Naphthenic Acid Corrosion

Naphthenic acid corrosion (NAC), along with high-temperature sulfidation, is commonly recognized as two primary damage mechanisms prevalent in the high-temperature sections of atmospheric and vacuum distillation units. The subsequent chapter offers an overview of the naphthenic acid corrosion mechanism, including process-corrosion relationships and solutions for corrosion mitigation.

General Information

Historically, the term ’naphthenic acids’ referred to organic acids featuring saturated aliphatic rings with a carbon atom count ranging from 6 to 20, primarily found in crude oil and distillation side cuts. However, its definition has evolved to encompass all types of organic acids, including those with both saturated and unsaturated rings, some with structures containing up to 50 carbon atoms (see Figure 1). The molecular weight (MW) of these acids varies approximately from 100 to nearly 600-700 atomic units (au).

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Tools

Below are our user-friendly calculators and integrity tools to estimate the corrosion of carbon steel and selected CRAs in hot sections of crude distillation unit.

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Crude Corrosivity-Corrology®

Crude Corrosivity-Corrology® is designed to provide rapid corrosion rate estimates for steels and Corrosion Resistant Alloys (CRAs) in hot crude service. It allows you to evaluate the impact of key process parameters like total sulfur, naphthenic acid concentration, temperature, and velocity on corrosion rates.

Crude-Corrology®

NOTICE: The provided tool is for advisory purposes only. Corrology Innovations Limited and its employees shall not be held liable for any damages, resulting from the use or inability to use the information provided.

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Integrity Risk Indicator (Corrology®-IRI: Crude Corrosivity)

The Challenge: Moving Beyond Linear Estimates

In hot crude service, traditional linear corrosion tracking often fails to capture the rapid transition to accelerated damage. As wall thickness decreases, risk does not grow steadily - it accelerates exponentially. Small changes in total naphthenic acid concentration, temperature, or velocity can also shift an alloy from a stable operating window into a severe-corrosion regime.

The Solution: Crude Corrosivity Integrity Risk Indicator

The Corrology®-IRI: Crude Corrosivity quantifies asset risk by calculating corrosion rates, service-life context, and empirical remaining-life indicators through a margin-aware, API 581-inspired engine. This tool functions as a sensitivity analysis engine, allowing you to evaluate how shifting total sulfur, naphthenic acid concentration, temperature, and velocity impact carbon steel and Corrosion Resistant Alloys (CRAs) in Predictive Mode to determine the most robust material for your specific operating window.

The Advantage

Access a high-speed, agile modeling engine designed for rapid, defensible engineering decisions:

Non-Linear Risk Scaling: Utilize a logarithmic-exponential transformation that reflects the true physical reality of risk escalation in late-life degradation states.
Dynamic Inspection Credit: Input your Inspection Effectiveness (Class A-E) to see how high-quality data reduces uncertainty and optimizes maintenance intervals.
Auto-Derived Prior Confidence: Prior confidence is automatically computed from the uninspected time interval (t_uncertainty): <3 years → High, 3–5 years → Medium, ≥5 years → Low. This applies in both Predictive and Inspection modes, ensuring API 581-consistent conservatism without manual override.
Precision Structural Floors: Take control of your model by setting Manual T-Min structural floors to match your equipment’s unique geometry.
Expert Calibration: Gain confidence with a model structured against a 6-point lifecycle truth set, ensuring accuracy from low-risk early life to critical thin-wall scenarios.
API 581-Aligned Risk Bands: Status and score-pill color are determined by the log probability of failure (log P_f) rather than the 0–100 score scale alone. Thresholds follow API 581 PoF magnitude ranges: log P_f ≤ −5 → Very Low; −5 < log P_f ≤ −4 → Low Risk; −4 < log P_f ≤ −3 → Moderate; log P_f > −3 → Critical. This ensures that improvements in inspection effectiveness are always reflected in the displayed risk band, even in high-age or thin-wall scenarios where the numerical score is saturated.

Note: The Integrity Risk Indicator shown below is a static preview of our Professional+ engine. Register for Professional+ to unlock Corrology®-IRI: Crude Corrosivity.

References

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