Adhesion characteristics and corrosion stability of epoxy coatings electrodeposited on phosphated hot-dip galvanized steel

Abstract

The influence of hot-dip galvanized steel (HDG) surface pretreatment with phosphate coatings on the corrosion stability and adhesion characteristics of epoxy coatings electrodeposited on HDG steel was investigated. Phosphate coatings were deposited on hot-dip galvanized steel from baths with different concentrations of NaF (0.1, 0.5 and 1.0 g dm-3) and at different temperatures (50, 65 and 80 °C). The influence of fluoride ion concentration in the phosphating bath, as well as the deposition temperature of the bath, on the adhesion characteristics and corrosion stability of epoxy coatings on phosphated HDG steel was investigated. The dry and wet adhesions were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test, while corrosion stability was investigated by electrochemical impedance spectroscopy (EIS). It was shown that under dry testing conditions all the samples exhibited very good adhesion. However, the different trends of adhesion loss of different protective systems during exposure to corrosive agent (3% NaCl solution) were observed. The lowest adhesion values were obtained for epoxy coating on hot-dip galvanized steel, while the phosphate pretreatments significantly increase the adhesion of epoxy coatings. The highest values of adhesion strength were obtained for epoxy coating on HDG steel modified by phosphate coating deposited at 65 °C with 0.5 g dm-3 NaF in the phosphating bath. EIS measurements in 3% NaCl solution are in accordance with adhesion measurements, i.e. greater values of charge-transfer resistance and smaller values of double-layer capacitance were obtained, as compared to other protective systems.