Potentiostatic deposition process of fluoride conversion film on AZ31 magnesium alloy in 0.1 M KF solution

Abstract

The coating process of fluoride conversion film deposited at −1.4V on AZ31 Mg alloy in 0.1M KF solution of pH 7.5 at room temperature was studied by in situ potentiostatic polarization, together with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Our results show that the coating process can be distinguished into five stages. The initial two stages are caused by the rapid discharge of the electrical double layer capacitance and the slow increase of the faradaic current density on AZ31 Mg alloy and Mg(OH)2 was detected in the corrosion product at these stages. The third process corresponds to the deposition of amorphous MgF2 film followed by the failure of the protection of MgF2 caused by Mg(OH)2 at the fourth process. Finally, the deposition of crystallized KMgF3 occurs. The SEM and XPS depth profile indicate that the deposited coating consists of two layers. The inner layer is composed of amorphous Mg(OH)2 and MgF2 with a thickness of about 300nm, and the outer one is comprised of Mg(OH)2, MgF2 and KMgF3 with thickness of about 260nm.