Plasma electrolytic oxidation of an Al-Cu-Li alloy in alkaline aluminate electrolytes: A competition between growth and dissolution for the initial ultra-thin films

Abstract

Plasma electrolytic oxidation (PEO) of an Al-Cu-Li alloy has been carried out with a pulsed-bipolar current regime in alkaline sodium aluminate electrolytes. Thick coatings can be developed under plasma discharges in electrolytes of 5g l−1 NaAlO2+1g l−1 KOH and 24g l−1 NaAlO2+1g l−1 KOH. In contrast, the alloy exhibits a conventional anodization behavior with the occurrence of galvanoluminescence in concentrated aluminate electrolyte of 32g l−1 NaAlO2+1g l−1 KOH, resulting in extremely thin nanoporous films and macropits on the alloy surface. It is proposed that, in the latter case, chemical attack from the electrolyte caused severer thinning of the original oxide film, paticularly at the weak sites of anodic constituent particles or precipitates, which then triggers strong field-assisted dissolution of the oxide film, preventing the further growth of the coating. However, after the formation of a thin pre-existing film(∼ 1-2μm) on the alloy in less concentrated electrolytes, then thick coatings develop more rapidly in 32g l−1 NaAlO2+1g l−1 KOH, since the field-assisted dissolution is only prevalent in extremely thin films and retreats quickly as the film thickens. Long-term immersion in aluminate electrolytes can eliminate all the weak points on the alloy surface and the oxide film is repaired, thus thicker PEO coatings can also be developed in concentrated electrolytes.