API 571 Damage Mechanisms: Complete Industry Guide [2026]

What API RP 571 actually is, and how damage mechanisms drive NDE method selection

Short answer: API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, is a recommended practice and not an inspection code. The current edition is the 3rd, published in 2020. It describes nearly 70 mechanisms: how each one forms, which materials and units it affects, its critical factors, what the damage looks like, how to prevent it, and how to inspect and monitor for it. It does not set inspection intervals, acceptance criteria, personnel levels or record-retention periods. Those come from API 510, 570 and 653, from the construction codes, and from the owner's programme.

This section corrects several statements earlier on this page and adds the part the page lacks: a working map from damage mechanism to NDE method, and how inspection teams use RP 571 in practice. The RP is copyrighted. What follows paraphrases its role and structure from API's public descriptions. Consult a licensed copy for mechanism details.

Corrections to the earlier sections of this page

The general template text above describes API 571 as if it were an NDE code. It is not, and the following points need correcting:

• "API 571 establishes requirements, methods and acceptance criteria for nondestructive examination." RP 571 has no NDE acceptance criteria. Flaw acceptance comes from the construction code (for example, ASME Section VIII or B31.3) for new work. For in-service equipment, the inspection code (API 510, 570 or 653) applies, together with an engineering assessment the owner commissions where needed, such as API 579-1/ASME FFS-1.
• Inspection frequencies "defined" by API 571. RP 571 gives no intervals. Intervals come from API 510 for pressure vessels, API 570 for piping and API 653 for tanks, or from an RBI programme the owner runs under API 580/581.
• Personnel Levels I, II and III, and recertification "mandated by the standard." These are ASNT and ISO 9712 concepts, applied through the employer's written practice, for example under SNT-TC-1A. RP 571 does not set personnel qualification rules.
• Record retention "as specified in API 571." RP 571 sets no retention period. Record requirements come from the inspection codes, OSHA process safety management and the owner's procedures.
• Use of alternative NDE methods requires "regulatory authority approval." RP 571 does not regulate method choice. The owner and inspector select techniques that can detect and size the expected damage, under qualified procedures.
• The scope list (aerospace, castings, "bolted connection pre-load"). RP 571 covers fixed equipment in refining. API notes that much of it also applies to petrochemical and other process industries, but it is not an aerospace or manufacturing standard.
• "API 510/570/575: in-service inspection of pressure vessels, piping, and tanks." API 575 is a recommended practice for inspecting atmospheric and low-pressure tanks. The in-service tank standard is API 653.

How RP 571 is organised

The 3rd edition (2020) reorganised the document. Earlier editions grouped mechanisms by category. Industry summaries of the 2020 edition report that it removed those subcategories and listed the mechanisms alphabetically. It merged a few mechanisms, such as folding vibration-induced fatigue into mechanical fatigue, withdrew one, and added several new ones, including brine corrosion, concentration cell corrosion and oxygenated water corrosion (non-boiler). Each mechanism entry follows a consistent pattern:

• a description of the damage,
• the materials affected,
• the critical factors that control whether and how fast it happens,
• the units or equipment where it is typically found,
• its appearance or morphology,
• prevention and mitigation,
• inspection and monitoring options, with related mechanisms and references.

The appearance and inspection entries matter most for NDE. They tell you whether the damage is general or localised thinning, surface-breaking cracking, embedded or subsurface damage, or a change in microstructure or properties. That classification decides which method can find the damage.

API describes the RP as guidance for pressure equipment integrity personnel, covering integrity from identifying damage through to conducting examinations. Owners use it to identify existing damage and future susceptibility, to build inspection and monitoring strategies, and as an input to RBI and fitness-for-service assessments. Those assessments are engineering work the owner commissions.

From mechanism to method: an NDE selection matrix

Choosing an inspection method starts with the damage morphology, not the method you already own. The table below groups common refinery mechanisms by the kind of damage they produce. For each group it shows the NDE methods usually used and where those methods fall short. It is a planning aid. The owner's damage mechanism review and the inspector of record decide the actual scope.

For a deeper look at method capability by mechanism, see NDE method selection by damage mechanism. Atlantis has separate articles on sulfidation, naphthenic acid corrosion, stress corrosion cracking variants and HTHA.

Have a damage mechanism review and need the NDE to match it? Send the circuit list and get a quote.

A worked example: one hydroprocessing train, four mechanisms, four different scopes

Take a typical hydrotreater reactor effluent train in a US refinery, described in general terms. The owner's corrosion engineer has already done the damage mechanism review, and the inspection team now needs a scope that matches it.

1. Reactor effluent air cooler and outlet piping: ammonium bisulfide corrosion. The damage is velocity-dependent and concentrates at tube inlets, return bends and downstream of injection points. Spot UT at CMLs on straight runs will miss it. The scope moves to encoded corrosion mapping at elbows, tees and the first diameters after changes in direction. Tube-side inspection of the cooler uses tube techniques, not shell UT.
2. Cold high-pressure separator: wet H₂S damage. RP 571 places this service at risk of HIC, SOHIC and SSC. The scope combines internal WFMT of welds and heat-affected zones at a turnaround with UT examination of plate for mid-wall blistering and stepwise cracking. On-stream, PAUT from outside can help monitor known HIC areas. The sour service interval logic is covered in API 510 intervals in wet H₂S service.
3. Feed/effluent exchanger shells in carbon steel: HTHA. If operating temperature and hydrogen partial pressure put carbon steel near or above the API RP 941 carbon steel curves, the risk is microscopic fissuring that thickness readings will never show. Inspection here uses advanced UT run by examiners trained specifically for HTHA. The materials decision stays with the owner's engineers.
4. Insulated cold piping downstream: CUI. The damage is external and patchy, driven by insulation and coating condition. The scope uses screening to find wet or damaged areas, then removes insulation and does UT or profile RT to size the wall loss. See CUI inspection.

One train, four mechanisms, four completely different NDE plans. A generic "UT thickness everywhere every few years" plan would have found only part of the problem. That is the practical value of RP 571: it forces the inspection plan to ask what the damage will look like before choosing how to look for it.

Inspection effectiveness: the question behind every method choice

Choosing the right method is only half the job. The other half is coverage and capability. Inspection planners, and RBI programmes where an owner runs one, judge an inspection by how likely it was to find the damage that was there. That depends on several factors:

• Coverage. What fraction of the susceptible area was examined: one CML per circuit, a scanned band, or 100% of the welds in the susceptible zone?
• Technique capability. Can this set-up detect the expected flaw size and orientation? Mid-wall HIC, for example, needs a different set-up from ID-connected SOHIC.
• Surface condition and access. Coatings, scale, temperature and insulation limit what is possible on-stream.
• Procedure and personnel. The technique should follow a written procedure approved under the employer's Level III, carried out by technicians qualified in that method. Mechanisms that need specialised skills, such as HTHA and early-stage cracking, deserve technique-specific demonstration.

An inspection report that records the mechanism targeted, coverage achieved, technique and procedure, and the limitations encountered gives the integrity team what it needs. A report that says only "UT performed, no recordable indications" does not.

What the inspection record should carry for the integrity team

• The damage mechanism or mechanisms the examination targeted, using the owner's terminology from the review.
• Method, technique, procedure number and revision, and the qualification level of the technician.
• Coverage achieved compared with coverage planned, with the reasons for any shortfall.
• Indications sized and located on a drawing that can be compared next cycle, plus encoded data files where encoded methods were used.
• Limitations such as surface condition, temperature, dead zones and access.

Those records feed the owner's API 510, 570 or 653 inspector and engineers. Corrosion control documents (API RP 970) and integrity operating windows (API RP 584) are the owner-side tools that keep the review current when process conditions change.

Want inspection reports that record mechanism, coverage and limitations for your inspector of record? Ask for a scope.

"API 571 workshop", course and certification questions

Many searches for API 571 are about certification. Here are the public facts from API's Individual Certification Programs API 571 page, checked in October 2026:

• The certification is API 571 Corrosion and Materials, and its body of knowledge is based on RP 571, 3rd edition (2020).
• The exam has 110 multiple-choice questions, of which 100 are scored. It runs 3.25 hours, is closed-book, and can be taken at a test centre or by remote proctoring.
• Holders of a current API 510, 570 or 653 certification qualify automatically. Other candidates qualify through a published combination of education and experience.
• Certification lasts three years, with recertification available before expiry.

API publishes the body of knowledge and the exam effectivity sheet, and those are the authoritative study scope. Workshops and prep courses are offered by many third parties, and their quality varies. Atlantis does not offer API 571 or other API certification courses. For NDE personnel who carry out the examinations these mechanisms call for, ASNT SNT-TC-1A method training is the complementary path. See NDT Level II training and ASNT Level III training.

How Atlantis supports this

Atlantis performs NDE matched to the damage mechanisms in an owner's review. That includes UT thickness and encoded corrosion mapping, PAUT and TOFD, WFMT, PT and ET, guided-wave screening, and CUI screening with prove-up. ASNT-certified technicians do the work under ASNT Level III procedure oversight. We deliver results to the owner's API 510, 570 or 653 inspector, who remains inspector of record. The damage mechanism review itself, RBI, and fitness-for-service assessment are the owner's engineering work, and we do not offer them. See our inspection services overview.

Need PAUT, TOFD or WFMT for cracking-susceptible equipment? Request a quote.

API 571 questions inspection teams ask

Is there an API 571 workshop?

API certifies against API 571 and publishes its body of knowledge. Workshops and prep courses come from third parties. Check any provider against API's published exam scope and effectivity sheet. Atlantis does not run API 571 training.

What is the current edition of API RP 571?

The 3rd edition, published in 2020. The API 571 Corrosion and Materials certification is based on it. Check API's catalogue for any later edition or addendum before relying on details.

How many damage mechanisms are in API 571?

Nearly 70 in the 2020 edition, by industry count. Confirm the exact list in a licensed copy, because some were merged or withdrawn and new ones added.

Does API 571 set inspection intervals?

No. Intervals come from API 510, 570 and 653, or from an owner's RBI programme. RP 571 tells you what damage to expect and how it can be inspected.

Does API 571 contain acceptance criteria?

No. Acceptance comes from the applicable construction or inspection code, or from an engineering assessment the owner commissions.

Which NDE method finds wet H₂S cracking?

WFMT is commonly used for surface-breaking cracking at welds. UT and PAUT are used for mid-wall HIC and SOHIC. The scope should match the forms identified in the owner's review.

Who should perform the damage mechanism review?

The owner's corrosion or materials engineers, with process and inspection input. NDE contractors carry out the examinations that come out of the review.

Do API 510, 570 and 653 inspectors need to know RP 571?

Yes. RP 571 content is part of the published bodies of knowledge for those certifications, and API offers API 571 as a separate certification as well.