The patented Keronite® process uses Plasma Electrolytic Oxidation (PEO) to transform the surface of light alloys such as aluminium and magnesium into a dense, hard ceramic with outstanding resistance to corrosion and wear. As a ceramic, the surface has other useful characteristics: it acts as a thermal barrier and an electrical insulator, and yet, unusually for a ceramic, the Keronite® layer remains flexible and resistant to cracking or chipping and provides extremely good adhesion for scratch-resistant topcoats.
Because of the flexibility and multi-functional nature of Keronite® ceramic surfaces, the industrial applications are extremely wide ranging: from automotive body and engine parts to consumer products, textile machinery, tooling and moulding applications, and more recently, architectural panels for the external cladding of buildings.
The process is very eco-friendly. It is completely chrome-free and the proprietary electrolyte solutions contain no ammonia or heavy metals. There is no hazardous waste, or the associated costs of disposal, and the process is both simple and safe to operate. Components treated with Keronite® can often be re-processed rather than replaced and initial tests by a leading automotive manufacturer indicate that treated parts present no problems when it comes to recycling.
During the process, a sophisticated electrical current is passed through a bath of Keronite® electrolyte solution. Parts are suspended from a bus bar and submerged in the electrolyte, inside a stainless steel electrode cage. A controlled plasma discharge is formed on the surface, fusing the oxides of the substrate alloy into a harder phase. Acoustic vibration in the tank works in synergy with the complex electrical pulses to ensure that the ceramic layer is as smooth, hard and compact as possible.
Because of the nature of the process, the Keronite® layer is self-regulating and a uniform thickness is automatically achieved, even along the edges and inner surfaces of complex shapes. This can often be an advantage over conventional dip processes, which can produce points of weakness around critical edges. As an immersion process, Keronite® has much greater throwing power than plasma-sprayed ceramic surfaces or other line-of-sight processes.
Processing time is dependent upon the thickness of the Keronite® required and the size of the parts being treated, but the ceramic layer will typically grow at around 1 micron per minute on aluminium and up to 4 microns per minute on magnesium surfaces. However, because there is no requirement for aggressive etching or other complex pre-treatments, productivity rates across the system as a whole are impressive.
The Keronite® layer grows both above and below the surface of the component being treated and when examined under an SEM, three distinct layers can be detected as follows:
- A thin intermediate layer <1 µm providing a strong, molecular bond between the metal substrate and the ceramic
- A hard, dense, functional layer of fused ceramic, which provides the protection against wear and corrosion
- An outer porous layer making up 10-20% of the total coating thickness which acts as a perfect base for top coats
Three times harder than hardened tool steel and almost twice as hard as hard chrome surfaces, the Keronite® layer on aluminium has a nanoscale microstructure and the characteristics of alumina. Depending upon the alloy used and the thickness of the coating applied, the hardness of Keronite® on aluminium can reach 2000 HV. The 2000 series alloys with a high copper content will generally produce the hardest Keronite® surfaces. Independent tests have demonstrated that Keronite® on aluminium is seven times more wear-resistant than hard anodising, far less prone to cracking, and easily out-performs electroless nickel in ball-on-disk tests.
On magnesium, tests conducted by the University of Cambridge concluded that hardness of around 700 HV will be achieved on a typical AZ91 alloy, making it harder than most hard anodised aluminium and enabling the much wider use of this strong, lightweight material. In two-body abrasive wear scenarios, magnesium alloy parts coated with Keronite have 20 times the wear-resistance of untreated metal, similar to that of case hardened steel. Keronite® also eliminates the high friction and galling normally associated with magnesium. Once polished, the Keronite® surface has a friction coefficient of less than 0.15 against steel.
Keronite as a ceramic is inert to most chemicals and corrosive environments and performs extremely well in terms of preventing corrosion of both aluminium and magnesium. Giving up to 1,000 hours of corrosion resistance to ASTM B117 rating 9, unsealed Keronite® on aluminium performs four times better than electroless nickel and twice as well as sealed hard anodising.
One of the key factors limiting the wider use of magnesium is its poor resistance to atmospheric and galvanic corrosion. Keronite® Ltd believes that it has the solution and that Keronite® ceramic can achieve results far superior to those of conventional conversion coatings or anodising techniques. Tests conducted by The Welding Institute (TWI) revealed that magnesium alloy AZ91 D with 35 microns of Keronite® can survive one month of immersion in a saline solution and 1,000 hours in a salt spray environment without significant visual evidence of corrosion attack.
Tests conducted by the University of Cambridge and rated in accordance with ASTM D1654 demonstrated that 10 microns of Keronite® on AZ91D magnesium alloy together with e-coat (McDermid Electrolac High Build XD4434 and BASF’s GV82/9438) or powder coat (H B Fuller P4M5229 polyester) consistently achieve a top rating of 10 after 750 hours in salt spray (ASTM B117), even on scribed samples.
In tests to determine resistance to galvanic corrosion, 10 microns of Keronite® on AZ91 cast magnesium together with a layer of powder coat produced the best results, scoring a rating of 10 after 2,000 hours, in accordance with ASTM D1654.
The thin outer layer of the Keronite® ceramic surface is more porous and therefore somewhat softer, rougher and more friable. This can easily be removed by gentle polishing using emery paper for improved tribological performance in high wear applications. Typically, half of the porous outer layer is removed to give a 1 micron Ra surface finish, but in many instances this technology layer can be beneficial as, it provides an ideal base for impregnation with decorative paints and lacquers, or functional composites such as PTFE, lubricants or other metals.
Independent tests conducted on Keronite® as a pre-treatment for adhesive bonding demonstrated that it can be used as an effective, eco-friendly alternative to phosphoric, chromic or sulphuric acid anodising. When used as a pre-treatment for A-class automotive paint finishes, there is an added benefit in that magnesium components treated with Keronite® can be painted in a conventional steel-body paint process, passing through the phosphate bath without fear of corrosion.
The scratch resistance of Keronite® is known to be superior to that of conventional pre-treatment systems because of the unique surface characteristics. Scratch tests conducted by the University of Hull demonstrated that Keronite® performs three times better than anodised magnesium. The adhesion of the composite layer to the substrate can be enhanced threefold by the application of an e-coat top coat, and improved almost ten times over with powder coat. Further tests have demonstrated that a solvent-based paint could not be removed at all from the Keronite® layer by means of the tape test described in ASTM D3359-97.
Atomically bonded to the substrate alloy, the Keronite® ceramic layer has adhesion properties similar to the fracture strength of the alloy itself. The risk of delamination is therefore minimal, making it suitable for applications where cracking and chipping of the surface can be problematic.
As a form of ceramic, Keronite® also provides an effective thermal barrier. Research scientists at the University of Cambridge have conducted a programme of research into the thermal barrier properties of Keronite® coatings, on both aluminium and magnesium. Using a 10 kW Keronit®e machine, they have determined that a layer of Keronite® on 6061 aluminium alloys has a thermal conductivity of approximately 1.6 +/- 0.3 W/m/K. At less than 10% of that of compressed alumina ceramic, and in the order of 1% of that of the parent metal, it is clear that Keronite® can offer an effective thermal barrier. In the case of magnesium, the conductivity is even lower, at 0.8W/m/K.
Tests conducted by the University of Sheffield demonstrated that unlike conventional anodising processes, Keronite® causes no more than a 10% reduction in the endurance limit of magnesium alloys. Because the Keronite® technology provides dense and uniform oxide ceramic layers with a fine-grained micro-structure, it is suitable for components such as magnesium wheels, which experience fatigue loading and can have a tendency to crack.