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CATEGORIES (articles) > Chassis & Bodywork Construction > Finishings > Anodising finish for metal surfaces

Anodising finish for metal surfaces


These inexpensive carabiners have an anodised aluminium surface, and come in many colors.

Anodising, or anodizing, is a technique used to modify the surface of a metal. It may provide greater corrosion resistance, create a different surface topography, and change the crystal structure close to the metal surface. The process derives its name from the fact that the part to be treated forms the anode portion of an electrical circuit in this electrolytic process.

Anodisation is frequently used to protect aluminium and titanium from abrasion and corrosion and to allow it to be dyed in a wide range of colors.


Anodized titanium

Anodized titanium is used in a recent generation of dental implants. Anodizing generates a thicker layer of titanium dioxide (>1 µm and up to >2.5 µm compared with much less than 1 µm for un-anodized specimens) and a characteristic surface topography. It has been suggested that both of these parameters improve the performance - longevity, stability - of dental implants, but the technology is still new and there is not yet clear clinical data to support these claims.

Anodizing is also said to impart anti-galling properties on threaded components.

Anodizing titanium generates an array of different colors without dyes. The color formed is dependent on the thickness of the oxide (which is determined by the anodising voltage); it is caused by the interference of light reflecting off the oxide surface with light traveling through it and reflecting off the underlying metal surface. Titanium nitride coatings can also be formed, which have a brown or golden color and have the same wear and corrosion benefits as anodization.


Anodised Niobium

This anodises in a similar fashion to titanium with a range of attractive colours being formed by interference at different film thicknesses. Again the film thickness is dependent on the anodising voltage. The only real use is on a special issue coin.


Anodized aluminium

Anodized aluminium can be found in cookware, cameras, and sporting goods due to its aesthetic and corrosion protection properties. Most aluminium aircraft parts including major components are anodised before assembly and painting.

The aluminium oxide coating is grown from and into the surface of the aluminium. Because of this it is not prone to peeling or cracking like organic coatings such as paint. Aluminium oxide also possesses excellent thermal and electrical insulation qualities.


Chemistry

Aluminium, when exposed to the atmosphere, forms a passive oxide layer, which provides moderate protection against corrosion. This layer is strongly adherent because it is chemically bound to the metal surface as compared to oxidation (corrosion) in steel, where rust puffs up and flakes off, constantly exposing new metal to corrosion. In its pure form aluminium self-passivates very effectively, but its alloys, especially 6000 series due to the magnesium content, are far more prone to atmospheric corrosion and therefore benefit from the protective quality of anodising.

Before being treated, the aluminium, if wrought, is cleaned in either a hot soak cleaner or in a solvent bath and may be etched in sodium hydroxide (normally with added Sodium Gluconate), ammonium bi-fluoride or brightened in a mix of acids. Cast alloys are normally best just cleaned due to the presence of intermetalics unless they are a high purity alloy such as LM0.

In anodizing aluminium, this aluminium oxide layer is made thicker by passing a DC current through a sulfuric acid solution, with the aluminium object serving as the anode (the positive electrode). The current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the aluminium anode, creating a buildup of aluminium oxide. Anodizing at 12 V DC, a piece of aluminium with an area of 1 square decimeter (about 15.5 square inches) can consume roughly 1 ampere of current. In commercial applications the voltage used is more normally in the region of 15 to 21 V.

Conditions such as acid concentration, solution temperature and current must be controlled to allow the formation of a consistent oxide layer, which can be many times thicker than would otherwise be formed. This oxide layer increases both the hardness and the corrosion resistance of the aluminium surface. The oxide forms as microscopic hexagonal "pipe" crystals of corundum, each having a central hexagonal pore (which is also the reason that an anodized part can take on color in the dyeing process). The film thickness can range from under 5 micrometres on bright decorative work to over 25 micrometres for architectural applications.

The older chromic acid method produces thinner, more opaque films that are softer, ductile, and to a degree self healing. They are harder to dye and may be applied as a pretreatment before painting. The method of film formation is different to using sulfuric acid in that the voltage is ramped up through the process cycle.


Dyeing

The iPod Mini is dyed using anodisation.

Where appearance is important, the oxide surface can be dyed before the sealing stage, as the dye enters the pores in the oxide surface. The number of dye colors is almost endless; however, the colours produced tend to vary according to the base alloy. Though some may prefer lighter colours, in practice they may be difficult to produce on certain alloys such as high silicon casting grades and 2000 series (with its high copper content). Another concern is the light fastness of organic dyestuffs, some colours (reds and blues) are particularly prone to fading. Black dyes and gold produced by inorganic means (ferric ammonium oxalate) are more light fast.

Alternatively, metal (usually tin) can be electrolytically deposited in the pores of the anodic coating to provide colors that are more light-fast (resistant to fading). Metal dye colors range from pale champagne to black. Bronze shades are preferred for architectural use.

Alternatively the colour may be produced integral to the film. This is done during the anodising process using organic acids mixed with the Sulfuric electrolyte and a pulsed current.

After dyeing, the surface is usually sealed by using hot water or steam, sometimes mixed with nickel acetate or other anti-bloom agents, to convert the oxide into its hydrated form. This reduces the porosity of the surface as the oxide swells. This also reduces or eliminates dye bleed out and can increase corrosion resistance. Sealing at 20C in Nickel/Cobalt salts, cold sealing, when the pores are closed by impregnation is also popular due to energy savings. Coatings sealed in this method are not suitable for adhesive bonding.


Mechanical considerations

Anodising will raise the surface, since the oxide created occupies more space than the base metal converted. This will generally not be of consequence except in the case of small holes threaded to accept screws. Anodising may cause the screws to bind, thus the threaded holes may need to be chased with a tap to restore the original dimensions. In the case of unthreaded holes that accept screws or pins a slightly oversized hole to allow for the dimension change may be appropriate.

Plasma electrolytic oxidation is a similar process, but where higher voltages are applied. This causes sparks to occur, and results in more crystalline coatings.






CATEGORIES (articles) > Chassis & Bodywork Construction > Finishings > Anodising finish for metal surfaces

 
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