Naphthenic Acid Corrosion (NAC) - Detection and Mitigation Key Strategies

Naphthenic acid corrosion (NAC) is a critical challenge in oil refining and processing, particularly in distillation units such as vacuum distillation towers. This corrosive phenomenon, driven by organic naphthenic acids present in crude oil, poses a significant threat to equipment longevity, operational efficiency, and safety in refineries. Understanding how to detect, assess, and mitigate NAC is essential for minimizing its impact.

What is Naphthenic Acid Corrosion (NAC)?

NAC is a high-temperature form of corrosion affecting materials exposed to certain fractions of crude oil containing naphthenic acids. This corrosion is most prevalent in crude and vacuum distillation units, where it can cause severe damage to carbon steel, low-alloy steels, stainless steels, and even high-performance nickel-based alloys. The challenge lies in the unpredictability of NAC, as its severity depends on a combination of factors: naphthenic acid concentration, sulfur content, temperature, and fluid velocity.

Detection and Assessment of NAC

Accurate detection of NAC requires a combination of advanced inspection techniques:

• Visual Testing (VT): Useful for accessible areas but limited in scope for detecting localized corrosion in hidden spots.
• Radiographic Testing (RT): Helps identify wall loss and corrosion in piping and equipment, especially in high-risk areas.
• Ultrasonic Thickness Testing (UT): Offers a detailed assessment of material loss, particularly effective in close-grid or scanning mode to capture localized thinning.
• Permanent Monitoring Systems: Utilizing sensors such as permanently mounted thickness gauges or electrical resistance probes, corrosion rates can be continuously monitored, helping to predict potential failures.

NAC is especially aggressive in areas with high-velocity or turbulent flows, such as pump internals, elbows, tees, and transfer lines. Therefore, inspections should focus on these critical areas to detect early signs of damage.

Mitigation Strategies for NAC

Preventing or reducing the effects of NAC can be approached in several ways:

1. Material Upgrades: When designing or upgrading units, selecting alloys with higher molybdenum content (e.g., 317L stainless steel or alloys containing 4-6% molybdenum) significantly enhances resistance to NAC.
2. Blending Crudes: By blending crudes to reduce the total acid number (TAN) or increasing sulfur content, refineries can minimize the corrosive potential of naphthenic acids. However, the relationship between TAN and NAC is complex, as some crudes with lower TAN may still cause severe corrosion.
3. Chemical Inhibitors: The use of high-temperature corrosion inhibitors has proven moderately successful, though their effectiveness varies. Careful selection and monitoring are essential to avoid negative impacts on downstream processes, such as catalyst poisoning.
4. Hydrogen Injection: Injecting hydrogen into hydroprocessing units can mitigate NAC in 300 series stainless steels, reducing the overall corrosivity of naphthenic acids.

The Role of Inspection and Maintenance

Routine and targeted inspections are critical to managing NAC in refinery systems. During scheduled shutdowns, comprehensive inspections can reveal areas most affected by corrosion. For instance, during a 2019 shutdown of a vacuum distillation tower at a Brazilian refinery, significant NAC was detected in regions exposed to both high temperatures and the synergistic effects of sulfur corrosion. This case highlighted the importance of coupling inspection data with operational adjustments to reduce future damage.

By adopting a proactive approach to NAC through regular monitoring, material upgrades, and chemical treatments, refineries can significantly reduce the risk of catastrophic failures and maintain safer, more efficient operations.

Feel free to engage in the conversation or share your thoughts below. Let’s work together to overcome the challenges of NAC!

#corrosion #oilrefining #inspection #NAC #materialsengineering #refinerymaintenance #chemicalinhibitors #assetintegrity #corrosionsolutions