What is Corrosion of Steel?

The term corrosion is defined as an act or process of gradual wearing away of a metal due to chemical or electro-chemical reaction by its surroundings such that the metal is converted into an oxide, salt or some other compound. A substance known as rust results from such a process. In other words, the corrosion indicates the deterioration and loss of material due to chemical attack.

What Causes Corrosion of Steel in Concrete?

Concrete provides an ideal protective environment to steel from corrosion. Due to high initial alkalinity, an extremely thin passive film of ferric-oxide (Fe₂O₃) is automatically formed on the surface of steel. This layer, though extremely thin, effectively protects the steel from corrosion. But unfortunately this layer is effective as long as the surrounding remain alkaline. Therefore, if we can manage to keep the environment alkaline, the corrosion of steel can effectively be prevented and the durability of the structure can be ensured. The alkaline medium can be maintained for a longer period by making the concrete impermeable.

Corrosion is a electro-chemical process in which one part of steel becomes anode and the other cathode. Fortunately, anode reaction can’t start till the passive ferric-oxide film is destroyed by acidic medium or it is made permeable by the action of chloride ions. Similarly the cathode reaction can’t start till sufficient supply of oxygen and water is available at the surface of steel. It can, therefore, be concluded that corrosion of steel can be prevented if the concrete is sufficiently impermeable so as to keep air, water and other such agents out of its reach.

How to Prevent Corrosion of Steel in Concrete?

The steel is the most liable to the corrosion and hence the study of steel corrosion is of paramount importance. To minimize the chances of development of corrosion of steel in concrete, the following preventive measures may be taken.

1. Avoiding heavily congested reinforcement especially at the intersection of beams and columns.
2. Avoiding the steel to come into contact with bricks, soil, wood and other porous non-alkaline materials
3. Avoiding the use of materials which accelerate the process of corrosion i.e. aggregates with high salt contents, water containing salts, etc.
4. Cleaning the reinforcement with wire-brush to remove the rust scales before placing of concrete
5. Maintaining a high degree of workmanship
6. Proper structural design with due provision of cover
7. Providing cathodic protection to the reinforcement by some suitable method
8. Providing surface coatings with paints, tars, asphalts, etc
9. Use of high quality and impermeable concrete
10. Using stone pebbles in place of badly made cover blocks
11. Using the correct water-cement ratio