SURFACE MODIFICATION OF ELECTROGALVANIZED STEELS BY ZINC PHOSPHATE CONVERSION COATINGS

When electrogalvanized steel (EGS) surfaces were treated by immersing them in a phosphating solution consisting of Zn-3(PO4)(2).4H(2)O, H3PO 4, CO(NO3)(2).6H(2)O, poly(acrylic acid) (p(AA)) and water, the result ing electrochemical reaction led to the creation of short-circuited ce lls with cobalt acting as the cathode and the galvanized (zinc) coatin g as the anode. These cells accelerate the anodic dissolution of zinc, which then rapidly precipitates embryonic zinc phosphate tetrahydrate (hopeite) crystals on the EGS surfaces, resulting in their complete c overage with fully grown hopeite crystals after only 5 s immersion. Th e hopeite layers formed not only serve to protect the galvanized coati ngs against NaCl-induced corrosion, but also contribute significantly to improving adhesion to the polyurethane (PU) topcoating. The reasons for the latter improvement were due primarily to the following: (1) t he interfacial chemical reaction between the p(AA) existing at the top surface of hopeite and the PU, and (2) the anchoring effects of the p enetration of PU into the rough hopeite crystal layers.