Desempenho à corrosão de corpos de prova de aço carbono revestidos com cromo trivalente e com posterior aplicação de um filme de passivação de cromo trivalente eletrolítico

Abstract

Currently, metallic electrodeposition is one of the most used ways to obtain protective barriers against corrosion media. For a long time, processes based on hexavalent chromium were used as a protective coating, but because it is an aggressive compound to humans and the environment, it was necessary to develop sustainable processes, such as the case of trivalent chromium. However, coatings based on trivalent chromium proved to be inefficient in terms of corrosive protection, thus it was necessary to apply chemical or electrolytic passivation so that there was an increase in corrosion resistance performance. Thus, the main objective of the present work is to evaluate the best condition for the application of a trivalent electrolytic passivation on a trivalent chromium coating. Within this context, it was decided to initially test the voltage conditions 4, 6 and 8 volts, which were based on the ideal condition of an electrolytic trivalent process already marketed by the supplier. A preliminary salt spray test was carried out, selecting the voltage conditions 4 and 6 V, which presented the best results. Scanning electron microscopy tests with field emission (SEM/FEG) and energy dispersion spectroscopy (EDS) were carried out to evaluate the morphology of the passivating film as well as the corrosion product after salt spray test. It was observed by scanning electron microscopy (SEM/FEG) the formation of micro cracks in the passivated specimens (CP's), at a lower intensity under the 4 V voltage condition, as well as the formation of corrosion products from the base material, iron. Also, electrochemical and corrosion tests were carried out, where it was possible to observe the tendency of formation of a passivating film and a slight improvement in corrosion resistance compared to the specimen without a passivating film. To evaluate the possible changes in the mechanical properties of CP's with a passive film compared to those without a passive layer, Vickers microhardness and pencil hardness tests were performed, where no changes were found. However, in the adhesion test, a small detachment was observed in the 4 V voltage condition, but it was not considered significant. The evaluations carried out in this work prove that the application of a passive film can cause an improvement in the resistance to corrosion. Furthermore, through the obtained results, it can be concluded that even with an improvement with the 4 V voltage condition, the applied conditions are not the most adequate as a film free of imperfections was not obtained. [resumo fornecido pelo autor]