Effect of sulphur and naphthenic acids on the corrosion of 9%Cr-1%Mo steels

摘要

Refiners are interested in opportunity crude oils because they are cheap. They are cheapbecause they contain naphthenic acids and sulphur, which are potentially corrosive.Laboratory tests were carried out to investigate the interaction between sulphur andnaphthenic acids, which gives rise to subtle changes in their corrosiveness depending ontheir relative concentrations. Corrosion tests of 9% Cr 1% Mo steel were carried out in asealed autoclave. Two oils containing greatly different amounts of sulphur were used.The concentration of naphthenic acids, which is low in both oils, was increased byadding a mixture of synthetic acids. At a fixed concentration of naphthenic acids, thereis a big difference in the appearance of the metal specimens after tests with the two oils.The high sulphur oil leads to the formation of thick black scales of sulphides, pointingto sulphidic corrosion, while the low sulphur oil leaves the surface of the metal smoothand free of damage, pointing to naphthenic acid corrosion. The difference in the weightof corrosion products that form on the metal surface and the sharp contrast in thebehaviour of the corrosion rate versus concentration of the acids supports the aboveconclusions. The experimental results are therefore in agreement with the rough modelthat predicts the onset of sulphidic corrosion as the concentration of sulphur increases.On the other hand, the concentrations of naphthenic acids that were required to startnaphthenic acid corrosion were far higher than those experienced in refinery plants. Theexperimental equipment used in these tests probably needs to be modified if theconditions set up in laboratory tests are to be made more similar to those of refineryplants.