PHOSPHATE COATING TECHNIQUES FOR CAR BODIES.

Abstract

Zinc phosphate coatings consist of hopeite, Zn//3(PO//4)//2-4H//2O and phosphyophyllite, Zn//2Fe(PO//4)//2-4H//2O. A higher phosphophyllite content is more favorable for preventing deterioration of wet adhesion in cationic ED painting due to the superior alkali resistance of phosphophyllite. Compared with the spray process, the full dip process is more advantageous in that more Fe**2** plus dissolved from the substrate during the initial stage of reaction can be incorporated into the coating. In a galvanized steel sheet, the absence of Fe**2** plus results in a coating consisting of hopeite alone. Hopeite can be excellent in alkali resistance in some grain orientations which are controllable by the galvanized layer and composition. A phosphate coating is better if the grain size is smaller and porosity lower. Such a coating is more available by the full dip process.