Advanced Polyelectrolyte-Modified Zinc Phosphate Coatings

The aim of our work was to develop polyelectrolyte-modified zinc phosphate conversion coatings to protect cold-rolled steels against corrosion. Accordingly, we formulated environmentally acceptable material systems for the zinc phosphating make-up solution, and developed the process technology to meet this purpose. The basic formulation consisted of 5 wt Zn3PO42 . 2H2O, 10 wt 86 H3PO4 and 85 water appropriate amounts of the MnNO32. 6H2O and FeSO4 . 7H2O as additives were incorporated into the basic formulation. The former had two important functions one was to create a large number of nucleate sites of embryonic Zn.Ph crystals on the steel surfaces, and the other was to act as an inhibitors of corrosion. The latter additive served to rapidly promote the rate of crystal growth. Poly acrylic acid, PAA, which is one of the anionic polyelectrolyte species, was employed as the replacement for the conventional Cr-related compounds which are commonly used in the rinsing solutions of Zn.Ph coatings. The pAA-rinsed Zn.Ph coatings not only displayed an excellent salt-spray resistance of 1000 hrs, but also contributed to the strong electrochemical affinity with the electro-deposited polymer topcoatings.