Stress corrosion cracking: what it means for the inspector

How it works

SCC is not general corrosion with a crack added. The metal can lose almost nothing to corrosion and still crack. The three conditions must overlap:

1. A susceptible material. Each form of SCC attacks certain alloys. Austenitic stainless steels crack in chloride environments. Carbon steel cracks in caustic and in amine service.
2. Tensile stress. The stress can come from pressure and load, but locked-in stress from welding and forming is a frequent culprit. A weld that was not stress relieved is a classic site.
3. A specific environment. The chemical matters. Chloride, caustic, amine, wet H2S and others each go with a particular set of materials and conditions, and temperature and concentration often play a part.

Because all three are required, SCC control works by removing one: choose a material that does not crack in that chemical, relieve the stress, or change the environment (for example, keep chlorides away from stainless steel).

Where it shows and how it is found

SCC cracks tend to be fine and may branch, sitting in or next to welds or at stress concentrations. They can run deep with little surface evidence.

• Surface crack methods such as wet fluorescent magnetic particle on ferritic steel, and liquid penetrant on non-magnetic stainless steel, find cracks that reach the surface.
• Angle-beam UT helps judge depth once a crack has been found.
• Surface preparation matters. Scale and deposits can hide tight cracks, so cleaning before examination is part of the job.

Mix-ups to avoid

• SCC versus fatigue. Fatigue cracks from repeated stress cycles and does not need a particular chemical. SCC needs the environment, and static stress is enough.
• SCC versus general thinning. SCC leaves the surface almost intact. General thinning removes metal visibly.
• Naming the SCC. Different chemicals give different named mechanisms. See chloride SCC for stainless steel, and caustic versus amine SCC for carbon steel.

On the API exam

API's effectivity sheet for September 2026 to May 2027 limits RP 571 questions to Section 2 plus 15 mechanism paragraphs. Three of the 15 are SCC types: amine SCC (3.3), caustic SCC (3.15) and chloride SCC (3.17). Wet H2S damage (3.67) also includes cracking forms. A common stem names a material and an environment and asks which mechanism fits, so the habit to build is pairing each chemical with its metal, and asking where the stress comes from.

Practice it

Practise sorting cracking scenarios by environment on the API 510 damage mechanism quiz. Compare it with Chloride stress corrosion cracking and Caustic vs amine stress corrosion cracking, or browse every term in the damage mechanism glossary.

Know the mechanisms? Check the rest of the Body of Knowledge

Take the free 10-question check. It is spread across the Body of Knowledge, needs no signup and explains every answer, so you see which domains to open first. The Long Seam course then adds a lesson for each domain, timed practice exams and a study plan from your exam date.

Sources

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• API 510 Exam Publications Effectivity Sheet, September 2026, January 2027 and May 2027, American Petroleum Institute
• API 510 Body of Knowledge, September 2026, January 2027 and May 2027, American Petroleum Institute
• API 510 Pressure Vessel Inspector: exam information, American Petroleum Institute
• API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 3rd edition, March 2020, paragraph 3.3, 3.15 and 3.17. Cited by number only; read it in your own copy.