Maya Ellison
Corrosion is a costly and dangerous issue, with the World Corrosion Organization estimating an annual global cost of $2.5 trillion. It is an electrochemical phenomenon that causes the breakdown of surfaces, leading to pitting and cracking. While it is commonly associated with metals, corrosion can also occur in other materials, such as ceramics and plastics. The susceptibility of an object to corrosion depends on its environment, the type of material, and the object's chemical composition. For example, the wrong type of plastic will deteriorate when exposed to certain chemicals. In this context, the term degradation is often used instead of corrosion. This process can be mitigated with the use of specific materials, such as Ceramco's A998 (99.8% alumina), which is non-corroding and oxidation-proof.
| Characteristics | Values |
|---|---|
| Metals corrode | Yes |
| Plastics corrode | Yes, but not as readily as metals |
| Ceramics corrode | Yes, but less likely than metals and plastics |
| Metals are good conductors | Yes |
| Plastics are good conductors | No |
| Ceramics are good conductors | No |
| Metals rust | Yes |
| Plastics rust | No |
| Ceramics rust | No |
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What You'll Learn
- Galvanic corrosion occurs when two metals are in contact and immersed in a common electrolyte
- Noble metals, like gold and silver, rarely corrode
- Ceramics can be used to prevent corrosion
- Plastic does not rust but can deteriorate when exposed to certain chemicals
- Corrosion prevention methods include passivation and chromate conversion
Galvanic corrosion occurs when two metals are in contact and immersed in a common electrolyte
Galvanic corrosion, also known as bimetallic corrosion or dissimilar metal corrosion, occurs when two metals are in electrical contact with each other and immersed in a common electrolyte. This process requires three elements: two different metals, an electrolyte, and an electrical conducting path between the metals. The electrolyte, usually water, creates a "conductive path" that allows ions to move and prevents charge build-up that would otherwise stop the reaction.
An example of galvanic corrosion is when zinc (galvanized steel) comes into direct contact with stainless steel and is exposed to an electrolyte like water. In this case, galvanic corrosion will occur, and the zinc will corrode more quickly than the stainless steel. The relative surface area of each exposed metal also plays a role in the rate of corrosion, with a larger anode and smaller cathode resulting in a higher current and faster corrosion.
To prevent galvanic corrosion, dissimilar metal combinations should be avoided in areas with prolonged moisture accumulation. If the use of dissimilar metals is necessary, they should be electrically insulated from each other. Additionally, the fasteners used to combine multiple metals should match the noblest of the metals being joined to slow down the corrosion process.
While galvanic corrosion specifically refers to the corrosion of metals, plastics, composites, and ceramics are also susceptible to corrosion. However, they do not rust or undergo the same electrochemical corrosion as metals due to their poor conductivity. The corrosion of these materials occurs through the deterioration of chemical properties and the chemical reaction of unstable molecules in the environment with the molecules of the non-metal materials.
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Noble metals, like gold and silver, rarely corrode
All metals corrode in the presence of regular environmental conditions, except for precious metals. Noble metals are a subset of precious metals and include gold, silver, and platinum. These metals are more stable and resistant to interactions with other chemicals, and therefore, more resistant to corrosion.
The galvanic series sorts metals on a hierarchical scale from noble to active. Active metals tend to interact more with the environment, while noble metals are more stable. Noble metals tend to be resistant to oxidation and other forms of corrosion, and this corrosion resistance is often considered a defining characteristic.
Gold, platinum, and the other platinum group metals (ruthenium, rhodium, palladium, osmium, and iridium) are most often classified as noble metals. Silver, copper, and mercury are sometimes included as noble metals, but each of these usually occurs in nature combined with sulfur. In atomic physics, for example, only copper, silver, and gold are considered noble metals because they have completely filled d-subshells, which makes them more stable.
While noble metals rarely corrode, there are some exceptions. Silver is subject to corrosion when present in the mouth, and copper is dissolved by nitric acid and aqueous potassium cyanide. Rhodium must be in a fine pulverized form to be dissolved, and palladium and silver are soluble in nitric acid.
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Ceramics can be used to prevent corrosion
Ceramics are highly resistant to corrosion and are therefore the preferred material for use in harsh environments. They are also lightweight, inexpensive, and can withstand high temperatures, pressures, and other extreme conditions. These properties make ceramics useful in several industries, including aerospace, gas, oil, and semiconductors.
Ceramics are already oxides, so they are almost entirely unaffected by the oxidation process leading to corrosion. This is because the chemical bonds in ceramics are already oxidized and, therefore, cannot be oxidized further. This makes ceramics especially useful in preventing corrosion in steel structures and piping. Concrete, for example, is used to protect steel structures and pipes, as well as the reinforcing steel rods within concrete itself. When concrete sets, it generates alkalis that produce a ferric oxide film over the steel surface, protecting it from corrosion.
Ceramics are also used to protect iron parts. Chrome oxide, a corrosion-resistant ceramic, can bond to glass and repair fractured glass linings of chemical tanks. Glass frequently lines valves, water and chemical tanks, and plumbing fixtures and appliances. Enamel coatings are also applied to steel and iron in chemical plants.
Ceramics are also used in the oil and gas industries. In these industries, ceramic materials are used in manufacturing pumps, transfer pipes, bushes and coupling sections, drilling tools, and wear-resistant products for drilling, exploration, and processing applications.
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Plastic does not rust but can deteriorate when exposed to certain chemicals
Unlike metals, plastics and ceramics do not rust. However, they are still susceptible to corrosion. Plastics and ceramics are poor conductors of electrons and are not susceptible to the same electrochemical corrosion as metals. Instead, they corrode through a process called uniform attack, where unstable molecules in the environment react chemically with the molecules that make up these non-metal materials.
Plastics and ceramics do not corrode as readily as metals, and the process is environmentally selective. While the chemical bonds in ceramics are already oxidized and cannot be oxidized further, plastics are vulnerable to degradation over time. This degradation can be classified as either physical or chemical. Physical degradation refers to changes in the bulk structure, such as cracking, embrittlement, and flaking, while chemical degradation refers to changes at the molecular level, such as bond cleavage or oxidation of long polymer chains to create new molecules.
The degradation rates of plastics can vary depending on the type of plastic and the environmental conditions. For example, a recent study found that polystyrene exposed to sunlight degrades much faster than previously estimated. The specific surface degradation rate (SSDR) is a metric used to harmonize disparate types of measurements and estimate half-lives for different types of plastics. SSDR values can vary widely, with some plastics in the marine environment having estimated half-lives ranging from 58 years for bottles to 1200 years for pipes.
While plastics do not rust, they can still deteriorate when exposed to certain chemicals and environmental factors. This deterioration can lead to changes in the physical and chemical properties of the plastic, resulting in degradation and mass loss over time.
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Corrosion prevention methods include passivation and chromate conversion
Corrosion is a common issue that affects a wide range of materials, including metals, plastics, and ceramics. While plastics and ceramics do not rust, they can still corrode through the deterioration of their chemical properties. Metals, on the other hand, are susceptible to electrochemical corrosion due to their conductivity.
Passivation is a technique used to prevent corrosion in metals. It involves coating the metal with a protective material, such as metal oxide, to create a shield that resists corrosion. This process can be achieved through chemical reactions or by allowing spontaneous oxidation to occur in the air. Passivation is particularly effective for stainless steel, where it removes contaminants and enhances the protective chromium oxide layer.
Chromate conversion is another method used to prevent corrosion. It involves applying a thin layer of chromium hydroxide to the metal surface, creating a barrier that protects against corrosion. This process can be applied to a variety of metals, including steel, aluminum, and zinc. Chromate conversion coatings can also serve as a primer, improving the adherence of paints and adhesives, and providing some resistance to abrasion and light chemical attack.
Both passivation and chromate conversion are effective methods for preventing corrosion in metals. By creating a protective layer, these techniques enhance the durability and longevity of metal products, ensuring they can withstand harsh environments without succumbing to the detrimental effects of corrosion.
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Frequently asked questions
Yes, all metals can corrode. Some, like pure iron, corrode quickly. However, this can be prevented by adding alloys to a pure metal or by carefully combining metals.
Plastics do not "rust" in response to corrosion. However, they can deteriorate when they come into contact with certain chemicals.
Ceramics can corrode, but this effect can be overcome with certain ceramic parts.
