This month we continue our #epicfail series on failure mechanisms looking at the consequence of sensitisation on austenitic stainless steels.

Carbon is an undesirable impurity in austenitic stainless steels due to its great thermodynamic affinity for chromium. Upon exposure to the temperature range of 450-900°C, chromium rich carbides precipitate along the austenite grain boundaries. During carbide precipitation, interstitial carbon diffuses rapidly to the grain boundaries. However, since chromium is a substitutional element and diffuses very slowly, there is insufficient time for chromium to diffuse to the carbides from all over the grains. A high concentration of chromium in the carbide particles decreases the chromium content immediately adjacent to the grain boundaries. In these areas, the concentration of chromium falls below 12 wt% (the level at which stainless steels acquire their ‘stainless’ characteristic) and thus the depleted areas become anodic in the presence of an electrolyte and are susceptible to corrosion. This local depletion in chromium is commonly termed sensitisation.

Figure 1 – Chromium carbide precipitation at austenite grain boundaries

Chromium carbide precipitation can occur during exposure to high operating temperatures. This is not deemed to be an issue if the operating temperatures remains above the dew point temperature. However, in the presence of moisture and air, intergranular attack can occur due to the localised depletion of chromium adjacent to the grain boundaries. For materials with a metal sulphide scale from exposure to sulphur containing species during operation, the scale may react with air and moisture to form polythionic acid. This in combination with a stress, residual or applied, can lead to a phenomenon known as polythionic stress corrosion cracking.

Figure 2 – Polythionic SCC crack

Chromium carbide precipitation can also occur by slow cooling from elevated temperature such as solution annealing during manufacture or welding. The phenomenon associated with welding is commonly termed weld decay where the heat affected zone is locally sensitised and thus susceptible to intergranular attack.

Susceptibility to sensitisation can be minimised by using a low carbon variant (i.e. 304L, 316L) or a stabilised grade alloyed with titanium (321) or niobium (347). These additions are stronger carbide formers than chromium and tie up the carbon so that chromium carbides are not formed. Titanium or niobium additions also reduce the solubility of carbon in austenite resulting in MC type carbides, where M represents titanium or niobium. Although these alternative materials reduce susceptibility to sensitisation at short exposure times to elevated temperature, chromium rich carbides still precipitate after long-term ageing. Therefore, careful management during start-up/shut-down would need to be implemented to minimise the risk of exposure to moisture.

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