Lap-joint corrosion of automotive coated materials in chloride media. Part3 - Electrogalvanized steel/galvanneal interface

The aim of this work is to study the lap-joint corrosion of zinc-coated aut omotive painting substrates namely electrogalvanized steel, galvanneal, and electrogalvanized/galvanneal assemblies. Parts 1 and 2 of the work were de dicated specifically to aspects concerning electrogalvanized steel and galv anneal, respectively. In this final part, the special case of mixed lap joi nts involving electrogalvanized steel and galvanneal is studied. By means o f salt-spray exposure and SEM/EDS and XRD analysis, the study suggests that the coupling of galvanneal with electrogalvanized steel contributes to dec reasing galvanneal corrosion inside lap joints, while electrogalvanized ste el suffers an increase in lap-joint corrosion. These phenomena seem to be d ue to significant transformations among the alloy phases and corrosion prod ucts, which seem to start with zinc consumption of the electrogalvanized st eel surface and, only subsequently, of the galvanneal surface. (C) 2000 Els evier Science S.A. All rights reserved.