Formation of corrosion resistant plasma electrolytic oxidation coatings on aluminium alloy with addition of sodium tungstate species

Abstract

Plasma electrolytic oxidation (PEO) coatings were formed in silicate based electrolytes without and with the addition of sodium tungstate on AA 6063 aluminium alloy. Microstructure, composition and corrosion resistance of PEO coatings were investigated by scanning electron microscopy, X-ray diffraction and electrochemical impedance spectroscopy and potentiodynamic polarisation test respectively. The effects of additive sodium tungstate were examined. The results showed that the additive containing PEO coatings were of dense structure with additional phase (WO3) and of less cracks than the additive free PEO coating. In addition, additive containing coatings were of better corrosion resistance than the additive free PEO coating, which was confirmed by electrochemical impedance spectroscopy and potentiodynamic polarisation tests. Furthermore, long time immersion test revealed that the PEO coated alloy with the addition of 12 g L− 1 sodium tungstate maintained high impedance over 82 h in 3.5 wt-%NaCl, while the PEO coating without additive was unable to protect the substrate after such long time immersion.