Further Developments Towards the Understanding of Cosmetic Automotive Corrosion Mechanisms on Painted Galvanized Sheet Steels

During the last decade many materials were developed to improve corrosion resistance of automotive vehicles as to cosmetic and perforation corrosion.

Tests were carried out based upon classic laboratory salt spray testing as well as on newly developed cyclic laboratory tests and on vehicle tests.

To translate the laboratory test results into practice it is necessary to understand the corrosion mechanisms occuring in laboratory experiments compared to the corrosion mechanisms in real practice.

This paper deals with the understanding of laboratory corrosion mechanisms in cyclic corrosion testing compared to some static outdoor corrosion tests. It is shown that delamination rate of scribed panels of painted hot dip galvanized, and galvannealed materials is highly controlled by the speed of anodic dissolution of the “zinc” due to the electrical characteristics of the corrosion products formed in the scribe and under the paint layer. These characteristics depend on the type of test. Microclimatical differences seem to control type of corrosion product so that more research is necessary to get information about the role of microclimate.

Again it is shown that not all accelerated laboratory corrosion tests yield realistic product rankings.