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Effect of Fluoride Ion on the Microstructure and Corrosion Resistance of Permanganate Conversion Coating on AZ91D Magnesium Alloy
Magnesium alloys,Conversion coating,Permanganate,Electrochemical analysis,Hydrogen evolution test,
|Publication Year :||2018|
Magnesium alloys have low density, high specific strength, good magnetic shielding, good heat dissipation, and biocompatibility properties. Among them, due to good mechanical properties and proper corrosion resistance, magnesium-aluminum alloys are widely used. However, the applications of magnesium-aluminum alloys are still limited by their high corrosion rate. Surface modifications are thus required to enhance their corrosion resistance. Therefore, this study aimed at improving a permanganate conversion solution by addition of fluoride ion to make it effectively enhance the corrosion resistance of AZ91D magnesium alloy. Due to the instability of the original permanganate conversion solution, this study investigated the effect of fluoride ion on the solution stability and on the microstructure and corrosion resistance of the permanganate conversion coatings. Moreover, the effect of conversion treatment time and concentration of fluoride ion were also investigated to find the optimal treatment condition.
The results show that the permanganate concentration in the conversion solution without addition of fluoride rapidly decreases due to the instability, and the addition of fluoride ion reduces the decrease in permanganate concentration and effectively enhances the solution stability. Because of the dual-phase galvanic effect, the conversion coatings formed without fluoride are heterogeneous, and plenty of MnO2 forms on the β phase accompanied by dehydration cracks, which leads to poor corrosion resistance. In contrast, because more MgF2 forms on the α phase and therefore inhibits the dual-phase galvanic effect. As a result, the conversion coating formed with fluoride is homogenous, thin, and nearly crack-free, which significantly improves the corrosion resistance. With longer conversion treatment time, the thicker coatings induce more dehydration cracks, and the dissolution of the coatings leaves pore on the surface. Both degrade the corrosion resistance. Hence, the best treatment time is 60 s. With lower fluoride ion concentrations, MgF2 formation is inadequate and dual-phase galvanic effect is not inhibited, and once again, plenty of MnO2 forms on the β phase accompanied by dehydration cracks. With higher fluoride concentrations, although dual-phase galvanic effect is inhibited, the β phases are etched by fluoride ion leaving pores on the surface. Hence, the coatings formed with high and low fluoride ion concentrations are both defective and have inferior corrosion resistance, and the best fluoride concentration is 0.02 M. The polarization resistance deduced from potentiodynamic polarization (PD) is consistent with the total impedance estimated from electrochemical impedance spectroscopy (EIS). Moreover, the corrosion current measured from PD is close to the equivalent corrosion rate measured from hydrogen evolution test, which indicates the corrosion analyses are related to each other and prove the accuracy. In addition, based on the equivalent circuit used in the EIS analysis, the simulated total impedance at frequency approaching zero can be directly used to evaluate the corrosion resistance.
|Appears in Collections:||材料科學與工程學系|
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