Pourbaix Examples TCEX40A, 40B, 40C, and 40E

This example shows you how to use Option 1 where you start a completely new calculation. It uses a single database. The Pourbaix diagram for 0.001 m Fe in a 0.1 m NaCl solution at 25 °C and 1 bar is automatically calculated and plotted.

A more advanced version of 40A can be found in TCEX40E, which also uses Option 1 but with multiple databases. However, licenses are required for three commercial Thermo‑Calc databases.

See POURBAIX_DIAGRAM Commands for Options 1 to 4.

A so-called Pourbaix diagram is actually a phase diagram with independently-varied electropotential (Eh) and acidity (pH), for an heterogeneous interaction system at a certain bulk composition (that is by default always set as 1 kg of water solving a specified amount of metals and other solutes), under defined temperature and pressure conditions. Some property diagrams, along various phase boundaries for the same interactions resulting from the same Pourbaix module calculation, are also plotted, without leaving Option 1.

At the end of this example, a new POLY3 file called POURBAIX.POLY3 is automatically saved. The following examples (TCEX40B and TCEX40C) use this same POLY3 file for the calculations. Therefore it is important to make copies of this file and rename these to TCEX40A.POLY3, TCEX40B.POLY3, and TCEX40C.POLY3. This must be done outside of the Thermo‑Calc software and after the TCEX40A calculation and plotting is complete. This is so that the required POLY3 file structure is not lost.

The reason you do not save from within Thermo‑Calc is because of how the file is saved. When the command sequence (GO POLY3>SAVE TCEX40A.POLY3>Y) is used, the POLY3 workspace of the POURBAIX calculation results in the POLY3 module monitor and all the initial equilibrium points and the mapping/stepping calculation results are lost as well as parts of the POLY3 file structure (e.g., some definitions of the previously defined POURBAIX-type calculation system).

A reminder that Linux and Mac are case sensitive; when applicable ensure you enter file extensions with capital letters.

This example shows you how to use Option 2 where you Open an old (existing) file and plot other property diagrams.

Example 40A calculated and plotted the Pourbaix diagram for 0.001 m Fe in a 0.1 m NaCl solution at 25 °C and 1 bar. Example 40B reads the original POURBAIX.POLY3 file , and uses the calculated results to plot property diagrams for the same heterogeneous interaction system.

For example 40B, the POLY3 file is not modified. However, it is still recommended that a copy of the original POLY3 file is used so you do not lose the file structure and calculated results.

If you have a POLY3 file generated after choosing Option 4, that file can also be used for Option 2, and vice versa.

This example shows you how to use Option 3 where you Open an old file and make another Pourbaix calculation. The previously calculated (and copied) POURBAIX.POLY3 file from TCEX40A is edited to make another Pourbaix calculation and plot.

The same chemical system is used (Fe-H₂O-NaCl) but with different P-T-X (pressure-temperature-composition) conditions and without the GASeous mixture phase. The new conditions are 0.01 m Fe in a 1 m NaCl solution at 200 °C and 1 kbar.

In this example, the GASeous mixture phase for the Pourbaix diagram is not considered.

The POLY3 file is modified and rewritten with the new conditions. As previously recommended, make a copy of the original POLY3 file and rename it (e.g.TCEX40C.POLY3) so you do not lose the file structure and new calculated results.

A reminder that Linux and Mac are case sensitive; when applicable ensure you enter file extensions with capital letters.

After the default Pourbaix diagram is generated and refined, you can also use Option 3 to plot property diagrams along different phase boundaries for the same system. These settings are possible from within Option 3 and you do not need to go back to Option 2.

This example shows you how to use Option 1 where you start a completely new calculation. It uses multiple databases (TCAQ, SSUB, and SSOL). The Pourbaix diagram for 0.001 m Fe₉₀Co₅Ni₅ alloy (9E-3 m Fe + 5E-4 m Co + 5E-4 m Ni) in an aqueous solution with 2E-2 m H₂S and 3 m NaCl at 25 °C and 1 bar is automatically calculated and plotted.

You need licenses for three Thermo‑Calc databases, i.e.

• A: AQS (TGG Aqueous Solution Database; HKF model) or TCAQ (TCS Aqueous Solution Database; SIT model);
• B: SSUB (SGTE Pure Substances Database)
• C: SSOL (SGTE Alloy Solutions Database) or TCFE (TCS Steels/Fe-Alloys Database) or similar.

The TCAQ database contains only an AQUEOUS solution phase (using the SIT model) and REF_ELECTRODE phase (as a reference state for electron in aqueous electrolyte systems). The SSUB database is appended for the GASeous mixture phase and some pure solid phases (so-called secondary phases formed during aqueous-involving interaction processes), and the SSOL database is appended for various solid solution phases.

The Pourbaix module inside the software also accepts the AQS2 aqueous solution database (using the complete Revised HKF model) in the Multiple Database Option, in addition to the TCAQ2 aqueous solution database (using the SIT model).