Corrosion & Environmental Degradation

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· If more than one reduction reaction is possible, their relative rate at a given over-potential is influenced by their electrode potentials.  · This situation is shown for cadmium and hydrogen,  possible products in the electro-deposition of cadmium from an oxygen free aqueous electrolyte. · The electrode potential for hydrogen is zero (by definition) and that for cadmium - 0.4 V.

From: Newey and Weaver, "Materials Principles and Practice,"  Butterworths (1990)

· The equilibrium exchange current for each system is shown by the vertical dotted line, and is 10 orders of magnitude smaller for hydrogen than cadmium. Increasing the overpotential above zero volts, the H+ reduction rate increases linearly, the Cd rate remaining at the equilibrium value until its electrode potential, -0.4V, is reached. Above this threshold, the Cd2+ reduction rate increases linearly with voltage. · At the overpotential of -0.65 V, the Cd-rate is 10+5 and the H-rate is 10-2, a seven order of magnitude difference. H-liberation will, therefore, be unimportant for this system.