In-situ Observation of Initial Stage Cosmetic Corrosion Behavior under Wet and Dry Conditions

Abstract

It is well known that the types of automotive corrosion can be divided into perforation corrosion and cosmetic corrosion. Although the mechanism of perforation corrosion has been studied extensively, the mechanism of cosmetic corrosion has not yet been clarified. The authors investigated the cosmetic corrosion behavior of cold-rolled steel sheets without galvanizing with an in-situ observation device. After coating the samples by cathodic electrodeposition （ED）, the sample surface was scratched with a cutter. Corrosion resistance was evaluated under cyclic corrosion test. It was found that the progress of under-film corrosion consisted of 3 steps. The 1st step occurs in the initial stage of the dry process in the 1st cycle. In this step, red rust gradually changed to black rust. The 2nd step occurs in the latter stage of the dry process, and under-film corrosion progressed from the scribed part. The 3rd step occurs in the wet process, and in this step, the tip of the under-film corrosion displayed swelling behavior. In the 2nd cycle, the 2nd step and 3rd step of the 1st cycle were repeated. Under-film corrosion progressed at almost the same rate as in the 1st cycle.