All Metals Want to Corrode
Corrosion is flow of electric current and can be stopped by opposite current
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 takes many forms, from left: chloride induced corrosion of concrete rebars, stress corrosion cracking, general corrosion
Corrosion of Steel Structures
Corrosion reaction in a liquid often involves steel or other allyed iron in water containing various salts in varying concertrations, e.g. clorides in sea water. Positively charges Ferrous ions break away from a metal surface to liquid while simultaneously negative electrons are released in metal. In water, the ferrous ions react with negative hydroxide ions (OH-) ultimately forming rust.
All negatively charged electrons released on the corroding anode surface must be used elsewhere on the surface of the metal structure in a cathodic reaction. The cathodic reaction in metal happens on the cathodic surfaces. The cathodic reaction releases negative ions, in case of water negative OH ions and the reaction with positive iron ions closes the electric circuit in liquid. The end result of this corrosion reaction is rust and iron oxides.
Corrosion always need an electrolyte, a conductive liquid for ions to move. E.g. in concrete, there is always moisture enough to keep corrosion in speed.
The corrosion resistance of stainless steels is based on a protective oxide film forming the metal surface. This natural passive film is not 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 gives rise to corrosion current.
In cathodic protection, opposite current is applied to the metal structure with a rectifier and anodes. Cathodic current passed through the metal surface keeps the oxide film stable and corrosion resistant even in oxidizing electrolytes.
Corrosion of Concrete Reinforcing Steel
Corrosion of reinforcing steel in concrete structures is a growing concern worldwide.
Massive resources are being spent each year in replacing deteriorated concrete structures. Corrosion is a slow but deceptive process because 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.
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.
What happens inside the concrete?
Concrete is not a homogenous material - its density and porosity vary considerably. Chloride ions are able to penetrate the pores of the concrete with varying degrees of concentration. This variation in concentration causes the reinforcement to have different electric potentials along its length. This is enough to set up a corrosion cell within the concrete.
The area of reinforcement with the most negative electric potential becomes the anode in the cell and that with the least electric potential, the cathode. Electrons will leave the anode and pass along the rebar to the cathode. The circuit is closed by ions moving from cathode through the pores of the concrete. Corrosion is electric current.
Rust expands and breaks down the concrete
The loss of electrons at the anode is known as corrosion and the products of the reaction is rust. Rust occupies a multiple volume compared to the original rebar. As the rust accumulates, enormous pressure develops within the concrete and destructive cracking of the structure begins.
Once the corrosion of the rebar has begun, sealants, overlays or coatings are useless in stopping further corrosion. The chloride ions serve only as an intermediary and are not consumed by the corrosion process. Thus, once chlorides have penetrated the concrete, corrosion will continue and will accelerate if the chloride concentration increases.
Only Cathodic Protection stops corrosion process inside the concrete
Cathodic Protection is achieved by placing a supplemental anode either on or within the concrete, joining this to the reinforcement via a small impressed DC current source. The rebar becomes the cathode and it cannot rust. Only anode rusts. The supplemental anode is ususally made of very noble metal which rusts very slowly - titanium oxide is mostly used. These anodes will last well in excess of 30 years.
To monitor the system, reference electrodes are embedded in the concrete close to the reinforcement. They provide a constant indication of the electric potential of the electric potential of the reinforcement and thus it is possible to tell if the system is doing its job properly, or not.