Corrosion is a universal occurrence that takes place through diverse forms in a metal. When an alloy or metal goes through a chemical reaction, as per the adjacent atmosphere, the end result is known to be as corrosion. There is no particular defined place where corrosion occurs. It is a universal occurrence and can occur in any dynamic environment. Corrosion is commonly referred to as rusting also. Rusting is a usual course of action, which gradually destructs a metal.
Different Types of Corrosion
There are different types of corrosion. And they differ according to the aspects which influence the course of corrosion. The corrosion types are categorized are Atmospheric, Microbial, Galvanic, Crevice, and Pitting.
- The most regular or universal form of corrosion is Atmospheric corrosion. This chemical reaction can be noticed in all over and all around us. The example of such corrosion can be seen over the top of a manhole.
- When metals or alloys come in contact with sludge or soil the corrosion occurs is Microbial corrosion. An illustration of this type of corrosion is the rails upon which train runs. As the rails lay in contact with soil it gets corroded with time.
- A chemical reaction that takes place when two contrasting metals come in contact of each other. This is termed as Galvanic corrosion or Bimetallic corrosion. The most common cause of this form the corrosion is when a car’s stainless rim and iron part corrodes.
- A type of corrosion that happens most widely is the Crevice corrosion. As the term suggests it generally takes place in a crevice. Crevice means a small crack, gap or split which remains in the structures of metal. This type of corrosion can be noticed between the metal connectors and train rails.
- The last common type of corrosion is Pitting. When the surface of a metal corrodes it creates pits, initially which are small and gradually gets bigger. If the process of corrosion goes on the small pits turn wider and deeper. An example of Pitting corrosion can be seen on the rear of goods transporting vehicle.
Causes of Corrosion
Let us now discuss the various logical reasons that cause corrosion.
- There are many factors that influence Atmospheric corrosion or Uniform corrosion. 1. The corrosive atmosphere, 2. The air, 3. The pollutants, 4. The exposure to sunlight, 5. The temperature, 6. Wetness and Humidity, 7. The surface of metal dampened by moisture, 8. Water molecules and its absorption, 9. The wetness caused by dew and rainwater, 10. The water used for splashing comprising of road salts (in cold nations).
- Microbial corrosion occurs in any metal’s or alloy’s surface when it comes in contact straightly with moist or soggy soil. If the soil remains waterlogged the surface of the metal remains damp for longer times. This more often happens where the soil contains a higher amount of clay. In such an environment, there is less oxygen and the bacteria that reduce sulfate is more. Due to the deficient arrangement of drainage, the deposits of slime also results in corrosion.
- The Galvanic corrosion takes place when two different metals come in contact on a corrosive surrounding. The long-time exposure of metals in such surrounding leads to the development of this corrosion. It also occurs when the coating on the surface is damaged or it has a faulty coating. In colder countries, the chloride in road salts intensifies the surroundings for corrosion.
- Crevice corrosion is the result of unrelated macro and microenvironments. The splits or cracks in metal entrap moisture which exaggerates corrosion. This occurs due to the deficiency of oxygen in the crevices. Crevice corrosion also occurs when pollutants of corrosive quality, from the air, is deposited on gaps or splits of metal.
- The main reason for the occurrence of Pitting corrosion is improper maintenance. The faulty coating of metal surface leads to corrosion. Moisture present in the protective coating of a metal surface allows the corrosive agents to penetrate.
Prevention of Corrosion
Since the causes of corrosion differ in each type of corrosion, the preference of precautionary actions for protecting the metals to differs.
- To prevent Atmospheric or Uniform corrosion the use of proper corrosion inhibitors should be given priority. The metal or alloy needs protection in some form of a coating, it may be polymer, cement or glass or painting. The metallic substance should be put less into use. The metal or alloy used should have corrosion resisting properties. Lastly, it is very important to curb pollutants in the atmosphere.
- Microbial Corrosion can be prevented when metals are not kept away from contacting the soil. It can be done so by putting some substantial amount of crushed stone or gravel as the layer. The surface of the metals needs to be kept clean through the use of various mechanical ways. The proper drainage system should be implemented to avoid deposits of slime.
- Galvanic corrosion also calls for the use of appropriate corrosion inhibitor. Another way to prevent such corrosion is to use excellent insulation so that two, unlike metals, don’t come in contact with each other. An intermediate metal can be used in between where such corrosions occur.
- To stop the occurrence Crevice corrosion the use of bare connectors of metal should be avoided. The nuts, bolts, and connectors of metal used should be coated in the anti-corrosive coating. Also, the use of insulating washers or gaskets of Teflon or rubber can serve the purpose. The surface of metals should be kept clean of debris or any other deposits.
- Pitting corrosion can be prevented by regular cleaning of the surface of metal or alloy. If any part gets corroded the use of liquid spray or foam having anti-corrosion qualities should be used. The defects in coating the metal surface must be avoided so it is advised to go for pitting defiant metals or alloys. In this occurrence also adjoining corrosion inhibitors can be of great help.
In order to prevent corrosion, there is a need to minimize the surge of corrosion. There are some resins which can resist the corrosion. These are chlorinated rubber, phenol catalyzed, vinyl and epoxy. Some pigments also help in resisting corrosion like iron oxide, china clay, talc, and mica.