Corrosion behavior of stainless and low-chromium steels and IN625 in molten nitrate salts at 600 °C

The corrosion resistance of two ferritic steels P91 and X20CrMoV11-1, two austenitic steels SS316 and SS347H and a Ni-alloy IN625 is studied during long term isothermal immersion in molten nitrate salts consisting of 40% KNO3 and 60% NaNO3. The corrosion test was conducted at 600 °C for a maximum immersion time of 5000 h. The corrosion behavior was determined by recording the weight changes of samples at different time intervals and metallographic methods. The corrosion products were investigated in detail via electron probe micro-analysis (EPMA), and scanning electron microscopy (SEM). Results showed that P91 and X20CrMoV11-1 do not offer a sufficient long term corrosion resistance in molten nitrate salts. Stainless steels SS316 and SS347H instead formed protective oxide scales in both salts with similar kinetics. However, dissolution of the formed oxide scale in the form of alkali ferrite was observed on stainless steels. A linear weight loss with a very slow kinetics was evident for IN625. However, IN625 showed the best protective behavior among alloys. The qualitative and quantitative EPMA results revealed that a multiphase oxide layer composed of iron-chromium spinel, iron oxides, and sodium ferrite formed on the stainless steel SS347H. A dense NiO layer was primarily formed on the IN625 surface.