Lap-joint corrosion of automotive coated materials in chloride media. Part 2 – Galvannealed steel

The main aim of this work is to study the lap-joint corrosion of present-day zinc-coated automotive painting substrates, namely electrogalvanized steel, galvanneal, and electrogalvanized/galvanneal assemblies. The first part of the work (E. Almeida, M. Morcillo, Surf. Coat. Technol., in press) was dedicated to specific aspects concerning electrogalvanized steel. In this second part, galvannealed steel is studied using special specimens with spot-welded lap-joints, before and after salt-spray exposure. The corrosion mechanisms proposed as a consequence of this study have been based on results obtained by visual observation, scanning electron microscopy (SEM) associated with X-ray energy dispersive spectrometry (EDS), X-ray diffraction (XRD), Auger spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The experimental results obtained showed that the use of galvannealing did not alter the main mechanisms or chemical reactions involved in lap-joint corrosion already observed for electrogalvanized steel. However, in the case of galvanneal, the corrosion propagation step seems to be significantly moderate.