Unit 2- Class 9- CORROSION (Corrosion Control)

CORROSION INHIBITORS:

Using certain chemical substance known as inhibitors can reduce rate of corrosion.  Inhibitors slow down the anodic or cathodic reactions by forming a protective film on these regions inhibitors mostly used to provide protection to systems in which corrosion environment re-circulated or confined for a long time like IC engine, re-circulated water system etc.


 ANODIC INHIBITORS:
These include oxidizing agents like chromates, molybdates, tungstates and nitrates.  These anions react with metal ions formed at anode during oxidation reaction forming sparingly soluble respective salts.  These compound are deposited on the anodic sites forming a protective films so that further preventing the anodic reactions.
    These inhibitors are found to be effective only when sufficient amount of inhibitors is added into corrosion medium.  So that entire anodic surface can covered other wise intense corrosion takes place.

ii) CATHODIC INHIBITIORS:
These can act by inhibiting the cathodic reaction, which involves liberation of hydrogen or absorption of oxygen.
If the cathodic reaction is liberation of H2 type then by adding organic inhibitors like amines, Urea, thiourea, heterocyclic compound.  They form a protective layer on cathodic region so that evolution of hydrogen gets retarded.  Evolution of H2 can also prevent by increasing hydrogen over voltage.
This can be achieved by adding oxides of arsenic or antimony.
           
If the cathodic reaction is oxygen absorption type then by adding certain oxidizing agent like N2H4 (hydrazine), Sodium sulphite, these compounds absorb oxygen as

       
        N2H4 + O2------------->N2 + 2H2O

      2Na2SO3 + O2----------->2Na2SO4

So that cathodic reaction is retarded otherwise by adding inorganic inhibitors like sulphates of Zinc, Mg, Mn etc. They reacts with OH- ions (which are liberated at cathode) forming insoluble hydroxides which form a protective film over cathodic area and hence corrosion rate reduced. Corrosion inhibitors have certain limitations
1. They contaminate environment
2. Many inhibitors are toxic in nature.
3. It can be used only in   closed systems
4. Generally inhibitors lose their efficiencies as conc. And temperature increases.

Cathodic protection:
It is a method of protecting the metal or alloy from corrosion and no part of it is allowed to act as anode.
The technique of offering cathodic protection to a specimen (metal) against corrosion by providing electrons from an external source.
There are two methods for providing electrons for cathodic reactions there are

1. Sacrificial anode method:
 The method involves the use of more active metals as sacrificial anode in contact with specimen (like iron, copper or brass). The active metals like Zn, Mg, Al, and their alloy acts as an auxiliary anode, and undergoes preferential corrosion protecting the metal structure. Here the anode metals are sacrificed to protect the metal, the method is known as sacrificial anode method, exhausted anodes are replaced by new ones as and when required.
Egs: 1. A Mg or Zn block connected to a buried oil tank
        2. Mg bars are connected to ocean going ships
         
Sacrificial anode methods are simple with low installation cost and do not require power supply but involves recurring expenditure of replacement of consumed anodes.
2. Impressed current method:
This is another method to provide cathodic protection by supplying electrons. These can be provided by a source of direct current.
The structure to be protected should be made negative (connected to negative terminal of a D.C source). Resin bonded graphite rod, platinised Ti, Pb are used as anode and connected to the positive terminal. The main structure being cathode does not undergo corrosion, and anode being inert remains unaffected.    

This technique is used to protect marine structure, water storage tanks and other gas or oil pipelines. This method is simple, can protect large metal area with low maintenance cost but expensive, because it needs high current.

Anodic protection:
Certain metals like Ti and alloy like stainless steel, which readily get passivated so that cathodic protection cannot be offered in such cases corrosion process, can be slowed down by use of anodic current. BY passing anodic current in these metals an oxide layer will grow and that oxide layer will protect the metals. The potential that is required to grow the oxide film and to protect the metal can be obtained from potential-current curve.

At a predetermined range of applied potential the changes observed in the potential and the corresponding changes in current are studied. In initial stages (AB) the current increases on increasing potential indicates the dissolution of metal so corrosion takes place. When the current reaches a critical point, (Icrit ), passivation, that is due to development of oxide layer sets in and that potential  ( Ep) is called passivating potential (Ep). Above Ep along BC current flow decreases to a very small value called passivating current ( Ip). It is the minimum protective current density to maintain passivation. At this point (C ) an increase in potential will not corrode the metal since the metal is in highly passive state. The small current (Ip) is sufficient to maintain the passivity so that corrosion rate brings down.
The anodic protection to a structure is applied by using device called potentiostat. It is an electronic device that maintains a metal at a constant potential with respect to a reference auxiliary Pt electrode and a reference calomel electrode.

The metal structure kept in a suitable oxidizing atmosphere acts as the working electrode ( anode). The potential is then slightly increased to Ep (passivating potential) for the initial corrosion to start. The potential is then slightly increased till the current decreases to very small value (Tp) indicating passivation of metal.  Now the potential is kept at constant Ep and the current is maintained at Ip.

    This method has an advantage as it requires very small current, but this method can be adopted to only passive metals like Ti, Ta,Al,Cr etc., and also by using this method corrosion rate cannot be reduced to zero.

    This method usually applied in the transportation of acid.

This ends the corrosion unit. In the next classes, we will start up with polymers. Until that revise the two units that have been done .