Corrosion Performance of AISI-309 Exposed to Molten Salts V2O5-Na2SO4at 700°C Applying EIS andRpElectrochemical Techniques

E F Diaz,O Sotelo-Mazón,S Mejía Sintillo,C Cuevas-Arteaga,N Flores-García

doi:10.1155/2015/826759

Abstract

The corrosion performance of AISI-309 exposed 5 days to molten salts 50 mol% V2O5-50 mol% Na2SO4at 700°C is reported in this paper. Such evaluation was made using three electrochemical techniques: potentiodynamic polarization curve (PC), electrochemical impedance spectroscopy (EIS), and linear polarization resistance (Rp). FromPC, the Tafel slopes,Icorr, andEcorrwere 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 Cr2O3layer over the metallic surface; after that, the dissolution of Cr2O3formed 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 Cr2O3layer.

Highlights

Many industrial components operate at high temperatures, in which severe corrosion is found because of the aggressiveness of the environments
A 5 μm thick oxygen layer is noted below the sulfur layer, maybe related to nickel, since the empty space presented by Cr and Fe is not evident in the mapping of nickel, so the layer of oxygen must be related to nickel, as it has been presented in some other V2O5Na2SO4 systems at high temperature [14]
Afterward such chromium was transformed into a passive layer of Cr2O3 as a reaction of the alloy to be protected, and this initially dense and coherent passive layer of chromium oxide was evenly dispersed with a trend to be dissolved, as it can be seen in the corresponding mapping

Summary

Introduction

Many industrial components operate at high temperatures, in which severe corrosion is found because of the aggressiveness of the environments. When using fossil fuels in energy generation systems, some chemical elements can form compounds with low melting points, which are called products of the combustion reactions. These compounds are deposited on the metal components of the system, and when the temperature is above its melting point, they can generate catastrophic corrosion phenomenon [4]. The hot corrosion phenomenon has been investigated since early 1940 in boilers and their components, internal combustion engines, gas turbines, and incinerators This issue gained importance and interest until 1960, when gas turbine engines for military aircraft in the Vietnam conflict suffered a severe corrosion [5]

Methods

Results

Conclusion