| Among light metals, the density of magnesium alloys (from 1.74 to 1.85 g/cm3) is approximately 35% smaller than Al-based alloys and 65% smaller than that of Ti-based alloys. Meanwhile they have unique characteristics of the best strength-to-weight ratio, higher modulus of elasticity, generally a better machinability, and can be recycled, thus they are of great value for applications in the transportation, aerospace, digital communication and biomedical, which are compared as the 21st Green Engineering Material. 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. Surface treatment technologies can greatly improve the corrosion resistance of metals. Thus, the research for magnesium alloy surface treatment will become very important, extremely has application value and economic value.Adding corrosion inhibitor is a simple and effective method to protect metals from corrosion, which needs small amounts and could attain the desired effect, besides it does not affect the performance of material itself. So in considering environmental protection and economic factors, the inhibiting effect for AZ91D magnesium alloy in 0.05 wt.% NaCl solution of the five inhibitors with different concentrations such as TU, ATU, TSC, Bz and SC with different additives was investigated by open circuit potential (OCP), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), computational method and ultraviolet spectrum. The possible inhibiting mechanisms were also discussed in detail. The results have important significance for selecting and using the inhibitors for magnesium alloys. The results revealed that TSC showed a good inhibiting effect, and TU as well as ATU slightly improved the corrosion resistance of AZ91D magnesium alloy in the studied solutions. A negative inhibiting effect with the addition of Bz or SC was also observed. The results obtained by electrochemical method were consistent with computational method and ultraviolet spectrum.Because of the corrosion inhibitors technology research for magnesium alloys is in the development stage, the five studied inhibitors can't meet the industrial standard, so it is necessary to continue to explore other surface treatment methods. 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, which could expand the application range of the materials. It is not suitable for direct electroplating due to the lower potential which could result in corrosion of magnesium alloys in the deposition solution, it needs special pretreatment before electrodeposition. Based on the summary of plating technology and the pretreatment technologies of magnesium alloys, Ni-Co-TiO2 composite coating was successfully electrodeposited on AZ91D magnesium alloy with the pretreatment of electroless nickel coating. The electrochemical corrosion behaviors of the composite coating in 0.6 mol·L-NaOH,0.6 mol·L-1 Na2SO4, and neutral 3.5 wt.% NaCl solutions were investigated. Surface morphologies, microstructure, and chemical compositions of the composite coating were also studied. The results indicated excellent protection for AZ91D magnesium alloy, and the performance of the composite coating was superior to the electroless nickel coating. Immersion tests were also introduction to investigate the long-term protection. Combined with the equivalent circuit model, the mechanism of composite coating degradation was discussed. |
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