Effects of ionic liquid Al coating and its subsequent heat treatment on the characteristics of Mg alloy

• Main Authors: Mu-Huan Chuang, 莊牧寰
• Other Authors: Wen-Ta Tsai
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/37021155440167691851

碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 97 === Electrodeposition of a metallic aluminum coating on the magnesium alloy(AZ91D) in AlCl3-EMIC at 30 oC has been investigated in this research. The effects of the AlCl3 to EMIC molar ratio, in the range from 50 %–50 % to 60 %–40 %, on the reaction between the ionic liquid and Mg substrate as well as the electrodeposition characteristics were investigated. Heat treatment and various coating thickness were applied to understand the effects on the substrate surface reaction and coating characteristics. The composition, structure and corrosion behavior of various conditions of deposited layers were analyzed. Morphologies and compositions of a bare Mg alloy and Al-coated samples were examined in a Philip XL-40FEG SEM and its auxiliary energy dispersive spectroscope. X-ray diffraction analysis was also performed to identify crystal structures. A pipe tube was utilized to apply the heat treatment at 450 oC with Ar flow as the protective gas. The corrosion behaviors of the specimens, inclusive of open circuit potential, electrochemical impedance spectra, and potentiodynamic polarization curves, were measured in 3.5 wt % NaCl solution utilizing Autolab and Solartron 1255 electrochemical measurement system. The results indicated that deposition bath concentration was a crucial factor governing the quality of deposited layers and their corrosion resistances. XRD analysis showed a preferential growth on the (200) plane at higher concentration of deposition bath. With regard to heat treatment effects on the coating layers, no obvious composition variation can be seen after the Al-coated layer was heat-treated for 10 minutes. A diffusion layer composed of Mg17Al12 formed at the interface between Al coating and Mg alloy substrate after heat treatment for thirty minutes；meanwhile, the prolonging of treatment time resulted in the increase of the diffusion layer thickness. The corrosion resistance of Al-coated Mg alloy was evaluated in 3.5 wt % NaCl solution. The results indicated that coating layer made contribution to the improvement of the corrosion resistance. The open circuit potential as well as polarization resistance of the Mg alloy ameliorated after the electrodeposition of the aluminum coating layer. A passive region was observed in the Al-coating alloy in the potentiodynamic polarization test. The open circuit potential of the specimens maintaining at 450 oC for 30 minutes revealed a fluctuated curve, which probably resulted from the uneven composition of the heat-treated deposits；A passive region and lower corrosion rates were observed via potentiodynamic polarization tests. Specimens holding at the same temperature for over 60 minutes appeared active owing to the existence of Mg17Al12 ; besides, potentiodynamic polarization results showed no passive region.These results proved that Mg contents of the Al-coated Mg alloy increased as the prolonging of the heat treatment time, hence resulting in the failure of the corrosion resistance.