99 Diseases of Pressure Equipment: Caustic Cracking

I already mentioned this common affliction in the introduction. Caustic cracking was long called caustic embrittlement, but since no embrittlement actually occurs that name is fading away. It’s one of many environmental cracking mechanisms that typically occur in carbon steel equipment, but can also afflict low alloy and austenitic stainless steel equipment. Caustic environments (NaOH & KOH) cause cracking most commonly in weldments because of high residual stresses, but also affects base metal with high residual stresses. It takes a certain temperature range and concentration of caustic to cause caustic cracking that has been long established in the caustic cracking chart in the NACE Corrosion Data Survey (1). Cracks can follow the heat-affected zone (HAZ) or they can be transverse through the weld. Cracks can be tight and difficult to find with PT, or they can be so wideopen that you only need your eyes to see them. On the outside surface, it’s not uncommon to see white crystalline deposits of caustic where a leak as occurred due to caustic cracking.

Typically when a caustic environment is expected, the materials engineer specifies that the equipment will be post-weld heat treated (PWHT). In my experience, the six most common reasons for caustic cracking are:

1. Steaming out non-PWHT equipment. To prevent this, operations needs to have a list of equipment that cannot be steam cleaned, but rather needs to be cleaned with hot water or other means.
2. Injecting concentrated caustic into process environments. To prevent this, properly designed and operated injection equipment is needed or lower concentrations of caustic can be used.
3. Inadequate PWHT. All too often, when we don’t have rigid QA/QC during the fabrication of equipment, PWHT will not be adequately performed, leaving higher residual stresses and higher susceptibility to caustic cracking than were expected in the design specification.
4. Caustic carry-over or concentration in steam systems. This can be a real nightmare and must prevented by continuous, high quality control of boiler feed water treatment and operating controls, i.e. excellent procedures and management systems.
5. Pipe bends, bellows, or coils that have high levels of residual stresses and then are exposed to hot caustic containing fluids. Can also occur when heat tracing coils touch caustic containing equipment.

Non-stress relieved welds are made on equipment that was originally PWHT’d. This is obviously a big no-no, but it keeps happening.
But there are several more reasons for experiencing caustic cracking and your RBI team needs to ask all the right questions about whether or not caustic cracking is a potential failure mode in your systems. Don’t forget the potential for sudden, inadvertent contamination with caustic from some unexpected source. Where are those sources and how can you prevent them from eventually causing caustic cracking?

If you are involved in the inspection of equipment, this old adage applies to you: You have either already experienced some cases of caustic cracking in your tenure, or you eventually will. Few afflictions are more prevalent in our industry.