Corrosion Characteristics of Iron-Nickel-Chromium Alloys in Molten Nitrate Salts Under Isothermal and Thermal Cycling Conditions

The corrosion degradation of metallic components in Concentrated Solar Power Plants (CSP) with molten salt poses significant threat to the continuous safe operation at high temperature. Updated understanding of the corrosion behavior, mechanisms and the properties of corrosion products formed are needed to explain and optimize the corrosion performance of metals. In this study, the corrosion behavior of 321 SS, 347 SS, Alloy 625 and Alloy 825, are investigated in nitrate salt for 7, 14 and 28 days under isothermal; at 565°C, and thermal cycling conditions (between 565 and 290°C) in air. Several corrosion and advance surface evaluation techniques were implemented to characterize the corrosion process. The results show that the corrosion rate after thermal cycles is lower than in isothermal condition. This was due to increased spallation of corrosion products under isothermal test condition than during thermal cycling tests; particularly in stainless steel samples after 28 days. Multilayer corrosion products were observed on the surface of stainless steels and Ni-based alloys from both isothermal and thermal cycling tests. For stainless steels, an outer layer of sodium iron oxide, intermediate iron oxide and an inner layer of iron chromium spinel were found. NiO and Cr-rich oxides that were detected on the surface of Ni-based alloy, serve as protective layers against further corrosion processes.