Delay in micro-discharges appearance during PEO of Al: Evidence of a mechanism of charge accumulation at the electrolyte/oxide interface

Abstract

PEO was conducted on Al by applying a pulsed bipolar current. The role of the cathodic polarization on the appearance of micro-discharges (MDs) and on the subsequent formation of the PEO oxide layers is investigated. Various ratios of the charge quantity RCQ = Q p / Q n (defined as the anodic Q p to cathodic Q n charge quantity ratio over one current pulse period) in the range [0.5 ; 6.0] were selected by changing the waveform parameters of the cathodic current while keeping the waveform of the anodic current unchanged. Results show that the appearance of MDs is delayed with respect to the rising edge of the anodic current; this delay strongly depends on both the processing time and the applied cathodic charge quantity. It is also evidenced that shorter delays promoted by high RCQ (RCQ > 1) values are associated with stronger MDs (large size and long life) that have detrimental effects on the formed PEO oxide layers. The thicker and the more compact oxide layer morphology is achieved with the intermediate RCQ (RCQ = 0.9) for which the delay of the MDs appearance is high and the MDs softer. Low RCQ (RCQ < 0.9) results in an earlier extinction of the MDs as the process goes on, which leads to poorly oxidized metal. A mechanism of charge accumulation taking place at the oxide / electrolyte interface and arising before the occurrence of dielectric breakdown is proposed to explain the ignition of MDs during pulsed bipolar PEO of aluminium. A close examination of the voltage-time response which can be adequately simulated with an equivalent RC circuit evidences the capacitive behaviour of the oxide layer and therefore confirms this proposed mechanism of charge accumulation.

Review of the Soft Sparking Issues in Plasma Electrolytic Oxidation

“…This observation is consistent with the general perception that microdischarges occur during anodic polarization, but rarely in cathodic polarization. Even more importantly, Figure 4 shows that the light emission is strongly affected by the R pn ratio [50]. If the light emissions at 30 min are compared, R pn = 0.5, 0.9, 1.5, 1.6, 6.0, the microdischarge activity is evidently suppressed at a low R pn ratio.…”

“…Using a pulse bipolar power, instead of AC, Martin and co-workers [50] recently substantiated many T&M inferences and filled in extra data on the substrate of 2214 aluminum alloy in a prototype solution. They further proved that setting a lower R pn ratio, expressed in Q p /Q n in Figure 4, delayed the microdischarge appearances in the anodic period and reduced the plasma intensity.…”

“…Five PEOs have been operated with Q p /Q n = 0.5, 0.9, 1.5, 1.6, 6.0 using a bipolar pulse current, with the voltages, currents, light emissions measured at 1, 8, 30 min. The preceding cathodic pulse delays the microdischarge activity of its following anodic pulse [50].…”

Phosphate Porous Coatings Enriched with Selected Elements via PEO Treatment on Titanium and Its Alloys: A Review

“…Other ways of modifying the surface are ion implantation [26][27][28][29][30][31], electrophoretic deposition [32], and laser surface treatment [33][34][35][36]. Development of materials engineering and technology in the following years has allowed new challenges to be posed regarding surface engineering, including, but not limited to, fabrication of porous coatings by the Plasma Electrolytic Oxidation (PEO) or Micro Arc Oxidation (MAO) [37] on light metals and alloys, such as aluminum [38][39][40][41][42][43][44] and its alloys [45][46][47][48][49][50][51][52][53][54], magnesium [38,39,[55][56][57] and its alloys [58][59][60][61][62][63][64], titanium and its alloys [57,[65][66][67][68][6...…”

Duplex surface treatment of metallic alloys combining cold-spray and plasma electrolytic oxidation technologies

“…The occurrence of this "soft" regime is associated with the gradual decrease in the voltage response, the gradual disappearance of visible discharges as well as an increase in the growth rate and a lowering of the oxide coating porosity [30].…”