Abstract
With the aim of establishing laboratory methods of scab corrosion tests for automobile bodies, the influences of wetness duration in environment and the shape of damage on 3-coated test panels have been studied by cyclic corrosion test (CCT).The wetness duration fraction was defined as the percentage of the wetness duration in one complete cyclic corrosion test.Painted test panels, subjected to the CCT in which the wetness duration fraction is more than 50%, develop blister widths on scribed, damaged panels of electrogalvanized steel that are larger than those formed on cold rolled steel. An opposite effect is observed when the wetness duration fraction is lower.Comparison of the above result with the data obtained from outdoor exposure test and actual car bodies has demonstrated that for scab corrosion test the wetness duration fraction in one cycle should be less than 50%.From the measurements of pH values around scribe damage and the galvanic couple current which flows between electropainted test panel and cold rolled bare steel, it is suggested that the scab corrosion propagates by the dissolution of substrate metal at the blistering front.
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