Hard anodising: strong coating, long life

Aluminium components are racked on an electrically conductive rack prior to commencing the anodising process.

Manufacturers’ Monthly speaks with Accurate Repetition Engineering design engineer Brett Turner about the long-term benefits of DECO Industrial’s hard anodising service.

Raw aluminium is susceptible to corrosion if exposed to pollutants. However, aluminium tends to naturally oxidise, forming a hard skin, almost as hard as diamonds, which protects the aluminium beneath from corrosion. To enhance corrosion protection for aluminium parts and machine components, as well as increasing their wear resistance, this layer can be made thicker through a process known as anodising.

Anodising is an electrolytic treatment process which grows an even aluminium oxide skin on the surface of the component by immersing it in an acid solution. The resulting coating increases the corrosion, scratch, and chemical resistance of the part.

For an even more durable aluminium oxide coating, aluminium parts can also be hard anodised. The hard anodising process involves growing a denser, more hardwearing aluminium oxide layer into and out of the surface of aluminium components. Achieved by lowering the temperature of the acid bath, this highly durable coating has narrower pores and significantly increases durability and corrosion resistance. Hard anodising is especially useful for tough engineering applications with high temperatures or high levels of wear.

DECO Industrial – the protective coating arm of DECO Australia, the aluminium finishing experts – offers both standard anodising and hard anodising capabilities to suit project requirements. As the only commercial hard anodiser in NSW – and one of only a handful of commercial hard anodisers in Australia – DECO has capabilities to hard anodise a range of custom parts up to 2.8m long.

During the anodising process, components are immersed in an electrolytic sulphuric acid solution, which causes an aluminium oxide coating to grow on the surface.

The hard anodising process imbues aluminium parts with high resistance to corrosion and mild acids and increases durability and heat resistance up to 800°C, which is especially beneficial for engine parts. Hard anodising improves the longevity and lifespan of aluminium components, reducing the frequency of replacement and allowing more efficient use of machinery with less downtime. This can be extremely beneficial for manufacturers and their customers.

One such company that has seen the benefits of DECO Industrial’s hard anodising for more than ten years is Accurate Repetition Engineering. The company manufactures CNC-machined metal components for their customers’ machines, servicing the engineering, defence and automotive sectors.

“The bulk of the product that we make would be primarily made out of aluminium,” said design engineer Brett Turner. “On one hand, we do bright anodising for appearance and protecting the aluminium, but the hard anodising that DECO do for us is quite specific because it is subject to wear.”

An example of a part requiring a wear-resistant coating would be a piston or a valve in a turbo charged car.

“They see quite a lot of cycles, and we found that the regular bright anodising will wear out after five years of use,” Turner said.

Accurate Repetition Engineering switched to DECO’s hard anodising 12 years ago, and according to Turner, since the company began using the process, the parts have never worn out.

“Hard anodising is much thicker and far more resistant to abrasion,” he said. “It’s a grey finish, so it’s not really used for a decorative appearance. The part that we hard anodise is housed within an assembly, so you don’t see it, but the wear characteristics are much better than regular bright anodising.”

Although hard anodising is a tougher finish than regular anodising, according to Turner, the vast majority of aluminium parts are protected by regular anodising. Some anodisers also offer an alternative “hard” anodised finish by adding chemicals to the acid baths – but this form of “hard” anodising is not as effective. “True” hard anodising is achieved by lowering the temperature of the acid baths to between -2 and 0 degrees Celsius, which creates a thicker aluminium oxide build up. Using ‘true’ hard anodising will result in greater durability – and greater longevity for components.

“The interesting thing in the automotive industry is everybody always incorrectly advertises hard anodising,” he said. “That term unfortunately is misused. Hard anodising is not regular anodising, and it’s not actually to do with the hardness.

“Companies will often advertise their parts as being hard anodised, but in fact they’re not. That’s something that we make sure we explain in our advertising.”

Turner said the standout quality of DECO Australia’s hard anodising service is the consistency, which ensures parts that are subject to “extremely fine” machining tolerances can function together.

“We effectively have six tolerances to deal with, and these parts have to fit together precisely,” he said. “The housing that it sits in is bright anodised, so we have machining tolerance, we have an anodised tolerance, and then we have the sleeve, which has machining tolerance inside and outside, anodising tolerance, and then a piston itself which is brass.

“All of those parts have to fit and slide together, and that can cause a bit of a headache, so the DECO consistency with the anodising thickness certainly is a benefit when we settle these things so we don’t have to worry about the parts not fitting together properly.”

Anodised and black dyed components after the anodising process has been completed.

DECO Industrial offer true, low-temperature hard anodising from 30 to 100 microns and tests its anodising processes to the US military standard MIL SPEC to ensure the highest quality.

DECO’s anodising process

  • The part is racked on a specially coated rack designed to conduct electric currents;
  • The aluminium part is chemically cleaned to remove surface contaminants;
  • The part is dipped in an acid solution with an electric current running through it, which creates a buildup of aluminium oxide; (for hard anodising the temperature of the bath is lowered to 0 degrees Celsius)
  • An aluminium oxide layer grows into and out of the product, converting the surface of the metal into a coating;
  • The microscopic pores of the aluminium oxide coating can be sealed or coloured or left as they are depending on the application.
  • The parts are inspected for quality before leaving DECO according to customer specifications.

Anodising applications

  • CNC machined parts ;
  • machinery components e.g. Cogs, plates, panels, bodies and cases;
  • interior or exterior applications;
  • window and door frames;
  • door handles;
  • shelf brackets;
  • appliance parts (eg. grills, pans, trays);
  • motor vehicle components (eg. Trims, wheel covers);
  • light fixtures.

For more information on DECO Australia’s hard anodising services visit deco.net.au/protective-coatings.

For more information on Accurate Repetition Engineering visit https://www.accuraterepetition.com.au/.