Performance and formation mechanism of micro-arc oxidation coating using novel calcium hydroxy-methionine on magnesium alloy

The excessively high corrosion rate of magnesium alloy in human body seriously inhibits their clinical applications. Micro-arc oxidation (MAO) can improve the corrosion resistance and bioactivity of Mg alloy. However, the generated wastewater during MAO treatment may threaten aquatic ecosystems. Calcium hydroxy-methionine (MHA-Ca), one kind of amino acid-chelated Ca, was selected for the first time as the Ca source and its influences on coating formation were systematically investigated. The results show that the coating produced in the base solution with 1 g/L MHA-Ca achieves excellent corrosion resistance. MHA-Ca is highly sensitive for the increased concentration and solution pH value, and can preferentially deposits on the sample surface where local corrosion takes place. MHA-Ca inhibits small spark development due to its absorption and precipitation on sample surface. 1 g/L MHA-Ca achieves the appropriate arc-suppressing effect and improves the coating uniformity with the decreased pore size. However, 2 g/L MHA-Ca exhibits a strong arc-suppressing role and MAO coating cannot be successfully developed due to the intensive spark discharge on the sample edges. These results will provide a basis for future screening of amino-acid-chelated Ca to prepare high corrosion-resistant coatings with high bioactivity.