Also known as “naphthenic acidity’, the total acidity is one of the crude oils features that most influences their price and processing. Crude oils having high acidity are usually cheaper than low-acidity crude oils and are important alternatives to worldwide refineries. Having origin in countries such as Brazil (post-salt), Russia, Venezuela, China and India, and also present in unconventional crude oils such as shale oils and sand oils, these acidic crude oils also tend to be heavier. While representing good opportunities to the refiners, these raw materials also bring with them a few challenges to the refining units, one of them being naphthenic corrosion.

The acidity of a crude oil is determined by the TAN (total acid number) which is the number of how much KOH is required to neutralize 1 g of crude oil. Crude oils having TAN higher than 0.5 mg/g are considered acidic. The acidity is promoted not only by naphthenic acids but also by other types of carboxylic acids and H₂S.

Naphthenic corrosion occurs in the absence of water and in a localized manner, characterized by pitting, with marked rims (Figure 1). The reaction of carboxylic acids with iron yields a corrosion product (salt) and hydrogen, according to the following reaction:

Given that Fe (RCOO)₂ compounds are highly soluble in hydrocarbons, a corrosion-erosion mechanism, that can lead to loss of containment, is installed without low thickness being detected in other portions of the equipment or pipe.

^{Figure 1 – pipe damaged by naphthenic corrosion.
Source: Webcorr – the corrosion clinic - https://www.corrosionclinic.com/types_of_corrosion/what-is-naphthenic-acidcorrosion_NAC_prediction_modeling.htm accessed on 04/04/2022}

The occurrence and intensity of naphthenic corrosion do not depend only on TAN, but also on the operating conditions and further characteristics of the crude oil composition. The most influential factors are:

1. Temperature – usually, naphthenic corrosion occurs in the 200^oC and 400^oC range, above 400°C carboxylic acids are degraded;
2. Sulfur content – in case the hydrocarbon contains H2S or other iron sulfide-forming (FeS) agent, a protecting layer can be formed. However, a sufficient concentration of these compounds is mandatory to keep the layer stable. That is how for systems such as distillation towers bottoms of low H2S concentration and in case of low sulfur, high acidity crude oils, naphthenic corrosion is more critical;
3. Rate – naphthenic corrosion tends to occur in high flow velocity regions, such as pipe curves and heat exchangers. This possibly happens because in these regions the FeS protecting layer degradation occurs easier;
4. Phase change – naphthenic corrosion occurs intensively on surfaces where phase changes or biphasic flow are in course, such as tower plates, bubblers, furnaces and heat exchangers.

At a refinery processing high acidity crude oils the occurrence of naphthenic corrosion will be more intense in the atmospheric and vacuum distillation units, where heavier hydrocarbons are at high temperature, but not high enough to destroy the acidic compounds.

In the FCC units the acids responsible for the naphthenic corrosion are destroyed in the converter, however the corrosion mechanism can be observed in the unit loading system, notably at the end of the pre-heating battery, loading furnace and riser load injection lines, where the temperatures exceed 200°C.

Prevention:

Prevention against naphthenic corrosion envisages several factors that should be considered and applied according to the objective of the refinery and the frequency of acidic crude oils processing. The main prevention forms are:

• Crude oil blend: acidity is controlled by mixing a high-acidity crude oil with a low-acidity crude oil to obtain a lower-than 0.5 mg/g TAN blend;
• Metallurgical compliance: for refineries having acidic crude oils processing as long-term objective, the replacement of carbon steel by low-carbon alloy steel and higher-than 2% molybdenum content aids in the equipment increased resistance to naphthenic corrosion. For distillation towers it is possible to provide resistant material linings;
• Utilization of corrosion inhibitors: corrosion inhibitors were developed to aid in this kind of corrosion;
• Load treatment: the enhanced availability of unusual loads provides several resources for destroying naphthenic acids in crude oil and heavy products, from the injection of huge amounts of soda (which generates a huge amount of effluent and is not the solution of choice) to thermal and catalytic treatments.

FCC S.A. has a team of experts ready to aid in case of changes in the cast of crude oil.