Effect of organic additives on structure and corrosion resistance of MAO coating

In order to improve the corrosion resistance of micro-arc oxidation (MAO) coated Mg alloys, some organic additives (3-aminopropyltrimethoxysilane (APTMS), glycerol, triethanolamine (TEA)) were doped in the silicate-based electrolyte and the microstructure and corrosion resistance were investigated. The surface and cross-section morphology feature, phase composition, elemental composition, and functional groups were examined by SEM, XRD, EDX, and FT-IR, respectively. The corrosion resistance was evaluated by electrochemical tests in 3.5% NaCl solution. The results showed that the addition of organic additives had little effect on the phase composition, while it influenced the thickness and microstructure of the coating and so improved the corrosion resistance. After the addition of APTMS, the coating thickness was increased, and the corrosion resistance became the best due to the smallest pore size, relatively dense microstructure and the formation of Mg-O-Si chemical bond with the long organic alkyl chains. The addition of TEA or glycerol makes the coating microstructure loose. Although the addition of TEA reveals larger pore size and micro-crack than addition of glycerol, its corrosion resistance was higher due to the largest thickness.