Understanding the Types of Corrosion
Types of Corrosion and How to Spot Them
Corrosion occurs when a chemical reaction takes place in an environment that surrounds materials and starts to cause slow deterioration and weakening of the material. Particular gasses from an environment that contact a certain metal ultimately determine the rate and form of corrosion. Below, find the nine most common types of corrosion and what they look like. It’s important to understand each in the wire and cable industry since it is one of the leading deteriorating factors.
1. General Attack Corrosion
This form of corrosion happens when a chemical or electrochemical reaction causes the deterioration of the exposed metal surface. Although general attack corrosion is the most prominent form in metals, it is also considered the safest, in that it can be predictable, manageable and often preventable.
2. Localized Corrosion
This type of corrosion is localized, targeting just one area on a metal structure. With localized corrosion, there are three different categories: pitting, crevice and filiform. Pitting is the result of corrosion causing a small hole (pit) in the metal, usually hard to find because of the small nature. The pitted area becomes anodic, while part of the remaining metal becomes cathodic, producing a localized galvanic reaction. Crevice corrosion is what happens in a crevice which is due to a lack of oxygen or an overly acidic area. It’s typically found around gaskets, washers, and clamps. Filiform corrosion can ultimately lead to structural weakness. It starts when water seeps through painted or plated surfaces, or anywhere there may be a defect in a surface coating.
3. Galvanic Corrosion
This form of corrosion will take place if there are two electrochemically different metals occupying the same space that are in electrical contact with one another and the metals are exposed to an electrolyte. If all of these scenarios are present, galvanic corrosion will happen rapidly to the less corrosive-resistant metal, becoming anode, where the more corrosive-resistant metal will see a decrease, becoming cathode.
4. Environmental Cracking Corrosion
Environmental cracking happens when the conditions that surround the metal start to have an effect on it. This can be from some conditions such as chemical, temperature, and stress-related surroundings. You may have heard of environmental corrosion referred to as stress corrosion cracking, corrosion fatigue, hydrogen corrosion or liquid metal embrittlement. The two most common cases of environmental cracking corrosion are seasonal cracking of brass and caustic embrittlement of steel.
5. Flow-Assisted Corrosion (FAC)
This corrosion is caused by the wind or water “washing away” the metal surface’s protective layer of oxide. Once the metal is exposed, flow-assisted corrosion transpires deteriorating the piece/object.
6. Intergranular Corrosion
An electrochemical attack on the grain boundaries of metal is what is known as intergranular corrosion. This often occurs due to impurities at the grain boundaries, enrichment of one of the alloying elements or depletion of one of these elements near the grain boundaries.
7. Selective Leaching Corrosion
This occurs when one element from a solid alloy is removed due to corrosion. Selective leaching most commonly happens when there is a selective removal of zinc in brass alloys, also known as dezincification.
8. Fretting Corrosion
Repeated wearing, weight and vibration on rough or uneven surfaces and asperities of contact surfaces affected by corrosion damage cause this type of corrosion. Impact machinery, bolted assemblies and bearings often see fretting corrosion, as well as surfaces that experience heavy vibration.
9. High Temperature Corrosion
This corrosion can be caused by oxidization in high temperatures, sulfidation, and carbonization. More commonly, high-temperature corrosion exists in gas turbines, diesel engines, and other machinery. Corrosion starts in these mechanics when, during combustion, vanadium or sulfates form compounds with a low melting point. The composites are corrosive towards metal alloys that are otherwise resistant to high temperatures and corrosion.