Study of the Corrosive Behavior of the AISI 1020 Steel in Acid Crude Oil by Microscopic Techniques (LM, AFM and SEM/EDX) and Raman Spectroscopy

Abstract

Microscopic techniques were combined to study the influence of corrosion rate on the morphologic behavior of AISI 1020 steel specimens submitted to thermal degradation of a typical acid crude oil (total acid number (TAN) = 2.1390 mg KOH g -1 and total sulfur (S) = 0.7778 wt.%). The techniques used were light microscopy (LM), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDX), atomic force microscopy (AFM) as well as Raman spectroscopy. Assays were performed in six different degradation time (t = 6, 12, 24, 36, 48 and 72 h) at 320 °C. After the exposure of the specimens to petroleum, a reduction above 37% in the TAN after t = 72 h was observed, with a maximum corrosion rate during the first periods of degradation (t = 6 and 12 h). Correlating the TAN and corrosion rate data with the microscopic data, the images of LM, AFM, and SEM/EDX showed that after 6 h of exposure to petroleum, a passivation film was formed on the surface of the steel. This film consisted of two layers, an external one, formed of FeS, and an internal one, composed of iron oxides and oxyhydroxides. However, after 48 h of thermal degradation, this morphology was altered to a single layer of FeS coating the steel surface.

Synergistic Effect of the Total Acid Number, S, Cl, and H2O on the Corrosion of AISI 1020 in Acidic Environments

“…Moreover, when the TAN level is 3, there is no S in the corrosion product and the content of chlorine is low. The abovementioned was because, under high acetic acid concentrations, the oxide scale formed during the corrosion process effectively prevents active sulfur and organic chlorine, as shown in Figure g–i …”

“…Generally, carbon steel and organic acids form oil-soluble corrosion products ( eq 7 ), but due to the presence of active sulfur, the formed FeS protective film hinders the corrosion of steels by naphthenic acid to some extent ( eq 8 ). 17 , 23 , 30 , 31 …”

“…The corrosion rates at the three levels corresponding to the TAN, S, Cl, and H 2 O all gradually decreased over time. After the protective corrosion products are formed on the metal surface, Cl can effectively penetrate with oxygen or sulfur and form a metal salt, which causes the surface film to crack and accelerates the progress of corrosion. , Obviously, chlorine will not only corrode the equipment but also influence the effect of the corrosion product protection film and destroy the integrity of the anticorrosive protection film . In the process of removing organic chlorine from oil products, a more selective dechlorination method should be used.…”

“…Contrary to the previous periods, an increase in the TAN leads to a decrease in the corrosion rate. The main reason is that the Fe­(RCOO) 2 film generated by the organic acid was gradually replaced by FeS, Fe 2 O 3 , and FeCl 3 , and the latter are more stable and insoluble in the oil phase. ,, Meanwhile, the presence of water provides ionization conditions for multiple impurities and generally promotes the corrosion process and eventually reaches a stable state of the film thickness . Overall, while chlorine and sulfur promote the corrosion of carbon steel, their corrosion products protect it from other impurities in sour oil as well.…”

“…The abovementioned was because, under high acetic acid concentrations, the oxide scale formed during the corrosion process effectively prevents active sulfur and organic chlorine, as shown in Figure 4g−i. 30 2.4. Interaction Effects.…”