Effect of electrolyte additives on anti-corrosion ability of micro-arc oxide coatings formed on magnesium alloy AZ91D

Abstract

Oxide coatings on AZ91D alloy were prepared using micro-arc oxidation techniques at a low applied voltage, in electrolyte solutions with hexamethylenetetramine and sodium borate additives. The different electrolyte solutions changed the applied voltage from 400 to 200 V and reduced the micro-pore size of the oxide coating on the AZ91D alloy. The corrosion current of the oxide coating changed from 3.6 × 10 − 5 to 4.2 × 10 − 7 A/cm 2 when the additives were added to the plating solution. When sodium borate was added to the plating solution, the Na content of the oxide coating was increased, improving its anti-corrosion ability. Moreover, the surface roughness (Ra) was reduced from 10.12 to 1.22 by the addition of 0.1 M hexamethylenetetramine to the electrolyte. The cracks in the oxide coating were also smoothed by the addition of 0.1 M hexamethylenetetramine, reducing the thermal stress. In this work, the addition of the aforementioned additives to the alkaline silicate phosphate electrolyte reduced the applied voltage and modified the oxide coating, resulting in better anti-corrosion ability.