DON26BZ01-NV028 — Overlay/Bond Coatings that Resist Hot Corrosion in Navy Gas Turbines
Award Maximum: $140,000 (Base) / $100,000 (Option) Period of Performance: 6 months (Base) + 6 months (Option) Phase Type: Phase I
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. The 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.
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. Two important factors that lead to stress corrosion cracking of single crystal nickel-based superalloys are the type of deposits that form on components and the concentration of SOx in the environment.
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. Initiate correlations that should begin to formulate the ICME model framework to create a coating that would avoid reactions leading to SCC. 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. 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. 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. 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. 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.
KEYWORDS: Hot Corrosion, Stress Corrosion Cracking, Environmental-Induced Cracking, Corrosion Fatigue, Gas Turbines, Superalloys