The effects of temperature, velocity and naphthenic acids concentration in the corrosive mechanism of 5%Cr-1%Mo steel were studied in a dynamic autoclave by Electrochemical Impedance spectroscopy and Potentiodynamic Polarization Curves with an electrochemical cell of three electrodes: reference and counter electrode was made of Au and working electrode was 5%Cr-1Mo sample. The sample was exposed to mineral oil-naphthenic acids solutions to different concentrations and atmospheric pressure. Contrary to the velocity effect, the temperature has a huge influence in the steel corrosion: to 180°C the steel exhibits lower corrosion rate, however, increasing temperature promotes the thermal composition of complex naphthenic acids, reflecting a reduction in electrochemical polarization resistance, increasing the corrosion rate. Moreover, the temperature is closely related to the concentration acid effect, since the kinetics of corrosion changes significantly when the system is subjected to higher temperatures. These results were complemented with surfaces characterizations by SEM and EDX.

Petrochemical industry has suffered great economic impact due to light crude oil reserves reduction, so refineries have been processing high acidity heavy crude oils. Studies of corrosion caused by naphthenic acids are interfered by presence of other corrosive agents contained in real crude oils, so naphthenic phenomenon must be isolated using synthetic crude oils. For this reason, in present work two high purity mineral oils were used to evaluate their efficiency as synthetic crude oil matrices in AISI/SAE–1020 steel naphthenic corrosion study. Temperature levels evaluated were 200 °C, 250 °C, and 300 °C, while exposure times evaluated were 5, 10 and 15 hours. Surface morphological characterization of AISI/SAE–1020 steel was carried out using scanning electron microscopy and X–ray diffraction. Gravimetric tests showed that AISI/SAE–1020 steel naphthenic corrosion rate increases with temperature and exposure time for one of the synthetic crude oils. However, results obtained for the other synthetic crude oil did not show increasing behaviour due to presence of sulfur traces in the oil, which caused an interference with AISI/SAE–1020 steel naphthenic corrosion study, reducing the reliability of gravimetric results so they cannot be extrapolated to operating conditions in distillation units.