Addressing the problem

Citation

Ford, T. (2002), "Addressing the problem", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 3. https://doi.org/10.1108/aeat.2002.12774caf.001

Addressing the problem

Keywords: Aircraft, Corrosion, Corrosion prevention

Over the years an increasing amount of information has become available regarding the effects of corrosion on aircraft. Together with the attention being given to the problems of ageing which have been emphasised by investigations into accidents and incidents, more recent symposia have sought to identify the major issues so that Corrosion Prevention and Control Programmes (CPCP) may be structured to have the best chance of addressing all aspects of this continuing concern.

A particularly cogent contribution was made to these activities at a conference a year or so ago by DERA which detailed some future challenges in aerospace corrosion control. Corrosion can compromise the structural integrity of an aircraft in a number of ways. Small areas of localised corrosion can introduce stress raisers causing the early initiation of fatigue cracks. Reduction in component section resulting from generalised corrosion will lead to losses in strength, ductility and stiffness. Bonded joints may be degraded following generation of stresses arising from corrosion product wedging. Failures in highly stressed parts may occur as a result of stress corrosion cracking and hydrogen embrittlement. Corrosion damage in hydraulic pipes and fuel lines may lead to the eventual loss of fluids and system failures. Thus, corrosion repair and rectification continues to be a major element is the life costs of both civil and military aircraft.

Starting at the build stage there are three aspects to corrosion protection: material selection; design; and protective schemes. In the first, mechanical and physical properties such bas tensile strength, fatigue resistance, density and stiffness are the first considerations. Poor material selection can lead to corrosion problems; one example is the aluminium-zinc-magnesium alloy 7075 which when used in its extruded form and heat treated to the T6 peak strength condition, is highly susceptible to stress corrosion cracking. Another instance is that of 2024 machined from thick plate where it has been found that exfoliation corrosion can develop. The approach has been to develop new tempers and alloys with reduced impurity levels which offer higher resistance to corrosion and stress corrosion cracking.

Poor design can also result in some corrosion problems. The presence of crevices, water traps and dissimilar metal contacts can also contribute to aircraft corrosion. These potential problems can be reduced or eliminated through the use of sealants and jointing compounds during the assembly of components, and the provision of drainage holes and filters to prevent the collection of moisture and aircraft fluids in internal areas.

The third aspect of aerospace corrosion control is the application of protective coatings to individual components and to the assembled airframe structure. The schemes can include pretreatment and chromate pigmented primer for aluminium alloys, cadmium plating and passivation and chromate pigmented primer for steels, among a number of measures for metals and alloys. They act as a barrierc layer but additionally provide a surface to which a paint scheme may be applied. Normal practice is 0 use a chromate pigmented primer typically 25 microns thick. In service, corrosion control depends on regular maintenance and inspection. Corrosion rectification may typically involve the blending out of corrosion damage, reprotection using localised repai schemes and the application of wax based supplementary protective schemes.

Maintaining aircraft structural integrity

Following entry into service, the maintaining of the high level of integrity of the aircraft is a continuing task which was outlined by Boeing at the same conference. A damage tolerant aircraft retains adequate residual strength to sustain anticipated toads in the presence of damage such as cracks or corrosion. Predetermined inspection cycles or the obvious malfunction of selected structure ensures that the operator can return the undamaged strength levels. A fail-safe aircraft can sustain a load higher than that of normal operating conditions until damage is found and repaired. Fail-safety is typically provided by multiple load path construction; if one part of the total element suffers damage, the other part can sustain the required load. A key element is the inspection process.

Boeing emphasise that corrosion is the deterioration of metal resulting from the reaction between the metal and its environment, as illustrated in Figure 1. Three conditions are present. Difference in electrical potential between anode and cathode (including dissimilar metals, crevice regions or grain boundaries); presence of an electrolyte (a liquid capable of conducting an electrical current); and metallic connection between anode and cathode (to provide a path for electron flow). Corrosion damage can be caused by ineffective protective finishes (as noted the proper selection, maintenance and application of the finish system and the use of corrosion inhibitors where applicable, can prevent damage by corrosion). Corrosion- causing contaminants and moisture will accumulate if drain passages become blocked or drain valves are inoperative.

Figure 1 Conditions necessary for corrosion to occur

Corrosion can be classified into different types, each requiring a best type of corrective action. Stress corrosion cracking occurs when structure is subject to a moist environment and a sustained stress. It tends to follow a single plane or path, usually related to the grain flow formed by rolling, extrudingand forging of parts during manufacture (Figure 2). Stress corrosion is minimized during the design process through material selection, recognition of secondary clamp-up stresses and proper corrosion prevention measures. In service, repeated reports prompt the manufacturer to develop service bulletin action.

Figure 2 Effect of precorrosion in service (10 years) on fatigue life in air

Exfoliation corrosion is similar to stress corrosion cracking in that it follows grain boundaries created during manufacture. However, exfoliation corrosion attacks many grain boundaries, resulting in a leafing or delamination effect. The volume of corrosion product can exceed 10 times the original material volume. In fastened joints, the pressures created can cause noticeable bulging or deformation of the structural member.

Galvanic corrosion results when two dissimilar metals are in contact or are otherwise connected in the presence of a corrosive medium. The most reactive metal corrodes after the protective finish system is damaged. In design, the use of dissimilar metals is minimized. When dissimilar metals cannot be avoided, they are isolated from one another by protective sealants. Maintenance of the sealants is essential to promote durability.

Concentration cell (or crevice) corrosion is a result of differences in the environment at a metal surface. It typically occurs in a crevice or stagnant area. A commonly encountered form is oxygen differential cell corrosion where the entrapped moisture in the crevice has a lower oxygen content than at the open surface. Additionally, when moisture and salt are present, chloride ions migrate to the oxygen-depleted zone (anode) inside the joint, creating an acidic and corrosive condition.

Filiform corrosion occurs as a network of threadlike filaments of corrosion products on the surface of a metal coated with a paint film. A form of crevice corrosion, it begins at a break in the paint film, typically around a fastener head. The development of more flexible paint systems such as polyurethane enamels has minimized the extent of this type of damage.

Pitting corrosion is a localized corrosion that begins on a metallic surface by galvanic or concentration cell mechanisms, the corrosion penetrating into the metal and forming a pit. The developing corrosion pits can act as stress concentrations that can evolve into fatigue or stress corrosion cracking.

Accidental damage to structure can have a variety of causes such as spills of liquids, impact, etc., and has to he removed as soon as possible. Whatever the cause of the defect, rapid detection, often using NDT methods, is essential to restore the level of structural integrity. Close inspection has to be undertaken after this process to ensure compliance with the Structural Repair Manual (SILM) of the aircraft.

NDE methods

Assessing corrosion damage by Non Destructive Examination (NDE) methods is most important, where the exposure of the materials in corrosive environments is detrimental to all mechanical properties. Traditionally, during maintenance cycles the aircraft components are disassembled in order to be inspected for corrosion damage and discontinuities according to design criteria. An alternative approach is the assessment of structural integrity "in situ" by NDE methods aiming at service cost reduction.

This contribution from Hellenic Aerospace Industries investigates the possibility of NDE methods such as Eddy-Current and Ultrasonic C-Scan for the detection and mapping of corrosion in aircraft light alloys. Specimens of quality Alloys were exposed in several corrosive environments and accelerated test performed in order to simulate corrosive conditions. Exfoliation (EXCO) and Alternate \Immersion Tests were applied to produce varying degrees and different types of corrosion in anodized and sealed specimens. All types of attack, e.g. pitting, intergranular and exfoliation, were investigated by the two methods mentioned and the results compared. This aims at the development of a database related to the limitations of NDE methods with respect to different characteristics of corrosion damage (type, severity of damage, size, depth, geometry, location, etc).

The specimens used for testing were of AI alloy 2024-T3 (solution heat treated and then cold worked in accordance with QQ-A-250/ 4E). From the results the measured impedance of specimens exposed in exfoliation tests, increases with the increment of time of exposure and the value of amplitude is higher than that of the alternate immersed specimens. This is owed to the type of corrosion (intergranular) resulting in the formation of delaminated layers that affect the magnetic field and the amplitude value.

The same specimens were inspected non- destructively by Ultrasonic C-Scan using Through Transmission and Mirror (Reflector Plate) techniques. The results for exfoliation specimen showed that these tests produce inspectable corrosion even after a few hours of immersion in the corrosive solution. For alternate immersion test however, several days are required for that inspectable damage by Ultrasonic C Scan. The pits created by this test, characterized by their small sizes, are difficult to inspect and are limited by the resolution of the inspection system.

The results show that Eddy Current is an appropriate method for the detection of anodized and sealed AI Alloy 2024 for a large frequency bandwidth (from 10 to 100 kHz. It is well known that the depth of penetration is inversely proportional to the frequency. The higher the frequency, he more the information about surface or near surface defects. By using lower frequencies, hidden corrosion can be detected. Impedance given by specimens produced from exfoliation tests is increasing almost linearly with the degreasin of the specimen depth. Due to non- uniformity of the alternate immersion specimen, the results gathered by the detection using Eddy Current are less straightforward.

Whereas exfoliation tests produce inspectable corrosion damage in a few hours, alternate immersion tests require several days to produce damage detectable by Utrasonics. Due to high attenuation of the signal passed brought the exfoliation specimen a higher gain is required for these NDE tests. This is because of the type of corrosion involved, which results in the formation of delaminated layers and small cracks (intergranular corrosion). Lower gain of signal is required for the specimens generated by alternate immersion tests, where the type of corrosion results in pitting. The optimum frequency for the detection of damage is 5MHz for through transmission test and 2MHz for the mirror tests.

Corrosion fatigue

This is a very complex phenomenon became two very different loading systems are acting together. In addition, its correct simulation in the lab is relatively easy if expensive for fatigue alone, but is nearly impossible for the combined condition. Notched components behave differently from unnotched material specimens in corrosion fatigue, therefore material tests are useless. It is necessary to test components with their protection systems under the relevant variable amplitude stress- time history and to simulate also the typical sequence: Corrosion-Corrosion Fatigue- Corrosion, etc., in order to obtain result which corresponds to the real behaviour in service, which is very difficult One way out of this problem would be to corrosion-fatigue test components which have suffered corrosion damage in service.

Attempts have been made in various programmes to take account of the factors involved. It is also noted that there are corrosion resistant materials in every Fe, AI and Ti-based materials group. However, as this German paper makes clear, this does not mean these materials are corrosion-fatigue resistant; on the contrary, all of them are highly sensitive to additional corrosion during fatigue, in certain states, e.g. welded. An example is the "seawater resistant" Al-Alloy 5456 which shows a disastrous effect of seawater in the unnotched state, especially in the onnotched state. On the other hand, some materials which are known to be corrosion sensitive are quite good in crack propagation under corrosion conditions. There are many unexplained questions with regard to corrosion fatigue (Figure 2).

It is suggested that in order to improve the present state-of-the-art in corrosion fatigue, the following are among the rules suggested: the component is tested not the material, and it is complete with its corrosion protection system and with any attached parts. Crevice and contact corrosion, if they occur, are included in this way. Simulate the mechanical stresses in service in the best way possible and increase the severity of corrosion by alternating the severity of the corrosion by alternating corrosion periods with drying periods. Increase the corrosion time. Fatigue test (in air or in the corrosive medium) components corroded in previous service with damaged protection systems, etc. Compare the result with the fatigue life of identical new components. It is finally suggested that "corrosion boxes" of about one half to one square metre are fitted over typical design details of the structure for the full scale fatigue test. It is thought that if these future full scale fatigue tests could be fitted with corrosion boxes, this would be the nearest approach to service possible.Please check the inserted running article title and author is appropriate.

Terry Ford