Corrosion is the process of deterioration of metal caused by the action of chemicals or electro-chemicals present in the surrounding atmosphere. Corrosion is a major problem especially in the construction industry where various metals are used for structural purposes. Different types of corrosion are explained in this article.
Types of Corrosion
Following are the 9 different types of corrosions that commonly occur in metals.
- Atmospheric Corrosion
- Erosion Corrosion
- Selective Corrosion
- Uniform Corrosion
- Pitting Corrosion
- Fretting Corrosion
- Stress Corrosion
- Inter-granular Corrosion
- Corrosion Fatigue
1. Atmospheric Corrosion
Atmospheric corrosion is a type of wet corrosion caused by the action of electrolytes. In this case, moisture present in the atmosphere, rainwater, etc. acts as electrolytes triggering corrosion of the exposed metal surface.
2. Erosion Corrosion
Erosion corrosion is caused by mechanical abrasion due to the relative movement between metal surfaces and corrosive fluids. In this case, the surface of metal gets deteriorated gradually by the abrasion of fast-moving fluids and cavities are also formed. This type of corrosion commonly seen in metal tubes carrying moving fluids in it.
3. Selective Corrosion
Selective corrosion occurs in alloys where one of the component metals is de-alloyed by the corrosive environment. This type of corrosion can be seen in brass alloy pipes where zinc is commonly used as another component and here zinc is de-alloyed. Similarly in the case of copper-nickel alloy tubes where nickel is de-alloyed by selective corrosion.
4. Uniform Corrosion
In case of uniform corrosion, a uniform layer of rust is formed on the surface of metals and it is extended over the whole surface area of metal. This type of corrosion can be seen in metals which are not protected by surface coating. Aluminum, zinc, lead, etc. are some metals commonly affected by uniform corrosion.
5. Pitting Corrosion
Pitting is the formation of rust pits or holes on the surface. Pitting corrosion is a localized form of corrosion where the corrosion is limited to small areas. The shapes of rust pits may not be similar but in most of the cases, they are hemispherical in shape.
Pitting corrosion occurs when the protective oxide layer of the surface gets damaged or due to structural defects in metal. It is considered more dangerous because it causes the failure of structure with a relatively low overall loss of material. It can be observed in steel, aluminum, nickel alloys, etc.
6. Fretting Corrosion
Fretting corrosion occurs at the contact area of the two materials which are joined together. This is developed when the contact area is subjected to slips and vibrations. This type of corrosion can be seen in bolted and riveted joints, clamped surface, etc.
7. Stress Corrosion
Stress corrosion caused by the combined action of a corrosive environment and mechanical stress on the surface of the material. In its initial stage, small cracks are developed and these finally leads to failure of the whole structure. This type of corrosion can be seen in stainless steel when they are stressed in chloride environments, in brass materials when they are stressed in the presence of ammonia, etc.
8. Inter-granular Corrosion
Inter-granular corrosion is the corrosion occurs along the grain boundaries and the grains are not affected in this case. It is caused when there is a noticeable difference in reactivity against impurities exists between grain boundaries and grains. This difference in reactivity occurs due to defective welding, heat treatment stainless steels, copper, etc.
9. Corrosion Fatigue
Fatigue of materials is defined as the failure of a material due to repeated application of stress. When the fatigue of metal is developed in the corrosive environment then it called corrosion fatigue. This can be prevented by improving the fatigue resistance of a material.