Anodic Film Formation on Zinc in Alkaline Electrolytes Containing Silicate and Tetraborate Ions

The reactivity of zinc in sodium hydroxide electrolytes containing sodium silicate and sodium tetraborate was studied by electrochemical and microstructural examinations. Impedance measurements indicate that a porous surface layer is formed on zinc immersed in sodium hydroxide at its open circuit potential and on polarization in the prepassive region. This surface layer becomes compact at potentials in the passive region. The anodic reactions of a zinc electrode in a sodium hydroxide containing electrolyte are controlled by the mass transport of zincate ions away from the electrode. Silicate and tetraborate ions suppress the anodic dissolution of zinc. In the presence of silicate ions, a compact silicate containing film is formed upon anodic polarization. The electrode rotation affects the microstructure of the surface layer both in silicate-free and silicate-containing sodium hydroxide solutions. On addition of tetraborate ions, the polarization resistance increases and the formation of a surface layer is achieved at increased applied potential.