Outline Curriculum (5 lectures) Each lecture  45 minutes

1. Outline Curriculum (5 lectures)Each lecture  45 minutes Lecture 1: An introduction in electrochemical coating Lecture 2: Electrodeposition of coating Lecture 3: Anodizing of valve metal Lecture 4: Electroless deposition of coating Lecture 5: Revision in electrochemical coating

2. Lecture 2 of 5Electrodeposition of Coating

3. Electrochemical Surface Engineering • An electro-chemical reaction • Cathode: Metals/alloys coatings • Anode: Soluble or insoluble • Conductive solution: ionic species • Transfer of electrons

4. An example of electroplating of copper Power Supply e- Copper Anode Steel Cathode Main reaction Cu2+ + 2e- Cu

5. Other possible electrochemical reactions Electrodeposition of copper Cu2+ + 2e- Cu Hydrogen evolution 2H+ + 2e-  H2 At the cathode At the anode Soluble anode Dissolution of copper Cu 2e- Cu2+ Insoluble anode Oxygen evolution H2O  2e-  2H+ + 0.5 O2 Overall reaction Cu2+ + H2O Cu + 2H+ + 0.5 O2

6. Definition: Electron transfer reactions • Oxidizing agent + n e- = Reducing agent • Oxidizing agents get reduced • Reducing agents get oxidized • Oxidation is a loss of electrons (OIL) • Reduction is a gain of electrons (RIG) OILRIG

7. Typical steps in the electroplating of metals • Cleaning with organic solvent or aqueous alkaline; to remove dirt or grease. • Is the surface is covered by oxides as a result of corrosion, clean with acid. • Rinse with water to neutralise the surface. • Electroplate metals under controlled condition. • Rinse with water and dry. • Additional step: heat treatment in air or vacuum environment

8. What is the Job of the Bath? • Provides an electrolyte • to conduct electricity, ionically • Provides a source of the metal to be plated • as dissolved metal salts leading to metal ions • Allows the anode reaction to take place • usually metal dissolution or oxygen evolution • Wets the cathode work-piece • allowing good adhesion to take place • Helps to stabilise temperature • acts as a heating/cooling bath

9. Typically, What is in a Bath?e.g., Watts Nickel • Ions of the metal to be plated, e.g. • Ni2+ (nickel ions) added mostly as the sulphate • Conductive electrolyte • NiSO4, boric acid, NiCl2 • Nickel anode dissolution promoter • NiCl2 provides chloride ions • pH buffer stops cathode getting too alkaline • Boric acid (H3BO3) • Additives • Wetters, levellers, brighteners, stress modifiers..

10. Current efficiency • pH changes accompany electrode reactions wherever H+ or OH- ions are involved. • In acid, hydrogen evolution occurs on the surface of cathode. This will result in a localised increase in pH near the surface of the electrode. • In acid, oxygen evolution occurs on the surface of anode. This will result in a drop of pH near the surface of the electrode. • pH buffer stops the cathode getting too alkaline. • Boric acid (H3BO3) 2H+ + 2e-  H2 H2O  2e-  2H+ + 0.5 O2 H+ Cathode H2 OH H2O  H+ + OH

11. Current efficiency • Is the ratio between the actual amount of metal deposit, Ma to that calculated theoretically from Faradays Law, Mt.

12. Parameters that may influence the quality of electrodeposits • Current density (low to high current) • The nature of anions/cations in the solution • Bath composition, temperature, fluid flow • Type of current waveform • the presence of impurities • physical and chemical nature of the substrate surface

13. An example of Current vs. Potential Curve for electroplating of metal

14. Typical Recipe and ConditionsWatts Nickel Component Concentration/g L-1 Nickel sulphate 330 Nickel chloride 45 Boric acid 40 Additives various Temperature 60 oC pH 4 Current density 2-10 A dm-2

15. Faraday’s Laws of Electrolysis Amount of material = amount of electrical energy n = amount of material q = electrical charge z = number of electrons F = Faraday constant

16. Faraday’s Laws of Electrolysis: Expanded Relationship n = amount of material w = mass of material M = molar mass of material I = current t = time z = number of electrons F = Faraday constant

17. Current, Current density, Surface area j = current density [mA cm-2] I = current [A] A = surface area of the electrode [cm2] jelectroplate = electroplating current density (metal electroplate) jcorrosion = corrosion current density (metal corrosion/dissolution)

18. Faraday’s Laws of Electrolysis: Average thickness w = weight (mass) of metal M = molar mass of metal I = current t = time z = number of electrons F = Faraday constant x = thickness of plating

19. Faraday’s Laws of Electrolysis: Average deposit thickness The thickness of plate depends on: - the current (I) - the time for which it passes (t) - the exposed area of the work-piece (A) - a constant (M/rAzF) which depends on the metal and the bath

20. Faraday’s Laws of Electrolysis: Question - Nickel Plating Nickel is plated from a Watts bath at a current density of 3 A dm-2. The current efficiency is 96%. The molar mass of nickel is 58.71 g mol-1. The density of nickel is 8.90 g cm-3. The Faraday constant is 96 485 C mol-1. What will be the averaged plating thickness in 1 hour?

21. Faraday’s Laws of Electrolysis: Answer - Nickel Plating Assume that the reaction is: Ni2+ + 2e- = Ni So, two electrons are involved for every Ni atom, and z = 2 The current density used in plating nickel is 96% of the total current, i.e., 0.96 x 3 A dm-2.

22. Faraday’s Laws of Electrolysis: Answer - Nickel Plating The average deposit thickness is given by:

23. Summary • Electrodeposition is a versatile coating technique. • There is a high degree of control over deposit thickness. • Many metals can be electroplated from aqueous baths. • So can some alloys, conductive polymers and composites. • Rates of electroplating can be expressed via Faraday’s Laws of electrolysis. Thank you for your attention!