8.6 Caustic Corrosion & Naphthenic Acid Corrosion
Key Takeaways
- Caustic corrosion is metal loss caused by concentrated sodium or potassium hydroxide, often where evaporation, deposits, poor mixing, or heat transfer concentrates caustic against steel.
- Caustic corrosion and caustic SCC are different damage forms: one is wall loss, while the other is crack growth driven by tensile stress in a susceptible environment.
- Naphthenic acid corrosion is high-temperature metal loss in certain crude and vacuum services; total acid number alone does not predict rate because temperature, velocity, sulfur species, and metallurgy interact.
- Naphthenic acid attack is often smooth, flow oriented, and severe at elbows, nozzles, reducers, transfer lines, and other turbulent high-velocity locations.
- Inspection planning must follow the corrosion circuit and operating history, using thickness mapping and repeatable high-risk locations rather than assuming uniform loss.
Caustic Corrosion and Naphthenic Acid Corrosion
Caustic corrosion and naphthenic acid corrosion are both metal-loss mechanisms selected in the current API RP 571 scope, but they occur in very different services. One is aqueous alkaline attack where hydroxide becomes concentrated; the other is high-temperature organic-acid attack in certain crude fractions. Correct diagnosis changes both inspection and mitigation.
1. Caustic corrosion
Caustic corrosion is general or localized metal loss caused by concentrated alkaline solutions, principally sodium hydroxide or potassium hydroxide. Carbon steel often performs acceptably in controlled dilute alkaline service, but rate can rise sharply when caustic concentrates because of evaporation, boiling, wet-dry cycling, poor mixing, leaks, deposits, or heat-transfer conditions.
| Concentration mechanism | Vulnerable location | Field clue |
|---|---|---|
| Evaporation or boiling | Heated surfaces, liquid lines, vapor-liquid interfaces | Local thinning near heat input or level |
| Poor chemical mixing | Injection quills, nozzles, downstream impingement | Directional or highly local attack |
| Deposits and crevices | Under solids, gaskets, attachments, deadlegs | Under-deposit pits or irregular wastage |
| Carryover or contamination | Equipment not intended for caustic service | New attack after process upset |
| Wet-dry cycling | Intermittently wetted hot areas | Concentration rings and local grooving |
Differentiate metal loss from caustic stress-corrosion cracking. Caustic corrosion removes measurable wall and may be smooth, gouged, or locally pitted. Caustic SCC produces cracks and depends on tensile stress, environment, temperature, and metallurgy. UT thickness mapping is appropriate for metal loss, while surface and volumetric crack methods may be required for SCC. Finding one form does not rule out the other.
Review caustic concentration, injection rate and geometry, temperature, water balance, deposits, heat tracing, startup/shutdown events, and mixing. Inspect injection zones, vessel waterlines, heated walls, deadlegs, low points, and areas beneath deposits. Mitigation may involve dilution and concentration control, improved injection/mixing, temperature control, deposit removal, compatible materials, and eliminating unintended caustic ingress.
2. Naphthenic acid corrosion
Naphthenic acid corrosion (NAC) is a high-temperature corrosion mechanism associated with certain petroleum crude fractions containing organic acids. It is encountered in hot crude and vacuum-unit circuits where the acids are present in a liquid hydrocarbon phase. It generally appears as rapid local wall loss rather than cracking.
Total acid number (TAN) is a useful screening property but not a stand-alone corrosion-rate predictor. Two feeds with similar TAN can behave differently because acid species, boiling range, temperature, sulfur chemistry, velocity, shear stress, vaporization, and metallurgy differ. Sulfur compounds can form iron-sulfide scale that sometimes reduces acid attack, while flow and turbulence can strip or prevent a protective scale. A feed change can therefore increase NAC even if a single bulk property changes little.
Morphology and locations
NAC often produces smooth, sharply bounded, flow-oriented thinning with grooves or a washed appearance. Target elbows, reducers, control-valve downstreams, pump discharge regions, transfer lines, furnace outlet and transfer zones, nozzles, impingement points, and vapor-liquid transition regions. Damage can be highly localized to the hottest liquid-wetted or most turbulent part of a circuit.
| Factor | Effect on inspection plan |
|---|---|
| Feed or blend change | Revisit susceptible circuits and baseline readings |
| Temperature profile shift | Move CMLs to the new liquid-phase hot zone |
| Increased throughput | Increase attention to high-shear fittings and nozzles |
| Changed sulfur behavior | Do not assume prior scale remains protective |
| Alloy substitution | Verify material by PMI and reassess local rate |
Use repeatable UT grids, encoded corrosion mapping, or profile methods at flow-disturbed locations. Trend short-term rates after a feed, rate, or temperature change because long-term averages can hide a recent increase. Visual examination during shutdown can identify flow direction and help distinguish smooth NAC from pitting or cracking.
Mitigation may include feed blending or selection, temperature/velocity changes, corrosion monitoring and inhibitors where qualified, improved metallurgy, and targeted replacement. Alloy performance is service specific; material selection must be based on applicable corrosion expertise and verified by a material-verification program.
3. Scenario method
First ask whether the environment is aqueous concentrated alkali or hot organic-acid crude. Then identify the concentrating or flow mechanism. Next match morphology and location. Finally select an examination that can find local loss and use recent operating history to set the rate.
Exam traps
- Do not confuse high pH caustic metal loss with caustic SCC.
- Do not use TAN as a complete NAC rate equation.
- Uniform CML spacing misses turbulence-driven NAC.
- A material upgrade only helps if the installed alloy is verified and compatible with the actual service.
Which condition most directly promotes caustic corrosion?
What distinguishes caustic corrosion from caustic SCC?
Why is TAN alone insufficient to predict naphthenic acid corrosion?
Where should an inspector prioritize NAC thickness mapping?