slides metal finishing

7/26/2019 Slides Metal Finishing

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Metal FinishingContents

Theory of electroplating

Polarization, decomposition potential & overvoltage

Characteristics of a good deposit

Factors influencing the nature of deposit

Methods of cleaning the metal surface

Hard chromium & decorative chromium plating

Electroless plating ! Copper plating

Metal Finishing - Dept. of Chemistry 1


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Learning Objectives

E"plain the theory of electroplating & factors affecting it

#ifferentiate electroplating & electrolessplating

#escri$e the methods of cleaning the metal surface

#iscuss the characteristics of a good deposit

%llustrate electroplating process of Cr & electrolessplatingof Cu

2Metal Finishing - Dept. of Chemistry


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Process of deposition of an adherent metallic

film of a no$le metal on a $ase metal orpolymer surface

Technological Importance

Methods of Metal finishing

Metal Finishing

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ELECTROPLATI!

Process in hich a thin layer of coating metal is deposited on a $ase

metal $y passing d'c' through an electrolytic soln', containing the

solu$le salt of the coating metal

Theor" of electroplating# (node, cathode, Electrolyte, #irect current

Mechanism of electroplating$ Three steps

%& '(l) diff(sion$ Mn* +b(l), Mn* +s(bstrate,

-& Atomic la"er formation$ Mn* *ne. Mad

/& Macro.deposition$ Mad * M +atoms, Ms(bstrate



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0chematic of electroplating bath



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Factors Governing ElectroplatingDecomposition otential !D.."

olari#ation

$vervoltage

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1ecomposition Potential Minim(m e2ternal potential at 3hich the electrol"sis c(rrent

begins to increase appreciabl" 4 contin(o(s electrol"sis sets in is

)no3n as decomposition potential of the electrol"te& Meas(rement

&Metal Finishing - Dept. of Chemistry


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Factor affecting 1P)trength of the current floing through the cell

Chemical nature of the electrodes

Physical nature of the electrodes

(ctivity of the electrolyte

($solute temperature

*ses of #P

'Metal Finishing - Dept. of Chemistry


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Overvoltage

#ifference $eteen the actual applied emf to $ring

a$out continuous electrolysis & the theoretical emfneeded for such electrolysis'

Factors affecting overvoltage$

+ature & physical state of the electrodes Current density

Temperature

+ature of the electrolyte +ature of the material deposited

(Metal Finishing - Dept. of Chemistry


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Polari5ation

EMF reuired for the electrolysis ill $e greater than the reversi$le

EMF of the corresponding galvanic cell' (n electrolytic cell thatreuires e"cess voltage over the theoretical is said to $e polarized &

the phenomenon is called polarization'

Concentration polari5ation

6inetic polari5ation

Factors

%mportance

1)Metal Finishing - Dept. of Chemistry


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Characteristics of a good deposit7 should $e $right & lustrous

- should $e uniform, nonporous & have good adhesion

should $e hard & ductile

should have fine grained structure

Factors affecting the nat(re of deposit$

Current density

Metal ion concentration

Temperature

pH(dditives

Throing poer



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Methods of cleaning the metal s(rface

Physical cleaning

Chemical Cleaning

Mechanical Cleaning

Pic.ling

Electropolishing



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Electroplating of chromi(m

#ecorative coating Hard coating

/ath composition Chromic acid 0123 g4 5

H1)6701'2 g4 5Trivalent chromium08g4

Chromic acid 0123 g4 5

H1)6701'2g45 Trivalentchromium08g4

6perating temp' 98991: ; 98991: ;

(node %nsolu$le P$)n alloy %nsolu$le P$)n alloy

Cathode (rticle to $e coated (rticle to $e coatedCurrent density 1373 m(


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Electrolessplating

Method of depositing a metal film on a su$strate surface

0conductor


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Electrolessplating

Advantages

More uniform coating ith $etterTP than electroplating

Electrical poer & other

accessories not reuired

Coating is harder & $etter earresistance

#eposits have compati$le

etta$ility

)uperior uality deposits

(dditives, levelers or thecomple" filtration method not

reuired

)imple euipment sufficient

Disadvantages

Cost of aste treatment high Freuency of dumping

electroless $ath high

Costs per unit eight of the

deposited metal are more +eeds pure chemicals

Chemical reductants e"pensive Metal salts & reductants are

thermodynamically not sta$le

%mpurities, dust or colloidal

particles promote decomposition

of $ath components



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Electroless plating of Copper

1%

* Coating sol+tion , C+$4 soln

* e/+cing agent , 0C0$

* +er, a$0 ochelle salt* Compleing agent, ED67 /i so/i+m

salt

* $ptim+m p0, 11* $ptim+m temperat+re, 25 8C


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Comparison of electroplating 4 electroless platingParticulars Electroplating Electrolessplating

#riving force Electrical energy #ecrease in free energy of theredo" reaction

Cathodic ?eaction Mn5 5 neM Mn5 5 neM

Cathode (rticle to $e plated (rticle to $e plated ithcatalytic surface

(node )eparate anode ?educing agent in the soln'

+ature of deposit Pure metal or alloy Metal ith reducing agent &o"idized products asimpurities

Thic.nesslimit

0@m4

8 833 8 ! 833

(pplica$ility (pplica$le toconductors only

Can $e used for conductors &nonconductors

Throing poer0TP4

Aess TPB cannot $eused for irregular

shapes & intricate

More TPB can $e used forirregular & uneven shaped

o$ects 1&


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0(mmar"

Metal finishing $y surface modification is an important aspect of

various engineering applications

Commonly used metal finishing techniues include electroplating &

electrolessplating

Theory of electroplating, electrolesssplating & the factors

influencing electroplating are discussed

Electroplating of chromium & electrolessplating of copper areillustrated

1'Metal Finishing - Dept. of Chemistry

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?evie Duestions 8' hat is electroplating #iscuss the steps involved in

electroplating

1' #escri$e the various purposes & applications ofelectroplating

9' #efine polarization' Aist the factors that affect it'

7' E"plain the effect of the folloing on the nature of thedepositB i4 current density ii4 pH iii4 metal ion concentration

iv4 throing poer

2' Mention the characteristics of a good deposit

=' Give reasons i4 Concentration polarization can $e

minimized $y constant stirring of the electrolyte ii4 #P of

ater is more than that reuired for its formation

1(Metal Finishing - Dept. of Chemistry

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