Avaliação de diferentes extratos de cascas de café como inibidores de corrosão do aço carbono 1020 em solução de HCl 1 mol L-1

Abstract

Corrosive processes reduce the performance and durability of metallic materials, causing excessive downtime for maintenance and replacement of equipment, generating financial damage for industries, as well as possible environmental and human losses. Acid solutions, used industrially in the processes of cleaning and pickling of metal structures and pipes, further favor corrosion. In order to delay or reduce the damages of corrosion, one of the most practiced and advantageous methods in the control and prevention of this problem is the use of corrosion inhibitors. However, most of the commercial inhibitors are expensive, also presenting high toxicity, causing problems to the environment and health. Thus, many studies have been carried out, aiming at the application of more efficient and ecologically viable inhibitors, called natural inhibitors. Among these, the inhibitors based on plants and vegetables extracts appear as promising alternatives, since they are obtained from renewable and biodegradable sources, easy to acquire, of low cost and, mainly, because of they are not toxic. In this Dissertation, the anticorrosive activity of coffee husks’ extracts (Coffea arabica L.), obtained from different extracting solvents (water, ethanol/water mixtures and propanone/HCL mixture) was evaluated in the protection of carbon steel in 1 mol L-1 HCl. For each type of extract, different concentrations (10, 20 and 30% v/v) were used and their anticorrosive performance was evaluated by electrochemical techniques (Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR) and Potentiodynamic Polarization (PP)), Weight Loss (WL) assays, being corroborated by the analysis of Total Phenolic compounds and Scanning Electron Microscopy (SEM). In the results obtained by EIS and LPR, an increase in the corrosion resistance of the steel in the acid medium was observed in the presence of the inhibitory extracts, regardless of the extractor solvent used, while the PP results showed that these extracts acted as mixed-type inhibitors. In WL tests, the values of the corrosion rates (Wcorr) increased with the temperature and decreased with the concentration of the inhibitor in the medium. The adsorption of the extracts on the carbon steel surface followed the Langmuir adsorption isotherm. Furthermore, the Ea values of the corrosive process increased in the presence of the extracts, indicating an adsorption process by physisorption and that the carbon steel corrosive process was less viable energetically in the presence of the inhibitory extracts, confirming their anticorrosive action. The samples morphologies evaluations confirmed that the addition of extracts led to a decrease in the formation of corrosion products on the surfaces of the specimens, resulting from the polyphenol molecules adsorption on the carbon steel surface, which corroborated the observed results in the other tests. The Inhibition Efficiency (IE%) values obtained in all of the tests used, increased with the addition of extracts and with their concentrations in the acid medium. The best results of IE% were observed, in the following order, considering the extractor solvent used: ethanol/water 60% v/v > ethanol/water 40% v/v > water >propanone/HCl. This series is in direct accordance with the total phenolic compounds in each extract. As a whole, the results obtained pointed to a high potential for the application of coffee husk extracts as corrosion inhibitors for carbon steel in an acid medium.