Influence of electrolyte ageing on the Plasma Electrolytic Oxidation of aluminium

The chemical stability of the electrolyte in Plasma Electrolytic Oxidation (PEO) has been the subject of much systematic investigation. The present study reports on experimental results on the ageing of a usual PEO electrolyte and its effect on the oxidation of the Al2214 aluminium alloy. A solution of potassium hydroxide ([KOH]=1gL−1) and sodium silicate ([Na2SiO3]=1.65gL−1) diluted in deionised water was either aged by running a process during 8h at most or not. Comparison between materials treated in either solution was made next. Cross-checked experimental results from analytical chemistry, plasma diagnostics and materials science clearly evidence the effect of ageing of the electrolyte on the PEO process behaviour. The transition from arc to “soft” regime occurs earlier (from 18min to 13min processing time) as the electrolyte gets older. The concomitant decrease in the electrolyte electrical conductivity (from 7.5mScm−1 to 6.8mScm−1) is correlated with changes in the ionic species content in the electrolyte. Fast video-imaging (125kfr/s) reveals that the micro-discharge characteristics are also sensitive since they become less numerous per area and unit time (from 220×103cm−2s−1 to 120×103cm−2s−1), exhibit shorter lifetime and have smaller sizes as the electrolyte ages. SEM observations of the as-grown coatings combined with roughness measures indicate a decrease in coating thickness (~20%) and roughness (~30%) while species from the electrolyte accumulate in the outer porous layer simultaneously with a gradual enrichment in α-Al2O3 phase in the inner compact layer (from 36% to 43%). Finally, within the PEO conditions that were used, it is shown that the ageing phenomenon of the PEO electrolyte starts to influence the process after 2h aged. •The chemical ageing of a usual KOH/Na2SiO3 PEO electrolyte is investigated.•Chemical ageing of the electrolyte becomes effective after 2–3h of processing.•As the electrolyte gets aged, the beneficial PEO “soft” regime appears earlier.•Oxide layers become thinner and more compact with aged electrolytes.•The development of the crystalline α-alumina is promoted with aged electrolytes.