Corrosion, which is caused by the action of chemicals or electro-chemicals in the surrounding environment, leads to the deterioration of metal. This process is a significant concern, particularly in the construction industry where various metals are employed for structural purposes.
There are several types of corrosion that one should be aware of. This article provides an explanation of these different types.
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 specific form of wet corrosion that occurs as a result of the action of electrolytes. This type of corrosion is triggered by the presence of moisture in the atmosphere, as well as rainwater and other similar factors. When these electrolytes come into contact with exposed metal surfaces, they can cause corrosion to occur. This process can have a significant impact on the overall durability and longevity of the metal, and it is important to take steps to mitigate the effects of atmospheric corrosion where possible.
2. Erosion Corrosion
Erosion corrosion is a type of corrosion that occurs as a result of the combined effect of mechanical abrasion and the corrosive action of fluids on metal surfaces. This type of corrosion occurs when fast-moving fluids come into contact with metal surfaces, causing gradual deterioration through abrasion. As a result of the abrasion, cavities are formed on the metal surface, leading to further deterioration.
Erosion corrosion is a common type of corrosion that is often observed in metal tubes that carry fluids. This type of corrosion can significantly reduce the lifespan of metal components and compromise their structural integrity. Therefore, it is important to take measures to prevent erosion corrosion, such as selecting suitable materials for the tubes and controlling the fluid velocity to reduce the impact of abrasion on metal surfaces.
3. Selective Corrosion
Selective corrosion is a type of corrosion that can occur in alloys where one of the component metals is de-alloyed by the corrosive environment. This phenomenon is commonly observed in brass alloy pipes where zinc is one of the component metals. The corrosion process in such cases results in the de-alloying of zinc. Copper-nickel alloy tubes are another example where selective corrosion can take place. In this case, nickel is the component metal that is susceptible to de-alloying due to corrosion.
4. Uniform Corrosion
Uniform corrosion is a type of corrosion that affects metals which are not protected by surface coatings. When a metal is affected by uniform corrosion, a uniform layer of rust is formed on its surface, which extends over the entire surface area of the metal. This means that the corrosion is evenly distributed across the surface of the metal, giving it a uniform appearance.
Metals such as aluminum, zinc, and lead are commonly affected by uniform corrosion. This type of corrosion can occur when these metals are exposed to corrosive environments for extended periods of time. If left unchecked, uniform corrosion can weaken the metal and compromise its structural integrity, which can be a serious safety concern in some applications.
To prevent uniform corrosion, it is important to protect metal surfaces with appropriate coatings or inhibitors. Regular inspection and maintenance of metal structures can also help to identify and address any corrosion issues before they become more serious problems. By taking proactive measures to prevent uniform corrosion, the lifespan and durability of metal components can be significantly improved.
5. Pitting Corrosion
Pitting is a type of corrosion that can occur on the surface of metals, resulting in the formation of rust pits or holes. This type of corrosion is localized, meaning that it only affects small areas of the metal surface. In many cases, the rust pits take on a hemispherical shape, though this can vary depending on the specific circumstances of the corrosion.
Pitting corrosion typically occurs when the protective oxide layer that naturally forms on metal surfaces is damaged or compromised in some way. This can happen due to a variety of factors, such as exposure to certain chemicals or environmental conditions, or the presence of structural defects within the metal itself. One of the reasons that pitting corrosion is considered particularly dangerous is that it can cause structural failure even with a relatively small overall loss of material.
Pitting corrosion can affect a wide range of metals, including steel, aluminum, and nickel alloys. Because of its potential to cause serious damage and failure of structures, it is an important concern in many industries that rely on metal materials for construction or manufacturing purposes.
6. Fretting Corrosion
Fretting corrosion is a phenomenon that occurs at the contact point of two joined materials. This type of corrosion is typically observed when the contact area is exposed to repeated slips and vibrations. Fretting corrosion is commonly found in bolted and riveted joints as well as clamped surfaces.
The corrosion process occurs due to the movement between the surfaces of the joined materials. When this happens, a layer of corrosion product builds up, and as the process continues, the corrosion damage becomes more severe. Fretting corrosion can ultimately lead to the failure of the joined materials if not addressed promptly.
To prevent fretting corrosion, it’s essential to minimize the movement between the surfaces of the joined materials. One way to do this is by using a lubricant that reduces friction and protects the surfaces from wear and tear. Alternatively, the materials can be treated with an anti-corrosion coating to prevent the buildup of corrosion products and prolong the lifespan of the joined materials.
7. Stress Corrosion
Stress corrosion occurs when a material is subjected to both a corrosive environment and mechanical stress on its surface. This phenomenon can lead to the development of small cracks in the material’s initial stage, which can eventually result in the failure of the entire structure. Examples of this type of corrosion include stainless steel that is stressed in chloride environments and brass materials that are stressed in the presence of ammonia. These materials are particularly susceptible to stress corrosion due to their composition and the specific conditions to which they are exposed. Therefore, it is essential to take appropriate measures to prevent stress corrosion and maintain the integrity and safety of the affected structures.
8. Inter-granular Corrosion
Inter-granular corrosion is a specific type of corrosion that occurs along the boundaries between grains, while the grains themselves remain unaffected. This phenomenon typically arises due to a significant difference in reactivity between the grain boundaries and the grains themselves, often resulting from factors such as defective welding, heat treatment, or the use of certain materials like stainless steel or copper.
One of the key characteristics of inter-granular corrosion is that it specifically affects the boundaries between grains, rather than the grains themselves. This can result in a unique pattern of corrosion that appears to follow the contours of the grain structure, leaving the individual grains largely intact while causing significant damage to the boundaries between them.
There are several different factors that can contribute to the development of inter-granular corrosion, including the presence of impurities or other contaminants in the material, as well as the specific properties of the grain boundaries themselves. In some cases, this type of corrosion may be exacerbated by factors such as high temperatures, exposure to certain chemicals or environments, or other external stressors.
Overall, the occurrence of inter-granular corrosion can have significant implications for the integrity and durability of a wide range of materials, particularly those that are commonly used in industrial or manufacturing settings. As such, it is important for engineers, materials scientists, and other experts to carefully consider the potential risks of this type of corrosion and take appropriate steps to mitigate or prevent its occurrence.
9. Corrosion Fatigue
Fatigue of materials occurs when a material fails due to repeated stress applications. When this fatigue occurs in a corrosive environment, it is referred to as corrosion fatigue. To prevent corrosion fatigue, it is essential to enhance the fatigue resistance of the material.
Fig 9: Corrosion Fatigue