Conventional plasma electrolytic oxidation treatments produce oxide coatings with micron-scale discharge pores, resulting in insulation and wear and corrosion resistance far below that expected of highly dense Al₂O₃ coatings. The introduction of cathodic polarization during the plasma electrolytic oxidation process, especially when the applied cathode-to-anode current ratio (Rpn) is greater than 1, triggers a unique plasma discharge phenomenon known as “soft sparking”. The soft spark discharge mode significantly improves the densification of the anode ceramic layer and facilitates the formation of the high-temperature α-Al₂O₃ phase within the coating. Although the soft spark discharge phenomenon has been known for a long time, the growth behavior of the coating under its discharge mode still needs to be studied and improved. In this paper, the growth behavior of the coating before and after soft spark discharge is investigated with the help of the micro-morphology, phase composition and element distribution of a homemade fixture. The results show that the ceramic layer grows mainly along the oxide–electrolyte direction before the soft spark discharge transformation; after the soft spark discharge, the ceramic layer grows along the oxide–substrate direction. It was also unexpectedly found that, under soft spark discharge, the silicon element only exists on the outside of the coating, which is caused by the large size and slow migration of SiO₃²⁻, which can only enter the ceramic layer and participate in the reaction through the discharge channel generated by the strong discharge. In addition, it was also found that the relative phase content of α-Al₂O₃ in the coating increased from 0.487 to 0.634 after 10 min of rotary spark discharge, which is an increase of 30.2% compared with that before the soft spark discharge transition. On the other hand, the relative phase content of α-Al₂O₃ in the coating decreased from 0.487 to 0.313 after 20 min of transfer spark discharge, which was a 55.6% decrease compared to that before the soft spark discharge transformation.

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).

The correlation between negative pulse and the black electrolyte properties of magnesium alloy micro-arc oxidation and the treated area was investigated by introducing a negative pulse electric field. The physical phase composition, microstructure, elemental distribution, and content of the coating were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results showed that the introduction of negative pulses favored the generation of MgO and MgSiO₃ contents in the coatings, and an increase in the MgO phase was found in the coatings formed in the failed electrolytes; the microporous size and microcracks of the coatings were gradually and significantly reduced; the average consumption of Cu ions was 0.0453 g/L·dm², which is only 26% of that in the unipolar condition; the introduction of the negative pulses significantly improved the “anomalous consumption” of Cu ions. The introduction of negative pulse can significantly improve the “abnormal consumption” of copper ions, which is attributed to the change in the electric field by negative pulse, which makes the cathode-enriched Cu ions migrate to the anode and reduces the reduction and precipitation of Cu ions at the cathode.

A plasma electrolytic oxidation (PEO) coating was produced on 6061 aluminum alloy within a silicate-containing electrolyte using a bipolar pulsed power supply. The impact of K₂ZrF₆ addition on the wear resistance of the coating was investigated. The phase composition, surface morphology, and elemental distribution of the coatings were assessed by means of X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and scanning electron microscopy (SEM). Experimental data revealed that the growth rate of the coating increased by 37.3% compared to that without K₂ZrF₆; the addition of K₂ZrF₆ favored the formation of mullite and enhanced the coating densification; it also improved the breakdown voltage of the coating, which increased by 46.0% compared to that without K₂ZrF₆; and it also demonstrated excellent abrasion resistance, with a reduction of 41.8% in the weight of the abrasion.