WFMT and ACFM for Surface Cracking in Sour and Amine Service

Short answer: wet fluorescent magnetic particle testing (WFMT) is the long-standing method of choice for surface-breaking cracks on the inside of carbon-steel vessels in wet H2S and amine service, but it needs vessel entry and blast-cleaned steel. Alternating current field measurement (ACFM) detects and estimates the depth of surface cracks through thin coatings with less preparation. Neither finds embedded HIC or SOHIC, which needs ultrasonic methods such as PAUT or TOFD.

This guide is for refinery, gas plant and petrochemical inspection engineers, API 510 inspectors and turnaround planners in the USA and Canada who have to choose a crack detection method for sour-water strippers, amine contactors and regenerators, overhead accumulators, flare knockout drums and similar equipment. It covers the damage mechanisms, the governing guidance, how WFMT and ACFM work and where each fails, and how a defensible scope combines surface and volumetric methods.

The cracking mechanisms you are looking for

Sour and amine environments produce several distinct damage mechanisms in carbon steel, and the method has to fit the mechanism. API RP 571 describes each of them; in summary:

• Sulfide stress cracking (SSC). Cracking of hard zones, typically weld metal and heat-affected zones with elevated hardness, under tensile stress in wet H2S. Surface-breaking, usually at welds.
• Hydrogen blistering and hydrogen-induced cracking (HIC). Atomic hydrogen from the corrosion reaction diffuses into the steel and collects at inclusions and laminations, forming blisters near the surface or stepwise internal cracks parallel to the plate surface. Largely embedded; not surface-breaking until it links up.
• Stress-oriented HIC (SOHIC). Stacked arrays of small HIC cracks aligned through-thickness, usually next to welds where residual stress is high. Can be embedded or connected to the surface.
• Amine stress corrosion cracking. Intergranular cracking of non-post-weld-heat-treated carbon steel welds and adjacent material exposed to lean amine solutions. Surface-breaking, often tight, frequently on the process side.
• Carbonate cracking. Related alkaline stress corrosion cracking found in some sour water and FCC environments, also surface-breaking at welds.

The practical split is simple: SSC, amine SCC and carbonate cracking start at the surface, so surface methods find them. HIC and much SOHIC start below the surface, so surface methods can miss them entirely. A 1995 refiner study published in Inspectioneering made this point after WFMT programmes failed to show the full extent of wet H2S damage, because some of it was not surface-breaking. The lesson still applies.

What the governing documents say

There is no single code clause that says "use WFMT". Method choice comes from a mix of owner programme, recommended practices and the in-service inspection code.

Note on a common mix-up: NACE SP0288 is the AMPP standard on inspection of lining application in steel and concrete equipment. It is not a wet fluorescent magnetic particle standard. For wet H2S crack inspection, the reference is SP0296.

How WFMT works and what it needs

WFMT is magnetic particle testing with fluorescent particles suspended in a liquid carrier, viewed under UV-A light in darkened conditions. The steel is magnetised, typically with an AC yoke for weld inspection; a surface-breaking crack disrupts the field and creates leakage that gathers particles into a bright line. Fluorescent particles give much higher contrast than visible dry powder, which is why WFMT finds tight cracks that dry MT or visual inspection miss.

The price of that sensitivity is preparation and access:

• Vessel entry. Most sour and amine cracking starts on the process side, so WFMT normally means a shutdown, decontamination, gas-freeing, confined-space entry and scaffolding.
• Surface preparation. Scale, deposits and coatings must be removed so particles can reach the crack; abrasive blasting to bare metal is common. That adds time and creates waste, and it can also remove evidence of corrosion product.
• Darkness and UV-A. Background light must be controlled and UV intensity checked, as the procedure requires.
• Coverage planning. Welds, attachment welds, nozzle welds and repair areas are the usual targets; the scope percentage and priorities come from the owner's programme and SP0296 guidance.

What WFMT gives you: excellent detection of surface-breaking cracks and clear length information. What it does not give you: crack depth, or any information about cracks that do not break the surface. Depth requires grinding exploration or ultrasonic sizing.

How ACFM works and where it fits

ACFM is an electromagnetic technique. The probe induces a uniform alternating current in a thin layer at the steel surface and measures two components of the magnetic field above it. A surface-breaking crack makes the current flow around and under it, which changes those field components in a recognisable pattern. Because the uniform field can be modelled mathematically, the instrument can estimate crack length and depth, not just presence.

ACFM's practical strengths for sour and amine work:

• Works through coatings. It does not need electrical contact with the steel, so it can inspect through paint and thin non-conductive coatings, reducing or avoiding blasting.
• Depth estimate. The model-based depth estimate supports prioritising grinding and repair, and gives the owner's engineer data for any assessment.
• Recorded data. Signals are stored, giving an auditable record that can be reviewed and compared at the next inspection.
• Weld geometry. Probes are designed for weld toes and caps, where SSC and amine cracking concentrate.

Its limits:

• It is a surface technique; embedded HIC and subsurface SOHIC are out of reach.
• Sensitivity to very short or very tight cracks may be below WFMT on clean steel; ASME Section V Article 15 is framed around detecting linear discontinuities above a minimum size, so check what the procedure is qualified for.
• Branched or clustered cracking, typical of some SCC, complicates depth sizing because the model assumes a simpler crack.
• Thick or conductive coatings, and some geometries, reduce performance. Procedure qualification on representative samples is essential.

WFMT vs ACFM vs ultrasonic methods: a method-to-mechanism table

Inspectioneering's damage mechanism summaries list WFMT, eddy current, RT and ACFM as effective for amine cracking and note that penetrant testing is not effective, because tight scale-filled cracks do not hold dye reliably. Ultrasonic methods also allow external examination without entry, which is attractive for screening between turnarounds. Our guide on NDE method selection by damage mechanism and the amine and caustic cracking guide go further on each.

A worked example, described qualitatively

An amine regenerator and its reflux accumulator are due for an internal inspection at a refinery turnaround. Records show the accumulator was built without post-weld heat treatment. A previous internal inspection reported a few short linear indications at a nozzle weld, ground out and repaired.

1. Planning. The owner's API 510 inspector, using SP0296 and API RP 945 guidance, sets priorities: all nozzle welds, the previous repair area, a sample of long seams and attachment welds on the process side. External shear-wave PAUT is scheduled on the long seams before shutdown to locate any embedded or root-side indications in advance.
2. Pre-shutdown. PAUT finds no reportable SOHIC, but flags one region of laminar reflectors that is followed up with straight-beam UT mapping, consistent with mid-wall HIC.
3. During the outage. After cleaning, WFMT covers the blasted nozzle and repair welds. Where the internal lining makes blasting undesirable, ACFM is used through the coating on selected welds.
4. Findings. WFMT shows two short indications at a nozzle toe; ACFM estimates their depth as shallow. The inspector directs exploratory grinding and repeats WFMT to confirm removal.
5. Engineering. The HIC map goes to the owner's engineer, who decides whether a fitness-for-service assessment is needed. That decision and any assessment belong to the owner, not the NDE provider.

The combination worked because each method covered a mechanism the others miss.

Procedures, qualification and documentation the inspector expects

Whichever method is used, the API 510 inspector will expect the record to stand on its own:

• written procedures for WFMT, ACFM and UT methods, approved under the employer's written practice and, where invoked, compliant with ASME Section V;
• personnel qualified under ASNT SNT-TC-1A or CP-189 in the relevant method, with ACFM-specific training for that technique;
• for WFMT: yoke lift test or field indicator checks, particle concentration, UV-A intensity and ambient light readings, surface preparation achieved;
• for ACFM: calibration block records, probe type, coating thickness measured, data files retained;
• indication logs with location (weld ID, clock position, distance from datum), length, and depth where the method provides it;
• marked-up drawings showing coverage, including areas not examined and why.

Coverage records are where most audits find gaps. If 20 percent of a weld could not be examined because of internals, the report should say so.

Turnaround planning: making the crack inspection fit the outage

Crack inspection in sour and amine units usually sits on the critical path of a turnaround, because it can only start after the vessel is decontaminated, opened, gas-freed and cleaned, and repairs cannot start until it is finished. A few planning choices make the difference between a scope that is completed and one that is cut back on day three.

• Do the volumetric work before shutdown. External PAUT, TOFD and straight-beam UT mapping can usually be performed while the unit is running, provided surface temperature is within the probe and couplant limits. Doing it early means HIC or SOHIC findings arrive while there is still time to plan repairs or engineering reviews.
• Agree surface preparation in the work pack. Blasting scope, blast media, waste handling and who inspects the blasted surface should be agreed with the cleaning contractor before the outage. WFMT crews standing idle waiting for blasting is a common cause of lost coverage.
• Plan lighting and power. WFMT needs darkness and UV-A lamps; internal lighting plans for other crafts often conflict. Schedule night shifts or blackout periods for MT inside large vessels.
• Use ACFM to protect coatings and schedule. Where an internal lining or coating is in good condition, ACFM through the coating can avoid stripping and recoating, which can save days of curing time on the critical path.
• Pre-agree indication handling. Decide in advance who directs grinding, how deep exploration may go before the inspector is called back, and what re-examination is required. That avoids delays while a crew waits for decisions.
• Book confined-space support. Attendants, rescue plans and gas testing for every entry need to be planned with the crack inspection crews, not added later.

These are owner and contractor coordination items rather than code requirements, but they are where crack inspection scopes are usually won or lost.

Common mistakes

• Treating a clean WFMT result as clearance for HIC/SOHIC. It is not; volumetric methods are needed.
• Using PT instead of WFMT on carbon steel. Penetrant is less reliable for tight, scale-filled environmental cracks.
• Inadequate cleaning. WFMT on poorly prepared steel gives false comfort.
• Reading ACFM depth estimates as exact. They are model-based estimates; branched cracking and geometry affect them. Confirm by grinding or UT where decisions depend on depth.
• Ignoring PWHT history. Non-PWHT welds in amine service deserve priority; check construction records.
• Undocumented coverage. If it is not on the drawing, the next inspector cannot rely on it.

Regulatory overlay: US and Canada

In US refineries and gas plants, sour and amine vessels in covered processes are under OSHA PSM, 29 CFR 1910.119(j), which requires inspection and testing following recognized and generally accepted good engineering practices; API 510, API RP 571 and AMPP SP0296 are commonly used to meet that expectation. EPA's Risk Management Program rule (40 CFR Part 68) applies at many of the same sites. State boiler and pressure vessel laws may also apply; confirm with your jurisdiction. In Canada, provincial regulators such as ABSA in Alberta and TSSA in Ontario oversee pressure equipment integrity management; owner-user programmes there must show how environmental cracking is managed.

How Atlantis supports this

Atlantis NDT performs crack-detection NDE under the owner's API 510 programme with ASNT-certified technicians and ASNT Level III-approved procedures: WFMT and other magnetic particle testing for refining, phased array UT and TOFD for embedded and root-side cracking, and UT mapping for HIC and blistering. If you need ACFM, tell us the equipment and coating when you request a quote and we will confirm scope. Indications are reported to your API 510 inspector, who remains inspector of record and decides on disposition, repair and any engineering assessment. See our pressure vessel inspection services, and request a quote; we respond within 24 hours.

Frequently asked questions

What is WFMT inspection?

Wet fluorescent magnetic particle testing: fluorescent particles in a liquid carrier are applied to magnetised steel and viewed under UV-A light. Surface-breaking cracks show as bright lines. It is widely used inside refinery vessels for wet H2S and amine cracking.

Why is WFMT used for wet H2S inspection?

It is highly sensitive to tight, surface-breaking cracks such as SSC at welds, and refiners have used it for decades. But it must be paired with ultrasonic methods because HIC and much SOHIC are not surface-breaking.

ACFM vs WFMT: which is better?

WFMT is generally more sensitive on clean bare steel. ACFM works through coatings with less preparation and estimates crack depth. Many programmes use both, choosing by access, coating and whether depth data is needed.

Can ACFM size crack depth?

It estimates depth using a model of the field around the crack. Estimates are useful for prioritisation; confirm by grinding or ultrasonic sizing where critical decisions depend on them.

Does WFMT detect HIC?

Only where cracking has reached the surface. Mid-wall HIC is found with straight-beam UT and PAUT.

Is penetrant testing acceptable for amine cracking?

Penetrant is generally regarded as not effective for amine SCC, because tight, scale-filled cracks do not hold dye reliably. WFMT, ACFM, eddy current or ultrasonic methods are preferred.

What standard covers wet H2S cracking inspection?

AMPP (formerly NACE) SP0296, current edition 2020, together with API RP 571 for mechanisms and API 510 for the in-service inspection framework.

Is NACE SP0288 the wet fluorescent magnetic particle standard?

No. SP0288 covers inspection of lining application in steel and concrete equipment. For wet H2S crack inspection, use SP0296.

Can amine cracking be found from outside the vessel?

Often yes, with angle-beam PAUT or TOFD targeted at process-side weld toes, which supports screening between turnarounds. Procedure qualification on representative samples is important.

Who decides if a crack can stay in service?

The owner's API 510 inspector and engineer. Remaining cracks may be ground out, repaired, or assessed by the owner's engineer under a fitness-for-service standard. The NDE provider reports; it does not disposition.

Planning a sour or amine vessel turnaround? Send us your vessel list for a crack inspection quote. Related reading: HIC and SOHIC explained, API 510 intervals in wet H2S service, and ASME Section V Article 7 MT requirements. Ask about PAUT for embedded cracking too.