8/3/2019 Jaimin Ppt

1/14

THROWING POWER AND COVERING POWEROF AN ELECTROCHEMICAL REACTIONS

Metallurgical & Materials Engineering Department,Faculty of Technology & Engineering,

The M.S. University of Baroda,Vadodara

GUIDED BY:Dr.V.V.MATHANE SIR

PREPARED BY:JAIMIN PANCHAL.V.

8/3/2019 Jaimin Ppt

2/14

ELECTROLYTIC CELL & ELECTROLYSIS:

Electrolytic cell is reverse of electrochemical cell.current issupplied to carry out the chemical reactions.

Electrolytic cell consists of following four main components:(1) electrolyte(2) anode(3) cathode(4) external power supply

An electrolytic cell should be design to meet followingrequirement:(1) it should consume little energy per unit weight of metal.(2) it should have a high output for a small volume and workingarea.(3) it should be corrosion resistant to the electrolyte.(4)it should be resistant to heat.

8/3/2019 Jaimin Ppt

3/14

ELECTROLYSIS may be defined as a process in which chemicalreaction are brought about on the electrode in contact with anelectrolyte by means of external electromotive force.

during electrolysis positive charged ions (cation)are attracted towards the negatively charged electrodes(cathode) while the negatively charged ions are attractedtowards the positively charged electrodes (anode).

8/3/2019 Jaimin Ppt

4/14

THROWING POWER: The closeness with which the complete uniformity isapproached in the deposit over the whole area of thecathode is assigned to a characteristic of system known asthrowing power.

It describes the ability to balance the current density ondifferent parts of cathode.

The throwing power is complex property and is influenced bymany factors given below.

8/3/2019 Jaimin Ppt

5/14

Primary current distribution : this expresses the way inwhich the current varies over different parts of the surface. In a working cell the concentration of the electrolyte

immediately adjacent to the cathode is weaker than in thebulk of the solution due to incomplete replacement of thedischarged ions. the concentration overpotential arises.Thus in corners and deep crevices, the combined effect oflocal high electrolytic resistance and concentrationoverpotential hinders the electrodeposition at these plates. There is another factor known as activation overpotentialat the electrodes counteracting the effect of electrolyticresistance and concentration overpotential. As activation overpotential is dependent on current density,

activation overpotential will be least in the recessed areaand greatest on the prostrusions of the cathode , thus due tothis factor there should be uniform deposition becausecurrent distribution is better than theoretical value.

8/3/2019 Jaimin Ppt

6/14

The third factor is the cathode efficiency which may eitherincrease or decrease with increasing current density . In thesecond case in the recessed area having lower density , therate of deposition will be higher as compared to protrusions ,increasing the tendency to have a uniform electrodeposit.

Thus throwing power is not specific or inherent property of asolution but is the resultant effect of the three factorsmentioned above.

8/3/2019 Jaimin Ppt

7/14

PHYSICAL FACTORS AFFECTING DISTRIBUTION OF METAL : Position of the electrode in relation to each other and theplating tank : it should be properly controlled . Shape of the anode : the shape should follow the maincontours of the cathode. Agitation : the electrolyte should be sufficiently agitated so asto minimize both local high electrolytic resistance and highconcentration overpotential over recessed areas of thecathode. Temperature : the throwing power can also be improved byvariation in the temperature .for example in case of chromiumplating solution , the throwing power decreases slightly withrise in temperature.

8/3/2019 Jaimin Ppt

8/14

CHEMICAL FACTOR :

Composition of electrolyte : the throwing power can beimproved by altering the composition of the electrolyte.Greater is the difficulty to obtain the metal ion from the bath, greater is the throwing power.

For example by adding the h2so4 to cuso4 bath , because ofcommon ion effect of so42- , the amount of cu ion in thesolution is expressed and thus h2so4 increases throwingpower.

8/3/2019 Jaimin Ppt

9/14

ELECTRICAL FACTOR : The solutions exhibiting higher cathode potentials havehigher throwing power. From current density and current efficiency two cases mayarise.Case(1) : cathode efficiency may increase with increasingcurrent density.Case(2) : cathode efficiency may decrease with increasingcurrent density. Solution of 2nd type will have a higher throwing power . Avery good example is chromium plating solution , thethrowing power of which is negative , and it is inferior to themajority of other plating solutions.

8/3/2019 Jaimin Ppt

10/14

COVERING POWER : It refers to the ability of the solutions to cover a cathodecompletely with deposited metal.

The term covering power should not be confused withcovering power.

It is the ability of a solution to initiate metal deposition atlow current density areas. It is the minimum current densityat which the metal deposition will start to form and with alower current density than the minimum , no point of thespecimen will be plated.

8/3/2019 Jaimin Ppt

11/14

THIS CHARACTERISTIC CAN BE CONTROLLED BYCONTROLLING THE FOLLOWING FACTORS Attainment of deposition potential at low current densityareas .

Concentration of metal ions in the solutions .

Over voltage of the metal .

over voltage of hydrogen on the basis metal : this differs forvarious metals. The covering power is favored in the ordersteel, brass, copper and nickel.

8/3/2019 Jaimin Ppt

12/14

Presence of carbon : carbon has an adverse effect. Thisexplains why high current densities are required fordepositing chromium on cast iron surface.

Composition of the electrolyte : covering power can also becontrolled by controlling composition of the bath. It has beenseen that the throwing power is favored by high chromic :sulphuric acid ratio . but for covering power the reverse istrue. For example for good throwing power the ratioaccepted is 125:1 . But for covering passive surface such asnickel, ratios of 90:1 or even 75:1 are preffered.

Flashing the basis metal with metal of high over voltage : tinhas higher over voltage than iron. Thus in the zinc platingprocess if malleable cast iron is flushed with tin , the higherhydrogen over voltage of tin will enable the zinc to cover thesurface more completely including recesses.

8/3/2019 Jaimin Ppt

13/14

pH value : covering power of acid baths is generally poor ,covering power of alkaline bath is usually good .

Effect of roughness and polishing : better deposit on a roughsurface can be obtained if the sharp points are rounded offby polishing . Any polishing operation which has the effect ofremoving high points will be advantageous.

8/3/2019 Jaimin Ppt

14/14

THANK YOU