Microstructure characteristics and degradation behaviors of Se/phosphate conversion coatings on Mg-Gd-Ag-Y-Zn alloys in the Hank's electrolyte

Abstract

In the present study, a selenium-phosphate based conversion coating (SPCC) was successfully developed on the dual rare-earth Mg-Gd-Ag-Y-Zn by immersion in specific acid phosphate bath. The compositions, morphologies, degradation behaviors as well as the formation/deposition mechanism of the SPCC were deeply analyzed. The results declared that the compositions of conversion coatings were the mixture of Mg3(PO4)2/η-Se, with the shallow hill-like structure and dispersive distribution, which was beneficial to enhance the adhesion and corrosion resistance. The electrochemical and static-immersion tests both revealed that the lowest corrosion dissolution rates of the SPCC coatings were realized through pretreatment at 70 °C, with higher Rct value (507.4 Ω·cm2) and lower corrosion rate of 0.603 mm/y. On the one hand, the appropriate temperature was conducive to increasing the concentrations of H2PO4−/SeO32− and accelerating the exchange rates of molecule active-movement in the conversion solution; On the other hand, the dense/stable selenium-phosphate conversion layers formed by redox reaction and hydrolysis reaction retarded the adsorption sites and penetration channels for corrosive ions.