PROTECTIVE ACTION OF CHROMATE CONVERSION COATINGS

The results of investigating the corrosion resistance of zinc in an aerated 5 percent NaCl solution with a pH of 7 show that colorless and iridescent chromate conversion coatings sustain a decrease in the cathodic process, i.e., the reduction of oxygen. Olive chromate coatings also retard the rate of partial anodic dissolution. The chromate coatings had no effect on the value of the anodic Tafel constant. A Cr6+ compound in the coating increases the initial rate of zinc oxidation. Visual changes in chromated surfaces originate by the corrosion of zinc. Black spots and stains are indicators of uniform corrosion, whereas white spots are the result of crevice corrosion.