PROJECTED CMMC LEVEL REQUIREMENT
Level 2 (Self)
TECHNOLOGY AREAS
None
MODERNIZATION PRIORITIES
Advanced Materials
|
Sustainment & Logistics
KEYWORDS
Hot Corrosion, Stress Corrosion Cracking, Environmental-Induced Cracking, Corrosion Fatigue, Gas Turbines, Superalloys
OBJECTIVE
Develop overlay or bond coatings and a coating model that enables longer service and prediction of corrosion, oxidation and overall degradation when exposed to marine Naval environments as a function of corrosivity, stress, and various temperature combinations via integrated computational material engineering (ICME), which will foster creation of new coatings resistant to these degradation modes.
DESCRIPTION
Marine gas turbine engines serve as primary and auxiliary power sources for several current classes of ships in the U.S. Navy. It is desirable for marine gas turbine engines to have a mean time between removals of 20,000 hours. While some engines have approached this goal, others have fallen significantly short. The main reason for this shortfall is various forms of hot corrosion (Type I and Type II) damage in the hot section turbine hardware due to intrusion of salts from the marine air and/or from sulfur in the gas turbine combustion fuels.
The synergistic effect of stress- and deposit-induced high temperature corrosion can lead to other corrosion mechanisms. Corrosion fatigue as well as fatigue often initiates at stress risers. Metallurgical examination of several failed marine gas turbine blades that had operated between 5,000 and 10,000 hours was performed and compared to unfailed blades with 18,000 operating hours from a similar marine engine. Deposition occurring at sites under the platform of unfailed turbine blades revealed pitting at those sites.
Further examination revealed poor coating quality (i.e., high porosity and variable thickness) under the platform of first stage turbine blades that allowed salts to permeate through the coating to the alloy surface and initiate hot corrosion. Further coating examination under the platform showed highly variable coating thicknesses (0-40 m) in the curved area of transition between the under platform and the blade stem. In a few cases, coatings were non-existent on the unfailed blades. The Cobalt Chromium Aluminum Yttrium (CoCrAlY) coating, when present, usually was porous or the available coating under the platform was highly contaminated due to lack of adequate spray deposition in these non-line-of-sight areas. CoCrAlY coating thicknesses at other sites along the blade stem were 35 m to 105 m (1.4 to 4.1 mils) and devoid of porosity. The corrosion that was observed under the platform in all cases was caused by Type II, low-temperature hot corrosion, which occurs in the temperature range of 649 -732 C (1,200 -1,350 F). Corrosion penetrated the porous coating and caused further undercutting of the coating along the coating/alloy substrate interface, Type II hot corrosion caused pitting at these locations under the platform, which caused stress risers where corrosion fatigue cracks initiated. These pits advanced through the blade stems to varying degrees.
The synergistic effect of stress- and deposit-induced high temperature corrosion leads to the premature failure of aero turbine blades reportedly due to stress corrosion cracking. The lower shank of aero gas turbine blades, which operates below 600 C is susceptible to this mode of failure. Two important factors that lead to stress corrosion cracking of single crystal nickel-based superalloys are the type of deposits that form on components (these include alkali chlorides and sulfates which are introduced through the environment) and the concentration of SOx in the environment. Therefore, it is important to understand the synergistic role of deposits and sulfur containing gases on the stress corrosion cracking susceptibility of single crystal nickel-based superalloys below 600 C.
PHASE I
Demonstrate an understanding of what differences and influences exist between aviation and marine propulsion. Determine the mechanism for the observed corrosion at 500 -550 C. if stress corrosion cracking (SCC) is the prevalent corrosion mechanism, determine the interplay with NaCl, Na2SO4, SOx, and stress. For a given concentration of the chemical compounds, determine at what stress initiates SCC. Initiate correlations that should begin to formulate the ICME model framework to create a coating that would avoid reactions leading to SCC. ( For shipboard operations this would be either an overlay or diffusion coatings. For aero applications, this would lead to creation of a bond coat that would also promote long thermal barrier coating (TBC) life (goal: > 5K hours) and assist in maximizing corrosion and oxidation resistance by changes in coating chemistry and structure while not impacting fatigue, SCC, or substrate strength of the substrate alloys. It is suggested that the starting TBC be yttria-stabilized zirconia.) Lastly, perform a short-term (~200 hours) high temperature test to validate the feasibility of the ICME model.
PHASE II
The ICME framework shall be further expanded to include compatibility of the TBC to different bond coats as well as further development, modification, and maturation of the ICME model. Collaboration with coating and engine gas turbine original equipment manufacturers (OEMs) is encouraged for advice and direction for further developments of the ICME models and strategies. Correlate into the ICME-derived model the interaction of chromium and aluminum content in a coating that leads to the formation of chromia or alumina scales. Coatings on several alloys shall be tested to determine coating compatibility and assess the impact of coatings on alloy substrate properties in a burner-rig or similar test environment that includes salt ingestion. Coatings shall be applied onto alloy substrates by at least one recognized commercial coating process (line-of-sight and/or non-line-of-sight). The expected deliverables will be: (1) optimized coating corrosion resistance to SCC for a given set of alloys and (2) an ICME-derived model that would predict and assist in the development of future overlay or bond coats to minimize SCC in gas turbine that are compatible with multiple alloy substrates.
PHASE III DUAL USE APPLICATIONS
The ICME model will be further developed and matured through the expansion of bond coat/overlay coat chemistry and structure with the selected strategies to mitigate salt interaction that could lead to SCC. TBCs are permeable so the bond coat must form an impermeable barrier to avoid salt interaction with the alloy substrate that would tend to cause SCC. Engage with a gas turbine engine OEM to have an appropriate bond coat-TBC system applied on select static and/or rotating engine components of a current Navy engine and testing in cycling temperature test. The expected deliverables will be: (1) a TBC(s) compatible to corrosion and hot corrosion-resistant bond coat substrates, (2) TBC(s) resistant to SCC in the marine environment, and (3) an ICME-derived model that would predict and assist in the development of future TBC systems (i.e., alloy, bond coat, TBC, TBC strategy to minimize SCC with marine aero engine operational environment). Engage with a marine engine OEM to further develop the TBC technology for incorporation into current and future Navy ship engines.
Development of long-lived TBC systems able to withstand hot corrosion and subsequent SCC at temperatures below 600 C for U.S. Navy applications will also enable more efficient service for commercial applications that employ industrial gas turbines. Marine gas turbine engines are industrial gas turbines that are intended for Naval use. Successful development of better coatings for the current alloys, capable of extended service in the highly corrosive Naval operating environment, should enable subsequent use in commercial applications such as cargo ships, cruise ships, ferries, and tankers if the business case justifies the results.
REFERENCES
Martinez, F.D. et al. "Chlorine-Induced Stress Corrosion Cracking of Crystal Superalloys at 550 C", High Temperature Corrosion of Materials, 101, 2024, pp.951-960. https://link.springer.com/content/pdf/10.1007/s11085-024-10282-7.pdf
Shifler, D.A. "Navy Turbine Engine Corrosion: Understanding and Mitigation." 2023 DoD Corrosion Prevention Technology and Innovation Symposium, August 14-17,2023, Tucson, AZ. https://www.dau.edu/sites/default/files/webform/documents/25016/DOD-AUG22-19822.pdf
Alvarado-Orozco, J.M.; Gleeson, B.; Pettit, F.S.; and Meier, G.H. "Reinterpretation of type II hot corrosion of Co-base alloys incorporating synergistic fluxing." Oxidation of Metals, 90, 2018, pp.527-553. https://ouci.dntb.gov.ua/en/works/40gaMMP7/
Kistler, E.; Chen, W.T.; Meier, G.H. and Gleeson, B. "A new solid-state mode of hot corrosion at temperatures below 700 C." MATERIALS AND CORROSION-WERKSTOFFE UND KORROSION, 70(8), 2019, pp. 1346-1359. https://onlinelibrary.wiley.com/doi/abs/10.1002/maco.201810751
QUESTIONS & ANSWERS
To ask a question, you must
log in or create an account
for the DSIP.
