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Anodising
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Anodising

Aluminium alloys are Anodised  to increase corrosion resistance, to increase surface hardness, and to allow dyeing (colouring), improved lubrication, or improved adhesion. The anodic layer is non-conductive.

When exposed to air at room temperature, or any other gas containing oxygen, pure aluminium self-passivates by forming a surface layer of amorphous aluminium oxide 2 to 3 nm thick, which provides very effective protection against corrosion. Aluminium alloys typically form a thicker oxide layer, 5-15 nm thick, but tend to be more susceptible to corrosion. Aluminium alloy parts are Anodised  to greatly increase the thickness of this layer for corrosion resistance. The corrosion resistance of aluminium alloys is significantly decreased by certain alloying elements or impurities: copper, iron, and silicon, so 2000, 4000, and 6000-series alloys tend to be most susceptible.

You will see anodised aluminium on aircraft parts, architectural materials, and various consumer products such as, mp3 players, flashlights, cookware, cameras, sporting goods, window frames, roofs, in electrolytic capacitors, and on many other products. Although anodizing only has moderate wear resistance, the deeper pores can better retain a lubricating film than a smooth surface would.

Anodised coatings have a much lower thermal conductivity and coefficient of linear expansion than aluminium. In typical commercial aluminium anodisation processes, the aluminium oxide is grown down into the surface and out from the surface by equal amounts. Anodising will increase the part dimensions on each surface by half of the oxide thickness. For example a coating that is (2 microns) thick, will increase the part dimensions by (1 microns) per surface. If the part is Anodised on all sides, then all linear dimensions will increase by the oxide thickness. Anodised aluminium surfaces are harder than aluminium but have low to moderate wear resistance, although this can be improved with thickness and sealing.

The Process

Anodising, is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called "anodising" because the part to be treated forms the anode electrode of an electrical circuit. Anodising increases corrosion resistance and wear resistance. Anodic films can be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light. Anodising is also used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors. Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, niobium, and tantalum. This process is not a useful treatment for iron or carbon steel because these metals exfoliate when oxidized; i.e. the iron oxide (also known as rust) flakes off, constantly exposing the underlying metal to corrosion.

Anodising changes the microscopic texture of the surface and changes the crystal structure of the metal near the surface. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Anodised  aluminium surfaces, for example, are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Anodic films are generally much stronger and more adherent than most types of paint and metal plating, but also more brittle. This makes them less likely to crack and peel from aging and wear, but more susceptible to cracking from thermal stress.

 

 
 
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