Corrosion is simply the degradation of metals caused by a chemical interaction with the environment surrounding them. It is a natural process where there is a gradual deterioration of metals leaving their current natural state. Generally, it is the destruction of materials (usually metals) due to chemical and/or electrochemical reactions.
Corrosion tends to turn materials back to a more thermodynamically stable state. This happens through their combination with other substances in the environment, particularly oxygen. It is from such a state that the materials are transformed through the metallurgic process of refinement and extraction by means of chemical, electric or heat work.
The most common metals used today in various fields of industries have a high affinity for oxygen. Their corrosion process exclusively depends on the phenomenon known as oxidation. Some metals are more prone to this process than others. For instance, metals such as platinum and gold hardly oxidize due to their low affinity for oxygen. They are commonly known as noble metals. On the other hand, metals such as iron easily oxidize due to its high affinity with oxygen.
Three Common Forms of Corrosion
The electrochemical nature of corrosion leads to diverse results. These are determined by the succession of mechanical, environmental and geometrical factors. Identifying the forms of corrosion is important in the diagnosis of the cause. This will help determine the process as well as the control, prevention and protection of the material from corrosion.
In this regard, it’s imperative for you to familiarize the types of corrosion. Corrosion is classified into three categories, namely:
- Generalized corrosion;
- Selective corrosion;
- Localized corrosion
Generalized corrosion affects the entire surface of the metal. It translates to a rapid slimming of the metallic wall that is in contact with the electrolyte. Depending on the superficial attack’s uniformity, it can either be generalized uniform or generalized non-uniform type of corrosion. These cavities can also be referred to as craters, criccas or ulcers. This condition depends on their external diameter and depth.
Localized corrosion, on one hand, only affects a part of the metal surface and causes a cavity formation on it. The punctures or pitting can be penetrating at times. The fissure or cricca can form transgranular corrosion or intergranular corrosion, depending on whether the cavity damages only the edge or penetrated through the metal.
The selective type of corrosion causes the dissolution of a particular part of the metal. For metallographic or chemical reasons, this type proves to be more easily attackable.
We can also speak of intergranular, crystallographic or interdendritic corrosion if the corroded metal turns to a crystalline element in the adjacent zone or in the grain borders.
De-alloying is another form of selective corrosion. This happens when one of the two metals of an alloy is attacked, leaving a frothy inconsistent residue of the metal in the alloy.
Prevention and Control of Corrosion in Metals
There are general methods of controlling or preventing corrosion in metals. Some of the widely accepted methods include:
- Usage of special construction metal that is highly corrosion-resistant.
- Application of inert barriers such as paint.
- Adjustment of the electrolyte and corrosive mediums chemistry.
- Employing methods such as anodic and cathodic protection.
- Using special inhibitors to control or avoid corrosion.
- Application of anticorrosive system.
An anticorrosive system consists of two components that form a set. The set works like a barrier which controls oxidation. Ideally, it prevents the attack by external factors
The two components of an anticorrosive system are:
- The primer or base. It acts as a cathodic corrosion inhibitor. This is due to its high zinc proportion that provides the system with enough resistance to substrate.
- The finishing. This consists of a layer or film that complements the system. It also provides a high resistance to the attacks of external factors.