Corrosion resistance of AZ91D magnesium alloy with electroless plating pretreatment and Ni-TiO2 composite coating

In this paper, a protective multilayer coating, with electroless Ni coating as bottom layer and electrodeposited Ni-TiO2 composite coating as top layer, was successfully prepared on AZ91D magnesium alloy by a combination of electroless and electrodeposition techniques. Scanning electron microscopy and X-ray diffraction were employed to investigate the surface, cross-section morphologies and phase structure of coatings, respectively. The electrochemical corrosion behaviors of coatings in 3.5 wt.% NaCl solutions were evaluated by electrochemical impedance spectroscopy, open circuit potential and potentiodynamic polarization techniques. The results showed that the corrosion process of Ni-TiO2 composite coating was mainly composed of three stages in the long-term immersion test in the aggressive media, and could afford better corrosion and mechanical protection for the AZ91D magnesium alloy compared with single electroless Ni coating. The micro-hardness of the Ni-TiO2 composite coating improved more than 5 times than that of the AZ91D magnesium alloy. (C) 2009 Elsevier Inc. All rights reserved.