Corrosion - Definition and types

Lots of metals are seen to change their colors, textures, and densities when exposed to air and moisture for an extended period of time. The picture above shows a car that was once new and shiny, but it has been broken down and roughened by reacting with components of the atmosphere. Another picture below shows a decaying and rough looking copper pipe that has experience partial degradation. In time, both metals will all be flakes.

 This two scenarios obviously shows that there is some form of invisible chemical reaction that leads to the irreversible damage of these metals when exposed to an open environment for a long time. Such chemical process is known as corrosion.

Definition of corrosion 

Corrosion is a process involving a metal being reconverted from it refined form to a  more chemically stable form such as oxides, hydroxides or sulfides  by natural means. 

During the process of corrosion, a metal is permanently and irreversibly degraded and broken down due to chemically and/ or electrochemically reacting with it environment.

Why does corrosion happens?

Metals are often gotten from their respective ores. Before any metal can be useful to create anything, it must first be extracted and refined from it ore. This ores can exist in form of oxides, sulfides, or salt. For example, iron(III)oxide or hematite is a common ore where iron is extracted, likewise in copper(I)sulfide for copper. These ores are most stable form in which  these metals can exist, and they always tend to remain in this stable forms.  So when extracted or refined for industrial purposes, they are less stable and they will look for any possible reaction in order to revert to stability.

Corrosion happens as the refined metals attempt to revert to their more stable raw or ore form.

Factors affecting corrosion

The reaction that causes corrosion can lead a metal to go back to it oxide or sulfide state when it react with an environments that contains traces of these compounds. The following are some factors influencing the rate or corrosion.

• Position of the metal in the electrochemical cell

The more reactive metals are easily corroded because when left exposed, it will have a higher tendency to to react with other elements like oxygen to form basic oxides. So the reactivity of the metal affects the corrosion

• Air or Oxygen.

It is the most important thing for corrosion i.e. oxidation of metal. More the exposure of metal to the air, more it will corrode. Most of the metals are corroded by the oxygen bust some of them by other gases like Copper by CO2, Silver by H2S, etc. For e.g. Copper develops a green colour coating on its surface due to the presence of air , moisture and carbon dioxide forms basic green malachite Cu(OH)2.CuCO3.

• Moisture

Moisture lead to formation of coordinate complex between metal oxide and water molecules like (Fe2O3. nH2O), etc. Moisture is present in the form of Water Vapour or capillary water in soils.

• Presence of other chemicals

Sometimes metals are corroded by the action of certain other chemicals like acids and bases.