г, trials Science Forum S n?S5-5476, Vol 877, pp 485-491 •1:n4028/wmv.scientißc.net/MSF.877.485 doi7n 17 Trans Tech Publications, Switzerland

Corrosion of Aluminum Aerospace Alloys J.T. Staley Element Materials Technology, 3200 South 166th Street, New Berlin, Wl 53151 [email protected]

Keywords: aluminum, corrosion, aerospace, aloha, testing, mitigation.

Abstract. The Junkers F13 airplane, which began production in 1919, was the first plane to be built using aluminum aerospace alloys. Nearly 100 years later, approximately 1,800 new planes are being

ou:U each ycar with aluminum aerospace alloys. For the five trillion or so dollars worth of existing aging airplanes, cost of aerospace corrosion in United States alone is an estimated 23 billion dollars per war. hi addition, hidden corrosion costs have contributed to a bigger impact in the commercial

aircraft industry. In 1988, in the corrosion sensitive environment of the Hawaiian islands, an Aloha Airlines 737 aircraft suffered an in-flight failure due to crevice corrosion in the lap joint of the fuselage. After this event, the aviation technical community launched a new era of advanced technology, improved procedures and higher standards for maintaining the world's aging and cuimding aircraft. This paper discusses types of corrosion that affect aluminum aerospace alloys including crevice corrosion, pitting, exfoliation, intergranular, stress corrosion cracking (SCC) and ..•orrosion fatigue. Standardized testing to determine if the alloy is susceptible to these types of corrosion is explained and examples of how to mitigate certain types of corrosion is discussed.

Introduction Alfred Wilm studied metallurgy near Berlin and to support the German war effort, was commissioned to develop an aluminum alloy that could be used for the manufacture of ammunition. For two years he investigated the possible strengthening of Al-Cu alloys by heat treatment. Then, one day in 1906, he was experimenting with an Al-Cu-Mn-Mg alloy, which he quenched as usual and was surprised that the hardness increased for four days then remained constant giving way to a new alloy patent and by 1908 commercial production began at Durener Mct.ilwerke. Contractions of the words "Durener" and "aluminum" led to the name "Duralumin" 'Iii' t!u' new alloy, which is still recognized today. In fact, airships like the Hindenburg and the woild's first all-metal transport aircraft (the Junkers F13) was made from Duralumin. The plane was a technological success; it was able to travel long distances and carry heavy loads. However, duralumin had severe corrosion problems especially in salt spray. [1,2] In 1916, Alcoa produced a modification of duralumin, designated "17S", which is still in production l'xld\ as alloy 2017. Further Alcoa work resulted in alloys 14S (2014) and 24S (2024). Alloy 24S exhibited significantly higher strength (vs 2017) and better elongation (vs 2014-T6). It was used in tue T3 (naturally aged + cold-worked) temper and became the primary alloy for the Douglas DC3 airplane, which was introduced in the 1930's. Although these newly developed aluminum alloys had significantly better corrosion resistance than duralumin, they were still very susceptible to one or mure types of corrosion. In fact, a corrosion resistant aluminum aerospace alloy for all wrought forms •tnd directions has yet to be developed.

Factors Influencing Corrosion Substances that cause corrosion of aluminum aerospace alloys are called corrosive agents. The most common corrosive agents are acids, alkalies, and salts. The atmosphere and water, the two most «>mmon media for these agents, may also act as corrosive agents. In general, moderately strong acids ^ill severely corrode most of the aluminum alloys used in airframes. Although alkalies, as a group,

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are generally not as corrosive as acids, aluminum is exceedingly prone to corrosive attack by many alkaline solutions unless the solutions contain a corrosion inhibitor. Particularly corrosive to aluminum are washing soda, potash (wood ashes), and lime (cement dust). Ammonia is an exception because aluminum alloys are highly resistant to it. Most salt solutions are good electrolytes and can promote corrosive attack of aluminum alloys. Exposure of airframe materials to salts or their solutions is extremely undesirable. Some factors that influence metal corrosion and the rate of corrosion are; type of metal, temper, grain direction, anode and cathode surface areas, temperature, humidity, presence of electrolytes, availability of oxygen, stress on the corroding metal and time of exposure to a corrosive environment. Aluminum Aerospace Alloys. In the aircraft industry, high-strength 2XXX and 7XXX series aluminum alloys (see Table 1 below) are commonly used for primary airframe structures (fuselage skins, stringers and frames, wing and empennage skins, spars and ribs), mechanical systems (landing gear legs, cylinders, forks and struts) and fluid systems (pressure vessels and connectors). Corrosion damage of the material is very essential to the structural integrity of the aircraft. Since the material of a component is subjected to corrosion, it is expected that its critical mechanical properties will vary with increasing service time, which must be taken into account for the structural integrity calculation of the component. The most widely used aluminum alloy in aerospace is the damage-tolerant Al 2024-T3 alloy. The location of the anodic path varies with the different alloy systems. In 2XXX series alloys, it is a narrow band on either side of the boundary that is depleted in copper; in copper-free 7XXX series alloys, it is generally considered to be the anodic zinc and magnesium bearing constituents on the grain boundary. 2XXX series aluminum alloys are especially sensitive to aqueous medium containing chloride ions (seawater) because such medium favor oxidation and pitting corrosion of these alloys. Because of the electrochemical nature of most corrosion processes, the solution potential relationships among the microstructural constituents of a particular alloy significantly affect its corrosion behavior. Solution potential is not affected significantly by second phase particles of microscopic size, but because these particles frequently have solution potentials differing from that of the solid solution matrix, localized galvanic cells may be formed between them and the matrix.

Table 1. Composition of alloys

2024 3.8 - 4.9 1.2-1.8 0.25 0.30 - 6,90 0.50 , 0.50 2124 3,8-4.9 1,2-1.8 0.25 0.30-0,90 0,30 0,20 2026 3.6 - 4.3 1.0-1,6 0.10 0.30 - 0.80 0.0S-0.2S 0,07 0.05 2524 4.0 - 4,5 1,2-1.6 0.15 0,45 - 0,70 0.12 0.06 7075 1.2 - 2.0 2.1-2.9 5.1-6.1 0,30 0.18 - 0.28 0.50 0.40 7050 2.0 - 2.6 1.8 - 2.6 5,7 - 6.7 0.10 0.08 - 0.15 0.04 0.15 0.12 7150 1.9 - 2,5 2.0 - 2.7 5.9 - 6.9 0.10 0.08 - 0.15 0.04 0.15 0,12 7475 1.2-1.9 1,9 - 2.6 5.2 - 6,2 0.06 0,18*-0.25 0.12 0.10 2099 2,4 - 3.0 1.6 - 2.0 0.1-0.S 0.4-1.0 0.1-0.5 0,05 - 0.12 0.07 0.05 2199 2,3 - 2,8 1.4-1,8 0.05 - 0.40 0.2- 0.9 ОЛ-О.5 0.05-0.12 0.07 0.05 2050 3.2 - 3.9 0.70-1.3 0.20 - 0.60 0.20 - 0.50 0.06 - 0.14 0.20 - 0.70 0.10 0.08 2195 3.7 - 4.3 0.80-1.2 0.25 - 0.80 0,25 ' 0,08 - 0.16 0,25 - 0,60 0.15 0.12

Corrosion Types General. General corrosion consumes aluminum uniformly and affects a large area versus other more local types of aluminum aerospace alloy corrosion. It occurs at a relatively slow rate but left unattended over a long period, can remove enough metal to cause structural concerns. Aluminum exposed to marine, tropical and industrial atmospheres has the greatest rate of general corrosion. Pitting. Pitting corrosion is a localized form of attack where pits develop in aluminum causing localized perforation of the alloy. Pitting corrosion is confined to a point or small area that takes the

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form of cavities. One area of the surface becomes anodic with respect to the rest of the surface. The its formed by this type of attack are generally very small and difficult to detect during routine inspection. ('ré*ice- Crevice corrosion occurs when localized changes in the corrosive environment exist and lead to accelerated localized attack. These changes are generated by the existence" of narrow crevices that contain a stagnant environment, which results in a difference in concentration of the cathode reactant between the crevice region and the external surface of the material. Crevices can be formed at joints between two materials, e.g. riveted or glued sections of an aircraft fuselage. G;il\:inic. Galvanic corrosion occurs when dissimilar metals are in direct electrical contact in a corrosive environment. This results in enhanced and aggressive corrosion of the less noble metal and protection of the more noble metal of the bimetallic couple. This type of corrosion can be recognized b\ severe corrosion near the junction of the two dissimilar metals, while the remaining surfaces are relatively corrosion-product free. The driving force for galvanic corrosion is a potential difference between the different materials. Stress Corrosion Cracking (SCC). Stress Corrosion Cracking is attributed primarily to the copper content. A higher copper content provides a modification in the relative differences in electrochemical potential between matrix and grain boundary regions by the diffusion of copper into particles during overaging. The combination of strength and corrosion characteristics is attributed to the size, spatial distribution and copper content of precipitates. Failure by stress corrosion cracking is frequently caused by simultaneous exposure to an apparently mild chemical environment and to a tensile stress well below the material's yield strength. )• \ioliation. Exfoliation corrosion is associated with high strength aluminum alloys. Alloys that have been worked heavily, with a microstructure of elongated, flattened grains, are particularly prone to this damage. Corrosion products building up along these grain boundaries exert pressure between the grains and the end result is a lifting or leafing effect. Damage often initiates at end grains encountered in machined edges, holes or grooves and can subsequently progress through an entire section. Corrosion Fatigue. Corrosion Fatigue is a process where a metal fractures prematurely under conditions of simultaneous corrosion and repeated cyclic loading at lower stress levels or fewer cycles than required in the absence of the corrosive environment. Corrosion can create discontinuities (pits, cracks, etc.) that act as origins of fatigue cracks with significant reductions in life at all stress levels. Corrosion effects are also well known to produce accelerated fatigue crack propagation. [4]

Meet! For Testing On April 28, 1988, a 19 year old Boeing 737-200 with 89,090 flights operated by Aloha Airlines Inc.. as flight 243, experienced an explosive decompression and structural failure at 24,000 feet while en route from Hilo, to Honolulu, Hawaii. Approximately 18 feet of the fuselage separated from the airplane during flight. The Safety Board determined that the fuselage failed catastrophically at a lap joint along a stringer allowing the upper fuselage to rip free. Multiple site damage (MSD) can be used to describe multiple fatigue cracks along a rivet line. It is probable that after a lap joint disbonded, numerous small corrosion fatigue cracks in the lap joint joined to form a large crack, which grew very fast and caused catastrophic failure. The Safety Board believes that Aloha Airlines had sufficient information to have implemented a maintenance program to detect and repair the lap joint damage. 1 he aviation industry and civil aviation authorities formed the Airworthiness Assurance Task Force ( ЛЛ1F) to address airworthiness issues relating to aging aircraft. Among the issues addressed by the task force was the need for corrosion programs. The AATF developed a Baseline Corrosion Prevention and Control Program (CPCP) applicable to each aging major transport airplane model that is Published in the manufacturer's documents. These Baseline Programs are a starting point where successful programs may safely evolve based on the operator's own service experience. The Baseline Programs recognize three levels of corrosion that are used to assess the CPCP effectiveness. Level 1 corrosion found during the accomplishment of the numbered Corrosion Tasks indicates an effective

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program. Level 2 corrosion indicates that program adjustments are necessary. Level 3 corrosion is an urgent airworthiness concern requiring expeditious action on the part of the operator to protect its entire model fleet. The FAA must be notified immediately upon determination of Level 3 Corrosion. In summary, the Safety Board believes that the Aloha Airlines maintenance department did not have sufficient manpower, technical knowledge, or required programs to meet its responsibility to ensure the continued structural integrity of its airplanes. Therefore, as a result of its investigation of this accident, the National Transportation Safety Board recommended that Aloha Airlines initiate a CPCP designed to afford maximum protection from the effects of harsh operating environments (as defined by the airplane manufacturer). Mandatory corrosion control programs were developed and introduced from the start of 1992. These required that all operators have in place prevention, inspection and testing systems sufficient to ensure that hazardous corrosion never occurs. [6] Another need for corrosion testing is for quality control and alloy development.

Testing Techniques Non-Destructive Testing (NDT) (UT, RT, PT, ET and VT). Non-destructive testing portable hardness and metallography can be conducted at one of Element's accredited non-destructive testing (NDT) labs or onsite on new or used aluminum alloy aerospace forms or components to find corrosion defects and lower corrosion costs without having to destroy the part. Ultrasonic (or UT) done mostly manually in air and automatically under water (or by immersion) testing full thickness by looking for indications based on the time it takes ultrasonic sound waves (or energy) to travel through aluminum alloys. Radiographic (or RT) uses high energy x-rays to travel through the part and leave a film record of indications based on density differences between the aluminum alloy and the indication. Penetrant (or PT) is an economical technique that uses red dye or fluorescent penetrant to penetrate into surface defects such as pitting or SCC corrosion. Electromagnetic (or ET) uses electrical conductivity and eddy currents to inspect for surface defects and corrosion as well as measure anodize or coating thickness. Finally, visual (or VT) uses magnifying lenses, stereo scopes, borescopes and scanning electron microscopes to inspect for surface defects. ASNT level II and III technicians do all the testing in accordance with prescribed standards and specifications. The job of the NDT inspector is to find the corrosion while it is still within acceptable limits. Without NDT, the cost of maintaining and flying in airplanes would increase dramatically, while the safety of flying would decrease. [5] Destructive Testing. Destructive testing is used for quality control and alloy development, which is critical for most components of a commercial airplane and that can be performed by Element in accordance to ASTM standards or customer requirements. General. General corrosion tests are typically performed on samples placed in salt spray or high humidity chambers where after a certain amount of time the surfaces are examined and the samples weighed. Pitting. Pitting corrosion is tested by preparing samples, putting them into a salt spray or high humidity chamber and evaluating them for pitting after so many hours of exposure. Micro examination can also be used. Crevice. Crevice corrosion of aluminum alloys is also tested by preparing samples with a crevice, putting them into a humidity or salt spray chamber and evaluating them for crevice, pitting and other forms of corrosion after so many hours of exposure. Micro examination can also be used. Galvanic. Galvanic corrosion is tested by connecting two dissimilar metals in an electrolyte, measuring potential difference and evaluating them for galvanic corrosion after so many hours of exposure. Micro examination can also be used as when Element's metallurgical engineers conduct coirosion testing of clad alloys to measure core metal diffusion and clad thickness. Stress Corrosion Cracking (SCC). Stress corrosion cracking testing is done by preparing samples, loading in fixture (which can be constant strain or constant load) alternating immersion in salt water and examinating samples for cracking after so many days. Micro examination is conducted if needed.

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|"\!'<>li:ition. Exfoliation corrosion is tested by preparing samples, corroding in acid, conducting a comparison and micro examination based on standard criteria. Corrosion Fatigue. Corrosion fatigue and fatigue crack propagation is tested by running these tests in air or argon and comparing to tests run in a particular corroding environment such as NaCl solution or high humidity.

Mitigalion Maintenance. Maintenance inspection programs should be created for all specific aircraft. Corrosion nrone areas should be cleaned, inspected, and treated more frequently than less corrosion prone areas. I nome exhaust gas deposits are very corrosive. Hence, inspection and maintenance of exhaust trail areas should be made. Exhaust deposit buildup on the upper and lower wing, aft fuselage, and in the horizontal tail surfaces will be considerably slower and sometimes completely absent from certain aircralï models. Except for special requirements in trouble areas, inspection for corrosion should be a part of routine maintenance inspections; i.e., daily or preflight. Mclad. \lclad is a composite of non-corrosion resistant core and corrosion resistant pure aluminum clad material approximately 5-10% of the core thickness bonded together on one or both sides during lun rollmir. Invented by Edgar Dix of Alcoa Laboratories around 1927 to solve corrosion problems with 2WX alloy aircraft sheet (Alclad solved many of the corrosion problems of duralumin during the 1930s) cladding is commonly used by the aircraft industry even today. Pure Al is most sacrificial io \l-( 'u alloys, which make cladding of these 2XXX alloys very effective against corrosion. In 1944, \lco.i produced Alclad alloy 75S-T6 (7075), which showed enough improvement in corrosion icsi-laiKv to be used on the B-29 bomber, which ended the war with Japan Anodi/ing. Anodizing refers to the process of electrolysis, which thickens aluminum oxide film that form. luiurally on the surface of aluminum thus creating another layer of protection for the aluminum from con ( ision. Typical tests for anodized coatings include coating weight, corrosion resistance, paint 'idhcsion. thickness and abrasion resistance. Coating. Coating is the most practical and effective means of protecting against corrosion of aluminum aircraft alloys and consists of a surface where a corrosion-inhibiting primer is applied. Drainage. Drainage of all structures is vital to prevent fluids from becoming trapped in crevices. The entire low er pressurized fuselage is drained by a system of valved drain holes. Boeing effectively eliminates the potential for joint crevice corrosion by sealing the fay surfaces with a polysulfide. Hnisbiiiji. sealing, and drainage provide most of the corrosion protection for airplane design. Corrosion-Inhibiting Compounds (CICs). Corrosion-Inhibiting Compounds offer additional protection, especially when periodically reapplied in service. Effective corrosion control is established by applying non-penetrating CICs onto aircraft paint cracks. CICs greatly retarded or inhibited corrosion development and have been introduced in corrosion maintenance programs such as CI'CPs CICs are found to discourage moisture entry until a more permanent treatment, such as a paint touch up or sealant is used. Surface Enhancement. Surface enhancement of metals, inducing a layer of surface compressive iesickuil stresses in metallic components, has long been recognized to enhance fatigue strength. The fatigue strength of many aircraft components is often improved by shot peening (SP) and low pressure burnishing (LPB). The depth of compression is critical in preventing failure from corrosion pits. If the overall depth of compression exceeds the depth of pitting then fatigue failure from pitting is militated. Whether used as a repair process or during initial manufacture, LPB increased the fatigue life of the specimens by greater than an order of magnitude compared to SP. Major structural forgings are shot peened to improve the fatigue life of aluminum parts and to reduce susceptibility to

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Perspective Improving corrosion resistance of aluminum alloys is promising from a temper and new alloy development outlook. Unlike the mitigation techniques mentioned above, new alloys and tempers are more of a root cause solution rather than a short term, possibly non efficient or effective fix. Tempers. Tempers such as T7XX alleviated stress corrosion and exfoliation problems inherent with the T6 temper. This is due to the discontinuous precipitates along grain boundaries in the overaged condition versus the continuous grain boundary phase for alloys tempered to T6. Alloys 7079-T6, 7075 -T6 and 2024 - T3 contributed to more than 90% of the corrosion service failures of all high-strength aluminum alloys. Changing 7075-T6 alloys to a T74 overaged temper provides excellent corrosion resistance, especially the resistance to SCC. Similarly, 2024 - T3 changed to 2024 - T8 has dramatically improved the corrosion resistance. This development continues today, with the T77 tempers being utilized with special alloy compositions to attain levels of strength and corrosion performance not matched by previous materials. Heat treatment practices such as retrogression and re-ageing have also improved the SCC and exfoliation resistance compared to T651 tempers. Another example to increase resistance to corrosion and SCC is the replacement of 7XXX-T651 aluminum plate on upper wing skins with 7XXX-T7751 plate, which is not as susceptible to corrosion.

Alloys. Alloys that are more corrosion resistant than current alloys need to be developed to eliminate the bandage type fixes such as cladding applied to non-corrosion resistant alloys. In the last ten years, there has been a renewed interest in Al-Li alloys. This new interest is being driven by the challenge to meet significantly higher performance requirements demanded by the new commercial aircraft currently under development. For example, a third-generation Al-Li alloy 2050-T84 was recently approved for lower wing structures. A main reason for this is that Al -Li alloys are more corrosion resistant than traditional aluminum aerospace alloys. [7] Recent alloy developments produced a new generation of Al-Li alloys that provide many property benefits such as excellent corrosion resistance by sometimes adding zinc to some Al-Li alloys to improve corrosion resistance. For fuselages, compared to 2024,2199 Al-Li plates have lower density and significantly better stress corrosion and exfoliation corrosion resistance. Due to the significantly better corrosion performance of 2199, the sheet product does not require cladding for corrosion protection, unlike the current 2024-T351 [8].

Summary Alfred Wilm discovered an Al-Cu-Mn-Mg alloy, "Duralumin", which was used for the world's first all-metal transport aircraft (the Junkers F13). The plane was a technological success but duralumin had severe corrosion problems especially in salt spray. Many factors influence aluminum aerospace alloy corrosion such as type of metal, temper, grain direction, temperature, humidity and availability of oxygen. Some common Al-Cu, Al-Zn and Al-Li aerospace alloys and their composition were given in a Table and some common types of corrosion affecting these alloys (General, Pitting, Crevice, Galvanic, SCC, Exfoliation and Fatigue) were discussed. The need for testing these alloys for the presence and susceptibility of corrosion is due in a large part to failure of a 19 year old Boeing 737-200 with numerous small corrosion fatigue cracks in a lap joint. Also, corrosion testing needed to help reduce corrosion including NDT (UT, RT, PT, ET and VT), portable hardness and metallography along with destructive testing of the common types of corrosion for quality control and alloy development was discussed. Mitigation of the corrosion discussed including (Maintenance, Alclad, Anodizing, Coating, Drainage, CICs and Surface Enhancement) was reviewed and the positive outlook of future aluminum aerospace alloy corrosion based on tempering and Al-Li alloy development was researched and presented.

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References

j 11 Inlormation on http://en.wikipedia.org/wiki/Duralumin [2] Information on http://www.centennialofflight.net/essay/Evolution of Technology/metalplane/ Techl5.htm j [>R ifessor Robert Sanders, Chongqing University course notes [4| Information on https://www.nace.org/uploadedFiles/CorrosionCentral/Aircrat%20Corrosion.pdf [5| Information on http://www.newsweek.com/boeings-737-aiфlane-prone-problems-63629 ],-,] information on http://avstop.com/stories/aloha.html [7] Information on http://www.elinorcorp.com/uploads/TALAT Lectures Corrosion Control of \luminum.pdf I н I In formation on https://www.alcoa.com/global/en/innovation/papers_patents/pdf7LMT2007 110.pdf

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