CHARACTERIZATION OF POLYACRYLIC-ACID MODIFIED ZINC PHOSPHATE CRYSTAL CONVERSION COATINGS

Raman spectroscopy and atomic force microscopy have been used to inves tigate the composition and surface structure of polyacrylic acid modif ied zinc phosphate crystal conversion coatings on steel. Zinc phosphat e coatings are used extensively to provide corrosion protection and to improve adherence of topcoatings to steel. Within the last few years it has been demonstrated that addition of high molecular weight polyac rylic acid (PAA) to the phosphating bath can significantly improve bot h resistance to corrosion and topcoat adherence. It has been reported that the addition of PAA reduces the size of crystallites, which leads to greater film ductility, and therefore to fewer sites for corrosive attack, and that organic molecular segments from the PAA are incorpor ated into the surface structure and provide additional adhesive bondin g with polymeric topcoats. In this work Raman spectra show the composi tions of both unmodified and PAA modified films to be zinc phosphate d ihydrate, Zn3(PO4)2.2H2O. Atomic force microscopy (AFM) was used to me asure the morphologies of single crystallite surfaces. Morphologies of the unmodified and modified films obtained by AFM are in general quit e similar, but subtle differences are apparent. (C) 1993 John Wiley & Sons, Inc.