What is Corrosion? – Definition and Prevention

What is Corrosion?

Corrosion is a natural process 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 the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reactions with their environment.

The ability of electrochemical processes to break compounds down into elements or to create new compounds can be destructive as well as productive. Corrosion is an all-too-common result of electrochemical reactions between materials and substances in their environment.

Corrosion 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.

When reduction and oxidation take place on different kinds of metal in contact with one another, the process is called galvanic corrosion. In electrolytic corrosion, which occurs most commonly in electronic equipment, water or other moisture becomes trapped between two electrical contacts that have an electrical voltage applied between them. The result is an unintended electrolytic cell.

Engineering Choice The Biggest Learning Platform

Take a metal structure such as the Statue of Liberty. It looks strong and permanent. Like nearly all metal objects, however, it can become unstable as it reacts with substances in its environment and deteriorates.

Corrosion

Chemistry of corrosion

The chemistry of corrosion is complex; it can be considered an electrochemical phenomenon. During corrosion at a particular spot on the surface of an object made of iron, oxidation takes place and that spot behaves as an anode.

The electrons released at this anodic spot move through the metal and go to another spot on the metal and reduce oxygen at that spot in presence of H+ (which is believed to be available from H2CO3 formed due to the dissolution of carbon dioxide from the air into water in moist air condition of the atmosphere.

Hydrogen ions in water may also be available due to the dissolution of other acidic oxides from the atmosphere). This spot behaves as a cathode.

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.

Some of the main and popular causes of corrosion are as follows:

  • Too much humidity or condensation of water vapor on metal surfaces are the primary causes of corrosion.
  • Corrosive gases such as chlorine, hydrogen oxides, ammonia, sulfur oxides, amongst others can result in corrosion of parts of electronic equipment, etc. Corrosion can also occur due to hydrogen and oxygen exposure.
  • Corrosion in steel can occur when it is placed under too much stress and the material develops a crack in it.
  • Metals exposed to electrical currents for a long time can experience electronic corrosion.
  • Exposure to dirt and bacteria can cause corrosion in metals.

Corrosion of Iron

Iron corrosion is the destruction of a metallic material that contains the element iron (Fe) due to chemical reactions with oxygen (O2) and water in the surrounding environment, 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.

Iron corrosion is generally characterized by the formation of rust due to an electrochemical process in the presence of water, oxygen, and an electrolytic salt solution. Therefore, the remediation of one or all of these reactant sources can be used to reduce the rate of corrosion in a given metal.

When iron reacts with water and oxygen, iron (II) hydroxide is formed. The latter further reacts with oxygen and water to then form hydrated iron (III) oxide – widely known as rust.

Types of Corrosion

There are many different types of corrosion that are visible to the naked eye:

  • Uniform Corrosion
  • Localized Corrosion
  • Galvanic Corrosion
  • Environmental Cracking
  • Flow-Assisted and Intergranular Corrosion
  • Fretting Corrosion
  • High-Temperature Corrosion
  • Soil Corrosion

These are the most common types of corrosion, let’s explain the underlying mechanism of each.

1. UNIFORM CORROSION

Uniform corrosion is the most common variant of corrosion. This corrosion occurs naturally when carbon steel deteriorates through a chemical or electrochemical reaction with the surrounding environment that deteriorates the entire surface, corroding it ‘uniformly.’ This type of corrosion is the most widespread, but is predictable and can be managed by using the appropriate preventative measures.

2. LOCALIZED CORROSION

Localized corrosion comes in many variations, such as pitting, crevice corrosion, and filiform corrosion.

Pitting corrosion

Pitting corrosion, also known as pitting, is another localized form of corrosion that occurs on metal surfaces. Pitting typically manifests itself as small diameter cavities or holes on the object’s surface while the remainder of the metallic surface remains unattached. This form of corrosion is also highly penetrative and is considered to be one of the most dangerous types of corrosion because it is difficult to predict and has a tendency to cause sudden and extreme failures.

Pitting usually originates on areas of the metal surface where inconsistencies in the protective passive film exist. These inconsistencies may be due to film damage, poor coating application or foreign deposits on the metal surface.

Areas where passivity has been reduced or lost now become the anode while the surrounding regions act as the cathode. In the presence of moisture, the anode and cathode form a corrosion cell where the anode (i.e., the areas unprotected by the passive film) corrodes. Because the corrosion is confined to a localized area, pitting tends to penetrate the thickness of the material

Crevice corrosion

Crevice corrosion is a highly penetrative type of localized corrosion that occurs in or directly adjacent to gaps or crevices on the surface of a metal. These crevices can be the result of a connection between two surfaces (metal to metal or metal to non-metal), or by an accumulation of deposits (dirt, mud, biofouling, etc.).

This type of corrosion is characterized by deterioration in the area of the crevice while the surrounding areas of the metal substrate remain unaffected. One of the main criteria for the development of crevice corrosion is the presence of stagnant water within the crevice. This lack of fluid movement gives rise to the depletion of dissolved oxygen and an abundance of positive ions in the crevice.

This leads to a series of electrochemical reactions that alters the composition of the fluid and makes it acidic in nature. The acidic liquid in the crevice breaks down the metal’s passive layer and renders it vulnerable to corrosion attack.

Filiform corrosion

This corrosion occurs under surfaces that have been painted or coated. Defects in the paint or coating allow water to intrude, thereby causing corrosion below the protective layer, resulting in a weakened structure.

3. GALVANIC CORROSION

Galvanic corrosion is the result of a very specific set of conditions. It is only found in environments where there are electrochemically dissimilar metals in electrical contact that are also exposed to an electrolyte. This corrosion happens when galvanic coupling occurs between the anodic and cathodic metals. The anode corrodes faster by being coupled, while the cathode deteriorates more slowly.

4. ENVIRONMENTAL CRACKING

This corrosion process occurs when environmental conditions arise that negatively affect carbon steel. Chemicals, stress, and temperatures can create conditions that produce stress corrosion cracking (SCC), corrosion fatigue, liquid metal embrittlement, and hydrogen-induced cracking.

5. FLOW-ASSISTED AND INTERGRANULAR CORROSION

Flow-assisted corrosion occurs when the protective oxide layer is dissolved over time by the flow of wind or water. This corrosion exposes the oxide on the surface of the metal, exposing subsequent layers to further corrosion.

Intergranular corrosion attacks the grain boundaries of metal, often as a result of metal impurities. Impurities are frequently present in higher concentrations near these grain boundaries, making them more susceptible to this type of corrosion. 

6. FRETTING CORROSION

This type of corrosion occurs as repeated weight, vibration, or wearing causes pits and grooves on the surface of the metal. This occurs most often in machinery parts in motion, or surfaces that suffer vibration as they are transported from place to place.

4. HIGH-TEMPERATURE CORROSION

High-temperature corrosion can occur from oxidization, sulfidation, or carbonization, or from fuels that contain vanadium. Sulfates can also form corrosive compounds that will attack carbon steel which is normally resistant to high temperatures and corrosion.

8. SOIL CORROSION

Soil corrosion is seen when carbon steel is exposed to moisture and oxygen in the surrounding soil. Soils with high moisture content, high electrical conductivity, high acidity, and high dissolved salts are the most corrosive.

Because carbon steel accounts for approximately 85% of total steel production worldwide, it becomes necessary to be knowledgeable about the things that cause it harm. Efforts to understand and manage carbon steel corrosion can help mitigate and assuage the high costs associated with this common concern.

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

There are several cost-effective ways to prevent corrosion including:

  • Use non-corrosive metals, such as stainless steel or aluminum.
  • Make sure the metal surface stays clean and dry.
  • Use drying agents.
  • Use a coating or barrier product such as grease, oil, paint or carbon fiber coating.
  • Lay a layer of backfill, for example limestone, with underground piping.
  • Use a sacrificial anode to provide a cathodic protection system.

You can prevent corrosion by selecting the right:

  • Metal Type
  • Protective Coating
  • Environmental Measures
  • Sacrificial Coatings
  • Corrosion Inhibitors
  • Design Modification

1. 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 charges 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. A number of 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 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.

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.

Passivation

In passivation, a light coat of protective material, such as metal oxide, creates a protective layer over the metal which acts as a barrier against corrosion. The formation of this layer is affected by environmental pH, temperature, and surrounding chemical composition.

A notable example of passivation is the Statue of Liberty, where a blue-green patina has formed which actually protects the copper underneath. Corrosion inhibitors are used in petroleum refining, chemical production, and water treatment works.

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.