Naphthenic Acid Corrosion 2026 — TAN + Sulfur Synergy Decoded

Naphthenic Acid Corrosion 2026 — TAN + Sulfur Synergy Decoded — naphthenic acid corrosion, TAN > 0.5, sulfur synergy, 9Cr-1Mo derating, refining crude unit applications. Informational guide by an ASNT NDT Level III.

Atlantis NDT · · Corrosion + Damage Mechanisms

Naphthenic Acid Corrosion 2026 — TAN + Sulfur Synergy Decoded

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.

Mechanism

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. Reference: API RP 571 (3rd ed.) paragraph 3.46.

Susceptibility Factors

  • Total acid number (TAN) of the crude or fraction; risk rises with TAN, though TAN alone does not predict corrosivity
  • Temperature in the susceptible range
  • Velocity and turbulence, two-phase flow and condensing conditions
  • Sulfur content and its interaction with acids
  • Alloy: carbon steel and low-Cr steels are most affected; molybdenum-bearing stainless steels such as 316 and 317 resist better

Detection Methods

  • UT thickness and corrosion mapping at high-velocity and turbulent locations
  • Profile radiography
  • Visual inspection during turnarounds
  • Corrosion probes and coupons, and monitoring crude TAN and sulfur

Technique selection should follow the damage mechanism, with procedures approved under the employer's written practice.

Prevention + Mitigation

  • Upgrading to molybdenum-bearing alloys in susceptible sections
  • Crude blending to manage TAN
  • High-temperature corrosion inhibitors in some services
  • Design to reduce velocity and turbulence

How RBI and FFS Use the Inspection Data

Owners 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.

FAQs

Q1: Which inspection method finds this damage?

A: UT thickness and corrosion mapping at high-velocity and turbulent locations; profile radiography.

Q2: Where is it described in the codes?

A: API RP 571 (3rd ed.) paragraph 3.46. API RP 571 describes the mechanism, critical factors, affected equipment and inspection and monitoring options.

Q3: Who decides the inspection interval?

A: 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.

Q4: How does Atlantis NDT help?

A: 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.

Related Codes, Qualification and Records

Recommended 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.

Related Damage Mechanisms

API 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.

Frequently Asked Questions

Q1: Which code edition of Naphthenic Acid Corrosion 2026 should we reference?

Use 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.

Q2: How does this fit our vertical operational model?

A: 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.

Q3: What is the typical implementation timeline?

Implementation time depends on the agreed configuration, data preparation, interfaces and acceptance tests. Confirm milestones and responsibilities in the proposal.

Q4: Does Atlantis NDT publish pricing?

Pricing depends on region, scope, delivery arrangements and team size. Request a proposal that identifies what is included, what needs configuration and the support provided.

Q5: How does Atlantis integrate with our existing stack?

A: 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.

Q6: How do I start?

A: 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.

Related Atlantis NDT Resources

Atlantis 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.

Running this as a programme, not a one-off

If you are responsible for an inspection programme rather than a single job, the recurring problem is rarely the code — it is keeping measured thickness, damage-mechanism assignment and next-inspection dates in one defensible place. Asset integrity management software covers keeping measured thickness readings per CML in one place, so the RBI (API 580/581) and fitness-for-service (API 579) work your integrity team or its specialists carry out starts from measured data rather than default rates. Atlantis supplies the NDT data and the software to hold it; it does not perform RBI or FFS assessments.

Atlantis NDT Products & Services

Where Atlantis can help

Atlantis provides NDT inspection, ASNT SNT-TC-1A training, Level III consulting and NDT software.