Corrosion control and prevention in commercial aircraft

Aircraft Corrosion Control

Corrosion is a major concern for commercial airline industries as well as military aircrafts. Undetected and untreated corrosion problems lead to structural failures and hence pose a great safety risk. Recent reports from a study conducted by the 'General Accounting Office' (GAO) on the impact of corrosion on U.S. military shows that corrosion costs consume around 5% ($10 to 20 billion) of the total military budget allocation of $400 billion. [Wynne, Michael W] Most commercial and military aircrafts have exceeded their stipulated age of 20 years and still continue to be in service. Corrosion related maintenance cost is escalating with the increasing age of the aircrafts. The annual corrosion cost for the U.S. air force which was quoted at $800 million in 1997 has increased steadily and crossed the billion mark. In the year 2000, aircraft corrosion cost touched $1.4 billion and corrosion was reported as the single largest cost driver for the U.S. navy. [Kennedy] Let us study the corrosion control issues and preventive strategies that are being followed by the airline industry in a little detail.

Corrosion Problem

Corrosion occurs because of the reaction between the materials with each other. Aluminum alloys which are used for building aircrafts are more reactive than the pure form of the metal and hence over a period of time they are vulnerable to corrosion due to atmospheric moisture and interaction with industrial fluids resulting in structural weaknesses. Pitting corrosion and cervical corrosion are the two important forms of corrosions observed in the aluminum alloys used for airplane construction. Pitting refers to the localized deterioration of the metal structures leading to increased stress concentration. Cervical corrosion occurs when a corrosive fluid gets inside the joints or penetrates the coated surface. Both the forms of corrosion are dangerous and can result in cracking of the metal surface. The problem is more pronounced in aging aircrafts which are exposed to corrosive materials over a long period of time. The 1988 Aloha Airline B-737-200 incident was an eye opener to the dangers of corrosion. Due to undetected corrosion the whole fuselage of the plane got separated in mid air. This incident led to the formation of the 'Aging Aircraft Task Force Steering Committee' which is currently known as the 'Airworthiness Assurance Working Group'. [Gerhardus H. Koch]

Corrosion Control by Coating

Using appropriate coating methods, galvanic corrosion of metal surfaces can be avoided. For example titanium surfaces are plated with cadmium and primed before they are attached to aluminum surfaces. The aluminum alloy surface is typically coated with corrosion inhibiting primers such as Skydrol-resistant epoxies. Before applying the coating the surface is anodized using phosphoric acid anodizer. [Gerhardus H. Koch] the most poplar method of corrosion prevention however is the use of chromate surface treatments of the aluminum structures. Using hexavalent-chromium compounds for surface coating is however an environmentally unsound practice as chromium is well-known as a highly toxic and carcinogenic agent. The U.S. air force has done extensive research to find an environmentally safer alternative to chromium as a surface coating agent. This research has shown promising results and already self-assembled, nano-phase particles (SNAP) is suggested as an environmentally safer and long lasting (30 years) alternative to the toxic chromium coating method. Electrochemical impedance spectroscopy (EIS) studies have confirmed that SNAP system offers better corrosion protection compared to chromate coating. [Donley]

Maintenance programs

Though tackling corrosion at the designing and manufacturing phase is key to successful control of the corrosion menace, proper maintenance also occupies a significant role in remedying the damage and in prolonging the life of the aircrafts. Detecting corrosion earlier would help avoid costly repair operations. There are numerous non-destructive forms of tests that are available to assess the level of damage without inducing any real damage to the aircraft. Ultrasonic testing, optical testing, eddy current testing, electrochemical impedance spectroscopy and radiography are some of the commonly used non-destructive testing procedures. By means of selective 'blend out', affected regions are removed and in case the damage exceeds the acceptable limit of 10% of the surface thickness the whole part is replaced. Typically, maintenance programs include daily inspection by the pilots, regular structural checks and periodic total inspection. [Gerhardus H. Koch]

Conclusion

Corrosion is a big problem for domestic and military aircrafts. Both government as well as private airlines are forced to spend a huge amount of money on maintenance operations. Corrosion preventive strategies are more cost efficient and performance boosting than remedial measures and the return on investment for corrosion control programs is very attractive. It would be forthright to implement the best possible corrosion control methods for our domestic and military aircrafts as it would not only improve the performance and life of these fleets but also be a huge cost saver on the maintenance budget. A common standard for corrosion control programs has to be evolved and all concerned industries should adhere to these standards. To address the problem more effectively it would be necessary for the government as well as the private sector which are involved avionics production to work in a cooperative manner.