Corrosion rate studies of AISI 1020 steel using linear, cyclic, and aromatic naphthenic acid standards

The use of total acid number (TAN) to measure the acidity of crude oil samples is a common practice. Daily practice in petroleum refineries shows that oil with a TAN less than 0.5 mg KOH g−1 can usually have unexpected corrosive behavior. Although naphthenic corrosion has been studied for many years, its process is not entirely understood. Therefore, this study investigated the influence of linear, cyclic and aromatic NAs molecules, evaluating the effect of their concentrations, molar weight (Mw), structural and electronic properties on the corrosion rate of AISI 1020 steel samples at 320 °C. The type and extension of corrosion were determined by surface characterization of the sample using optical microscopy (LM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The results showed that even when TAN was below the detection limit, the corrosion rate (CR) value was significant (CR < 0.19 mm year−1) and considered high for the industry. For concentrations of benzoic (BENA) and stearic (STA) acids higher than 5000 ppm, the corrosion rate varied proportionally with the TAN. Also, there was an inverse proportional correlation between molecular weight (Mw), the number of aromatic rings and NAs corrosiveness. In general, NAs standards containing a lower molar volume were found to cause a higher corrosion rate. SEM micrographs revealed changes in the surface area of all samples exposed to linear, cyclic and aromatic NAs, indicating the formation of a protective layer under the surface of the carbon steel specimen. AFM imaging showed drastic changes in the topographic profile and phase images characterized by the formation of valleys and peaks, as well as variations in the mechanical properties of the surface of the AISI 1020 steel sample. Results obtained in this study confirm the influence of the electronic-structural properties of the molecules of organic acids on the naphthenic corrosion.