Chrome-Free Coatings, The Future is Not Without its Challenges

In some areas of the aerospace industry, military and defense operators are well ahead of their commercial aviation counterparts when it comes to innovation and new product development. In others, it tends to be the other way around.

When it comes to the drive towards sustainability, however, and protecting the environment, both are on a similar trajectory, and this is particularly evident in the field of aerospace coatings.

The Chromium Legacy

New product innovation in aerospace coatings has come a long way in a comparatively short period of time. The largest area of innovation is around the reduction of harmful chemicals that have a long history of proven performance.

Chromate-containing primers have more than 50 years of flight data for the most critical aircraft components. Going chromate-free, specifically removing Chromium VI, is one challenge; becoming non-chrome completely, and removing all types of chrome from the total system including the surface treatments is a more complex challenge altogether.

While it seems a subtle difference, chromate-free versus chrome-free, it is a huge change in how aircraft are protected and requires a major mind shift within the industry. Chrome has been the standard for corrosion prevention of metal parts. Chrome is a known carcinogen and can be dangerous in its initial application, but even more so when an aircraft exterior protected by a chromate primer is stripped and sanded back to bare metal or composite.

The US Military’s first step on the journey to moving away from chromate primers was the addition of “Class N” to NAVAIR’s MIL-PRF 23377. This is the principal performance specification that covers the requirements for corrosion-inhibiting, chemical, and solvent-resistant, solvent-borne epoxy primer coatings, used across multiple branches of the military. While the new class introduced chrome-free primers to the military, the system still required the use of chromate pre-treatments.

A hybrid solution paves the way

NAVAIR’s MIL-PRF 23377 CL N is a hybrid solution in the journey to remove harmful chemicals from the industry. It is, in effect, the ‘bridge’ that starts reducing the use and reliance on

chrome and lays the foundation of trust needed to shift to a chrome-free world. That initial step from NAVIAR paved the way for the US Air Force to release MIL-PRF-32239. Senior leaders in the US Air Force committed to moving to chrome-free systems for all their aircraft. Both of these specifications have led to the introduction and development of new coatings, including Aerodur HS 2118, Aerodur 2122, Aerodur 2100 MgRP, and the soon-to-be-launched AkzoNobel MIL-PRF-23377 Class-N primer.

MIL-PRF-32239 established the requirement of a chrome-free coating system, consisting of a non-chrome surface treatment, a chrome-free primer, and a highly durable single-stage topcoat for aircraft exteriors. The adoption of this specification within the Air Force has accelerated change. With the release of this new specification, many platforms immediately started evaluating the technologies available, and thanks to the successes already realized within the commercial industry, the Air Force has moved quickly to make chrome coatings a thing of the past.

An excellent example of how the Air Force is moving away from chrome is the Boeing-built KC-46 tanker, a military version of the Boeing 767. Combined efforts between The Boeing Company and the Air Force have led to the qualification of Aerodur HS 2118 CF primer from both organizations. The primer not only provides excellent corrosion protection, but it also meets military and commercial aerospace requirements for fluid resistance (including hydraulic fluids) and optimizes the system adhesion of the exterior decorative finish. In this example, Aerodur HS 2118 CF is a technology born out of the commercial aviation space and from which the military is immediately benefiting.

Building the Foundations

The acceptance of chrome-free systems within the industry has not been a short journey nor has it been without its challenges. Like any major change, acceptance takes time, testing, failure, redevelopment, more testing, and proof. AkzoNobel released its first versions of chromate-free exterior primers in the late 1990s and has spent the last 30 years making the technology better. First efforts are rarely the best, but with the release of second, third, and fourth-generation coatings the technology has improved to the point where it is on par with chrome coatings.

Aerodur 2100 MgRP, is an example of an earlier generation of a chrome-free primer. 

When initially released for trial, evaluators were so surprised by the efficacy of its performance, that many doubted the results and had the product re-tested. While its corrosion-inhibiting properties were superb, the product presented application challenges that early adopters were unwilling to accommodate.

Chromate primers are non-discriminating during application and will go anywhere and everywhere, which is advantageous. Chrome-free primers, on the other hand, tend to provide

protection ‘locally’ and are fairly specific to the pre-treatment, the paint stack, and the substrate they are trying to protect.

Painters who had become used to the performance of one coating over another now found themselves with a primer that appeared less forgiving in its application, certainly at the beginning. Now those same painters are more familiar with the product, and its idiosyncrasies, they can optimize their own and the product’s performance. This learning curve was necessary for Aerodur 2100 MgRP to gain acceptance. While initially developed for the military, Aerodur 2100 MgRP has found its place in the world of General Aviation.

Protected Investment

The continued development of new and more robust chrome-free solutions was not hampered by the pandemic. the steep decline of the commercial aviation space is well documented, however, the US Military’s mission never changed, and all branches are dedicated to adopting safer and more environmentally friendly alternatives to chromates.

Changes in procurement strategy and the guaranteed investment of the U.S. Government are helping organizations commit to the long timelines and investment needed to change the industry. Universities, Research Organizations, and Governments around the world are also committing to long-term investment and partnerships with corporations to make the removal of chrome in aircraft coatings a global movement. This partnership between the private and public sectors is how companies, including our own, have been able to commit to longer-term investments in Research and Development (R&D).

As of today, the vast majority of accepted chrome-free solutions were designed for the exterior of the aircraft. However, internal structural components and fuel tanks are the next step on the journey. These applications are being actively targeted, and solutions are being developed now. The AMS 27725, a multi-segment, industry specification for protecting fuel tanks, has been the first place for new chrome-free products to land. AkzoNobel’s Aerolith P27CF primer, which features a Lithium-based inhibitor package in preference to chromates, has been approved on this specification. Not only has it been proven successful in protecting fuel tanks, similar to the non-discriminating properties of chromate-primers, Aerolith P27CF has also shown to be extremely effective at protecting structural parts in preventing corrosion.

Challenging Future

Certainly, the future is not without its challenges. The use of Lithium as part of the corrosion inhibitor package achieves good results, but worldwide demand and capacity for Lithium present their own issues. The difficulty is not that there isn’t enough Lithium to go around, but rather mining and production capacity is limited, while the metal is much in demand. This means the defense sector is having to compete for raw materials in the short term against

“When it comes to the drive towards sustainability, however, and protecting the environment, both are on a similar trajectory, and this is particularly evident in the field of aerospace coatings.”

huge growth sectors such as the Electrical Vehicles market, which has a voracious appetite for consuming everything in its path.

Specifications are also evolving and can sometimes appear contradictory. The low-gloss/flat matt requirement for the topcoat used on the F16 is a good example. The low gloss has to be balanced with the need for the coating to be ‘fine grind’, but a coating whose particles are finely ground invariably delivers a ‘shinier’ finish! This required considerable innovation from the R&D teams to resolve, and military specifiers understand there is often a ‘trade-off’ to be had, both in finish and performance.

What we are likely to see over time is the hybrid MIL-PRF 23377 specification superseded by specifications akin to MIL-PRF 32239. Whereas currently the chrome-free pre-treatments available are principally used to clean and promote adhesion to increase the effectiveness of the primer and topcoat, they will, in turn, evolve to include their own inhibitors, thus further improving performance.

We will also see even further increased synergies between the commercial and military aviation sectors. This is already evident from the examples quoted earlier. But they are also apparent in the field of Urban Mobility and the development of ‘flying cars’ and Uber-style VTOL vehicles in the General Aviation space.

Constructing a large number of smaller airplanes will mean significant changes in manufacturing processes, and greater use of robots. ‘Traditional’ painting processes allow for the paint to be mixed and inducted, to achieve its ‘best state’ before being applied by a team of painters. Fast, automated lines, such as those already used in the automotive sector, will require coatings that require zero induction time for the robots to apply, and that is a significant hurdle yet to be overcome.

A chrome-free future

Military leaders have understandably made sustainability and the safety of their personnel a key priority and as such are very ‘bought in’ to a chrome-free future.

The introduction of chrome-free primers, however, is only the first step on what promises to be a long and exciting journey. Our vision is bold: to remove all chromates from the entire paint system, and to develop products that are not just focused on the airplane’s exterior surfaces, but also its structure.

Innovation is being driven by sustainability, which means that all future products must deliver the performance benefits that the military demand. They must protect their assets, their people, and the environment.

A new generation of chrome-free structural primers, for example, is already being tested and evaluated, which means the mission has already started, and momentum is building. Both the military and the civil aviation pioneers have the desire for a more sustainable future and are learning from each other, and that desire has to be matched to appropriate and unambiguous legislation. When it comes to future innovations and partnerships, the winners will be those firms that can continue to invest and have the widest portfolio of products to flex and adapt across multiple markets and platforms.