High quality oxide-layers on Al-alloy by micro-arc oxidation using hybrid voltages

Abstract

Abstract Oxide layer was grown on 6061 Al alloy by micro arc oxidation (MAO) using a power supply of hybrid voltages: direct (V DC ) and alternative (V AC ) ones. The power supply was set at a fixed V AC voltage maintained at a value between 240 and 320 V coupled with an in-series V DC which is auto-tuned to keep the constant current density 5 A dm − 2 . This led to varying ratios of final V DC versus maximum V AC (V DC-final /V AC-max ) as the major processing parameter. Alkaline solution containing electrolytes of Na 2 SiO 3 , NaH 2 PO 2 ·H 2 O, and Na 2 WO 4 ·2H 2 O was used. All MAO samples show an oxide layer thickness around 10 μm after 30 min operation time. For samples MAO with V AC-max 320 V SEM morphology indicates a smoother oxide-layer surface with uniform micro-pores, which have mean diameters less than 1 μm. XRD patterns show the main oxide-phases of hexagonal α-Al 2 O 3 , cubic γ-Al 2 O 3 and mullite (Al 2 O 3 · n SiO 2 ). Micro-hardness of MAO samples increases from 960 HV (V AC 240 V) to 1480 HV (V AC 320 V). Corrosion resistance of the MAO layer is significantly improved with reducing V DC-final /V AC-max . Samples by V AC-max 320 V show the minimum corrosion current density 6.28 × 10 − 9 A cm − 2 , corresponding to the highest polarization resistance 2.20 × 10 8 Ω cm 2 . Comparing with MAO layers obtained by using V DC only, the oxide-layer by hybrid voltages is a little thinner, much harder and much more corrosion resistant.