What Does Corrosion Mean?
Corrosion, as applied to metals, is a chemical process that converts a refined metal into one of its more chemically stable compounds, usually an oxide, a hydroxide or sulfide. This oxidation or other reaction leads to a slow, long-term deterioration in metal performance through thinning, especially within the built environment. Corrosion processes usually take the form of electrochemical reactions between the metal and its surrounding environment (water, air or both). Hence corrosion is regarded as a natural chemical phenomenon.
The term 'corrosion' is applicable mainly to refined metals. But it may also be applicable to reinforced concrete where metals are encased in concrete, and on some occasions polymers.
The Impact of Corrosion
The global cost of corrosion runs into trillions of dollars every year and accounts on average for about 3-4% of every country's GDP. Mitigating the losses due to corrosion either through control or prevention is a huge business that is characterized by various strategies, including the use of non-corroding metals in the first place, as well as driers. Surface coatings that have been formulated specifically for anticorrosive applications are a huge business in their own right. An average figure from various up-to-the-minute sources places the market for anticorrosive coatings at about US$ 40 billion by 2027 and these kinds of measures can recoup 15-35% of corrosion losses. Major industries that are affected by corrosion are those connected with oil & gas, chemical processing, food etc.
How does rust differ from corrosion?
In chemical terms, rusting is the conversion of iron or steel into brown-red hydrated iron(III) oxide in the presence of aerial oxygen and water. Rust crumbles away and this is what makes rust the most visible and familiar form of corrosion. Rusting applies only to iron and steel.
Corrosionpedia Explains Corrosion
There are eight main forms of corrosion, and some others that are less common:
- Rusting
- Uniform or General Corrosion Galvanic Corrosion (Dissimilar Metal Corrosion or Bimetallic Corrosion) Pitting Corrosion
Crevice Corrosion
Selective Corrosion
Stress Corrosion
Erosion Corrosion
Intergranular Corrosion (Weld Decay)
Fretting Corrosion
Microbially Induced Corrosion (MIC)
Sulfidation Corrosion
Corrosion Under Insulation (CUI)
Concrete Corrosion
Uniform corrosion or general corrosion takes place consistently over a wide surface area of exposed metal. This type of corrosion is the most widespread and visible type. Because it is both predictable and readily detectable, uniform corrosion poses the lowest risk to structural deterioration. Uniform corrosion occurs on unpainted metals, typically aluminum, lead or zinc.
Galvanic corrosion occurs when two different metals form an electrical contact in an electrolyte. The electrochemical difference between the two metals establishes an anode and cathode arrangement. The metal cathode is protected while the metal anode becomes corroded. Because of this, the principle of galvanic corrosion may be employed as a corrosion mitigation technology through the use of a sacrificial anode. (link to corrosion mitigation?)
Pitting Corrosion
Pitting corrosion is the formation of holes or cavities on a metal surface. Pitting corrosion may start in two ways, either from the damaged layer of protective metal oxide (e.g. in aluminum or stainless steel), or from structural defects in the metal itself. Exposure of the bare metal in a corrosive aqueous environment in air leads to oxidation and pit formation.
Crevice Corrosion
Crevice corrosion is similar to pitting corrosion but occurs as a result of the ingress of corrosive environments (e.g. seawater) into hard-to-reach places, whereas pitting may be visible on a surface. Crevice corrosion is a localized corrosion. It may also occur in the space between two metals and result in failure of metal fixings and joints.
Selective Corrosion
Selective corrosion occurs in alloys where one metal is corroded within an alloy, but not the other. In brass, zinc is de-alloyed but not copper. In copper-nickel alloys, nickel is de-alloyed but not copper.
Stress Corrosion
Stress corrosion is caused by a combination of mechanical stresses and corrosive environments (e.g. ammonium or chloride ions) that leads to small cracks in structures. These cracks eventually propagate into larger cracks by joining up, leading to structural failure.
Erosion Corrosion
Erosion corrosion is similar to stress corrosion. It results from the transmission of corrosive chemicals through pipelines and the abrasive effects of their flow. This combination of corrosion and abrasion leads to cavities within the pipe.
Intergranular Corrosion (Weld Decay)
Intergranular corrosion occurs only in metals and alloys at extremely high temperatures (typically > 887°F) and therefore it is usually associated with metals and alloys that have been heat treated, welded or those operating in very high-temperature environments. Intergranular corrosion arises from the mis-orientation of grains within metal or alloy structures, as high temperatures lead to structural changes within the metal. Its occurrence at the granular or microscopic level renders it invisible to the naked eye, but it is reversible.
Fretting Corrosion
Fretting corrosion occurs where two different materials are joined together and they are in an environment where there is movement or vibration. Fretting corrosion occurs at the interfaces where the different materials are in contact.
Microbially Induced Corrosion (MIC)
Microbially induced corrosion comes from the presence of bacteria in pipes, tanks, vessels, wiring and tubing and results in pitting (hole formation). MIC occurs only on carbon steel or stainless steel and early detection is needed to avoid it compromising structural integrity.
Sulfidation Corrosion
Sulfidation corrosion is especially associated with the refining industry. Sulfidation processes take place at high temperatures as a result of chemical reactions contained within crude oil components.
Corrosion Under Insulation (CUI)
Corrosion under insulation occurs principally in steel components in process plants and pipes. Where plants and pipes are insulated there is often a space where water can get in the presence of air, leading to undetected corrosion beneath insulation. Beyond the presence of water and air, any available free chloride from the insulation material may accelerate the corrosion process and special care must be exercised in the choice of applied coatings and selection of insulation.
Concrete Corrosion
Concrete corrosion occurs when the steel bars – or other embedded metals – in reinforced concrete become corroded. Concrete contains both water and air so may act as an electrolyte, providing the conditions for corrosion to occur. Steel bars in concrete are contained in a high-pH cement paste that reduces very effectively, but not completely, corrosion of the bars from taking place through creating a protective oxide layer (passivation).
Reinforced concrete corrodes in two ways. One route depends upon two processes taking place. The first process is based on dissolved chloride ions (e.g. from seawater or de-icing salts) that reach and then penetrate into the protective oxide layer, making the metal increasingly vulnerable to corrosion. The second process is carbonation. This process is the reaction of aerial carbon dioxide with calcium hydroxide leading to calcium carbonate formation, thereby reducing the pH from 12-13 to around pH 8. Under these much weaker alkaline conditions, the steel in reinforced concrete becomes much more susceptible to corrosion.
The second way that concrete corrodes is through galvanic corrosion. This may occur where both aluminum and steel have been embedded in concrete. Other processes also lead to the deterioration of concrete but those are not corrosion processes and lie outside the scope of this explanation.
Corrosion Mitigation Industry
Corrosion mitigation is the prevention of corrosion through the application of corrosion-inhibiting products or solutions applied to the surface.
One of the most cost-effective ways of mitigating corrosion is the use of surface coatings, which may be specially formulated liquid industrial paints, powder coatings or plastic coatings. The global protective coatings sector, which loosely incorporates these coating classes, has been estimated to be worth around US$ 13 billion in 2023. Despite considerable consolidation in the coatings sector, this industry segment remains very fragmented with a large number of players in most countries because there are many formulation chemistries (e.g. polymers, polyurethanes, epoxies, polyureas, zinc, aluminum and chromium) and end-use applications. Industry sectors that protective coatings companies serve include: construction and infrastructure, oil & gas, industrial, marine, automotive, power generation and mining.
Corrosion inhibitors are compounds that may also be used to significantly slow down the corrosion process. The corrosion inhibitors sector is an US$ 8 billion industry that serves the pulp and paper, oil & gas, petroleum and chemical processing industries. Corrosion inhibitors arrive in many different forms, including anodic, cathodic, mixed and volatile types - volatile types being used in steam pipes where they can be carried through high-temperature pipework. These compounds may be either organic or inorganic in nature, and usually contain nitrogen, oxygen or sulfur.
The use of sacrificial anodes is another common technique in corrosion prevention and is mainly associated with marine applications. It takes the form of a highly reactive metal (usually magnesium, aluminum or zinc) which is sacrificed in order to protect another less reactive metal, echoing the principle of galvanic corrosion.
