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8E.pdf - ANODISING OF ALUMINIUM Anodising is a process for...

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VIII-Metals-E-Anodising Aluminium-1ANODISING OF ALUMINIUMAnodising is a process for producing decorative and protective films on articles made ofaluminium and its alloys.The article is made the anode of an electolytic cell with aqueoussulfuric acid as electrolyte where the following overall oxidation reaction occurs.2Al+3H2OAl2O3+6H++6e-A dense even layer of oxide about 0.08 μm thick is formed rapidly, followed much moreslowlywith a more porous layer up to 25μm thick.Before anodising the surface of the article must be horoughly cleaned, normally using adetergent based process, and etched with a solutionn of sodium hydroxide.After anodising the surface may be coloured with a dye or by an electrolytic method usingappropriate metal cations, and then sealed by placing in boiling water, the pores in theoxide layer being closed off.INTRODUCTIONAnodising is a process for producing decorative and protective films on articles made fromalumnium and its alloys.It is essentially a process where a thick film of aluminium oxide isbuilt up on the surface of the aluminium through the use of a direct current electrical supply.In the majority of anodising plants in New Zealand it is carried out in an electrolyte bathcontaining sulphuric acid with aluminium sheet cathodes and the work to be anodisedattached to the anode (Figure 1).Anode (+)Cathode (-) — Aluminium sheetElectrolyte (sulfuric acid)Jig carrying articles to be anodisedFigure 1 - A typical anodising cell(Articles have to be securely fastened to ensureelectrical contact during the anodising)When the current is flowing in the cell the following sequence of events is believed to occur.Sulphuric acid begins to decompose, the hydrogen ions moving to the cathode where theyare reduced to hydrogen gas:2H++2e-H2(g)(1)Simultaneously, negatively charged anions, i.e. hydroxide, sulphate and maybe oxide ionsmove to the anode.The electrical charge in the circuit causes positively charged aluminiumions (Al3+) to be generated in the anode and in turn move toward the cathode.At the anodesurface they react with the oxide/hydroxide ions to form aluminium oxide (in the case of thehydroxide ion, hydrogen ions are released into the solution).
VIII-Metals-E-Anodising Aluminium-2Equations of the anode reactionsAlAl3++3e-(2)2Al3++3O2-Al2O3(3)2Al3++3OH-Al2O3+3H+(4)For which the overall process is:2Al+3H2OAl2O3+6H++6e-(5)The sulphate ions also play some part as the oxide coating contains 12 - 15% sulphate ions.It is suggested that the sulphate ions facilitate the movement of hydrogen ions reducing thecell voltages required.THE DEVELOPMENT OF THE ALUMINIUM OXIDE LAYERFresh aluminium reacts readily with oxygen to produce aluminium oxide.Once formed theoxide remains firmly bonded to the surface forming an impenetrable layer.Consequently,further reaction ceases.The film is very thin (0.01!m), and despite its tenacity it can beremoved by abrasion and chemical corrosion.

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Term
Spring
Professor
carlson
Tags
Chemistry, Alloy, Aluminium, Oxide, Sodium hydroxide, Anodising

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