Corrosion and protection of magnesium alloy AZ91 and its Al2O3-reinforced composite

Abstract

The present project aims at studying and comparing the corrosion behaviour of a newly developed metal matrix composite (AZ91C/Al2O3 MMC) in marine environments with its matrix alloy, and at investigating the feasibility of a number of corrosion protection systems for the MMC. Using potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS) and immersion tests, it was found that the corrosion resistance of the MMC in 3.5% NaCl solution at pH 10.5 was much lower than that of the matrix alloy, with a corrosion rate of about 5 times that of the matrix (published in CORROSION REVIEWS vol. 16 (1998) P. 43-52). In order to enhance the corrosion resistance of AZ91C and its MMC, three protection systems were developed, each representing a generic class of surface modification: (a) a metallic coating system consisting of electroless Ni-P coating with Zn/Cu precoating or chemical etching pretreatment, (b) an organic coating system consisting of a primer and a BMI (bismaleimide) topcoat, and (c) laser surface melting accomplished by an Nd:YAG laser. The efficacy of these protection measures was evaluated by the copper-accelerated acetic-acid salt spray (CASS) test, supplemented by other tests such as adhesion test, potentiodynamic polarization measurement and electrochemical impedance spectroscopy. It was found that the Ni-P coating and the primer/BMI coating were efficient in providing corrosion protection to both AZ91C and its AZ91C/Al2O3 MMC in chloride-containing environments. The presence of Al2O3 in the MMC did not cause adverse effects in these coating systems. Laser surface melting, which created a modified surface layer on the specimens, also led to a significant increase in the corrosion resistance of AZ91C and its AZ91C/Al2O3 MMC, though the increase was not comparable to that in the case of electroless Ni plating or BMI coating.