CORROSION AND THE ANTI CORROSION MEASURES TO THE DESTRUCTION OF STEEL OR ALLOY , DUE TO IT'S ENVOIRNMENT.

CORROSION :-- Corrosion refers to the destruction of a metal or alloy by chemical change , electrochemical reaction or physical dissolution due to its envoirnment.Corrosion of metal in concrete is an electrochemical process and requires that moisture be present .Moisture in concrete may be available from two sources : (a) when fresh concrete is placed , it contains mixing water which is only partly used up as the cement hydrates . The remaining water migrates to exposed surfaces and is available for corrosion.(b) if hardened concrete is not  dense and impervious, moisture may penetrate into the interior from the envoirnment.  Porous concrete may permit the passage of carbon dioxide from the air into the concrete .This reacts with calcium hydroxide to form calcium carbonate which lowers the alkalinity (pH) of concrete and leads to corrosion of steel .This type of corrosion manifests itself as white flakes on the concrete surface .Another type of Corrosion is caused by a gaseous or liquid corrosive envoirnment such as acidic gases and solution of sulphides and chlorides .The product of steel corrosion occupies a volume several times greater than the volume of the reinforcing steel .It exerts considerable radial pressure on the adjoining concrete layer , as a consequence , cracking results along the reinforcing bars and concrete spalls . Experience has shown that reinforcing steel is protected from corrosion by concrete .The degree of protection afforded by the concrete depends on the thickness of concrete ,that is, cover to steel ,quality of concrete and its water cement ratio .When steel is encased in concrete ,a protective iron oxide film forms at the steel concrete interface as a result of high pH value of about 12.5 of the concrete .This film is effective in preventing corrosion as long as this pH value is maintained .It is possible to maintain high alkalinity of concrete by avoiding high water cement ratio ,low cement content, thin cover over the reinforcing steel and poor compaction of concrete. It is strongly recommended that any material for use in making concrete that is suspected of containing chloride be analysed and rejected , if chlorides present exceeds the permissible values. These chlorides may originate from concrete admixtures, sea water , beach sand,or self containing aggregate.