Aerospace’s Conflict with Corrosion and Chromates

Aerospace’s Conflict with Corrosion and Chromates

The continuous conflict with corrosion within the aerospace industry needs to be managed across the whole supply chain to ensure aircraft can fly for as long as possible and as safely as possible. But that’s just the start of the battle.

Corrosion and wear are extremely significant factors within the aerospace industry, not only because aircraft protective requirements are incredibly high, but corrosion is expensive and costs the industry a staggering $2 trillion per year. Alongside this, it’s now faced with increasing legislative demands and sustainability requirements that need to be met.

More than 1,700 substances are under regulatory control in the aerospace industry, including surface treatments and materials such as paints, sealants and fire protection because they are carcinogenic, mutagenic or toxic for reproduction.

Many of the corrosion inhibiting products in use contain hexavalent chromium (Cr6+) for example chemical conversion coatings and primers. European and UK REACH legislation already restricts the use of hexavalent chromium containing substances to aerospace due to its carcinogenicity. The authorisation for this use is set to expire in 2024, beyond which their use in the UK and EU could be prohibited. The tight regulation of these compounds means that suitable alternatives must be investigated and implemented to ensure that product performance is maintained with respect to corrosion performance.

So, what can be done?

A number of attempts have been made to introduce Cr6+ free materials and processes, however, the solutions have been found not to be as robust as expected, due in part, to the lack of validated and accelerated testing methods, resulting in unexpected failures.

These regulations could be seen as a barrier to market, however, Marshall, global leaders in C-130 aircraft maintenance and support, are looking to implement REACH compliant solutions that are reliable, innovative and designed for sustainability.

Through sound methodology, Marshall has been evaluating and risk assessing chromate free products, in order to provide customers with robust alternatives to Cr6+ chemicals that can be utilised throughout their supply chain and accepted by military customers.

Chris Botting, Materials and Environmental Engineering Manager at Marshall adds:

“A key feature on aircraft structures is their ability to absorb and dissipate lightning strikes. In order to do this, there needs to be a conductive pathway between all the panels on the aircraft. This is known as electrical bonding.

"The majority of aircraft structure is comprised of aluminium alloys which are anodised and painted to protect them from corrosion. Aluminium is a highly reactive metal that readily forms a natural oxide under normal conditions. This oxide is insulative and prevents electrical bonding from occurring. To overcome this a chemical conversion coating is applied to the surface after deoxidising which inhibits the growth of this oxide film and allows an electrical current to flow.

"Without this protective conductive layer, the aluminium would corrode resulting in deterioration in the mechanical properties. The most commonly used chemical conversion coating to allow electrical bonding is based on Cr6+. Marshall has introduced a Cr6+ free alternative into our special processes division which utilises a trivalent chromate conversion coating (Cr3+) which is non-carcinogenic.”

Chemical processes and products are present in every imaginable industrial ecosystem in Europe today, from health and energy, to transport and housing, which makes the need for safer, more sustainable alternatives hugely important. They are key components of materials used in solar panels, electric batteries, wind turbines and building insulation materials, in addition to playing a vital role in the everyday needs of society, from medicines to clothes, and of huge benefit to the economy.

The UK and EU already have sophisticated chemicals laws in place, but global chemicals production and widespread use of chemicals are expected to double by 2030. These are important drivers why Marshall is focused on innovation and assessment for safer and more sustainable chemicals in the products, projects and services it offers.

Marshall engineering and design departments have long-established working relationships with partners and customers addressing safe-and-sustainable-by-design solutions to deliver alternatives to Cr6+ - restrictions on which have the potential to change future aeronautical engineering completely.

Whilst REACH regulation and the EU Chemical Strategy boost innovation and investment to better protect humans and the environment from harmful chemicals, Marshall through its practical experience with chemical regulation also views these as opportunities to strengthen value chains and promote the need for alternative chemical choices to customers.