ENHANCING CORROSION PROTECTION OF AZ91D SURFACE VIA DIFFUSION, PA-PVD AND DUPLEX TECHNIQUE: A COMPARISON STUDY

Abstract

Magnesium and its alloys are widely known metal due to its excellent properties such as low density, high specific strength, and high recyclability. However, its corrosion resistance is low compare to other metals. Thus, in this paper, three techniques known as diffusion, plasma-assist physical vapor deposition (PA-PVD) and duplex were selectively applied onto AZ91D surface as an attempt to enhance its corrosion resistance. For the diffusion, AZ91D was exposed to oxygen gas at a constant rate of 5 scfh at 400°C for 2 hours in a tube furnace. For the PA-PVD, AZ91D surface was deposited with CrO hard coating of 1 to 2 microns in thickness using RF magnetron sputtering. For the duplex, AZ91D surface was treated with a sequential step of diffusion followed by PA-PVD technique. The morphology and elemental composition of AZ91D was analysed using SEM-EDX. The case-depth was evaluated using microhardness tester and its corrosion rate were calculated from the Tafel plot. Results showed that the corrosion rate of AZ91D surface was found to perform best with the diffusion treatment followed by the duplex and the PA-PVD. However, for the duplex, a slightly lower performance of the corrosion rate was notice due to the reduction of the oxide zone thickness underneath the CrO coating caused by the erosion process of ion bombardment during the PA-PVD sputtering process.