Phosphate coating techniques for car bodies.

Abstract

In an attempt to prevent rust on car bodies, various improvements have been made through such measures as use of precoated steel sheets, transfer of phosphate treatment toward full dipping, incorporation of cationic ED primer, and adoption of wax injection.Since introduction into Japan in 1978, the full dip process for phosphate treatment of car bodies has been subjected to many studies in the aspect of production techniques as well as in chemicals.Treating practices for the practical application have substantially been established, and the process has already been diffused to 58% of all the car body production lines in Japan.The phosphate coating plays a particularly important role because of its location between the steel substrate and the painted film. More specifically, it should be capable of coping with new properties of new steel sheet materials and new paint materials not experienced by the industry.Zinc phosphate coating essentially consists of hopeite, Zn3(PO4)2• 4H2O and phosphophyllite, Zn2Fe(PO4)2•4H2O: a higher phosphophyllite content is more favorable for preventing deterioration of wet adhesion in cationic ED painting. This is due to the superior alkali resistance of phosphophyllite. As compared with the spray process, the full dip process is more advantageous in that more Fe2+ dissolved from the substrate during the initial stage of reaction can be incorporated into the coating.In a galvanized steel sheet, in contrast, the absence of Fe2+ source results in a coating consisting of hopeite alone. Hopeite can however be excellent in alkali resistance in some grain orientations which are controllable by the galvanized layer of substrate and the composition of chemicals. A phosphate coating is more excellent in corrosion resistance, if the grain size is smaller and the porosity is lower. Such a coating is more easily available by the full dip process than by the spray process.