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Corrosion thermodynamics — Pourbaix diagram

Pourbaix E vs pH stability map (Fe-H₂O · 25 °C)

Plots the operating point in pH × E coordinates against Fe-water region boundaries (immunity / corrosion / passivation). The classical thermodynamic screening before any kinetic E_p assessment.

Inputs

—
0 = strong acid · 7 = neutral · 14 = strong base
mV vs SHE
Free corrosion potential. Add +244 mV to convert from SCE; +199 mV from Ag/AgCl(sat).
O₂-saturated, ~50 mV vs SCE → +275 mV vs SHE

Results

Pourbaix diagram — Fe-H₂O · 25 °C · 10⁻⁶ M activity
H₂ lineO₂ lineIMMUNITY (Fe stable)CORROSIONFe²⁺ / Fe³⁺ aqPASSIVATIONFe₂O₃ / Fe(OH)₂02468101214-1.0-0.50.00.51.0pHE (V vs SHE)(pH 8.1, 220 mV)Pourbaix Atlas 1974 · Fe-water · simplified contours at 10⁻⁶ M
Operating point (pH 8.1, E 220 mV vs SHE) → PASSIVATION
RegionCORROSION
E (V vs SHE)0.220V
H₂ line at this pH-0.480V
O₂ line at this pH0.749V
Recommendation: Active corrosion of carbon steel — use CRA (≥316L for 0 ppm Cl, ≥duplex for sea water)
Pourbaix is purely thermodynamic — it tells you what could form at equilibrium. For kinetic verdicts (pitting potential E_p, CPT, SCC) run bayesian_pitting_potential.
Download all inputs + calculations + outputs as a single PDF.