Pitting is one of the most important local corrosion forms causing the failure of passive metals. It is hard to be detected and develops fast. Therefore it is necessary to investigate the mechanism of pitting. It has been found that MnS inclusions are related to pitting behavior in stainless steel. However, the detail about the trigger mechanism is still uncertain.
Transmission electron microscopy (TEM) was used to investigate the structure and defects of MnS in SUS 316F stainless steel. The results show that there are several kinds of defects in MnS inclusions, such as cracks, twins, dislocations and oxide particles. The lattice type of the oxides is face-centered cubic (FCC) and the structure is determined to be spinel.
The interfacial composition was analyzed by scanning transmission electron microscopy (STEM) equipped with X-ray energy dispersive spectrometry (EDX). The results indicate that there is no Cr-depleted zone around MnS inclusions. Therefore the Cr-depleted zone is not the main cause of pitting in stainless steel.
After immersion in NaCl solution, it is found that some regions of the MnS become amorphous and Mn content decrease evidently in the amorphous regions indicating a Mn partly dissolution. Oxides are more stable than MnS. As a result, the MnS around them is expected to dissolve faster.
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