Effect of phosphate post-treatment on corrosion behavior of cerium-based conversion coated Mg-4.0Y-4.0Zn-0.5Zr-0.2Ca (wt.%) alloys

Abstract

The present study aims to investigate the effect of cerium-based conversion coating and phosphate post-treatment on the corrosion behavior of Mg-4.0Y-4.0Zn-0.5Zr-0.2Ca (wt.%) alloy (WZ44) in chlorinated solution. In this regard, as-cast and forged WZ44 alloy was subjected to cerium salt bath treatment at ambient followed by NaH2PO4 solution at 85 °C. The morphological evolution and chemical composition of the composite coating were examined using field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), two and three-dimensional surface topography, and X-ray photoelectron spectroscopy (XPS) reveal the formation of complex phase mixture of CeO2, Ce2O3, CePO4, Mg(OH)2, and unknown hydrated compounds. The as-prepared cerium conversion coating possesses nodular CeO2 precipitates with abundant mud-cracks features originating due to hydrogen evolution and film dehydration-induced tensile stress. However, phosphate treatment develops a relatively smoother and compact coating with minimum surface roughness and conversion of nodular CeO2 precipitates into CePO4, albeit with few fine cracks. The tape adhesion and scratch test disclose that coating delamination predominant in cerium conversion coated specimens was reduced drastically by phosphate treatment with ∼16–23 % improvement in scratch hardness. Corrosion in terms of mass loss and potentiodynamic polarization (PDP) highlights that phosphate treatment improves the corrosion rate by ∼25–65 % compared to blank. Electrochemical impedance spectroscopy (EIS) demonstrates that coating (Rcoat) and polarization resistance (RP) at the film-metal interface was substantially increased to ∼953 Ω cm2 and ∼729 Ω cm2 for phosphate-treated specimens with a progressive increase in semicircular capacitive loop diameter indicating surface passivation. Overall, a duplex coating strategy with cerium salt bath followed by phosphate treatment can essentially develop a compact double-layer that effectively minimizes severe corrosion of WZ44 alloy in a 0.1 M NaCl electrolyte.