Stainless steels grain boundary composition

The effect of carbon on the corrosion of stainless steels in liquid sodium depends upon the test conditions and the composition of the steels . Stabilised stainless steels tend to pick up carbon from sodium, leading to a degree of carburisation which corresponds to the carbon activity in the liquid metal. Conversely, unstabilised stainless steels suffer slight decarburisation when exposed to very pure sodium. The decarburisation may promote corrosion in the surface region of the material and, under creep rupture conditions, can lead to cavity formation at the grain boundaries and decreased strength. 

Figure 1.9. FIM micrograph showing boron segregation to a grain boundary in austenitic 316L stainless steel, together with atom probe composition profiles for boron (Karlsson and Norden 1988).

Etching of stainless steel represents the most typical case of the application of electrolytic etching. This technique has been used on die 18/8 stainless steels in order to obtain a reliable and rapid measure of their sensitization to intergranular corrosion after a thermal treatment at 600 to 900°C. Several organic electrolytes such as oxalic acid (1) and tartaric acid (2) as well as inorganic electrolytes such as chromic acid (3) and sodium cyanide (4) have been employed. All these electrolytes attack preferentially the grain boundaries in which a second phase has precipitated. The composition of this phase responsible for the etching is not always known. 

Thermal treatment of stainless steel produces grain boundary carbides and depletion of chromium from the grain boundaries, causing the alloy to become sensitized to intergranular corrosion. A sensitization diagram for type 304 stainless steel with various carbon compositions is shown in Fig. 9.21 [93]. 

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