Caustic Stress Corrosion Cracking, often referred to as caustic cracking or caustic embrittlement (although it is not technically an embrittlement damage mechanism), is a form of stress corrosion cracking (SCC) caused by the combined effects of a caustic environment and either applied or residual tensile stress. Caustic cracking is characterized by surface-breaking crack-like flaws that most often occur adjacent to non-post weld heat treated (PWHT) welds in pressure equipment exposed to caustic (e.g., caustic soda or caustic potash). Carbon, low-alloy, and 300 series stainless steels are particularly susceptible to caustic cracking, while nickel base alloys are generally more resistant, although not entirely immune. Cracking in carbon steels is usually intergranular (although in rare cases it can be transgranular) and it tends to exhibit multiple, oxide-filled cracks. In 300 series stainless steels, cracking is most often transgranular, and it can be difficult to distinguish from chloride SCC.
Areas Susceptible to Caustic SCC
Most modern cases of caustic cracking are observed in carbon steel equipment near non-PWHT welds (in the heat-affected zone (HAZ) or base metal directly adjacent to the HAZ). Typical locations in common refining units to monitor for caustic cracking include:
- Hydrofluoric and Sulfuric Acid Alkylation Units
- Crude Distillation Units
- Merox (Mercaptan Oxidation) Units
- Sour Water Stripper Units
- Boilers with Inadequate Water Chemistry Controls
- Improperly Heat Traced Piping
In boiler feed water systems, even trace amounts of caustic can become concentrated in boiler tubes due to wet-dry cycling, and this can lead to tube failures. Cracking can also initiate at rolled tube-to-tubesheet junctions in boilers where caustic concentration may also take place. Furthermore, caustic may be present in other downstream units (e.g., steam condensate piping) because of unintended caustic carryover.
Caustic SCC Inspection
When inspecting for caustic cracking it is difficult to follow a strict set of guidelines. These cracks can follow the 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 with 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.
Caustic SCC is covered in more detail in API RP 571 - Damage Mechanisms Affecting Fixed Equipment in the Refining Industry.
References
- Prueter, P., 2022, “Damage Control: Stress Corrosion Cracking Detection,” Inspectioneering Journal, 28(2), pp. 45-52.
Related Topics
- Amine Stress Corrosion Cracking
- Ammonia Stress Corrosion Cracking
- Carbonate Stress Corrosion Cracking
- Chloride Stress Corrosion Cracking
- Polythionic Acid Stress Corrosion Cracking (PASCC)
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