Aluminium has long been valued for its lightness, something that has made it a first-choice material for the construction of aircraft. However, there has been a price to pay for its use.
To prevent the corrosion that occurs when aluminium is exposed to the atmosphere, which could ultimately lead to catastrophic failure at altitude chromate coatings have been used on aluminium surfaces as a way of keeping air at bay for over half a century.
Unfortunately, these coatings have carcinogenic qualities, leading to numerous incidents including the high profile case involving environmental activist Erin Brockovich, given wide publicity through the film named after her and released in 2000.
The toxic and adverse health effects of chromium have been known since the 1890s. However, these chromate coatings were only banned from use in consumer and vehicle applications.
Industries such as defence and aerospace were exempted from the ban. With no viable alternative, there was little option if the cost, both in human and financial terms that could result from weakened aluminium and the airborne failure of planes, was to be avoided. Consequently, the use chromate coatings have continued to this day.
But now there is an alternative, thanks to the efforts of a research team at the University of Nevada, who have developed an environmentally-friendly coating that offers a viable and safer way forward.
Though its formulation has not yet been fully optimised, the team are confident that this new coating will soon be widely adopted by the aerospace industry. And given the coating’s performance characteristics, they have little doubt that it offers the answer to what has been a long-term and intransigent problem.
The solution has been a long time coming, primarily because of the need for an alternative to exhibit one of the prime characteristics of chromate coatings – the ability to ‘heal’ itself. This means that coating components near to a scratch or other damage will migrate to it, effectively protecting the aluminium beneath and preventing its exposure to air.
The new molybdate-based coating can achieve this self-healing with equal effectiveness, yet is environmentally friendly and can be applied to all aluminium products and surfaces.
To determine that molybdate was present after a scratch test and to demonstrate that the aluminium had self-healed, the development team used a combination of advanced surface analytical techniques including Raman microspectroscopy, Fourier transform infrared spectroscopy, energy dispersive spectroscopy, secondary ion mass spectroscopy and X-ray photoelectron spectroscopy along with electrochemical testing.
The development of the new coating is the culmination of 14 years’ hard work by lead scientist and Assistant Professor of Materials Science and Engineering at the University of Nevada, Dev Chidambaram. The current formulation has been arrived at after the testing of over 300 previous formulations.
Its widespread adoption by the aerospace industry will be a welcome step in removing the use of a carcinogen from the environment. However, it remains part of the manufacturing process in other industries.