SPECIAL EDUCATION PROGRAM IMPROVEMENT 274

The electrochemical nature of corrosion leads to diverse forms of assault. These are determined by the succession of a series of environmental, mechanical and geometrical factors. The identification of the form of corrosion is of vital importance for the diagnosis of the cause that determines the corrosive process, as well as the prevention, control, and protection of the element.

According to this, the corrosion classification goes as follows:

- Generalized corrosion.

- Localized corrosion.

- Selective corrosion.

• 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: