The Physical Properties of Phosphate Coatings and Their Correlation with Potential/Time Measurements

Abstract

SummaryPhosphate coatings produced in a conventional zinc phosphate solution containing nitric acid, with or without nickel nitrate as accelerator, have been monitored by potential measurements and examined by coating weight, porosity, corrosion performance and paint adhesion tests and electron microscopical techniques. A first potential minimum is associated with complete precipitation of phosphate film and indicates the achieving of full coating weight, minimum porosity and maximum zinc concentration. Subsequent recrystallisation is indicated by a potential rise and fall. Additions of nitrate and nickel to the basic solution accelerate the first stages of dissolution and precipitation, markedly reduce the grain size of the coating but do not improve the adhesion of paint films.