No Slide Title · • Types of current : (1) Direct current plating (2) Pulse plating In dc-plating, constant current is used, and the rate of arrival of metal ions depends on

Electroplating/

Electrodeposition

Wei Yan

ABC’s of Electrochemistry

03/22/2012

2 Ohio University - Avionics Engineering Center

• Introduction

• Electroplating Setup

• Importance of Electrodeposition

• Electrochemistry Fundamentals

• Factors affecting the Electrodeposits

• Conclusions

OUTLINE

3

Introduction

Electroplating? Electrodeposition? Electrolytic deposition?

• Electroplating is often also called "electrodeposition,” a short version of

“electrolytic deposition,” and the two terms are used interchangeably.

• It’s a process using electrical current to reduce cations of a desired

material from a solution and to coat the material as a thin film onto a

conductive substrate surface.

Reference:

http://electrochem.cwru.edu/encycl/art-e01-electroplat.htm

Center for Electrochemical Engineering Research, Ohio University

Definition

4

Electroplating Setup

1. Is the core part of the

electroplating.

2. Cathode: the object to

be plated

3. Anode: (1)dissolvable

anode (2) inert

anode

4. Electrolyte/plating

bath


Electrolytic Cell

Reference: http://electrochem.cwru.edu/encycl/art-e01-electroplat.htm

5

Table =The metals can be plated


Part of Periodic Table. Metals inside the frame

can be electrodeposited from aqueous solutions

6

Importance of Electrodeposition

• 1. Decoration: Coating a more expensive metal onto a base metal

surface in order to improve the appearance. Such as jewelry,

furniture fitting

• 2. Protection: Corrosion-resistant coatings such as chromium

plating of automobile parts

• 3. Electroforming: Manufacture of sieves, screens, dry shaver

heads and dies.

• 4. Enhancement: coatings with improved electrical and thermal

conductivity, reflectivity etc.

In CEER,

Electrodeposition of Metals in Catalyst Synthesis


Applications

7

Electrochemistry Fundamentals


Reference: http://chem1.eng.wayne.edu/~yhuang/Papers/Book_Plating_ECHP

Anode Cathode

Anode Cathode

A typical electroplating setup for plating copper one mistake?

8



• Cathode:

Cathode

• Anode:

Reduction reaction

Oxidation reaction Anode

Anode and Cathode

9


According to the Faraday’s first law and second law of

electrolysis, the amount of material deposited on an

electrode is proportional to the amount of electricity used.


Faraday’s Laws of Electrolysis

m is the number of moles of the metals

reduced by charge Q;

F is Faraday constant

Mw is the atomic weight

W is the weight of deposit

10


• Current efficiency = 100 * WAct/WTheo

Wact is the weight of metal deposited or dissolved

WTheo is the corresponding weight to be expected from Faraday’s

laws if there is no side reaction.

Cathode efficiency

Anode efficiency

Current distribution


Current Efficiency, and Current Distribution

11


• Overpotential is the difference in the electrode potential of

an electrode between its equilibrium potential and its

operating potential when a current is flowing.

• It represents the extra energy needed to force the

electrode reaction to proceed at a required rate.

• A slow reaction will require a larger overpotential for a given

current density than a fast reaction.


Overpotential and overvoltage

Reference : http://chem1.eng.wayne.edu/~yhuang/Papers/Book_Plating_ECHP

12


• Overvoltage is the difference between cell voltage and the

open circuit voltage (OCP) when a current is flowing.

• It represents the extra energy needed to force the cell

reaction to proceed at a required rate.

• The overvoltage is the sum of the overpotentials of the two

electrodes of the cell and the ohmic loss of the cell.

Unfortunately, the terms overvoltage and overpotential are

sometimes used interchangeably.

.


Overpotential and overvoltage

13



How to determine the potential of deposition

Fig. Typical voltammogram for a single-metal deposition


14


• How to determine the potential of deposition for two metals

codeposition ?

Key point- the onset of the reduction peaks in CV

(1) If the two reduction peaks of metals are not separated, both

of metals can be reduced when operating potential is more

negative than the onset potential.

(2) If the two reduction peaks of metals are clearly separated,

both of metals can be reduced (codeposition) at a more

negative potential than onset potential of the less noble

metal


15




Fig. CV for Ni-Co codeposition in the plating bath containing 0.06M NiCl2 and

0.06M CoCl2 at 25C.

Reference: Chi-chang Hu and Allen Bai, J.Electrochem.Soc. 149(615) 2002

E < -0.75 V, cathodic deposition

-0.75V <E< -0.3V double layer

response

E > -0.3V anodic dissolution

16




Fig. CV for Ni-Fe codeposition in the plating bath containing 0.06M NiCl2 and

0.06M FeCl2 at 25C.


E < -1.00V Fe and Ni

codeposition

-1.00V <E< -0.6 V Ni only

deposition


Factors affecting the Electrodeposits

18


• Surface Preparation

• Physical & chemical nature of substrate surface

• Deposition temperature

• Deposition current / deposition potential

• Bath composition

• Power supply current waveform


19


“ one can make a poor coating perform with excellent

pretreatment, but one cannot make an excellent coating

perform with poor pretreatment”

Three basic steps:

1. Surface cleaning – chemical way and mechanical way

2. Surface Modification

3. Rinsing



Surface Preparation

20


Current density of deposition can make an influence on

(1) Current efficiency

(2) Thickness of deposit

(3) Structure of deposit

(4) Composition of deposit


Deposition current

21



Deposition current- Example

Reference: M.J.Rahman, S.R.Sen. J. Mechanical. Engineering. 40(1) 2009

22


• Ingredients of a Plating Bath

(1) Metal salt solution-To provide a source of the metal or metals being

deposited.

(2) Various Additives

• The purposes of Additives:

1.To form complexes with ions of the depositing metal

2. To provide conductivity.

3. To stabilize the solution e.g. against hydrolysis.

4. To act as a buffer to stabilize the pH.

5. To modify or regulate the physical form of the deposit.


Bath composition and additives

23



Bath composition and additives- Example

Apart from a buffer, boric acid can inhibit zinc deposition

and promote nickel discharge

without boric acid with boric acid

Reference: M.J.Rahman, S.R.Sen. J. Mechanical. Engineering. 40(1) 2009

24


• Types of current : (1) Direct current plating (2) Pulse plating

In dc-plating, constant current is used, and the rate of arrival of metal ions

depends on their diffusion coefficient (electrode-to-part spacing and agitation).

• The pulse current (PC) and pulse-reverse current (PRC) are used to get the

deposit with reduced porosity, and finer grains , and to minimize the use of

additives and , contamination, etc.

• The morphology of some metal and alloy deposits were found to be superior to the

dc-plated deposits.


Power supply current waveform

25


• Characteristics of Pulse Currents (PC)

• PC, where all the pulses are in one

direction (with no polarity)

Each pulse consists of an ON-time (TON)

during which potential /current is applied,

and an OFF-time (TOFF) during which

zero current is applied



Reference: M.S. Chandrasekar, M. Pushpavanam. Electrochim Acta 53 (2008)

3313–3322

26


• Characteristics of PRC

• PRC, where anodic and cathodic pulses

are mixed.

• To characterize a train of pulse-reverse

current waveform, key parameters need

to be known.

(1) The cathodic peak pulse current

density (Ic); The cathodic pulse length

(Tc)

(2) The anodic current density (IAA); The

anodic pulse time (TAA)

(3) IA = (Ic x Tc + IAA TAA) / (Tc + TAA)

average current density

(4) T is the cycle time



Reference: M.S. Chandrasekar, M.

Pushpavanam. Electrochim Acta 53 (2008)

3313–3322

27



Fig. CV for Ni-Fe codeposition in the plating bath containing 0.06M NiCl2 and

0.06M FeCl2 at 25C.


E < -1.00V Fe and Ni

codeposition

-1.00V <E< -0.6 V Ni only

deposition


Conclusions

29

Conclusions

1. “Electroplating/ Electrodeposition: historically old,

technologically advanced” D.Gabe said.

2. The purpose of electroplating is to produce a qualified coating

with the desirable attributes. Each kind of coating needs a

special electroplating process with a given application.

3. It is easy to get the coatings of metals, but it is not easy to get

the coatings with good quality. Key points are those factors

affecting the electrodeposits, which is crucial for the final

deposits.


Questions?

Documents

No Slide Title · • Types of current : (1) Direct current plating (2) Pulse plating In dc-plating, constant current is used, and the rate of arrival of metal ions depends on