Citrate Conversion Coating on AZ31 Magnesium Alloys
Date Issued
2015
Date
2015
Author(s)
Chu, Yu-Ren
Abstract
Magnesium alloys are known for their high specific strength, high specific stiffness, and good biocompatibility. However, their engineering applications are still limited, due to their extremely high corrosion rate when exposed in corrosive environment. In order to develop an environmental friendly passivation process for magnesium alloys, a citrate conversion coating system has been developed in this study. To investigate the formation mechanism of the citrate conversion coating on AZ31 magnesium alloy, the conversion coating process is performed in both aqueous and glycerin solutions. As the result, citrate conversion coatings formed in aqueous and glycerin solutions are similar in both microstructure and corrosion properties, which includes the reduced corrosion current density in potentiodynamic polarization and the elevated total impedance in electrochemical impedance measurement. Despite the similarity of pure citrate conversion coating, the effect of cupric ion shows significant difference in aqueous and glycerine solutions. In glycerin solution, the copper-rich clusters are formed on the citrate conversion coating with cupric ion added, and an apparently higher corrosion potential in potentiodynamic polarization can be observed. On the other hand, oxide particles are formed with cupric added in aqueous solution, which leads to an intense increase in corrosion current density. A post treatment using a citrate-based polyamide-imide (PAI) is also proposed in this study. It was found that the impedance of the AZ31 in EIS increases from around 5,000 to 10,000 Ωcm2 after citrate-based conversion coating, and can be further elevated to near 20,000 Ωcm2 with the PAI post treatment.
Subjects
Magnesium
Citrate
Polyamide-imide
Conversion Coating
EIS
Type
thesis
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