Study on the Formation Mechanism of Micro-arc Oxidation Coatings on Magnesium Alloys

摘要

This article concerned with the formation mechanism of the micro-arc oxidization (MAO) coating on magnesium alloys. The micro-arc oxidation ceramic coating on AZ91 magnesium alloy was prepared in a silicate alkaline solution (Na_2SiO_3:5-30g/l, NaOH:1-5g/1, KF:5-8g/1, Na3C_6H_5O_7:0.5-2g/l, EDTA: 0.5-2g/l). The experiments were carried out in AC current galvanostatic mode with a current density of 20-50A/dm2. The microstructure of the MAO coating was characterized by means of transmission electron microscopy (TEM). Microcrystallite and/or nanocrystallite were found in the MAO coatings, the size of the former was about 50-200nm and the latter was about 20-50nm. Cellular structure was observed in the coatings, which was composed of a lot of microano grains. The grains grew preferentially in some directions during the MAO process, and the cellular structure is formed by the grains growing in a laminar way. The cellular structures also grew and aggregated by laminar way to form the whole MAO coatings.