An­odiz­ing

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Alu­minium is a re­ally use­ful metal be­cause it is light and strong. In many ap­pli­ca­tions it is su­pe­rior to steel, not least be­cause it can be left out in the weather for years with­out sig­nif­i­cantly de­te­ri­o­rat­ing.

Steel in sim­i­lar sit­u­a­tions would need a pro­tec­tive coat­ing, such as paint or zinc. The sur­faces of both alu­minium and steel ox­i­dize when ex­posed to the oxy­gen in the air. The dif­fer­ence is that the layer of alu­minium ox­ide formed on alu­minium is ‘water­proof’: it is im­per­me­able to wa­ter. If there were no ox­ide coat­ing, the alu­minium would rapidly re­act with wa­ter, form­ing alu­minium hy­drox­ide and hy­dro­gen gas, de­stroy­ing the alu­minium. The iron ox­ide that forms on the sur­face of steel is per­me­able — wa­ter can travel through it, form­ing hy­drated iron III ox­ide, which we call ‘rust’. Even­tu­ally the steel will be de­stroyed as it rusts away.

The layer of alu­minium ox­ide is only a few atoms thick, but is tough and can be pol­ished, giv­ing the alu­minium an at­trac­tive ap­pear­ance. Over time the sur­face of the alu­minium weathers and de­te­ri­o­rates. It has been found that a thicker layer of ox­ide bet­ter pro­tects the metal from weath­er­ing. Pass­ing a low-volt­age DC elec­tric cur­rent through an acid elec­trol­y­sis bath with the alu­minium be­ing the an­ode pro­duces highly re­ac­tive oxy­gen atoms on the sur­face of the alu­minium. The oxy­gen re­acts with the alu­minium to form alu­minium ox­ide. This in­creases the thick­ness of the ox­ide layer, greatly ex­tend­ing the time that the alu­minium looks shiny and new. Dyes can be added to the an­odiz­ing bath to colour the ox­ide layer.

For more see this ar­ti­cle on

web­site: the-shed.nz/ home/2019/8/25/colour-an­odise-thata­lu­minium-al­loy.

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