Inhibitive Effect of Sulfate and Phosphate on the Pitting of Type 304 Stainless Steel in High Temperature NaCl Solution

Effect of sulfate and phosphate on the pitting behavior of type 304 stainless steel has been studied in chloride solutions at 423 and 523K. The inhibitive work was evaluated in terms of concentration (or activity) of inhibiter required to inhibit pitting as a function of chloride concentration. The relation was expressed conventionally as log [Cl-]=αlog [inh]+β, where α and β, based on the molarity at 298K, were 0.64 and -1.07, respectively, at 423K and 0.67 and -0.95, respectively, at 523K. The values for phosphate were 1.03 and 0.01, respectively, at 423K. The relation for sulfate at 423K is restricted to 0.5M of sulfate, above which pitting is rather promoted possibly because of the increase of activity coefficient of chloride ion and of the deviation from the adsorption isotherm. Increase in temperature requires more concentration of inhibiter to inhibit pitting; β decreases with temperature while α is not very sensitive to it.