Chemical, Physical and Electrochemical Characterization of Two Stainless Steels Exposed in NaVO3 Molten Salt at 700°C

Background: Hot corrosion is a phenomenon seen in gas turbines using residual fuel oils, which contains vanadium. Austenitic stainless steels are ones of the most used materials in components and devices of boilers, gas turbines and furnaces burning contaminated fuel oil. The materials selection is made in great extend due to a good corrosion resistance and low cost. Some works have been done in order to determine the performance of stainless steels under molten salts at high temperatures, obtaining good corrosion resistance, that is why the evaluation of the corrosion rate and the type of corrosion suffered by two austenitic stainless steels is important in order to determine their possible selection under hot corrosion conditions. Methods: The corrosion resistance of the materials was obtained applying the linear polarization resistance and the electrochemical impedance spectroscopy during 100 hours. In order to determine some aspects of the corrosion mechanism, potentiodynamic polarization measurements were also obtained. Physical and Chemical analyses were carried out utilizing the Scanning Electron Spectroscopy and the Xray Diffraction technique. Results: It was determined that AISI-316H developed a Cr2O3 layer, which was dissolved by the electrolyte, so that, this alloy suffered intergranular corrosion due to the precipitation of Cr23C6, such as it has been reported extensively. AISI-347H presented the Kirkendall effect, suffering an important depletion of Fe and in a less extent of Cr, leading to a mixed corrosion process. These behaviors were corroborated with the electrochemical techniques. Conclusion: AISI-316H presented intergranular corrosion, whereas AISI-347H suffered a mixed corrosion type, observing areas indistinctively localized and uniformly corroded. The electrochemical techniques indicated that AISI-316H is more corrosion resistance and both alloys were diffusion controlled systems.