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Formation And Characterization Of Ce-Mn Conversion Coating On AZ91Magnesium Alloy At Room Temperature

Posted on:2013-07-28Degree:MasterType:Thesis
Country:ChinaCandidate:Y H PanFull Text:PDF
GTID:2231330374974901Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
As the lightest metallic structural materials, magnesium alloys have been widely used in the industries of automobile and electrical products. However, the wider applications of magnesium alloys are restricted due to their poor corrosion resistance. Therefore, surface protections are very important to determine the quality of the magnesium products. Among many surface protection routes, the technology of painting on chemical conversion coating is widely used in the industries. As the conventional chemical conversion route, the chromate conversion coating has been restricted due to the toxicity of hexavalent chromium compounds. Therefore, it is very urgent that an environmental-friendly technique is developed to replace the chromate conversion coating. Among many non-chrome chemical conversion technologies, rare earth conversion coating is one of the promising alternate for chromate conversion coating. Based on the present research status, rare earth conversion treatment also has some problems, such as long conversion time and high operating temperature. In this thesis, an environmentally-friendly Ce-Mn chemical conversion coating was developed on AZ91magnesium alloy. This coating could be quickly formed at room temperature.This research was firstly based on the results on rare earth conversion coating on aluminum alloy carried out by our research group. The AZ91magnesium alloy was immersed in solution containing Ce(NO3)3, KMnO4and NH4HF2. Using orthogonal test and single factor experiment, an optimized forming process at room temperature was obtained: Ce(NO3)35g/L、KMnO42g/L、NH4HF20.1g/L、operating time of2min. But when NH4HF2was added to the solution, the Ce-Mn conversion coating was formed more quickly and the conversion solution was instability. To control the formation rate of Ce-Mn conversion coating and improve the stability of the conversion solution, some additives were added to the solution to investigate and the optimal additive and its concentration were determined. By contrast, the results show that NaB4O7·10H2O is a better additive. And the conversion solution was also optimized. The optimal formula for the conversion bath is:Ce(NO3)35~8g/L, KMnO41.4~2.5g/L, Na2B4O7.10H2O0.2g/L.The surface morphology, structure and composition of the Ce-Mn conversion coating were analyzed by SEM, EDS, XPS and XRD. The results show that the coating is integrated and uniform from macroscopic view but contains some micro-cracks. With the extension of immersion time, the thickness of the coating increases, but the cracks in the coating also increases and the coating even peels off locally. The Ce-Mn conversion coating is mainly composed of amorphous oxide or hydroxide of Ce, Mn and a little Mg. The element of Ce in the conversion coating existes with Ce3+and Ce4+. The elements of Ce and Mn are deposited on the surface of magnesium alloy at the same time.The evolution of the open circuit potential was tracked. The potential increase sharply towards more positive value in1minute after being immersed, and then increase slowly mainly from one to two minutes. After about two minutes, the potential approach constant value. By investigation of the coating formation and analyzing the composition of the coating, the formation mechanism of Ce-Mn conversion coating was discussed. The corrosion resistance of the coating was investigated by polarization curve and electrochemical impedance spectroscopy (EIS). The results show that the corrosion resistant coating could be obtained by immersing for two minutes. With the extension of immersion time, the corrosion resistance is almost the same. The conversion coating could improve the corrosion potential, the charge transfer resistance and the passivation interval and decrease the corrosion current density of magnesium alloy. The Ce-Mn conversion coating could effectively protect the magnesium alloy to avoid corrosion.
Keywords/Search Tags:magnesium alloy, Ce-Mn conversion coating, corrosion resistance, roomtemperature, composition
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