POLARIZATION : POLARIZATION BY :-
AJAY SINGH PARIHAR
ROLL NO. 05
VII SEM Slide 2: CORROSION
CORROSION IS DESTRUCTION OF MATERIAL BY CHEMICAL ,ELECTRO-CHEMICAL,AND METALLUREGICAL INTERACTION WITH THE ENVIRONMENT.
THE REACTION BETWEEN A METAL AND ITS ENVIRONMENT WHICH RESULTS IN A TRANSFORMATION OF THE METALLIC TO THE COMBINED STATE. DRIVING FORCE OF A CORROSION REACTION(Corrosion Tendencies) : DRIVING FORCE OF A CORROSION REACTION(Corrosion Tendencies) A driving force is necessary for electrons to flow between the anodes and the cathodes.
This driving force is the difference in potential between the anodic and cathodic sites.
The potential is a measure of this tendency. Slide 4: Any change in the potential of an electrode from its equilibrium value is known as POLARISATION.
EQUILIBRIUM POTENTIAL:- when a metal is immersed in a solution of its own ions there will be an outgoing pressure of ions trying to enter the and an ingoing pressure of ions trying to enter the metal . Sooner or later these opposing pressures will come to equilibrium and ions will enter and leave the metal at equal rates; the potential of the metal at this point
is called the EQUILIBRIUM POTENTIAL. Slide 5: Now
if the piece of metal becomes the part of a galvanic cell this equilibrium is upset and the reaction will proceed in definite direction .
if the metal becomes more positive (anodic) it will corrode ; but if it becomes more negative (cathodic) the metal will plate out from solution.
These are called “anodic polarization” and “cathodic polarization” respectively. Slide 6: At an electrode it is possible to distinguish mainly 3 types of polarization :
Concentration Polarization Resistance polarization : Resistance polarization It is due to resistances in the cell circuit and results from- any IR drop at the electrode/solution interface ,
Or from the deposition of solid precipitates at cathode,
Or the formation of films of high resistivity on the surfaces of anodes. Concentration Polarization : Concentration Polarization Concentration polarization is the polarization component that is caused by concentration changes in the environment adjacent to the surface .
When a chemical species participating in a corrosion process is in short supply, the mass transport of that species to the corroding surface can become rate controlling.
A frequent case of concentration polarization occurs when the cathodic processes depend on the reduction of dissolved oxygen since it is usually in low concentration, i.e. in parts per million (ppm). Activation Polarization : Activation Polarization The term activation polarization makes reference to retarding factors that are an inherent part of the kinetics of all electrochemical reactions.
it is logarithmically dependent on current density i ,and the relationship and the relevant equation is of the form
µ = a + b log I
this is known as TAFEL EQ. POLARIZATION CURVE : POLARIZATION CURVE