Microstructure and Electrochemical Characterizations of Mn-Ce Conversion Coating on LZ91 Magnesium Alloy
Authors: Chang Ko-Lun , Jian Shun-Yi , Ger M.D. , Sheu Hung-Hua 
Published: February 2020

In the past few years, the energy crisis and environmental issues have been emphasized. In order to reduce the greenhouse effect and enhance the efficiency of energy consumption, each country has enacted relative laws to decline the emission of carbon dioxide year by year. Magnesium alloys, the lightest structural metal, have found their potential use in the aerospace, automobile, tool, and electronic industries due to their attractive damping capacity, high specific strength and specific stiffness, as well as good electromagnetic shielding characteristics. In comparison to the AZ (with aluminum and zinc as the major alloy elements) series magnesium alloys, the LZ (with lithium and zinc as the major alloy elements) series magnesium alloys have been developed by substituting aluminum with lithium. However, because of the difference in the standard reduction potential between the α phase and the β phase and the presence of active lithium, Mg-Li alloys display poor corrosion resistance. Surface modification is thus essential for Mg-Li alloys.

In this study, the main theme is aimed at the improvement of phosphate/permanganate conversion coating. With substituting phosphate for cerium (III), magnesium dissolved more magnesium ions and released more electrons to enhance the oxidation-reduction reaction. The conversion coating showed better adhesion and less severe crack by virtue of increasing the chemical reactivity and lowering the immersion time, expecting to replace hexavalent chromate conversion coating and reduce the cost. The LZ91 magnesium alloys were mechanically grinded up to 1200 grade silica emery paper, rinsed with deionized water, and dried with an air stream. The conversion coating treatment was conducted in the solution composed potassium permanganate (KMnO4) and cerium nitrate (Ce(NO3)3). The pH of the solution was about 1.5-4.5. After conversion coating treatment, the LZ91 plate was rinsed in deionized water and left drying overnight at room temperature. Based on the microstructural, and XPS, the formation and deposition mechanism of the Mn-Ce conversion coating on LZ91 is discussed in detail. The corrosion resistance of the coating was characterized by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and salt spray tests (SST). The results of the electrochemical measurement and SST demonstrated that the corrosion resistance of LZ91 alloy was markedly improved by the Mn-Ce conversion treatment.

Figure 1. shows the potentiodynamic polarization curves of bare LZ91 and the various Mn-Ce coated LZ91 plates (for pH=1.5) in 0.05 M NaCl and 0.1 M Na2SO4 solution. In general, the cathodic reaction in the polarization curve is related to the evolution of hydrogen and the anodic polarization curve is the characteristics corresponding to the corrosion resistance of the coating. Both the cathodic and anodic reactions were inhibited in the presence of the conversion coating, indicating the coating effectively enhances the corrosion resistance of the LZ91. And, the values of electrochemical corrosion parameters were shown in Table 1. In contrast with the un-coated LZ91 alloy, the corrosion current density Icorr
of the Mn-Ce coated LZ91 plates was decreased by one orders of magnitude (with the value from 46.9 µA/cm2 to 1.78 µA/cm2), which functioned as a barrier.

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Figure 1