Effects of Chromate Yellow-black Oxidation Interval on Quality and Corrosion Resistance of AZ91D Film Layer
In order to further optimize the preparation process of the chromate conversion coating,the corrosion resistance of AZ91D magnesium alloy was improved by adjusting the time interval between yellow oxidation and black oxidation.The film morphology,phase composition,element distribution and electrochemical characteristics of alloy samples with time intervals of 0 h,12 h and 24 h were systematically studied,and the black oxidation film-forming behavior and reaction mechanism after different time intervals were revealed.The results show that a micron scale conversion film with network microcrack structure is formed on the surface of AZ91D alloy after black oxidation.If the time interval is too short,the hydrogen evolution of the matrix significantly destroys the integrity of the black oxide film,the microcrack size is large,and the film thickness is relatively thin.However,the time interval is too long,the substrate oxide film is too dry to adhere the conversion solution.The black film cracks are reduced,but the surface is rough,and a large amount of Mn3O4 particles are deposited.When the interval time is 12 h,the surface coating of AZ91D alloy is relatively uniform and dense.The electrochemical test shows that the corrosion resistance of the sample is the best at this time.Compared with untreated magnesium alloy,the self-corrosion current density decreases by two orders of magnitude,and the self-corrosion potential is moving to-1.176 V,increased by 19.51%.
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