Iron Pourbaix Diagram: Corrosion, Immunity & Passivation

Operating Point

Solution pH 7.0

Electrode potential E -0.20 V

Fe²⁺ activity 10⁻⁶ M

Animation speed 1.0×

Pourbaix Diagram

Click/drag the diagram to set pH & E

Live State

IMMUNITY

Corrosion rate (rel.)

0.0

Passive film

Boundary E₁ (Fe/Fe²⁺)

-0.62 V

Boundary pH₂

8.8

How it works

A Pourbaix diagram maps which chemical species is thermodynamically stable at a given electrode potential (E, vs. SHE) and pH. This simplified iron-water diagram uses three species — solid Fe, dissolved Fe²⁺, and solid Fe(OH)₂ — and derives every boundary from the Nernst equation and a solubility product, so it is not decorative: it is computed live from your sliders.

Fe → Fe²⁺ + 2e⁻   (E° = -0.440 V, n = 2)
E₁ = -0.440 + (0.0592/2)·log[Fe²⁺]   (horizontal — no H⁺ involved)

Fe²⁺ + 2OH⁻ ⇌ Fe(OH)₂   (Ksp ≈ 4.9×10⁻¹⁷)
pH₂ = 5.85 + pFe/2,  pFe = -log[Fe²⁺]   (vertical — no electrons involved)

Fe(OH)₂ + 2H⁺ + 2e⁻ → Fe + 2H₂O
E₃(pH) = E₁ - 0.0592·(pH - pH₂)   (sloped, -59 mV/pH, continuous with E₁ at pH₂)

Immunity (blue) — E below E₁ (or below E₃ past pH₂): metallic iron is the stable species, the electrode is thermodynamically inert. Cathodic protection works by pushing E down into this region.
Corrosion (orange) — E above E₁ at pH below pH₂: Fe²⁺ is the stable species, so the surface actively dissolves into solution.
Passivation (green) — E above E₃ at pH above pH₂: solid Fe(OH)₂ is stable and forms a thin protective film that slows further attack, even though the underlying metal is not immune.
The dashed lines are water's own stability window (H⁺/H₂ at E = -0.0592·pH and O₂/H₂O at E = 1.23 - 0.0592·pH) — real aqueous corrosion cells operate between them.

This is a genuine simplification of the full Fe-H₂O Pourbaix atlas (which also includes Fe³⁺, Fe₂O₃, Fe₃O₄ and HFeO₂⁻): it keeps the three-region shape every corrosion-engineering course teaches first, with boundaries you can verify by hand from the equations above.

Fe²⁺ ions leaving surface Oxide passivation film Metallic electrode

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.