Improving the Chemical Stability of Al Alloy through the Densification of the Alumina Layer Assisted by SiF 6 2- Anion Hydrolysis.

In this work, a high-density alumina layer with high chemical stability was successfully developed by controlling the hydrolysis of hexafluorosilicate (SiF 6 2- ) anions through the addition of various concentrations of sodium citrate (SCi) into the electrolyte of plasma electrolysis (PE). To achieve this aim, the substrate samples were anodized in alkaline aluminate-SiF 6 2- -based electrolytes with 0, 5, and 10 g/L of SCi. The presence of SCi anions in the electrolyte led to the formation of a thick adsorbed electrochemical double layer (EDL) on the substrate surface. The EDL not only affected the movement of SiF 6 2- anions towards the anode but also influenced their hydrolysis reaction, which in turn led to a controllable sealing of structural defects with the hydrolysis products, namely SiO 2 and AlF 3 . Among three different oxide layers, the oxide layer obtained from the electrolyte with 5 g/L SCi showed the highest chemical stability in a corrosive solution, which was linked to the fact that a considerable increase in the compactness of the oxide layers was obtained by the incorporation of SiO 2 and AlF 3 . The mechanism underlying the effects of SCi on triggering the hydrolysis of SiF 6 2- anions and factors affecting chemical stability are discussed based on the experimental data and computational analysis.