“An effective engineering lies in controlling of corrosion rather preventing it, because it is not possible to eliminate corrosion completely”
Differential Metal Corrosion (Galvanic Corrosion)
As its name should suggest this kind of corrosion occurs when two different type of metals are in contact. Here if we observe the process of working of Electrochemical (Galvanic) cell we can see we have the two, anode and cathode are joined so we don’t need any salt bridge this time.
As discussed earlier also, the combined state of metal is it is natural state because it is more stable that the metal state and hence the corrosion is a spontaneous process (Thermodynamics) Because of it we have the top quote.
Lower of the two electrode becomes the anode while one at the higher potential become the cathode
- The driving force is the difference in tendency of oxidization and reduction, so which ever metal has a higher reduction tendency will get reduce which means will form the hydroxide or oxide i.e. will corrode.
- So we can conclude from that, that higher is the driving force or the difference in tendency (Reduction potential) the faster will be the process of corrosion will occur.
Differential Aeration Corrosion
This type of corrosion is due to electrochemical attack on the metal surface, exposed to an electrolyte of varying concentrations or of varying aeration. This may be the result of local difference in metal-ion concentration, which causes slow diffusion of metal ions, produced by corrosion.
We have the fairly common reactions in corrosion,
Fe ———- > Fe+2 + 2e– (oxidation of metal anodic reaction)
O2 + 2e– +H2O ——— > 2OH– (Reduction cathodic reaction)
Both the reaction can and do occur in both part of metal in and out of water, But what matters is the difference in the amount i.e. we need water and metal which is sufficient for corrosion all across, but the other part of oxygen in water is short, so oxygen or air is like here the limiting reagent of this reaction.
So the reduction or the cathodic part in one over the top, so this is the cathodic part, while one inside is the anodic part.
Now, the circuit is completed by migration of ions, through the electrolyte, and the flow of electron through the metal and the medium from anode to cathode.
There is a similar example of such type of corrosion when a drop of water makes a similar case when it is on a metal piece.
General facts about differential aeration corrosion:
- Corrosion may be accelerated in apparently inaccessible places, because the oxygen deficient areas serve as anodes and, therefore cracks or crevices serve as foci for corrosion,
- Corrosion is accelerated under accumulation of dirt sand, scale or other, contamination, the corrosion will be non-uniform just because as the sand and dirt do the lost of air contact.
- Metal exposed to aqueous media corrode under blocks of wood or pieces of glass, which screen that portion of metal from oxygen access. It is going to cause localized pitting attack becomes more intensified with time as the corrosion produce
There is a lot of types and a few more we need to discuss in the next part of this lecture part -II of types of corrosion.
Abhishek kumar jha
(Chemistry at Utkarshini)