Unit 2- Class 4- CORROSION (Factors affecting rate of corrosion)

 In this class let us study the factors affecting rate of corrosion .

Factors affecting rate of corrosion: 

Several factors affecting the rate of the corrosion, which can be divided into two parts 
1.    Factors affecting on metal (related to the metal)
2.    Factors affecting on corrosive environment.


Factors affecting the metals:
1.    Nature of the metal: The tendency of the metal to undergo corrosion is mainly dependent on the nature of the metal. IN general the metal with lower electrode potential have more reactive and more susceptible for corrosion and metal with high electrode potential are less reactive and less susceptible for corrosion for egs: metals like K, Na, Mg, Zn etc have low electrode potential are undergo corrosion very easily, where as noble metals like Ag, Au, Pt have higher electrode potential, their corrosion rate are negligible but there are few exception for this general trend as some metals show the property of passivity like Al, Cr, Ti, Ta etc.
2.    Surface state of the metal or nature of the corrosion product (passivity):
The corrosion product is usually the oxide of the metal; the nature of the product determines the rate of further corrosion process.
If the oxide layer, which forms on the surface, is stoichiometric, highly insoluble and non-porous in nature with low ionic and electronic conductivity then that type of products layer effectively prevents further corrosion, which acts as a protective film. For egs: Al, Cr, Ti develop such a layer on their surface and become passive to corrosion and some metal like Ta, Zr and Mo not only forms such a protective layers but are capable of self repairing oxide films when it is damaged. Hence these are extremely passive metals.
If the oxide layer forms on the metal surface is non-stochiometric, soluble, unstable and porous in nature and have an appreciable conductivity, they cannot control corrosion on the metal surface for egs: oxide layer formed on metals like Zn, Fe, Mg etc.
3.    Anodic and Cathodic area:
The rate of the corrosion is greatly influenced by the relative sizes of cathodic and anodic areas.
If the metal has smaller the anodic area and larger the cathodic area exposed to corrosive atmosphere, more intense and faster is the corrosion occurring at anodic area because at anode oxidation takes place and electrons are liberated. At the cathode these electrons are consumed. When anode is smaller and cathode region is larger all the liberated electrons at anode are rapidly consumed. This process makes the anodic reaction to takes place at its maximum rate thus increasing the corrosion rate. If the cathode is smaller and reverse process takes place decrease rate of corrosion.
For egs: If tin (Sn) coated on iron (Fe) and in that some area are not covered or some pin holes are left, there forms smaller anodic area and larger cathodic area because tin is cathodic with respect to iron so intense localized corrosion takes place. On the other hand if Zn coated to Fe then if there are some pin holes are there creates larger anodic area and smaller cathodic area because Fe is cathodic with respect to zinc so that rate of corrosion is very less.
4.    Hydrogen over voltage:
A metal with low hydrogen over voltage on its surface is more susceptible for corrosion. When the cathodic reaction is hydrogen evolution type with low hydrogen over voltage, liberation of H2 gas is more easier so that cathodic reaction is very fast, that makes anodic reaction faster hence overall corrosion process is very fast. If the H2 over voltage is high so cathodic reaction is slow hence corrosion reaction also slower.