Several metallic materials in course of the time develop chemical changes on their surface, if left unused, or stagnant. These changes are known as corrosion. Corrosion is a process that leads metals to a gradual degradation. It will happen on iron and its alloys such as steel. Rusting of iron is one of them. Iron objects react with the oxygen present in the air and develop rust in a humid environment. The other examples of corrosion are tarnish on silver and the blue-green patina on copper.
Rust is basically an iron oxide. Mostly, red oxide is formed by the redox reaction between oxygen and iron in the presence of air moisture and water. It was researched that surface rust is friable and flaky and does not provide any protection to the iron. ‘Green rust’ is developed when iron reacts with chloride in the presence of water and oxygen which is mostly found in the underwater iron pillars.
What is the Chemistry Behind the Rusting of Iron?
The formation of rust takes place in the presence of water and oxygen on iron or some of its alloys. The reaction needs a considerable long time to develop. The formation of bonds between iron atoms and oxygen atoms makes iron oxides. The rusting of iron includes an upsurge in the oxidation state of iron with a loss of electrons. The rust chemical formula can be written as Fe2O3.3H2O (hydrated iron (III) oxide).
Rusting of iron reaction: 4Fe + 3O2 → 2Fe2O3
Fe2O3 reacts with water and forms Fe2O3.3H2O.
Rust is developed from two different iron oxides that are different due to their oxidation state in the iron atom. These oxides are
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Iron (II) oxide or ferrous oxide – The oxidation state of iron in this compound is +2 and its chemical formula is FeO.
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Iron (III) oxide or ferric oxide – in which the iron atom shows an oxidation state of +3. The chemical formula of this compound is Fe2O3.
We all know that oxygen is an excellent oxidizing agent while iron is a reducing agent. So, the iron atoms willingly provide their electrons to oxygen when exposed to it. The rusting of iron involves the process:
Fe → Fe2+ + 2e–
The oxidation state of iron changes due to the oxygen atom in the presence of water.
The ferrous ions get oxidized to ferric ions in presence of moisture and air, also generating hydroxyl ions and yielding ferric hydroxide.
4Fe2+ + O2+ 2H2O →4Fe3+ + 4OH–
Fe3+ +3OH– → Fe (OH)3
Fe (OH)3 converts into Fe2O3.3H2O.
Factors that Affect Rusting of Iron
All the chemical reactions of rusting are dependent on the presence of water and oxygen. The rusting of iron can be controlled by restricting the amount of oxygen and water surrounding the metal.
Why is Rusting an Undesirable Phenomenon?
Rusting is known as a great destroyer of things. It can destroy cars and other vehicles, sink ships, fell bridges, spark the fire, and destroy everything that is made up of iron or its alloys.
The entire piece of metal may disintegrate and be turned to rust when left unattended for extended periods. This can cause many problems as iron is used to construct buildings, bridges, automobiles, etc. Rust makes the metal weaker as oxidized metal is weaker than the original metal itself. It also makes the metal brittle and puts it at risk of breaking.
Iron is also used to make water pipes and storage tanks. If this iron rusts, the pipes can get damaged. They can also increase the amount of iron oxide in the water being transported. Rust also acts as a breeding ground for bacteria. If a person is injured by rusted iron, he/she could be at risk for tetanus.
How can Rusting be Prevented?
Rusting can be prevented by many methods. One method is to keep iron from corrosion by painting it. The layers of paint resist oxygen and water to form rust on the surface of iron as paint prevents iron from contacting them directly. The iron is protected from corrosion as long as the paint is there. Oil-based paints are hassle-free and the most highly recommended. Alternatively, any organic paint with a 15-25µm thickness may be used to prevent rust.
Alternatively, rusting can be prevented by thermoplastic or a thermoset polymer powder coating on the iron surface. Powder coating is considered superior to paint as it gives a thicker protective layer. Spraying a dry, organic powder onto the iron surface and heating the iron to the melting point of the powder. Once melted, the powder creates an even layer over the iron surface. Common materials used for powder coating include vinyl, polyester, nylon, acrylic, urethane, and epoxy-based organic materials.
Other strategies include iron alloying with other metals. For example, stainless steel is mostly made up of iron with a little amount of chromium.
In a different strategy, iron is galvanized or zinc-plated. Zinc has a lower reduction potential which enables it to oxidize more easily than iron. Zinc is a more active metal. This process is known as galvanization. The metal (iron) is covered with another metal such as zinc to form a protective layer. Galvanization can be done in two ways:
Hot-dip Galvanization: that involves dipping the iron into a very hot bath of melted zinc
Electro-Galvanization: Involves using zinc metal as an anode, iron as the cathode, and passing electricity through a zinc solution to apply an even coating of zinc on the iron surface.
Electro-galvanization is the preferred method of galvanization today as it produces an even coating, unlike the hot-dip method.
Cathodic Protection
An important method to protect iron from rusting is to make it a galvanic cell cathode. This process is known as cathodic protection. It can be used for metals, not only for iron. In this process, iron is connected with a more active metal such as magnesium or zinc. The more active metals have a lower reduction potential. Then, the other metal (iron) behaves as a cathode and does not get oxidized. This process is highly u
seful to the storage of iron tanks underwater when anodes are monitored properly and replaced timely. This process is also used to protect metal parts of water heaters.