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, and nitrate) on pitting behavior of type 304 (UNS S30400) stainless steel (SS) were examined to characterize their performance in high-temperature 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 characterize 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 not be predicted because pitting could not be inhibited as effectively at lower temperatures as at higher temperatures, as in the case of molybdate and phosphate. Temperature-vs-current density (CD) curves at given potentials were extended beyond 373 K using an autoclave system and were measured in addition to potential-vs-CD curves. The combined data provided extensive information on the inhibitive action of each oxoacid salt. Sulfate seemed to work through adsorption, while molybdate and phosphate were incorporated into the passive film, which proceeded more extensively at higher temperatures. Nitrate had a critical potential to inhibit pitting below which it scarcely worked, although its role was not understood satisfactorily. This explained why nitrate did not 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).