Basic Settings and Definitions

In a homogeneous aqueous solution or an aqueous-bearing heterogeneous interaction system, the most essential definition is for system-components there must be H2O, H+1 and ZE (electron) plus those for elements dissolved in aqueous solution (such as Na, Cl, S) and associated in interacting metals/alloys (such as Fe, Cr, Mn, Mg, Ni, Al, Si, Zn). Three fundamental phases in a system are the AQUEOUS solution, the GAS mixture and the REF_ELECTRODE. The REF_ELECTRODE phase is used for setting the reference state for electrostatic potential in the system and for calculating the Eh condition (defined as MUR(ZE)/RNF). Other phases should be appropriately selected and retrieved from critically-assessed databases that cover not only the target phases (solution or stoichiometric) in interacting metals/alloys but also the secondary phases (solution or stoichiometric). The two public databases PAQ and PAQS are specially designed as single-database choices that cover all kinds of phases necessary for calculations. However, these databases are each limited to a framework of a very small number of elements. When it comes to simulations of complex multicomponent systems with a wide variety of elements and phases, the thermodynamic data must be selected and retrieved from several databases: AQUEOUS solution and REF_ELCTRODE phases must be retrieved from TCAQ or AQS; alloy phases (such as FCC_A1, BCC_A2, HCP_A3, CEMENTITE) from alloy solution databases (such as SSOL for general alloy phases, TCFE for steel/Fe-alloy phases, TCAL for Al-based alloy phases; TCMG for Mg-based alloy phases, and TCNI for Ni-based superalloy phases); gaseous mixture phase and secondary phases from specific substance or solution databases (such as SSUB for GAS phase and various solid compound phases or TCOX for oxide solution phases). Note that the REF_ELECTRODE phase should always be suspended in equilibrium calculations, while GAS phase could be set as ENTERED, SUSPENDED or DORMANT, depending on the purpose of the calculation.

When defining an interaction system, the initial condition for the H2O component is always set as 1 kg of water. The initial composition conditions for dissolving and interacting elements are normally defined in moles (such as n(Fe)=0.009, n(Cr)=5E-4, n(Ni)=3E-4, n(Mn)=5E-5, n(S)=5E-5, n(Na)=3, n(Cl)=3). This makes it straightforward and convenient to count various related aqueous solution properties based on molality. The initial conditions for the H+1 and ZE components can be given as molar compositions (such as n(H+1)=0, n(ZE)=0) or their activities or potentials (such as lnACR(H+1)=-9.21, MUR(ZE)=8400).

The pH and Eh properties of the aqueous solution in the interacting system are always defined in the following way:

pH = -log10[ACR(H+1,AQUEOUS)*AH2O]]Eh = MUR(ZE)/RNF

The symbol AH2O is the molecular weight of solvent H2O (equals 55.508435) and RNF is the Faraday constant (equals 96485.309).

The activity of the solvent water (ACRH2O, Aw), the osmatic coefficient of aqueous solution (OSMC, Os), electronic affinity (Ah), electronic activity log10ACRe (pe), ionic strength (IS), total aqueous concentration, in molality (TM) and total alkaline concentrations under two definitions (At1/AT2) are calculated and listed for each equilibrium state.

POLY3 calculations for mass balances in Thermo‑Calc are always based on site-fractions. Consequently, when functions for describing various properties of aqueous solutes are defined, such as molality (MLi), activity coefficient (RCi) and activity (AIi), they should be converted to molality-based quantities:

MLi = Y(AQUEOUS,i)*AH2O/YH2O

RCi = ACR(i,AQUEOUS)*YH2O/Y(AQUEOUS,i)

AIi = ACR(i,AQUEOUS)*AH2O

Here, YH2O is the site-fraction of solvent H2O and AH2O equals 55.508435.

Many more variables, functions and tables can be entered for various purposes. For instance, an equilibrium constant for a homogeneous reaction or a solubility product for a solid dissolution reaction can be entered.