Abstract
The incorporation of cobalt (Co2+) and nickel (Ni2+) ions in a poly(acrylic)acid (p(AA))-modified zinc phosphate solution promoted the growth and development of crystalline zinc phosphate (Zn·Ph) conversion coatings deposited on steel surfaces. This enhancement was due primarily to the preferential uptake of Co2+ and Ni2+ ions by p(AA), to form the Co- and Ni-OOC- salt complexes, rather than complexes with zinc ions. The electron trapping behaviors of Co2+ and Ni2+ dissociated from the salt complexes, and the hydroxides in the NaCl solution inhibited the cathodic reaction on Zn·Ph. In particular, the conversion coatings derived from the nickel-modified phosphate solution were characterized by the formation of a coating consisting of hopeite as a major crystal phase, zinc orthophosphate dihydrate as a minor phase, and an amorphous iron-rich phosphate phase; this coating provided an extensive coverage over the steel, which contributed to protecting it from corrosion.
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