| Abstract: | Passivation of spec-pure Sn was studied in aerated, unstirred solutions of acid sulphate (H2SO4/Na2SO4) and neutral suphate (Na2SO4) using the potentiodynamic technique from an initial cathodic potential (?ve to open circuit potential) in the anodic direction and backwards to the initial potential. Only one anodic peak is observed in all results. The effect of etching is manifested in pure 0.1 M H2SO4 and in the solution containing 75% acid and 25% by vol 0.1 M Na2SO4. In all solutions, a grayish white film devlops on the Sn anode, and a white precipitate is formed in solution. Data are complied for the corrosion potential Ecor, the cathodic and anodic Tafel slopes, the corrosion cd Icor, the peak potential and the peak cd for acid sulphate solutions. The effect of scan rate has been demonstrated in 0.1 M Na2SO4 in the range 1-100 m V/sec for unstirred and 1-50 m V/sec for stirred solutions. The plot of peak cd Icc (Ip) vs the square root of the scan rate gives a striaght line with positive slope indicating diffusion kinetics for the anode film formation. USe of thermodynamic data leads to the possible formation of SnO, SnO2, Sn(OH)2 and Sn(OH)4 in the anode film. The limitations of the thermodynamic treatment is discussed. It is concluded that film formation is governed by a dissolution/precipitation mechanism based on diffusion kinetics. In the absence of the results of surface analysis, the anode film may be visualized to be composed of the oxides and hydroxides of Sn (thermodynamic evidence) together with sulphate anions (kinetic evidence). O2 evolution has not been detected up to 2.5 V (SCE), and this throws some light on the poor electron conductance of the anode film, and the probable dominance of lonic conductance during film growth. |
