How does sensitization occur in austenitic stainless steels, and what corrosion risk does it pose?

• A. Increase in nickel content causing pitting
• B. Formation of protective oxide at high temps reducing corrosion
• C. Chromium carbide precipitation at grain boundaries during high-temp exposure leading to boundary chromium depletion, increasing intergranular corrosion susceptibility
• D. Hydrogen absorption causing stress corrosion cracking

Answer: (C)

Sensitization in austenitic stainless steels happens when chromium carbides precipitate at grain boundaries during exposure to high temperatures, about 450–850°C. This precipitation pulls chromium out of the surrounding grain interior, creating chromium-depleted zones at the boundaries. Those boundary regions lose their ability to form the protective chromium-rich oxide film, so the grain boundaries become more susceptible to intergranular corrosion. In environments that are aggressive or under tensile stress, this can lead to intergranular corrosion and, in some cases, intergranular stress corrosion cracking. Preventing sensitization involves using low-carbon or stabilized alloys and heat treatments (like solution annealing and rapid cooling) to dissolve or avoid carbide formation. The other ideas don’t fit: increasing nickel doesn’t cause pitting in this context, a protective oxide forming at high temperatures would reduce corrosion, and hydrogen absorption leads to hydrogen-related cracking rather than sensitization-driven intergranular attack.