Understanding Pourbaix Diagrams: Corrosion and Metal Stability

Pourbaix diagrams are valuable tools in the field of electrochemistry, providing insights into the behavior of metals in aqueous environments. These diagrams categorize the stability of metals and their ionic forms based on two critical parameters: pH and electrochemical potential. According to an internationally agreed convention, a metal is considered to be actively corroding when the equilibrium activity of soluble ions, such as Fe²⁺ or Fe³⁺, exceeds 10⁻⁶.

In a typical Pourbaix diagram, three distinct domains can be identified: domains of immunity, corrosion, and passivity. The first domain, immunity, indicates that the metal is stable and not prone to corrosion under specific conditions. Conversely, in the corrosion domain, soluble ions are the stable species, suggesting the metal will corrode if the kinetics allow for it. Lastly, the passivity domain is where an insoluble protective layer forms, shielding the metal from corrosion.

Figures illustrating Pourbaix diagrams for various metals—such as aluminum, zinc, copper, tin, and nickel—can be found in the corresponding literature. Each diagram displays the stability of the metal alongside the stability of water, which is essential for understanding the environmental conditions that affect corrosion rates. For example, the copper-water system exhibits a notable characteristic: while Cu²⁺ ions are represented, the Cu⁺ ion does not have a domain of stability in aqueous solution, highlighting how the stability of ions can depend on chemical environments.

The diagrams not only represent stable species but also outline the potential for reactions between different oxidation states. For instance, the copper-water system shows that under certain conditions, Cu⁺ ions can disproportionate to form Cu²⁺ ions and solid copper. This reaction is governed by the Gibbs free energy change, which can be calculated using the standard electrode potentials for relevant reactions.

In summary, Pourbaix diagrams serve as essential references for understanding how various metals behave in water, predicting their corrosion tendencies, and defining conditions that promote or inhibit their stability. This knowledge is critical for industries that rely on metal structures and components, allowing for better materials selection and corrosion prevention strategies.