Pourbaix Diagram Region Identifier

Identify the thermodynamic region (Immunity, Corrosion, Passivation) for iron in water from pH and electrode potential.

🗺️ Pourbaix Diagram Region Identifier
pH2
-
014
V
Region
—
Immunity/Corrosion line
—
Corrosion/Passivation line
—
Step-by-step working

🗺️ What is the Pourbaix Diagram Region Identifier?

This Pourbaix diagram region identifier tells you which thermodynamic region, Immunity, Corrosion, or Passivation, a given pH and electrode potential falls into for iron in water, using the classic simplified Fe-H2O Pourbaix diagram. Enter the environment's pH and the metal's operating potential to instantly see which iron species (metallic Fe, dissolved Fe2+, or solid Fe2O3) is thermodynamically favored there.

Corrosion engineers use Pourbaix diagrams to choose cathodic protection potentials, to understand why a structure corrodes in one environment but not another, and to explain unexpected field failures where the same alloy behaves completely differently at two different pH levels. A pipeline engineer setting a cathodic protection potential, for instance, wants that potential to sit clearly inside the Immunity region for the local soil pH.

A common misconception is that the Passivation region guarantees protection. It only means an oxide is thermodynamically stable at that point, not that the oxide actually forms a dense, adherent, protective film. Ordinary rust on carbon steel is technically an oxide in a thermodynamic passivation region, yet it does not protect the underlying metal the way the passive chromium oxide film on stainless steel does.

This calculator is useful for corrosion engineering students, materials scientists, and anyone trying to build intuition for how pH and potential together control whether a metal survives or dissolves, without manually deriving Nernst equation boundary lines by hand.

📐 Formula

Eimmunity  =  −0.617 V (constant)
Fe²⁺ + 2e⁻ → Fe boundary, from the Nernst equation with [Fe²⁺] = 10−6 M, pH-independent since no H⁺ appears in the reaction
Epassivation  =  1.083 − 0.1773 × pH
Fe₂O₃ + 6H⁺ + 2e⁻ → 2Fe²⁺ + 3H₂O boundary, from the Nernst equation with [Fe²⁺] = 10−6 M
Region rule: E < Eimmunity → Immunity; Eimmunity ≤ E < Epassivation → Corrosion; E ≥ Epassivation → Passivation
Example: pH=1, E=0.0V: passivation line = 0.906V, so E falls in the Corrosion region (per simplified Fe-H₂O Pourbaix diagram boundaries)

📖 How to Use This Calculator

Steps

1
Enter the environment's pH - Enter the solution pH, from 0 (strongly acidic) to 14 (strongly alkaline).
2
Enter the electrode potential - Enter the potential E in volts versus the standard hydrogen electrode (SHE).
3
Read the region classification - See whether the point falls in Immunity, Corrosion, or Passivation, and the boundary values at that pH.
4
Check the diagram - See your point plotted on the simplified Fe-H2O Pourbaix diagram alongside both boundary lines and the water stability lines.

💡 Example Calculations

Example 1 - Cathodically protected iron in acidic soil

1
pH = 2, E = −1.0 V
2
Immunity line = −0.617 V. Since E = −1.0 V is below this line, the point is in the Immunity region
3
Metallic iron is thermodynamically stable and will not corrode at this potential
Region = Immunity
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Example 2 - Unprotected iron in acidic water near open-circuit potential

1
pH = 1, E = 0.0 V
2
Passivation line = 1.083 − 0.1773 × 1 = 0.906 V. Since −0.617 ≤ 0.0 < 0.906, the point is in the Corrosion region
3
Dissolved Fe²⁺ is the stable species, so the iron actively corrodes
Region = Corrosion
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Example 3 - Iron in strongly alkaline, oxidizing conditions

1
pH = 12, E = 1.0 V
2
Passivation line = 1.083 − 0.1773 × 12 = −1.045 V. Since E = 1.0 V is above this line, the point is in the Passivation region
3
Solid Fe₂O₃ is the thermodynamically stable species at this pH and potential
Region = Passivation
Try this example →

❓ Frequently Asked Questions

What is a Pourbaix diagram?+
A Pourbaix diagram (potential-pH diagram) maps which chemical form of a metal, the metal itself, a dissolved ion, or a solid oxide, is thermodynamically stable at each combination of electrode potential and pH. It was developed by Marcel Pourbaix and is a core tool in corrosion science.
What does the Immunity region mean?+
The Immunity region is where the metallic form of the element is thermodynamically stable. In this region the metal will not corrode no matter how long it is exposed, because oxidation to any dissolved or solid corrosion product is not thermodynamically favorable.
What does the Corrosion region mean?+
The Corrosion region is where a soluble ion (like Fe2+) is the thermodynamically stable species. In this region the metal will actively dissolve into the electrolyte over time, since staying as solid metal or forming a stable oxide is not favored.
What does the Passivation region mean on a Pourbaix diagram?+
The Passivation region is where a solid oxide or hydroxide is thermodynamically stable rather than the bare metal or a dissolved ion. This does not automatically mean the metal is protected in practice; the oxide must also be adherent and non-porous to actually slow corrosion.
Why does the passivation boundary depend on pH but the immunity boundary does not?+
The Fe2O3/Fe2+ passivation reaction involves H+ ions (Fe2O3 + 6H+ + 2e- -> 2Fe2+ + 3H2O), so its equilibrium potential shifts with pH via the Nernst equation. The Fe2+/Fe reduction reaction involves no H+, so its equilibrium potential is independent of pH.
Can a metal in the Passivation region still corrode?+
Yes, if the oxide film is porous, cracked, or non-adherent, the underlying metal can keep corroding even though the oxide is thermodynamically favored. This is exactly why plain steel rust, though technically in a Pourbaix passivation region, is not a protective film like the passive layer on stainless steel.
How is cathodic protection related to the Pourbaix diagram?+
Cathodic protection lowers a structure's electrode potential, pushing its operating point down into the Immunity region of the Pourbaix diagram, where the metal itself becomes the thermodynamically stable species and corrosion is suppressed.
Why do Pourbaix diagrams use ion activity of 1e-6 M for the boundaries?+
1e-6 M is a conventional practical threshold used to define the boundary between metal being considered dissolved versus negligibly soluble. Different activity conventions shift the exact boundary lines slightly, which is why published Pourbaix diagrams can vary a little between sources.
Are Pourbaix diagrams accurate for real corrosion prediction?+
Pourbaix diagrams show thermodynamic stability only. They say nothing about reaction rate (kinetics), so a metal in the Corrosion region might corrode very slowly if the kinetics are sluggish, and a metal in the Passivation region might still corrode fast if the oxide film is not protective.
What are the water stability lines shown on the diagram?+
The two dashed reference lines mark where water itself becomes thermodynamically unstable: below the lower line, water reduces to hydrogen gas; above the upper line, water oxidizes to oxygen gas. Most aqueous corrosion of interest happens between these two lines.
Tags: Pourbaix Diagram Calculator Fe-H2o Pourbaix Diagram Corrosion Region Identifier Passivation Immunity Corrosion Calculator Formula Free Online

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What is a Pourbaix diagram?

A Pourbaix diagram (potential-pH diagram) maps which chemical form of a metal, the metal itself, a dissolved ion, or a solid oxide, is thermodynamically stable at each combination of electrode potential and pH. It was developed by Marcel Pourbaix and is a core tool in corrosion science.

What does the Immunity region mean?

The Immunity region is where the metallic form of the element is thermodynamically stable. In this region the metal will not corrode no matter how long it is exposed, because oxidation to any dissolved or solid corrosion product is not thermodynamically favorable.

What does the Corrosion region mean?

The Corrosion region is where a soluble ion (like Fe2+) is the thermodynamically stable species. In this region the metal will actively dissolve into the electrolyte over time, since staying as solid metal or forming a stable oxide is not favored.

What does the Passivation region mean on a Pourbaix diagram?

The Passivation region is where a solid oxide or hydroxide is thermodynamically stable rather than the bare metal or a dissolved ion. This does not automatically mean the metal is protected in practice; the oxide must also be adherent and non-porous to actually slow corrosion.

Why does the passivation boundary depend on pH but the immunity boundary does not?

The Fe2O3/Fe2+ passivation reaction involves H+ ions (Fe2O3 + 6H+ + 2e- -> 2Fe2+ + 3H2O), so its equilibrium potential shifts with pH via the Nernst equation. The Fe2+/Fe reduction reaction involves no H+, so its equilibrium potential is independent of pH.

Can a metal in the Passivation region still corrode?

Yes, if the oxide film is porous, cracked, or non-adherent, the underlying metal can keep corroding even though the oxide is thermodynamically favored. This is exactly why plain steel rust, though technically in a Pourbaix passivation region, is not a protective film like the passive layer on stainless steel.

How is cathodic protection related to the Pourbaix diagram?

Cathodic protection lowers a structure's electrode potential, pushing its operating point down into the Immunity region of the Pourbaix diagram, where the metal itself becomes the thermodynamically stable species and corrosion is suppressed.

Why do Pourbaix diagrams use ion activity of 1e-6 M for the boundaries?

1e-6 M is a conventional practical threshold used to define the boundary between metal being considered dissolved versus negligibly soluble. Different activity conventions shift the exact boundary lines slightly, which is why published Pourbaix diagrams can vary a little between sources.

Are Pourbaix diagrams accurate for real corrosion prediction?

Pourbaix diagrams show thermodynamic stability only. They say nothing about reaction rate (kinetics), so a metal in the Corrosion region might corrode very slowly if the kinetics are sluggish, and a metal in the Passivation region might still corrode fast if the oxide film is not protective.

What are the water stability lines shown on the diagram?

The two dashed reference lines mark where water itself becomes thermodynamically unstable: below the lower line, water reduces to hydrogen gas; above the upper line, water oxidizes to oxygen gas. Most aqueous corrosion of interest happens between these two lines.

📌 Quick Tips

💡A Pourbaix diagram is a map of pH versus electrode potential showing which chemical species (metal, dissolved ion, or oxide) is thermodynamically stable at each point.
💡Immunity means the metal itself is stable and will not corrode, regardless of how long it sits in that environment.
💡Passivation on a Pourbaix diagram means an oxide is thermodynamically stable, not that it necessarily forms a protective film. Rust on plain steel is technically in a thermodynamic passivation region, but the oxide is porous and non-protective, so the steel keeps corroding underneath.
💡This simplified two-boundary model is built for the classic Fe-H2O system and ignores extra species that appear at very high pH (above about 10-12), such as soluble ferrite ions, so treat results near the edges of the pH range as approximate.
💡Real corrosion control often works by pushing a system's operating point into the Immunity region (cathodic protection, lowering potential) or into a genuinely protective Passivation region (raising potential and pH, as in passivating stainless steel).