The formation process of calcium tripolyphosphate films on stainless steel electrodes from aqueous solutions containing Na₅P₃O₁₀ and Ca (NO₃) ₂ has been studied by the use of galvanostatic method. It is shown that pH of aqueous solutions containing Na₅P₃O₁₀ increases with increasing temperature. The increase in pH is due to the change in the stability constant of H₅P₃O₁₀ with temperature and due to its hydrolysis.
The rate of film formation depends on the degree of supersaturation of calcium tripolyphosphate in solutions. The rate of the film formation increases with an increase in pH, since the degree of supersaturation increases with increasing pH. Too much increase in pH, however, causes a decrease in the film formation rate because of the suppressing effect of a precipitation on the supersaturation.
The films can be formed even at a very low current density when pH is adjusted to appropriate values.