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. Zinc alloy coated type sheets such as Zn-Ni, Zn-Fe double-layer, galvannealed were more corrosion resistant than the pure zinc type sheets when compared at the same coating weight. Organic coated Zn-Ni sheet showed the highest resistance.