A Study of the Electrolyte Composition Influence on the Structure and Properties of MAO Coatings Formed on AMg6 Alloy

Abstract

The influence of electrolysis conditions at different electrolyte compositions on the phase formation and properties of coatings obtained by microarc oxidation (MAO) on an aluminum alloy AMg6 was studied. For electrolysis, three types of electrolytes were used: alkaline electrolyte ((KOH) solution in distilled water), silicate electrolyte (with different percentages of Na 2 SiO 3 component) and complex alkaline silicate electrolyte with liquid glass (1÷12 g/l Na 2 SiO 3 ) and potassium hydroxide (1÷6 g/l KOH). An analysis of the results showed that the choice of electrolyte type and conditions of the microarc oxidation process allows a wide variation in the phase-structural state, thickness and properties of the AMg6 aluminum alloy. The criterion for the expected phase-structural state of the coatings as a result of microarc oxidation is the completeness of the γ–Al 2 O 3 →α–Al 2 O 3 transformation process during coating formation. The use of an alkaline electrolyte does not allow achieving a high hardness of the coating due to the formation of the γ-Al 2 O 3 phase and the absence of thermodynamic conditions for the γ–Al 2 O 3 →α–Al 2 O 3 transition. When using a silicate electrolyte, it is possible to significantly increase the growth rate of the coating, but at the same time, the presence of a large specific Si concentration stimulates the formation of mullite and an amorphous-like phase. The use of a combined alkaline silicate electrolyte (with different percentages of KOH+Na 2 SiO 3 ) with a low content (6 g/l) of Na 2 SiO 3 in solution stimulates the formation of mullite. This is manifested to the greatest extent with the lowest content (1 g/l) of the KOH component. At a higher content (2 g/l) of the KOH component, the processes characteristic of an alkaline electrolyte become dominant. This leads to an incomplete transformation reaction and the formation of only the γ-Al 2 O 3 phase. The achievement of the thermodynamic conditions of the γ–Al 2 O 3 →α–Al 2 O 3 conversion became possible with an increase in the specific Na 2 SiO 3 content in the electrolyte solution to 12 g/l. In this case, MAO coatings were formed on the AMg6 alloy with the highest hardness of 1500 kg/mm 2 and high electric strength of 12 V/μm