According to the laws of physics, all metallic materials have a tendency to return to their original form, ore, from which they are refined by using lots of energy. Nature favors the state of least bound energy. When metal corrodes, the in-built energy is released.
In a corrosion reaction, typically ferrous ions break away while simultaneously electrons are released. In water, the ferrous ions react with oxygen and hydroxide ultimately forming rust. All electrons released at the corroding anode must be used elsewhere in the metal structure in a cathodic reaction. This flow of electrons creates electric current which can be suppressed by feeding the same amount of current to the opposite direction.
Corrosion reaction in a liquid often involves steel or other alloyed iron in water containing various salts, e.g. chlorides in sea water. Positively charged ferrous ions break away from a metal surface to liquid while simultaneously negative electrons are released in metal. This anodic reaction disintegrates metal atoms, causing the material loss that we see as corrosion damage. In natural waters, the end result of anodic reaction is formation of rust.
All negatively charged electrons released on the corroding anode will be consumed elsewhere on the surface of the metal structure in a cathodic reaction. The metal atom does not participate to this cathodic reaction, and thus no corrosion occurs on cathodic sites.
In impressed current cathodic protection, direct current is applied to the metal structure with a rectifier and auxiliary anodes. All anodic reactions are transferred to the surface of auxiliary anode made of insoluble material. Cathodic current passed through the metal surface keeps the metal atoms in immune status, unable to release electrons and thus unable to corrode.
The corrosion resistance of stainless steels is based on a protective oxide film forming the metal surface. This naturally passive film is not always resistant to attack by chlorides in warm oxidizing electrolytes. Chloride breaks the surface film, causing a potential difference between the more noble oxide film and the less noble base metal at the exposed spots. The potential difference between active and passive sites creates corrosion current. In this case, cathodic protection current keeps the oxide film stable and corrosion resistant even in oxidizing electrolytes.
Massive resources are being spent each year in replacing deteriorated concrete structures. Concrete corrosion is a slow but deceptive process as it may have advanced for a long time inside the concrete structure before it is visible.
New concrete is alkaline and generally an ideal environment for steel. However, when aggressive chlorides from the salt laden air, sea water or road salt penetrate the concrete to the reinforcing steel, corrosion becomes active. In marine environment concrete structures are very prone to deterioration by chloride attack.
Corrosion always need an electrolyte, a conductive liquid for ions to move. In concrete, there is always moisture enough to keep corrosion in speed.
Once the corrosion of the rebar has begun, sealants, overlays or coatings are useless in stopping further corrosion. The only rehabilitation technique that has proven to stop corrosion regardless of the chloride content of the concrete is Cathodic Protection.