Anodic Behavior and Passivation of Tin In Solutions Containing Phosphoric Acid

A study has been made of the anodic behavior and passivation of tin in phosphoric acid solutions. Effects on passivation when the solution additionally contains nitric acid or potassium nitrate have also been noted. Under the conditions of these experiments, a stannic oxide film normally covers the tin electrode. Film porosity amounts to 2.6 percent of the total apparent area of the electrode surface. Passivation time, and hence the dissolution rate, of tin anodes in phosphoric acid solutions is hardly affected by temperature within the range 20-60 C, but increases linearly at higher temperatures. Inclusion of low concentrations of nitric acid or potassium nitrate (the nitrate ion) in the phosphoric acid bath showed an activating effect on the tin electrode. At higher concentrations of the nitrate ion, passivation is attained more quickly through chemical means, but a very porous film is formed; thus, continued dissolution of metal is not avoided. 3.8.3, 6.3.14