Definition - What does Cathodic Polarization mean?
This is a corrosion control method where the potential of either the anode or the cathode, or both, is changed. This minimizes loss of metal and reduces the driving force of corrosion reaction. Corrosion protection is achieved when the potential difference is reduced to a minimum.
Corrosionpedia explains Cathodic Polarization
It should be noted that a cathodic reaction will occur when there is a certain potential at the cathode. When this potential is achieved, hydrogen gas bubbles from the cathode, indicating a reduction process. Hydrogen over-voltage is the difference in potential between the hydrogen electrode and the cathode in the same solution and at equilibrium.
Hydrogen over-voltage is affected by temperature and the roughness of the metal surface. An increase in temperature and surface roughness will result in a decrease in the hydrogen over-voltage.
The most common figures for over-voltage include:
· Platinum = 0.12 V
· Aluminum = 0.19 V
· Nickel = 0.24 V
· Iron = 0.27 V
· Silver = 0.29 V
Over-voltage in low pH waters with high hydrogen ions can be easily controlled. Since some metals do not corrode in some acidic media, the absorbed hydrogen is responsible for depolarizing the cell. This reduces the driving force for the corrosion reaction. Hydrogen removal from the cathode depolarizes the corrosion reaction, and in turn increases the chance of metal loss.
In high pH levels where hydrogen over-voltage cannot be overcome, in natural waters, the rate of cathodic reaction is controlled by the dissolved oxygen. A corrosion reaction can only be polarized when the amount of the dissolving oxygen to the surface of the metal is controllable. Closed systems are easily protected as the dissolved oxygen forms an oxide, thus no further oxygen is available. The main cathodic reactions can be controlled by keeping the pH alkaline to maximize hydrogen over-voltage.