Calculator Inputs
Example Data Table
| Case | pH | Potential | Fe2 activity | Likely reading |
|---|---|---|---|---|
| Reducing water | 7.0 | -0.70 V | 1.0E-6 | Immunity may be favored. |
| Mild corrosion | 5.5 | -0.20 V | 1.0E-6 | Ferrous dissolution may occur. |
| Alkaline passive range | 10.0 | 0.10 V | 1.0E-6 | Hydroxide film may form. |
Formula Used
The calculator uses a simplified Nernst form for each boundary:
E = E0 - (2.303RT / nF) log10(Q)
For pH dependent reactions, hydrogen ion activity is replaced by pH. The temperature factor is:
k = 2.303RT / F
Common lines include:
- Hydrogen line: E = -k pH
- Oxygen line: E = 1.229 + k log10(pO2) / 4 - k pH
- Fe2/Fe: E = -0.447 + k log10(aFe2) / 2
- Fe3/Fe2: E = 0.771 + k log10(aFe3 / aFe2)
- Hydroxide boundaries use Ksp and pKw values.
This model assumes pure solids and water have unit activity.
How to Use This Calculator
- Enter the pH range for the diagram table.
- Add your selected pH and applied potential.
- Enter temperature, iron activities, oxygen pressure, pKw, and Ksp values.
- Press the calculate button.
- Review the result box above the form.
- Check the SVG diagram and boundary table.
- Use CSV or PDF links for reporting.
Understanding an Iron Pourbaix Diagram
A Pourbaix diagram maps metal stability against pH and potential. For iron, it helps show when iron may stay immune, dissolve, or form protective oxide films. This calculator gives a practical estimate. It is not a substitute for laboratory testing. Real water contains salts, gases, flow, and deposits. Those factors can shift boundaries.
Why pH Matters
pH controls hydrogen activity. Low pH usually supports active iron dissolution. High pH can favor hydroxide and oxide films. Those films may reduce attack, but only when they remain stable. The diagram helps compare these zones at a glance.
Why Potential Matters
Potential shows the oxidizing or reducing character of the environment. A low potential favors metallic iron. A higher potential may push iron toward ferrous or ferric ions. Very oxidizing water can also support passive ferric hydroxide. The water stability limits mark hydrogen and oxygen evolution. Values outside those lines can still occur, but water reactions become important.
How This Calculator Works
The tool uses simplified Nernst equations. It applies the selected temperature factor, pH range, ion activities, and solubility products. It then estimates common boundaries for iron, ferrous ions, ferric ions, and hydroxide films. The SVG plot is drawn from calculated table points. Exported files use the same values shown on the page.
Best Use Cases
Use this page for study, early screening, corrosion notes, and quick reports. It is useful when comparing how concentration or temperature changes affect iron behavior. Change one input at a time. Then review the new table and diagram. This makes trends easier to see.
Reading the Results
Start with the selected pH row. Compare the entered potential with iron, ferrous, ferric, and passive boundaries. A lower value often suggests immunity. A middle value can suggest corrosion. A higher value can suggest passivation or ferric corrosion. Use the zone label as a guide, not a final verdict today safely.
Important Limits
A real Pourbaix diagram can include magnetite, hematite, complex ions, carbonate species, chloride effects, and kinetic limits. This page focuses on a compact educational model. It uses selected iron species and assumes unit activity for solids and water. For engineering decisions, confirm the result with standards, experiments, and site water chemistry.
FAQs
What is an iron Pourbaix diagram?
It is a stability map for iron. It compares pH and electrode potential. It shows where iron may be immune, corrode, or form passive films.
Is this calculator suitable for real corrosion design?
It is best for education and early screening. Real systems need water chemistry, flow, deposits, alloy details, and testing before final decisions.
What does immunity mean?
Immunity means metallic iron is thermodynamically favored. It does not guarantee zero corrosion, because local chemistry and kinetics may still matter.
What does passivation mean?
Passivation means an oxide or hydroxide film may be stable. Such films can slow corrosion when they remain adherent and undamaged.
Why do Fe2 and Fe3 activities matter?
Ion activity shifts Nernst boundaries. Lower activity often moves dissolution lines and changes the apparent corrosion or passivation region.
Why include oxygen pressure?
Oxygen pressure affects the oxygen water stability line. More oxygen usually raises oxidizing conditions and can influence passive film formation.
What is pKw?
pKw describes water ionization. It affects hydroxide activity and therefore changes calculated precipitation boundaries for iron hydroxides.
Why are exported values useful?
CSV files help with spreadsheets. PDF files help with quick reports. Both use the same calculated values shown on the page.