Electrochemical Polarization and Passivation of Tin in neutral solutions of chloride,bromide and iodide ions

Passivation of commercial grade tin (99.5%) was studied in 0.1 M KCl, KBr and KI solutions using the potentiodynamic technique involving cathodic and anodic polarization at various scan rates. While low scan rates (0.1 mV/sec) lead to the complete deterioration of the anode in chloride solutions, higher scan rates of 1 and 5 mV/sec lead to the appearance of two anodic peaks. These peaks disappear at still higher rates (10 mV/sec). In bromide solutions, two anodic peaks appear with scan rates of 1 and 5 mV/sec, and only one peak is observed with 50 mV/sec. A white precipitate is formed in both chloride and bromide solutions during anodic scanning. No gas evolution (O₂, Cl₂ or Br₂) could be detected up to anode potentials of 3–4 V (SCE). At scan rates of 1 mV/sec and higher values, pitting does not occur, and thick anode films are formed. The film is gray in chloride solutions, and yellowish gray in bromide solutions. Values of the corrosion potential Ecor are recorded for the forward and reverse scanning. The corrosion cd Icor is calculated both by the Tafel extrapolation procedure and by the linear polarization technique. The latter gives more accurate results. In Cl^? and Br^? solutions, the first anodic peak associated with passivation is more anodic than the equilibrium potentials for the tin/tin oxides or hydroxides. Passivation is explained as a result of the formation of the sparingly soluble salts Sn(OH)Cl, Sn(OH)Br which appear both in the precipitate and in the anode film. The passivating films are constituted at least partially of these basic salts, and contain a proportion of the oxides and hydroxides of Sn. From the effect of scan rates on the characteristics of the anodic peaks, it appears that the formation of thick anode films is governed by the diffusion kinetics for porous films. Of the three possibilities considered as explanations for the second anodic peak (oxidation of impurities, oxidation of halides to oxyacids, and structural changes in the film) the latter appears to be the most probable one. Thick anodic films are not formed in iodide solutions (scan rates 1, 5, 10 mV/sec), and the anode surface remains very bright. Dense evolution of I₂ occurs during anodic polarization, and apart from I₂ which dissolves in solution, the latter remains clear. Only one peak is observed in the forward anodic scanning. The polarization resistance technique gives Icor = 0. Thermodynamic calculations show that I₃^?, HIO, IO^?, HIO₃, IO₃^?, HIO₄, and IO₄^? occur in 0.1 M KI solutions at potentials less anodic than the potential of the anodic peak (1.32 to 1.37 V, vs SCE). This peak potential is more anodic than the quilibrium potential for I₂ evolution.