Inhibition of steel and galvanised steel corrosion by zinc and calcium ions in the presence of phosphate

Mixtures of zinc, calcium and phosphate ions have been found to be effective (>90%) in inhibiting the corrosion of steel and zinc during immersion in acid rain solution and when used as pigment additions to organic coatings. The presence of calcium ions, in addition to zinc ions, appears to be fundamental in the inhibition mechanism since, in the absence of calcium, the inhibition efficiency is 50% or less. Although some anodic inhibition occurs, the predominant mechanism appears to be classical cathodic inhibition caused by the precipitation of a sparingly soluble, thin, but persistent, solid film. Surface analysis, by XPS, confirms that phosphorus is present in the form of phosphate species containing zinc and calcium. Also, the atomic ratios of zinc:calcium:phosphorus present in the inhibitive film strongly suggest that it consists of a mixture of CaZn2(PO4)2 (Scholzite) and Zn3(PO4)2 (Hopeite) in a 1 : 1 ratio.