Anodic oxidation of aluminium in sulphuric acid containing aluminium sulphate or magnesium sulphate

Abstract The effect of Al 2 (SO 4 ) 3 or MgSO 4 in the sulphuric acid electrolyte on the formation and dissolution of anodic films on aluminium has been investigated. When the sulphate concentration was increased, kinetic viscosity of the electrolyte became higher and the anode surface temperature during anodizing rose gradually. Increase in the concentration of the sulphate suppressed chemical dissolution of the anodic film. As the sulphate concentration was increased, hardness of the film generally increased at first and then decreased, while the coefficient of static friction of the film decreased. From measurement of microroughness and electro-microscopic observations, it was found that by the addition of the sulphate, the surface became more microscopically even and the porosity near the outer surface was reduced. The uniformity of the film was decreased with increased in sulphate concentration.