Microstructure and Electrochemical Characterizations of Permanganate Conversion Coating on AZ31 Magnesium Alloy

Permanganate conversion coating treatment is one of the promising alternatives to hexavalent chromium conversion coating treatment for magnesium alloys. However, the permanganate conversion coating on magnesium generally contains defects such as cracks, which deteriorate the coating adhesion and corrosion resistance.^1-6 In the present study, we reported the fabrication of a compact permanganate conversion coating on an AZ31 magnesium alloy in strongly acidic potassium permanganate solution. Moreover, the crack formation was largely avoided. Consequently, the corrosion resistance of the coating was improved. 

It has been well known that Mn(IV) oxides precipitate in the solution containing permanganate (MnO₄^–) and manganous ions (Mn²⁺), also known as the Guyard reaction.⁷  This reaction has been shown to be autocatalytic in acid solution as driven by the presence of colloidal Mn(VI) oxides.  This study also detailed the effects of the addition of Mn(NO₃)₂ or MnSO₄ in the permanganate solution on the microstructure and growth of the conversion coating on the AZ31 magnesium alloy.

 Prior to conversion coating treatment, the AZ31 magnesium alloys were mechanically abraded up to 1200 grade emery paper. Subsequently, the specimen was rinsed in deionized water and dried with an air stream. The nitrate-containing bath was performed in the solution composed of 0.1 M potassium permanganate (KMnO₄), 0.025 M manganese nitrate (Mn(NO₃)₂), and 0.02M potassium dihydrogen phosphate (KH₂PO₄) at room temperature for 60 s. The sulfate-containing bath was performed in the solution composed of 0.1 M KMnO₄, 0.025 M MnSO₄, and 0.02M KH₂PO₄ at room temperature for 60 s. After conversion coating treatment, the AZ31 plate was rinsed in deionized water and left drying overnight at room temperature. SEM, TEM, and EDX were employed to analyze the surface morphology and characterize the microstructure of the coating. The corrosion properties of the different conversion coatings were evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. Finally, the EIS was conducted after immersion in 3.5 wt.% NaCl solution for different time periods up to 24 h.

Figure 1 shows that the permanganate coated AZ31 displays a light brown color. As a result, the conversion coating has completely covered the AZ31 magnesium alloy after 60 s of immersion in the permanganate solution containing nitrate or sulfate anions.

Figure 2(a) and (b) show the cross-sectional TEM characterization of the AZ31 after 60 s immersion in the permanganate bath containing nitrate and sulfate anions, respectively. Both coatings have comparable thickness. Moreover, the SAED of the compact layer comprised diffused halos regardless of the bath composition, suggesting that all the coatings had poor crystallinity. The P content was the main difference between the two conversion coatings.

The nitrate-containing conversion coating effectively enhances the corrosion resistance of AZ31 magnesium alloys, as evaluated by EIS and potentiodynamic polarization measurements. The corrosion behaviors of the two coatings are also detailed using the EIS characterizations after different time periods of immersion in 3.5 wt.% NaCl solution.

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