Composite film formed on magnesium alloy AZ31 by chemical conversion from molybdate/phosphate/fluorinate aqueous solution toward corrosion protection

Anticorrosive molybdate films composed mainly of Mg(OH)2, MoO2, MoO3, and MgF2 were formed on magnesium alloy AZ31 by chemical conversion. The film had relatively compact surfaces and a thickness of approximately 800 nm. The XPS studies revealed that the film contained Mg, Al, Mo, O, P, S, and F elements. The corrosion resistant performances of the films were investigated by electrochemical measurements. The potentiodynamic polarization curves showed that anodic current densities and corrosion potential of the Mo coated AZ31 were lower and more positive than those of bare AZ31. The EIS measurement and appropriate equivalent circuit models revealed that the corrosion resistance was improved.