Rusting – definition, formation, enhancers, and prevention

Rusting – definition, formation, enhancers, and prevention

Rusting is an oxidation process that occurs very slowly in iron-on exposure to moisture. Upon exposure of a piece of metal made of iron or its alloy, over time, a reddish-brown substance called rust (a chemical compound) begins to cover the metal surface; a process called corrosion.

How is rust formed?

Rust can only form on iron in the presence of water or moisture. This moisture combines with the iron to give a chemical compound. The chemical equation below summarizes the process that occurs in the formation of rust.

4Fe + 3O2 + xH2O → 2Fe2O3.xH2O

Where
Fe represents iron metal
3O2 represents three molecules of oxygen gas.
xH2O represents some molecules of water (x can be any value) and
2Fe2O3.xH2O represents hydrated iron (III) oxide (rust).

From the equation above, it can be seen that both oxygen and water are necessary in the formation of rust.

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Enhancers of rust formation

Apart from these factors above, some other chemicals highlighted below are known to increase the formation of rust.

  • Carbon dioxide (CO2)
    Carbon dioxide is present in the atmosphere and is contained in the air we breathe out. It combines with water to form carbonic acid.

CO2 + H20 → H2CO3

Where
CO2 represents carbon dioxide
H2O represents water and
H2CO3 represents carbonic acid.

Carbonic acid then dissociates (is split) to give hydrogen ion (H+) and bicarbonate ion (HCO3). The bicarbonate ion formed attacks the iron and causes corrosion.

  • Acidic environment
    When the surrounding pH is low, it leads to a process similar to that described above by carbonic acid.
  • Salt
    Salty water corrodes metals even faster than normal water; this is because salty water contains ions, hence it increases the rate at which metal ions are disintegrated.
  • Increased temperature
    Rusting is a chemical process, and many chemical processes are enhanced by an increase in temperature. An increase in temperature increases the energy available for the reaction to proceed.

Disadvantages and Dangers of rusting

  • Reduction of metal quality.
    The process of rust formation is one that gradually ‘eats up’ the metal and reduces its viability and use. Moreover, rusting is a continuous process and once it begins, unless prevented, it continues until the metal is destroyed totally; as a result, if corrosive metals are used for important structures, these structures are likely going to get worse with time. Rusted iron has a lesser quality and cannot be used for its due purposes
  • Environmental pollution.
    Metals cannot decay as they are non-biodegradable; it becomes worse when the metal begins to rust, because in addition to the fact that it cannot be used, it cannot decay and as the corrosion progresses it leads to a continuous release of chemicals to the atmosphere and the soil which can be toxic.
  • Waste of resources.
    If you have a cup made of iron, and the cup begins to rust, you’ll have to change it and this will cost you a part of your resources.

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How can rusting be prevented?

  • Coating
    Just like humans wear coats, iron can also be coated with substances that will prevent contact with oxygen and moisture. The most common form of coating is painting; it provides a layer over the iron and prevents corrosion. Grease and oil can also be applied as coats over metal; other forms of coating include blueing and application of powder coats such as epoxy or vinyl to prevent rust.
  • Galvanizing
    This is also a form of coating that is done with a metal that resists corrosion more than iron especially zinc or aluminium. Galvanizing prevents direct attack to the metal as the metal used to cover the iron is corroded instead of iron.
  • Alloying
    An alloy is a substance formed when two or more substances (one of them being a metal) are mixed together. These substances are melted, and combined to give a new product that has desirable features.
    Examples of alloys are:
    Brass – an alloy of copper and zinc
    Stainless steel – an alloy of iron, chromium and nickel
    Bronze – an alloy of copper, aluminium, manganese, nickel, zinc and other non-metals.
    Amalgam – an alloy of mercury with another metal.

When metals are alloyed into other forms, the rate at which corrosion occurs is totally slowed down. Metallic cups are made with steel rather than iron because of this purpose.

The best way of preventing rust in any form of metal is to keep it in a cool and dry place.

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