Alternating current oxidation of Ti–6Al–4V alloy in oxalic acid for corrosion resistant surface finishing

Abstract

In this work, alternating current (AC) voltage (VAC) is applied to Ti–6Al–4V alloy in aqueous oxalic acid dihydrate solution to grow passive oxide films. The oxide layers are formed with VAC values in the range of 10–80 V. The resulting surface oxide layers have estimated thicknesses in the 100–500 nm range and are expected to have a two-layer structure consisting of a porous outer layer and a compact inner layer. The AC anodization process is demonstrated to have a slight effect on the surface roughness. Porous regions are observed on the films formed at lower and high AC voltages (i.e., VAC 10–30 and 80 V). The corrosion behavior in Ringer’s solution over 48 h of exposure is studied with electrochemical impedance spectroscopy. The AC anodized samples have higher impedance values than the untreated alloy by more than an order of magnitude. The oxides formed at VAC = 40 − 70 V exhibit optimal resistance to corrosion as demonstrated by the overall impedance values, the polarization resistance values, and the capacitive behavior.