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Caustic Stress Corrosion Cracking

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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

  1. Prueter, P., 2022, “Damage Control: Stress Corrosion Cracking Detection,” Inspectioneering Journal, 28(2), pp. 45-52.

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Articles about Caustic Stress Corrosion Cracking
July/August 2022 Inspectioneering Journal

This final installment of a 3-part series on SCC mitigation will provide practical, actionable steps to improve long-term equipment reliability through design, fabrication/welding, heat treatment, maintenance/repair, and process operating practices.

May/June 2022 Inspectioneering Journal

Part 2 of the three-part series on stress corrosion cracking (SCC) that examines common engineering and FFS methods that can be leveraged to understand and evaluate the propensity for an existing form of SCC to lead to loss of containment.

March/April 2022 Inspectioneering Journal

This article provides an overview of some of the more common forms of stress corrosion cracking (SCC) with a focus on effective inspection methods for detecting these forms of cracking.

May/June 2020 Inspectioneering Journal

There are many places where brittle fracture risk can sneak into your plant and many reasons why a new or revised brittle fracture assessment may be required when reviewing your pressure systems. Don’t overlook this dangerous failure mechanism.

Authors: Greg Garic
January/February 2003 Inspectioneering Journal

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.

Authors: John Reynolds
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