Ternary Molten Salt Thermal Decomposition Temperature Testing

Ternary molten salt thermal decomposition temperature testing establishes the safe upper operating limit of three-component nitrate, nitrite, carbonate, and chloride salt mixtures used for heat transfer and thermal energy storage. By combining thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and evolved gas analysis, the test quantifies decomposition onset and peak temperatures, mass-loss behavior, and degradation kinetics that govern long-term stability in concentrated solar power plants, molten salt reactors, and industrial heating systems.

Technical Overview

Ternary molten salts are eutectic or near-eutectic mixtures of three inorganic salts formulated to achieve a lower melting point and a wider liquid operating range than the individual constituents or their binary combinations. Because they store large amounts of sensible and latent heat at atmospheric pressure, ternary salts such as Hitec (NaNO2–NaNO3–KNO3), Hitec XL (Ca(NO3)2–NaNO3–KNO3), and lithium-containing nitrate eutectics are widely used as heat transfer fluids and thermal storage media.

The thermal decomposition temperature is the lowest temperature at which the salt melt begins to chemically degrade with a measurable loss of mass, releasing gases such as O2, NO, NO2, CO2, or HCl. It is fundamentally different from the melting point: a salt can be fully molten and chemically stable, and decomposition temperature defines the ceiling above which the melt is no longer chemically stable. For nitrate systems, decomposition typically begins with the equilibrium reaction nitrate to nitrite plus oxygen at roughly 550 to 620 °C in inert gas, followed at higher temperatures by conversion of nitrite to oxide with release of nitrogen oxides. Each decomposition step permanently shifts the salt composition, changes the melting point, accelerates corrosion of containment materials, and can generate pressurized off-gas.

Accurate determination of the decomposition temperature is therefore essential for setting the maximum operating temperature of a plant, defining safety margins (typically 50 to 100 °C below the measured onset), qualifying new salt formulations and suppliers, and assessing the remaining service life of in-service salts. Testing is performed according to recognized methods such as ASTM E2550, ISO 11358, and related thermal analysis standards.

Testing Samples

Ternary Nitrate–Nitrite Salts (Hitec-Type Mixtures)

The classic Hitec formulation of approximately 53 wt% KNO3, 40 wt% NaNO2, and 7 wt% NaNO3 melts near 142 °C and is commonly specified for service up to about 450 °C. Because the nitrite component is chemically the least stable, careful determination of the decomposition onset, typically between 550 and 620 °C depending on atmosphere and heating rate, is critical for both design and operational safety.

Ternary Nitrate Salts

Calcium nitrate based mixtures (Hitec XL type) and lithium nitrate containing eutectics (LiNO3–NaNO3–KNO3) offer melting points between roughly 120 and 220 °C. Lithium nitrate is thermally less stable than sodium or potassium nitrate, so the decomposition onset of these eutectics may appear as low as 480 to 550 °C and must be verified experimentally for each formulation.

Ternary Carbonate Salts

Lithium, sodium, and potassium carbonate mixtures melt near 400 °C and are candidate media for next-generation, higher-temperature storage. Decomposition proceeds via carbonate to oxide plus CO2 release, with measurable onset commonly occurring above 700 °C under inert gas, and the exact value is strongly influenced by the CO2 partial pressure of the test atmosphere.

Ternary Chloride and Fluoride Salts

Anhydrous chloride systems such as MgCl2–KCl–NaCl and fluoride systems such as LiF–NaF–KF (FLiNaK) are stable to temperatures well above 800 °C when dry, but they are highly hygroscopic and can release corrosive HCl or HF during heating if moisture is present, making decomposition testing and pre-test drying indispensable.

Sample Forms and Requirements

Acceptable sample forms include anhydrous powder, granules, crushed solid blocks, drilled cores from storage tanks, and pre-melted and quenched flakes. A typical test program requires 10 to 50 g of homogenized material, with a minimum of 2 to 5 g for screening. Hygroscopic salts are pre-dried under vacuum or dry nitrogen, typically at 105 to 300 °C depending on the salt system, and are handled under dry conditions to prevent moisture pickup before measurement.

Testing Items

• Decomposition onset temperature determined by the extrapolated tangent method
• Peak decomposition temperature from the derivative thermogravimetric (DTG) curve
• Completion temperature of each decomposition stage and identification of multi-step behavior
• Cumulative and stepwise mass-loss percentages
• Maximum mass-loss rate
• Apparent activation energy and pre-exponential factor by Kissinger, Ozawa–Flynn–Wall, and Friedman methods
• Identification and release profiles of evolved gases (O2, NO, NO2, N2, CO2, HCl, HF) by mass spectrometry or FTIR
• Isothermal stability at specified service temperatures with long-duration mass-loss monitoring
• Thermal endurance extrapolation for service lifetimes of 5,000 to 30,000 hours
• Comparative stability of fresh, aged, cycled, and contaminated salts
• Optional companion measurements: melting point, enthalpy of fusion, specific heat, and post-test composition by ion chromatography and XRD

Testing Methods

Thermogravimetric Analysis (TGA)

A 5 to 20 mg sample is heated in an open alumina or platinum crucible under high-purity nitrogen or argon purge at 40 to 60 mL/min, using heating rates of 5, 10, or 20 °C/min from ambient to 800–1000 °C. The decomposition onset is reported as the intersection of the extended baseline and the leading-edge tangent of the mass-loss step, or alternatively as the temperature at which a defined mass-loss threshold (for example 1% or 5%) is reached. Testing follows ASTM E2550 and ISO 11358 practices.

Simultaneous TGA–DSC (STA)

Simultaneous thermal analysis records mass loss and heat flow in a single experiment, allowing endothermic decomposition events to be distinguished from melting, solid-state transitions, and evaporative losses, which is particularly valuable for multi-component salts that exhibit overlapping events.

Evolved Gas Analysis (TGA–MS / TGA–FTIR)

Coupling the thermobalance to a mass spectrometer or FTIR gas cell identifies the decomposition products in real time, confirming the degradation mechanism, for example oxygen evolution from nitrate reduction, NOx release from nitrite breakdown, CO2 from carbonates, or HCl from moist chlorides, and providing evidence for regulatory and safety documentation.

Isothermal and Cyclic Stability Testing

Candidate service temperatures are verified by holding the salt isothermally for 100 to 500 hours while monitoring the mass-loss rate, and by thermal cycling between the solidification and operating temperatures. Post-test chemical analysis quantifies composition drift, nitrite oxidation, and oxide accumulation, which establishes the true long-term temperature tolerance of the melt.

Kinetic Analysis and Thermal Endurance

Multi-heating-rate experiments are evaluated with the Kissinger method (ASTM E2890) and model-free approaches (Ozawa–Flynn–Wall, Friedman) to derive apparent activation energies and pre-exponential factors. These parameters allow extrapolation of decomposition rates to service conditions and support thermal endurance assessments consistent with ASTM E1641 and ASTM E1877.

Quality Assurance and Calibration

Temperature and enthalpy calibration is performed with certified melting-point standards (indium, zinc, aluminum) and Curie-point magnetic standards (nickel, perkalloy, iron) for the thermobalance. Baseline runs with matched empty crucibles, balance calibration, and replicate measurements ensure onset temperature repeatability typically within ±2 to 5 °C.

Applications

• Qualification and receipt inspection of storage media for concentrated solar power (CSP) plants
• Molten salt reactor coolant and fuel salt chemistry and safety programs
• Design and safety-margin definition for thermal energy storage systems
• Development and optimization of new ternary eutectic formulations
• Condition assessment of in-service, thermally cycled, or aged salts
• Evaluation of impurity, moisture, and cover-gas effects on salt stability
• Supplier certification and third-party quality inspection for salt producers
• Input data for hazard analyses, HAZOP studies, and thermal runaway assessments

FAQ

What is the difference between melting point and thermal decomposition temperature?

The melting point is a physical phase transition at which the solid becomes liquid without chemical change, whereas the decomposition temperature is the point at which the salt chemically breaks down with gas evolution and permanent mass loss. A ternary salt may be molten and stable across a wide window; the decomposition temperature defines the upper boundary of that window and must be measured separately.

What decomposition temperatures are typical for common ternary salts?

Under nitrogen at 10 °C/min, Hitec-type nitrate–nitrite salts typically show an onset between 550 and 620 °C, lithium-containing nitrate eutectics may begin to decompose as low as 480 to 550 °C, ternary carbonates generally remain stable above 700 °C, and dry chloride or fluoride systems can exceed 800 °C. Actual values depend on composition, purity, atmosphere, and heating rate, so measurement of the specific batch is always required.

Why is the test usually performed in inert atmosphere, and can it be done in air?

An inert purge isolates thermal decomposition from oxidation reactions, providing the intrinsic stability limit. Testing in air or synthetic process gas is available and recommended when the salt will actually be exposed to those atmospheres, since oxygen suppresses nitrate decomposition but promotes nitrite oxidation, and CO2 shifts carbonate equilibria, so the measured onset can differ by tens of degrees between atmospheres.

How much sample is required and how should it be prepared?

A full test package generally requires 10 to 50 g of homogenized salt, with a 2 to 5 g minimum for a single dynamic TGA scan. Samples should be pre-dried under vacuum or dry inert gas, stored in sealed containers, and, for hygroscopic salts, handled in a glovebox to prevent moisture contamination that would distort the mass-loss curve.

What accuracy and repeatability can be expected?

With calibrated instruments and proper technique, replicate onset temperatures on the same sample typically agree within ±2 to 5 °C, and cross-laboratory reproducibility is generally within ±5 to 10 °C. Kinetic parameters carry somewhat wider uncertainty bands, which are reported together with the statistical confidence intervals.

How long does the testing take?

Dynamic TGA, STA, and evolved gas analyses are usually completed within 5 to 10 working days, including calibration and data reduction. Isothermal endurance and thermal cycling programs of 100 to 500 hours require two to eight weeks depending on the hold temperature and duration.

Can aged, contaminated, or partially decomposed salts be tested?

Yes. Testing in-service or off-specification material is a common request, and results for such samples are compared against fresh reference salt to quantify nitrite depletion, oxide accumulation, or moisture effects, providing a reliable basis for continued use, reconditioning, or replacement decisions.