Corrosion in Metals - Principles and Prevention

Corrosion is the tendency of metals to return to their natural condition, abandoning their current one, that is: the destruction or deterioration of a material because of chemical and/or   electrochemical reactions.

A chemical reaction or dry environment reaction can occur by the contact with vapors or gases, without the presence of liquids. With frequency, the dry environment reaction is closely associated with high temperatures.

A electrochemical reaction or humid environment reaction, is present in liquid atmospheres, by sprayed or immersed means.

Corrosion processes tend to return materials back to a more thermodynamically stable state by their combination with substances in the environment, particularly with oxygen. It is from such a state that the materials were transformed through the metallurgic process of extraction and refinement, by the means of the supply of electric, chemical or heat work.

The most widely used metals for technology have a great affinity for oxygen and their corrosion process depends exclusively on the phenomenon denominated “oxidation”. Some metals are more prone to oxidation than others.

For example: metals like gold or platinum hardly oxidize because of their low affinity with oxygen. They are known as noble metals. Other metals as iron oxidize easily due to their high affinity with oxygen.

Classification

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:

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.