Pourbaix diagram

A potential–pH diagram that maps the thermodynamically favored forms of an element in aqueous solution. Its regions distinguish stable dissolved species, solids, and immunity to corrosion under specified conditions.

Nernst equation: An equation relating an electrode potential to the activities of oxidized and reduced species. Potential-dependent equilibrium lines follow the Nernst equation, often varying with pH.
Corrosion: The deterioration of a material through chemical or electrochemical reactions with its environment. Metal stability regions help predict when corrosion products or dissolved ions are favored.
pH: A logarithmic measure of hydrogen-ion activity in a solution. It is the horizontal coordinate and controls proton-coupled equilibria.
Ellingham diagram: A plot of standard Gibbs energy changes for oxidation reactions against temperature. It maps oxide stability chiefly against temperature, rather than aqueous potential and pH.
Electrochemical potential: The energy change associated with transferring a charged species between electrochemical environments. Potential axes express the energetic favorability of electron-transfer reactions.
Passivation: The formation of a surface film that reduces a material’s reaction rate with its environment. Stable oxide regions suggest conditions where protective films may form.
Oxidation–reduction reaction: A chemical reaction involving transfer of electrons between species. Redox equilibria generate potential-dependent boundaries between oxidation states.
Evans diagram: A plot of electrode potential against logarithmic corrosion current density for electrochemical reactions. It estimates corrosion behavior and rates, whereas a Pourbaix diagram shows equilibrium stability.
Acid–base equilibrium: The balance between protonated and deprotonated forms of a substance in solution. pH-dependent boundaries separate species that differ by protonation.
Immunity (corrosion): A condition in which a metal is thermodynamically favored over its corrosion products. The metal’s stability field is conventionally labeled its immunity region.

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