| Magnesium(Mg) alloys have been extensively used in automotive, aerospace, electronic industry and other manufacturing fields due to their high specific properties, low density and high damping capacity. It was regarded as “Green Engineering Materials” in the 21 st century. However, Mg and its alloys had high electrochemical activities; its poor corrosion resistance limited the application of Mg. The appropriate surface treatment was considered as one of inexpensive and effective methods to improve the corrosion performance of Mg alloys. This article carried rare-earth conversion treatment and sol-gel post-treatment on AZ31 Mg alloy to develop Nd-based conversion coating and Nd-Al composite coating.The chemical conversion forming process chose AZ31 Mg alloy as substrate, Nd(NO3)3 as main salt and H2O2 as film forming promoter. The best rare-earth conversion coating forming process was: Nd(NO3)3 5 g/L, H2O2 5 ml/L, film forming time 7 min and temperature 40 oC, respectively, which was studied by orthogonal experiment and the single factor experiment, using spot corrosion and potentiodynamic polarization curve as the evaluation index. The influence of dip times, drying time and temperature, sintering time and temperature on the performance of sol-gel coating was investigated by the single factor experiment, which was utilized potentiodynamic polarization curve as the evaluation index. The best post-treatment was: 3 dip times, drying time 1 h and temperature 60 oC, sintering time 0.5 h and temperature 150 oC in sequence.The surface and cross-section morphology was analyzed by SEM, the results showed that, for Nd-based conversion coating, micro cracks distributed on the surface, and three kinds of cracks(inner crack, middle crack and surface crack) were found in the conversion coating; for Nd-Al composite coating, Al-gel covered on Nd-based conversion coating and filled the micro cracks. The consequence of EDS, XPS and XRD indicated that, the main components of Nd-based conversion coating were Nd2O3 and a little Mg O, the core components of Nd-Al composite coating were Nd2O3 and Al2O3.The corrosion resistance of AZ31 Mg alloy, Nd-based conversion coating and Nd-Al composite coating was examined by open circuit potential test, potentiodynamic polarization curve and hydrogen evolution experiment. The results illustrated that Nd-based conversion coating and Nd-Al composite coating can significantly reduce the corrosion rate and improve the corrosion resistance of AZ31 Mg alloy.Covering on AZ31 matrix, Nd-based conversion coating effectively impeded the contact between corrosive media and the substrate. Filling the micro crack of Nd-based conversion coating, Nd-Al composite coating better isolated the substrate from the corrosive media. According to EIS, the corrosion process of AZ31 Mg alloy, Nd-based conversion coating and Nd-Al composite coating could be divided into three stages: early stage(corrosive medium permeating), intermediate stage(corrosion initiation and propagation), and later stage(serious corrosion). |
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