Title: The cosmetic corrosion mechanism and corrosion resistance of zinc alloy coated steel sheet

Authors: Kimitaki Hayashi

Addresses: Nippon Steel Corporation, R&D Laboratories, 5-10-1 Fuchinobe, Sagamihara, Kanagawa, 229 Japan

Abstract: Corrosion of motor car bodies is classified as cosmetic corrosion and perforation corrosion. It is well-known that corrosion resistance is required in terms of both these corrosion types. Cosmetic corrosion especially is controlled by under-film corrosion. Zinc coated steel sheet is one remedy for preventing under-film corrosion. However, under-film corrosion resistance has not been fully elucidated on the basis of corrosion mechanism. In this report, experiments were conducted for various zinc and zinc alloy coated steel sheets to investigate (1) the under-film corrosion mechanism of zinc coated steel sheet, with special attention to its initial stage and (2) the reason why zinc alloy coated steel sheets show excellent corrosion resistance as compared with zinc coated steel sheet. EPMA analysis was conducted for zinc and zinc alloy coated steel sheets subjected to the atmospheric exposure test. The observed area is located at the vicinity of the tip of the corrosion. X-ray analysis showed that CI is selectively distributed on the coating layer and that neither the coating layer nor the steel sheet is oxidized. SE image showed that the CI-covered coating layer becomes friable. These suggest that Zn in the coating layer changed to chloride at the initial stage of the corrosion. At a later stage, ZnO was observed in the corrosion layer. The above observations suggest that Zn is transformed once into chloride and subsequently the chloride is transformed into ZnO through hydroxide. On this basis, it was made clear that iron substrate corroded after zinc coating lost the ability of sacrificial corrosion protection by the corrosion caused by chlorine ion penetration into the coating layer. Furthermore, the same result was obtained also for zinc alloy coated steel sheets. However, it was observed in the atmospheric exposure test that zinc alloy (Zn-Ni, Zn-Fe) coated steel sheets showed excellent corrosion resistance as compared with zinc coated steel sheet. This result was explained in terms of zinc alloy coatings being more durable against chlorine ion penetration into the coating layer than zinc coating.

Keywords: chlorine ion penetration; cosmetic corrosion; perforation corrosion; steel sheet; zinc alloy coating; zinc coating; car bodies; car body corrosion; automobile industry; automotive corrosion; corrosion resistance.

DOI: 10.1504/IJMPT.1991.036645

International Journal of Materials and Product Technology, 1991 Vol.6 No.1, pp.9 - 25

Published online: 05 Nov 2010 *

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