Aqueous Solutions - Thermo-Calc Software

TCAQ4 for Low PTX Conditions

TCAQ4 is a comprehensive database that includes all necessary phases (aqueous, gas, metallic solutions, and oxides) for calculating thermodynamic properties of complex aqueous solutions in low PTX conditions (up to 100 bar, 350 °C and 3 molality). The database is used in applications such as materials corrosion processes, hydro-metallurgy, aqueous chemistry, geochemistry, and environmental chemistry.

TCAQ4 Quick Overview

PROPERTIES: Gibbs Energy and derived aqueous properties
ELEMENTS: 91 total, 76 in the aqueous phase. Those listed in bold are included in the aqueous phase:

Ag, Al, Am, Ar, As, Au, B, Ba, Be, Bi, Br, C, Ca, Cd, Ce, Cl, Co, Cr, Cs, Cu, Dy, Er, Eu, F, Fe, Ga, Gd, Ge, H, He, Hf, Hg, Ho, I, In, Ir, K, Kr, La, Li, Lu, Mg, Mn, Mo, N, Na, Nb, Nd, Ne, Ni, Np, O, Os, P, Pa, Pb, Pd, Pm, Pr, Pt, Pu, Ra, Rb, Re, Rh, Ru, S, Sb, Sc, Se, Si, Sm, Sn, Sr, Ta, Tb, Tc, Te, Th, Ti, Tl, Tm, U, V, W, Xe, Y, Yb, Ze, Zn, Zr
ASSESSED PHASES: 1637. The database includes all the phases necessary to complete complex aqueous calculations. See the technical information sheet for details.

The aqueous solution phase in TCAQ4 is described by the SIT (Specific Interaction Theory) model.

TCAQ4 was developed to be used with Thermo-Calc and all available SDKs.

Questions You Can Answer Using TCAQ4 with Thermo-Calc:

How do acidity (pH) and redox potential (Eh) conditions affect the corrosion of a given alloy?
What aqueous species are formed under different electrochemical conditions (pH-Eh) for a given alloy?
What solid phases are most stable under different electrochemical conditions (pH-Eh)?
What solid phases are metastable during the aqueous corrosion?
How does the range of passivity vary for different alloy compositions or molalities?
How does the temperature and pressure affect the corrosion of a given alloy?
What aqueous species are expected when water of a given pH is equilibrated with a deposit of a given chemical composition?
How would the specification vary with the local Eh value (the depth) of the water reservoir?
How does pH vary with temperature for a given aqueous solution?

Designed to Be Used with the Aqueous Calculator

TCAQ4 is designed to be used with the Aqueous Calculator, a powerful tool in the graphical mode of Thermo-Calc that simplifies and accelerates advanced aqueous calculations, including Pourbaix diagrams. When used with the TCAQ4 database, the template automatically applies specialized settings for corrosion calculations, enabling fast setup of complex calculations with minimal manual input.

Learn more about the Aqueous Calculator

Highlights of the Latest Release

The latest version of our Aqueous Solutions Database, TCAQ4, was released with Thermo-Calc 2026a, in January 2026. The database was fully redesigned and significantly expanded to be used with the new Aqueous Calculator, so much so that it can be considered an entirely new database from TCAQ3.

Fully redesigned to be used with the new Aqueous Calculator for detailed modeling of corrosion processes across diverse conditions.
15 new elements: Am, Bi, Ge, Hf, Ir, Nb, Np, Pa, Pm, Pu, Rh, Ta, Tc, Ti, and Zr
While this database contains 91 elements, not all elements are available for all phases, as detailed below:

AQUEOUS: 76 elements

GAS: 27 elements

BCC_A2, FCC_A1, and HCP_A3: 65 elements

Stoichiometric oxide or hydroxide phases: 86 elements for 1614 phases

Terminal solid solution phases corresponding to stable element states: 24 elements for 17 phases
1635 new phases (1637 total)
Unlike previous versions of this database, TCAQ4 does not require additional databases to be appended to complete complex aqueous calculations

Read the Release Notes for More Information

AQS2 for High PTX Conditions

AQS2 is a database suitable for calculating thermodynamic properties of complex aqueous solutions in high PTX (up to 5 kbar, 1000 °C, and 6 molality of equivalent NaCl at ambient PT conditions or higher concentrations at higher PT) conditions. It is used in applications such as materials corrosion processes, hydro-metallurgy, aqueous chemistry, geochemistry, and environmental chemistry.

Unlike the TCAQ4 database, which is a complete stand-alone solution, the AQS2 database requires additional databases to be appended to it to complete calculations. The appended database depends on which systems you are interested in.

The aqueous solution phase in AQS2 is described by the complete Revised HKF (Helgeson-Kirkham-Flowers) model.

PROPERTIES: Gibbs Energy
ELEMENTS: 82
ASSESSED PHASES: See the technical information sheet

AQS2 was developed to be used with Thermo-Calc and all available SDKs.

Technical Information Sheet for AQS2

Aqueous Solutions Databases