Microstructure and Corrosion Resistance of Aluminized Coatings on Magnesium Alloy by Pulsed Bias Magnetron Sputtering

Abstract

Aluminized coatings were prepared on AZ91D magnesium alloy using pulsed bias magnetron sputtering. Effects of deposition temperature on the microstructure and corrosion resistance were studied. The microstructure, element distribution and hardness of the coatings were characterized using optical microscopy (OM), X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GDOES), and microhardness tester, respectively. The corrosion behavior of the aluminized coatings was studied by measuring anodic polarization curves in 0.5 wt% NaCl solution and compared with that of bare AZ91D. Results show that the aluminized coatings are composed of a deposition layer and a diffusion layer. With deposition temperature increasing, the thickness of deposition layer decreases while diffusion layer thickness increases. The interdiffusion of Al element with other elements in AZ91D magnesium alloy substrate results in a metallurgical bonding of the coatings with substrate. The coatings are mainly composed of γ-Al and α-Mg phase. A small amount of β-Al12Mg17 phase appears at higher temperature of 450°C. The surface microhardness of magnesium alloy is obviously improved by the aluminized coatings. The hardness gradually decreases along the depth of aluminized coatings and the hardness gradient becomes smooth with the increasing of deposition temperature. The corrosion resistance of magnesium alloy is increased considerably by the aluminized coatings with more positive corrosion potential and an order of magnitude decrease of the corrosion rate than the bare magnesium alloy.