Metal Finishing

1. Metal Finishing

2. Process of electro deposition of an adherent metallic film of uniform thickness on the surface of the substrate for the purpose of modifying its surface properties.

3. OBJECTIVES • In general a metal or alloy used for a purpose does not meet all the requirements. • Metal finishing is one of the methods employed to improve their lacking properties.

4. Improving resistance to corrosion, scratch or abrasion and wear. • Enhancing decorative appearance. • Improving solder ability and electrical properties • Reducing frictional loss etc.

5. Metal finishing was introduced as a decorative finish, but the increasing demand for parts with prescribed specifications has led to vast technological developments in the field.

6. Techniques of metal finishing: Electroplating of metals & alloys. Electro less plating of metals & alloys.

7. ELECTROPLATING • The process by which coating metal is deposited on the base metal by passing direct current through an electrolytic solution, containing the soluble salt of the coating metal. • Common metals used are Cu, Ni, Zn, Au, Pt etc. • Used in industries for producing metal coating.

8. Factors Governing Electroplating: • Decomposition Potential • Polarization • Over voltage

9. DECOMPOSITION POTENTIAL. • The minimum voltage required for the continuous electrolysis of an electrolyte in an electrolytic cell. • In the electrolysis of water, a pair of Pt electrodes is immersed in a solution of an acid. It is found experimentally that a potential of about 1.7 V must be applied to the cell before there sets in a continuous evolution of H2 and O2.

10. Reason: • During electrolysis accumulation of products of electrolysis around the electrodes causes a back emf which opposes the applied emf. • To overcome the back emf, the applied emf will have to be increased ;then only electrolytic deposition starts again.

11. Measurement of ED • Gradually increasing voltage is applied between the electrodes. • Milliammeter reading is noted down for every voltage applied.

12. Factors Influencing ED • Decomposition potentials of different electrolytes are different. • Strength of the current flowing through the cell • Chemical nature of the electrodes. • Physical nature of the electrodes • Activity of the electrolyte • Absolute temperature.

13. Uses of decomposition potential • Used to carry out all electrolytic processes. (electroplating, electrorefining etc.) • Used in the separation of ions from a solution by electrolysis. • Two types of ions can be discharged simultaneously by equalising their decomposition potentials.

14. OVER VOLTAGE • The decomposition of an electrolyte is expected to start as soon as applied potential reaches the value of reversible emf of the cell. • But when products discharged at the electrodes are gases, then the actual decomposition potential is invariably much higher than its theoretical reversible electrode potential. • The excess voltage is referred to as over voltage of the gas.

15. Definition • Over voltage is the difference between the actual applied emf to bring about continuous electrolysis and the theoretical emf needed for such electrolysis. • For instance the reversible potential of oxygen gas with smooth Pt electrode is 1.23 V. But actual gas evolution takes place at a potential of 1.68 V. The excess 0.45 V is the over voltage of oxygen on smooth Pt surface.

16. Factors Affecting Over voltage: • Electrode surface. • Nature of the electrolyte. • Temperature. • Current density.

17. Of the many steps involved in gas evolution at electrodes, at least one is rather a slow process and requires energy. • Electrolysis of water: (i) H+ + e -----> H (fast) (ii) H + H -----> H2(slow) The step (ii) being slow, consumes energy and accounts for overvoltage.

18. POLARIZATION • Development of over potential in an electrolytic cell is called polarization. • Electrolytic processes occur at the electrodes of the cell. • Polarization sets in the cell when the electrode processes become slow and irreversible. • Magnitude of over potential is directly proportional to the extent of polarization.

19. Types Of Polarization: The observed polarization can be of two types. *Concentration polarization. *kinetic or activation polarization.

20. CONCENTRATION POLARIZATION • An electrochemical reaction occurs only at the surface of the two electrodes. • Adjacent to the electrode surface reactant concentration decreases. • The reactant species are transported to the electrode surface by processes like diffusion, migration and convection from the bulk of the solution.

21. Current flowing through the cell drops when the rate of transport is insufficient to match the electrode reaction. • A potential than the theoretical value is required to maintain the current at the necessary level. This phenomenon is called concentration polarization.

22. Steps: *Diffusion of reactant species towards the electrode. *Diffusion of product away from the electrode cause concentration polarization. • This can be minimized by: * Increasing the convection. * By raising the temperature. * Mechanical agitation of the solution.

23. ACTIVATION POLARIZATION • This is caused when any of the following steps become slow and need activation energy. *Adsorption of the reactant on the electrode. *Charge transfer across the metal-solution interface. *Desorption of the product from the electrode become slow and need activation energy.

24. A higher potential than the theoretical value is to be applied to maintain current at the necessary level. • It is not possible to eliminate or minimize kinetic polarization by any means.

25. Activation polarization is *Low: When metal deposition or metal dissolution is the electrode process. *High: When the electrode process is a gas (H2 ,O2, etc) evolution,

26. Theory Of Electroplating. • The electroplating device is essentially an electrolytic cell, in which two electrodes, anode and cathode are dipped in an electrolyte solution.

27. Essential Components. • A d.c. electrical power source. • Anode : Inert material / coating metal itself. • Cathode : Article to be plated. • Electrolytic bath: Solution of conducting salt, metal to be plated in a soluble form, buffer and additives. • Container: Made up of rubber lined steel, wood or concrete.

28. Theory • Anode : Made of coating metal. • Cathode: Article to be plated. • Electrolyte: CuSO4 solution. CuSO4 Cu2+ + SO42- *On passing current, Cu2+ + 2e Cu (Cathode) Cu + SO42- CuSO4 + 2e.(Anode) Thus there is a continuous replenishment of electrolytic salt during electrolysis.

29. If the anode is made of some inert material like graphite, then electrolytic salt is added continuously in order to maintain proper coating metal ion concentration in the bath. • The process of electroplating goes on nonstop, since the anode is not consumed.

30. Characteristics Of A Good Deposit. • The deposit should be bright and lustrous. • The deposit should be continuous, uniform, non porous and adhesive. • It should be hard and ductile. • It should be of fine grained nature.

31. Factors Influencing The Nature Of Deposit. • Current Density: An optimum current density should be applied. If low- *leads to slow process *results in coarse grained deposit. If high- *leads to rough and treed deposit. *results in burnt and spongy deposit.

32. Metal salt and electrolyte concentration: • Optimum electrolyte concentration should be maintained in bath to get a better deposit. • The advantages of using a strong solution: *higher conductivity. * cathode efficiency. * Firm and adherent deposits.

33. Temperature: • It should be between35oc and 65oc. • Lower temperature: *reduces the solubility of the metal salt. *reduces the mobility of the ions. • Higher temperature *affects the lining of the equipment, electrode surface and the substrate.

34. Agitation: • Agitation of the solution brings up a fresh supply of metal salts to the cathode. • It also sweeps away gas bubbles which may otherwise cause pits. • Rapid agitation may *detach the particles. * stir up the sludge. *results in rough and porous deposits.

35. pH of the electrolytic bath: *Suitable pH is maintained using appropriate buffers. • In Ni plating borate buffer is used to maintain pH 4.5. • At low pH more H2 evolution takes place on the cathode. • At higher pH precipitation of hydroxides of metal takes place.

36. Addition agents: The wide range of additives added in low concentrations serve to electroplating bath modify the structure and properties of the electrodeposit. • Function: Makes the deposit smooth, fine grained, hard and adherent.

37. Types of additives: • Complexing agent: Eg: Cyanide, sulphamate ions. • Brightners: Eg: Aromatic sulphonates, thiourea. • Wetting Agents: Eg: Sodium lauryl sulphate.

38. Levellers: Produce a level deposit by getting adsorbed at places where rapid deposition of metal takes place. • Structure Modifiers: These additives change the structure of the deposit.

39. Throwing power: It is used as a measure of the distribution of the deposit . • This property is particularly important when the article is of an irregular shape. • This can be improved by: * separating the distances between the electrodes. * agitating the solution to minimize high local electrolytic resistance.

40. Metal ion concentration: Low metal ion concentrations give adherent coating films. • This can be achieved by: * addition of a compound with a common ion. * formation of complex compounds. • Eg: Double cyanides of Na or K.

41. Methods of cleaning the metal surface. • A good adherent deposit is obtained only if the base metal surface is free from dirt and extraneous matter. • The following methods are employed to clean the metal surfaces. • Solvent cleaning: To dissolve oils and greases. Eg: CCl4,CH2Cl2,CHCl=CCl2. • Alkali cleaning: To remove minute organic residues.Eg: NaOH,Na2CO3,Na3PO4.

42. Mechanical Cleaning: Involves removal of the oxide layer or rust and other inorganic deposits. Eg: Impact tools like sandpaper, chisels, knife scrapers, wire brushes etc. • Pickling: Removal of oxide films by means of an acid. Eg: 10--30% sulphuric acid is used for this acid dip. It is again rinsed with hot water.

43. Electropolishing: The metal to be cleaned iss made as anode in a suitable acid solution. *A surface layer of the metal gets dissolved along with the impurities. *This helps to remove surface irregularities. *The metal is then thoroughly rinsed with water, dried and used for electroplating.

44. Applications. • Better appearance. • Protection against corrosion. • To achieve desired engineering effect. • Plating on non-metallics. • Electroforming.

45. Electroplating of Chromium. Two types of chromium plating: *Decorative Chromium:- A thin deposit of Cr (0.25 --0.75/um ) is applied over either copper-nickel or nickel undercoat. *Hard Chromium:- Involves deposition of a thick coating of chromium directly over the substrate. The usual thickness range of hard chrome is 2.5---300/um.

46. Decorative chromium. • Bath composition- 250g chromic acid + 2.5g H2SO4 per liter of the solution and 1g trivalent chromium • Operating temperature - 45--55oC. • Current density - 145--430 A/ft2 • Anode - Insoluble anodes like Pb, Pb- 6%Sb,Pb--7% Sn. • Cathode - Article to be plated

47. Hard chromium. • Bath composition- 250g chromic acid + 2.5g H2SO4 per liter of the solution and 1g trivalent chromium • Operating temperature - 50--55oC. • Current density – 290--580 A/ft2 • Anode - Insoluble anodes like Pb, Pb- 6%Sb,Pb--7% Sn. • Cathode - Article to be plated

48. In the case of chromium plating inert anodes are used *Reason: • The metal going into the solution is five times of the metal getting deposited, resulting in the building up of excessive chromic acid (CrIII) concentration. • This leads to imbalance of bath composition. • Chromium metal becomes passive in the strongly acidic bath and a black deposit is formed on the cathode.

49. Chromium plating produces an attractive, wear resistant and abrasion resistant surface. • Chromium plating is not continuous; it is micro porous. Hence an undercoat of copper and/or nickel is essential. • Applications: *Decorative chromium provides a durable finish (on automobiles, furniture, surgical & dental instruments). *Hard chromium is applied to many components in almost every industry.

50. Electroless Plating. • It is a method of depositing a metal or alloy over a substrate (conductor or nonconductor) by controlled chemical reduction of the metal ions by a suitable reducing agent without using electrical energy. • Metal + Reducing agent --- Metal+ Oxidized product. *It is also termed as autocatalytic plating.