What Does Polarization Mean?
Polarization is a mechanism that typically results in a change in the potential of an electrode during electrolysis, when the anode’s potential becomes nobler than that of the cathode. It has the effect (based on conditions) of decreasing the output voltage of batteries, and increasing the voltage required for electrolysis cells or lowering currents.
Polarization can also be described as a kinetic deviation from equilibrium due to an electric current passing through a galvanic cell. Polarization may occur at the cathode (cathodic polarization) or at the anode (anodic polarization). Cathodic polarization is more common.
Corrosionpedia Explains Polarization
Polarization as an electrochemical phenomenon is of importance in the corrosion process. For all metals and alloys in any aqueous environment, cathodic polarization always reduces the corrosion rate. Cathodic protection refers to the application of a cathodic polarization to a corroding system.
Polarization can occur in three ways:
- Concentration polarization
- Resistance polarization
- Activation polarization
Concentration polarization of an electrode is the result of the formation of a diffusion layer adjacent to the surface of the electrode where there is a gradient of ion concentration. Diffusion of the ions through the layers controls the electrochemical reaction and is important for processes such as electroplating and corrosion. Concentration polarization may be reduced by increasing agitation or raising the temperature of the electrolyte.
Resistance polarization describes the potential drop due to the high resistivity of the electrolyte surrounding the electrode. It may also be a result of the insulation effect of the film on the electrode surface formed by the reaction products. Resistance polarization is expressed by the Ohm's law.
Activation polarization occurs when the electrochemical reaction proceeds through several successive steps. The speed of the overall reaction is determined by the slowest step (known as the rate-determining step) of the process. For example, in a hydrogen reduction reaction, the reaction may proceed as follows:
- Hydrogen ions are absorbed from the solution onto the anode surface.
- Electron transfer occurs from the anode to the hydrogen ions to form hydrogen.
- The hydrogen atoms form hydrogen gas molecules.
- Hydrogen gas bubbles are formed.