Molten salt corrosion of alloys often causes destruction of coating layers (by decomposing or transforming corrosion-protection and thermal-barrier layers) and even alloy matrixes.
A coated Cr2O3 layer is the typical protective layer on the surfaces of stainless steels, Ni-based superalloys, or other alloys, under normal circumstances. However, such a layer, if exposed to molten salts at elevated temperature conditions under some specific salty environments (such as marine, salt lake, and salty rock bed environments), may be damaged by aggressive molten salty agents, resulting in the alloy materials possibly being exposed to further corrosion attacks by other oxidizing or reducing substances in their application life-cycles.
Thermodynamic calculations using Thermo-Calc and the SALT1 database appended to the SSUB database can effectively help in predicting dissolution of a Cr2O3 layer caused by molten salt corrosion. The amount of stable phases (first image) and Cr partitions in various stable phases (second image) as a function of temperature are shown. The calculation was done for the heterogeneous equilibrium state in a multicomponent Cr-C-H-O-S-N-Na-Cl system, which originally consists of 0.05 mole of Cr2O3 solid, a salt mixture (0.01 mole of NaCl, 0.46247 mole of Na2SO4 and 0.03253 mole of Na2CO3), and 431.16 g of C-H-O-N gaseous mixture (note that all such settings are actual data on chemical compositions of the interacting NaCl-Na2SO4-Na2CO3 salt mixture and C-H-O-N gas mixture inside a specific gas turbine severing in an chemical engineering process).
The calculation results show that at an operation temperature of 750 °C, the Cr2O3 layer dissolves (with a remaining amount of 0.031756 mole), forming a crystalline Na2CrO4-Na2SO4 solid solution (Hexagonal) and a liquid mixture (Ionic_Liquid) that are stable with an equilibrated gaseous mixture. As the temperature increases, the molten salt corrosion becomes more serious, and when 1200 °C is reached, the entire Cr2O3 layer on the alloy surface may be completely destroyed if exposed for a long period under such a corrosive environment.
Stable phases remaining/formed (left) and Cr-partition in various phases (right) as a function of operation temperature condition, during the molten salt corrosion of the Cr2O3 layer (on surfaces of stainless steels, superalloys, or other alloys) when it is attacked by a NaCl-Na2SO4-Na2CO3 salt mixture and a C-H-O-N gaseous mixture, simultaneously. The calculation can be made using Thermo-Calc and the SALT1 database appended to the SSUB database.