What Does Naphthenic Acid Corrosion (NAC) Mean?
Naphthenic acid corrosion (NAC) is a non-aqueous corrosion process caused by naphthenic acids.
Naphthenic acid corrosion is a problem for refineries processing crudes containing high levels of naphthenic acid. It most typically affects crude and vacuum units.
Naphthenic acid corrosion behavior can be time variant, localized and difficult to predict.
The type of equipment in distillation units subject to NAC attack includes:
- Crude feedstock heaters
- Transfer lines
- Feed and reflux sections of columns
- Atmospheric and vacuum columns
- Heat exchangers/condensers
Corrosionpedia Explains Naphthenic Acid Corrosion (NAC)
Naphthenic acid corrosion occurs primarily in high-velocity areas of crude distillation units in the 430°F to 750°F (220°C to 400°C) temperature range. No corrosion damage is usually found at temperatures above 750°F (400°C), most likely because of the decomposition of naphthenic acids or protection from the coke formed at the metal surface.
NAC can be retarded by sulfur-containing compounds inherently present in crude oil fractions in the form of soluble sulfides. Although these sulfides can be corrosive as well, they can also form iron sulfide scale on metal surfaces, potentially offering a degree of protection against naphthenic acid attack.
Crude oils with a high content of naphthenic acids are often referred to as high total acid number (TAN) crude oils or high-acid crude oil (HAC).
NAC is differentiated from sulfidic corrosion by the nature of the corrosion (pitting and impingement) and by its severe attack at high velocities in crude distillation units.
Mitigation methods for naphthenic acid corrosion in distillation units includes:
- Upgrading materials
- Process control
Adding molybdenum to stainless steel can produce alloys resistant to NAC, and phosphorus-based inhibitors are also used for mitigation.