Experimental and numerical studies of single manganese sulfide inclusions in stainless steel during initiation of pitting corrosion

A combination of microelectrochemical techniques and mathematical modeling were used to study the dissolution behavior of single MnS inclusions and their role on the initiation of pitting corrosion of stainless steel. The key results of the microelectrochemical measurements are that the no metastable pits form in thiosulfate solutions without chloride, chloride ion catalyzes inclusion dissolution and causes metastable pits, the thiosulfate ion accelerates inclusion dissolution when it is present above a critical concentration and causes stable pitting, at low concentrations the chloride ion inhibits the aggressive nature of the thiosulfate ion and stifles stable pitting, at high concentrations the thiosulfate ion inhibits the ability of the chloride ion to cause metastable pitting events, the xanthate ion inhibits the rate of inclusion dissolution, and that stable pitting depends on inclusion geometry.; Small wires of W, Ag, and Pt were placed within the microelectrochemical cell above single MnS inclusions to detect the pH, sulfide, and thiosulfate. In chloride-free electrolytes, the pH was found to decrease 0.5 units as a result of MnS dissolution. In chloride-containing electrolytes, metastable pitting events were found to cause rapid decreases in the pH. However, sustained pH drops were found to occur after the onset of stable pitting. Sulfide was only detected in a pH 2 electrolyte. A Pt wire along with a iodide/triiodide couple was used to detect thiosulfate.; Mathematical modeling was used to simulate the dissolution of a single MnS inclusion within a microelectrochemical cell. The model allowed for the evaluation of the hypothesis of mechanism of pit initiation based a critical concentration of thiosulfate which results in depassivation of the stainless steel. The model supported the view that local acidification occurs after the initiation of pitting rather than trigger initiation. In addition, the model supported the hypothesis that pit initiation occurs due to the accumulation of an aggressive dissolved sulfur species, such as thiosulfate, as a result of inclusion dissolution. Critical pitting potentials were predicted based on a critical thiosulfate concentration and compared to experimentally measured values at single inclusions.