The influence of hexamethylenetetramine on the structure and corrosion resistance of phosphate coating on AZ31 magnesium alloy

Purpose This paper aims to report the influence of hexamethylenetetramine (HMTA) on phosphate coatings formed on AZ31 magnesium alloys. Design/methodology/approach These phosphate coatings were obtained by immersing magnesium alloys in phosphate baths with HMTA. The morphology and composition of the phosphate coatings were investigated via scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. Findings The phosphate coatings were mainly composed of CaHPO4·2H2O. The HMTA concentration in the phosphate bath influenced the crystallization and corrosion resistance of the phosphate coating. Originality/value The polarization curve shows that the anti-corrosion qualities of the phosphate coating were optimal when the HMTA concentration was 1.0 g/L in the phosphate bath. Electrochemical impedance spectroscopy (EIS) shows that the electrochemical impedances increased gradually when the HMTA concentration varied from 1.0 to 3.0 g/L.