Molten Salts Databases
Thermo-Calc Software offers two thermodynamic and properties databases for molten salts: TCSALT1 and SALT1. Both databases can be used for applications such as hot salt corrosion. TCSALT1 is better suited for processes involved with recycling aluminum where fluxes are used and the SALT1 database is better suited for applications like high energy lamp design.
TCSALT1 for Fluorides (F) and Chlorides (Cl)
Applications of TCSALT1 to TCS Molten Salts Database
Calculate the Liquidus Projection of the Molten Salt
TCSALT can be used to study how the liquidus of the Molten Salt varies with composition. Figure 1 demonstrates this by calculating the liquidus projection of a KCl-KF-NaCl-NaF molten salt.
Figure 1: Calculated reciprocal KCl-KF-NaCl-NaF liquidus projection.
Investigate How the Flux Composition Influences Element Reduction
TCSALT1 can also be used to study how the flux composition influences element reduction. In this example, the Ca and Na reduction in an Al-4.5% Mg melt with an initial amount of 200ppm Ca and 200ppm Na for a KCl-MgCl2 flux is presented. By increasing the MgCl2 content in the flux you can remove the Ca and Na from the Al melt and simultaneously maintain the Mg content, as shown in figure 3. Please note that this calculation was performed by combining TCSALT1 with TCAL9 databases.
Figure 2: Ca and Na content in the Aluminum melt when reacting with a KCl-MgCl2 flux with varying MgCl2 content.
Figure 3: Mg content in the Aluminum melt when reacting with a KCl-MgCl2 flux with varying MgCl2 content. As seen the Mg content is maintained at ~4.5wt%.
SALT1 for applications such as Hot Salt Corrosion of Alloys
Applications of SALT1 in Molten Salt Corrosion
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.
Figure 4: 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.
