Iron Pourbaix Diagram 2D: 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 the Fe/Fe²⁺/Fe(OH)₂ triple point)

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/red) — E above E₁ at pH below pH₂: Fe²⁺ is the stable species, so the surface actively dissolves into solution — shown here as ions detaching from the electrode's cross-section.
Passivation (green) — E above E₃ at pH above pH₂: solid Fe(OH)₂ is stable and forms a thin protective ring around the electrode 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. Drag the electrode view below to rotate it and watch ions detach from whichever face is nearest you.

Fe²⁺ ions leaving surface Oxide passivation ring Metallic electrode

drag to rotate view