Polarity Reversal of Iron and Tin in Aqueous Malic Acid

Isolated Open Circuit Potential (OCP) measurements are performed on pure tin and iron in aqueous malic acid solutions (1 - 2x10-3mol dm-3). In de-aerated aqueous malic acid solutions ≥1.56x10-2mol dm-3 tin was found to become sacrificial with respect to iron, reversing the polarity predictions on the basis of tabulated EOCP values. In the presence of oxygen polarity reversal is not established. Additions of Sn2+ >10-4 mol dm-3 to aerated 0.1 mol dm-3 aqueous malic acid influences iron to be cathodic with respect tin. However, in the absence of oxygen tin becomes cathodic with respect to iron in 0.1 mol dm-3 malic acid solutions containing Sn2+ >10-4 mol dm-3. Voltammetric measurements indicate a self-limiting displacement mechanism of Sn2+ on the iron surface. Ex-situ XPS surface measurements confirm tin oxide coverage on iron following short immersion times in 0.1 mol dm-3 malic acid containing Sn2+ = 10-3 mol dm-3.