Damage mechanism glossary: API 510 and 570 terms
Pressure equipment inspectors work from a set of damage mechanisms: what each one is, where it shows up, what finds it and how it differs from the ones it resembles. This glossary groups those terms and gives each a full page with a short drawing, how to spot it, and the mix-ups that cost points. It also covers the terms that turn thickness readings into a corrosion rate, a remaining life and a margin. The detail behind the mechanisms is in API RP 571, and inspection practice is in API RP 572. Every page here is in our own words and cites paragraph numbers only, so keep your own copies of both at the editions on the effectivity sheet. The "On the sheet" column below shows which RP 571 paragraphs are among the 15 on API's sheet for September 2026 to May 2027. For the one-line notes on those 15 in one list, see the API 571 damage mechanisms list for API 510.
Thinning, low to moderate temperature
Metal loss from the surface. The questions are where the water or flow is, and whether the loss is even or local.
| Term | In one line | On the sheet | Full page |
|---|---|---|---|
| Corrosion under insulation | Wet insulation holding water against steel, out of sight | 3.22 | Corrosion under insulation |
| Erosion-corrosion | Fast or turbulent flow wearing metal and stripping its protective film | 3.27 | Erosion-corrosion |
| Microbiologically influenced corrosion | Pits under microbial deposits in stagnant or slow water | Not on sheet | Microbiologically influenced corrosion |
| General vs localized corrosion | Even loss across a surface versus pits and local attack | Concept | General vs localized corrosion |
| Hydrochloric acid corrosion | Strong acid where the first water condenses in a chloride-bearing vapor | 3.37 | Hydrochloric acid corrosion |
| Boiler water and condensate corrosion | Dissolved oxygen and carbon dioxide attacking steam and condensate systems | 3.9 | Boiler water and condensate corrosion |
| Atmospheric corrosion | Air and moisture on exposed outdoor steel, worst at coating breaks | 3.8 | Atmospheric corrosion |
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Cracking from the environment
Cracks that need a chemical, a stress and the right metal. Name the chemical and the metal first.
| Term | In one line | On the sheet | Full page |
|---|---|---|---|
| Stress corrosion cracking | Cracking that needs a susceptible metal, tensile stress and a specific environment | 3.3, 3.15, 3.17 | Stress corrosion cracking |
| Chloride stress corrosion cracking | Branching cracks in warm, stressed austenitic stainless steel with chlorides | 3.17 | Chloride stress corrosion cracking |
| Caustic vs amine stress corrosion cracking | Weld-side cracking in carbon steel, told apart by the process chemical | 3.3, 3.15 | Caustic vs amine stress corrosion cracking |
| Wet H2S damage | Blisters, stepwise cracks and weld-zone cracks in wet sour service | 3.67 | Wet H2S damage |
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High-temperature damage
Damage that needs heat. Some thins the wall, and some works inside it where thickness readings do not show it.
| Term | In one line | On the sheet | Full page |
|---|---|---|---|
| Sulfidation | Hot sulfur compounds thinning carbon and low-alloy steel | 3.61 | Sulfidation |
| High-temperature hydrogen attack | Hot hydrogen opening voids inside the wall while the surface looks fine | 3.36 | High-temperature hydrogen attack |
| Creep | Slow stretching of hot metal under steady stress | Not on sheet | Creep |
| Naphthenic acid corrosion | Organic acids in hot crude cutting local pits and grooves, worst at high velocity | 3.46 | Naphthenic acid corrosion |
Fracture and cycling
Failure by a single event in cold steel, or by many small load cycles.
| Term | In one line | On the sheet | Full page |
|---|---|---|---|
| Brittle fracture | Sudden cracking of cold steel with a flaw and stress | 3.11 | Brittle fracture |
| Mechanical fatigue | Cracks grown by repeated stress cycles, often from vibration | Not on sheet | Mechanical fatigue |
Remaining life terms
The terms that turn thickness readings into a rate, a life and a margin.
| Term | In one line | On the sheet | Full page |
|---|---|---|---|
| Short-term vs long-term corrosion rate | Two ways to turn thickness readings into a rate | Calculation (7.1) | Short-term vs long-term corrosion rate |
| Remaining life | Years until the wall reaches its required thickness at the chosen rate | Calculation (7.2) | Remaining life |
| Corrosion allowance vs minimum thickness | Design margin for corrosion versus the thickness the loads need | Calculation | Corrosion allowance vs minimum thickness |
How to use this glossary
- Start from the symptom. Is the loss even or local, inside or outside, hot or cold, wet or dry? Those four questions cut the list down quickly.
- Name the pair. Most mechanisms are a metal plus a condition: 300-series stainless steel with chlorides, carbon steel with caustic at a weld, hot carbon steel with sulfur. Learn the pairs.
- Match the method. Surface cracks, internal damage and thinning each need a different examination approach, and the Body of Knowledge expects you to connect them.
- Check your window's list. Entries marked "Not on sheet" are background for this cycle, and the list has changed between cycles. Confirm yours on the sheet for your exam window.
To drill the mechanisms, use the API 510 damage mechanism quiz. For the numbers behind the last group, try the corrosion rate calculator, the remaining life calculator and the thickness survey analyzer. Our post on corrosion under insulation adds field detail on the most common hidden loss.
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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 RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 3rd edition, March 2020. Cited by paragraph number only; read it in your own copy.
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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