The Investigation of the Electrode Soft-Mode Impact of Micro-Arc Oxidation (MAO) on the Structure, Phase Composition and Corrosion Properties of AlCa2.5Mn1.5 Aluminum Alloy Sheets with MAO Coating of Different Thicknesses

Coatings with thicknesses of 25, 50, and 100 μm were formed in an alkaline-silicate electrolyte at a constant anodic current density of 10 A/dm2 in the Soft-mode. A multi-technique approach, including SEM-EDX, XRD, EIS, and microhardness testing, was employed to characterize the coatings. The coatings with thicknesses of 50 and 100 μm consisted of three layers: a base layer, a pancake layer, and an outer porous layer. The outer layer of the coating is enriched in silicon (from electrolytes) and alloying elements of the substrate, due to the occurrence of micro-arc discharges, type-B discharges, which eject plasma containing these elements into the surface layers of the coating. Phase analysis reveals a progressive increase in the α-Al2O3 modification with an increase in coating thickness, which correlates with enhanced microhardness. Electrochemical testing in a 3% NaCl solution demonstrates that the 100 μm Soft-mode coating provides the highest corrosion protection, significantly outperforming both thinner coatings and Arc-mode counterparts. In the Soft-mode, small “external” discharges organize into moving groups, enabling uniform thickness build-up. A model of these discharges is proposed, interpreting them as “type B discharge light” that does not reach the surface due to the presence on the surface of the “pancake-like” layer formed by type-B discharges of a hydrogen-saturated porous outer layer.