Preparation Of Composite Coating And The Corrosion Resistance For AZ91D Magnesium Alloys

Magnesium alloys, which are considered as time metal materials of 21 century, have unique characteristics of low density, the best strength-to-weight ratio of the commonly die-cast metals, generally a better machinability, and often a higher production rate, thus they are of great value for applications in theerospace, automotive industries and functional materials. However, its insufficient corrosion resistance as an actually usable metal for its relatively low standard potential and high reactive nature makes it easily oxidized in air, which strongly limited its applications. Thus, it is of great significance to make great efforts to increase the corrosion resistance of the magnesium alloys.In the conventional surface treatments, the electrochemical deposition is a promising method for the protection of magnesium alloys due to many pre-dominant performances such as decorative appearance, corrosion resistance, solderability, electrical conductivity, high microhardness and anti-wearing. While it is well known that magnesium alloys are "difficult to plate metal" for the oxide film with low electrical conductivity and bonding ability which is always present on the light metal surface, prevents the adhesion of the metallic coating. In addition, magnesium alloys are susceptible to be corroded in the deposition solution not containing chromate and fluoride. Furthermore, the coating has to be pore-free due to the potential difference between magnesium alloys substrate and metal coating which increases the difficultly of deposition. Therefore, based on the summary of corrosion mechanism and the currently surface protection technologies of magnesium alloy, several pretreatments were adopt to investigate the corrosion resistance technologies on AZ91D magnesium alloys, the main contents as follows.Firstly, considerning environment-friendly, low cost technology, a stannate chemical conversion process followed by an activation procedure was employed as the pre-treatment process for AZ91D magnesium alloy substrate. Zn was electroplated onto the pre-treated AZ91D magnesium alloy surface from pyrophosphate bath to improve the corrosion resistance and the solderability. The surface morphologies of conversion coating and zinc coating were examined with scanning electron microscope (SEM). The phase composition of conversion coating was investigated by X-ray diffraction (XRD).The zinc coating on top is even and filled with distorted polygonal which is 1-2μm in diameter and the coating thickness is about 10μm. The electrochemical corrosion behavior of the coatings in neutral 3.5 wt.% NaCl solution was investigated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The experimental results showed that the corrosion resistance of the AZ91D substrate was improved by the zinc coating. To improve the corrosion resistance of AZ91D magnesium alloy, a protectivemultilayer coating, with electroless Ni coating as bottomlayer and sol-gel SiO2 film as top layer, was successfully prepared on AZ91D magnesium alloy by a combination of electroless and sol-gel techniques. The experiment tests showed that, surface of the composite coating was compact, and its corrosion resistance was also further improved compared with AZ91D magnesium substrate and electroless Ni coating. In the long-term immersion test, it was maintained in the corrosive electrolyte for 120 hours. Two steps were involved in the dregradation process, the top sol film failed and followed by the inner layer in long period of immersion in corrosive electrolyte.Due to the inhere limitation of the electroless Ni coating, a protectivemultilayer coating, with electroless Ni coating as bottomlayer and electrodeposited Ni coating as top layer, was successfully prepared on AZ91D magnesium alloy by a combination of electroless and electrodeposition techniques. The test results showed that, the process was exercisable and reasonable, the surface morphologies was uniformly and compact. The experimental results obtained by electrochemical behaviors showed that, the immersion test in neutral 3.5 wt.% NaCl solution last for 72 hours. The corrosion resistance of AZ91D magnesium substate was improved by the metal coating compared with other technologies.Electrodeposition of nanoparticles within metal matrix produces composite coatings with the attractive properties such as high wear resistance, corrosion resistance and electrocatalysis. Thus, TiO2 nanoparticles were codeposited with Ni matrix to obtain a Ni-TiO2 composite coating. A protectivemultilayer coating with electroless Ni coating as bottomlayer and electrodeposited Ni-TiO2 composite coating as top layer was successfully prepared on AZ91D magnesium alloy. The results showed that Ni-TiO2 composite coating 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. The immersion test of Ni-TiO2 composite coating in neutral 3.5 wt.% NaCl solution last for 96 hours.