MEHRAN UNIVERSITY OF ENGINEERING AND TECHNOLOGY, JAMSHORO

DEPARTMENT OF METALLURGY AND MATERIALS DEPARTMENT OF METALLURGY AND MATERIALS ENGINEERINGENGINEERING

Corrosion in aqueous solutions proceeds by an electrochemical process, and anodic and cathodic electrochemical reactions must occur simultaneously.

No net overall charge builds up on the metal as a result of corrosion since the rate of the anodic and cathodic reactions are equal.

Anodic reactions involve oxidation of metal to its ions, e.g. for steel the following reaction occurs.M → Mn+ + ne-

The cathodic process involves reduction and several reactions are possible depending on the nature of the solution to which the metal is exposed.

In acidic water, where hydrogen ions (H+) are plentiful, the following reaction occurs.

2H+ + 2e- → H2

For an acid solution having dissolved oxygen, according to following reaction probably reduction occurs.O2 + 4H+ + 4e- → 2H2O

However, unless the water is deaerated reduction of oxygen is the most likely process, again producing alkali at the surface of the metal.

O2 + 2H2O + 4e- → 4(OH)-

Any metal ions present in the solution may also be reduced; for ions that can exist in more than one valence state (multivalent ions), reduction may occur by following reaction in which the metal ion decreases its valence state by accepting an electron.

Mn+ + e- → M(n-1)+

Or a metal may be totally reduced from ionic to a neutral metallic state according to

Mn+ + ne- → M