Effective regime of microarc oxidation (MAO) on AZ 91 magnesium alloy

Abstract

8 1 At the present time, the relative volume of produc� tion and use of magnesium alloys is continuously growing due to their lower density compared to other structural materials (1). Nevertheless the use of parts made of these Mg alloys require anticorrosive coatings on their surfaces. To obtain such protective coatings one of the most promising method consists of Microarc Oxidation (MAO) (2, 3) with a subsequent sealing or varnishing and painting of the surface since microarc coating (MAC) mostly serves as a high� porous layer for further finishing treatment on magne� sium alloys products. It was established (4-6) that the addition of fluo� rinecontaining chemical components in basic elec� trolytes leads to the improvement of the mechanical and tribological properties of microarc oxidation coat� ings formed on the surface of magnesium alloys, increases the efficiency of the MAO process and con� sequently reduces the power consumption of the pro� cess. In this work (3) it is shown that within alkaline phosphate electrolytes, the growth rate of microarc oxidation coating on the surface of AZ91 magnesium alloy is significantly higher (not less than 1.4 times) for a process conducted in the anodic regime than in the anodiccathodic regime. Thus, at the stage of sparking discharges, growth coverage is slowed down with the advent of microarc discharges on a surface of the working electrode. It is obvious that the performance of the process of microarc oxidation of magnesium alloys increases sig� nificantly if it is conducted by passing a direct current between the electrodes in basic electrolytes with the introduction of fluorides. Therefore, the main objec� tive of this work is the investigation of the process to obtain MAO precoats on the surface of magnesium 1 The article was translated by the authors. alloys. This work also aims at establishing technologi� cal parameters that allow us to conduct the process with higher performances. Therefore the optimal con� centration of ammonium fluoride in the base alkaline phosphate electrolyte, the current density and the shape of the current waveform in galvanostatic mode are investigated