Influences of edge effect on microstructure and corrosion behaviour of PEO coating

Abstract

Plasma electrolytic oxidation (PEO) coatings were prepared on AA1060 alloy substrate by a galvanostatic regime in an alkaline sodium hexametaphosphate ((NaPO3)6) electrolyte, and the influences of electric field edge effect on the microstructure, surface roughness, semiconducting behaviour and corrosion behaviour of the coatings were investigated by means of scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM), Mott–Schottky analysis and electrochemical impedance spectroscopy (EIS) tests. The results showed that an ordered porous film was formed before the presence of micro-discharges during the PEO treatment. The electric field strength at the specimen edge region was about twice as large as that in the specimen centre region, which resulted in the differences in the microstructure, surface roughness and corrosion behaviour of the specimens at the edge and centre regions. All PEO-coated specimens showed n-type semiconducting behaviour, and the coating at the edge region had a lower carrier concentration.