The API RP 571 damage mechanisms on the API 510 exam, with where each shows and how it is found

What the current sheet lists

The API 510 effectivity sheet for September 2026, January 2027 and May 2027 names API RP 571, 3rd edition, March 2020, and limits the exam to Section 2 (terms and definitions) plus 15 mechanism paragraphs:

The Body of Knowledge for the same windows repeats the same Section 2 and 15 paragraphs in its Section V, with the same three entries in red, and says test questions on RP 571 come from those portions only. The two documents agree on this list.

What "shown in red" means, and what actually changed

Neither document defines the red text. Here is how the current sheet compares with API's sheets for the two previous cycles:

• The September 2025, January 2026 and May 2026 sheet lists the same 15 paragraphs as the current one, with 3.9, 3.35 and 3.46 already in red.
• The September 2024, January 2025 and May 2025 sheet also lists 15, but a different 15. It has 3.20 (cooling water corrosion), 3.43 (mechanical fatigue, including vibration-induced fatigue) and 3.58 (sour water corrosion, acidic), and it does not have 3.9, 3.35 or 3.46.

So the RP 571 swap happened at the September 2025 cycle. The current cycle carries that list over unchanged, and the red marking on those three paragraphs carries over with it. On the 2025 sheet the three added paragraphs are the red ones. On the current sheet the red RP 571 lines sit next to red text for API 510 Errata 2, RP 577 Errata 1 and the 2025 ASME edition, which did change this cycle. If you studied from a September 2025 cycle list, your RP 571 scope is the same. If you studied from a 2024 cycle course or question set, drop 3.20, 3.43 and 3.58 and add the three red ones.

One smaller wording difference: the 2024 sheet lists the wet H2S forms ending in "SCC", while the 2025 and current sheets end in "SSC" (sulfide stress cracking). The paragraph number, 3.67, is the same.

Where each mechanism shows and how it is found

These notes are grouped by type rather than in RP 571's order. They describe general patterns from inspection practice. For each mechanism, the RP 571 paragraph covers affected materials, critical factors, appearance, prevention and inspection, so build your cards from it.

Environmental cracking

• 3.3 Amine SCC. Shows in carbon steel at welds and heat-affected zones that were not stress relieved, in amine treating units. Commonly found with wet fluorescent magnetic particle on cleaned weld surfaces, with angle-beam UT to judge depth.
• 3.15 Caustic SCC. Shows at carbon steel welds that were not stress relieved, and near steam-traced or heated spots where caustic concentrates. Commonly found with wet fluorescent magnetic particle; penetrant can miss tight, scale-filled cracks.
• 3.17 Chloride SCC. Shows in 300-series stainless steel exposed to chlorides while warm, often outside under wet insulation or in crevices and exchanger tubes. Commonly found with penetrant testing on the surface, with eddy current used on tubing.
• 3.67 Wet H2S damage. Shows in carbon steel in wet sour service: blisters and stepwise cracks in plate, cracking beside welds, and cracking at hard weld zones. Commonly found with wet fluorescent magnetic particle for surface cracks and straight-beam UT for blisters and internal laminar cracks.

Thinning at low and moderate temperatures

• 3.8 Atmospheric corrosion. Shows on outside surfaces where paint has failed or water sits: saddles, supports, nameplate edges and low ledges. Commonly found by visual examination, then UT thickness readings where it looks worst.
• 3.9 Boiler water and steam condensate corrosion. Shows inside condensate return lines, deaerators and feedwater equipment, often as pitting. Commonly found with internal visual examination where access allows and UT thickness readings or profile radiography from outside.
• 3.14 Caustic corrosion. Shows as local gouging where caustic concentrates, for example at injection points and under deposits on heated surfaces. Commonly found with UT thickness scanning and radiography, since spot readings can miss a narrow groove.
• 3.22 Corrosion under insulation. Shows where water gets past jacketing: penetrations, nozzles, support rings, skirts and the low side of the vessel. Commonly found by visual examination with insulation removed at likely spots, plus profile radiography or UT; screening methods help decide where to strip. Our post on corrosion under insulation covers where it hides, the warning signs and how inspection planning handles it.
• 3.27 Erosion/erosion-corrosion. Shows where flow turns or speeds up: inlet nozzles, impingement areas opposite them, internals and fittings downstream of valves. Commonly found with UT thickness grids and visual examination of internals.
• 3.37 Hydrochloric acid corrosion. Shows where water first condenses in a stream carrying chlorides, such as overhead systems and effluent coolers. Commonly found with UT thickness scanning and radiography focused on the condensation zone.

High-temperature damage

• 3.35 High-temperature H2/H2S corrosion. Shows as fairly even thinning in hot hydroprocessing equipment such as reactor effluent piping and exchangers. Commonly found with UT thickness readings at fixed monitoring locations, trended over time.
• 3.36 High-temperature hydrogen attack. Shows in carbon and low-alloy steels in hot hydrogen service, often first near welds. Hard to find early with surface methods; specialized UT techniques performed by qualified examiners are the usual approach.
• 3.46 Naphthenic acid corrosion. Shows in hot crude and vacuum unit equipment, worst where velocity is high, as local pitting or grooving. Commonly found with UT thickness scanning and radiography, backed by process monitoring.
• 3.61 Sulfidation. Shows as thinning of carbon and low-alloy steel in hot sulfur-bearing streams, sometimes worse in a single component made from the wrong material. Commonly found with UT thickness readings, with positive material identification (the RP 578 topic on the same sheet) to catch the wrong alloy.

Fracture

• 3.11 Brittle fracture. A sudden failure risk in carbon and low-alloy steel at low metal temperature, most often during startup, pressure testing or depressuring. It isn't found by examination in advance; it is managed by checking metal temperature against the material's limits, and UT or magnetic particle can find the existing flaws a fracture would start from.

Section 2 is terms and definitions. It has no mechanism of its own, but a closed-book question can turn on a term from it, so read it once with the mechanism paragraphs.

How the examination methods line up with the rest of the sheet

The current sheet limits ASME Section V to Articles 1, 2, 6, 7 and 23, with Article 23 limited to SE-797. Article 2 is radiography, Article 6 is liquid penetrant, Article 7 is magnetic particle and SE-797 is ultrasonic thickness measurement. The Body of Knowledge adds in its Section III that magnetic particle covers yoke and prod techniques only, and its list of items candidates are not expected to know (Section I.B, item 18) includes NDE requirements for acoustic emission, eddy current and motion radiography.

That gives a useful split when you study. Methods in our notes such as eddy current, guided wave or specialized UT for hydrogen attack belong to the damage mechanism side of your reading. Code rules for radiography, penetrant, magnetic particle and UT thickness belong to the Section V side. RP 572, also on the sheet, covers inspection practice for vessels and is the place to connect the two.

A worked example: erosion at an inlet nozzle

All numbers here are invented. A vessel's inlet nozzle has a UT reading of 0.412 in. at one inspection and 0.367 in. at the next, 5.0 years later. The required thickness for the nozzle is 0.300 in.

• Metal loss: 0.412 - 0.367 = 0.045 in.
• Corrosion rate: 0.045 / 5.0 = 0.009 in. per year.
• Remaining life: (0.367 - 0.300) / 0.009 = about 7.4 years.

The Body of Knowledge points to API 510 paragraphs 7.4, 7.1 and 7.2 for metal loss, corrosion rate and remaining life, and to Section 6 for the inspection interval that follows. Open those in your own copy for the rules. The mechanism matters as much as the arithmetic: an erosion pattern is local, so a single reading point can understate the loss next to it, which is why the notes above lean on scanning and grids. You can check this kind of working with the corrosion rate calculator.

How to study the 15

1. Make one card per paragraph number with five headings: materials, where on the vessel, what it looks like, how it's found, and how it's prevented. Fill each card from RP 571 in your own words.
2. Compare your "how it's found" line with the notes above and with RP 572. Where they differ, RP 571 and RP 572 win.
3. Practice going from a symptom to a paragraph number, for example "cracking at a non-stress-relieved weld in caustic service" to 3.15, since the mechanism names on the sheet are close enough to confuse under time pressure. The API 510 damage mechanism quiz gives you 18 scenarios on these 15 paragraphs, with every answer explained.
4. Recheck the sheet when your exam window changes. The RP 571 list changed at the September 2025 cycle and can change again.

For the full edition list and section limits for every publication, read our effectivity sheet guide. For how the five Body of Knowledge sections fit together, see the Body of Knowledge guide.

See where you stand

The free 10-question check takes a few minutes, needs no signup and explains every answer, so you see which Body of Knowledge 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.

Related exam prep

• NDT methods compared on Angle Beam: how the NDT methods find the damage described here.

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, which links the current sheet and Body of Knowledge