The study of the iron phosphate coatings porosity in bufered borate solution

Abstract

The iron phosphate layers on the low carbon steel surface slows steel corrosion in aggressive environments. The quality of the iron phosphate layer depends on the fraction of the total surface area covered by phosphate coating. Various factors affect this coverage fraction, in particular the composition of the deposition bath, bath temperature and additives used in the electrolyte. In this study phosphating was carried in the phosphate solution (NaOH + H3PO4 + NaNO2, pH 3.8) at different temperatures (30-70oC). The porosity of phosphate coatings on low carbon steel was determined using VAD technique in the borate solution. Surface morphology of the steel was studied using SEM and AFM technique. Anodic polarization showed that phosphate coatings formed on the steel surface resulted in the decrease of the steel dissolution rate. This effect of the phosphate coatings was more pronounced for coatings deposited at higher temperatures of phosphate solution. The decrease in the steel dissolution rate is caused by the decrease of coating porosity. The addition of NaNO2 in the phosphate solution significantly decreased phosphate coating porosity. The lowest porosity of phosphate coating (35.1%), for deposition time of 5 min, was obtained with 1.0 g dm-3 NaNO2 in the phosphate solution, at 50oC. The study of the phosphate coatings morphology showed that coating was evenly dispersed on the whole steel surface. There were two characteristic forms of phosphates present: laminated structure with needle-like forms. The average concentration of specific elements in the phosphate coatings were: oxygen 37.6%, phosphor 3.26% and iron 59.14%.