8.7 Amine/Caustic SCC, HCl, Sour Water, and MIC (API RP 571)

8.7 Amine/Caustic SCC, HCl, Sour Water, and MIC (API RP 571)

Six alkaline, acidic, and biological corrosion mechanisms from API RP 571 appear on the February 2026 API 570 Body of Knowledge. Inspectors must distinguish them by morphology, location, susceptible materials, inspection approach, and mitigation—not by memorizing section numbers alone. This section covers §3.3 Amine SCC, §3.14 Caustic Corrosion, §3.15 Caustic SCC, §3.37 Hydrochloric Acid Corrosion, §3.58 Sour Water Corrosion (Acidic), and §3.45 Microbiologically Influenced Corrosion (MIC).

Amine Stress Corrosion Cracking (API RP 571 §3.3)

Amine SCC affects carbon steel and low-alloy steels in alkanolamine sweetening systems (MEA, DEA, MDEA, etc.) when welds and heat-affected zones are high hardness or high residual stress and have not received adequate PWHT.

Morphology and Mechanism

• Intergranular cracking, often initiating at weld toes, HAZ, or hard spots.
• Cracks may be branching but are typically associated with alkaline amine chemistry, not chlorides.
• Attack accelerated by lean amine (higher acid gas loading), elevated temperature, and CO2/H2S content.

Preferred Locations

• Amine contactor/regenerator overhead and bottoms lines.
• Rich/lean exchanger channel covers and inlet nozzles.
• Reboiler and stripper piping.
• Heat-affected zones of welds not stress-relieved.

Susceptible Materials

• Carbon steel (most common); low-alloy steels without PWHT.
• Hardness typically > 200 BHN in HAZ is a red flag.

Inspection and Mitigation

• WFMT/MT and PT on welds; angle-beam UT for volumetric detection.
• Hardness testing of welds and HAZ.
• Mitigation: PWHT per B31.3 for amine service, control weld procedure to limit hardness, consider stainless upgrades only where fully evaluated for other damage modes.

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Caustic Corrosion (API RP 571 §3.14)

Caustic corrosion is general or localized metal loss in sodium hydroxide (NaOH) or potassium hydroxide (KOH) solutions—not cracking driven primarily by stress.

Morphology and Mechanism

• Uniform thinning, grooving, or impingement attack where velocity strips protective oxide.
• Accelerated at high temperature, high concentration, and high velocity (especially > 15–20 ft/s in concentrated caustic per industry guidance).

Preferred Locations

• Caustic transfer lines downstream of pumps.
• Elbows and tees in concentrated caustic service.
• Valve bodies and orifice locations.

Susceptible Materials

• Carbon steel (forms protective magnetite at moderate conditions; breaks down at high velocity/temperature).
• Copper alloys in some dilute caustic; nickel for severe service.

Inspection and Mitigation

• UT thickness at high-velocity fittings.
• Mitigation: velocity limits, dilution, temperature control, upgrade to nickel alloys in hot concentrated service.

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Caustic Stress Corrosion Cracking (API RP 571 §3.15)

Caustic SCC (often called caustic embrittlement) is stress-driven cracking in caustic solutions—distinct from caustic corrosion thinning.

Morphology and Mechanism

• Intergranular cracking in stressed regions (welds, bends, supports).
• Occurs in specific NaOH concentration/temperature windows (classically 20–50% NaOH at elevated temperature, though exam focus is concept not exact diagram memorization).
• Residual weld stress or external mechanical stress required.

Preferred Locations

• Steam-traced caustic lines (concentration increases from water boil-off).
• Weld HAZ on caustic piping.
• Cold-worked bends without stress relief.

Susceptible Materials

• Carbon steel; low-alloy steels.

Inspection and Mitigation

• PT/MT on welds; review PWHT records.
• Mitigation: PWHT/stress relief, avoid steam tracing without engineering controls, maintain dilute caustic where possible.

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Hydrochloric Acid Corrosion (API RP 571 §3.37)

HCl corrosion is aggressive general and pitting attack from hydrochloric acid in process streams, overhead systems, or acid gas environments.

Morphology and Mechanism

• Rapid uniform thinning and severe pitting.
• No protective scale on carbon steel in strong HCl.

Preferred Locations

• HCl injection points and downstream mixing zones.
• Overhead condenser systems where HCl condenses.
• Acid gas treater and regenerator overhead piping.

Susceptible Materials

• Carbon steel (very low resistance).
• Copper alloys in some dilute acid; nickel alloys (Hastelloy C, etc.) for severe service.

Inspection and Mitigation

• UT at injection points; visual for pitting.
• Mitigation: alloy upgrade, water wash/neutralization, inhibitors, lining.

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Sour Water Corrosion — Acidic (API RP 571 §3.58)

Acidic sour water contains dissolved H2S, NH3, CO2, and often chlorides, producing low-pH aqueous condensate in refinery overhead systems.

Morphology and Mechanism

• General thinning, under-deposit corrosion, pitting.
• Ammonium bisulfide and acidic salts accelerate attack.

Preferred Locations

• Refinery overhead systems: accumulators, reflux lines, sour water draw-offs.
• Condenser outlet and separator piping.

Susceptible Materials

• Carbon steel; 300-series SS may suffer chloride SCC if chlorides present (contrast with amine SCC on CS).

Inspection and Mitigation

• UT at low points; corrosion coupon/sour water sampling (pH, NH4HS, chloride).
• Mitigation: pH control, neutralizer injection, corrosion inhibitors, alloy upgrades in severe circuits.

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Microbiologically Influenced Corrosion (API RP 571 §3.45)

MIC results from microbial activity (sulfate-reducing bacteria, acid-producing bacteria, iron-oxidizing bacteria) creating localized chemical conditions under biofilms.

Morphology and Mechanism

• Pitting under tubercles (mound-like corrosion products).
• Under-deposit attack; often very localized with adjacent unaffected metal.

Preferred Locations

• Stagnant water: dead legs, hydrotest water left in lines, cooling water, buried water lines, soil with organic content.

Susceptible Materials

• Carbon steel most common; stainless in some environments.

Inspection and Mitigation

• Visual tubercles; UT at pits; bacterial culture and metallography for confirmation.
• Mitigation: biocides, pigging, oxygen control, avoid stagnant hydrotest water, CP where applicable.

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Comparison Table

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Worked Scenario: Amine SCC vs. Caustic SCC vs. Chloride SCC

An inspector examines a crack at the toe of a circumferential weld on a 2-inch carbon steel line.

Case data:

Eliminate chloride SCC: Cl-SCC requires austenitic stainless (300-series) at temperature with chlorides and stress. This is carbon steel—chloride SCC is not the primary mechanism (though chlorides can accelerate other attack).

Eliminate caustic SCC: No NaOH/KOH service; no steam-traced caustic concentration. Caustic SCC is unlikely.

Diagnose amine SCC: Carbon steel, hard HAZ (225 BHN), unstress-relieved repair weld, intergranular cracking in amine service at elevated temperature—classic amine SCC (§3.3).

Required actions: Engineer evaluates fitness-for-service; likely remove cracked weld, re-weld with qualified WPS, perform PWHT, hardness verify ≤ 200 BHN, extend MT/PT to similar welds in circuit, update inspection plan per API 570.

This distinction—material + environment + morphology + hardness—is exactly what API 570 expects when RP 571 mechanisms overlap in appearance.