Outline Curriculum (5 lectures)Each lecture 45 minutesLecture 1: An introduction in electrochemical coatingLecture 2: Electrodeposition of coatingLecture 3: Anodizing of valve metalLecture 4: Electroless deposition of coatingLecture 5: Revision in electrochemical coating

Lecture 3 of 5

Anodizing of Valve Metal

AnodizingAn 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.

Anodising: What and Why?Deliberately producing a stable oxide coating by anodic treatment of a metal surfaceCoating is usually non-conductingthermally and electrically insulatingOxide is usually protectiveagainst corrosion or wear or heatAnodised film can be post-treatedwith dye, polymer, lubricant

Types of Surface Contamination - Dirt: Sources?Oils, greases and waxesMetal oxide (or sulphide or chloride) filmsMetal particlesFlowed surface layers may be glassyMetallurgical defectsChemicals (including sweat)

Special Pre-treatmentsChemical polishingto give a bright finishElectrolytic polishingcapable of a mirror finish Electrograiningused to allow aluminium to pick up inkMicroetchingdeliberate micro-roughening of, e.g., siliconPlating strikese.g., Woods nickel on stainless before Watts nickel

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.

What does an anodised film look like ?Depending on conditions, oxide film thickness can be, e.g., 1-30 micronTransmission electron micrograph(TEM) cross section of an anodised film on Al150 V for 70 minutes in 0.5M phosphoric acidBarrier layerPorous layerAluminium substrate

Anodising of aluminium:state of a substance (s),(l),(g)

2Al(s) + 3H2O(aq) - 6e- = Al2O3(s) + 6H+(aq) AluminiumWaterElectronsAluminium oxideProtonsReactantsProductsNote the different phases and the phase changes during reaction.

Anodising of aluminium:reacting quantities

2Al + 3H2O - 6e- = Al2O3 + 6H+2 atom 3 molecules 6 molecules 1 molecule 6 ions2 mol 3 mol 6 mol 1 mol 6 mol2 x 26.98 g 3 x 18.016 g 6F 1 x 101.96 g 6 x 1.008 gReactantsProducts

Reactions during anodising of aluminium: note phase changesAnode (aluminium)

2Al(s) + 3H2O(l) = Al2O3(s) + 6H+(l) + 6e- 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)

Applications of Anodising?Heat sinksThermal, oxidation resistantPots and pansDecorativeArchitectural panelsDecorative and corrosion resistantEngineeringWear resistant, corrosion resistant

What are the Control Variables?ProcessPre- and post-treatment, temperature, timeElectrolyteComposition, temperatureMetalType of alloy, surface finish

Types of AnodisingDecoratived.c in sulfuric acidHardd.c in chromic or phosphoric acidPlasmaa.c. in near neutral salts

Plasma electrolytic oxidationAnodic 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.

Post-treatment following anodising (how and why)Seal (Boiling water) : proses utk menutup pori-pori Dye (Organics) : proses pewarnaan, 50-60 oC ImpregnatePTFE (Anti-stick) MoS2 (Self-lubricating)

ConclusionsAnodising 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.