Investigation of the Passivity of Tin in Neutral Media by cyclic galvanostatic polarization

In order to throw more light on the passivity of Sn in neutral media, two types of measurements have been performed in aerated unstirred KCl, Na₂SO₄ solutions (both 0.1, 0.5, 1.0, and 2.0 M) and in phosphate buffer (pH 6.6–6.7 for all), namely open-circuit measurements and cyclic galvanostatic polarization. The direction of change of the open-circuit potential with time in both KCl an Na₂SO₄ solutions indicates film repair and passivation (positive shift in potential). Addition of either KCl or Na₂SO₄ to phosphate buffer produces two opposite effects: (i) a positive potential shift in the case of chloride, and (ii) a negative potential shift in the case of sulphate (film destruction and activation). The behaviour of cyclic galvanostatic results (at 0.8 mA/cm²) differs considerably in phosphate buffer than in chloride and sulphate solutions. Thus, while in phosphate buffer the anodic half-cycle shows a passivation arrest and an anodic peak, the same half-cycle is manifested by a plateau in both chloride and sulphate solutions. Moreover, two cathodic arrests have been detected in the cathodic half-cycle in phosphate buffer, while three cathodic arrests are observed in the same half-cycle in chloride and sulphate solutions. The presence of cathodic arrests in chloride and sulphate solutions indicates the reduction of oxidized species originally formed during the anodic plateau. Data are compiled for the potential and duration of the arrests and plateaus in all three solutions, and the effects of electrolyte concentration and the number of successive cycles on these parameters are discussed. Although the combination of electrochemical results and thermodynamic calculations can only serve as an approximate indication to the possible occurrence of stoichiometric Sn(IV), Sn(II) oxides and hydroxides in the passive film such as in the case of phosphate buffer, yet the exact composition of the film can only be ascertained by surface analysis (eg XPS). However, electrochemical and thermodynamic means have been helpful in the present work in predicting the possible occurrence of species other than oxides and hydroxides (such as incorporated anions and oxy-salts) in the anode film formed in chloride and sulphate solutions in agreement with our previous potentiodynamic work. Although surface analysis can indicate the presence of elements and their oxidation states, yet no work has been cited in the literature on the calculations relevant to the presence of oxysalts in the film, and hence electrochemical and thermodynamic means are more helpful in this respect.