Completed Circuit

Definition

The completed circuit is the state in which all four corrosion cell components are present and energized, so corrosion products form and chemistry leaves a visible record. It marks a closed electrical loop, not just material loss.

Near a mild steel surface, ferrous ions meet hydroxide and combine as Fe2+ plus 2OH- giving Fe(OH)2 at the interface where ions are produced and consumed. With oxygen present, ferrous hydroxide is further oxidized into more stable iron oxides and oxyhydroxides, the red-brown rust seen as films, tubercles, or deposits. The appearance of these products signals that electrons moved through the metal, ions moved through the water, charge balanced, and energy was released.

Related concepts

Corrosion Cell · Structural collapse of metals (rust formation)

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Part of The Blueprint of Water · ⚡ electrochemistry