Morphological evolution and burning behavior of oxide coating fabricated on aluminum immersed in etidronic acid at high current density

Micro arcs on metal surface immersed in alkaline electrolyte have been studied by many works, but that in acidic electrolyte are rarely reported. In this work, anodizing of aluminum was carried out in etidronic acid (HEDP) at high current density to investigate the formation behavior of coating. SEM images indicate that the burning of initial film is actually caused by local oxide cracking due to the increase of compressive stress. The soft spark would appear when voltage reached 300 V, then change to the micro arcs at 400 V, and finally manufacture the outer micro-arc oxide (MAO) layer. Hereafter, the automatic extinguishing of micro arcs gives rise to the formation of inner anodic aluminum oxide (AAO) layer. Moreover, it is evidenced that plasma discharge has few contributions to coating thickening. The barrier layer provides the raw materials for the growth of outer MAO layer. Based on these results, the growth behavior of coating processed in HEDP at high current density is proposed and discussed. •Anodizing and mico-arc oxidation in etidronic acid at high current density•Initial anodic film burning is caused by stress-induce-cracking instead of discharges.•Alternation of micro-arc oxidation and anodzing leads to the MAO/AAO composite coating.•Plasma discharges have few contributions to coating growth thickening.•Growth kinetics of MAO/AAO composite coating comes from the ion migration.