Identification of MnCr2O4 nano-octahedron in catalysing pitting corrosion of austenitic stainless steels
Pitting corrosion of stainless steels, one of the classical problems in materials science and electrochemistry, is generally believed to
originate from the local dissolution in MnS inclusions, which are more or less ubiquitous in stainless steels. However, the initial location
where MnS dissolution preferentially occurs is known to be unpredictable, which makes pitting corrosion a major concern. In this work
we show, at an atomic scale, the initial site where MnS starts to dissolve in the presence of salt water. Using in situ ex-environment transmission
electron microscopy (TEM), we found a number of nano-sized octahedral MnCr2O4 crystals (with a spinel structure and a space
group of Fd 3m) embedded in the MnS medium, generating local MnCr2O4/MnS nano-galvanic cells. The TEM experiments combined
with first-principles calculations clarified that the nano-octahedron, enclosed by eight {1 1 1} facets with metal terminations, is “malignant”,
and this acts as the reactive site and catalyses the dissolution of MnS. This work not only uncovers the origin of MnS dissolution
in stainless steels, but also presents an atomic-scale evolution in a material’s failure which may occur in a wide range of engineering alloys
and biomedical instruments serving in wet environments.
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