(Digital Presentation) Experimental Analysis of Pit Initiation at MnS Inclusion in Low-Alloy Steel Using in-Situ Raman Spectroscopy

Low-alloy steel is an indispensable material for industry and infrastructure and is widely used in many fields because it can save cost and elemental resources. However, one of the disadvantages of low-alloy steel is its limited corrosion resistance compared to that of high alloys. Specifically, a major weakness is its high susceptibility to pitting in an environment containing Cl - ions. MnS inclusions are well known as preferential pitting sites because of its poor stability against chemical dissolution and ease of deformation by rolling. A lot of studies have investigated the mechanisms of pit initiation at MnS so far. Some detail analyses revealed that S species such as S 2 O 3 2- , HS - and elemental S were generated due to dissolution of MnS, and they were harmful to pit initiation and active dissolution. However, these studies did not focus on ‘when’ those S species were generated. In addition, most of these previous studies focused on pitting in high alloys such as stainless steel and Ni-based alloy, so the mechanism of pit initiation in low-alloy steel is not yet fully understood. This study aims to clarify the chronology between MnS dissolution, S species generation and pit initiation in low-alloy steel. The microscopic potentiodynamic polarization test was carried out in 20 mM NaCl added boric-borate buffer solution at pH 8.0. To analysis S species dissolved from MnS inclusions during the electrochemical test, in-situ observation using an optical microscope with high magnification and Raman spectroscopy were employed. The polarization curve started at cathodic region, and then the corrosion potential was observed. In the anodic region, active dissolution and passive regions appeared in the curve as the potential increased. Finally, surges in the current density, attributed to pit initiation, was observed. In-situ observation ascertained that pit initiated at MnS inclusion whose longer diameter was 20 µm. Interval analysis of Raman spectroscopy during the test detected elemental S about 0.1 V below the pitting potential. Analysis of the test solution after the polarization test also detected elemental S, although other S species such as S 2 O 3 2- and HS - were not detected. To confirm the morphology of the pit, cross-sectional observation was carried out using FIB/SEM. The pit was found to grow hemispherically, and corrosion product composed by Fe, Mn, O, and S was formed on the inner wall of the pit. This study clarified that, MnS started to di