Although the term "naphthenic acids'' was originally used to describe acids that contain naphthenic rings, today this term is used in a more general sense and refers to all components in the acid extractable fraction. In crude oil, naphthenic acids exist as a complex mixture of compounds with broad polydispersity with respect to both molecular weight and structure. There has been increasing interest in the naphthenic acids in crude oil because of the corrosion problems that cause during oil refining. Herein, two powerful analytical tools, negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, ESI(-)-FT-ICR MS and atomic force microscopy (AFM), were combined to monitor the thermal degradation of naphthenic acids and their corrosion effects on AISI 1020 steel, respectively. Two different acidic crude oils (TAN = 2.38 and 4.79 mg KOH g(-1), and total sulfur = 0.7993 and 1.0220 wt%) have been submitted to thermal treatment at 280, 300 and 350 degrees C during 2, 4 and 6 h, and characterized by ESI(-)-FT-ICR MS, total acid number (TAN), and total sulfur. The AISI 1020 steel was analyzed by scanning electron microscopy (SEM) and AFM. Generally, heating the crude oil at 350 degrees C in a period of 6 h, it was observed that a high efficiency (congruent to 80%) and selectivity of thermal decarboxylation process was monitored by decay of TAN (4.79 -> 0.44 mg KOH g(-1)). ESI(-)-FT-ICR MS results showed that naphthenic acid species remained after the heating have DBE ranging 1-12 and carbon number from C-15 to C-45. AFM topographic profile evidenced that the naphthenic acid corrosion of the crude oil with TAN of 4.73 mg KOH g(-1) on AISI 1020 steel was profoundly altered and a marked reduction in peak to peak height values (obtained by subtracting the value of the lowest peak by the highest peak in the topographic area examined). Optical images and microphotographs confirmed the presence of irregularities, characterizing the corrosion mechanism as pitting type. The naphthenic corrosion was also evidenced in samples with low TAN value (0.44 mg KOH g(-1)). (C) 2014 Elsevier Ltd. All rights reserved.