Intergranular corrosion, is a localized attack at and adjacent to grain boundaries within the metal, with relatively little corrosion of the grains. Intergranular
corrosion occurs when material in the grain boundaries of certain alloys is less resistant to the corroding agent than the grains themselves. Penetration of the
corroding medium follows grain boundaries and destroys the bond between grains. As corrosion proceeds, the grains fall out and the metal or alloy disintegrates.
Some stainless steels are susceptible to inter-granular corrosion if heated to temperatures in the 800°F to 1500°F (426 to 815°C) range, which is referred to as the
sensitization range.
Key Properties of Intergranular Corrosion
- Is caused by precipitation of carbides and depletion of chromium at grain boundaries:
- Knife Line Attack (KLA) is found at interface of weld pool and base metal; caused by rapid cooling.
- Weld decay is slightly removed <6 mm (1/4 in) from weld pool
Note: Sensitized austenitic stainless steels contain chromium carbides at the grain boundaries due to improper heat treatment during manufacturing or as a result of welding of high carbon grades without post-weld heat treatment (e.g. AISI 316 vs. AISI 316L). This may result in IGC of the steel.
Where does it occur?
- Weld heat-affected zones
- Stainless steel fabrications
- Process equipment
How to Control Intergranular Corrosion?
Intergranular Corrosion may be controlled in a variety of ways, including:
- Heat treatment
- Addition of stabilizing elements
- Use alloy with lower carbon content (TP316L vs. TP316 stainless steel)
Source: Chesterton ARC Manuals
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