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Chloride stress corrosion cracking: what it means for the inspector

Long Seam editorial teamSources checked 3 min read

Chloride stress corrosion cracking is cracking of austenitic stainless steel exposed to chlorides while the metal is warm and under tensile stress. The cracks are fine and branching, and they can run deep with little metal loss on the surface, which makes them easy to miss without a surface crack test.

Cross-section of a stainless steel wall under wet insulation carrying chlorides, with several fine branching cracks running from the outer surface into the wall and little metal loss at the surface
Fine branching cracks run in from the surface while the surface itself shows little loss.

How it works

Austenitic (300-series) stainless steels resist general corrosion well, but they are vulnerable to cracking in chloride-bearing water when the metal is warm and under tensile stress. All three SCC conditions are met: a susceptible material, stress, and a specific environment (see stress corrosion cracking).

Chlorides arrive in many ways: process water, cooling water, seawater, and chlorides leached from insulation. Temperature matters because the cracking is far more likely at elevated metal temperature than at ambient, and stress usually comes from welding, bending or pressure. The cracks start at the surface, often at pits or crevices, and spread in a branching, tree-like pattern, mostly across the grains.

The damage does not look like corrosion. The wall is not visibly thinned, and a leak may be the first sign that anything is wrong.

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Where it shows and how it is found

  • Under insulation and jacketing. Wet insulation carrying chlorides against warm stainless steel is a well-known setup, which links this mechanism to corrosion under insulation. The insulation must come off, at least at selected spots, to examine the metal.
  • Crevices and deposits. Gaps under supports, at tubesheets and beneath deposits let chlorides concentrate.
  • Heat exchanger tubes. Eddy current testing is used on tubing.
  • Surface methods. Liquid penetrant testing is the practical method for stainless steel, because stainless steel is not magnetic and magnetic particle testing does not apply to it. Angle-beam UT can help size a crack.

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Mix-ups to avoid

  • Chloride SCC versus caustic and amine SCC. Those attack carbon steel. Chloride SCC attacks austenitic stainless steel.
  • Chloride SCC versus pitting. Chlorides also cause pitting in stainless steel. Pitting is a hole-forming attack with metal loss. SCC is cracking.
  • Choosing the method. Because the base metal is not magnetic, penetrant testing, not magnetic particle, is the usual surface method for the cracks.

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. Chloride SCC is RP 571 paragraph 3.17 and is one of the 15. Look for the clue pair of 300-series stainless steel and a chloride source, often with insulation, warmth and a weld or crevice. The Body of Knowledge also covers the examination method side, so connect each cracking mechanism with its method.

Practice it

See where chloride SCC sits among the 15 mechanisms in the API 571 damage mechanisms list for API 510. Compare it with Stress corrosion cracking and Corrosion under insulation, or browse every term in the damage mechanism glossary.

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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