Corrosion is a huge burden in many industries, and defence is certainly not immune.
Aircraft are particularly susceptible as they are largely made of aluminium alloys. These exotic materials have some tremendous properties (like strength-to-weight), but can corrode readily when exposed to moisture and salt. When corrosion occurs, it can diminish the strength of the structure and – perhaps most significantly – reduce the aircraft life by promoting fatigue cracks.
A landmark study found that corrosion caused 687 aircraft incidents in the US from 1975 to 1995, leading to 81 casualties. In Australia, an F/A-18 lost a flap in flight due to extensive direct and incidental corrosion damage on the attachment lugs. Luckily, despite further damage to the airframe from the departing flap, the pilot was able to land the aircraft safely 1.
Corrosion is certainly not ignored by the ADF, who routinely perform extensive inspection and repair. It’s not uncommon for 10% of an aircraft fleet to be on the ground receiving corrosion maintenance at any one time. In one instance, the aircraft manufacturer is mandating extensive inspection regimes where up to 75% of all maintenance actions are for corrosion control 2.
We performed a detailed study into the scale of the problem and found that aircraft corrosion costs the ADF approximately AUD240 million per year. For the US DoD, the figure is over USD5 billion per year for aircraft, and over USD3 billion per year for ships 3.
But surely corrosion is only an issue for old aircraft with lots of aluminium alloy? Not so! In fact, we may have more problems with newer acquisitions…
While the latest aircraft structures can be largely constructed of materials that are not susceptible to corrosion on their own (such as titanium and carbon composites), a mix of materials can cause major complications. The current ADF fleet has profound examples of different structural metals driving galvanic corrosion. If the metals are not electrically isolated, one sacrifices itself to protect the other – just like how a battery works.
Even by themselves, novel aluminium alloys can be more susceptible to particular forms of corrosion. This wouldn’t be too bad if traditional paint coatings were used, but multiple testing programmes (including our own) show the new environmentally-friendly primers are markedly inferior.
Further, while new manufacturing processes allow for more elaborate and efficient designs, they may ultimately impede repair of the more vulnerable structure. A common method to remove corrosion is to grind it out, but the highly optimised components offer very little that can be removed. This means increased component replacement, which could be even more costly and timely than usual due to the highly integrated structure. All of this should be considered knowing that many ADF aircraft are stationed at locations around the coast considered ‘severe’ by international corrosion classification.
While improving design can reduce corrosion impacts, so can better maintenance practices. The ideal is to reduce inspections and transition to a condition-based philosophy – essentially to have the aircraft inform the operators when it needs maintenance, rather than have maintainers inspect it to a rigid schedule. If we can make this work, then the time and cost of maintenance is reduced as only necessary work is carried out. At a minimum, it allows for corrosion inspections to be deferred when exposures have been benign. Such a system is called Corrosion Prognostic Health Management (CPHM), and it needs to know the current and future status of corrosion for optimum maintenance scheduling.
The F-35 program recognised they needed a CPHM system, and BAE Systems Australia has played a key role for the global fleet of aircraft since 2009. To understand the current status, we have delivered almost 800 corrosion sensors, with 2 installed on all aircraft made. The sensors are made by our colleagues at BAE Systems in the UK, and are a smart ‘witness plate’ – it accumulates corrosion activity over time without the need for constant power or logging. They degrade with the platform, and report their status each time the aircraft is turned on.
These sensors provide the current status. To support prediction of future status we developed and supplied a prognostics model since 2016. This was a challenging task as the aircraft uses a new environmentally friendly primer that behaves completely differently to normal primers. We collaborated with the Defence, Science and Technology Group to refine our existing models, and subsequently won the Defence Industry Innovation Award at the 2017 Avalon Airshow.
Corrosion is a challenging problem now, and will remain so into the future. BAE Systems Australia is developing leading-edge CPHM technologies, and we are proud our solutions have been selected by the largest Defence program in the world.
In following blogs I’ll describe how we’ve developed a generic CPHM system to help optimise maintenance across a wide variety of other platforms.
Dr. Aaron Sudholz is the Engineering Manager of the Corrosion Group in Richmond, VIC.
1. B.R. Crawford, “A Proposed Roadmap for Transitioning DSTO’s Corrosion Structural Integrity Research into the ADF Fleet”, Aircraft Airworthiness and Sustainment Conference, session no. 11, (Brisbane, Australia, 2011).
2. A. McCreath, “The Changing Nature of Military Sustainment”, Aircraft Airworthiness and Sustainment Conference, panel session no. 2, (Brisbane, Australia, 2014).
3. D.A. Foreman, R. Baty, E.F. Herzberg, A.R. Kelly, M.V. Kumaran, N.T. O’Meara, “The Annual Cost of Corrosion for Air Force Aircraft and Missile Equipment”, LMI Government Consulting, Report MEC81T2, 2009