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Na2CO3―induced Gas Evolution Reaction and Morphology Modulation on Magnesium Alloy during Micro―arc Oxidation

Author(s): WANG Xiao-Bo, TIAN Xiu-Bo, GONG Chun-Zhi, YANG Shi-Qin

Journal: Journal of Inorganic Materials
ISSN 1000-324X

Volume: 26;
Issue: 7;
Start page: 721;
Date: 2011;
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Keywords: micro-arc oxidation; gas evolution; magnesium alloy; sodium carbonate; modulation effect

Surface morphology of the ceramic coatings can be modulated by gas evolution on anodes during micro arc oxidation (MAO) processes. A novel technique to enhance gas evolution using Na2CO3―containing solution was proposed. MAO ceramic coatings were fabricated on AZ31 magnesium alloy using Na2CO3 as a special additive in Na3PO4+KOH +NaF base electrolyte. The effects of Na2CO3 addition on the reaction of gas evolution near the anode, morphology modulation and chemical composition were investigated by using GC―MS, SEM, XRD and FTIR, respectively. The results show that both O2 and CO2 are evolved from the anode with Na2CO3 addition during MAO process. MgCO3 is found in the coating during the anodic oxidation before MAO, while only MgO is detected in the coating after MAO process. It is attributed to the decomposition of MgCO3 to MgO and CO2 due to local high temperature. Na2CO3 addition has slight effect on the microstructure of the coating and MgO is the main phase no matter whether Na2CO3 is utilized. With the addition of Na2CO3 the number of larger―size pores decreases and porosity in the coatings increases.
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