Factors Controlling Naphthenic Acid Corrosion

A laboratory study was conducted to elucidate the influence of chemical and physical parameters on corrosion of type 1018 carbon steel (CS, UNS G10180) and 5% Cr-0.5% Mo steel in oils containing naphthenic acids (NAs) for application to crude oil refinery systems. Effects of test duration, temperature, and acid concentration were assessed for a range of single acids of varying carbon numbers and for NA mixtures in mineral oil (MO) and in heavy vacuum gas oil (HGVO). In addition, a limited study of the effect of hydrogen sulfide (H2S) addition to the acid-oil mixture was conducted. Use of the total acid number (TAN) as a measure of corrosiveness of a crude oil was discredited further. For the same TAN value, molecular size and structure of the acid were shown to have an important influence. Tests conducted in HGVO showed lower corrosion rates than in MO, suggesting inhibition caused by S species in the oil or the steric hindrance of naphtheno-aromatic acids. In oil containing the mixture of NAs, the corrosion rate of type 1018 CS was lower than that for 5% Cr-0.5% Mo steel. The 0.1% H2S that passed through the acid-oil mixtures had an inhibiting effect on corrosion. Predicting corrosiveness of a crude oil from the measurement of TAN, distribution of NA composition, and S content and form was particularly challenging. The simple tests used were informative, but further work will be required to establish a standard test method that can provide an adequate ranking of crudes.