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.
What is Corrosion?
Corrosion is defined as a natural process that causes the transformation of pure metals into undesirable substances when they react with substances like water or air. This reaction causes damage and disintegration of the metal, starting from the portion of the metal exposed to the environment and spreading to the entire bulk of the metal.
Corrosion is usually an undesirable phenomenon since it negatively affects the desirable properties of the metal. For example, iron is known to have good tensile strength and rigidity (especially alloyed with a few other elements).
However, when subjected to rusting, iron objects become brittle, flaky, and structurally unsound. On the other hand, corrosion is a diffusion-controlled process, and it mostly occurs on exposed surfaces.
Therefore, in some cases, attempts are made to reduce the activity of the exposed surface and increase a material’s corrosion resistance. Processes such as passivation and chromate conversion are used for this purpose. However, some corrosion mechanisms are not always visible, and they are even less predictable.
On the other hand, corrosion can be classified as an electrochemical process since it usually involves redox reactions between the metal and certain atmospheric agents such as water, oxygen, sulphur dioxide, etc.
How & Why Does Corrosion Occur?
Corrosion occurs when atoms of the entire metallic surface become oxidized as a reaction to their environment. Hence, metals that are easily oxidized tend to corrode much faster than others. This is called general corrosion.
Localized corrosion, on the other hand, occurs only on a small area of the metallic surface. Sometimes this corrosion can even lead to small holes, or “pitting”.
Additionally, other kinds include galvanic corrosion, where only one of the two metals is affected. This happens especially when two different types of metal are placed in the vicinity of some kind of liquidized electrolyte. In such circumstances, one metal’s molecules may be drawn towards the other metal, thus causing corrosion in the other.
So, we can see that corrosion is largely determined by environmental factors, the gases they are surrounded by and other elements and metals that might be around. With that in mind, there are a few ways we can slow down or even prevent this process from happening.
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.
Do All Metals Corrode?
Most metals corrode. Some like iron corrode quickly, while others naturally corrode more slowly (e.g. zinc) or corrode slowly because they are an alloy of various metals (e.g. stainless steel).
A small group of metals, called the Noble Metals, are much less reactive than others. As a result, they rarely corrode. They are, in fact, the only metals that can be found in nature in their pure form. The Noble Metals, not surprisingly, are often very valuable. They include palladium, silver, platinum, and gold.
Types of Corrosion
Many different types of corrosion 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.
i). 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
2). 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.
3). 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.
#7. 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.
The rusting of iron can be prevented by greasing, painting, galvanizing, anodizing, or oiling the surface. These methods can be classified into the following categories:
- Galvanization: Galvanized metal is coated with a thin layer of zinc to protect it against corrosion. The zinc oxidizes when it is exposed to air creating a protective coating on the metal surface.
- Alloying: It is the method of improving the properties of a metal by mixing the metal with another metal or nonmetal. When iron is alloyed with chromium and nickel in stainless steel is obtained. Stainless steel does not rust at all.
- Painting: Rusting of iron can be easily prevented by coating the surface with paint which protects iron from air and moisture.
- Greasing/Oiling: When some grease oil is applied to the surface of an iron object, then air and moisture cannot come in contact with it, and hence rusting is prevented.
Natural protection
Some metals acquire a natural passivity, or corrosion resistance. This occurs when the metal reacts with, or corrodes in, the oxygen in air. The result is a thin oxide film that blocks the metal’s tendency to undergo further reaction.
The patina that forms on copper and the weathering of certain sculpture materials are examples of this. The protection fails if the thin film is damaged or destroyed by structural stress — on a bridge, for example — or by scraping or scratching.
In such cases the material may repassivate, but if that is not possible, only parts of the object corrode. Then the damage is often worse because it is concentrated at these sites.
Harmful corrosion can be prevented in numerous ways. Electrical currents can produce passive films on metals that do not normally have them. Some metals are more stable in particular environments than others, and scientists have invented alloys such as stainless steel to improve performance under particular conditions.
Some metals can be treated with lasers to give them a non-crystalline structure, which resists corrosion. In galvanization, iron or steel is coated with the more active zinc; this forms a galvanic cell where the zinc corrodes rather than the iron.
Other metals are protected by electroplating with an inert or passivating metal. Non-metallic coatings — plastics, paints, and oils — can also prevent corrosion.