For nearly a century, materials manufacturers have known that metals exposed to demanding environments can be protected by a simple yet extremely effective process known as anodizing. By immersing metals such as aluminium, titanium and magnesium in a solution that conducts electricity and applying an electric current, the natural oxide on the metal surface thickens and creates a protective outer layer.
Although anodized layers are typically limited to a few microns in thickness and are porous, this simple process can alter the surface appearance of a metal and, importantly, vastly improve its resistance to corrosion and wear. Today, anodized metals enjoy varied and far-reaching applications stretching from construction, through the automotive industry, to consumer products.
More recent modifications have improved this process still further: by using stronger electric fields in a process known as plasma electrolytic oxidation (PEO), a wider range of coatings can be produced, and performance can be enhanced. Coatings produced in this way on aluminium, for instance, incorporate a very hard, crystalline phase of aluminium oxide called corundum – known in its naturally occurring form as the minerals sapphire and ruby – that is second only in hardness to diamond.
WHAT IS PEO?
Plasma electrolytic oxide coatings are hard, dense, wear-resistant, and well-adhered oxide coatings for metals such as aluminium and magnesium. The process by which they are grown may also be referred to as micro-arc oxidation or spark discharge anodizing. Essentially, it involves the modification of a conventional anodically grown oxide film by the application of an electric field greater than the dielectric breakdown field for the oxide. Discharges occur, and the resulting plasma-chemical reactions contribute to the growth of the coating.
Like anodizing, the PEO process requires immersion of the components in a bath, and uses electrical current to grow a uniform layer of oxide on the surface of the metal, but that’s where the similarity ends. The electrolytes are non-toxic and are free from any heavy metals. They are typically just very dilute aqueous solutions, with just a few litres per gram of salts which can be disposed of at no environmental or commercial cost. The applied electrical currents are generally pulsed at high frequencies and at voltages of over 200V.
These conditions are specially selected so as to give rise to millions of very short-live microscopic discharges – just like bolts of lightning – through the oxide layer. These expose the oxide to temperatures over twice as hot as the surface of the sun (Ref: Dunleavy). It is this intense local injection of energy which can be used to transform the oxide into high performance engineering ceramics such as crystalline alumina.
Whereas conventional anodising can only produce amorphous aluminum oxide, the PEO process can produce a phase more than five times harder (1600-2000 HV0.1) , making the resulting coating far more wear resistant than a hard-anodised layer. The coating is therefore harder than any steels ‑ or even glass and sand ‑ so there are very few things which can scratch or erode it, even on magnesium.
There are many examples of PEO coating enabling the replacement of steel parts with aluminum parts, where the lighter replacement’s harder surface means it can even significantly out-last the original. One such example is sprockets for off-road bikes, which last three-to-four times as long as the heavier steel parts which they replace.
Because the process involves melting and re-solidification of the ceramic, it has a very different structure from a conventional anodized layer. The layer doesn’t show the usual columnar pores and opening cracks on sharp corners; instead it gives continuous cover. This is particularly important in the corrosion protection of complex geometries such as threads, or the wear protection of deliberately rough or textured surfaces.
Examples of such applications include aluminum winch drums for yacht racing, and a specially formulated black coating for Mavic’s high performance aluminum racing cycle rims.
The absence of columnar pores or cracks in the coating structure also has benefits in terms of fatigue performance as PEO coatings usually result in minimal fatigue debit, and certainly less than any conventional anodising process. This has been demonstrated across a wide range of aluminum, magnesium and titanium alloys, and even on more exotic alloys such as the AMC640xa metal matrix composite selected by the European Space Agency for large structural members on the International Space Station.
Indeed, the ability of the PEO process to coat a wide range of alloys is itself an advantage: conventional anodising processes are less effective on high copper alloys such as 2XXX or 7XXX series aluminum, due to copper precipitates in the substrate inhibiting the growth of a protective oxide film but under PEO processing, there is no such limitation.
CHROME FREE PROTECTION OF MAGNESIUM
For years, chromate conversion coating has been the standard pre-treatment for paint in the corrosion protection of magnesium alloys, but environmental regulation is increasingly discouraging its use. PEO provides a non-toxic alternative as extensive testing has been undertaken to compare the relative performance of this and other candidate systems for chromate replacement. One such example is Ford Motor Co. research (reference: Blanchard) which concluded that PEO could even out-perform the chromates. As such, the PEO-based systems meet or exceed the AMS-2466 specification.
Although more costly than mere chemical passivation, some PEO processes are used in the automotive industry as a chromate replacement wherever corrosion-resistance standards are strong enough to demand it. This includes motorsport where it is the most widely used magnesium coating in Formula 1, but also several mainstream automotive applications such as the Corvette roof, processed by IHC in Detroit.
In aerospace, PEO processes are qualified for use as a pre-treatment for resins for various helicopter gearboxes, where there is a need to meet or surpass AMS-2466 in terms of corrosion protection, with minimal fatigue debit. Some PEO processes for magnesium can harden the surface to over 900 HV0.1, and thus offer the additional benefit of enhanced scratch and wear protection.
TITANIUM COATINGS
Another recent aerospace approval is the use of certain PEO process for the refurbishment of civil airliner landing gear bearing carriers, made from Ti6Al4V. The refurbished parts offer improved wear performance over the original system (ref: Martini), and improved anti-galling protection.
The use of PEO on titanium, however, is predominantly as a more functional surface in such areas as dental implants ‑ where it presents significantly enhanced tissue adhesion ‑ and as a photocatalyst in water purification, and in photoelectrochemical cells.
These are just a couple of the examples of PEO’s increasingly diverse engineering solutions, which also include thermal management, high or low temperature electrical insulation, low-reflectance optical coatings, and even protection of aluminum against reactive plasmas in semiconductor manufacturing equipment.
In summary, PEO is a very versatile technology which presents a wide range of solutions to surface engineering challenges for light metals.
Dr. Curran is with the Department of Materials Science & Metallurgy, Cambridge University, Pembroke Street, Cambridge, and is the global research and development manager for Keronite International Ltd. He can be reached at jac64@cam.ac.uk
