How do Pourbaix diagrams assist in predicting corrosion? Use them to map stable phases and guide electrochemical testing.

Potential-pH (Pourbaix) diagrams help researchers predict which corrosion or oxidation state is thermodynamically favored at a given electrode potential and solution pH. By locating test conditions on the diagram, researchers can identify whether a metal is likely to remain metallic (immunity), dissolve as soluble ions (active corrosion), or form an insoluble oxide or hydroxide film (passivation). Overlaying the water-stability limits also shows whether hydrogen or oxygen evolution may compete with the metal reactions.

A Pourbaix diagram is a thermodynamic map, not a direct corrosion-rate measurement. It defines the likely stable phase under specified pH and potential conditions, while kinetics, film properties, mass transport, and solution chemistry determine how quickly and reliably that state is reached.

How Pourbaix Diagrams Map Corrosion Behavior

Potential and pH Define the Test Location

The vertical axis represents electrode potential, typically referenced to the normal hydrogen electrode (NHE). The horizontal axis represents pH, which captures the acidity or alkalinity of the aqueous environment.

Every combination of potential and pH corresponds to a location on the diagram. That location falls within a stability region associated with a metallic phase, dissolved ions, or an oxide or hydroxide compound.

Stability Regions Identify Favored Species

For a metal such as iron, different regions may indicate stability of:

• Elemental iron, corresponding to immunity from thermodynamically favored dissolution.
• Soluble ions, such as Fe²⁺ or Fe³⁺, corresponding to active corrosion.
• Insoluble hydroxides or oxides, such as Fe(OH)₂ or Fe(OH)₃, corresponding to possible passivation.

The boundaries between these regions represent conditions where two species are in thermodynamic equilibrium.

How Researchers Interpret Corrosion and Oxidation

Predicting Active Dissolution

If the test condition lies in a soluble-ion region, the metal has a thermodynamic driving force to oxidize and enter solution.

For iron, a region favoring Fe²⁺ or Fe³⁺ suggests that anodic dissolution is more stable than retention of the metallic phase. This helps researchers anticipate corrosion-prone combinations of potential and pH.

Identifying Possible Passivation

A region dominated by an insoluble oxide or hydroxide indicates that an oxidized surface compound is thermodynamically favored.

This does not prove that a dense, adherent, protective film will form. However, it identifies conditions where passivation is possible and provides a basis for interpreting reductions in anodic current during electrochemical testing.

Recognizing Immune Conditions

When the metallic phase is the stable species, the diagram indicates an immune region.

Under those conditions, oxidation of the metal is not thermodynamically favored relative to the competing phases represented in the diagram. Researchers can use this information to distinguish potentially stable metallic behavior from conditions that favor dissolution or film formation.

Why the Water Stability Lines Matter

Hydrogen Evolution at Low Potentials

The lower water-stability boundary represents conditions where water or protons can be reduced to produce hydrogen.

If a corrosion test operates near or below this line, hydrogen evolution may occur alongside metal reactions. This can affect measured current and may contribute to hydrogen uptake or embrittlement in susceptible materials.

Oxygen Evolution at High Potentials

The upper boundary represents oxygen evolution from water.

At sufficiently oxidizing potentials, oxygen production can compete with metal oxidation and influence the measured electrochemical response. The water lines therefore provide context for determining whether the applied potential is practically compatible with aqueous testing.

Separating Metal Behavior from Water Reactions

Overlaying the metal diagram with the water-stability region allows researchers to ask two questions:

1. Is the metal thermodynamically stable, dissolving, or forming an oxide?
2. Is the surrounding water itself stable, or are hydrogen or oxygen evolution reactions expected?

This distinction is essential when interpreting polarization curves, imposed-potential tests, and other electrochemical measurements.

How Diagrams Support Electrochemical Materials Testing

Selecting Test Conditions

Researchers can use a Pourbaix diagram before testing to select meaningful combinations of pH and potential.

For example, they may choose conditions inside a predicted dissolution region to study active corrosion, an oxide-stability region to investigate passivation, or a metallic region to examine immunity.

Interpreting Transitions in Measured Current

A change in electrochemical current may reflect a transition between thermodynamically favored states.

Movement toward a soluble-ion region can be associated with increasing anodic dissolution, while entry into an oxide or hydroxide region may correspond to film formation and a lower steady-state current. The diagram helps connect these electrical responses to possible chemical transformations at the surface.

Comparing Materials or Environments

Pourbaix diagrams also provide a common framework for comparing different metals or environmental conditions.

Changing the pH shifts the test location horizontally; changing the applied potential shifts it vertically. This makes it easier to reason about how acidification, alkalization, or potential control changes the expected oxidation behavior.

Understanding the Trade-offs

Thermodynamic Stability Does Not Predict Corrosion Rate

A Pourbaix diagram shows which phase is thermodynamically favored, but it does not indicate how fast a reaction occurs.

A metal may lie in a corrosion region yet corrode slowly because of activation barriers, mass-transfer limitations, or a persistent surface film. Conversely, a nominally passive phase may not form quickly enough to prevent substantial initial dissolution.

Passivation Is Not Always Protective

An oxide or hydroxide can be thermodynamically stable without being compact, adherent, or electronically insulating.

Protectiveness depends on film structure, defects, hydration, stress, conductivity, and environmental chemistry. Localized attack, such as pitting or crevice corrosion, may also occur even when the bulk condition falls within a nominal passive region.

Real Solutions Contain More Than One Chemical Species

Basic diagrams often simplify the environment by considering selected species and specified activities.

Chloride, complexing agents, dissolved gases, alloying elements, and precipitation reactions can alter stability boundaries or introduce additional corrosion products. Researchers should therefore treat a simple diagram as a baseline model rather than a complete description of a complex electrolyte.

Boundary Conditions Are Not Sharp Experimental Switches

The lines on a diagram represent equilibrium boundaries. Actual surfaces may remain metastable, and measured transitions may be displaced by polarization history, surface preparation, solution flow, and reaction kinetics.

The diagram is most valuable when combined with open-circuit measurements, polarization data, impedance results, surface analysis, and chemical characterization.

Making the Right Choice for Your Goal

Use the diagram as a planning and interpretation tool, then validate its predictions experimentally.

• If your primary focus is predicting active corrosion: Identify regions where soluble metal ions are stable and test whether anodic current, dissolved-metal concentration, or surface loss increases under those conditions.
• If your primary focus is evaluating passivation: Target oxide or hydroxide stability regions and verify film formation and protectiveness using current response and surface characterization.
• If your primary focus is avoiding unwanted water reactions: Compare the applied potential with the hydrogen- and oxygen-evolution lines before interpreting current as metal corrosion alone.
• If your primary focus is modeling a real service environment: Incorporate relevant species, activities, temperature effects, and alloy chemistry rather than relying only on a simplified metal-water diagram.

Used with kinetic and surface measurements, Pourbaix diagrams let researchers connect electrochemical test conditions to the likely corrosion and oxidation states of metallic materials.

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