Performance of Finish-Coated Galvanized Steel Sheets for Automotive Bodies

Abstract

Abstract High performance and cost reduction are principal concerns for vehicle manufacturers. In addition to shape, the exterior finish is the first thing a consumer sees when looking at a vehicle. The quality and durability of the finish is a reflection of the quality and durability of the vehicle itself. In this work, the behavior of two finish coat systems was studied. Finish coat system 1 included a powder coat, a waterborne base coat and a clear coat, and finish coat system 2 included the same powder coat, a solvent-based base coat and a clear coat. These finish coats were applied on unprimed and pre-primed electrogalvanized, hot dip galvanized and galvannealed steel. Before application, for each finish coat two activation treatments were considered: a conventional zinc phosphate for automotive industry and an experimental pretreatment. Three primers were considered. Primer 1 is an organic zinc rich silicate with a low thickness (near 4 μm), primer 2 is a very thin (near 2 μm) water-based primer filled with graphite and primer 3 is a conventional electrophoretic applied primer system. To evaluate the performance of finish coats, the samples were submitted to a cyclic corrosion test, and the paint coating adhesion was evaluated, before and after exposure, by a cross-cut test method. Better adhesion was verified on the finish coat with a waterborne base coat, a system environmentally more convenient. Both finish coats presented good anticorrosive behavior, without significant defects on undamaged areas. Regarding the scribe area, the performance seems sometimes to be better with the application of the conventional zinc phosphate pretreatment compared to the experimental pretreatment.