Most of the metals are naturally available in a chemically shared condition which we know as ORE. It is very usual to get an oxide, a carbonate, a sulphide or another compound in nature. In another words, we can say that most of the metals are naturally available in a mixed state. The reason for this mixed/compound condition is their low energy states as ores or other compounds. In order to take apart a metal from its ores, (for ex – Ferrous oxide), it is required to provide a huge amount of energy. This is achieved by heating with the help of a reducing agent in a blast furnace upto 1600 °C. Pure Metals are normally in high-energy states.
According to Thermodynamics, there is a strong tendency of elements having high energy state to transform into low energy states. It is why, metals recombine with other elements easily available in the environment which in turn further leads to the process of corrosion. The most common type of corrosion is Oxidation Corrosion.
In case of metals, they react with belligerent kind of other elements to form stable metallic oxides. This type of reaction is called Oxidation corrosion. So, we can say that Oxidation corrosion is the degradation of a metal from high energy state to low energy state by an electrochemical reaction with its environment.
Oxidation Corrosion mostly occurs in two common ways:
(a) over the entire surface of the metal i.e Generalised oxidation Corrosion
(b) in local spots or areas called pitting i.e Localised oxidation Corrosion
Generalized Oxidation Corrosion:
This uniform corrosion over the entire surface of the metal is very rare. It results in overall thinning of the metal which has little effect. It happens only in acidic conditions.
Localised Oxidation Corrosion:
The most common and the most affecting form of localised oxidation corrosion is pitting. Pitting is so called as this oxidation corrosion happens in only one single location on the surface and creates a pit or small cavity. This type of oxidation corrosion attack is hard to prevent and difficult to detect before structural failure is met due to cracking. Pipes are the usual victims and most often compromised due to pitting.