It is well documented that manganese sulfides (MnS) among non-metallic inclusions (NMI) act as initiation sites for pitting corrosion in commercial stainless steels (SSs). However, not only the influences of MnS characteristics such as its size and shape but also the effects of other NMIs such as oxides and nitrides on the pitting corrosion have rarely studied. In this study, we examined quantitatively the effects of NMIs such as MnS, oxide and nitrides on the pitting corrosion in 2507 duplex SS and 409L SS through an electrochemical noise analysis (ENA), micro-probe techniques, and EDS analysis.
2507 DSS (25Cr-7Ni-3Mo-0.25N) alloys were cast with various misch metal (MM) content (0, 0.022, 0.067, and 0.078 wt%) to control the distribution and morphology of (Mn, Cr)-oxysulfides in the alloy. With an increase in MM concentration to 0.067 wt%, the total amount and the number of (Mn, Cr)-oxysulfides decreased, the size of (Mn, Cr)-oxysulfides fell down from 7-8 ㎛ to 1-2 ㎛, and (Mn, Cr)-oxysulfides get round shape. However, in the alloy with 0.078 wt% MM, the total amount and the number of NMIs increased with their shape changed to needle-like shape. The resistance to pitting corrosion increased with MM concentration to 0.067 wt%, but the specimen with 0.078 wt% MM showed the lowest resistance to pitting corrosion when evaluated in terms of the pitting potential determined from potentiodynamic polarization response, and of metastable pitting event density from ENA in 4 M NaCl solution. The change in the resistance to pitting corrosion of the alloys depended on not the total amount of (Mn, Cr)-oxysulfides but the number of (Mn, Cr)-oxysulphides lager than 1.5 ㎛ in diameter. It was observed from an examination using a micro-droplet cell that the pitting corrosion initiated at the interface between (Mn, Cr)-oxysulphides and matrix. It was found that there is no preference to shape of NMIs for initiation state of pitting corrosion irrespective of whether the NMI is round or angular.
Commercial 409L SSs employed in this study were found to be different in the amount, morphology and type of NMI, depending on the decarburization method applied in the refining process. Total amount of NMIs in the alloy produced by argon oxygen decarburization (AOD) was much larger than that in the alloy produced by vacuum oxygen decarburization (VOD). NMIs in the AOD alloy were round (Ti, Ca)-oxide with 10 ㎛ ~15 ㎛ in diameter, however, those in the VOD alloy were angular Ti-nitride less than 2 ㎛ in size. The differences in total amount and type of NMIs caused large differences in the resistance to pitting corrosion of the alloy, i.e., the resistance to pitting corrosion of VOD alloy was much higher than that of AOD alloy when evaluated in terms of the pitting potential determined from potentiodynamic polarization response and metastable pitting event density from ENA in 0.5 M NaCl solution. It was found from the micro-probe technique using a micro droplet cell that pitting corrosion initiated at the interface between round (Ti, Ca)-oxide and matrix. On the other hand,Ti-nitrides in VOD alloy were inactive to pitting corrosion. For 409 alloy refined by AOD, the resistance to pitting corrosion increased linearly with Ti content in the (Ti, Ca)-oxide, but decreased with Ca content in the (Ti, Ca)-oxide.
