Effect of cerium-based conversion coating on corrosion behavior of squeeze cast Mg-4 wt% Y alloy in 0.1 M NaCl solution

Abstract

Cerium-based conversion coating (CeCC) has been utilized to improve the corrosion resistance of squeeze cast Mg-4wt%Y alloy in 0.1 M NaCl solution. The morphology and composition of the coating analyzed using field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), glancing angle X-ray diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS) demonstrate the formation of few microns thick pseudo-crystalline phase(s) composed of mainly Mg, O, C and Y species, assigned to hydrated Y(III) (Y(OH)3/Y2O3) CeO2, Mg(OH)2, and lower amount of PO43− ions with mud-crack morphology. The duration of the treatment in cerium nitrate salt bath has a pronounced effect on the evolution of the coating with 30 s treatment demonstrated the formation of uniform coating with dehydrated cracks that becomes loose and porous with nodular CeO2 precipitates for longer duration (1800 s). Potentiodynamic polarization (PDP) and mass loss test reveals that CeCC effectively reduces the corrosion rate by ~50–70% compared to base alloy, with the pitting corrosion around the second phase being the dominant mechanism due to the formation of the micro-galvanic couple between α-Mg phase and Mg24Y5 precipitates. The deep pit formation was severe around Y-rich cathodic sites due to the formation of Y(OH)3/Y2O3 as the corrosion products confirmed by EDX and elemental mapping. Electrochemical impedance spectroscopy revealed that corrosion protection was provided mainly by the barrier layer at the coating/Mg interface with a progressive increase in diameter of the capacitive loop up to 24 h commensurate with the surge in double-layer film resistance (RP) and free corrosion potential (Ecorr) shift towards noble value. Overall, cerium nitrate salt bath treatment in the presence of hydrogen peroxide for short duration is effective to retard the severe corrosion of Mg-4wt%Y alloy in 0.1 M NaCl solution.

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