Potentiodynamic and cyclic voltametric studies on the Passivity of Tin in neutral phosphate buffer

Passivity of tin has been investigated in phosphate buffer (pH 6.7) using potentiodynamic and cyclic voltammetric techniques. The formation of thin transparent electronically conducting passive film on which O₂ is evolved in the transpassive range is indicated. The solution remains clear. Potentiodynamic measurements performed from −1.2 to 2.5 V (SCE) and backward at scan rates between 1 and 100 mV/sec give one anodic peak. In the reverse scanning, two cathodic peaks appear at low scan rates and disappear completely at higher rates. Values of the open dircuit potential, the corrosion potentials Ecor (forward and reverse), the anodic peak potential Ep and the anodic peak cd Ip are recorded. The results are compared with our previous work in neutral chloride, bromide, iodide, and sulphate solutions. Thermodynamic data are used to predict the possible oxidized species constituting the anode film. Cyclic voltammetric measurements have been carried out in the potential range between H₂ and O₂ evolution, and in limited potential ranges at scan rates between 1 and 200 mV/sec. One anodic peak is detected in all cases. While a short arrest and a cathodic peak appear in the results between H₂ and O₂ evolution, two distinct cathodic peaks appear up to 100 mV/sec in the results taken between –1.2 and −0.224 V. Plots of Ip vs the square root of scan rate are linear, and indicate diffusion-controlled kinetics for the reactions initiating passivity. Linear relations are observed for Ep vs log scan rate indicating the irreversible nature of the anodic processes. While the formation of SnO and SnO₂ is predicted from thermodynamics at potentials more negative than Ep, the formation of Sn(OH)₄ is predicted at potentials more positive than Ep. The conclusions drawn from the results obtained with the two different techniques are generally the same. The results are compared with and discussed on the light of previous work using cyclic voltammetry.