Comparative Study On Characterization Of Corrosion Resistance Of Micro-arc Oxidation Coatings On Az91d Magnesium Alloys

Micro-arc oxidation (MAO) coatings on AZ91D magnesium alloys were prepared in silicate solution with same process. Drop test, immersion test, salt spray test and electrochemical tests (Cyclic Voltammetry, Tafel technique, Open Circuit Potential and Electrochemical Impedance Spectroscopy) were adopted to characterize the corrosion resistance of coatings, when suitable test methods were found out. compared study them.Drop test has been considered as a quick way to detect the corrosion resistance of MAO coatings, but fixed the dropping area, colour changing of drop liquid in the first place was adopted as proper counting point, and the concentration of nitric acid in drop liquid needed to be doubled when meeting coatings with good corrosion resistance in order to stimulate a corrosion action faster. At the same time, the corrosion resistance of MAO coatings can be more accurately characterized if drop liquid on unit area coatings was equal. The anti-corrosion ability of MAO coatings couldn't be presented properly with immersion test because it is hard to decide which evaluation criteria, weight loss or weight gain, should be taken due to corroded products might be stuck in coatings with porous microstructure and were hard eliminated completely. During salt spray test, adopting "corrosion rate of xx hours", "time of corrosion-pitting began to appear" and "appearance quality" as assessment criterias together were reasonable. And, "appearance quality" were the most important index, because it reflected macro status of MAO coatings. Electrochemical tests have been considered to carry out the overall characterization of the corrosion resistance of MAO coatings properly, and more information such as corrosion potential, corrosion current and impedance etc could be obtained from the tests of Cyclic Voltammetry (CV), Tafel technique, Open Circuit Potential (OCP) and Electrochemical Impedance Spectroscopy (EIS), which will enhance the further research on the corrosion behavior of coatings.Kinds of tested datas came from continuous experiment of CV, Tafel, OCP and EIS in turn were similar to which came from individual experiment of each kind of electrochemical tests, so, adopting continuous experiment detected the corrosion resistance of MAO coatings was feasible. And, continuous experiment could enhanced the further research on the corrosion behavior of coatings because it obtained more kinds of corrosion resistance datas on the same place on the same sample. Moreover, the results of drop test, CV, Tafel and EIS test could be confirmed with each other very well, and the results of drop test and EIS test could be confirmed with each other preferably.The corrosion resistance of AZ91D magnesium alloys after MAO treatment is improved dramatically. These samples, which oxidation for 60 min and thickness for 27μm±2μm, appearance quality was nine when them tested by 240h salt spray test, but appearance quality was six when AZ91D magnesium alloys only tested by 28h salt spray test. The corrosion resistance of AZ91D magnesium alloys with MAO coatings improved two orders of magnitude than AZ91D magnesium alloys without MAO coatings tested by Drop test and CV test, respectively. Corrosion potential of AZ91D magnesium alloys with MAO coatings improved 154mV than AZ91D magnesium alloys without MAO coatings tested by Tafel test, corrosion current density of AZ91D magnesium alloys with MAO coatings reduced two orders of magnitude than AZ91D magnesium alloys without MAO coatings tested by Tafel test, polarization resistance of AZ91D magnesium alloys with MAO coatings improved four orders of magnitude than AZ91D magnesium alloys without MAO coatings tested by Tafel test. And, open circuit potential of AZ91D magnesium alloys with MAO coatings improved 106mV than AZ91D magnesium alloys without MAO coatings. Finally, impedance value of AZ91D magnesium alloys with MAO coatings improved four orders of magnitude than AZ91D magnesium alloys without MAO coatings tested by EIS test.Thickness of surface close to cathode was slightly greater than thickness of surface far from cathode, and difference of two surfaces'thickness was less than 4μm. At the same time, the corrosion resistance of surface close to cathode was slightly well than the other surface, and difference of two surfaces'corrosion resistance was less than one orders of magnitude.