Evaluation of the Corrosion Rate of Zn Coated Steel Sheets for Automotive Body Use 971003

The corrosion mechanism of zinc coated steel sheets in automotive bodies was studied in field vehicle tests and several types of accelerated tests. Perforation corrosion starts in unpainted areas of lapped parts, and proceeds in the following steps: i) galvanic protection by the Zn coating, ii) protection by corrosion products, and iii) corrosion of the steel substrate and perforation. Although the corrosion processes were the same in all the cases tested, the corrosion rate depended significantly on the environment, such as atmospheric exposure conditions and the part of the automotive body. In accelerated corrosion environments, Zn coating is largely ineffective against perforation corrosion because galvanic protection and protection by corrosion products cannot be maintained over the long term. Conversely, coating is highly effective under actual atmospheric conditions which produce slow corrosion rates because galvanic protection and protection by corrosion products continue to protect the steel substrate over an extended period. Accelerated tests may be an accurate predictor of corrosion in cold rolled steel, but may not be representative of corrosion in coated products. On the other hand, cold rolled steel can be used to extrapolate the rate of corrosion in coated products. Therefore to evaluate the effects of Zn coatings quantitatively, the authors suggested an index of for corrosion resistance relative to cold rolled steel sheets under each atmospheric conditions. It is considered possible to estimate actual perforation corrosion life by using this index.