08-16-2017, 09:44 PM
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corrosion
The process of decay of metal by environmental attack is called corrosion.
The basic reason for this attack is most of the metals (except Pt, Au, Ag) exist in nature in the form of their ores as oxides, chlorides, silicates, carbonates etc.,
Metals have the natural tendency to go back to their combined states, as a result when metal is exposed environmental conditions forms stable compounds of metals, known as corrosion.
The process of corrosion is reverse of metal extraction.
Effects of the corrosion process
CHEMICAL (OR)DRY CORROSION
OXIDATION CORROSION
If the formed metal oxide is unstable corrosion is not occur.
If the formed metal oxide is volatile ,fresh metal surface is rapidly exposed and converted into metal oxide and evaporated.
If the formed metal oxide layer is porous , under laying metal is attacked and converted in to metal oxide. the total metal is converted into metal oxide form.
Corrosion by other gases
Dry or chemical corrosion
This occurs at dry conditions
Corrosion is uniform
It is a slow process
It involves direct chemical attack
Explained by absorption mechanism
Wet or electrochemical corrosion
This occurs at wet conditions (electrolytic medium)
Corrosion Is not uniform
It is a rapid process
It involves formation of electro chemical cells
Explained by mechanism of electro chemical reactions