8.4 Wet H2S Damage, Brittle Fracture & Erosion-Corrosion

Wet H2S Damage, Brittle Fracture & Erosion-Corrosion

These three selected API RP 571 topics create very different integrity threats. Wet H2S can create internal hydrogen damage or cracking with little general corrosion. Brittle fracture can propagate suddenly when toughness is inadequate. Erosion-corrosion produces directional, localized metal loss where a moving phase strips protective scale. Correct identification begins with morphology and location, then uses service conditions as confirmation.

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1. Wet H2S Damage Family

In aqueous sour service, corrosion reactions generate atomic hydrogen at the steel surface. Hydrogen atoms can enter carbon and low-alloy steel before recombining. Their destination, the steel microstructure, and the presence or absence of tensile stress determine the observed mechanism.

Hydrogen Blistering

Hydrogen atoms diffuse to laminations, inclusions, or other internal discontinuities and recombine as molecular hydrogen. Molecular hydrogen cannot diffuse readily through steel, so pressure builds and separates the metal into visible blisters. Applied tensile stress is not required. Blisters can remain isolated or link with other damage.

Hydrogen-Induced Cracking (HIC)

HIC consists of planar cracks associated with inclusions or laminations. Adjacent cracks can link in a stepwise pattern through the wall. Like blistering, ordinary HIC can develop from internal hydrogen pressure without applied tensile stress. Cleaner steels with controlled inclusion shape are more resistant, but service chemistry remains essential.

Stress-Oriented HIC (SOHIC)

SOHIC is an array of small HIC-like cracks stacked through thickness and oriented by tensile stress. It is often associated with welds and heat-affected zones because residual stress supplies the orientation force. The stacked pattern can be especially dangerous because it may extend across a substantial fraction of the wall.

Sulfide Stress Cracking (SSC)

SSC is hydrogen-assisted cracking of susceptible high-strength or high-hardness material under tensile stress in wet H2S service. Hard weld metal and HAZ microstructures are frequent concerns. Service-specific material standards and engineering specifications commonly impose hardness controls; 22 HRC is a widely used carbon- and low-alloy-steel screening limit, but the applicable material, weld, service, and governing specification must be checked rather than converting one number into a universal rule.

Inspection: Visual examination can find blisters but cannot establish their full extent. UT scanning and mapping can locate laminations and blister geometry. Angle-beam or other qualified ultrasonic techniques are used for crack detection and sizing; surface MT may find surface-connected cracking in ferromagnetic steel. Technique selection must account for crack orientation and surface condition.

Mitigation: Control water and sour chemistry where possible, use appropriate clean or HIC-resistant steel, control weld hardness and hydrogen, reduce tensile residual stress where engineering requirements call for it, and prevent contamination or process excursions that increase hydrogen charging.

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2. Brittle Fracture

Brittle fracture is rapid crack propagation with little macroscopic plastic deformation. It is most credible when four factors coincide:

1. a material with inadequate notch toughness at the actual metal temperature;
2. tensile stress from pressure, residual welding stress, thermal gradients, or a combination;
3. a crack-like flaw or sharp stress concentration; and
4. sufficient restraint or thickness to limit plastic relaxation.

Carbon and low-alloy steels undergo a ductile-to-brittle transition as temperature decreases. Heavy sections tend to have greater constraint. Startup, shutdown, hydrotest, autorefrigeration, and cold-weather depressuring can create governing low-temperature cases even when normal operation is warm.

ASME Section VIII Figure UCS-66 and related rules establish impact-test exemption temperatures based on material assignment, governing thickness, heat treatment, and stress ratio. The vessel's stamped MDMT, actual temperature-pressure history, weld details, repairs, and material documentation belong in the evaluation. An inspector should not assume that a warm design operating temperature eliminates brittle-fracture risk during a different operating case.

Inspection and prevention: Find and evaluate crack-like flaws, confirm actual material and heat treatment, observe pressure-test and startup temperature controls, review repairs or alterations for toughness effects, and enforce the permitted pressure-temperature envelope. Raising metal temperature before applying high pressure reduces risk; the exact limit comes from the vessel's applicable evaluation, not a generic rule of thumb.

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3. Erosion and Erosion-Corrosion

Erosion is mechanical surface removal by fluid, droplets, entrained solids, or bubbles. Erosion-corrosion occurs when flow repeatedly strips a protective oxide or sulfide film and exposes fresh metal to renewed corrosion. The result is commonly localized and directional rather than uniform.

High-priority locations include:

• inlet nozzles and the shell or internals directly opposite a jet;
• elbows, tees, reducers, control-valve outlets, and changes in flow direction;
• flashing or two-phase zones where droplets or bubbles collapse;
• slurry and catalyst paths carrying abrasive solids;
• tray outlets, downcomers, spargers, and impingement devices;
• restrictions and areas of high turbulence or local velocity.

Morphology may include smooth directional grooves, horseshoe-shaped scars, scallops, rounded depressions, or highly polished areas. The damage pattern often points downstream and follows the flow field. A failed or missing impingement plate can shift damage into the pressure boundary.

Inspection: Do not rely on one spot reading. Use targeted UT grids, encoded scanning, profile radiography where suitable, or direct dimensional profiling to locate the true minimum. Map both the nozzle and the downstream impact zone. Examine sacrificial internals and attachments because their failure may expose the shell even before shell loss becomes obvious.

Mitigation: Reduce velocity or turbulence, eliminate flashing where feasible, remove entrained solids, restore or redesign impingement protection, smooth abrupt flow transitions, increase local thickness, or select a more erosion-corrosion-resistant material. A material upgrade alone may not solve an extreme flow problem.

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4. Identification Workflow

Ask four questions in sequence:

1. Is the indication a bulge/lamination, a crack, or wall loss?
2. Does the service contain liquid water and H2S, expose ferritic steel to low temperature, or create high-energy flow?
3. Is the location a stressed weld region, a thick constrained detail, or a jet/turbulence path?
4. Can the selected NDE method find the expected orientation and true minimum?

Blisters or stepwise internal cracks in wet sour service support hydrogen damage. Rapid cleavage risk at low metal temperature supports brittle fracture. Smooth directional thinning at an inlet or change in flow supports erosion-corrosion. Service history confirms the diagnosis; morphology and location prevent a plausible but wrong label.