What Does Sensitization Mean?
Sensitization is the loss of alloy integrity. It results from chromium depletion in the vicinity of carbides precipitated at grain boundaries. This causes the steel or alloy to become susceptible to intergranular corrosion or intergranular stress corrosion cracking (SCC).
Sensitization requires specific combinations of:
Corrosionpedia Explains Sensitization
Sensitization happens when a stainless steel is held at a temperature of 425 - 815°C and chromium carbides precipitate at the grain boundaries. This precipitation happens because the carbides are insoluble at these temperatures. In order for the carbide to precipitate, it must obtain chromium from the surrounding metal. This means that there is a chromium-depleted zone around the grain boundaries.
Sensitization can occur during welding or annealing after cold working. It can also occur in ferritic and duplex stainless steels, though the thermal profile that results in the formation of precipitates is different than for austenitic stainless steels.
Sensitization is particularly important in welded metals. This is because the welding zone experiences temperatures in the range that can cause sensitization, which causes localized intergranular corrosion at the weld site. If a sensitized alloy is exposed to a corrosive environment, the areas near the grain boundaries are preferentially attacked. As the corrosion proceeds, the grains fall out and the metal loses strength.
Sensitization can be prevented by:
- Reducing the carbon content
- Adding stabilizers such as niobium or titanium
- Reducing the time of exposure to the critical temperature range
Non-sensitized austenitic stainless steels are resistant to intergranular attack in almost all environments.
Common methods to check the susceptibility of stainless steels to intergranular attack are described in ASTM A2621 for austenitic steels and ASTM A7632 for ferritic steels. Practice A in ASTM A262 and Practice W in ASTM A763 have rules for etch tests in oxalic acid.