Calculating a Pourbaix Diagram

The POURBAIX module lets you construct Pourbaix diagrams and other types of property diagrams for heterogeneous interaction systems with aqueous solution phases.

When calculating a Pourbaix diagram, you need to have a database with an aqueous solution phase (that is, a phase with water as the dissolving solvent and with aqueous cation/anion and complex species as the dissolved solutes). Due to restrictions of aqueous solution models (SIT, HKF, Pitzer) used in the Thermo‑Calc software, the aqueous solution database must be designed in the same Thermo‑Calc database format as that used in the PAQ, PAQS, TCAQ and AQS databases for the aqueous solution phase.

Pourbaix diagram calculations are done in the advanced Pourbaix module.

  1. At the SYS prompt, enter goto_module POURBAIX.
  2. At the Need any instruction on the POURBAIX module? prompt, press <Enter> to skip or enter Y to learn more.
  3. Press <Enter> at the prompt Enforce a PAUSE after plotting when running a MACRO?.
  4. Press <Enter> to accept the default 1 to Start a completely new POURBAIX diagram calculation or enter 3 to Open an old file and make another POURBAIX calculation.

    To choose 3, you must have already done a Pourbaix calculation and have it saved in a POLY3 file. The new calculation uses the same system definition as the previous calculation, but the other conditions, such as the bulk composition, temperature and pressure, can be modified in the next step.

  5. Type Y or N to set whether to Consider the GAS phase (gaseous mixture phase) in the calculation.

    The gaseous mixture phase should at least contain H2O, O2 and H2. If a GAS phase is not considered, then only the interactions between the aqueous solution phase and various solid phases (alloy phases and secondary phases) are calculated. Such a calculation may not accurately present all the heterogeneous interactions. Some secondary phases are therefore usually needed. Such phases exist in the PAQ or PAQS Public Aqueous Solution Databases. They can also be appended from the SSUB/SSOL/TCFE or other appropriate substances/solutions databases.

  6. Type Y or N to set whether to Use single database (Y) or a multiple database (N).

    If you choose to use a single database, at the Combined Database: /PAQ2/ prompt specify a database that is specially developed for the Pourbaix module’s diagram calculations.

    If you choose to use a multiple database, then you are prompted to specify an aqueous solution database and to append any additional required data and select necessary phases, such as various interacting alloy (solid) solution phases, and secondary (solid and liquid) phases, for example.

  7. Specify all your solutes and the molality of each solute when prompted to do so. This can be done in terms of elements (such as Fe, Ni, Na, Cl, C or S) or arbitrary complex species (such as NaCl, CO2, CoCl3, Fe0.93S, NaSO4-1, or H2SO4). You can enter the element/species and the molarity on one line (for an example, NaCl 2.5).

    The definition of your system and its initial bulk composition is now presented.

    The first letter of the element abbreviation must always be upper case. Any second letters must be lower case. When you have finished your specification, press <Enter> when asked to specify Next solute.

  8. Type Y to confirm the defined system and its bulk composition.

    The thermodynamic data for your system is now retrieved from the database(s) chosen.

  9. Answer the questions that you are prompted regarding various parameters of your calculation. You are asked to do any of the following:
    • Rejectphase(s)or Restore phases(s)
    • Check for miscibility gaps on any phase (Should any phase have a miscibility gap check?)
    • Enforce the Global Minimization Technique in calculations?
    • Save all functions, variables and tables in POLY3 file?
    • Set numerical limits on the calculation
    • Confirm defined conditions?
    • Accept the default settings for two mapping variables
  10. The Pourbaix calculation starts. Answer the question Any missing phase boundary you could possibly think of? Type Y or N to set whether you want to add starting points to the calculation. If you suspect that the calculation has missed some phase boundaries, this may help the program to detect them.
  11. Answer questions about how the diagram is plotted and how to save the results of your calculation. You are asked to do any of the following:
    • Change the pH/Eh steps for smoother curves?
    • Zoom in on a specific region of the diagram
    • Change the curve-label option for the diagram
    • Add label-texts onto the Pourbaix diagram
    • Change the subtitle of the diagram
    • Change the axis-texts
    • Further refine the diagram in the POST Module
    • Print the diagram (save a Hard copy of the diagram)
    • Save the X-Y coordinates of curve on a text file
    • Modify the diagram
    • Calculate and plot another Pourbaix diagram (Any more diagram?)