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The perforation corrosion of steel and galvanized steels due to the cyclic corrosion test (CCT) has been analyzed by the extreme-value statistics. The plots of the maximum values of perforation corrosion depth and the cumulative probability gave straight lines and they were fitted to the Gumbel (double exponential probability) distribution. The decrease of the slope of these lines with CCT time suggested that uniform corrosion occurred in an early stage of CCT but it changed to uneven corrosion with CCT time. The period of the uniform corrosion corresponds to the incubation time, tj, of perforation corrosion and the ti of the steel was extended proportionately with zinc coating weight. The perforation corrosion rate of Zn-Ni alloy electroplated steel after the t; was lower than those of other materials. This phenomenon seems to be attributable to formation of a protective corrosion product of ZnCl3 ·4Zn(OH)2 and a good adhesion of the product to the steel.

A new double-layer electroplated steel sheet has been developed. The steel sheet is composed of an upper Zn-Fe (80-90%) alloy layer which has good paint adhesion and a lower Zn-Ni (10-15%) alloy layer which has good corrosion resistance. Paintability, corrosion resistance and workability of the steel sheet were investigated. This coated steel sheet was revealed to be suitable for the automobile use.

A thin organic coated steel sheet (HI-SUPER-DN) has been recently developed for automotive body panel applications. The steel sheet consists of a Zn-Ni alloy-plated layer, a chromate film and a thin organic coated layer, which is composed of colloidal silica, organic polymer and an organic additive. Corrosion resistance, formability of the coated layer, weidability and electropaintability of the steel sheet required for automotive body panel were studied. An increase in the colloidal silica in the organic coated layer improves corrosion resistance. However, the formability of the organic coated layer deteriorates with such as increase in the colloidal silica. We have optimized the ratio of colloidal silica to organic polymer so as to balance both the corrosion resistance and the formability of the organic coated layer. Futhermore, the bake-hardenability of the steel sheet was maintained by a relatively low temperature baking of the chromate film and the organic coated layer.

In order to simulate the perforation corrosion of an automobile, hem model samples made of various kinds of coated steel sheets were set inside of the door outer panel and the door was exposed in the open air with once a week 5% salt spraying. After two years, the model samples were disassembled for investigation. Perforation corrosion occurred most severely just above the lapped portion of outer and inner specimens. Red rust samples taken from the hem model and from a field vehicle run in Detroit did not show clear pearks of spectra by X-ray diffraction analysis, whearas the one formed by a conventional cyclic corrosion test showed peaks of Fe3O4. These facts indicate that this new test well simulates the corrosion environment of an actual automobile hem portion. Corrosion resistance of many kinds of coated steel sheets was studied by this new test method. Perforation depth became smaller as a coating weight of pure zinc electroplated steel sheet increased.