What is Overvoltage?
Overvoltage is an important irreversible electrode process. In the reversible electrode process small current flows. In electrolysis, the potential at which metal ions start depositing at appreciating rates on the cathode is very close to the reversible electrode potential of the same metal.
But in some cases, particularly in the case of the evolution of gases say hydrogen at cathode or oxygen at anode, the potential is much higher than the reversible potential of hydrogen or oxygen electrode in the same solution.
The difference between the potentials of the electrodes when gas evolution is actually observed and the theoretical value for the same evolution is called overvoltage.
This overvoltage is sometimes referred to as the bubble overvoltage as it is observed just at the point at which gas bubbles begin to appear. The values for hydrogen overvoltage varies largely with the nature of metal. The occurrence of overvoltage indicates that some step involved in the process of evolution of hydrogen at the cathode is slow.
Application of Overvoltage:
(1) Electrodeposition of Metals in Aqueous Solution-Consider the electrolysis of solution which is one molar with respect to ZnSO4 as well as H2SO4. The reversible standard electrode potential of hydrogen and zinc are 0.00V and -0.76V. H2 alone should be liberated at the cathode. The deposition of zinc on the cathode should be possible only after all the H+ ions have been reduced to H2 gas. It has been found that Zn starts depositing along with the liberation of H2 in spite of a large difference in their reversible potentials. The reason for this is that in the high overvoltage for H2 evolution at Zn cathode, the value for which is about 0.7V. The value is very close to the reversible potential of zinc. This explains the almost simultaneous evolution of hydrogen and deposition of Zn. Zn can be deposited in an acidic solution. If there were no hydrogen overvoltage, it would not have been possible to do so.
(2) Corrosion of Metals- Consider the dissolution of lead and zinc in HCl. The standard electrode potential of lead and zinc is -0.13V and -0.76V. But in actual practice, neither lead nor Zn dissolves in HCl and no evolution of H2 occurs. This is because of hydrogen overvoltage. The bubble overvoltage of hydrogen over lead is 0.64V and Zn is 0.70V.
If the pure Zn is connected to a piece of copper and both are dipped in a dilute acid solution, the dissolution of Zn takes place very readily and H2 is evolved at the copper electrode. While the Zn electrode dissolves to produce Zn+2 ions the H+ ions are discharged and evolve as hydrogen gas at the copper electrode, the bubble overvoltage of copper is 0.2V. Corrosion or dissolution of metals takes place in any other base metal if it contains a metal with low hydrogen overvoltage as an impurity.
If a base metal contains a metal with high hydrogen overvoltage, it will dissolve in acid solution readily in the presence of air or oxygen. This is the reason why iron present in tin plates gets readily corroded in the air. But in the absence of oxygen or air, it does not dissolve at all because of high hydrogen overvoltage over the tin.
Corrosion may be inhibited by coating the metal surface with an impermeable layer of a suitable material such as paint. This method fails if the paint becomes porous because the atmospheric oxygen then gets access to the exposed metal and corrosion occurs beneath the layer of the paint.
