Corrosion cracking of mild steel in contact with alkaline solutions; sometimes called boiler embrittlement. The failure is usually intergranular and usually occurs with concentrated sodium hydroxide when specific impurities such as silicates act as accelerators. Stresses above the elastic limit are especially conducive to the problem.

Caustic embrittlement is a form of stress corrosion responsible for occasional boiler failures. The objective in hot water and steam boiler operations is to avoid handling acidic water because of metal loss by uniform corrosion. Therefore, small amounts of alkali (caustic soda) are added to the water periodically to keep the PH at about 10.5, at which level there will be practically no corrosion on plain steel. However, if an excess of alkali is added, such water at elevated operating temperatures can cause stress corrosion cracking. In these situations, the steel does not become embrittled; its still-solid grains remain ductile. Instead, intergranular attack penetrates the steel, weakening the steel section and reducing its capacity to act in a ductile manner. This intergranular attack is recognized as stress corrosion cracking that has developed by an anodic mechanism.

X