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Anodizing
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====Electrolysis==== The anodized aluminium layer is created by passing a [[direct current]] through an electrolytic solution, with the aluminium object serving as the anode (the positive electrode in an electrolytic cell). The current releases [[hydrogen]] at the [[cathode]] (the negative electrode) and [[oxygen]] at the surface of the aluminium anode, creating a build-up of aluminium oxide. [[Alternating current]] and pulsed current is also possible but rarely used. The voltage required by various solutions may range from 1 to 300 V DC, although most fall in the range of 15 to 21 V. Higher voltages are typically required for thicker coatings formed in sulfuric and organic acid. The anodizing current varies with the area of aluminium being anodized and typically ranges from 30 to 300 [[ampere|A]]/[[meter|m]]<sup>2</sup>. Aluminium anodizing (eloxal or '''El'''ectrolytic '''Ox'''idation of '''Al'''uminium)<ref>{{Cite web|title=Anodizing - WELCO Welding & Coating Solutions - Bruck i.d. Opf.|url=https://www.welco.eu/en/services/anodizing|access-date=2021-04-12|website=www.welco.eu}}</ref> is usually performed in an [[acid]]ic solution, typically sulphuric acid or chromic acid, which slowly [[Dissolution (chemistry)|dissolve]]s the aluminium oxide. The acid action is balanced with the oxidation rate to form a coating with nanopores, 10β150 nm in diameter.<ref name="Edwards"/> These pores are what allow the electrolyte solution and current to reach the aluminium [[Substrate (materials science)|substrate]] and continue growing the coating to greater thickness beyond what is produced by auto-passivation.<ref name="sheasby ch6">{{harvnb|Sheasby|Pinner|2001|pp=327β425}}.</ref> These pores allow for the dye to be absorbed, however, this must be followed by sealing or the dye will not stay. Dye is typically followed up by a clean nickel acetate seal. Because the dye is only superficial, the underlying oxide may continue to provide corrosion protection even if minor wear and scratches break through the dyed layer.{{citation needed|date=November 2015}} Conditions such as electrolyte concentration, acidity, solution temperature, and current must be controlled to allow the formation of a consistent oxide layer. Harder, thicker films tend to be produced by more concentrated solutions at lower temperatures with higher voltages and currents. The film thickness can range from under 0.5 [[micrometre|micrometer]]s for bright decorative work up to 150 micrometers for architectural applications.
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