Evaluating interactions of second phases with chemical conversion coating on Mg-Zn-RE (Gd, Y) alloy for surface protection

Abstract

Magnesium alloys are highly suitable for lightweight manufacturing and applications due to their impressive mechanical properties and high strength-to-weight ratio. However, their vulnerability to corrosion restricts their practical applications. Using a chemical conversion process, a calcium-phosphate-vanadium (Ca-P-V) composite coating was fabricated on Mg-Zn-RE (Gd, Y) alloy effectively in this research. SEM and EDX analysis of the base alloy and coated sample were carried out, and the coated surface showed different phases from different concentrations of vanadium. XRD analysis of the base alloy and coated samples were carried out, phases and crystal structure were determined. XPS analysis was performed to evaluate the interaction of coating materials on the surface of the sample and their binding energies. Ca3(PO4)2, CaCO3, and MgCO3, made up the majority of the composite coating which had a limited number of hydroxides of V(V) distributed into it. Electrochemical tests were also conducted to determine how the concentration of vanadium affected the substrate's ability to resist corrosion. Both electrochemical tests and scratch immersion tests in a 3.5 wt % NaCl solution were utilized to examine the corrosion behavior of the conversion coating. The findings demonstrated that the coated samples could self-heal in addition to having superior corrosion resistance. A unique composite layer was formed on the scratched area as a result of the Ca, P, and V ions that are liberated from the coated surface migrating in the corrosion solution. Moreover, another Mg alloy sample with different alloying percentages was coated and characterized to evaluate and compare the interaction of second phases of base alloy and coated sample.