In this article,
- we will define what is corrosion
- we will enumerate the different types of corrosion and how a metal deteriorates in each type
- In a future article, we will also discuss the countermeasures we should consider in order to prevent such corrosion
It is a natural process, which converts refined metal to their more stable oxide. It is the gradual destruction of materials (usually metals) by chemical reaction with their environment.
In the most common use of the word, this means electrochemical oxidation of metal in reaction with an oxidant such asÂ oxygen. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion.
1) General Corrosion (Uniform Attack)
Uniform attack is the most common form of corrosion. It is normally characterized by a chemical or electrochemical reaction which proceeds uniformly over the entire exposed surface or over a large area. The metal becomes thinner and eventually fails. Uniform attack, or general corrosion, accounts for the greatest amount of metal destruction by corrosion, but is considered as a safe form of corrosion, due to the fact that it is predictable, manageable and often preventable.
2) Galvanic Corrosion
Galvanic corrosion, or dissimilar metal corrosion, occurs when two different metals are located together in a corrosive electrolyte. A galvanic couple forms between the two metals, where one metal becomes the anode and the other the cathode. The anode, or sacrificial metal, corrodes and deteriorates faster than it would alone, while the cathode deteriorates more slowly than it would otherwise.
These conditions must exist for galvanic corrosion to occur:
– Anodic metal (active metal)
– Cathodic metal (noble metal)
– Electrical path
– Electrolyte (e.g. water)
3) Localized Corrosion
Unlike general corrosion, localized corrosion specifically targets one area of the metal structure. There are three types of localized corrosion:
- Pitting corrosion
- Crevice corrosion
- Filiform corrosion
Pitting is a form of extremely localized attack that results in holes in the metal. These holes may be small or large in diameter, but in most cases they are relatively small. Pits are sometimes isolated or so close together that they look like a rough surface. Generally a pit may be described as a cavity or hole with the surface diameter about the same as or less than the depth. Pitting is one of the most destructive and insidious forms of corrosion. It causes equipment to fail because of perforation with only a small percent of weight loss of the entire surface. It is often difficult to detect pits because of their small size and because the pits are often covered with corrosion products. Pitting is particularly vicious because it is a localized and intense form of corrosion, and failures often occur with extreme suddenness.
Intense localized corrosion frequently occurs within crevices and other shielded areas on metal surfaces exposed to corrosives. This type of attack is usually associated with small volumes of stagnant solution caused by holes, gasket surfaces, lap joint, surface deposits, and crevices under bolt and rivet heads. Acidic conditions, or a depletion of oxygen in a crevice can lead to crevice corrosion. This form of corrosion is sometimes called deposit or gasket corrosion.
Filiform corrosion occurs under painted or plated surfaces when water breaches the coating. It begins at small defects in the coating and spreads to cause structural weakness.
4) Intergranular Corrosion
Inter-granular corrosion is a chemical or electrochemical attack on the grain boundaries of a metal. This often occurs due to impurities in the metal, which tend to be present in higher contents near grain boundaries. These boundaries can be more vulnerable to corrosion than the bulk of the metal. The alloy disintegrates (grains fall out) and/or loses its strength.
5) Environmental Cracking
Environmental cracking is a corrosion process that can result from a combination of environmental conditions affecting the metal.
Chemical, temperature and stress-related conditions can result in these types of environmental corrosion:
- Stress corrosion cracking (SCC)
- Corrosion fatigue
- Hydrogen-induced cracking (HIC)
- Hydrogen embrittlement
Stress Corrosion Cracking (SCC)
Stress corrosion cracking is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. The tensile stresses may be in the form of directly applied stresses or in the form of residual stresses. SCC usually occurs in certain alloy-environment-stress combinations.
Corrosion fatigue is the result of the combined action of an alternating or cycling stresses and a corrosive environment. The fatigue process is thought to cause rupture of the protective passive film, upon which corrosion is accelerated. If the metal is simultaneously exposed to a corrosive environment, the failure can take place at even lower loads and after shorter time.
Hydrogen-Induced Cracking (HIC) and Hydrogen Embrittlement
Atomic hydrogen (H) is the smallest atom and it is small enough to diffuse readily through a metallic structure. When the crystal lattice is in contact or is saturated with atomic hydrogen, the mechanical properties of many metals and alloys are diminished.
Processes or conditions involving wet hydrogen sulfide (H2S), i.e. sour services, and the high incidence of sulfide in HIC has resulted in the term sulfide stress cracking (SSC).
The embrittlement of metal or alloy by atomic hydrogen involves the ingress of hydrogen into a component, an event that can seriously reduce the ductility and load-bearing capacity, cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Hydrogen embrittlement occurs in a number of forms but the common features are an applied tensile stress and hydrogen dissolved in the metal.
6) Selective Leaching (De-Alloying)
Selective leaching is the removal of one element from a solid alloy by corrosion processes. The most common example is the selective removal of zinc in brass alloys (dezincification). Similar processes occur in other alloy systems in which aluminum, iron, cobalt, chromium, and other elements are removed.
7) Erosion Corrosion
Erosion corrosion is the acceleration or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface. Generally, this movement is quite rapid, and mechanical wear effects or abrasion are involved. Erosion corrosion is characterized in appearance by grooves, gullies, waves, rounded holes, and valleys and usually exhibits a directional pattern.
8) Fretting Corrosion
Fretting corrosion occurs as a result of repeated wearing, weight and/or vibration on an uneven, rough surface. Corrosion, resulting in pits and grooves, occurs on the surface. Fretting corrosion is often found in rotation and impact machinery, bolted assemblies and bearings, as well as to surfaces exposed to vibration during transportation.
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