Lap-joint corrosion of automotive coated materials in chloride media. Part 3 — Electrogalvanized steel/galvanneal interface

The aim of this work is to study the lap-joint corrosion of zinc-coated automotive painting substrates namely electrogalvanized steel, galvanneal, and electrogalvanized/galvanneal assemblies. Parts 1 and 2 of the work were dedicated specifically to aspects concerning electrogalvanized steel and galvanneal, respectively. In this final part, the special case of mixed lap joints involving electrogalvanized steel and galvanneal is studied. By means of salt-spray exposure and SEM/EDS and XRD analysis, the study suggests that the coupling of galvanneal with electrogalvanized steel contributes to decreasing galvanneal corrosion inside lap joints, while electrogalvanized steel suffers an increase in lap-joint corrosion. These phenomena seem to be due to significant transformations among the alloy phases and corrosion products, which seem to start with zinc consumption of the electrogalvanized steel surface and, only subsequently, of the galvanneal surface.