Pourbaix Diagrams
A Pourbaix diagram (E-pH diagram) shows, for a given element dissolved in water, the predominance regions of each species (metal, ion, oxide, hydroxide) as a function of electrochemical potential E and pH. Developed by Marcel Pourbaix in the 1940s, these diagrams are indispensable in corrosion science and hydrogeology.
Constructing the Diagram
For each couple involving two species, the boundary corresponds to the equilibrium condition:
1. Horizontal boundaries: purely electronic couples (no H⁺ involved). E.g. Fe³⁺/Fe²⁺: E = E° − (0.0592/1) log ([Fe²⁺]/[Fe³⁺]), pH-independent.
2. Oblique boundaries: couples involving both protons and electrons. The boundary follows: E = E° − (0.0592 × m/n) × pH (m: number of H⁺, n: number of e⁻).
3. Vertical boundaries: acid-base equilibria (no electron exchange). E.g. Fe³⁺ ⇌ Fe(OH)₃: pH-dependent only.
Two reference lines delimit the thermodynamic stability domain of water: - Line (a): 2 H⁺ + 2 e⁻ → H₂ (E = −0.0592 pH) - Line (b): O₂ + 4 H⁺ + 4 e⁻ → 2 H₂O (E = 1.23 − 0.0592 pH)
Reading the Domains
On the Iron (Fe) diagram — the canonical example — three main zones are identified:
| Zone | Stable species | Practical meaning |
|---|---|---|
| Immunity | Fe(s) | The metal is thermodynamically stable: no corrosion. |
| Corrosion | Fe²⁺, Fe³⁺ | Ferrous/ferric ions predominate: the metal dissolves. |
| Passivation | Fe₂O₃, Fe₃O₄, Fe(OH)₃ | A solid oxide/hydroxide forms and protects the metal. |
Passivation is the mechanism exploited in stainless steel (Chromium (Cr) and Nickel (Ni) extend the passive domain) and in cathodic protection of metal structures.
Applications in Corrosion Science
Electrochemical corrosion occurs when the ambient conditions (E, pH) place the metal in its corrosion domain. Prevention strategies:
- Shift E toward immunity: cathodic protection (galvanic Zinc (Zn) on pipelines, sacrificial anodes).
- Shift E toward passivation: anodic inhibitors, oxide coatings.
- Adjust pH: alkalisation of cooling circuits (alkaline concrete protects steel reinforcement).
Example: reinforced concrete. Fresh concrete pH ≈ 12–13 places iron in the passive domain. Carbonation (atmospheric CO₂ lowers pH) can shift the system into the corrosion zone, causing rusting of rebars in aging structures.
Limitations of Pourbaix Diagrams
Pourbaix diagrams are thermodynamic tools: they indicate whether a reaction is possible, not how fast it proceeds. A metal in the corrosion zone may be kinetically passive in practice (compact oxide film). Conversely, a metal in the immunity zone can corrode if complexing agents (chlorides, cyanides) stabilise ions in solution, shifting the domain boundaries.