Avaliação da ação corrosiva de diferentes biodieseis sobre o aço AISI 316 utilizando métodos eletroquímicos e planejamento estatístico

Abstract

Biodiesel and diesel are organic and corrosive substances that can attack metal fuel tanks, connecting rods, piping systems, transportation and storage of steel, among other metallic constituents. Currently, metal and steel, such as the AISI 316 steel, are being increasingly used in tanks for transport and storage of biofuels. With such assumptions, the study aimed to evaluate and compare the corrosive action of different biodiesels on steel AISI 316. The methodology consisted of physic-chemical analysis of soybean oil, sunflower oil and castor oil. The biodiesels were synthesized by the transesterification reaction by the methyl and ethyl routes, making use of NaOH as catalyst. Biofuels were characterized following the guidelines of the American Society of Testing Materials (ASTM), Fourier Transform Infrared Spectrometry (FTIR). Also it evaluated B7 commercially available. The thermal stabilities were evaluated by Termogravimetric Analysis (TG, DTG and DTA) and Differential Scanning Calorimetry Dynamics (DSC). The corrosion test consisted of immersing the steel under biofuel sat different times. It were used Linear Potentiodynamic Polarization (PPL) measurements and Electrochemical Impedance Spectroscopy (EIS) for the electrochemical evaluation of corrosion. Subsequently, the steel was analyzed by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-Ray Spectrometer (EDS), and was carried out chemical analysis via X-Ray Fluorescence (XRF). Findings showed that biodiesels were like stated by National Agency of Petroleum (NAP) standards. The sets of the techniques indicated a weight loss, and surface analysis evidenced general pitting corrosion. With the polarization curves and electrochemical impedance spectroscopy, it was observed that all biofuels are corrosive agents, therefore, reducers. The B7 is the fluid that corrodes more the samples, which by having a nucleation of oxide form the metal passivation after being attacked by biofuels. By XRF, it was observed that biofuels have been contaminated with transition metals when exposed to the steel, causing the formation of free radicals that induce autoxidation, forming hydroperoxides, which was confirmed by changes in vibrations of FTIR spectra. Statistical analysis indicated that the model is acceptable and reliable, because the noise analysis is close to the standard range. The Response Surface Methodology (RSM) indicated that the immersion time is the factor that most influences the corrosion rate in biodiesels synthesized by methyl route, while the acid value is what most influences in ethyl esters.