Volume 3, Issue 6 (November 1975)
Characterization of Galvanized Sheet Steel for Automotive Vehicle Bodies
Commercially available galvanised steel sheet materials, obtained from several sources and representing sinc coatings of conventional spangles (sinc crystal faces), minimized spangles, and diffusion alloy (galvanneal), were characterized for quality and performance. Thicknesses of the galvanised coatings were determined by chemical stripping (procedures for which are offered), by microscopic examination of cross-sectioned specimen, and by magnetic and eddy-current measuring devices. These methods were compared for reproducibility and suitability for general use. Corrosion resistances of the galvanised steels were determined following specific periods of exposure of specimens in 5% salt-fog environment.
Bare galvanized steel sheets having a minimum coating thickness of 14 µm and averaging about 22 µm completely resisted rusting for 72 h in salt-fog. Since longer exposure (96 h) in some cases resulted in rusting of the steel substrate, the 72-h exposure period was recommended for quality control purposes. The adhesive and protective value of a conventional paint system applied to the galvanized materials are discussed. Bead and spot weldings of the galvanized sheets were performed successfully; typical and feasible welding procedures are described. The amenability of the various galvanized materials to forming or shaping without flaking of or damage to the coatings was demonstrated.
Conclusions regarding the suitability of the test methods and their adoption are offered. Recommendations for use of the materials, application of preferred thickness measuring methods, and acceptance of corrosion resistance qualifying criteria are presented.