Corrosion, element-selective, austenitic stainless

Secondly, we focused on the role of oxygen impurities in element-selective corrosion of austenitic stainless steels in liquid sodium. Calculation was carried out for an oxygen impurity associated with an Na atom located at the site on top of a Cr atom at the Fe(OOl) surface in sodium. The result shows that a positively charged Cr atom will be released selectively into sodium by an 0 anion. 

Mechanism of the element-selective corrosion of austenitic stainless steels... 

Secondly, we examined the mechanism of element-selective corrosion of austenitic stainless steels in liquid sodium. We performed calculations for a system where an 0 impurity atom accompanying Na atom approaches Or atom at an Fe(OOl) surface exposed to liquid sodium. The energy levels attributed to the Cr 3d orbitals are located above the Fermi level, while the energy levels originated from the 0 2p orbitals are below the Fermi level. Furthermore, the level for the Na 3s orbital is above the Fermi level. By the energy differences of these orbitals, the Cr atom is charged positively, while an 0 atom accompa-... 

In the first mechanism type I), the precipitate is inert or even cathodic with respect to a zone immediately adjacent, which is depleted in elements that promote passivation. The depleted zone being anodic with respect to the rest of the surface it selectively corrodes. Type-I intergranular corrosion is particularly obnoxious under conditions where the zone adjacent to the grain boundary becomes active, while the remainder of the surface remains passive. Intergranular corrosion of austenitic stainless steel due to the precipitation of chromium carbide is the best-known example of type I intergranular corrosion. 

Stainless steels containing chromium (18%) and nickel (8%) are well-known in this category. Alloying elements exceed 12% in this category. Chromium nickel steels, 3000 series are well known for their excellent corrosion resistance. Steels of the type 304, 304L, 347, 316, 316L, are used in corrosive aqueous solutions of acids and in marine environment. The composition and properties of selected chromium nickel austenitic steels is shown in Table 9.10. 

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