Effect of acid traces on hydrothermal sealing of anodising layers on 2024 aluminium alloy

The corrosion properties of hydrothermally sealed anodised 2024 aluminium alloy specimens in tartaric–sulphuric acid (TSA) and sulphuric acid (SA) are investigated using electrochemical techniques. A new mechanism is proposed to describe the improvements observed for the sealed porous layer grown in the mixed electrolyte. This mechanism involves the dissociation of the residual organic acid present inside the pores as a consequence of the pH increase during hydrothermal sealing (HT). Tartrate ions form chelate complexes with Cu(II) cations embedded in the oxide film. This results in a decrease in the heterogeneity of the film and consequently, better protective properties.