A crust you can trust

Magnesium, traditionally avoided by automotive manufacturers due to its lack of resistance to corrosion, could now be widely used thanks to a new surface treatment technique. Ben Hargreaves reports.

The replacement of steel with other materials in the automotive industry has become a matter of increasing importance in recent times.

Targets for reduced fuel consumption and new environmental legislation, coupled with the desire to accommodate ever greater amounts of the gadgetry demanded by consumers under the bonnet, have all seen automotive OEMs looking for lighter alternatives.

It is aluminium, of course, that has so far made the biggest impact as a replacement metal. Audi’s A8 and A2 models, the world’s first cars to be predominantly fashioned from the material, arrived to considerable fanfare when they rolled off the production line in 1997 and 1999 respectively. Jaguar has continued the trend with this year’s XJ, which similarly features an aluminium construction.

Comparatively unheralded but also beginning to play an important role in car manufacture is the metal magnesium. The favourite of schoolchildren in chemistry lessons the world over due to its spectacular, white-flared ignition, magnesium is also highly attractive to car makers.

As with aluminium, the reason is related to weight: magnesium is the lightest structural metal in the world, one and half times lighter than aluminium and approximately four times lighter than steel. It’s easy to see why weight-conscious automotive companies are keen on the material.

According to Neil Ridley, director of Arup Automotive, which provides consultancy services to the car industry, magnesium is already being used in components such as car beams, door casings, tailgates and engine blocks.

Estimates suggest that the current average volume of magnesium in cars – about 4kg – will go up to about 65kg over the next 20 years. ‘That represents quite a significant weight saving if the magnesium replaces steel,’ Ridley said. ‘And if you can make the vehicle and engine lighter you don’t have to produce as much power – it’s really a virtuous circle.’

Despite its obvious advantages, there are also a number of problems associated with magnesium that have seen its take-up in the car industry lag behind that of aluminium.

The first of these is cost. In the past it has proved to be more expensive to produce than steel and aluminium and its price has been subject to considerable fluctuation. However, the price issue has been resolved to some extent by more production of magnesium coming online and cheaper production processes – its expense has halved over the past decade and is now comparable with aluminium.

But also problematic is the material’s lack of resistance to wear and corrosion. Now it seems that a UK firm, Cambridge-based Keronite, may have come up with a solution that will see magnesium making a greater impact in all sorts of areas of manufacturing.

Keronite holds the rights to a technique called plasma electrolytic oxidation (PEO). Originally developed in Russia but brought over to the UK by its inventors in the late 1990s, PEO produces a hard, corrosion-resistant surface that is extremely effective on light alloys, including those of aluminium, titanium, and magnesium.

The process works by submerging a component in an electrolytic bath. A modulated electric current is applied that creates a plasma discharge on the component’s surface, converting the surface of the alloy being treated into its oxide. The energy in the discharge then converts the oxide into an extremely tough ceramic.

‘Typically the technique creates a surface about three times harder than that produced by hard anodising,’ said Graham Peel, chief executive of Keronite, ‘and comparable in hardness to other techniques such as plasma spraying. However unlike plasma spraying which requires line-of-sight treatment, because this is a bath process you are not restricted in the complexity of the components you can treat.’

Keronite claims that magnesium coated using the process is about 10 times harder than in its untreated state, and although PEO is more expensive than other processes like hard anodising, manufacturers are likely to find the non-toxicity of the process attractive.

Since some methods for surface-treating magnesium use harmful acids, the expense of handling and disposing of the chemicals is less with PEO. And with increasing pressure on car and other manufacturers to lessen the ecological impact of their activities, this is only likely to improve the chances of its adoption.

Magnesium is also highly recyclable, another factor which makes it attractive to automotive companies as legislation such as the European End of Life Vehicle Directive begins to bite. ‘It’s easy to get magnesium back out of the car at the end of its life,’ said Ridley.

‘These environmental and sustainable issues are part of the car companies’ brand image nowadays. They really are a crucial selling point. We produced a sustainability report for Ford on the new Jaguar, and they reprinted it in the sales brochure for the car.’

Peel said that Keronite is currently in dialogue ‘with all the major’ automotive companies about its process.

Although it is early days, he expects magnesium components coated using the firm’s technology to find their way into production vehicles by 2005.

In other industries, the process is already being used on light alloy parts that have replaced steel and iron ones, in situations where the usual surface treatment methods haven’t produced a hard enough finish, or where they have not been used at all because the shape of a component was too complex.

‘What we have here is an enabling technology,’ Peel said. ‘We’re seeing a much bigger take up of magnesium in all sorts of industries from consumer products through to mainstream automotive. We’re taking materials such as magnesium into areas that just would not have been possible before the development of our process.’

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