Corrosion Resistant Alloys


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Corrosion Resistant Alloys

  1. 1. Corrosion Resistant Alloys for Naval Aviation Applications William E. Frazier Introduction The materials selected and engineered for use in Navy and Marine Corps aircraft are driven by the unique maritime operational requirements and harsh corrosive environment in which the aircraft operate, Figure 1. Carrier based aircraft experience six times the structural loads of land based aircraft. In fact, carrier based aircraft are designed for a landing sink rate of 27 feet/second and a catapult launch acceleration of 150 mph in 2.1 seconds. The environment in which these carrier-based aircraft operate because of the extremely deleterious combination of salt water, sand and high humidity is the most severe natural corrosive environment known. Figure 1 Navy Operational Environment – Bow Wave Breaking Over the Deck of an Aircraft Carrier The cost of corrosion for DoD is between $10 -$20B per year DoD [1]. The cost of corrosion to the DoN has been estimated at $4.4B [2]. The impact of corrosion on naval aviation is huge. Over 100 million work hours and nearly $1 billion have been spent by the Naval Air Systems Command (NAVAIR) from 1994 to 2004 on corrosion related problems [3, 4]. Further it is important to understand that the financial impact of corrosion is just one aspect of corrosion’s pernicious effects. Corrosion also affects
  2. 2. safety and the number of aircraft ready for tasking. Examination of Figure 2 shows corrosion damage to a wing fold lugs, attachment points for the wing structure. Figure 2 Sever Corrosion: Missing Wing Fold Lugs Corrosion Mitigation Efforts Improved maintenance and systems designs have reduced corrosion related problems. Advanced paints, sealants and corrosion preventative compounds have also provided protection and also have mitigated the effects of corrosion. However, paints and sealants can be breached or imperfectly applied thus exposing the underlying structure to the harsh naval environment. The approach to corrosion control has traditionally been reactive and not proactive. Thus, designers have typically not considered corrosion until late in the design process. Rose et al. divided DoD response to corrosion into four categories: Repair and Maintenance; Planned Corrosion Maintenance; CPAC Methods; Design-in Resistance [5]. The Table 1 shown below outlines some of the merits and risks associated with two of these approaches. Designing in corrosion resistance clearly provides the benefits of reduced life-cycle cost and enhanced readiness; however, the up- front cost may be greater. Table 1. Risks Associated with Corrosion Mitigation Strategies Design-in Corrosion Resistance Repair and Replace Higher upfront cost, Less Lower upfront cost, more maintenance/repair required, Lower life- maintenance/repair, much higher life cycle cycle costs, longer service life, higher cost, shorter service life, lower reliability, reliability, higher readiness, lower risk, lower readiness, higher risk, extended system life
  3. 3. The Corrosion Resistant Alloy Workshop In December 2005, the program executive office, PEO(A), sponsored a NAE Rotorcraft/Vertical Lift Technology Workshop. That workshop identified Corrosion, Erosion, and Environmental Degradation as critical issues requiring research and development [5]. On November 8-9 2006, NAVAIR conducted a government-industry workshop on the development of corrosion resistant alloys. The workshop was aligned with the Naval Aviation Enterprise Science and Technology Strategic Plan [7], and in support the Command’s Fleet driven metric, Aircraft Ready For Tasking At Reduced Cost [8]. Workshop Structure and Methodology The Workshop focused on three classes of light weight structural aerospace alloys: aluminum, ultra high strength steel and cast magnesium. The Workshop provided a forum for experts from NAE, DoD, industry, and academia to share their views and recommendations. The principal goal was to identify potential S&T approaches for the development of corrosion resistant alloys. Over 75 national and international experts registered to participate including representatives from the US, UK, Canada, and Korea. Participants included representatives from the DoD services, from the steel, aluminum and magnesium alloys producers; from US airframers and from academia, Table 2. Table 2. Participating Organization Government Industry Academia • Air Force •ALCOA • Pratt & Whitney • Brown University • Army • Allegheny Ludlum • MagElectron • Carnegie Mellon • DARPA • Allison • Navmar • Drexel Transmission •JSF • Northrop Grumman • Georgia Tech • Allvac • NSWC • Tagnite • Loyola Marymount • Alcan, Boeing • NIST • Ques Tek • Penn State Univ. • Carpenter • NRL Technology • Sikorsky • University of Virginia • OSD • Goodrich • Swagelok • Pohang University • ONR • Granta • Navy Metalworking Center • OPNAV • Lamda Research • PEO(A) • Lockheed • NAVAIR The GOTChA (Goals, Objectives, Technical Challenges, and Approaches) methodology was used to structure the workshop and help focus the efforts of the participants, Figure 3. The workshop organizers identified a challenging materials property goal for each of
  4. 4. the three alloy systems that, if achieved, could significantly impact naval aviation. For example, the goal for steel was “Ultra High Strength Intrinsically Corrosion Resistant Steels for Enhanced Readiness, Improved Performance and Lower Life Cycle Costs.” Similar goals were established for the aluminum and magnesium alloy systems. • Three Discussion Groups GOTChA Process – Aluminum – Magnesium – Steel GOAL Navy Defined • Led by Facilitator TECHNICAL Navy Defined, OBJECTIVES Workshop Validated – Selected audience (~20) TECHNICAL Workshop – Guided the discussion CHALLENGES Developed – Note taker(s) RESEARCH APPROACHES Figure 3 GOTChA Methodology For each alloy system, three specific materials property objectives were validated and refined by the participants of the workshop. The workshop participants then engaged in an intense dialog in order to determine what were the scientific and technical challenges associated with achieving these objectives. For each technical objective, the participants then prioritized the challenges. The workshop participants then identified and prioritized viable scientific and technical approaches. Thus, the workshop provides a clear linkage between goals, objectives, technical challenges and possible approaches. Results of the Workshop Aluminum Alloy Working Group: The participants in the aluminum workshop sessions validated and amended three S&T objectives for achieving ultra high strength, inherently corrosion resistant aluminum alloys for Navy aircraft, Table 3. These objectives were based on the goal for aluminum alloys. The objectives were: (1) Aluminum alloy with strength of 7075-T6, good toughness, and “immune” to stress corrosion cracking (threshold stress >75% YS). (2) Aluminum alloys with strength of 7075-T6 requiring no anodized coating which produces a fatigue debit. (3) An alloy with three times the corrosion fatigue resistance of 7050-T7XX. The participants of the aluminum working group identified the technical challenges for each objective and categorized them as follows. For the objective 1: (1) Lack of quantitative modeling that accepts material’s inputs, (2) Non-heat treatable Al alloy with specific mechanical strength similar to 7075-T6, (3) Lack of a corrosion intensity factor and lack of ability to select accelerated corrosion or electrochemical testing to reproduce real life results. For the objective 2: (1) How to get a optimized surface layer- De- alloying, cladding, gradient, (2) Self-healing surface oxides via micro-alloying or other strategies, (3) Incomplete understanding of how micro-structural features affect corrosion
  5. 5. behavior. For the objective 3: (1) Lack of understanding of micro-structural features that gives rise to crack initiation sites, (2) Lack of understanding of damage induced by corrosion and its transition to crack growth, (3) Short term challenge: Fine tuning 2x9x Al alloys, (4) Lack of understanding of oxide formation and integrity on the crack growth behavior in the corrosive environment. The participants then developed and prioritized technical approaches to overcome the technical challenges. Based on the various technical approaches that were discussed, critical research areas were established which if carried out, would result in achieving the technical objectives for the aluminum alloys, see Table 3. Table 3. Critical Research Areas for Aluminum High Strength Aluminum Goal: High Strength Intrinsically Corrosion Resistant Aluminum Alloys for Enhanced Readiness, Improved Performance and Lower Life Cycle Costs Objectives: 1. SCC improvements-The threshold strength of 7075-T6 is 75% of the yield strength. No sacrifice in specific strength and toughness values. 2. 7075-T6 mechanical properties, corrosion resistance equivalent to anodizing and no fatigue debit. 3. 3X improvement in corrosion fatigue as compared to 7050-T7xx. Critical Research Areas 1. Al-Mg and Al-Mg-Li Base Alloy Research. Al-Mg base alloys have excellent corrosion resistance but do not meet aerospace specific strength requirements. Research is required to improve alloy strength while retaining corrosion resistance. Improved understanding of the effects of tertiary and quaternary alloy additions on precipitate structure, morphology, matrix-precipitate interface and dislocation-precipitate interaction are required. 2. Quantitative Corrosion Models. Establishment of quantitative corrosion modeling that accepts material’s input and enables to predict real time corrosion with accelerated test results. 3. Optimized surface layer. Modification of surface layer composition can significantly improve corrosion resistance without sacrificing the other mechanical properties. Understanding the corrosion mechanisms associated with varying composition is necessary to optimized surface. The surface modification can be accomplished via laser, spray, vapor deposition, nitriding, carburization and surface diffusion. 4. Development of self healing surface. Some oxide layer can be healable in the presence of selected micro-alloying or other strategies. It is necessary to explore the mechanisms of self healing and understand the characteristics of oxide layers based on the presence of other elements. 5. Corrosion Damage Crack Initiation and Propagation. The mechanisms associated with corrosion crack initiation and propagation are not well
  6. 6. understood. Mechanistic and statistic analysis of the crack initiation sites associated with corrosion damage in terms of its size and morphology and establishing the mechanisms associated with the formation of crack initiation are required. 6. Aluminum Lithium Alloy Design. The lack of a mechanistic understanding of the fatigue performance in corrosive environment for current Al-Cu-Li alloys 2098, 2050, 2198, 2099, 2199, and 2195 is impeding the optimization of this class of alloys. Third generation aluminum-lithium alloys exhibit very good fatigue and stress corrosion cracking resistance, but are far from achieving their full potential. Cast Magnesium Alloy Working Group: Cast magnesium alloys are widely used in rotorcraft as gearbox and transmission housings materials. Their low density and high structural stiffness provide designers a means saving hundreds of pounds of weight per rotary aircraft in comparison to aluminum. While magnesium alloys have a successful track record of reducing structural weight, their position in the electrochemical series renders magnesium parts susceptible to severe galvanic corrosion. This is particularly true in the Navy’s harsh operational environment. The participants in the magnesium working group validate and modified the S&T objectives for achieving highly corrosion resistant magnesium alloys for enhanced readiness, improved performance and lower life cycle costs, Table 4. The critical technical challenges associated with achieving the objectives were discussed and technical approaches identified and prioritized. From the product of working group, following critical research areas were developed, Table GM. • Qualify and Implement EV31A, Tagnite, and Rockhard, • Research and Implementation of Cathodic Protection Schemes, • Magnesium with a Self-Healing Passive Surface, • Advanced Computer Modeling to Invent Corrosion Resistant, High Performance Magnesium Alloys • Corrosion Damage Repair Innovations Table 4. Critical Research Areas for Cast Magnesium Alloys Cast Magnesium Alloys Goal: Highly corrosion resistant magnesium alloys for Enhanced Readiness, Improved Performance and Lower Life Cycle Costs Objectives: 1. A cast magnesium transmission gearbox with a 50% decease in life-cycle cost compared to baseline ZE41A or AZ91C alloys 2. A magnesium casting alloy/attach point system that provides galvanic performance superior to aluminum/steel 3. A magnesium alloy designed from first principles that mimics the oxide structure of
  7. 7. aluminum oxide (stainless magnesium) Critical Research Areas 1. Qualify and Implement EV31A, Tagnite, and Rockhard. The life cycle cost saving of 50% or more as compared to AZ91E alloy/Dow 17 surface treatment system, could be achieved through the implementation of state-of-the-art alloys and materials protection schemes. Experimental work is required to resolve technical issues (especially concerning applying these technologies to in-service systems) and validate system performance. . 2. Research and Implementation of Cathodic Protection Schemes. While coating protection schemes in galvanic couplings are primarily focused on the anode or magnesium alloy, very little consideration has been given to treating the cathodic attach points that are typically comprised of aluminum, titanium and steel. Fundamental galvanic corrosion studies are required to elucidate the pay- off of novel surface modification techniques to decrease the galvanic potential with magnesium alloys. Potential avenues include including the use of anodic plates, transition joints and surface treatment of the cathode. 3. Magnesium with a Self-Healing Passive Surface. Unlike aluminum alloys, magnesium alloys do not develop a protective, stable passive surface oxide layer. Instead, magnesium oxide tends to expand and crack, resulting in an unstable and friable oxide that offers little resistance to general and galvanic corrosion. A fundamental research effort to understand oxide stability criteria, oxidation kinetics, materials property data (crystallographic and thermodynamic) and candidate alloying additions has potential to provide a suite of corrosion resistant magnesium alloys with performance equivalent to aluminum alloy castings. 4. Advanced Computer Modeling to Invent Corrosion Resistant, High Performance Magnesium Alloys. The evolution of magnesium alloy design has progressed to provide continuous improvement in strength, fatigue strength, creep resistance and general corrosion resistance. With the use of advanced computer modeling technology, there is an opportunity to use alloy design first principles to develop a model that considers a) the effect of various alloying additions on strength, fatigue and creep and b) the resulting structure of the magnesium oxide layer. Past work in alloy design, empirical property data and oxidation characteristics would be used to guide these fundamental efforts. 5. Corrosion Damage Repair Innovations. Advancements in the aforementioned critical research areas will still require a means to repair corrosion and incidental maintenance damage, particularly in the depots and on ships. A fundamental processing study is required to understand the effectiveness of novel repair methods such as aluminum cold spray and brush-on Tagnite treatments. Of primary importance is the elucidation of galvanic and general corrosion behavior subsequent to repair processing. Ultra High Strength Steel Alloy Working Group: The participants in the steel workshop sessions validated and modified the S&T objectives for achieving the goal of
  8. 8. developing an ultra high strength intrinsically corrosion steels for enhanced readiness, improved performance, and lower life cycle cost, Table 5. The critical technical challenges associated with achieving the objectives were discussed and technical approaches identified and prioritized. The technical challenges for each objective were categorized into the following areas: (1) mechanisms and modeling, (2) manufacturing (3) materials qualification (4) design and (5) return on investment. From the product of working group, the following critical research areas were developed, Table 5 Table 5: Critical Research Areas for Ultra High Strength Steel Alloys Ultra High Strength Steel Goal: Ultra High Strength Intrinsically Corrosion Resistant Steels for Enhanced Readiness, Improved Performance and Lower Life Cycle Costs Objectives: 1. AerMet 100-type alloys mechanical properties with 3X improvement in KISCC. (For example, KISCC = 70 ksi(in)1/2 by RSL method in 3.5% NaCl, PH 7.3, induced potential at OCP) 2. Ultra High Strength Stainless Steels (corrosion resistance equal to or better than 15-5 alloy and with TS = 290 ksi, YS = 250 ksi, KIC = 110 ksi(in)1/2, KISCC = 70 ksi(in)1/2 3. Bearing steels with corrosion performance similar to 15-5 alloy and wear resistance 2X that of 52100 steel and/or Pyrowear 53. Critical Research Areas 1. Transgranular Cracking in the Presence of Hydrogen. The lack of a mechanistic understanding of the effect of hydrogen on transgranular cracking is a major impediment to improving the SCC of Ultrahigh Strength Steels. Research is required in order to develop that understanding and theory based models. Multi-scale computational methods must be developed for alloy optimization that include electrochemical uptake of hydrogen, trap binding energies and crack tip micromechanics. 2. Hydrogen Uptake Control in Ultra High Strength Steels. The inability to more precisely control hydrogen uptake in production lot quantities is a significant challenge to meeting the material property objectives. Materials processing research that incorporates design solutions involving impurity control, segregation control, cost control and thermal mechanical processing is required. 3. Passivation of Ultra High Strength Steels. Today’s ultra high strength steels do not develop a corrosion resistant passive surface layer. Innovative research is required in order to establish, understand, and model passivity in order to create a stainless ultra high strength steel. Theory should relate the roles of alloy chemistry, environment and microstructure to passivation and should address multi-component models for passivatiuon. 4. Tribology of Passivated Steel Surfaces. The lack of understanding of the relationship between the lubrication systems and wear resistance of bearing steels with a passivated surface layer impedes the development of high wear resistance, corrosion resistant bearing stainless steels. Research is needed to establish functionally graded (or monolithic) materials that integrate the tribological effects on
  9. 9. passivation for applications to bearings. Research is also required on optimizing lubrication system behavior that is compatible with the passivation layer/method. Modeling of interactions between lubricant additives and oxide layers under in- service operating conditions of temperature and pressure is needed. 5. Inherently Corrosion Resistant, Ultra High Strength Steels. Ultra high strength steels that are inherently corrosion resistant do not exist today. Research into new materials systems (amorphous, nanocrystalline materials) as well as research into high strength high nitrogen austenitic stainless steels have the potential to produce materials that will achieve the properties needed. Research is needed into formulating alloy designs and processing methodology to produce steels by utilizing new material systems (e.g., amorphous, nanocrystalline materials) and processing routes (e.g., powder metallurgy). Systemic Research Needs Three important overarching areas of research emerged from the working groups. These include: Multi-scale computational modeling and simulation: Advanced computational tools are required in order to enable physical and chemical mechanisms, occurring at the atomic through macroscopic size ranges, to be related to macroscopic material properties. The integration of computational tools (e.g., microstructural, process, mechanistic, cost) is essential in order to fully enable alloy by design. Mechanistic understanding of fundamental physical and chemical interactions: There are many mechanisms associated with alloy corrosion behaviors that are poorly understood, e.g., binding energy of hydrogen in steel. Thermo-physical data and thermodynamic phase stability: The development of thermo- physical data is the foundation upon which useful computational models and mechanistic understanding of material behavior must be built. Summary NAVAIR hosted a Corrosion Resistant Alloy Workshop. A well balanced group of over 75 national and international expert participated in this event including representatives from the US, UK, Canada, and Korea. Participants included representatives from each of the DoD services, NIST, from the major steel, aluminum, and magnesium alloy producers, US airframers, and academia. The results of this event provide a foundation upon which to build a robust corrosion resistant alloy research and development program. A program that supports the NAE Strategic Plan, PEO(A) rotorcraft O&S needs, and DoD corrosion mitigation efforts. I encourage industry-academia and government agency collaboration and dialog. Obviously, the work required to develop “intrinsically” corrosion resistant alloys is
  10. 10. ambitious and has just begun. It will take a concerted effort of a diverse group of concerned experts to adequate address this issue. Acknowledgements I would like to thank the workshop organizers, session chairpersons, and recorders, viz., Mr. Ken Clark, Prof. Omar Es-Said, Dr. Eui Lee, Dr. Will Marsden, Ms. Denise Piastrelli, Mr. Irv Shaffer, Dr. Suresh Verma, Dr. Jeffrey Waldman, and Dr. Daniel Wintersheidt. I especially wish to thank the technical experts from industry, academia, and the government, who made this event a success. References 1. Lewis Sloter, “Materials Research of Corrosion Performance: The future is Bright,” (Presentation at the Navy Corrosion Resistant Alloy Workshop, Patuxent River, MD 8-9 Nov 2006). 2. David A. Shefler, “Office of Naval Research Corrosion Control S&T Program,” (Presentation at the Navy Corrosion Resistant Alloy Workshop, Patuxent River, MD 8-9 Nov 2006). 3. S. Spadafora and C. Matzdorf, “Impact of Corrosion on the Naval Aviation Enterprise,” (Brief to Commander NAVAIR, Patuxent River, MD, 2005) 4. “Methodology for the Prediction of Corrosion Costs” (Report Prepared for NAWCAD under contract number N68335-05-C-0225; NAVMAR Applied Sciences Corporation, Warminster, PA Oct 2005) 5. David H. Rose, David Brumbaugh, Benjamin D. Craig, and Richard A. Lane, “Designers Tack Note! Improved System Corrosion Resistance Reduces Life-Cycle Costs,” AMTIAC Quarterly, 9(3)(2005), 3-13. 6. LTCOL Stephen Waugh and Suresh Verma, “NAE Rotorcraft/Vertical Lift Technology Workshop Overview and Results,” (NAE PEO(A), Patuxent River, MD, January 25, 2006). 7. VADM James M. Zortman, VADM Walter B. Massngburg, RADM Thomas J. Kilcline, “Naval Aviation Enterprise Science and Technology Strategic Plan,” (NAE, Patuxent River, MD, July 1, 2006). 8. Commander, Naval Air Systems Command, (Echelon II Visit Presentation to Admiral Clark, Chief of Naval Operations, March 25, 2005).
  11. 11. List of Tables Table 1. Risks Associated with Corrosion Mitigation Strategies Table 2. Participating Organization Table 3. Critical Research Areas for Aluminum Table 4. Critical Research Areas for Cast Magnesium Alloys Table 5: Critical Research Areas for Ultra High Strength Steel Alloys List of Figures Figure 1: Navy Operational Environment – Bow Wave Breaking Over the Deck of an Aircraft Carrier Figure 2: Sever Corrosion: Missing Wing Fold Lugs Figure 3: GOTChA Methodology