Pourbaix Diagram Calculator

Pourbaix diagrams, also known as E-pH diagrams or potential-pH diagrams, are thermodynamic maps that show the stability regions of different chemical species of a metal as a function of electrode potential (E) and solution pH. Developed by Belgian-born chemist Marcel Pourbaix in the 1940s, these diagrams are indispensable tools in corrosion science, electrochemistry, and materials engineering.

A Pourbaix diagram divides the E-pH space into regions corresponding to different thermodynamically stable forms of a metal: metallic metal (immunity region), soluble metal ions (corrosion region), and solid oxides or hydroxides (passivation region). The boundaries between these regions represent equilibrium conditions where two species coexist. By knowing the pH and potential of a given environment, one can predict whether a metal will corrode, remain protected by a passive film, or exist in its metallic state.

Pourbaix diagrams are constructed using the Nernst equation and thermodynamic data (Gibbs free energies of formation) for all relevant species. They are valid at 25°C by default and assume unit activity for dissolved species. While they provide valuable thermodynamic guidance, Pourbaix diagrams do not account for kinetic factors such as corrosion rate or the rate of passivation.

Pourbaix diagrams are built from electrochemical equilibrium equations that relate potential, pH, and ion concentration. The hydrogen evolution and oxygen evolution lines define the stability domain of water and are essential reference boundaries on every Pourbaix diagram.

For metal dissolution reactions, the Nernst equation gives the equilibrium potential as a function of ion concentration and pH. The boundary between immunity and corrosion regions shifts with ion concentration, while passivation boundaries depend on oxide solubility products.

Pourbaix diagrams contain three fundamental stability regions, each with distinct implications for metal behavior:

This Pourbaix diagram calculator analyzes the corrosion behavior of common engineering metals:

1. Select a Metal: Choose from iron (Fe), aluminum (Al), zinc (Zn), copper (Cu), or nickel (Ni).
2. Enter pH: Specify the solution pH from 0 (strongly acidic) to 14 (strongly alkaline).
3. Enter Electrode Potential: Set the potential in volts vs. SHE, typically ranging from −2 V to +2 V.
4. Select Ion Concentration: Choose the dissolved metal ion concentration (1 M to 10⁻⁶ M), which shifts the immunity/corrosion boundary.
5. View Results: The calculator determines the stability region, dominant species, and provides engineering recommendations for corrosion protection.

Pourbaix diagrams are essential in designing cathodic protection systems for pipelines, ship hulls, and underground storage tanks. By shifting the metal's potential into the immunity region through impressed current or sacrificial anodes, corrosion can be prevented. The diagrams help determine the required protection potential for different metals in various soil and water conditions.

In chemical processing, Pourbaix diagrams guide the selection of materials for reactors, heat exchangers, and storage vessels. For example, the aluminum Pourbaix diagram shows that aluminum is passive between pH 4 and 9, explaining why aluminum containers are suitable for neutral solutions but corrode in strongly acidic or alkaline environments. In biomedical engineering, titanium's favorable passivation behavior across a wide pH range contributes to its excellent biocompatibility for implants.