Types of Corrosion Encountered in Structural Steel Elements

Types of corrosion encountered in structural steel elements are:

1. Pitting corrosion
2. Crevice corrosion
3. Bimetallic corrosion
4. Stress corrosion
5. Fretting corrosion
6. Bacterial corrosion

Pitting corrosion:

The anodic areas form a corrosion pit. This can occur with mild steel immersed in water or soil. This common type of corrosion is essentially due to the presence of moisture aided by improper detailing or constant exposure to alternate wetting and drying. This form of corrosion could easily be tackled by encouraging rapid drainage by proper detailing and allowing free flow of air, which would dry out the surface.

Crevice corrosion:

The oxygen content of water trapped in a crevice is less than that of water which is exposed to air. Because of this the crevice becomes anodic with respect to surrounding metal and hence the corrosion starts inside the crevice.

Bimetallic corrosion:

When two dissimilar metals (for e.g. Iron and Aluminium) are joined together in an electrolyte, an electrical current passes between them and the corrosion occurs. This is because, metals in general could be arranged, depending on their electric potential, into a table called the ‘galvanic series’. The farther the metals in the galvanic series, the greater the potential differences between them causing the anodic metal to corrode. A common example is the use of steel screws in stainless steel members and also using steel bolts in aluminium members. Obviously such a contact between dissimilar metals should be avoided in detailing.

Stress corrosion:

This occurs under the simultaneous influence of a static tensile stress and a specific corrosive environment. Stress makes some spots in a body more anodic (especially the stress concentration zones) compared with the rest. The crack tip is the anodic part and it corrodes to make the crack wider. This corrosion is not common with ferrous metals though some stainless steels are susceptible to this.

Fretting corrosion:

If two oxide coated films or rusted surfaces are rubbed together, the oxide film can be mechanically removed from high spots between the contacting surfaces. These exposed points become active anodes compared with the rest of the surfaces and initiate corrosion. This type corrosion is common in mechanical components.

Bacterial corrosion:

This can occur in soils and water as a result of microbiological activity. Bacterial corrosion is most common in pipelines, buried structures and offshore structures.