The corrosion and passivation of tin in borate solutions and the effect of halide ions

The corrosion and passivation of tin in sodium borate (Na2B4O7) solutions at pH = 8.35 were studied by potentiodynamic and cyclic voltammetry techniques. The effect of the following factors on the anodic dissolution and passivation of tin metal are discussed: concentration of borate ion, pH, potential scan rate, successive cyclic voltammetry, switching potential and progressive additions of halide ions. The primary passivation is attained when the metal is covered with a film of Sn(OH)2 and/or SnO. Permanent passivity occurs when Sn(OH)4 and/or SnO2 forms a continuous layer on the electrode surface. The anodic dissolution reactions are controlled by diffusion. In all experiments, the aggressive action of the halides decreased in the order Cl− > Br−. SEM examination confirms the occurrence of pitting corrosion in presence of halide ions. The pitting potential (Epit) decreases with increasing halide concentration.