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 Zn3(PO4)2-4H2O, H3PO4, Co(NO3)2·6H2O, poly(acrylic acid) (p(AA)) and water, the resulting electrochemical reaction led to the creation of short-circuited cells with cobalt acting as the cathode and the galvanized (zinc) coating 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 coverage with fully grown hopeite crystals after only 5 s immersion. The hopeite layers formed not only serve to protect the galvanized coatings 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) the interfacial chemical reaction between the p(AA) existing at the top surface of hopeite and the PU, and (2) the anchoring effects of the penetration of PU into the rough hopeite crystal layers.
KeywordsPolyurethane Acrylic Acid H3PO4 Interfacial Chemical Reaction Anodic Dissolution
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