Caustic Stress Corrosion Cracking, also referred to as Caustic Embrittlement, is a form of degradation that is caused when a component operates in a caustic environment. It is one of the most prevalent afflictions in the industry and one of many environmental cracking mechanisms that typically occur in carbon steel equipment, but it can also afflict low alloy and austenitic stainless steel equipment. Caustic environments (those that contain high amounts of NaOH & KOH) cause cracking most commonly in weldments because of high residual stresses, but can also affect base metal with high residual stresses.
When inspecting for cracks caused by caustic cracking it is difficult to follow a strict set of guidelines. These cracks can follow the heat-affected zone (HAZ) or they can be transverse through the weld. They can sometimes be small and difficult to find even with penetrant testing, or they can be so large and visible that they can be detected using a simple visual inspection. On the outside surface though, it’s not uncommon to see white crystalline deposits of caustic where a leak has occurred due to caustic cracking.
A number of factors can contribute to whether caustic cracking will affect a particular piece of equipment. Steaming out non-post-weld heat treated (PWHT) equipment can make it more vulnerable to caustic cracking. Similarly, inadequate PWHT can also create higher residual stresses and thus higher susceptibility to caustic cracking. Injecting concentrated caustic into process environments is another big problem. Caustic carry-over or concentration in steam systems is also incredibly dangerous and needs to be prevented by continuous, high quality control of boiler feed water treatment and operating controls. Finally, pipe bends, bellows, or coils that have high levels of residual stresses and are exposed to hot caustic containing fluids are also quite susceptible.
This topic is covered in more detail in API RP 571 - Damage Mechanisms Affecting Fixed Equipment in the Refining Industry.
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January/February 2003 Inspectioneering Journal
By John Reynolds at Intertek
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. |
