There are different theories of Corrosion and different methods of defining corrosion. In our view Corrosion can be defined in two different ways (1) Corrosion is the processes of transforming the TRANSFORMED MATERIAL to its ORIGINAL STATE, under the INFLUNCE of the ENVIRONMENT. (2) Corrosion can be defined as DETERIORATION of MATERIALS (in our case building materials) under INFLUENCE of the ENVIRONMENT. Most of the CIVIL ENGINNERS are generally worried about the following, as they form main part of the CIVIL ENGINEERING STRUCTURES. (A) CORROSION OF CONCRETE. (B) CORROSION OF STRUCTURAL STEEL. (C) CORROSION OF STEEL EMBEDDED IN CONCRETE