Caustic vs amine stress corrosion cracking: what it means for the inspector

How it works

Both mechanisms follow the same pattern as other SCC types: carbon steel (the susceptible material), tensile stress (frequently residual weld stress) and a specific environment. For caustic SCC the environment is a hot caustic solution. For amine SCC it is an amine solution used in gas treating.

The residual stress from welding is the shared factor. A weld that has been through post-weld heat treatment (stress relief) has much less locked-in stress, and that is why stress relief is the standard defence against both mechanisms. Welds that were not stress relieved, and the heat-affected zone beside them, are where cracks begin. The cracks usually run parallel to the weld, in the base metal next to it, and they are often a network of fine cracks.

Where each one shows

• Caustic SCC. Caustic is used and injected in several refinery and chemical processes. Cracking is associated with places where caustic concentrates, such as near heated surfaces, steam tracing or steam-out, and at injection points. The caustic concentrate is what does the damage, so a dilute solution in a cool line can be a much lower risk than a hot, concentrated one.
• Amine SCC. Amine treating units remove acid gas, and cracking is found in carbon steel in that service, again at welds and heat-affected zones that were not stress relieved.

How the cracks are found

Both forms are found with wet fluorescent magnetic particle testing on cleaned weld surfaces, because the steel is magnetic and the method is sensitive to tight cracks. Angle-beam UT helps judge crack depth. Liquid penetrant testing is less reliable for these cracks, since they can be tight and filled with scale. Preparing the surface properly before examination is part of getting a reliable result.

Mix-ups to avoid

• Caustic versus amine. Same metal, same weld stress, same crack pattern. The service tells you which name applies, so read the stem for the chemical.
• Either one versus chloride SCC. Chloride SCC attacks stainless steel, not carbon steel.
• Caustic SCC versus caustic corrosion. RP 571 has both. Caustic corrosion (paragraph 3.14) is local thinning or gouging where caustic concentrates. Caustic SCC (3.15) is cracking.

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. Both are on it: amine SCC is paragraph 3.3 and caustic SCC is paragraph 3.15. Questions usually give the service and the weld condition and ask for the mechanism or the preferred examination method. See the broader pattern on the stress corrosion cracking page.

Practice it

Work through a caustic and an amine scenario on the API 510 damage mechanism quiz. Compare it with Stress corrosion cracking and Chloride 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

Show the list

• 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 and 3.15. Cited by number only; read it in your own copy.