Corrosion Resistance in Aviation: Critical Metal Selection for Longevity

Modern aircraft experiences an extraordinary range of environmental stresses during a typical lifetime. Every flight cycle can subject materials to stresses, rivalled in few industries. Selecting materials for corrosion resistance becomes something of a vital discussion point. 

And rather than simply choosing "the most corrosion-resistant" option, engineers must balance alloy characteristics against specific environmental exposures, structural requirements, and practical economic considerations. Understanding these trade-offs forms the foundation of successful long-term aircraft design.

So, which are the more common and better materials to choose from? Well here at Dynamic Metals, we’ve combined our decades of experience below to help you make that decision. 

Aluminium Alloys: Strength and Ease of Corrosion Balancing

Aluminium has become one of the most utilised materials for the construction of airframes. However, not all types of aluminium alloys have the same levels of resistance to corrosion, and subsequently not created equally. 

The 2000 series of alloying has copper addition up to 4.5%, and this gives alloy 2024 an exceptional strength-to-weight ratio. Unfortunately, copper greatly decreases the resistance to corrosion. The alloys formed need to be coated with thin surface claddings of pure aluminium to ensure protection which leads to corrosion, dropping the strength gains to be more than 50% down.

The 7000 series alloys gain even greater strength with the additives of zinc and magnesium. 7075-T6 has tensile strength above 570 MPa. 7075-T6 is also more prone to stress corrosion cracking when compared to the rest of the alloys. The T73 temper which is more overaged than T6, loses close to 10% strength and significantly improves the overaged stress corrosion resistance.

Alloys that contain magnesium and do not contain copper belong to the 5000-series group. These types are better than the other types of alloys because they have superior resistance to corrosion. Applications with moderate loads and severe corrosion exposure are served by fuel tanks, leading edges, and belly skins.

Stainless Steels: When Corrosion Resistance Proves Critical

Stainless steel serves where the need for corrosion resistance exceeds the capabilities of aluminium, even with the added weight concerns. Components of the landing gear, hydraulic system fittings, and all fasteners installed on the structures of an aircraft depend on stainless grades for their long-term reliability.

In high alloy steels, as with precipitation hardened steels like 17-4PH and 15-5PH, the loss of corrosion resistance suffers only a weakening of the alloy. Heat treatment leads to a retention of the chromium oxide protection, sustaining the high strengths, greater than 1100 MPa. These materials endure the most severe loading in corrosion critical applications and dominate the most serious of these; components of the landing gear and fittings of the primary flight control.

Titanium: Exceptional corrosion resistance in extreme conditions

Compared to other structural metals, the corrosion resistance of titanium is exceptional. The thin, stable and closely adhering oxide film that forms on titanium surfaces offers excellent resistance to atmospheric corrosion, seawater and most acids.

The popular titanium alloy, Ti-6Al-4V, showcases excellent corrosion resistance in multiple exposures. It is highly suited for landing gear structures, engine mounts, and wing attachment fittings, as it is compatible with hydraulic fluid and resistant to stress corrosion cracking and aviation fuels, where corrosion failures are not acceptable.

Near-alpha titanium alloys containing tin and zirconium have even better elevated-temperature oxidation resistance. Beta titanium alloys retain excellent corrosion characteristics while providing processing advantages for complex geometries.

Titanium fasteners have special value in corrosion prevention. Galvanic concerns are eliminated and superior thread retention and fatigue performance are provided by installing titanium bolts in aluminium structures.

Nickel Alloys: Performance in Extreme Environments

Regarding which materials to use in engine systems, nickel-based alloys are the most popular for withstanding a variety of conditions over a long period of time. Most other materials fall apart under the extreme temperature ranges, corrosive environments, and combinations of the two. When this happens, nickel-based alloys are the materials of choice.

An alloy of nickel - Inconel 625, stands out for the brilliance of the design. It can survive the fiercest of environments and temperatures. It withstands: extreme oxidizing, acidic, saline water, and even heat.

Nevertheless, Inconel 718 is far more predominant and is much more commonplace and used because of its better, far stronger qualities. Although the better concentration of iron, Inconel 718 still possesses the very admirable quality of corrosion resistance. With Inconel 718, turbine cases, engine mounts and even high temperature fasteners - are able to provide decades of useful service under extremely harsh conditions. These elements, within other materials, which often find themselves in extremes, such as high temperatures proves to be very useful.

A good example would be components in specialised hydraulic fluids, de-icing systems, or auxiliary power systems. In these components, alloys of Hastelloy C-276 will provide great resistance to aggressive chemicals, acids, chlorides and reducing environments.

Philosophy of Material Selection  

Selecting the materials necessary to resist corrosion requires an orderly analysis of exposure conditions, consequences of failure, and life-cycle economics. Primary structures need materials which have proven durable over time, which for aviation means aluminium alloys with proper cladding and surface treatment.  

Corrosion resistant materials of superior quality are beneficial for fittings, and fasteners. While small components may seem cheap, they lead to expensive labour costs when replacement is needed. Corrosion resistant fasteners should always be specified to prevent issues down the road. Environmental exposure is highly dependent on location: skins and fuselage frames are inflicted with road debris, fluids and moisture; upper surfaces are showered with UV radiation and rain; under the spell of the engine nacelles are combustion products, and the poor unconditioned exposed to high temperatures and hydraulic fluids.  

Dynamic Metals provides various alloys, including aluminium, nickel alloy, stainless steel, and titanium. We also supply alloy steel, maraging steel, cobalt alloy and more. Our technical specialists know the atmospheric conditions around structures of airframes. We can recommend suitable materials for their best possible application. These materials come fully traceable, and the documents certifying them will prove aerospace traceable certifiable mechanical properties and corrosion resistance.  

In Conclusion...  

Whether components of the aircraft get replaced early or the aircraft achieves its design service lives, corrosion resistance is the main factor. Material selection, of course, is about balancing corrosion, performance, and other factors like strength, weight, and cost.  

No single material optimizes all parameters, as success comes with a thorough understanding of specific alloys and their exposure to a complex environment, Also, success comes with the application of the right surface protection system. Given proper inspection and maintenance, the right material selection can let aircraft structures serve for decades while preserving their structural integrity and safety margin.  

The materials defending aircraft against corrosion is a fine example of smart, demanding metallurgical engineering. Advanced, sophisticated systems with proper understanding of selection criteria can deliver systems with best structural durability and operational economics.