Effect of ZrO2 and L-Cys nanoparticles as dopants in sol-gel of mesoporous silica coating for corrosion protection of AZ61 magnesium alloy

Abstract

Sol-gel coatings based on GPTMS-TMOS precursor, including as dopants L-Cysteine and ZrO2 in different concentrations, were applied on the surface of AZ61 magnesium alloy. Their corrosion resistance was studied in 0.6M NaCl solution, by immersion up to 14 days. XRD spectra revealed Mg(OH)2 as the main corrosion product on the coated surface, while on the untreated AZ61 in addition there were several compounds of Zn with chloride. The localized corrosion attack on the untreated AZ61 was expressed by cracks and caverns, while on the dip-coated surface the corrosion was mostly through pitting. Two non-destructive electrochemical methods were employed, contrasting the electrochemical behavior of coated AZ61 with that of uncoated alloy. The tendency in the changes of the corrosion potential at open circuit correlated positively with SEM-EDS and XRD analysis. The EIS diagrams were fitted to equivalent-circuit model and the obtained corrosion resistance Rcorr (Rs + Rct) values strongly decreased over time. The ZrO2 and L-Cysteine effect is influenced by the pH changes of the solution, Zeta potential surface charge, chemisorption and desorption processes, internal stress in the sol-gel precursor, as well as the change in its structure, after the encapsulation of both dopants.