPMA analysis.