What is Corrosion and its types? | Corrosion

What is Corrosion?

Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment.

Corrosion is when a refined metal is naturally converted to a more stable form such as its oxide, hydroxide, or sulfide state this leads to deterioration of the material. It is a dangerous and extremely costly problem. Because of it, buildings and bridges can collapse, oil pipelines break, chemical plants leak, and bathrooms flood.

Corroded electrical contacts can cause fires and other problems, corroded medical implants may lead to blood poisoning. And air pollution has caused corrosion damage to works of art around the world. Corrosion threatens the safe disposal of radioactive waste that must be stored in containers for tens of thousands of years.

The most common kinds of corrosion result from electrochemical reactions. General corrosion occurs when most or all of the atoms on the same metal surface are oxidized, damaging the entire surface.

Most metals are easily oxidized: they tend to lose electrons to oxygen (and other substances) in the air or in water. As oxygen is reduced (gains electrons), it forms an oxide with the metal.


Causes of Corrosion

Metal corrodes when it reacts with another substance such as oxygen, hydrogen, an electrical current, or even dirt and bacteria. Corrosion can also happen when metals like steel are placed under too much stress causing the material to crack.

Corrosion of Iron

The most common type of iron corrosion occurs when it is exposed to oxygen and the presence of water, which creates a red iron oxide commonly called rust. Rust can also affect iron alloys such as steel.

The rusting of iron can also occur when iron reacts with chloride in an oxygen-deprived environment, while green rust, which is another type of corrosion, can be formed directly from metallic iron or iron hydroxide.

Types of Corrosion

Eight Forms of Corrosion

  • Uniform Attack
  • Galvanic or Two-Metal Corrosion
  • Crevice Corrosion
  • Pitting.
  • Intergranular Corrosion.
  • Selective leaching
  • Erosion Corrosion.
  • Stress-corrosion cracking.

1. Uniform Attack

The uniform attack is the most common form of corrosion. It is normally characterized by a chemical or electrochemical reaction that proceeds uniformly over the entire exposed surface or over a large area.

The metal becomes thinner and eventually fails. For example, a piece of steel or zinc immersed in dilute sulfuric acid will normally dissolve at a uniform rate over its entire surface. A sheet iron roof will show essentially the same degree of rusting over its entire outside surface.

Uniform attack, or general overall corrosion, represents the greatest destruction of metal on a tonnage basis. This form of corrosion, however, is not of too great concern from the technical standpoint, because the life of equipment can be accurately estimated on the basis of comparatively simple tests. Merely immersing specimens in the fluid involved is often sufficient.

Uniform attack can be prevented or reduced by:

  • proper materials, including coatings,
  • Inhibitors, or
  • cathodic protection.

2. Galvanic or Two-Metal Corrosion

A potential difference usually exists between two dissimilar metals when they are immersed in a corrosive or conductive solution. If these metals are placed in contact (or otherwise electrically connected), this potential difference produces electron flow between them.

Corrosion of the less corrosion-resistant metal is usually increased and attack of the more resistant material is decreased, as compared with the behavior of these metals when they are not in contact. The less resistant metal becomes anodic and the more resistant metal cathodic.

Usually the cathode or cathodic metal corrodes very little or not at all in this type of couple. Because of the electric currents and dissimilar metals involved, this form of corrosion is called galvanic, or two-metal, corrosion. It is electrochemical corrosion, but we shall restrict the term galvanic to dissimilar-metal effects for purposes of clarity.

3. Crevice Corrosion

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 joints, surface deposits, and crevices under bolt and rivet heads.

As a result, this form of corrosion is called crevice corrosion or, sometimes, deposit or gasket corrosion.

4. Pitting

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 a 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 weight loss of the entire structure. It is often difficult to detect pits because of their small size and because the pits are often covered with corrosion products.

In addition, it is difficult to measure quantitatively and compare the extent of pitting because of the varying depths and numbers of pits that may occur under identical conditions. Pitting is also difficult to predict by laboratory tests.

Sometimes the pits require a long time several months or a year to show up in actual service. Pitting is particularly vicious because it is a localized and intense form of corrosion, and failures often occur with extreme suddenness.

5. Intergranular Corrosion

Grain boundary effects are of little or no consequence in most applications or uses of metals. If a metal corrodes, uniform attack results since grain boundaries are usually only slightly more reactive than the matrix.

However, under certain conditions, grain interfaces are very reactive, and intergranular corrosion results. Localized attack at and adjacent to grain boundaries, with relatively little corrosion of the grains, is intergranular corrosion. The alloy disintegrates (grains fall out) and/or loses its strength.

Intergranular corrosion can be caused by impurities at the grain boundaries, enrichment of one of the alloying elements, or depletion of one of these elements in the grain-boundary areas. Small amounts of iron in aluminum, wherein the solubility of iron is low, have been shown to segregate in the grain boundaries and cause intergranular corrosion.

It has been shown that based on surface tension considerations the zinc content of brass is higher at the grain boundaries. Depletion of chromium in the grain-boundary regions results in intergranular corrosion of stainless steels.

6. Selective leaching

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. Selective leaching is the general term to describe these processes, and its use precludes the creation of terms such as dealuminumification, decalcification, etc.

Parting is a metallurgical term that is sometimes applied, but selective leaching is preferred.

7. Erosion Corrosion

Erosion corrosion is the acceleration or increase in the 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.

Metal is removed from the surface as dissolved ions, or it forms solid corrosion products which are mechanically swept from the metal surface. Sometimes, movement of the environment decreases corrosion, particularly when a localized attack occurs under stagnant conditions, but this is not erosion-corrosion because deterioration is not increased.

Erosion corrosion is characterized in appearance by grooves, gullies, waves, rounded holes, and valleys and usually exhibits a directional pattern. In many cases, failures because of erosion-corrosion occur in a relatively short time, and they are unexpected largely because evaluation corrosion tests were run under static conditions or because the erosion effects were not considered.

8. Stress-corrosion cracking

Stress-corrosion cracking refers to cracking caused by the simultaneous presence of tensile stress and a specific corrosive medium. Many investigators have classified all cracking failures occurring in corrosive mediums as stress-corrosion cracking, including failures due to hydrogen embrittlement.

However, these two types of cracking failures respond differently to environmental variables. To illustrate, cathodic protection is an effective method for preventing stress-corrosion cracking whereas it rapidly accelerates hydrogen-embrittlement effects.

Hence, the importance of considering stress-corrosion cracking and hydrogen embrittlement as separate phenomena is obvious.

Effect of Corrosion

Some of the effects of corrosion include a significant deterioration of natural and historic monuments as well as an increased risk of catastrophic equipment failures. Air pollution causes corrosion, and it’s becoming worse worldwide.

The annual worldwide cost of metallic corrosion is estimated to be over $2 trillion, yet experts believe 25 – 30% could be prevented with proper corrosion protection. Poorly planned construction projects can lead to a corroded structure needing to be replaced, which is a waste of natural resources and contradictory to global concerns over sustainability. In addition, corrosion can lead to safety concerns, loss of life, additional indirect costs, and damage to reputation. 

Direct effects of corrosion may include:

  • Damage to commercial airplanes or vehicle electronics
  • Damage to hard disks and computers used to control complicated processes (e.g. power plants, petrochemical facilities or pulp and paper mills).
  • Damage to server rooms and data centres.
  • Damage to  museum artefacts
  • Costs of repairing or replacing household equipment that fails

How to Prevent Corrosion

You can prevent corrosion by selecting the right:

  • Choose the right Metal Type.
  • Protective Coating
  • Environmental Measures
  • Sacrificial Coatings
  • Corrosion Inhibitors
  • Metal Plating
  • Design Modification

1. Choose the right Metal Type

One simple way to prevent corrosion is to use a corrosion-resistant metal such as aluminum or stainless steel. Depending on the application, these metals can be used to reduce the need for additional corrosion protection.

2. Protective Coatings

The application of a paint coating is a cost-effective way of preventing corrosion. Paint coatings act as a barrier to prevent the transfer of electrochemical charge from the corrosive solution to the metal underneath.

Another possibility is applying a powder coating. In this process, a dry powder is applied to the clean metal surface. The metal is then heated which fuses the powder into a smooth unbroken film. Several different powder compositions can be used, including acrylic, polyester, epoxy, nylon, and urethane.

3. Environmental Measures

Corrosion is caused by a chemical reaction between the metal and gases in the surrounding environment. By taking measures to control the environment, these unwanted reactions can be minimized.

This can be as simple as reducing exposure to rain or seawater, or more complex measures, such as controlling the amounts of sulfur, chlorine, or oxygen in the surrounding environment.

An example of this would be would be treating the water in water boilers with softeners to adjust hardness, alkalinity, or oxygen content.

4. Sacrificial Coatings

Sacrificial coating involves coating the metal with an additional metal type that is more likely to oxidize; hence the term “sacrificial coating.”

There are two main techniques for achieving sacrificial coating: cathodic protection and anodic protection.

Cathodic Protection:

The most common example of cathodic protection is the coating of iron alloy steel with zinc, a process known as galvanizing. Zinc is a more active metal than steel, and when it starts to corrode it oxides which inhibits the corrosion of the steel.

This method is known as cathodic protection because it works by making the steel the cathode of an electrochemical cell. Cathodic protection is used for steel pipelines carrying water or fuel, water heater tanks, ship hulls, and offshore oil platforms.

Anodic Protection:

Anodic protection involves coating the iron alloy steel with a less active metal, such as tin. Tin will not corrode, so the steel will be protected as long as the tin coating is in place. This method is known as anodic protection because it makes the steel the anode of an electrochemical cell.

Anodic protection is often applied to carbon steel storage tanks used to store sulfuric acid and 50% caustic soda. In these environments, cathodic protection is not suitable due to extremely high current requirements.

5. Corrosion Inhibitors

Corrosion inhibitors are chemicals that react with the surface of the metal or the surrounding gases to suppress the electrochemical reactions leading to corrosion. They work by being applied to the surface of a metal where they form a protective film.

Inhibitors can be applied as a solution or as a protective coating using dispersion techniques. Corrosion inhibitors are commonly applied via a process known as passivation.

6. Metal Plating

Plating is very similar to coating as a thin layer of metal is applied to the metal you actually want to protect. As well as preventing corrosion, the metal layer provides good aesthetic finishes.

There are four types of metallic plating:

  • Electroplating: Where a thin layer of metal such as chromium or nickel is put on the substrate metal via an electrolyte bath.
  • Mechanical Plating: this involves cold welding metal powder to the substrate metal.
  • Electroless: A coating metal like nickel or cobalt is placed on a substrate metal using a non-electric chemical reaction.
  • Hot Dipping: The simplest coating technique which involves immersing the substrate in a molten bath of the protective metal.

7. Design Modification

Design modifications can help reduce corrosion and improve the durability of any existing protective anti-corrosive coatings. Ideally, designs should avoid trapping dust and water, encourage the movement of air, and avoid open crevices. Ensuring the metal is accessible for regular maintenance will also increase longevity.

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