Electrochemical Analysis of Temperature, Concentration and Velocity Effects in Naphthenic Acid Corrosion of 5%Cr-1%Mo Steel
When heavy oil are submitted to high temperature and stirring conditions as found in the furnaces that belongs to preheating-section in refining units, corrosive mechanisms such as produced by naphthenic acids evolves to attack the material. The corrosion resistance of steels that are used in these applications is improved by adding elements such as Cr and Mo, since these tends to form oxides that change the kinetics of corrosion and provide major stability to the material structure.
The temperature, velocity and naphthenic acids concentration effects in the corrosive mechanism of 5%Cr-1%Mo steel were studied in a dynamic autoclave by Electrochemical Impedance spectroscopy (EIS), Linear Polarization Resistance 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 (0.0-3.0 mg KOH/g) and 200 psi 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 resistance charge transfer between metal-electrolyte Rm/e, 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 Scanning electron microscopy and dispersive X-ray analysis (SEM/EDX) and were observed major attacks on steel to higher temperatures. Moreover, the electrolytes were characterized by Fourier transform infrared spectroscopy (FTIR) and mass spectrometry techniques.