Phosphate Precipitation in Corrosion Protection: Reaction Mechanisms

The precipitation of phosphate is an important problem in both scale formation and corrosion inhibition. The formation of zinc phosphate barrier films in the latter case is a well-known technique, but the mechanism of the process is still poorly understood. Calcium phosphate phases which precipitate in solutions containing calcium and phosphate are markedly dependent upon such factors as pH, supersaturation, ionic strength, temperature, and the nature and extent of solid phases already present. Their formation may be determined much more by kinetic factors than by pure thermodynamic considerations. Metastable phases may persist for considerable periods in supersaturated solutions. In this paper, methods are described for the highly reproducible kinetic study of the precipitation reactions of the phosphates in the presence of solid surfaces. A recently developed constant composition technique throws new light on the mechanism of calcium phosphate precipitation and enables solid stoichiometry and rates of reaction to be determined precisely, even at very low supersaturation which is of significance in the field. A knowledge of the mechanism of precipitation is essential for the effective design and control of phosphate precipitation for corrosion protection. The constant composition method offers a unique opportunity for quantitative studies of these systems.