Corrosion performance of AISI-309 exposed to molten salts [V.sub.2][O.sub.5]-[Na.sub.2]S[O.sub.4] at 700°C applying EIS and [R.sub.p] electrochemical techniques

The corrosion performance of AISI-309 exposed 5 days to molten salts 50 mol% [V.sub.2][O.sub.5]-50 mol% [Na.sub.2]S[O.sub.4] at 700°C is reported in this paper. Such evaluation was made usingthree electrochemical techniques: potentiodynamic polarization curve ([P.sub.C]), electrochemical impedance spectroscopy (EIS), and linear polarization resistance ([R.sub.p]). From [P.sub.C,]the Tafel slopes, [I.sub.corr], and [E.sub.corr] were obtained. From Nyquist and Bode plots, it was possible to determine two different stages; the first one showed just one loop, which indicated the initial formation of [Cr.sub.2][O.sub.3] layer over the metallic surface; after that, the dissolution of [Cr.sub.2][O.sub.3] formed a porous layer, which became part of the corrosion products; at the same time a NiO layer combined with sulfur was forming, which was suggested as the second stage, represented by two capacitive loops. EIS plots were in agreement with the physical characterization made from SEM and EDS analyses. Fitting of EIS experimental data allowed us to propose two electrical circuits, being in concordance with the corrosion stages. Parameters obtained from the simulation of EIS data are also reported. From the results, it was stated that AISI-309 suffered intergranular corrosion due to the presence of sulfur, which diffused to the metallic surface through a porous [Cr.sub.2][O.sub.3] layer.