Maintenance Matters: Massive Metal Termites

Corrosion of aircraft structures is one of the most costly maintenance problems for the aging aircraft fleet.


Corrosion of aircraft structures is one of the most costly maintenance problems for the aging aircraft fleet. Maintenance planners need to make efficient decisions to assess corrosion damage and project a structure's future corrosion condition. This prediction uses two inputs: the current corrosion condition (determined primarily by visual or nondestructive inspection (NDI), such as eddy current, ultrasonic, etc.), and the rate of corrosion growth.

The various forms of localized corrosion, including pitting corrosion, crevice corrosion, stress corrosion cracking (SCC), and common fatigue, are particularly destructive and act like metal termites. They frequently occur without any outward sign of damage and are usually discovered coincidentally, severely impacting maintenance scheduling. When left undetected, they can result in sudden and catastrophic structural failures. Thus, it is important to develop an effective inspection and maintenance scheduling program that takes advantage of life extension technologies like corrosion preventative compounds (CPCs).

With the constraints of actual aircraft environments and inaccessible structural configurations, it becomes necessary to consider CPCs. The impact of localized corrosion can be predicted by analyzing several sampling locations within a limited area. Predicted maximum localized corrosion depths can be used to evaluate the residual life of the structure by applying the appropriate CPC in a known environment and with openly exposed surfaces. To predict corrosion damage in hidden aircraft structures with an unknown internal microenvironment can be more difficult, but CPCs are at least some protection. It's like the microenvironment drinks up the CPCs.

Corrosion studies

There have been some studies of lap joint environmental conditions and lap joint corrosion. One way to describe the existence of crevice corrosion within a lap joint is either as-built (i.e. pristine) or as-is (i.e. current aged) condition. The rate of corrosion development for any given condition is mainly influenced by the environment to which the aircraft is exposed. To predict the corrosion damage to the lap joint in a given time period, two major components should be available: information on the as-is corrosion condition of the specific aircraft assembly from NDI and the corrosion rate measurements of as-is lap joints as a function of environmental parameters (chloride concentrations, humidity, temperature, time of wetness, etc.).

CPCs function through some combination of film formation, wicking into hard-to-access areas, water displacement, and corrosion inhibition capability. The relative capabilities of these performance parameters will vary with the material. Likewise, the relative need for a particular performance characteristic may vary with the application. CPC use can occur at any level of maintenance activity.

Initial efforts are being made to address CPC performance via laboratory testing with the objective of finding what works best. This task is very complex because there are countless specific applications, exposure types, and severities, etc. There may be hidden or exposed areas subjected to continuous or alternating environments with a variety of contaminants. This is further complicated by the fact that only a small number of different CPCs will be used on a specific aircraft type. A CPC which performs the best for one performance parameter may not be as desirable as a product which performs only moderately well over all of the parameters.

Corrosion conditions might have their origins from multiple damage modes; for example, environmentally assisted cracking (EAC), intergranular corrosion (IGC), crevice corrosion, exfoliation, pitting corrosion (PC), and general surface attack. Multiple electrochemical, mechanical, and metallurgical factors have a significant effect on the rate of corrosion, and, as a result, on the structural integrity of the aircraft structure.

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