Product category:
Materials and components
News Release from: Keronite | Subject: Keronite
Edited by the Engineeringtalk Editorial
Team on 19 September 2002
Green solution for corrosion-free
magnesium
A new surface pretreatment technology for magnesium components adds a layer of ceramic that is not only extremely hard and corrosion resistant, but is also protected from phosphate attack.
The magnesium market is going through a period of transformation The production of aluminium alloys is no longer the main application for primary magnesium
This article was originally published on Engineeringtalk on 6 Mar 2003 at 8.00am (UK)
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The plasma electrolytic oxidation process transforms lighter metals such as aluminium and magnesium into a hard, dense ceramic that is extremely durable and resistant to corrosion and wear.
It was overtaken by die casting for the first time in 2001 when over 100,000 tonnes were shipped to die casters around the world.
This represents around a third of all magnesium production.
The market for cast magnesium components has been growing at around 15% per annum in recent years and forecasts for the next decade are even more bullish.
Much of this increase in demand is coming from the automotive sector where designers are clearly starting to appreciate the benefits of this under-rated light alloy.
The use of magnesium in cars is expected to increase from an average of less than 4kg per vehicle today to at least 65kg by 2020 and as much as 150kg in some models.
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There are a number of benefits that magnesium brings, not only to the automotive industry but also when used in electronics, luggage, hand tools and other applications where portability is important.
First, magnesium is the lightest of all structural metals: magnesium components equal in strength are 40% lighter than steel and 20% lighter than their aluminium counterparts.
This enables car manufacturers to meet regulatory requirements for lighter weight vehicles and a reduction in emissions, despite the demand for more and more onboard equipment.
Secondly magnesium has excellent mechanical properties: good tensile and fatigue strength as well as valuable elongation qualities.
This makes it suitable for the consolidation of parts, enabling manufacturers to reduce the number of joints and costly steps in the production process.
Compared with aluminium, magnesium is very ductile and demonstrates good shock or impact resistance and as such, is ideal for use in automotive safety applications.
It also has unique damping qualities, absorbing energy such as noise or vibration: qualities important in any transport or electronic application.
The small draft angle facilitates the production of near net shapes, even for the most complex of high precision components and this helps to keep machining costs to a minimum.
Nowadays, any part that can be made from plastic can be cast in magnesium and because thinner walled components can be used, the volume and cost of raw materials can usually be reduced.
In terms of physical properties, magnesium has other advantages over aluminium: it has a lower density and a much lower melting temperature.
Higher casting rates can be achieved, die life is extended by a factor of three or four, and machining costs are further reduced.
Modern magnesium alloys are easier and cheaper to recycle than most plastics, and this is becoming more and more important with the increasing trend towards total life cycle analysis, particularly among the major car manufacturers faced with regulations such as the End of Life Vehicle Directive in Europe.
With supplies of high quality casting materials in abundance, and more high-volume, low-cost capacity shortly coming on stream, magnesium prices eventually fell below those of aluminium in 2001 and are expected to remain low throughout the next decade.
The last few years have seen a real breakthrough in the development of new casting alloys: primary producers are shipping consistently good quality material and scrap rates at the casting stage have been cut dramatically.
Improved casting technology has reduced costs, increased productivity rates and eliminated much of the waste seen in the past.
Magnesium is for the first time being considered as a viable alternative to plastic.
Magnesium producers have long been under attack by the green lobby because of their use of the dangerous greenhouse gas SF6 in the primary production process.
The new facilities are being constructed to run much greener technology and other producers have undertaken to reduce and gradually eliminate these harmful practices.
Further downstream, improvements in die casting scrap rates have also been viewed favourably.
Some of the larger casting companies now have very efficient in-house recycling systems where waste has been all but eliminated.
The one thing slowing the growth in the magnesium market is the reputation it has for being so highly susceptible to corrosion.
Galvanic corrosion is a particular concern and even the smallest surface scratch can quickly cause problems when magnesium is used together with steel or other metals.
Because of this, it is practically impossible to use magnesium alloys without first treating the surface and most parts need to be given some type of chemical coating even before finishing.
Without some kind of pretreatment to protect the parts during storage and shipment and to and improve the adhesive qualities of the surface, paint applied directly to magnesium is subject to unsightly alkaline peeling.
This can have serious consequences for a finished component, detracting considerably from its perceived value.
Acid treatments have traditionally been used to protect the surface of magnesium.
These include phosphoric, chromic or hydrofluoric acid solutions which, although generally successful in enhancing the durability and adhesion of the topcoat, give little protection from corrosion or wear.
Some of these processes have serious health hazards associated with them and are slowly being phased out: chromic acid is regulated as a known carcinogen and the use of fluorides is now strictly controlled by OSHA.
Phosphate treatments have been found to be unsuitable for high volume, automated lines.
The phosphate process, particularly where there are midprocess stoppages, can cause magnesium parts to start dissolving in the phosphating tank, quickly killing the bath and leaving the surface of the magnesium part pitted and more susceptible to corrosion.
Unlike aluminium, magnesium cannot easily be isolated from the phosphate solution.
This causes scrap rates to rise, productivity to fall and costs to increase considerably.
A new pretreatment technology, developed by Keronite of Cambridge UK, can overcome these problems.
A magnesium component pretreated with a layer of Keronite ceramic is not only extremely hard and corrosion resistant, but is also protected from phosphate attack.
The Keronite process uses a similar bath and power supply to those used in traditional anodising, but differs in that it creates a plasma discharge on the surface of the substrate metal.
Independent tests have repeatedly shown that Keronite produces an extremely strong bond between coating and base metal.
As a result it provides far superior scratch-resistance and much better wear and corrosion resistance than any other commercially available process.
The Keronite technology also has environmental advantages: it uses an electrolyte comprising 98% demineralised water and because of the low concentration, the process can be interrupted at any time without damaging the components being treated.
Moreover, after a simple post-treatment to precipitate out any solids, Keronite licensees have the approval of local water authorities to dispose of used solutions directly into mains drainage systems, eliminating the need for expensive waste treatment.
The Keronite process transforms the surface of the substrate alloy into a hard, dense ceramic.
The outer layer is porous, providing an ideal base for subsequent finishing with paint or lacquer, for adhesives or for composite coatings such as PTFE or electroless nickel.
The inner layer, being extremely dense, provides excellent protection from corrosion.
Independent tests have demonstrated that magnesium parts coated with Keronite will withstand 1000h in salt spray without the slightest sign of blistering, corrosion or creep from the scribe line.
The world-beating combination of new low-cost magnesium alloys, improved casting and moulding techniques and Keronite surface treatment technology opens up new possibilities for magnesium as a replacement for steel, plastic and other light alloys such as aluminium or titanium.
Design engineers are starting to see the benefits of working with magnesium for transport casings such as cam covers and gear box housing; for external parts such as automotive roof rails; and for consumer applications such as spectacle frames.
As confidence grows, so does the number of new applications: Keronite has made possible the development of magnesium-framed wheelchairs and bicycles, magnesium cases for binoculars and personal electronics such as mobile Ophones and lap top computers.
The possibilities like the supplies of magnesium - are endless.
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