STAINLESS-STEEL FITTING IN THIOSULFATE SOLUTIONS WITH ELECTROCHEMICALNOISE

Resistance of stainless steels (SS) to generalized corrosion is provid ed by growth of a protective thin film of mixed oxides. In aqueous sol utions, impurities can destroy this passive layer locally and increase the intensity of localized attack. Thiosulfate ions (S2O32-) have bee n recognized as potentially detrimental to the stability of protective films on SS. Forms of localized corrosion with far-reaching consequen ces have been detected in the nuclear, petroleum, and pulp and paper i ndustries. Although S2O32- ions alone have not been found aggressive e nough to cause damage to SS, they have been shown to act in synergy wi th other common ions, such as chlorides or sulfates, to cause localize d corrosion. Understanding the chronology of initial events, speculate d to be connected to local variations in the anodic and cathodic react ion rates, is fundamental in studying localized corrosion. During loca lized corrosion, electrochemical noise (EN) seems to be generated by s tochastic processes, such as passivation breakdown and repassivation e vents, and deterministic processes caused by film formation or pit pro pagation processes. The study of EN particularly is adapted to monitor ing onset of events that characterize localized corrosion. Type 304 SS (UNS S30400) specimens exposed to chloride (Cl-) and S2O32- ions were used to generate EN signals.