NANUET SCHOOL DISTRICT COMMUNITY EDUCATION

Generalized corrosion affects the whole surface of the metal and translates into a more or less rapid slimming of the metallic wall in contact with the electrolyte. Depending on the uniformity of the superficial attack, it can be differentiated into generalized uniform and generalized not uniform corrosions.

Localized corrosion affects a limited part of the metallic surface and causes cavity formation. These cavities, depending on their external diameter vs. depth relation, are named ulcers, craters, pittings, or criccas. The pitting or punctures, can sometimes be penetrating. The cricca or fissure can be intergranular or transgranular, depending if it follows the edge of the grain or through it.

Selective corrosion produces the preferential dissolution of a certain part of the metal that, for chemical or metallographic reasons, proves to be more easily attackable.

We can speak of cristalographic, intergranular or interdendritic corrosion, if the corroded material results in certain crystalline species in the grain borders or the immediately adjacent zone.

Another form of selective corrosion, the dealloying, happens when the preferential dissolution of one of two components of an alloy is verified, leaving an inconsistent and frothy residue of the other metal in the alloy.

• Methods of control and prevention:
• The general methods for corrosion control more widely accepted on the industry are: the use of special construction materials resistant to corrosion, the application of inert barriers as paint, the use of methods of cathodic or anodic protection, the adjustments on the electrolyte or corrosive medium’s chemistry, the application of specific inhibitors to control corrosion, and the application of anticorrosive systems.
• An anticorrosive system is made up of two components which form a set. This set works as a barrier that controls oxidation and prevents the attack of external factors.

The components of an anticorrosive system are:

• - Firstly, the base or primer, whose function is to provide the system with the sufficient adherence to the substrate and to act as a cathodic corrosion inhibitor (because of its high proportion of zinc).

- The finishing, which consists of a film or layer that complements the system and provides a high resistance to the attack of the environment.