Corrosion behaviour of the Mg65Y10Cu15Ag10 bulk metallic glass

Amorphous and multiphase crystalline Mg65Y10Cu15Ag10 samples were subjected to potentiodynamic polarisation in electrolytes with pH = 5-8.4 and 13. Potentiostatic current transients were recorded for studying anodic passive layers growth mechanisms. Surface states were characterised by Auger electron spectroscopy (AES) depth profiling and atomic force microscopy (AFM). The corrosion behaviour of the quaternary alloy samples containing silver is markedly different compared to that of the earlier studied Mg65Y10Cu25 alloy. The free corrosion potential is more noble and corrosion current densities are lower. Stable anodic passivation was observed in electrolytes with pH greater than or equal to 6, whereas in acidic media dissolution sets in. In neutral to weakly alkaline electrolytes, a mainly high field driven layer growth occurs which was similarly observed for the ternary alloy. In strongly alkaline electrolyte, the growth mechanism tended to shift from diffusion control (ternary alloy) to a more high field driven growth pattern due to the presence of silver in the alloy. Passive layers formed in weakly alkaline electrolyte under participation of all alloying elements, especially copper, are thick and porous. Magnesium and yttrium oxide/hydroxides are the main constituents of the thinner passive layer formed in the highly alkaline electrolyte, but also traces of silver oxides were detected. Also, for this quaternary alloy composition, the multiphase crystalline material exhibits a poorer stability and passivation ability than the single amorphous phase. (C) 2003 Elsevier B.V. All rights reserved.