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Alloyed pleasures

Technical feature Aluminium

Anodised aluminium is undergoing a renaissance thanks to technological developments that offer improved consistency and a wider range of colours. Dave Parker reports.

Until 1886 aluminium was no more than a chemical curiosity, a light, soft, brittle metal with an electrical conductivity 60% that of copper and so reactive that it immediately formed a protective skin of aluminium oxide when exposed to the air.

Then, following the development of large scale electrical generation, came the invention of electrolytic smelting. Industrial scale production of aluminium began. Alloys of aluminium and materials such as silicon, magnesium or manganese were found to have very desirable properties - and anodising was invented.

'Basically the idea was to enhance the natural film of aluminium oxide with a thicker, denser coating of oxide which made the surface harder and more durable, ' explains Alcan product manager Robin Furneaux.

'But anodising produces a transparent film about 25 micron thick - which obviously suggested that it could be coloured in some way to enhance its appearance.'

By the 1980s most anodisers were able to offer a limited range of stable colour finishes based on metallic pigments, mostly various shades of bronze. Alcan called its process for producing these effects Anolok. But by the end of the decade bronze tints were beginning to fall from favour with architects and specifiers. In the search for new tints, Alcan experimented with new techniques, eventually perfecting a process which yielded new shades of blue-grey and a unique diffraction effect that added extra sparkle.

Anolok II was licensed to two anodisers in the UK - LHT and Heywood Metal Finishers. The original range of colours proved very popular, but demand for even lighter tints continued.

Now Pale Blue Grey has been added to the range and looks likely to be at least as popular as the original darker shades.

All methods of applying tints and colour to the anodic coating rely on the fact that at one point near the end of the anodising process, the film of aluminium oxide is full of deep pores. Normally these are filled by hydration products after the anodised aluminium goes through its final stage, immersion in boiling water. So in the early days the aluminium was simply dipped into vats of organic dyes or reactive inorganic 'dyes' such as ferric ammonium oxalate.

Furneaux explains: 'Unfortunately the organic dyes weren't colourfast and the inorganics reacted right on the surface of the film, making them prone to damage from abrasion and so on.

'What was needed was a technique that put a stable colouring agent well down into the pores, where it would be much better protected.'

By the mid-1960s Alcan and others were experimenting with the electrodeposition of other metals into the pores. Nickel, tin and cobalt were all tried. All three gave a broadly similar spectrum of bronzes, ending in a near total black. By the mid-70s Alcan had settled on cobalt for its patented Anolok process.

This was claimed to be a much superior process to the alternative 'integral colouring system', which relied on expensive organic acids and a lot more energy to completely fill the pores with dye. Soon, however, all anodisers were faced with the challenge of polyester powder coating, which gave a wide range of much more consistent colours - although durability and corrosion resistance were not as good as anodisation.

Furneaux confirms that one of the biggest technical problems with anodising has always been consistency of colour. Minute variations in the chemical composition of the alloy, the complex geometry of some extrusions and variations in power supply made achieving total consistency very difficult.

Alcan's latest solution to this problem is its J57S alloy, specially formulated, the company claims, to give a consistent, more metallic finish to anodised sheets and extrusions.

More was needed, however. A new palette of colours had to be developed. Alcan looked again at the basics of the Anolok process and introduced another stage: anodising in phosphoric acid after preliminary anodisation in traditional sulphuric acid.

'This produces pores with a bell shaped section, much wider at the base, ' Furneaux explains.

'So when a metal is deposited there, its surface is much flatter than in the original Anolok process.

'We discovered we were getting interference effects between light reflecting from the deposited metal and light reflecting off the aluminium substrate, which gave some very exciting visual effects.'

Nickel was the chosen metal, producing the blue-grey tints Alcan reckoned the market was looking for. Alcan marketing manager John MacNamara says this new range is not the only reason anodised aluminium is coming back into fashion.

'There's growing appreciation that aluminium itself has environmental benefits [see box].

And anodising as a technique uses no unpleasant organic solvents, while its residues, mainly aluminium hydroxide sludge, can be recycled economically.'

Green aluminium?

Green pigments are notoriously difficult to handle, whether for fabrics, wallpaper or aluminium.

According to Graham Cheetham, product development manager at Uxbridge-based anodiser LHT, his company will produce a green anodised aluminium if asked - 'but it's not our favourite colour'.

Cheetham says: 'We produce more than a dozen colours using the Sandalor process - blues, reds, turquoises, even tangerine - but green is the most challenging because mottling is always a risk.'

The Sandalor process combines both metal and organic pigments.

As in the original Anolok process (see main story), cobalt is electrolytically deposited at the base of the pores in the anodic film to produce a 'base coat' ranging from pale bronze to almost black. Then a high pressure spray of ultraviolet stable organic pigment is applied as a top coat before the coloured product is steam sealed.

With the cobalt at the bottom of the pores and the translucent dyes higher up both contributing to the final appearance, LHT is sufficiently convinced of the durability of the latest generation of pigments to offer a 25-year guarantee on all Sandalor products.

Green aluminium?

Producing aluminium from bauxite ore is an energy hungry process. The ore, mainly from Australia, has to be chemically converted into pure alumina (A l2O3 ) then shipped to the smelters, usually in the northern hemisphere.

There the alumina is dissolved in molten cryolite (sodium aluminium fluorite) at 1000C and electrolytically converted to pure aluminium in cells consuming up to 500kW of electrical energy each.

Both carbon dioxide and carbon monoxide are emitted during the process. Each tonne of aluminium produced started life as five tonnes of bauxite and directly consumes at least 14,000kWh of power during the smelting process. Add in the energy used in mining and transportation, and primary aluminium production looks far from environmentally friendly.

Aluminium producers, however, point to several key mitigating factors. Two in particular are directly relevant to greenhouse gas production. The first is that smelting is increasingly concentrated in areas where there are abundant supplies of cheap, non-polluting hydro electricity. Producers claim that between 1990 and 1998, for example, the emissions of greenhouse gases due to primary aluminium production in the UK fell by 65%.

Even greater savings come from increased recycling. Aluminium can be recycled almost indefinitely without measurable loss of performance.

Already more than 70% of UK aluminium building components are recycled, a process that uses 95% less energy than primary smelting.

There are subtler advantages too. Cars, HGVs and ferries that substitute aluminium for structural steel use less fossil fuel. Aluminium building components need less energy to transport and erect. And aluminium, particularly when anodised, has a very long working life.

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