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Anodising

Anodising (anodizing) covers a range of techniques, with different electrolytes. But each is an electrolytic process, primarily for aluminium alloys, transforming the surface layer into a hard aluminium oxide. The part is made the anode in an acid bath, with lead cathodes, and a closely controlled DC voltage is applied across them.

The main industrial processes are:

Sulphuric acid anodising

Sulphuric acid anodizing produces layers 10 to 15 thick. Typical applications are:

Chromic acid anodising

Chromic acid anodizing is typically 1 to 2 thick, often used as a pre-treatment prior to painting, as well as for some corrosion protection. Typical applications are:

Tartaric/sulphuric acid anodising

Tartaric sulphuric anodising is typically 5 to 7 thick, promoted as an alternative to chromic acid anodizing. Typical applications are:

Hard anodising

Hard anodizing is typically 25 to 50 thick, used for providing wear resistance where surface contact loads are light, for instance in the low stress abrasive wear caused by food or pharmaceutical products. It is also used for corrosion protection and thermal or electrical insulation. It can be combined with polymers for a wider range of low friction and non-stick applications (see the web pages on the Apticote 350 and Apticote 355 ranges). Typical applications are:

Coating structure

Anodising takes place at the metal interface, so that the oxide grows out of the surface, converted from the metal. Since the oxide has twice the volume of its parent metal, the coating grows 50% out from the original surface and 50% into the original surface. In consequence, the increase in the size of the anodised part is half that of the coating thickness, a factor to be taken into account when considering size tolerances.

As the coating grows, the electrolyte and electrical current must continue to gain access to the oxide metal interface, producing more oxide. Since aluminium oxide is an insulator, this could be a problem, but the coating structure forms a matrix of porous (sub micron size) columns, from the surface to the interface, and oxide conversion continues at the base of the columns. As the coating thickness builds, and the electrical conductivity to the interface decreases, the rate of coating growth falls exponentially, so that the process cannot continue indefinitely.

Hard anodised coatings are usually sealed to improve their corrosion protection, that is filling or hydrating the pores in the columns, using either hot water, chromates or acetates.

The columns can also be filled with an ultra-fine aqueous dispersion of PTFE, improving the friction and wear properties of the coating (Apticote 350), as well as its corrosion protection.

The most dense coatings are produced from pure aluminium alloys. The presence of alloying elements like copper and silicon produces macro-porosity in the coating, reducing its corrosion protection properties.

Substrates

Some of the more common alloys that are regularly hard anodised are:

Generally, the purer the alloy, the harder the coating. Alloys with silicon greater than 10% are unsuitable, as are alloys with more than 5% copper.

Thickness

A typical hard anodised layer is 50 (0.002 inches), but the Apticote process, with special jigging and power supplies, allows thicker coatings up to 75. The unique Apticote 300M, which involves proprietary processing parameters, can produce coatings up to 150 thick, and is ideal as a thermal barrier. (Note that only 50% of the coating grows outwards see above).

Surface finish and coverage

It is important that the alloy surface be as smooth as possible before anodising. The subsequent increase in roughness after anodizing depends on the alloy, the purest giving the smoothest coatings. E.g. on 6082, a 50 coating will increase an original value of 0.2 Ra to 0.4 Ra

In terms of coverage, with the Apticote 300N process Poeton can successfully anodise down blind holes and along the inside of tubes to a depth equal to 10 X diameter. A small area is left uncoated at the electrical contact mounting points.

Sharp corners give problems, since the coating grows in two orthogonal directions and tends to crack. All corners should therefore be radiused for best results.

Hardness

The coating is essentially alumina, with a compositional hardness of over 2500Hv (Vickers Hardness). But anodised layers have an open structure; so that the measured micro-hardness is typically 400 to 500Hv (25 or 50g load).

The purest alloys give the highest hardness and wear resistance, since alloying elements like copper and silicon cause additional porosity in the layer. Poeton Apticote 300M is softer (around 300Hv) deliberately so. It is designed for more ductility and thickness, as a thermal or electrical barrier.

Corrosion resistance

Hard anodizing on the purest alloys, like 6082, gives moderate corrosion protection, further enhanced by di-chromate sealing.

Coating salt mist endurance

For the best corrosion protection with anodised coatings (>1000 hrs), consult our web pages on anodic polymer composites (Apticote 350).

Wear resistance

Apticote 300N hard anodising is ideal for low stress abrasion situations, where the inherent hardness of the alumina coating can resist wear from a wide range of products, provided the loads are light.

The coating can provide wear resistance superior to that of hardened steel, so that it is suitable for use with abrasive food and pharmaceutical products.

Machining

Hard anodised coatings can be honed, ground or lapped, using light cuts. If tolerances are wide, polishing by hand can remove superficial roughness and bring back the original finish. When tolerances are tight, grinding or honing with fine SiC abrasive is recommended.

For polishing and lapping, use Boron Carbide abrasive in heavy oil or petroleum jelly. With an alloy like 6082, a finish of 0.05 Ra is achievable.

Process control

Regardless of the process type, anodising requires strict controls on the bath parameters, including composition, build-up of dissolved metals, temperature and pH. There is also a tight specification on the electrical parameters - the DC voltage, it's ramp-up and ramp down, the allowable AC ripple, the anodising time and the maximum time period parts can be in the bath before anodising is commenced. Any of the electrical parameters can vary according to the aluminium alloy being processed.

Poeton Apticote anodising, irrespective of the type, benefits from the following advantages:

Mechanical Strength/Fatigue Strength

Losses in tensile strength due to anodising are very slight and have not prevented the widespread use of aluminium into many high-tech industries. However, anodised aluminium is not recommended for high-impact situations. Hard anodising can reduce the fatigue strength of the base material by up to 47%, depending on the alloy and the coating thickness. However, this loss can be held at 16-20 % by modifying the sealing procedure, provided a little reduction in abrasion resistance can be tolerated. Where fatigue mechanisms exist, the components benefit from ceramic shot peening before coating.

Dielectric Strength

Breakdown voltage increases with film thickness. Coatings normally withstand voltages in excess of 1,000volts DC. Unlike most insulators, Apticote 300 hard anodised coatings can operate at temperatures up to 2,000°C. Its combination of high dielectric strength and thermal stability make Apticote 300 the ideal surface for mounting electrical components.

Heat resistance

Hard anodic coatings can withstand short exposures up to 2000°C, making the Apticote 300M process ideal for rocket venturi. Long exposure to high temperature, such as hard anodised pistons, shows no adverse affect on performance, other than surface crazing.

The coefficient of thermal expansion of the coating is five times that of the base metal.

Pitfalls

When sending work for hard anodising, please take note of the following:

Other anodizing bath formulations for aluminium

Anodising can be undertaken in various organic acids, notably oxalic and sulfosalicylic acid for colouring, phosphoric acid, as a preparation for adhesives, and in borate or tartrate baths for electrolytic capacitors. Plasma electrolytic oxidation is also a form of anodising, achieved at high voltage with a variety of waveforms.

Other alloys that can be anodised

Titanium is anodised for colouring, the different hues being a result of interference colours produced by the very thin oxide (around 0.1 micron), with reds, greens and blues being common. Magnesium is anodised for improved corrosion resistance and as a precursor to painting. Thickness is typically 5 microns.

Environmental aspects

Potential environmental problems occur only with the chromic acid anodising variety process, with the use of a hexavalent chrome electrolyte (chromic acid). Hexavalent chrome is considered to be toxic and carcinogenic, and there are some claims of a higher level of cancers amongst operators working with the chemical. Unlike Chrome plating, which also uses this electrolyte, buoyant gas emissions from the anodising bath are low, so that inhalation hazards are slight.

In a drive to remove the use of hexavalent chrome from all processes associated with the production of aircraft, the manufacturers are planning to eliminate chromic acid anodising as a process. It is being replaced by tartaric/sulphuric acid anodising, with Airbus leading the way in Europe. The electrolyte is relatively benign (tartaric acid is use in wine making), and tests have shown that the new process produces anodised layers with the required corrosion protection and paint bonding properties.

The most common anodising effluents from non-chromate baths is aluminium hydroxide and aluminium sulphate, both of which are recycled for various industrial uses.

Contact us for complete information regarding EU directives on the use of chromic acid anodising, Health & Safety information and reputable sources of independent advice.

Can I Specify chromic acid anodising?

Yes, there remain applications in defence and aerospace that will continue to use it for the next few years. But it will be phased out on future Airbus work. For instance, the Airbus 350 plane is declared as free from any process using hexavalent chrome.

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