EFFECTS OF TEMPERATURE AND POTENTIAL ON THE INHIBITIVE ACTION OF OXOACID SALTS FOR PITTING IN HIGH-TEMPERATURE CHLORIDE SOLUTIONS

The effects of several oxoacid salts (sulfate, molybdate, phosphate, a nd nitrate) on pitting behavior of type 304 (UNS S30400) stainless ste el (SS) were examined to characterize their performance in high-temper ature chloride solutions through measurement of pitting temperatures ( T-p) and potentials (E-p). Measurement of E-p at a given temperature, as usually adopted, did not provide sufficient information to characte rize oxoacid salts as pitting inhibitors because the unique potential dependency for the inhibitive action could be missed, as was the case for nitrate. In addition, whether pitting occurred on heating could no t be predicted because pitting could not be inhibited as effectively a t lower temperatures as at higher temperatures, as in the case of moly bdate and phosphate. Temperature-vs-current density (CD) curves at giv en potentials were extended beyond 373 K using an autoclave system and were measured in addition to potential-vs-CD curves. The combined dat a provided extensive information on the inhibitive action of each oxoa cid salt. Sulfate seemed to work through adsorption, while molybdate a nd phosphate were incorporated into the passive film, which proceeded more extensively at higher temperatures. Nitrate had a critical potent ial to inhibit pitting below which it scarcely worked, although its ro le was not understood satisfactorily. This explained why nitrate did n ot shift E-p at 423 K as extensively as at 298 K, where the E-p in 0.5 M sodium chloride (NaCl) solution was as low as -70 mV vs a standard hydrogen electrode (SHE).