8.3 Amine, Caustic & Chloride Stress Corrosion Cracking

Amine, Caustic & Chloride Stress Corrosion Cracking

The current API 510 blueprint selects three stress-corrosion mechanisms from API RP 571: amine stress corrosion cracking, caustic stress corrosion cracking, and chloride stress corrosion cracking. Each requires the same three-part framework:

1. a susceptible material or microstructure;
2. a specific chemical environment; and
3. sustained tensile stress, either applied or residual.

These are cracking mechanisms, not ordinary uniform corrosion. A component can retain nearly full general wall thickness while a tight crack threatens integrity. Inspection plans must therefore target crack location and orientation, not only average thickness.

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1. Amine Stress Corrosion Cracking

Amine SCC is most often associated with carbon steel equipment and piping in aqueous alkanolamine systems used to remove acid gases. Cracking commonly develops at or near welds, attachments, heat-affected zones, and highly stressed cold-worked regions. Residual welding stress is a major susceptibility factor, so cracking is frequently associated with weldments that were not effectively stress relieved.

The environment and operating history matter. Concentration, acid-gas loading, contaminants, temperature, local wetting, and solution chemistry can change susceptibility. Do not convert a risk-based mitigation recommendation into a universal rule that every amine-service weld has exactly the same mandatory heat treatment. The examination task is to recognize the mechanism and apply the effective edition of RP 571 and the applicable construction, repair, and engineering requirements to the actual service.

Typical morphology is surface-connected, oxide-filled cracking near welds and HAZs. Cracks may run parallel to a weld and can be difficult to see after ordinary surface cleaning.

Inspection: Wet fluorescent magnetic particle testing is effective for surface-breaking cracks in ferromagnetic carbon steel when surface condition permits. Angle-beam ultrasonic techniques can help size or detect cracks below the surface. Visual examination and thickness readings alone are not sufficient to rule out SCC.

Mitigation: Effective stress-relief heat treatment, control of solution chemistry and contaminants, appropriate material selection, and reducing stress concentrators can reduce susceptibility. Repairs must address both the crack and its cause; simply grinding or welding over a crack without changing the environment/stress combination invites recurrence.

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2. Caustic Stress Corrosion Cracking

Caustic SCC occurs when concentrated sodium or potassium hydroxide contacts a susceptible material under tensile stress. Carbon steel and low-alloy steel are common concerns; some stainless steels and nickel alloys have service-dependent susceptibility ranges. Caustic can concentrate at heat-transfer surfaces, beneath deposits, at leaks, in crevices, or where evaporation occurs, so the local chemistry may be much more severe than a bulk sample suggests.

Cracking in carbon steel is commonly surface connected and may be predominantly intergranular. High-residual-stress locations—weld toes, HAZs, attachments, bends, and cold-worked details—deserve priority. A network of fine cracks can be hidden by deposits or corrosion scale.

Inspection: After suitable cleaning, WFMT is often selected for carbon steel. PT may be used on nonmagnetic alloys with an appropriate surface. Angle-beam UT can characterize deeper cracking when the procedure is qualified for the geometry and expected orientation.

Mitigation: Control caustic concentration and temperature, prevent local concentration beneath deposits or at heat-transfer hot spots, reduce residual stresses where engineering rules call for it, and select a more resistant material when service severity warrants. Because susceptibility depends on material, concentration, temperature, and stress, avoid memorizing one universal temperature boundary.

Caustic corrosion and caustic SCC are separate RP 571 mechanisms. Caustic corrosion is metal loss; caustic SCC is cracking under tensile stress. They can occur in related environments, but their damage morphology and inspection response differ.

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3. Chloride Stress Corrosion Cracking

Chloride SCC primarily affects 300-series austenitic stainless steels exposed to aqueous chlorides while under tensile stress. Oxygen or another oxidizing influence, increasing temperature, chloride concentration, evaporation, and wet-dry cycling can increase likelihood. It can occur at temperatures below commonly quoted screening values when conditions are severe, so a single temperature should not be treated as an absolute threshold.

Cracks are classically highly branched and predominantly transgranular. They often initiate at pits, crevices, deposits, insulation breaches, or other locations where chlorides concentrate. Weld residual stress can make HAZs and attachment details vulnerable even without high applied stress.

Inspection: PT can reveal surface-breaking cracks on clean stainless steel. Eddy current may be useful for thin sections or tubing. Angle-beam UT can assist with crack detection and sizing, although complex branching and geometry demand a suitable technique and qualified examiner. Ordinary UT thickness readings do not establish the absence of chloride SCC.

Mitigation: Exclude chlorides and moisture where practical, prevent concentration under deposits or insulation, use compatible insulation and coatings, control wash-water quality, reduce tensile stress, or upgrade to a more resistant alloy based on engineering evaluation.

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4. Comparison and Inspection Logic

When an exam scenario presents cracking with little general wall loss, identify the material first, then the environment, crack morphology, and stress source. Select an examination method capable of finding cracks in that material. Finally, choose mitigation that removes or controls at least one of the three necessary factors. This structured approach is more reliable than relying on a single remembered temperature or concentration.