INVESTIGATING THE CORROSION BEHAVIOUR OF ZX10 MAGNESIUM ALLOY IN VARIOUS CORROSIVE MEDIA AND THE CORROSION RESISTANCE OF MAGNESIUM OXIDE COATINGS

Abstract

At present, magnesium alloys are receiving considerable attention in the fields of computer parts,automotive,aerospace components, and biomedical applications because of their low density, high specific strength and bio-compatibility. Among them, the ZX10 magnesium alloy exhibits excellent mechanical properties. However, due to their poor corrosion resistance and insufficient mechanical properties when exposed to different aggressive environment, the engineering application of magnesium alloys is limited and requires appropriate surface treatment. This study, therefore, focused on the degradation behaviour of ZX10 magnesium alloy in different corrosive media at room temperature and the development of protective MgO coating on ZX10 magnesium alloy. The degradation behaviour and corrosion rate of ZX10 magnesium alloy in different aggressive media with varying pH values, were investigated by employing the Tafel curve and electrochemical impedance techniques, and analyzed with the Scanning Electron Microscope (SEM). The chemistry and diffraction phases of adhering corrosion products were also investigated by utilizing the X-ray diffraction (XRD) technique. MgO coatings were electrodeposited on magnesium alloy using cathodic electrolytic method and subsequent calcination at different temperatures to improve the corrosion resistance. The protective performance of the MgO coatings was investigated by potentiodynamic polarization, and electrochemical impedance spectroscopy and a correlation was drawn between calcination temperature of coatings, corrosion resistance, and mechanical hardness. A modeled calcination mechanism, as it relates to barrier performance at low and high thermal conditions, has been illustrated based on collected experimental evidence. Results showed that magnesium alloy demonstrated varying rates of corrosion and passivation within a range of pH of corrosion media. The acidic systems showed the highest rate of metal corrosion while the lowest corrosion rate was observed for the alkaline systems. Localized corrosion has been observed by SEM, while electrochemical impedance spectroscopy and potentiodynamic polarization techniques reveal increased anodic polarization and charge transfer, respectively. The metal uniformly corroded for each examined corrodent over an extended period of time; however, the early corrosion stages reveal localized formation of oxide films at grain boundaries. The protective MgO coating was successfully developed and optimized on ZX10 magnesium alloy. The internal tensile stress was produced in electrodeposition because of the occlusion of hydrogen atoms in the coatings. Results showed that at calcination temperature of 250 o C, the internal tensile stress was released leading to crystallization of oxide coating layers with compact and continuous multi-layered micro-structures. At higher temperatures (around 500 o C), recrystallization of the MgO coating increased coating porosity and broadened inter-diffusion at micro-defects. Surface and bulk pores and cracks have been observed at 500 o C as clear evidence of the consequence of over-calcination. Enhanced corrosion resistance was observed for the MgO coatings obtained at 250 o C calcination temperature after chloride-induced corrosion tests and its corrosion current density is one thousand times smaller than that of bare magnesium alloy.