Caustic cracking is a serious form of boiler metal failure it occurs when there is a high level of stress of the boiler metal and concentrated alkalinity.

Boiler tubes that fail as a result of caustic cracking show failure around the circumference of the boiler tube.

Caustic cracks occur because high levels of caustic attack the carbide particles between the microscopic grain boundaries of the metal, failure is often sudden and catastrophic.
Conditions for this to occur is when the caustic levels reach 50,000 mg/litre, the following sections describe how this can occur, poor system design and bad water treatment practices can account for a lot of problems.

Caustic cracking can be prevented by either the use of a sodium nitrate or a coordinated phosphate/pH program.