Galvanic Corrosion Cell

What is modelled

Zinc is less noble than copper, so in a conducting electrolyte it dissolves (Zn to Zn2+ + 2e-) while oxygen is reduced at the copper. Electrons flow through the external wire and load; ions carry the current through the electrolyte. The current is set by the cell voltage, the series resistances and, above all, by how much dissolved oxygen can reach the cathode.

Equations

• Series resistance R = Rsol + Ranode + Rcathode + Rload, with Rsol = d / (κ √(Aa Ac)).
• Mixed-potential balance E = I R + η, transport overpotential η = -b ln(1 - I / Ilim), solved by bisection.
• Oxygen-limited current Ilim ∝ Ac c(O2) f(mixing) √T.
• Faraday: m = Q M / (z F) with M = 65.38 g/mol, z = 2, F = 96485 C/mol.

Checks run in the page

• Zinc loss equals Q M / (2 F) after 18 simulated hours.
• With plenty of oxygen and a large load the current is E / R_total (Ohm limit).
• The solved current satisfies the mixed-potential equation with a tiny residual and an open circuit carries no current.

Honest simplifications

Lumped, spatially uniform cell: no real pit growth, no crevice or concentration cells, an empirical passivation film and an empirical oxygen limit. The drawn ions and the zinc thinning are illustrative; the current and mass loss come from the model. Not a corrosion-life predictor.

• Switch from seawater to fresh water: the ionic resistance jumps and the current collapses.
• Raise the load to 150 ohm and read the Ohm-limit check; set it to 0 and watch the oxygen limit take over.
• Open the circuit (untick Circuit closed): the load line and the zinc loss stop.