FACTORS CONTROLLING NAPHTHENIC ACID CORROSION

A laboratory study was conducted to elucidate the influence of chemica l and physical parameters on corrosion of type 1018 carbon steel (CS, UNS G10180) and 5% Cr-0.5% Mo steel in oils containing naphthenic acid s (NAs) for application to crude oil refinery systems. Effects of test duration, temperature, and acid concentration were assessed for a ran ge of single acids of varying carbon numbers and for NA mixtures in mi neral oil (MO) and in heavy vacuum gas oil (HGVO). In addition, a limi ted 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 me asure of corrosiveness of a crude oil was discredited further. For the same TAN value, molecular size and structure of the acid were shown t o have an important influence. Tests conducted in HGVO showed lower co rrosion rates than in MO, suggesting inhibition caused by S species in the oil or the steric hindrance of naphtheno-aromatic acids. In oil c ontaining the mixture of NAs, the corrosion rate of type 1018 CS was l ower than that for 5% Cr-0.5% Mo steel. The 0.1% H2S that passed throu gh the acid-oil mixtures had an inhibiting effect on corrosion. Predic ting corrosiveness of a crude oil from the measurement of TAN, distrib ution of NA composition, and S content and form was particularly chall enging. The simple tests used were informative, but further work will be required to establish a standard test method that can provide an ad equate ranking of crudes.