Sodium enrichment of an Al–Mg alloy surface after alkaline etching

This paper reports a case of sodium enrichment of the surface of an Al–Mg alloy that contains sodium impurities in its bulk and has been subjected to etching treatment in an alkaline solution. This phenomenon has only occurred in the case of the aforementioned Al–Mg alloy and has not been observed in other tested Al–Cu, Al–Mg–Si or pure Al type alloys, despite the fact that these also contain sodium as an impurity. The origin of the sodium ions incorporated in the aluminium oxide film that covers the surface of the Al–Mg alloy is discussed, and an explanation is suggested for this difference in behaviour compared with the other alloys.