AZ31 마그네슘 합금의 환경 친화적 화성 처리 피막의 특성 평가

Abstract

Magnesium alloys possess many excellent properties, such as low density, high specific strength, shielding effectiveness, good casting ability, machinability, and damping capacity, etc. However, magnesium alloys are chemically reactive and liable to suffer severe corrosion during service, which seriously limits their application. Surface modification treatments are hence indispensable for improving the corrosion resistance of magnesium alloys. Among the various surface modification techniques, chemical conversion coating is widely applied for its low cost and simplicity in operation. The most popular and effective conversion process is the chromate conversion treatment. However, because of high toxicity of the hexavalent chromium compounds, the chromate conversion treatment has been strictly restricted all over the world. Therefore, to develop alternative environmentally-friendly treatment processes instead of the chromate conversion treatment is very necessary and urgent. A chromium-free conversion coating for AZ31 magnesium alloy has been obtained by using a acidic phosphate-permanganate solution, which has been developed with acid pickling. Examination has been carried out on the conversion coatings for morphology, composition and corrosion resistance. The morphology of the conversion coated layer was observed relatively uniform and continuous layer with thickness 1.8 to 2.7㎛ by using optical microscope and SEM. The chemical composition of conversion coating was mainly consisted of Mg, O, P, K, Al and Mn by EDS analysis. It was found that the corrosion resistance of the AZ31 magnesium alloy has been improved by the acidic phosphate-permanganate conversion treatment from electrochemical polarization measurements. A uniform chromium-free conversion coating treated by a alkaline phosphate-permanganate solution was formed on the AZ31 magnesium alloy. The alkaline phosphate-permanganate conversion coating was mainly composed of elements Mg, O, P, Al and Mn. The results of the electrochemical polarization measurements demonstrated that the corrosion resistance of the AZ31 magnesium alloy has been improved by the alkaline phosphate-permanganate conversion treatment. The Ce-based conversion coating formed by immersion in a solution containing cerium chloride and nitric acid on AZ31 magnesium alloy has been studied. The change effect of acid pickling on the morphology and the corrosion resistance of the cerium conversion coating was investigated. The chemical composition and phase structure of conversion coating layer were determined via optical microscopy, SEM, EDS, XPS and XRD. The main elements of the conversion coating of AZ31 Mg alloy are Mg, O, Al, Ce and Zn by EDS analysis. The corrosion resistance of the Ce-based conversion coating for AZ31 Mg alloy substrates in 3.5 wt% NaCl solutions was found by potentiodynamic polarization, open circuit potential monitoring. The electrochemical polarization results showed that the Ce-based conversion coating reduced the corrosion activity of the AZ31 Mg alloy substrates in the presence of chloride ions.