Distinctions between the Activating Effects of Sulfuric and Phosphoric Acids on the Process of Zinc Chromating

Abstract

Activating effect of anions of sulfuric and phosphoric acids (both in the range from 0.025 to 0.200 mol/l) on the process of zinc chromating in acid (pH 1.1) 0.2 M CrO3 solution is studied by analytical methods. A general balance of oxidized zinc (Zn2+) and reduced chromium (Cr3+) and their distribution between the solution and chromate film for different solution compositions, as well as the elemental composition throughout the film's depth, are determined. It is found that the zinc oxidation and Cr6+ reduction reactions do not proceed in the absence of SO2– 4 ions (that is, when only PO3– 4 ions are present), so that the chromate film cannot form. However, the PO3– 4 ions combined with SO2– 4 ions increase the Zn2+ and Cr3+ ions concentration in the solution significantly, while their concentrations in the film correspondingly decrease. SO2– 4 ions activate the zinc surface because they form soluble complex compounds with Zn2+ and Cr3+ ions and increase the part of the surface, on which the cathodic reduction of Cr6+ to Cr3+ occurs. The activating action of phosphoric acid is caused by the increase in the total (analytical) concentration of H+ ion in the solution; hence, the deposition of the Cr3+ and Zn2+ hydroxide compounds onto zinc is retarded, due to the increased near-surface concentration of H+ ion.