N242-094 TITLE: Anti-Corrosion Coating for Gas Turbine Compressor Components Operating in Marine Environments

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Sustainment

OBJECTIVE: Develop and demonstrate a chemically and mechanically robust coating system or other surface treatment for sustained protection of engine compressor components, such as mating compressor stages, cases, and vane tracks, from corrosion in naval aero engine operation.

DESCRIPTION: In naval aviation, aircraft operate in and around marine atmospheric environments often with high humidity and salt content, which accelerates corrosion degradation of aircraft components. High compressor operation temperatures relative to ambient conditions create a cyclic environment within the engine, in which water can evaporate and cool between flight and ground time cycles, leading to the accumulation of salts and other contaminants. This cyclic environment further accelerates corrosion mechanisms. As a consequence of this cyclic, marine atmospheric environment, multiple components throughout military aircraft propulsion systems require frequent repair and/or replacement due to severe corrosion and pitting, which leads to high maintenance costs, increased engine removals, and reduced aircraft readiness.

The Department of the Navy is seeking the development and/or demonstration of a coating system or other surface treatment for sustained protection of compressor components from corrosion in naval aero engine operation. Such a proposed solution must have the mechanical durability to provide corrosion protection along surfaces in contact and under some loading from other components. Specific components of concern are low-pressure and high-pressure compressor stages and cases and are typically composed of martensitic stainless steels (17-4PH and Jethete M152), titanium (Ti-6Al-4V), and Inconels (IN600, IN718, IN909) [Note: specific alloys will be identified upon project award]. Proposed solutions may constitute either the development of a new solution or a demonstration leveraging an existing solution in naval aviation application.

A brief description of the target application of this solution is provided below:

1. Low Pressure Compressor Stage or Case Flanges: Individual stages of the low pressure compressor casing are joined together in a flange configuration, where one stage is bolted to another along a mating rim or collar on each stage. This flange joint can link dissimilar materials of the stages, bolts, and other supporting structures within the engine. Typically, these flanges can involve material combinations of stainless steel and titanium mating surfaces bolted together with Inconel hardware. In most cases, corrosion and pitting occurs on the stainless steel flange surface. In addition to potential galvanic interactions, the flange joint may be susceptible to crevice corrosion. Vibration of the engine can also generate fretting between the bolted surfaces. Greases or lubricants are generally not used along the mating faces to maintain a high coefficient of friction. Temperatures in the low pressure compressor stage vary with specific engine platform but may reach as high as 180°C in operation.

2. High Pressure Compressor Stage or Case Flanges: Similar to the low pressure compressor, individual stages of the high pressure compressor casing are joined together in a flange configuration. This flange joint can link dissimilar materials of the stages, bolts, and other supporting structures within the engine. Typically, these flanges can involve material combinations of stainless steel, Inconel, and titanium mating surfaces bolted together with Inconel hardware. In most cases, corrosion and pitting occurs on the stainless steel or Inconel flange surface. Again, similar to the low pressure flanges, the high pressure compressor flanges may be susceptible to galvanic corrosion, crevice corrosion, and to vibration and fretting damage. Temperatures in the high pressure compressor stage vary with specific engine platform but may reach as high as 500°C in operation.

3. High Pressure Compressor Vane Tracks: Along sections of the high pressure compressor case, there are grooves or vane tracks into which stator vane sections slide into place. Shims are used to ensure a tight fit between the vane sections and the vane tracks. Again, the conditions at this contact point may establish conditions for galvanic corrosion, crevice corrosion, and fretting-related degradation. Pitting and other corrosion products form within the vane tracks, at times degrading the contact with the vane section and other times locking the vane section in place.

4. Compressor Stage or Case Free Surfaces: In addition to the contact and connection points between compressor stages, both low and high pressure, there is pitting occurring on exposed free surfaces along the interior-facing (gas pathway) and exterior-facing surfaces. This pitting may be observed adjacent to areas of contact with dissimilar materials (flange or other bolted connections) and far from areas with other material contact. Corrosion and pitting are also observed around the full circumference of the compressor.

Common among these applications are exposure to elevated temperature, exposure to salt water (sea spray, atmospheric, water wash, etc.), exposure to cyclic engine conditions (cycle on, take-off, land, etc.), exposure to mechanical contact conditions (wear, fretting, vibration, etc.) with potentially dissimilar materials, among others. Thus, solutions are desired which address multiple key challenges associated with these conditions. Each challenge is discussed briefly below and is listed in order of descending priority:

1. Environmental Corrosion Protection within a Naval Aviation Engine Operation Environment: Compressor environmental conditions cycle between ambient conditions when the aircraft is at rest on the ground and the elevated temperature conditions of take-off and in-flight operations. Ambient conditions vary globally but primary areas of operation are sub-tropical, maritime, or coastal environments with moderate to high humidity, high salt content, and exposure to other marine atmospheric contaminants. Take-off or in-flight operation conditions can see temperatures rise as high as 180°C in the low-pressure compressor and 500°C in the high pressure compressor. The environment is further complicated by the design of the compressor, which creates distinct local environments for corrosion. First, water tends to drain from the upper section of the compressor and pool in the lower section. Second, corrosion occurs both on open, exposed surfaces and along mating or contacting surfaces (i.e., stage flanges and vane tracks), which may trap water, salts, or other contaminants. The differences in local environment across the compressor are reflected by the multiple corrosion mechanisms observed: uniform corrosion, oxidation/rust, pitting, crevice corrosion, and galvanic corrosion.

2. Galvanic Corrosion Protection: As mentioned, corrosion is being observed at mating or contacting surfaces, including across the bolted flange interface of connecting compressor stages. In addition to the environmental factors, the engine design may introduce galvanic coupling of dissimilar materials. The stainless steel and Inconel cases are bolted to each other and to other components, like titanium support structures, within the engine, often with Inconel hardware. Changing the material design may not be possible, so coating or surface treatments should be capable of addressing both galvanic and environmental corrosion. (Note: Specific layering combinations of materials and dimensions can be provided upon project award.)

3. Mechanical Durability and Resistance to Flaking and Delamination: Solutions may be applied to mating surfaces (i.e., flanges and vane tracks) where durability to fretting or mechanical contact loading may be required. Solutions applied to mating surfaces may also be required to maintain equivalent coefficient of friction with the underlying substrate material to maintain consistent wear and load transfer performance. Solutions may also be applied to exposed surfaces along the gas flow pathway where resistance to flaking and impact damage may be required.

4. Minimal Coating or Surface Treatment Thickness: Solutions may be applied to mating surfaces and, in such instances, solution thickness may be constrained by allowable tolerances in component and/or engine system design. Solutions of an applied material thickness of less than 300 microns may be required.

5. Environmental Health and Safety Conscious: Current and forthcoming aviation regulations may restrict the use of hexavalent chromium and other hazardous materials in material systems used either in engine manufacture or repair. Chromate-based coatings have long been the standard for corrosion coatings, but the coating itself, or volatilization of the coating into its by-products, may contain hazardous materials like hexavalent chromium. Solutions may be required to comply with these health and safety regulations and be free of, or seek to minimize, hexavalent chromium and other hazardous materials.

6. Suitability for Different Compressor Applications: The proposed solution should seek to target application across the different identified compressor stages and vane tracks listed previously and across the multiple alloys used in those compressor stages and vane tracks. Severe pitting is currently being experienced across the low-pressure and high-pressure compressor cases and vane tracks made of various stainless steel and Inconel alloys. While it may be possible, even optimal, to tailor a specific solution to each individual application, solutions which address multiple applications and materials may receive greater priority. (Note: Component dimensions, flange configurations, material heat treatments, and other information can be provided upon project award.)

7. Coating Removal: Ease of coating removal by chemical stripping, grinding, or other common process will assist inspection, repair of engine components, and minimize maintenance costs.

PHASE I: Develop an initial design of a new solution or refine the design of an existing solution by identifying an approach to evaluate the technical design and feasibility to accomplish long-term resistance to corrosion damage of compressor stages, flanges, and vane tracks in a naval aero-engine propulsion system. Perform some preliminary evaluation of the proposed solution concept with the aim to demonstrate the potential benefits of the solution if granted Phase II and Phase III support. Conduct the following analyses:

1. Technical Challenges Assessment: Perform a thorough review of the technical challenges facing a proposed solution. Consider what technical data about the engine operation environment, the application components, the materials, etc. may be necessary to complete an evaluation of any solution. Consider what technical data may or may not be available and how limited availability of that data may affect the project success. (Note: It is the nature of some military platforms that certain technical information may not be disclosed, but available technical data (material, heat treatment, basic dimensions, etc.) may be shared upon project award.) Identify and assess the different challenges posed by the specific engine applications (compressor case, flange, vane tracks) and environment conditions, material contact conditions, and external environment (local climate, sea spray, etc.). Identify and assess the most promising solutions, coatings, or surface treatments to address these technical challenges. Consider if one or multiple solutions may be necessary to address these challenges, different applications, and different materials. Based on the assessment of technical challenges and coating/surface treatment options, propose one or more solutions for that specific application(s) to evaluate and characterize further.

2. Solution Feasibility Assessment: Identify a strategy or method to evaluate the proposed solution(s), which best accommodate the breadth of technical challenges, compressor applications, and materials previously discussed. (Note: The methodology may incorporate all experimentation or a coupled experimentation-computation approach, but some experimental characterization (coupon-level testing) of solution performance is requested. The methodology should suitably capture the complex environmental factors (cyclic exposure to salt and/or other contaminants, humidity, and temperature) and contact mechanics (flange joints, fretting, etc.) of the compressor applications and proposed solution(s). Identify a test and performance matrix, which will be used to score or evaluate the solution(s).) The solution(s) should be evaluated based on quantifiable metrics or a combination of quantifiable and qualitative metrics identified on the basis of the technical challenges posed by the environment and contact conditions, capture corrosion resistance, wear resistance, and friction properties. The feasibility assessment should identify work and tasking to be completed both in Phase I as part of a preliminary evaluation and in subsequent Phases of work, if awarded. (Note: OEM participation, while not required in Phase I, may benefit the development of the feasibility assessment and may help align the feasibility assessment with the Transition Plan developed if Phase III is awarded.)

3. Preliminary Feasibility Evaluation: Include a preliminary evaluation of the proposed solution(s) based on the feasibility assessment. (Note: While the scope of this evaluation may not be as broad as work identified for Phase II or Phase III, the objective of the preliminary evaluation should be to demonstrate the potential of the proposed solution(s), to identify the further benefits and improvements that may be achieved with subsequent phases of work, and to identify the potential risks. This preliminary evaluation may incorporate experimental or computational methods and should serve both as an evaluation of the solution(s) and of the proposed methods for the solution feasibility assessment.)

4. Risk Assessment: Identify potential risks with the proposed solution(s) and the evaluation strategy or method based on both the preliminary evaluation and other proposed solution assessment methodologies. Account for programmatic and technical risks to the development and evaluation of the proposed solution(s) and identify and describe the operational risks to the implementation of the proposed solution(s) in naval aero-engines. Develop a risk mitigation plan that outlines specific strategies and measures that will be employed to address those risks throughout the course of this project.

5. Project Schedule: Develop a detailed project plan and schedule for the tasks and activities for subsequent phases of the project, including Phase II (Prototype Development and Testing) and Phase III (Full-Scale Validation and Transition). Outline specific tasks, milestones, and objectives to be completed in each phase, including any decision points or milestones that may inform how or when the previously identified risk mitigation plan should be consulted. Identify the resources and expertise required for successful completion of each phase. Develop an anticipated timeline of each phase tasks and activities.

6. Program Cost Analysis: Conduct a preliminary cost analysis for the development and evaluation of the proposed solution(s). Include estimates for any required research and development, prototyping, and testing costs. Estimate the costs associated with identified risk mitigation activities.

Upon completion of Phase I, the feasibility assessment and project schedule for the proposed solution(s) will serve as the foundation for subsequent phases of this project, providing a clear roadmap for development and evaluation of the proposed solution(s) in a naval aero-engine environment.

PHASE II: Develop a prototype of the proposed solution(s) and conduct a feasibility evaluation to assess performance with respect to the seven technical challenges listed in the Description and to other technical challenges identified in Phase I as well as adaptability to multiple compressor applications. Perform either an experimentation or coupled computation/experimentation approach to refine, test, and optimize the proposed solution(s) based on the feasibility assessment prepared under Phase I. A scrap component validation of the downselected or refined solution(s) shall be performed to confirm the corrosion and mechanical durability performance of the proposed solution(s). OEM participation is recommended to facilitate the evaluation and solution design optimization. This approach should include the following:

1. Detailed Solution Design: Develop a detailed design of the proposed solution(s) to address the corrosion, mechanical durability, and other technical challenges presented by the compressor applications. This design should describe the application process and finishing steps for the solution(s), e.g., if the solution is a coating, identify the surface preparation, application method, and number of layers required. Identify all details of the solution design necessary to fully describe its preparation, application, and form. This design is expected to be identified over the course of Phase II via multiple iterations and refinements.

2. Design Optimization: The solution feasibility assessment and approach should incorporate an experimentation or computation based strategy for solution design refinement and optimization. This strategy should specifically include design refinement based on solution performance addressing the technical challenges identified in Phase I and suitability to multiple compressor applications. Analyze the available data and/or performance of the design to iterative refine and improve the design. Periodically, evaluate the design progression against the project milestones and pursue risk mitigation activities as appropriate to address any identified issues or to maximize potential benefits.

3. Prototype Fabrication: Based on the design optimization, fabricate a prototype solution on material coupons. Coupons should be manufactured to replicate the compressor materials, including alloy and heat treatment (this information will be provided upon project award). Prepare the coupons in accordance with the detailed solution design as if preparing the actual compressor components. Sufficient quantities of coupons should be prepared at minimum to experimentally evaluate the prototype via coupon-level testing. Once the proposed solution is refined or downselected to its final iteration, scrap component sections may be provided for prototype demonstration of the solution(s) in the same evaluation procedure as the test coupons.

4. Coupon and Scrap Component Testing and Characterization: The solution feasibility assessment is required to include at minimum, coupon-level experimental testing and characterization of the prototype design. Experimental testing may also be part of the design refinement and optimization; however, for experimental evaluation of the prototype design, coupon-level testing should carefully simulate exposure of the solution(s) to the complicated compressor environment, including, but not limited to, characterization of the impact of cyclic environmental (humidity, salt, water, temperature, etc.) exposure, galvanic pair with other compressor alloys, and mechanical loading and/or fretting from bolted flanges and vane tracks. OEM participation is encouraged to provide assistance with details of the engine operation environment and evaluation of the test results. Evaluate the coupon performance in accordance with the feasibility assessment prepared in Phase I. Once coupon-level testing has satisfied the conditions of the solution feasibility assessment, sectioned pieces of scrapped compressor components may be supplied for subsequent validation on actual component hardware. The components should be subjected to any surface preparation, including any surface grinding to remove pre-existing damage, prior to applying the solution. The performance of the solution(s) on the scrapped component should be evaluated according to the same solution feasibility assessment and with OEM engagement.

5. Updated Risk Assessment: Revisit and update the risk assessment and mitigation plan developed in Phase I based on the solution design and prototype development undertaken in Phase II. Identify any new risks and mitigation strategies that may have arisen.

6. Phase III Planning: Develop a detailed plan for Phase III (Full-Scale Validation and Transition), outlining specific tasks, milestones, and resources that will be required. Scrapped compressor components will be made available for full-scale testing and validation. Identify any testing requirements or validation of the solution necessary for operation on naval aero-engines not performed under this program, and develop a plan for transition to the Navy. OEM participation is highly encouraged to identify OEM-specific testing requirements.

PHASE III DUAL USE APPLICATIONS: Collaborate with engine OEMs to develop and implement a transition plan for the proposed solution(s) to the Navy and OEMs and to confirm the corrosion and mechanical durability performance of the proposed solution(s). This work should include:

1. Transition Plan: Develop a plan to transition the proposed solution(s) to the Navy, including documentation of how the material surface should be prepared prior to application, how the solution(s) are to be applied, and any post-application finishing steps. Collaborate with engine OEMs to identify all test and characterization requirements to validate and transition the proposed solution(s) to engine compressor components for in-flight operation, including any coupon level testing and/or field service evaluation. Identify any transition pathways to any non-aviation or non-military applications. The proposed corrosion solution(s) may be applicable across multiple industries, including commercial aviation and propulsion, automotive, and marine propulsion, OEM involvement in prior phases of work was specifically encouraged to capture broad commercial and military requirements in the solution feasibility assessment for coupon testing to smartly tailor the evaluation process to maximize benefits to multiple applications. The transition plan should build off this prior work and include any other test requirements as appropriate. Identify any solution-specific health and safety precautions. Identify a solution inspection plan, including how Navy fleet maintainers should inspect the solution for deposition defects, damage in use, corrosion, or other degradation.

2. Transition Test Matrix: Based on the transition plan, collaborate with the OEM to identify a test matrix for evaluation of the proposed solution(s). This test matrix may consist of an updated solution feasibility assessment developed and applied in Phases I and II based on OEM input, but it should reflect the updated test requirements identified in the transition plan. The objective of the test matrix is to specify the experimental methods and success criteria of the requirements laid out in the transition plan, e.g., if the transition plan should identify the requirement to perform an accelerated environmental engine test, the test matrix should specify the target environmental conditions, mission operation cycle, and other test parameters as recommended by the OEM.

3. Solution Validation: Based on the transition plan and test matrix, some of the identified transition requirements may be satisfied by the coupon or scrapped component testing; however, some requirements may remain unsatisfied. Any unsatisfied transition requirements should be addressed according to the transition test matrix pending available funding, component supply, and other test hardware.

REFERENCES:

1. Beavers, J.A.; Koch, G.H. and Berry, W.E. "Corrosion of metals in marine environments." Metals and Ceramics Information Center, 1986, pp. 1-746. https://apps.dtic.mil/sti/trecms/pdf/ADA171167.pdf
2. DeMasi-Marcin, J.T. and Gupta, D.K. "Protective coatings in the gas turbine engine." Surface and Coatings Technology, Volume 68-69, December 1994, pp. 1-9. https://www.sciencedirect.com/science/article/pii/0257897294901295
3. Gleeson, B. "High-temperature corrosion of metallic alloys and coatings." Materials Science and Technology: A Comprehensive Treatment: Corrosion and Environmental Degradation, 2000, pp.173-224. https://onlinelibrary.wiley.com/doi/abs/10.1002/9783527619306.ch14
4. Schneider, K.; Bauer, R. and Grunling, H.W. "Corrosion and failure mechanisms of coatings for gas turbine applications." Thin Solid Films, Vol 54, Issue 3, 1978. pp. 359-367. https://www.sciencedirect.com/science/article/abs/pii/004060907890398X

KEYWORDS: Naval Aviation Propulsion; Compressor; Turbine; Marine Atmosphere Corrosion; Stainless Steels; Nickel Superalloys; Corrosion Protective Coatings; Cyclic Environment Conditions

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