Plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate-tungstate electrolytes and the coating formation mechanism

Abstract

Plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy under pulsed bipolar regimes has been carried out in an aluminate electrolyte with the addition of 0–25 g l−1 Na2WO4·2H2O. Black coatings are formed with the addition of tungstate. Sequential anodizing has also been adopted to investigate the coating formation mechanisms by tracing the elemental distribution of W and Al in the coatings. The coatings develop an outer layer, inner layer and a barrier layer after a certain period of PEO. At the later stage of the PEO, the coating grows inwardly, which was accompanied by the strong penetrating discharges. The penetrating discharges have caused significant anion deposition, and the electrolyte species, such as W and Al, can be transported to the coating/substrate interface instantly. The anodic current density within the penetrating discharge channels is estimated to be ∼104 A cm−2, which is high enough to melt the coating materials beneath the pancake structure and cause the direct thermal decomposition of water and hence the anomalous gas evolution reported for PEO. X-ray photoelectron spectroscopy (XPS) denies that free state W exists in PEO coatings.