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 Lecture 5 of 5 Revision in electrochemical coating Type of electrochemical processes for the production of surface coatings • Electroplating – Uses external power sources to reduce metal ions to metal deposit. – Requires the substrate to be electrically conductive. • Electroless deposition – Uses chemical reaction to reduce metal ions. – Requires substrate to be catalytically active. – Does not requires an external power sources to initiate deposition. • Immersion deposition – Metal ion is reduced from the solution by exchange with metallic substrate. – Type of metal that can be deposited depends on the metal substrate and metal ions in solution. (Electromotive Series Table). – Displacement Reaction. Type of electrochemical processes for the production of surface coatings • Anodizing – Anodic oxidation of metal to form metal oxide. – Metal oxide forms at the anode. – Requires the external supply of high voltage, e.g. 10 to 100 V to form oxide layer. – Oxide layer thickness, e.g. 10 to 100 m. • Plasma electrolytic oxidation – Anodic oxidation of metal to form metal oxide. – Uses higher voltage than anodizing, e.g. 100 to 1000 V. – Metal oxide forms at the anode. – Thicker oxide layer than anodizing, e.g. 100 to 500 m. Current efficiency • The percentage of the current which goes to the useful electroplating reaction. • Is the ratio between the actual amount of metal deposit, Ma to that calculated theoretically from Faradays Law, Mt. Ma Current efficiency 100% Mt Faraday’s laws of electrolysis Faradays Law: The mass of metal electroplated is directly related to the number of coulombs passed through the electrochemical reactions. amount of material = amount of electrical energy q n zF n = amount of material q = electrical charge z = number of electrons F = Faraday constant Faraday’s Laws of Electrolysis: Units check q n zF [C] [mol] 1 [C mol ] Faraday’s Laws of Electrolysis: Expanded Relationship q n zF w It M zF n = amount of material, mol w = mass of material, g M = molar mass of material, g mol-1 I = current, A t = time, s z = number of electrons F = Faraday constant, 96 485 C mol-1 A Worked Example: Facts • 200 cm3 of an acidic solution of 0.1M copper sulphate pentahydrate (CuSO4.5H2O) is electrolysed using inert electrodes. • Molar mass of copper, M = 63.54 g mol-1 • Faraday constant, F = 96 485 C mol-1 A Worked Example: Questions on reactions • A sketch is useful. • What is the cathode reaction? • What is the anode reaction? • What is the cell reaction? • At which electrode does reduction occur? Answers on reactions • Cathode reaction: Cu2+ + 2e- = Cu • Anode reaction: H2O - 2e- = 2H+ + 1/2O2 • Cell reaction: Cu2+(l) + H2O(l) = Cu(s) + 2H+(l) + 1/2O2(g) • Reduction (electron gain) occurs at cathode A Worked Example: Questions on Concentration • What is the concentration of dissolved metal in units of – mol dm-3 – g dm-3 – ppm Answers on Concentration • The concentration of dissolved metal is the same as that of the compound, i.e.: c = 0.1M = 0.1 mol dm-3 • The concentration on a mass basis is: c = (0.1 mol dm-3)(63.54 g mol-1) c = 6.354 g dm-3 • Expressed as parts per million (= mg dm-3): c = 6.354 x 103 mg dm-3 c = 6354 ppm A Worked Example: Questions on mass • What is the mass of dissolved metal in g? Answers on Mass • The mass of dissolved copper, w, is given by the product of concentration (on a mass basis) and solution volume: w = (6.354 g dm-3)(200 cm3) w = (6.354 g dm-3)(0.200 dm3) w = 1.271 g A Worked Example: Questions on Electrical Charge • Calculate the electrical charge needed to remove all of the copper from solution. Answers on Electrical Charge, q q = nzF n = (0.1 mol dm-3)(0.200 dm3) n = 0.0200 mol -1 q = (0.0200 mol)(2)(96485 C mol ) q = 3859.4 C A Worked Example: Questions on Rate of Removal • If the current used is 1.0 A, calculate the rate of copper deposition in – g s-1 – kg day-1 Answers on Rate of Removal w It M zF So, the rate of change of mass is given by: w MI t zF Answers on Rate of Removal 1 w (63.54 g mol )(1.0 A ) 1 t ( 2)(96485 C mol ) w 4 1 3.293 x 10 g s t Answers on Rate of Cu Removal w 4 3 1 3.293 x 10 x 10 kg s t w 3.293 x 10 7 x 60 x 60 x 24 kg day 1 t w 0.02845 kg day 1 t How long does it take to deposit 1 m over 100 cm2?) Answers on Time to Deposit 1 m Copper w 1 0.02845 kg day t How long does it take to deposit 1 m over 100 cm2?) Methods – either: (a) rearrange earlier equation or (b) use w/t expression Time to Deposit 1 m Cu on 100 cm2 (a) Rearrange earlier equation w M .I t z .F to give w.z .F t M .I But the density of metal is the ratio of mass to volume w V and the volume of metal deposited is the product of thickness and area So w .x.A t and t .x.A.z .F V x .A M .I .x.A.z .F M .I ( 8.96 g cm3 )(1 x 10 4 cm )(100cm2 )( 2 )( 96 485C m ol1 ) t ( 63.53 g m ol1 )(1 A ) t 272s 4 m in Time to Deposit 1 m Cu on 100 cm2 (b) From our earlier result w 0.02845 kg day 1 t So, in one day, over a 100 cm2 area w 28.45 g / 100 cm 2 A This mass of deposit has a thickness of 2 0.2845g cm x 0.03175cm 317.5 m 3 8.96 g cm 317.5 m are deposited in 1 day so 1 m is deposited in a much shorter time: 1 x 24 x 60 x 60 t s 272 s 317 .5