Aspects of the adhesion and corrosion resistance of polyelectrolyte-chemisorbed zinc phosphate conversion coatings

Abstract

The ability of polyelectrolyte macromolecules to suppress the crystal growth of zinc phosphate (Zn · Ph) conversion coatings depends primarily on the functional pendant groups. The extent of segmental chemisorption of macromolecules having carboxylic and sulphonic acid groups on the embryonic crystal faces was found to be considerably higher than that of macromolecules containing amine groups. The reaction products formed by intermolecular reactions between amide groups in polyurethane coatings and carboxylic acid groups on the outermost surface of polyelectrolyte-modified Zn · Ph in Zn · Ph-to-polymer adhesive joint systems played an essential role in developing interfacial adhesive forces. A highly dense precipitation of Zn · Ph derived from a zinc orthophosphate dihydrate-based phosphating solution contributed significantly to reducing the corrosion rate of cold-rolled steel. It also was determined that the presence of an internally diffused polyelectrolyte in the Zn · Ph layers further enhances the resistance to corrosion of Zn · Ph itself.