What Does Exchange Current Mean?
Exchange current refers to the rate that charges, either positive or negative, enter or leave a metal's surface when an electrode reaches dynamic equilibrium in an electrolyte. It is the current at equilibrium, or the rate that oxidized and reduced species transfer electrons with the electrode.
At the equilibrium current, the forward and reverse reactions progress at the same rate, i.e., the rate of forward reaction (anodic) balances the rate of reverse reaction (cathodic). Exchange current has a significant influence on corrosion measurement.
Corrosionpedia Explains Exchange Current
Exchange current is the equilibrium current where the rate of the forward reaction is equal to the rate of the backward reaction on the metal electrode. It is the magnitude of current where the forward and reverse reactions are equal. For example, when a metal is immersed in an electrolyte, a dynamic equilibrium is established across the interface with a potential difference between the metal and the electrolyte. The metal atoms are ionized and electrons are produced as follows:
M ---> Mn+ + ne
where M represents metal atoms. The metal is left with a negative charge and its positively charged metal ions (Mn+) in the electrolyte are attracted back toward the metal surface. Since the rate of ionization on the metal surface becomes equal to the rate of discharge across the contact layer, a potential difference and dynamic equilibrium between the metal and the solution is established.
Factors that affect the exchange current include:
- Forward reaction: The atoms that are at energetically unfavorable positions, such as at grain boundaries, dislocations and half planes, are able to detach themselves and participate in the reaction.
- Electrode composition: The exchange current depends on the composition of the electrode. The exchange current is different for different electrodes.
- Surface roughness: Larger surface areas provide a high exchange current.
- Impurities: The presence of trace impurities causes current fluctuations.
From the exchange current we can calculate the exchange current density, which is very useful for measuring corrosion potential. If the current density at the anode is smaller than the current density at the cathode, the current applied will be equal to the current density at the cathode and only the cathode will be polarized.