Estudo da curva de polarização cí­clica da liga de níquel Inconel 625 em solução de NaCl.

Abstract

A Deus pela constante atuação em minha vida. À Professora Dra. Zehbour Panossian que, com imensa dedicação, carinho e sabedoria, orientou este trabalho.A todos os amigos do Laboratório de Corrosão e Proteção, pelo incentivo e apoio, principalmente à A todas as pessoas que direta, ou, indiretamente colaboraram com este trabalho. molibdato para o óxido MoO 3 decorrente da forte acidificação do eletrólito verificado nesse trecho da curva. Ainda, após a inversão da polarização, ocorre a transição da corrente anódica para catódica em um potencial próximo de 0,720 V (ECS). Do potencial de 0,600 V (ECS) até 0,200 V (ECS), observa-se uma região de passivação secundária, atribuída à formação do MoO 3 de maneira mais significativa. Ao final a densidade de corrente volta a aumentar, indicando o início da oxidação/dissolução do MoO 3 para Mo 3+ . Palavras-chave: liga de níquel, polarização, Inconel 625. ABSTRACT Cyclic potentiodynamic polarization curves of the nickel alloy Inconel 625 in 3.56 % NaCl solution, pH 7.5, deaerated and at ambient temperature, obtained according to the ASTM G61, were studied in order to interpret them, especially after the reversion of the scanning direction. For this study, complementary analyzes by MEV/EDS and by Raman spectroscopy of the working electrode surfaces as well as EDS analyses of the electrolyte collected near the working electrode were carried out. The pH values and O 2 content of the electrolyte near the working electrode were also obtained. From the open circuit potential (OPC) up to breakdown potential (E b ), at about 0,600 V (ECS), the polarization curve presents a passive behavior with a tendency to reach the current density of 10 -6 A/cm². In this region, the thickening of the passive film takes place mainly due to the formation of Cr 2 O 3 . After E b , a change of the curve slope occurs due to the oxygen evolution reaction which is confirmed by the acidification of the electrolyte. At the 0,750 V (ECS) potential, an inflection of the curve is observed causing a discrete decrease in the current which was attributed to the formation of MoO 4 2-(gel) which, in turn, avoids the incorporation of anions as Clor OHin the passive film. This fact makes the oxidation/dissolution of the passive film difficult but does not avoid it since Cr and Ni ions were detected in the electrolyte after the inflection, characterizing a transpassive region. After the reversion of the scanning direction at the potential 1,130 V (ECS), the curve shows a positive hysteresis with lower current densities than the values obtained at the same potential applied during the direct polarization. This indicates the occurrence of repassivation mainly due to the formation of a Mo rich layer on the alloy whose formation can be attributed to the transformation of the molybdate to the MoO 3 because of the strong acidification of the electrolyte. After the reversion of the polarization, a transition from the anodic current to cathodic occurs at a potential close to 0,720 V (ECS). From the potential of 0,600 V (ECS) ...