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.
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.
CPCs and prevention
Corrosion occurs when the external environment contacts aircraft structure/components and that environment acts on the material to degrade its properties or performance. This exposure of specific aircraft materials and types of structure is influenced by the environment on the ground and in flight. CPCs were developed as a practical method of reducing the effects associated with environmental exposure. Various types of commercially available CPCs are applied to a wide variety of corrosion-prone structural components. Some of these CPCs are specified by the aircraft manufacturer's programs and are identified by numerous specifications and processes.
Corrosion prevention involves either protecting the aircraft from the contact or via mitigating the effects of the contact. Corrosion control involves slowing the erosion process once it has begun or correcting the damage. The best approach is to keep the environment away from the structure and components. Thus, the severity of that environment can be lessened. Corrosion prevention has many aspects, from washing and sensing, to corrosion inhibiting paint systems. This also includes the use of CPCs as a barrier to prevent contact with the environment. CPCs have been used for many years in aerospace applications to provide a temporary protective barrier when aircraft coatings systems such as FR primers are no longer protecting and they could not be immediately reapplied.
CPCs were typically inexpensive oily or waxy materials which provided some temporary barrier to environmental exposure. Some of these products contained corrosion inhibitors. However, good performing corrosion inhibitor packages tended to be expensive and/or environmentally unfriendly. Products were primarily judged for a particular application by the viscosity and the characteristics of the films that were formed.
More recently, the perceived functionality of these CPCs has shifted from a perception of a temporary barrier to being an integral part of any coatings system. The use of CPCs may be considered a top-layer or first-line protection and are therefore expected to be maintained and applied more frequently so that metal termites don't drink them rendering them ineffective. The function and effect of CPCs are shorter term than that of the paint or primer. Thus, while CPCs may be considered part of the coatings system, their role is different from the paint system. Likewise, they differ from a traditional maintenance chemical yet still have some of those attributes.
The general concepts of barrier formation and inhibition are simple enough. However, when specific materials and processes are considered, in the context of specific structural applications, the requirements for their use become more complex. While the properties of the CPCs themselves are important, they may not be as critical as the frequencies and processes by which they are applied.
The shift of the CPC paradigm has resulted in many misapplications, overstated claims, and confusion of some aircraft owners/operators. In many cases, marketing claims have far outweighed performance in determining which particular type of CPC should have been chosen. This is accentuated by the belief of some users that if it is more expensive it must be better. CPCs have been marketed as a stand-alone silver bullet to solve corrosion problems, rather than as a key component in a comprehensive corrosion program that involves many other things. This has been very counterproductive to the use of these materials, which do offer significant benefit when properly used. Improper use of these products and processes has left some disillusioned and unwilling to incorporate these materials into their corrosion prevention and maintenance programs.
One of the major ingredients for success of a CPC is a belief by the user that the material will help in the battle against corrosion. If the user believes this, then the CPC will be reapplied frequently, and in so doing any emerging corrosion will be found and repaired before it becomes major. As well, the area will be cleaned more frequently, reducing the potential for corrosion even before the CPC is applied.
Anecdotal and subjective claims of benefits by various users, coupled with some reported problems, indicate that various CPC types are needed in different aircraft locations. There has been only limited objective documentation of field experience. This experience seemed to indicate varying successes, which were driven by both materials and processes. Thus, multiple types of commercial off-the-shelf products may be used. While CPCs have traditionally been considered for internal applications only, the rapidly changing usage scenarios dictated that the external locations should also have CPCs applied.
Application and removal
CPCs come in a myriad of specific applications, each with different requirements. The different types are drying/hardening, drying/nonhardening, and thick and thin film. Several material characteristics become apparent. There is an obvious need for using a drying type CPC in areas where the collection of dirt and debris is a concern. However, what is less obvious is the need for a hard vs. soft film in areas such as wheel wells where the debris may become embedded.
The compatibility of various CPCs with other CPCs and aircraft fluids should also be considered. Some materials become gooey and pasty when exposed to hydraulic fluids. Not only do they collect dirt, but other debris such as metal shavings and pieces of insulation can get stuck. There is also the potential to transfer the residue to people and cargo. Sticky residue can also trap foreign object debris (FOD) and cause foreign object damage. Compatibility is also of concern with aircraft materials such as wiring insulation, seals, gaskets, and composite materials. Many current specifications do not address compatibility to ensure that damage does not result.
CPCs may be brushed on, fogged, or sprayed on via aerosol can or pressure sprayers. This creates an array of associated issues involving the control of film thickness. Experience has shown that severe corrosion damage is induced when CPCs plug drain paths and water or other fluids accumulate. The reapplication frequency is important to obtain the best results. The material's requirements should be met to allow for reapplication during regular maintenance cycles. External areas and those subject to loss of the film from routine operations should have the CPC reapplied subsequent to washing and at all scheduled corrosion maintenance checks. Internal and difficult-to-access areas should have reapplication whenever these areas are opened.
Removal of a CPC from exposed surfaces may be preferable when applying a thin film CPC over faying surfaces and lap joints. The objective is for the CPCs to wick into those faying surfaces. This process leaves the CPC inside the joints and hidden areas while leaving the exterior areas dry and clean. Most CPCs have little or no UV resistance, weather poorly, and are rapidly removed when in the air stream. Thus, these products are generally not acceptable for external applications in their current form. It is essential that the area to be treated is first cleaned and the corrosion removed. The barrier properties of the CPC may initially retard the corrosion growth rate but the corrosion won't be stopped. Many of these materials, particularly if applied thicker than recommended, will mask the ability to visually detect corrosion until it is more advanced and corrosion comes through the CPC.
An evaluation of CPCs should consider both cost avoidance and cost savings. Tracking aircraft corrosion maintenance actions often indicates that recordkeeping systems fail to document corrosion maintenance. Initial benefits over a short period of time may not be sustained if reapplication cycles are not short enough or if the ability to visually detect corrosion is impaired. And actual results vs. manufacturers' claims might not measure up based on the proposed increased inspection and maintenance cycle times.
Experience and testing indicate significant benefits with the proper use of CPCs as an integral part of a corrosion maintenance program. On the other hand, using CPCs alone or as a one-time application will yield little or no benefit.
Visual inspection for aircraft
Here are some general tips from Advisory Circular, AC 43-204, Aug. 14. 1997.
Preliminary inspection: A preliminary inspection of the overall general area should be performed for cleanliness, presence of foreign objects, deformed or missing fasteners, security of parts, corrosion, and damage. If the configuration or location of the part conceals the area to be inspected, it is appropriate to use visual aids such as a mirror or borescope.
Precleaning: The areas or surface of parts to be inspected should be cleaned without damaging any surface treatment which may be present. Contaminants that might hinder the discovery of existing surface indications should be removed. Some cleaning methods may remove indications of damage; care should be used if the cleaning tends to smear or hide possible indications of trouble. Surface coatings may have to be removed at a later time if other NDI techniques are required to verify any indications that are found. Some typical cleaning materials and methods used to prepare parts for visual inspection are detergent cleaners, alkaline cleaners, vapor degreasing, solvent cleaners, mechanical cleaning, paint removers, steam cleaning, and ultrasonic cleaning.
Corrosion treatment: Any corrosion found in the preliminary inspection should be removed before starting a close visual inspection. Manufacturers' handbooks, when available, are a good general guide for treatment of corrosion. Recommendations of AC 43-4A, if appropriate, should be used on aircraft for which the manufacturer has not published a recommended corrosion inspection schedule or treatment program. The AC contains a summary of current available data regarding identification and treatment of corrosion on aircraft structure and engine materials. Types of corrosion damage detectable visually are also given.
- Engineering Corrosion Prediction Model for Aircraft Structures, Sept. 17, 2002, 6th Joint FAA/DoD/NASA Aging Aircraft Conference, San Francisco, CA.
- Managing Internal and External Aging Aircraft Exposures, Sept. 17, 2002, 6th Joint FAA/DoD/NASA Aging Aircraft Conference, San Francisco, CA.
- The Use of Corrosion Prevention Compounds for Arresting the Growth of Corrosion in Aluminum Alloys, 1996, 4th International Aerospace Corrosion and Control Symposium, Jakarta, Indonesia.
- Performance Assessments of Corrosion Prevention Compounds using Laboratory Tests, Sept. 19, 2002, 6th Joint FAA/DoD/NASA Aging Aircraft Conference, San Francisco, CA.
- Fred Workley is the president of Workley Aircraft and Maintenance Inc. in Alexandria, VA, Indianapolis, IN, and San Jose, CA. He holds an A&P certificate with an Inspection Authorization, general radio telephone license, a technician plus license, ATP, FE, CFI-I, and advance and instrument ground instructor licenses.