CORROSION PROTECTION OF AEROSPACE GRADE
CORROSION PROTECTION OF AEROSPACE GRADE MAGNESIUM ALLOYELEKTRON 43 FOR USE IN AIRCRAFT CABIN INTERIORSSarah S. BaillioThesis Prepared for the Degree ofMASTER OF SCIENCEUNIVERSITY OF NORTH TEXASAugust 2013APPROVED:Peter Collins, Major ProfessorJincheng Du, Committee MemberNarendra Dahotre, Committee Memberand Chairman of the Departmentof Materials Science andEngineeringCostas Tsatsoulis, Dean of the Collegeof EngineeringMark Wardell, Dean of the ToulouseGraduate School
Baillio, Sarah S. Corrosion protection of aerospace grade magnesium alloyElektron 43 for use in aircraft cabin interiors. Master of Science (Materials Scienceand Engineering), August 2013, 100 pp., 17 tables, 88 illustrations, bibliography, 74titles.Magnesium alloys exhibit desirable properties for use in transportationtechnology. In particular, the low density and high specific strength of these alloys is ofinterest to the aerospace community. However, the concerns of flammability andsusceptibility to corrosion have limited the use of magnesium alloys within the aircraftcabin. This work studies a magnesium alloy containing rare earth elements designed toincrease resistance to ignition while lowering rate of corrosion. The microstructure ofthe alloy was documented using scanning electron microscopy. Specimens underwentsalt spray testing and the corrosion products were examined using energy dispersivespectroscopy.
Copyright 2013bySarah S. Baillioii
ACKNOWLEDGEMENTSSincere appreciation is extended to Tim Marker, Federal Aviation Administration.Images depicting the full-scale flammability testing of magnesium alloys in aircraftseating are used with his permission. I would also like to thank Tracee Friess and thedynamic test lab group at the National Institute for Aviation Research, Wichita StateUniversity. Images depicting dynamic testing of aircraft seats and salt fog corrosionequipment are used with their permission.Gratitude is also expressed to Zodiac Seats US, LLC, and Magnesium Elektronfor funding and supply of materials. To Bruce Gwynne, Paul Lyon, and Bruce Davis ofMagnesium Elektron: Thank you for your support, feedback, and kindess.Special appreciation is due my advisor, Dr. Peter Collins, and my committeemembers, Dr. Nahendra Dahotre and Dr. Jincheng Du.Above all, I would like to thank my family for their unwavering support during thisproject. Thank you, Jon, for helping me through this! I love you.iii
TABLE OF CONTENTSPageACKNOWLEDGEMENTS . iiiLIST OF TABLES . viiLIST OF FIGURES . viiiCHAPTER 1 INTRODUCTION . 11.1. Usage of Magnesium in Commercial Airplanes . 11.2. Alloy Selection . 21.3. Qualification . 31.3.1. Flammability . 41.3.2. Corrosion . 111.3.3. Mechanical Properties . 13CHAPTER 2 HISTORY OF MAGNESIUM . 162.1. Elemental Properties . 172.2. Production . 192.2.1. Electrolysis . 222.2.2. Thermal Reduction. 232.2.3. Carbothermic Reduction . 262.3. Recycling . 272.4. Alloy Development . 282.4.1. Factors Affecting Magnesium Production . 292.4.2. Usage . 302.4.3. Pricing . 32iv
2.5. Modern Alloys . 332.5.1. Nomenclature . 362.6. Properties of Magnesium and Its Alloys . 372.6.1. Alloys Based on the Mg-Al System . 382.6.2. Alloys Based on the Mg-Zn System . 382.6.3. Alloys Based on the Mg-Y System . 392.7. Corrosion of Magnesium. 392.7.1. Measuring Corrosion Rate . 402.7.2. General Corrosion. 412.7.3. Galvanic Corrosion . 422.7.4. Localized Attack . 452.7.5. Stress Corrosion Cracking . 492.8. Elektron 43 (WE43C) . 532.9. Treatment of Elektron 43 . 53CHAPTER 3 METHODOLOGY . 553.1. Sample Preparation . 553.2. Salt Spray . 563.3. Powder Coat Thickness . 573.4. Adhesion Testing . 583.5. Polishing . 583.6. Optical Microscopy . 593.7. Environmental Scanning Electron Microscopy. 60v
CHAPTER 4 RESULTS AND DISCUSSION . 614.1. Results of Testing: Salt spray . 614.2. Results of Testing: Coating Thickness . 684.3. Results of Testing: Coating Adhesion . 704.4. Results of Optical Microscopy . 734.5. Results of Environmental Scanning Electron Microscopy . 744.5.1. Analysis following Salt Spray Testing, Bare Samples . 794.5.2. Analysis following Salt Spray Testing, Trivalent Chromate Sample 834.5.3. Analysis following Salt Spray Testing, Powder Coated Samples . 87CHAPTER 5 CONCLUSION . 895.1. Summary . 895.2. Recommendations . 90APPENDIX COPYRIGHT NOTICES . 92BIBLIOGRAPHY . 94vi
LIST OF TABLESPageTable 1-1: Corrosion potential values for common engineering metals . 12Table 2-1: Most common minerals used in magnesium production . 20Table 2-2: China vs. US magnesium pricing . 32Table 2-3: Effects of alloying elements on magnesium . 35Table 2-4: Nomenclature of magnesium alloys . 36Table 2-5: Temper designations . 37Table 2-6: Mechanical properties of 99.9 wt% pure magnesium . 38Table 2-7: Heavy metal tolerance for magnesium alloys . 42Table 2-8: Chemical composition limits (weight percent) . 53Table 3-1: Sample Preparation . 55Table 4-1: Results of salt spray testing – Elektron 43 magnesium . 62Table 4-2: Results of salt spray testing – 7075 aluminum . 63Table 4-3: Powder coat thickness . 68Table 4-4: Results of adhesion testing . 70Table 4-5: Elemental composition of spheroidal precipitate . 75Table 4-6: Elemental composition of cubic precipitate . 76Table 4-7: Chemical analysis of cracks in sample M9 . 86vii
LIST OF FIGURESPageFigure 1-1 Test arrangement for engine casting ignition tests. 4Figure 1-2 Oil-Fired burner test configuration. 6Figure 1-3 Primary structural components of commercial aircraft seats . 7Figure 1-4 Fuselage mockup for baseline testing . 8Figure 1-5 Fuselage mock-up following baseline testing . 8Figure 1-6 Cabin survivability comparison. 9Figure 1-7 Fuselage mock-up following test with WE43 magnesium . 10Figure 1-8 Illustration of yaw, pitch, and roll . 14Figure 1-9 Aircraft seating dynamic test set-up . 15Figure 2-1 Magnesium metal shavings. 17Figure 2-2 Hexagonal close-packed crystal structure . 18Figure 2-3 Dolomite . 21Figure 2-4 Brucite . 21Figure 2-5 Carnallite . 21Figure 2-6 Magnesite . 21Figure 2-7 Olivine . 21Figure 2-8 Magnesium crystals produced by vapor deposition . 25Figure 2-9 Worldwide magnesium production trends . 29Figure 2-10 Convair B-36, the “Magnesium Cloud” . 31Figure 2-11 Corrosion film formed after salt spray on magnesium alloy . 41Figure 2-12 Galvanic corrosion of magnesium adjacent to steel fastener . 43Figure 2-13 Faying surface seal . 44viii
Figure 2-14 Crevice Corrosion . 47Figure 2-15 Intergranular corrosion in an aluminum alloy with a steel fastener. 48Figure 2-16 Filiform corrosion . 49Figure 2-17 Stress corrosion cracking initiated at a pit . 51Figure 3-1 Approved salt fog chamber per ASTM B117 . 57Figure 3-2 Allied M-Prep polishing system . 59Figure 3-3 Nikon Eclipse ME600 Microscope. 59Figure 3-4 FEI Quanta ESEM with EDS . 60Figure 4-1 M1 – M5, L to R, before . 64Figure 4-2 M1 – M5, L to R, after . 64Figure 4-3 M6 – M10, L to R, before . 64Figure 4-4 M6 – M10, L to R, after . 64Figure 4-5 M11 – M15, L to R, before . 64Figure 4-6 M11 – M15, L to R, after . 64Figure 4-7 M1, Bare; before . 65Figure 4-8 M1, Bare; after . 65Figure 4-9 M8, Trivalent chromate coat, before . 65Figure 4-10 M8, Trivalent chromate coat, after . 65Figure 4-11 M11, Trivalent chromate coat TGIC powder coat, before . 65Figure 4-12 M11, Trivalent chromate coat TGIC powder coat, after . 65Figure 4-13 A1 – A5, L to R, before. 66Figure 4-14 A1 – A5, L to R, after . 66Figure 4-15 A6 – A10, L to R, before. 66ix
Figure 4-16 A6 – A10, L to R, after . 66Figure 4-17 A11 – A18, L to R, before. 66Figure 4-18 A11 – A18, L to R, after. 66Figure 4-19 A4, Bare, before . 67Figure 4-20 A4, Bare, after . 67Figure 4-21 A6, Trivalent chromate treated, before . 67Figure 4-22 A6, Trivalent chromate treated, after . 67Figure 4-23 A14, Trivalent chromate TGIC powder coat, before . 67Figure 4-24 A14, Trivalent chromate TGIC powder coat, after . 67Figure 4-25 M12, powder coat thickness verification . 69Figure 4-26 M16, before . 71Figure 4-27 M16, after . 71Figure 4-28 M17, before . 71Figure 4-29 M17, after . 71Figure 4-30 M18, before . 71Figure 4-31 M18, after . 71Figure 4-32 A16, before . 72Figure 4-33 A16, after . 72Figure 4-34 A17, before . 72Figure 4-35 A17, after . 72Figure 4-36 A18, before . 72Figure 4-37 A18, after . 72Figure 4-38 Precipitate distribution . 73x
Figure 4-39 Cubic Precipitates . 74Figure 4-40 Magnesium matrix. 74Figure 4-41 Yttrium precipitates . 74Figure 4-42 Zirconium precipitates . 74Figure 4-43 Spheroidal precipitates. 75Figure 4-44 BSE image illustrating dynamic recrystallization . 76Figure 4-45 Parallel precipitates . 77Figure 4-46 Uniform structure of matrix . 78Figure 4-47 M2, bare: Corrosion products. 79Figure 4-48 Chemical analysis of corrosion products – sample M2 . 80Figure 4-49 Crevice in sample M2 . 81Figure 4-50 Chemical analysis of corrosion products in crevice – sample M2 . 81Figure 4-51 Area of increased corrosion resistance, sample M2 . 82Figure 4-52 Corrosion products, sample M9, trivalent chromium coating. 83Figure 4-53 Chemical analysis of sample M9. 84Figure 4-54 Cracking of trivalent chromate film . 85Figure 4-55 Trivalent chromate coating on magnesium, no salt spray testing. 86Figure 4-56 Trivalent chromate coating on magnesium, after salt spray testing . 86Figure 4-57 Sample M12, powder coated . 87Figure 4-58 Chemical analysis of powder coated sample M9 . 88xi
CHAPTER 1INTRODUCTIONIn this work experimental data showing the effect of protective coatings on amagnesium alloy is presented and analyzed. Criteria are established to determine theeffectiveness of two aerospace grade coating systems in preventing corrosion.Microstructural evaluation and theoretical data are used to assess the corrosionresistance of the alloy and recommendations for commercial usage are given.This work focuses on the qualification of one magnesium alloy system for use incommercial aircraft cabin interiors. The necessary requirements for qualification as wellas potential issues are discussed. This introduction primarily serves to advise thereader on these requirements and issues. Brief historical information which providesbackground and reasoning is also given.1.1.Usage of Magnesium in Commercial AirplanesThe identification and qualification of lightweight materials is a driving factor intransportation technology. Magnesium, as the lightest structural metal, is a strongcandidate for applications requiring low weight. With an average density of 1.8 g/cm3,magnesium alloys have only one-quarter the density of stainless and carbon steels andtwo-thirds the density of aluminum alloys [4, 8, 21]. The alloys of magnesium alsoexhibit high specific strength, excellent machinability, and capability to absorb vibrationand impact [1, 2, 4]. Alloy development in recent years has focused on materials withhigh strength at elevated temperatures and good creep resistance [6, 11].1
Magnesium castings are currently in widespread use within the engine andtransmission systems of transport airplanes, primarily in the form of cast structuralhousings [9]. However, regulations exist which forbid the use of magnesium within theaircraft cabin interior [31]. The Federal Aviation Administration (FAA), as overseer ofthe regulations pertaining to aircraft safety, has worked with industry groups to analyzethe possibility of magnesium alloy usage within the cabin interior and evaluate anypotential impact on the established level of safety [29].1.2.Alloy SelectionThere are two traditional systems of magnesium casting alloys: those producedwith aluminum and those without. The earliest documented alloying elements werealuminum, zinc, and manganese [46]. In fact, the Mg-Al-Zn system remains one of themost widely used compositions for casting alloys, with a notable example being AZ91.Although the aluminum-based systems exhibit good casting characteristics, they showlow response to age hardening and have relatively low strength properties [1].In magnesium systems for wrought alloys, there are again two major systems:those produced with zirconium and those without. The wrought alloys generally fall intothe same categories as casting alloys with the exception that they can be obtained in anumber of tempers [54]. In the past, wrought alloys containing thorium were used toproduce missile and spacecraft components requiring creep resistance at elevatedtemperatures. However, due to the radioactive nature of thorium, these alloys are nowconsidered to be obsolete for purposes of future design [55].2
Magnesium alloys, in general, exhibit high strength-to-weight ratios, low density,good weldability, and excellent damping characteristics [2]. Properties such asincreased creep resistance and superior performance at elevated temperature requirealloying elements such as rare earth metals [1, 3]. Corrosion behavior can also beaffected positively by the addition of rare earth metals [6]. These factors lead to theselection of magnesium alloyed with rare earths metals. Magnesium Elektron,headquartered in Manchester, UK, has developed a rare earth alloy designated asWE43C. The trade name for this material is Elektron 43. This paper will focus on thecorrosion characteristics of the wrought Elektron 43 alloy.1.3.QualificationIn March of 2007, the FAA presented information to the International AircraftMaterials Fire Test Working Groups (IAMFTWG) indicating that there was a revivedindustry interest in revisiting the use of magnesium alloys in the aircraft cabin interior[29]. In October of the same year, during the Fifth Triennial International Fire and CabinSafety Research Conference, the FAA presented initial test results of magnesium barsfor flammability. Magnesium Elektron provided all samples for testing. Six differentalloy systems were evaluated during the initial testing: AZ31, AZ80, ZE41, ZE10, WE43,and Elektron 21 [53].3
1.3.1. FlammabilityIn the early stages of qualification, efforts were focused on exploring theflammability properties of magnesium alloys. A study conducted by the FAA in 1964explored the ignition and burning characteristics of four magnesium casting alloys:AZ61A, AZ31B, AZ80A, and ZK60A [52]. The goal of the testing was to reproduce theconditions of an aircraft power plant fire. To this end, a test apparatus was developedwhich used a commercial conversion-type oil burner to produce a flame temperature ofapproximately 2000 F (Figure 1-1).Figure 1-1Test arrangement for engine casting ignition tests [52]Initial testing using a small Fisher-type burner and conducted on magnesiumbars of varying thickness determined that time to ignition did not vary between specificalloys but was directly related to specimen thickness and airflow over the specimenduring the testing. The tests of engine castings weighing between twenty-three and4
ninety-one pounds confirmed these observations. Self-heating of the alloys andsubsequent combustion was noted to begin at approximately 1000 F. Due to theconnection between component thickness and time to ignition, it was noted that small,thin-walled magnesium alloy components had the potential to represent a serious firehazard.In re-opening the discussion of magnesium alloy usage, the FAA noted thatmaterials technology has advanced significantly and newer magnesium alloys havediffering levels of susceptibility to ignition. The focus of research conducted by theFederal Aviation Administration Technical Center (FAATC) is to ensure that anychanges to material, process, or design guidelines will not reduce the level of safety.With this mandate, the FAATC worked with members of industry and raw materialproducers to develop testing protocols and establish safety parameters for the usage ofmagnesium alloys in the cabins of commercial aircraft.As with the 1964 study, the FAATC began testing using an oil-fired burner typelaboratory scale test apparatus. The burner was configured according to an existingstandard dedicated to the qualification of aircraft seat cushions to flammabilityrequirements, Title 14 Code of Federal Regulations (CFR) Part 25.853(c) Appendix FPart II [33] (Figure 1-2).5
Figure 1-2Oil-Fired burner test configuration [29]Six alloys were selected for analysis, including the newest iterations of AZ80 andAZ31. Testing showed a direct correlation between each alloy’s upper melting rangeand the time required for the sample to melt. Further, it was shown that alloys includingrare earth components, such as WE43, exhibited more resistance to ignition thantraditional alloy systems. The FAATC also tested specimens representing machinedcomponents of aircraft seating systems, such as legs and spreaders, to further evaluatethe importance of consistent part thickness. It was determined that milling andorientation had no impact on the flammability of the samples. However, very thinsections (ranging from 0.0625 to 0.125 inch in thickness) were shown to be verysusceptible to ignition and, once ignited, to burn until consumed.Using the knowledge gained from the initial testing sequences, the FAATCproceeded to full-scale testing. With the input of the industry team, it was determinedthat primary seat components were a focus for potential usage of magnesium alloys inthe aircraft cabin. Primary seat components include large machined parts, such aslegs, spreaders, baggage bars, seat back hoops and cross tubes (Figure 1-3). The6
most weight could be saved by substituting magnesium alloys in place of the traditionalaluminum in these parts due to their comparatively substantial mass in the seatstructure.Figure 1-3Primary structural components of commercial aircraft seatsImage credit: Wichita State University, National Institute for Aviation ResearchThe test fuselage for the full-scale testing consisted of a twenty-foot long steelcylinder which was inserted between two halves of a Boeing 707 fuselage. A standardsize opening, forty by eighty inches, was used to represent a break in the fuselage. Apan containing fifty-five gallons of JP-8 fuel was placed directly outside of this openingand ignited to produce the fire source. The fuselage mock-up included paneling andcarpeting with three rows of triple seats centered around the fire opening (Figure 1-4).7
Initial full-scale testing was conducted using seats constructed of traditional materials inorder to establish a basis by which to measure the impact of the usage of magnesiumalloys (Figure 1-5). Once this baseline was established, additional testing was carriedout on seats fabricated with two magnesium alloys: AZ31 and WE43. These alloyswere selected based on the results of the laboratory scale testing to represent the worstand best performing material, respectively.Figure 1-4Fuselage mockup for baseline testingImage credit: Marker, FAA [29]Figure 1-5Fuselage mock-up following baseline testingImage credit: Marker, FAA [29]8
A survivability model using regression equations to determine a fractionaleffective dose for incapacitation was used to predict the amount of time a human has toescape an aircraft fire. This model takes into acco
Magnesium alloys exhibit desirable properties for use in transportation technology. In particular, the low density and high specific strength of these alloys is of interest to the aerospace community. However, the concerns of flammability and susceptibility to corrosion have limited the use
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