Preparation and characteristics of phosphate conversion films on AZ31B magnesium alloy

Three types of phosphate conversion films, Ca–P, Mn–P, and Ca–Mn–P films, were prepared on AZ31B magnesium alloy from phosphate aqueous solutions containing Ca, Mn, or Ca and Mn ions. Chemical and corrosive resistant properties of the films were characterized using XRD, FT-IR, XPS, XRF, GD-OES, SEM, and potentiodynamic polarization curve measurements. No crystalline phase originated from the film was observed in XRD patterns for all samples. XRF studies revealed that elemental P was included in all films. FT-IR and XPS spectra demonstrated that the Ca–P and Mn–P films were composed of MgO, Mg(OH)2, Mg3(PO4)2, CaCO3, Ca3(PO4)2, Ca(OH)2, CaHPO4, and MgO, Mg(OH)2, Mg3(PO4)2, MnHPO4, Mn(OH)2, respectively, and the Ca–Mn–P films contained MgO, Mg(OH)2, Mg3(PO4)2, CaCO3, Ca3(PO4)2, Ca(OH)2, CaHPO4, MnHPO4, Mn(OH)2. The formation mechanism for all films was also proposed. Potentiodynamic polarization curves indicated that the corrosion current density of the Mn–P film was found to show the lowest value among all films prepared in this study.