Effects of Current Output Modes on Corrosion Resistance of Micro-Arc Oxidation Black Coatings on Aluminum Alloy

In this work, micro-arc oxidation (MAO) under constant- and gradient-current modes was used to modify the surface of 6061 aluminum alloy. A black coating was created in situ on the alloy surface by controlling the spark discharge parameters during MAO. Using an electrochemical workstation (Metrohm Autolab, PGSTAT302 N, Herisau, Switzerland), energy-dispersive spectroscopy (EDS, JEOL, JSM-IT500A, Tokyo Metropolis, Japan), and scanning electron microscopy (SEM, JEOL, JSM-7900F, Tokyo Metropolis, Japan), the effects of the current output modes on the coating growth rate, energy consumption, colorimetric parameters (L*, a*, b*), microstructure, and corrosion resistance were methodically examined. The findings showed that the gradient-current mode (6 → 4 → 2 A/dm2) greatly lowered the micropore size (from 3.89 μm to 1.52 μm) and improved the coating compactness (porosity dropped by 40%), and all coatings satisfied the necessary blackness criterion (L* < 30). Additionally, this mode achieved excellent corrosion resistance, as demonstrated by a one-order-of-magnitude reduction in the corrosion current density (2.55 × 10−8 A/cm2 vs. 2.34 × 10−7 A/cm2), while minimizing the energy consumption (2.37 kW·h/m2·μm vs. 3.45 kW·h/m2·μm for constant current).