Despite all this, VW stopped production of the Lupo magnesium tailgate with engineers citing the cost of corrosion protection as the main obstacle to future magnesium usage. Currently, most magnesium parts are concentrated in well protected regions of the car that are not exposed to corrosive environments. This includes seat frames, steering wheel armatures, instrument panel beams and other parts where a safe environment and high component integration are available.

Further away from the cockpit, e.g. door frames, cabriolet and sunroof frames, corrosion protection becomes an issue. In such parts, manufacturers can consider Keronite as one of the best anti-corrosion pre-treatments available for magnesium.

Why Keronite?

Corrosion resistance: Keronite corrosion performance on magnesium has been extensively tested by the likes of BMW, Ford, VW, General Motors, Honda and Daimler Chrysler and compared to other treatments. These manufacturers have found that Keronite has performed particularly well in both standard and galvanic corrosion and this has led to BMW issuing a ‘Freigabe’ for Keronite on magnesium parts.

Integrity on edges: Edges are often the starting point for corrosion. Anodising pre-treatments are columnar and prone to crack on edges, which provide entry points for corrosive electrolytes. To compound this, paint often thins on edges due to surface tension effects, so the top coat is thinnest in the place where the pre-treatment is cracked. Keronite has an amorphous structure which does not crack on edges. Also, it maintains a largely uniform thickness all over a component and, in fact, is slightly thicker on the edges. All this means Keronite will provide good protection even when dissimilar metals are fixed to the magnesium, because it can provide an effective chemical barrier preventing a galvanic cell from being formed.

Adhesion to the substrate: Because it is a conversion coating, Keronite is atomically bonded to the substrate.

Paintability: The porous top surface of Keronite provides an excellent key for almost any type of top coat and has been used as a pre-treatment to provide an A-class finish on body exterior panels. The excellent adhesion properties are illustrated in the table below. In these tests, cast magnesium parts were treated with Keronite and then sprayed with three different powder top coats. In all cases, the tape pull test registered a pass even after 100 hours of 100% humidity exposure at 50º C.
Phosphate bath compatibility: Our automotive customers would sometimes like the ability to pass magnesium parts through the conventional steel body paint line. The first stage of such a line is typically a zinc phosphate or iron phosphate bath of high acidity (typically pH 2) used as a pre-treatment for steel. Normally, this would not be suitable for magnesium parts, which would be quickly corroded in such an acid environment. However, our customers have found that if the components are pre-treated with Keronite and then passed through the zinc phosphate bath, they can continue through the line to be painted and subsequently survive long salt spray endurance such as 15 weeks in the German VDA cyclic corrosion test with no visible corrosion.

Electropainting: Although Keronite itself is an electrical insulator, it can still be electropainted or e-coated if certain precautions are taken and, indeed, a layer of Keronite followed by an epoxy-based e-coat can give very effective protection to magnesium at low cost.

Scratch resistance: The first line of defence in protecting magnesium against corrosion is to apply a fully protective coating and to keep it intact. One of the side benefits of the Keronite hardness on magnesium (circa 350 – 700 HV) is that it gives high scratch resistance. The tests by Hull University show that 10 µm of Keronite on its own already requires three times more force than an equivalent thickness of anodising to break through to the magnesium substrate. This can then be increased tenfold when the Keronite layer is impregnated with an organic topcoat such as e-coat, PTFE or powder-coat. This makes Keronite-coated parts are less likely to become damaged during assembly, maintenance or general wear and tear.

Industrial implementation

Two criticisms that have been levelled at PEO processes in general is that they are expensive and not scalable to the high volumes required for the automotive industry. At Keronite, we have concentrated our R&D efforts for the last three years in developing the magnesium process to address these two issues directly.

Cost effectiveness: The latest Keronite process for magnesium reduces PEO costs in two ways. Firstly the coating speed is in the range 2 – 4 µm per minute – up to three times higher than previous generations of the process. Secondly, we have improved the performance of the coating so that thinner layers are now required to achieve similar performance. The preliminary results were published at the 2003 DGM conference on Magnesium. This combination means that Keronite is often the cheapest solution that actually works. We also have several examples where customers are switching to Keronite because they can save money by eliminating other processes or manufacturing steps. For example, one customer found that 6 µm of Keronite gave higher corrosion performance than 25 µm of another PEO process + e-coat.

Full system cost: The cost of Keronite should always be viewed as part of the cost of protecting the whole system against corrosion. It has been demonstrated that the use of Keronite when protecting against corrosion can lead to elimination of costly washers, gaskets and snubbers. This has been proven to provide an overall cost down for a system when compared to the use of alternative corrosion protection coatings.

Scalability: The fully automated Keronite production line recently developed and installed in Bicester, UK at the premises of our partner, Powdertech Ltd, uses our new generation process and is capable of processing up to 3.6 m2 of magnesium parts with Keronite every 5 minutes.

Recycling: A full recycling study on Keronite-coated automotive magnesium parts is yet to be completed. However, the Keronite coating contains no heavy metals or other constituents that would have a detrimental effect on the properties of the commonly used magnesium alloys. This has been borne out in practice by chemical analysis of a sample of AM60B coated with Keronite and melted. The recommendation being that Keronite coated parts can effectively be recycled as per current practice for un coated components.

Turnkey supply
There is more to making magnesium parts than just Keronite! By working in a network with the rest of the supply chain we can serve each other in a synergistic way to provide a better product to our end customers. Keronite has co-operation agreements with Meridian for diecast parts and Timminco for extruded parts. By tonnage, Meridian is the world’s largest manufacturer of diecast magnesium parts for the automotive industry and pioneered the magnesium Front End Module: a one-piece casting that fixes to the front longitudinals of a vehicle, tying it together with good structural integrity and supporting ancilliaries like lamps and radiator. Timminco provides a range of wrought and extruded Magnesium products for customers who are looking for higher strength, as well as cast aluminium road wheels.

Nanomag
Keronite participated in the Framework 5 European NANOMAG project (Development of Innovative Nanostructural Materials for Magnesium Component Protection) funded by the EU under the banner of Competitive and Sustainable Growth initiatives. The objective of this programme was to develop new corrosion and abrasion resistant coatings for the protection of magnesium parts using a clean, environmentally-friendly and economic process. Keronite was assessed alongside PAPVD, CVD, Sol-Gel and Anomag coatings. The project was aimed at the automotive and aerospace industries which have a large contribution to CO emissions and other pollution and so are important targets for weight reduction. As well as technical performance, the project looked at life cycle cost and environmental impact of each of the coatings. The final report is available upon request.