Intergranular corrosion occurs when austenitic stainless steels are exposed to temperatures in the range 550-850°C where chromium carbides (Fe, Cr)23 C6 can precipitate in the grain boundaries. The chromium content of the carbide precipitates is very high, and since the diffusion rate of chromium in the austenite is low, the alloy adjacent to the grain boundary becomes chromium depleted. Since chromium is essential to passivity, the chromium-depleted region becomes less corrosion resistant than the matrix. In a corrosive environment the depleted area may be depassivated and corrosion will take place in very narrow areas along the grain boundaries. A stainless steel, which has been heat-treated in a way that produces grain boundary precipitates and adjacent chromium depleted zones, is said to be “sensitized”. Sensitization might occur as a result of welding, or of hot forming at an inappropriate temperature.
Measures to increase the resistance of stainless steels to intergranular corrosion caused by chromium carbide precipitation are solution annealing, lowering the carbon content and alloying with titanium or niobium (stabilizing).
Stainless steels are usually delivered from the steel producer in the solution-annealed condition. Solution annealing means heating to temperatures in the range 1000 – 1200°C, at which chromium carbides are dissolved, followed by rapid cooling in air or water. Such an operation leaves the carbon in solid solution in the steel. Steel with a sensitized structure may be recovered by a further solution annealing procedure. A low carbon content (< 0.03%) extends the time required for significant sensitization and is the second measure to decrease the risk of intergranular corrosion. Modern steel making methods enable much lower carbon contents to be achieved in steels than in the past (304L, 316L).
8. Intergranular corrosion attack.
Stabilized steels, containing titanium or niobium, show good resistance to intergranular corrosion even though their carbon contents may be fairly high. This is due to the fact that titanium and niobium form carbides more easily than chromium. Such carbides precipitate randomly in the grains and no carbon is available to form chromium carbide precipitates in the grain boundaries. Low carbon grades and stabilized grades can be considered as equally resistant to intergranular corrosion, unless exposed for long periods at temperatures above 500°C, in which case stabilized grades are to be preferred.
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