Pourbaix diagram
/ poor-BAY /
If you want to know what form an element takes in water, two things rule it: how oxidizing the surroundings are, and how acidic they are. A Pourbaix diagram is a map with exactly these two axes — electrode potential up the side, pH across the bottom — and it shades in which species of the element is the most stable in each region. It is, in effect, a weather map of redox chemistry in water: tell me the potential and the pH, and the map tells me what survives there.
Each region of the map is a field of stability for one species; the boundaries between them are lines you can read. A horizontal boundary is a pure redox change, independent of pH (only electrons move). A vertical boundary is a pure acid-base change, independent of potential (only H+ moves). A sloping boundary is a reaction involving both electrons and H+ together, and its slope follows directly from the Nernst equation. The whole map is usually framed by two dashed lines marking the stability window of water itself — above the upper line water is oxidized to O2, below the lower line it is reduced to H2 — so anything outside that band cannot persist in water.
Pourbaix diagrams are indispensable in corrosion science, geochemistry, and hydrometallurgy: they show at a glance whether a metal will corrode, stay immune (its metal form is stable), or passivate (cloak itself in a protective insoluble oxide) at a given potential and pH. The crucial honest caveat is that they are thermodynamic maps only. They tell you what is stable, not how fast it forms or dissolves — a metal can lie deep in its corrosion field yet survive for years because the kinetics are slow, or because a passive film that the equilibrium diagram does not capture protects it.
On the iron Pourbaix diagram, at low pH and moderate potential the stable form is dissolved Fe2+ (it corrodes), but at high pH iron sits inside a region of solid Fe2O3/Fe3O4 — a passivating oxide that protects it.
Immunity, corrosion, and passivation each occupy their own region of the E-pH map.
A Pourbaix diagram maps thermodynamic stability, not kinetics. A metal can survive far inside its 'corrosion' region because the reaction is slow or because a protective film not shown by the equilibrium diagram forms in practice.