In this work, the correlation between the composition of heavy distillation cuts and the corrosion process was analyzed. According to the results of electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR MS), naphthenic acid species were detected in the seven cuts studied, with m/z and DBE values ranging from 100 to 300 and from 2 to 4, respectively. The relative abundance of heavy naphthenic acids increase with distillation cut temperature (from 188 to 315 degrees C). In the residue, naphthenic acids (02 class) and carbazoles (N class) are detected as polar species. The corrosion mechanism for AISI 1020 steel in the heavy oil and distillation cuts was determined by electrochemical impedance spectroscopy and scanning electron microscopy. The equivalent electrical circuits proposed for the dehydrated oil, its distillation cuts and the residue are based on observations obtained from the Nyquist and Bode diagrams. Resistance values decrease from oil to cuts in function of increasing distillation temperature. The metal film resistance values increase with the distillation cut temperature and are compatible with the formation of passive films. According to the microphotographs, the localized corrosion is characterized as alveolar for cuts 2-5 and pitting for cuts 6-7. (C) 2012 Elsevier Ltd. All rights reserved.