Electrochemical behaviors of magnesium alloy with phosphate conversion coating in NaCl solutions

Abstract: A composite conversion coating was prepared on magnesium alloy by the only one-step immersion treatment. The characteristics of the conversion coating were investigated by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results indicate that the composite conversion coating consists of magnesium hydroxide, magnesium phosphate and manganese phosphate. The electrochemical behavior of the conversion coating was investigated systematically by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurement in different NaCl solutions. Polarization measurements and EIS results reveal that the magnesium alloy with the conversion coating have better corrosion resistance compared to the bare magnesium alloy in these conditions. And the corrosion rate of the magnesium alloy with conversion coating increases consistently with the chloride ion concentration. In alkaline conditions, the magnesium alloy with conversion coating has superior corrosion resistance by the synergistic effects between Mg(OH)₂ film and conversion coating. Moreover, the electrochemical corrosion mechanism of the magnesium alloy was analyzed with respect to the conversion coating in a Cl⁻ containing environment.