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