Intergranular Corrosion
What is Intergranular Corrosion in Stainless Steel?
Intergranular corrosion (IGC) is a localised form of corrosion that occurs along the grain boundaries of stainless steel. These boundaries, where individual crystals or grains meet, can become susceptible to attack under certain conditions, particularly when chromium carbides precipitate during improper heat treatment or welding.
How to Detect Intergranular Corrosion
Intergranular corrosion (IGC) often begins beneath the surface, making it hard to spot visually. Detection typically involves specialised methods such as bend tests (like ASTM A262 Practice E), which reveal cracking due to grain boundary attack, or the oxalic acid etch test, which highlights sensitisation under a microscope. Microscopic examination helps identify carbide formation or chromium-depleted zones, while electrochemical testing offers precise data on the material’s susceptibility.
How to Reduce the Risk of Intergranular Corrosion
To minimise the risk of intergranular corrosion in stainless steel, it is essential to select the right grade. Low-carbon variants like 304L or 316L help prevent chromium carbide formation. Post-weld heat treatments like solution annealing followed by rapid cooling can restore corrosion resistance by dissolving harmful precipitates. Additionally, using controlled welding techniques reduces exposure to sensitising temperatures.