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Outline Curriculum (5 lectures) Each lecture  45 minutes

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.

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Outline Curriculum (5 lectures) Each lecture  45 minutes

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  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 3 of 5Anodizing of Valve Metal

  3. Anodizing • An electrolytic passivation process. • An anodic oxidation of metal at the anode, e.g. Al, Ti, Mg and is usually accompanied by hydrogen evolution at an inert electrode (the cathode), e.g. Ti, stainless steel. • Typical cell voltage 5 to 100 V.

  4. Anodising: What and Why? • Deliberately producing a stable oxide coating • by anodic treatment of a metal surface • Coating is usually non-conducting • thermally and electrically insulating • Oxide is usually protective • against corrosion or wear or heat • Anodised film can be post-treated • with dye, polymer, lubricant…

  5. Types of Surface Contamination - ‘Dirt’: Sources? • Oils, greases and waxes • Metal oxide (or sulphide or chloride) films • Metal particles • ‘Flowed’ surface layers may be ‘glassy’ • Metallurgical defects • Chemicals (including sweat)

  6. Special Pre-treatments • Chemical polishing • to give a bright finish • Electrolytic polishing • capable of a mirror finish • Electrograining • used to allow aluminium to pick up ink • Microetching • deliberate micro-roughening of, e.g., silicon • Plating strikes • e.g., Wood’s nickel on stainless before Watts nickel

  7. Anodizing At excessively high current density, a secondary anodic reaction, O2 evolution takes place at a significant rate. This may give rise to problems such as pitting of the anodised layer and an increased possibility of hazardous O2/H2 mixture.

  8. What does an anodised film look like ? Transmission electron micrograph (TEM) cross section of an anodised film on Al 150 V for 70 minutes in 0.5M phosphoric acid Porous layer Barrier layer Aluminium substrate Depending on conditions, oxide film thickness can be, e.g., 1-30 micron

  9. Anodising of aluminium:state of a substance (s),(l),(g) Products Reactants 2Al(s) + 3H2O(aq) - 6e- = Al2O3(s) + 6H+(aq) Aluminium oxide Protons Aluminium Water Electrons Note the different phases and the phase changes during reaction.

  10. Anodising of aluminium:reacting quantities Products Reactants 2Al + 3H2O - 6e- = Al2O3 + 6H+ 2 atom 3 molecules 6 molecules 1 molecule 6 ions 2 mol 3 mol 6 mol 1 mol 6 mol 2 x 26.98 g 3 x 18.016 g 6F 1 x 101.96 g 6 x 1.008 g

  11. Reactions during anodising of aluminium: note phase changes • Anode (aluminium) 2Al(s) + 3H2O(l) - 6e- = Al2O3(s) + 6H+(l) • Cathode (e.g., stainless steel) 6H+(l)+ 6e- = 3H2 (g) • Cell (overall process) 2Al(s) + 3H2O(l) + 6H+(l)= Al2O3(s)+ 6H+(l)+ 3H2(g)

  12. Applications of Anodising? • Heat sinks • Thermal, oxidation resistant • Pots and pans • Decorative • Architectural panels • Decorative and corrosion resistant • Engineering • Wear resistant, corrosion resistant

  13. What are the Control Variables? • Process • Pre- and post-treatment, temperature, time • Electrolyte • Composition, temperature… • Metal • Type of alloy, surface finish…

  14. Types of Anodising • Decorative • d.c in sulfuric acid • Hard • d.c in chromic or phosphoric acid • Plasma • a.c. in near neutral salts • 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.

  15. Post-treatment following anodising (how and why) • Seal (Boiling water) • Dye (Organics) • Impregnate • PTFE(Anti-stick) • MoS2(Self-lubricating)

  16. Conclusions • Anodising is important in surface finishing. • Uses: decorative and engineering applications. • Capacitors to architectural panels are involved. • Pre-treatment is important. • Good process control is essential. • So is adequate post-treatment. • Use d.c., a.c. and plasma electrolysis techniques.

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