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Filiform corrosion phenomenon and its prevention method for 5000 series aluminum alloy sheet have been investigated. The painted aluminum alloy sheets were subject significantly to filiform corrosion caused by formation of a mottled and coarse zinc phosphate film during chemical conversion process. On the other hand, galvanized aluminum alloy sheet showed an uniform and fine phosphate film in a brief time and the filiform corrosion resistance was improved markedly. The test results of press-forming and corrosion for the prototype engine hoods made of this newly developed galvanized sheets have revealed a good formability and an excellent filiform corrosion resistance.

New vapor deposited zinc alloy coatings, which are Zn-Mg, Zn-Al, Zn-Cr, Zn-Ni and Zn-Ti coatings, have been developed by vapor deposition pilot-line. Morphology of surface structure, depth profile of alloy composition and crystal structure have been studied for these coatings. Zn-Mg alloy coated steel sheet showed the best corrosion resistance. The corrosion behavior of this coating has been investigted. The coating surface is densely covered with a corrosion product layer composed of ZnC12·4Zn(OH)2, which could play a role as a protective barrier film. The corrosion potential remains active enough to give the substrate galvanic protection for a long time in a corrosive environment. Attention has been directed to corrosion resistance for automobile body panels in recent years and several types of zinc coated steel sheets has been developed for automotive usage.

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.