Stainless Steel Pitting in Thiosulfate Solutions with Electrochemical Noise

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Abstract
Resistance of stainless steels (SS) to generalized corrosion is provided by growth of a protective thin film of mixed oxides. In aqueous solutions, impurities can destroy this passive layer locally and increase the intensity of localized attack. Thiosulfate ions (S2O32−) have been recognized as potentially detrimental to the stability of protective films on SS. Forms of localized corrosion with far-reaching consequences have been detected in the nuclear, petroleum, and pulp and paper industries. Although S2O32− ions alone have not been found aggressive enough to cause damage to SS, they have been shown to act in synergy with other common ions, such as chlorides or sulfates, to cause localized corrosion. Understanding the chronology of initial events, speculated to be connected to local variations in the anodic and cathodic reaction rates, is fundamental in studying localized corrosion. During localized corrosion, electrochemical noise (EN) seems to be generated by stochastic processes, such as passivation breakdown and repassivation events, and deterministic processes caused by film formation or pit propagation processes. The study of EN particularly is adapted to monitoring 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.

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