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Common Types of Corrosion

Nearly all of the metal items we use on a daily basis are susceptible to corrosion, which is a normal occurrence. All metal objects, including those found in vehicles, airplanes, medical equipment, buildings, infrastructure, appliances, and energy distribution networks, are susceptible to corrosion. If an alloy is exposed to various conditions at various locations within a system, it may experience more than one type of corrosion.

What is corrosion ?

Corrosion is the process of metal corrosion brought on by the presence of chemicals or electrochemicals in the environment. It is the degradation of a substance brought on by chemical interactions between that substance and its environment.

Why do metals corrode?

It takes energy (being used in a blast furnace) to separate the metallic elements from the compounds they are found in naturally for the majority of metals to exist in their metallic state. The atoms that make up a metal in its metastable state desire to revert to the stable state that needs the least energy to maintain because entropy rises and free energy must fall due to the law that free energy must increase as entropy rises. Corrosion is the process by which materials lose energy by oxidation after being extracted from their initial low-energy (ore) form.

This is accomplished through an electron exchange that takes place between the environment and a metal . When a metal corrodes, it normally loses electrons, turning oxidized, while the ambient elements interacting with it gain electrons, turning reduced. Unfortunately, because corrosion is a more stable condition than uncorroded state, practically all situations will produce corrosion to some extent in metals.

Types of corrosion

  • Stress corrosion cracking

You examine your component with an eyelet and see tiny, barely noticeable fractures on the corners. This is a landmark sign of stress corrosion, a corrosion type that is infamously terrible. Stress corrosion happens when a material is exposed to both tensile loads and a corrosive environment. All metals have various tensile limitations or ranges, and stress corrosion is more likely to develop the closer the metal is to these limits.

  • Pitting corrosion

This kind of corrosion develops when a tiny crack appears in the metal. The resulting hole or cavity turns anodic, whereas the metal in its immediate surroundings turns cathodic. This causes a localized galvanic reaction, which causes the metal to fail structurally. Pitting can be hard to find and handle because other elements created during the formation of corrosion frequently obscure it.

  • Erosion corrosion

The relative movement between metal surfaces and corrosive fluids results in mechanical abrasion, which is what causes erosion corrosion. The abrasion of the fast-moving fluids in this situation gradually degrades the metal’s surface, and cavities also develop. Metal tubes conveying moving fluids frequently experience this form of corrosion. It is easily distinguished from other materials by the metal substrate’s directional look of grooves, gullies, craters, and valleys.

  • Crevise corrosion

On the metal’s surface, this kind of corrosion develops in gaps or crevices. Crevice corrosion can also impact everything that the metal is “connected” to, such as welded metal on metal or even metal bonded to non-metal objects. Most frequently, contaminants that speed up the corrosion process fill these cracks. The main suspicion to watch out for is water (which is typically stagnant). This water is the catalyst for the electrochemical reactions that will ultimately cause the material to break down and fail.

  • Selective leaching

Selective corrosion happens in alloys when the corrosive environment de-alloys one of the component metals. Brass alloy pipes that frequently include zinc as an additional component and where zinc is de-alloyed exhibit this sort of corrosion. Similar circumstances apply to copper-nickel alloy tubes where nickel is selectively corroded out of the alloy.

  • Uniform corrosion

Because of this corrosion, the metal will gradually degrade and dissolve, losing some of its “thickness.” The metal will eventually crack because of it, becoming brittle. This corrosion happens naturally when carbon steel deteriorates as a result of a chemical or electrochemical reaction with the environment, corroding it “uniformly” across the entire surface.

  • Intergranular corrosion

In the instance of intergranular corrosion, the grains are not harmed; the corrosion happens along the grain boundaries. It develops when grain borders and grains have a distinct variation in their reactivity to contaminants. Because of improper welding, improper heat treatment of stainless steels, copper, etc., there is a difference in reactivity.

  • Galvanic/bimetallic corrosion

Bimetallic corrosion happens when two metals that are very different from one another are in contact for a very long time. Bimetallic couple cells are produced as a result of the different electrode potentials between the metals. In a bimetallic couple cell, one metal functions as the cathode and the other as the anode.

In the galvanic series, the potential impact on bimetallic corrosion of the two materials increases with the distance between two metals.

Thus, the type of corrosion that is most likely to develop on a specific metal surface depends mostly on the exposure environment as well as the physical and chemical characteristics of the metal. Therefore, it is crucial to comprehend the mechanisms involved in each type of corrosion in order to guarantee that the best preventative and mitigation strategies are used when appropriate.

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