naphthenic acid corrosion, TAN > 0.5, sulfur synergy, 9Cr-1Mo derating, refining crude unit applications. Catalogued by API 571 as a major damage mechanism affecting fixed equipment in refining and petrochem. This guide walks through mechanism, susceptibility, detection, and prevention.
\nNaphthenic acid corrosion (NAC) is high-temperature corrosion caused by organic (naphthenic) acids in crude oil, typically between about 425°F and 750°F (220°C to 400°C). It often appears as localized pitting, grooving or sharp-edged attack in high-velocity or turbulent areas and interacts with sulfidation, since sulfur compounds can form protective sulfide scales. Reference: API RP 571 (3rd ed.) paragraph 3.46.
\nTechnique selection should follow the damage mechanism, with procedures approved under the employer's written practice.
\nOwners that run risk-based inspection (API RP 580/581) use damage-mechanism susceptibility and measured inspection results to set inspection priorities and intervals, and damage beyond code acceptance may be referred for an engineering fitness-for-service assessment (API 579-1/ASME FFS-1). Both are owner and engineering responsibilities; Atlantis NDT does not provide RBI or FFS services. The Atlantis Digital Twin platform can display the underlying inspection data on a 3D model.
\nA: UT thickness and corrosion mapping at high-velocity and turbulent locations; profile radiography.
\nA: API RP 571 (3rd ed.) paragraph 3.46. API RP 571 describes the mechanism, critical factors, affected equipment and inspection and monitoring options.
\nA: The owner's authorized inspector under API 510, 570 or 653, or the owner's RBI assessment where one is used. Atlantis NDT does not provide RBI services.
\nA: With ASNT Level III consulting on NDE procedures and technique selection, ASNT SNT-TC-1A training for the technicians who perform the examinations, and inspection software (Atlantis ERP and the Digital Twin platform) that holds the inspection records.
\nRecommended practices and guides inform an inspection programme; binding requirements come from the in-service codes (API 510, API 570, API 653), the construction codes and the regulations that apply to the asset. NDE personnel are qualified under the employer's written practice (commonly ASNT SNT-TC-1A, ANSI/ASNT CP-189 or ISO 9712), and NDE procedures are approved by the employer's NDT Level III. Acceptance of indications follows the governing code; damage beyond its rules may be referred for an engineering fitness-for-service assessment (API 579-1/ASME FFS-1), which Atlantis NDT does not perform. Keep procedure revisions, calibration records and personnel certifications traceable; Atlantis ERP can hold these records.
\n\nAPI RP 571 (3rd edition, 2020) catalogues the damage mechanisms affecting fixed equipment in refining; its numbering changed from earlier editions, so cite the edition you use. Related mechanisms to screen alongside this one include sulfidation, naphthenic acid corrosion, high-temperature hydrogen attack, wet H2S damage, amine and chloride SCC, CUI and MIC.
\nUse the edition required by the contract, governing requirements and applicable jurisdiction. Record that edition in the procedure and evaluate changes before adopting a later publication.
\nA: Atlantis NDT delivers vertical-specific procedure packs + inspector cert rosters + asset-integrity playbooks. Free 30-min discovery call scopes your specific asset class + damage mechanisms + code requirements + delivery model preference.
\nA: 4-20 weeks depending on team size + scope. SMB: 4 weeks. Mid-size: 8-12 weeks. Enterprise multi-region: 12-20 weeks. Atlantis NDT delivers phased rollout with clear milestone gates.
\nA: No — pricing varies by region, scope, delivery model, and team size. Atlantis NDT is positioned as affordable, accessible, and fully customizable. Free 30-min consultation + tailored quote.
\nA: Atlantis ERP has an open REST API, so it connects to SAP, Maximo, NetSuite or any other system that accepts API connections; each integration is scoped with you during implementation. Free integration scoping consultation.
\nA: Request a free 30-min consultation at /contact. We reply within one business day. We scope your asset class + damage mechanisms + code requirements + delivery model preference + integration needs in a single call.
\nAtlantis NDT — led by Anoop Rayavarapu (ASNT NDT Level III in UT, RT, MT, PT and VT). Free consultation. request a free consultation. Pricing varies by region and scope.
","order":0,"createdAt":"2026-06-24","updatedAt":"2026-10-18","quickAnswer":{"question":"What is Naphthenic Acid Corrosion 2026 and how do I detect it?","answer":"Naphthenic acid corrosion (NAC) is high-temperature corrosion caused by organic (naphthenic) acids in crude oil, typically between about 425°F and 750°F (220°C to 400°C). It often appears as localized pitting, grooving or sharp-edged attack in high-velocity or turbulent areas and interacts with sulfidation, since sulfur compounds can form protective sulfide scales. Informational guide by an ASNT NDT Level III; RBI and FFS decisions stay with the owner.","bullets":["Total acid number (TAN) of the crude or fraction; risk rises with TAN, though TAN alone does not predict corrosivity","Detection: UT thickness and corrosion mapping at high-velocity and turbulent locations","Mitigation: Upgrading to molybdenum-bearing alloys in susceptible sections"]}}