What Does Depolarization Mean?
Depolarization is an electrochemical phenomenon that involves the removal or prevention of polarization in a substance or of polarization arising from the field due to the charges induced on the surface of a dielectric when an external field is applied.
In the depolarization process, depolarizers are used to remove hydrogen, and therefore, they help to keep the voltage at a high level in a polarization cell.
For ferrous-based materials, oxygen depolarization is the determining factor in almost all cooling water situations, since pH is maintained at levels where hydrogen evolution effects are minimal.
Corrosionpedia Explains Depolarization
Depolarization is the reduction of a counter-electromotive force by removing or diminishing the cause of polarization.
Since in the polarization process hydrogen gas evolves during reaction with metal and water, which coats the cathode and slows corrosion, dissolved oxygen in the water is able to react with the hydrogen gas surrounding the cathode:
Hydrogen gas + Oxygen = Water
Depolarization removes the hydrogen gas surrounding the cathode and speeds up the corrosion process. Therefore, high levels of dissolved oxygen in water cause more corrosion.
Oxygen, even in very small amounts, may cause serious corrosion in:
- Feed water lines
- Stage heaters, economizers
- Steam-operated equipment
- Condensable piping
Oxygen reacts by consuming electrons at the cathode, causing cathodic depolarization and enhancing the rate of corrosion. Cathodic corrosion can be controlled by prevention of depolarization, and creating other forms of barriers, such as coatings. Chemicals which eliminate oxygen from the closed systems are called scavengers.
Hydrogen is metabolized by many species, causing depolarization of the corrosion cell, similar to the action caused by dissolved oxygen. The properties of steel and other metallic materials in acidic environments are of interest when applied to ground water with sulphate-reducing bacteria and water contaminated by H2S. The mechanism is believed to involve both direct attack of the steel by hydrogen sulfide and cathodic depolarization aided by the presence of bacteria.
In the battery industry, the term "depolarizer" has been used to denote a substance used in a primary cell to prevent buildup of hydrogen gas bubbles. A battery depolarizer takes up electrons during discharge of the cell.