(Invited) Micro-Electrochemistry of Pit Initiation at Non-Metallic Inclusions of Stainless Steels and Roles of Alloying Elements in Improving Corrosion Resistance
Authors: Muto Izumi , Sugawara Yu , Hara Nobuyoshi 
Published: December 2020

A micro-electrochemical system for in situ high-resolution optical microscopy was developed and applied to the real-time observation of pit initiation at MnS inclusions in Type 304 stainless steel in NaCl solutions. Metastable and stable pits were found to be initiated at the MnS/steel boundaries during potentiodynamic polarization. After polarization, it was confirmed that deep trenches were generated at the boundaries. Therefore, metastable and stable pits were determined to be initiated in the trenches. From the real-time observation, it was found that the initial rounded form of metastable and stable pits became polygonal in shape within 1 s. After that, the dissolution proceeded in the depth direction with no change in the appearance of the pit as observed externally. In the case of the metastable pitting, the duration of this stage was approximately 1.5 s, and then the pit repassivated, and the polygonal metastable pit remained. The in-depth growth stage for stable pitting was relatively longer (approx. 3.5 s), and the pit grew deeply into the steel matrix and wrapped beneath the inclusion, leading to the formation of a large occluded cavity, in which the corrosivity considerably exceeded the critical conditions for autocatalytic pit growth.
The anodic dissolution behavior of CrS, TiS, and Ti4C2S2 inclusions was compared with that of MnS. The dissolution potential of CrS inclusions was in the transpassive region of stainless steels. Stable pits were initiated in this potential region. In contrast, TiS and Ti4C2S2 inclusions did not dissolve in the passive or the trans-passive regions of stainless steels, and no pits were initiated at these inclusions in NaCl solutions. The calculations made from the potential-pH diagrams for CrS, TiS, and Ti4C2S2 systems indicated that Cr-oxide or Ti-oxide enriched surface films would form, and this was confirmed by surface analysis. The Cr-oxide or Ti-oxide enriched layers on the inclusions inhibited the anodic dissolution of the inclusions in the passive or trans-passive regions of stainless steels. In contrast, MnS inclusions dissolved in NaCl solutions, and pits were initiated at the inclusions. While MnS inclusion surfaces were covered by Mn-oxides, the corrosion resistance of Mn-oxides was not sufficient to prevent inclusion dissolution and subsequent pit initiation. The oxide enriched surface layers on the sulfide and carbo-sulfide inclusions were shown to significant enhance pit initiation resistance at the inclusions.
In addition to the above findings, the roles of the addition of Ce and Mo were studied. From the results of micro-scale polarization, thermodynamic calculations, and scanning electron microscopy, it was suggested that the Ce3+ ions inhibit trench formation at the inclusion/steel matrix boundary, resulting in improved pitting corrosion resistance at sulfide inclusions. And also, the active dissolution rate of the steel was suppressed by Mo alloying. This suggests that, even after trenching at high potentials, Mo alloying inhibits the initiation of pitting inside the trenches.
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