EXTREME ENVIRONMENT VALIDATION
Molten Salt Validation Campaign
At HybriMet™ LLC, we solve one of the single biggest bottlenecks preventing the commercial scaling of next-generation energy systems: mechanical component failure in extreme environments.
We design and manufacture proprietary Metal Matrix Composites (MMCs) engineered specifically for hard wear surfaces and bearings and corrosion and erosion resistant coatings. Our structural composites maintain exceptional mechanical integrity and dimensional stability at continuous operating temperatures up to 750°C.
What sets us apart from theoretical material startups is that our technology is already empirically validated. We possess existing static corrosion and wear data in both chloride and fluoride salt environments. This means our materials are uniquely demonstrated to survive the degradation pathways found in Gen IV Molten Salt Reactors, high-temperature Concentrating Solar Power systems, Industrial Materials and Critical Elements refining, and Thermal Energy Storage.
HybriMet™ MMC directly extends critical pump, valve, bearing and tank lifespan.
During a eight year development and material optimization program HybriMet™ completed static corrosion tests and dynamic tribology, wear and erosion tests on solid HybriMet™ MMC and on superalloys HVOF coated with HybriMet™ MMC and then validated the results by building and exercising a 35 GPM molten salt pump.
HybriMet™ REDUCES YOUR RISK for the commercialization of molten salt systems.
Corrosion
Over ten HybriMet™ MMC compositions and several alloy substrates were evaluated to optimize the composition for 750C chloride salt applications. HybriMet™ NiWC3b™ and NiWC3b2™ were selected as the best performing candidate materials.
We chose to use Haynes 230 as a control in these tests to be able to relate our test data to the corrosion rates developed at UWisc and by other investigators and to normalize our tests to remove variation. Substrates tested included Inconel 617 and 625.
The CSP Gen 3 nominal salt composition is 40/40/20 mol% for KCl/MgCl2/NaCl. This salt purified and filtered, at 750°C was used for corrosion testing. The tests were designed as screening tests with multiple materials and coatings. Due to the ionic nature of the salt, material transfer occurs between specimens. The specimen geometry varied greatly, and some were very large and were used for post exposure tensile, flexure and compression testing.
Molten chloride salt corrosion tests were run on NiWC3b™ coated IN625 and IN617 bend strip specimens exposed to 750⁰C chloride salt for 250 hours. These specimens were measured and weighed and analyzed microscopically optically and using SEM and EDS.
IN625 substrate remains intact under the coating and does not exhibit Cr migration through the coating. There was no observable loss of adhesion of the coating to the substrate. The NiWC3b™ coating protected the IN625 substrate very well. Both the IN625 and IN617 uncoated substrates had chromium depletion at the grain boundaries to a depth of 20 µm after the 250 hour test.
In these photos, HybriMet™ NiWC3b™ coated IN617 is on the left and coated IN625 is on the right. SEM photos are on the top and EDS are on the bottom. For each material a cross section of the coated surface is on the left and the uncoated back side is on the right. Note the Cr depletion on the back side and there is no corrosion on the NiWC3b™. Some Cr migrated to the NiWC3b™ surface.
In addition to these bend strips, flex bars and tensile specimens in the same corrosion vessel for 250 hours showed negligible change when tested at RT and at 750C.
Pump Reservoir
A salt loop used in this test campaign features a 12” diameter NiWC3b™ coated 316SS vessel as the lower pump reservoir. This vessel saved $13,890 and 6 weeks over an IN625 vessel which would have cost $21,082 just for the material.
Upon opening the pump reservoir to extract the pump, the NiWC3b™ coated vessel was visually inspected. The vessel was intact and well protected from corrosion with no salt level line or indication of corrosion on the interior walls of the vessel.
Wear
A 720 °C pin-on-disk wear test apparatus designed by High Temperature Systems Design and HybriMet™ and operated in the University of Wisconsin – Madison Salt Laboratories is the first instrument to be able to test material friction and wear rates in this molten salt environment. Pin-on-disk is a basic tribology test and is used to rank materials by their wear performance. While most tribologists attempt to accelerate these tests to predict component life by increasing load, speed and temperature or by decreasing lubrication, accelerated testing in a 720 °C molten salt environment was limited to increased load. Speed is usually related to a temperature increase; in our application the salt temperature over-rides any frictional temperature increase. A 96 hour pin on disk test indicates that we have a useful bearing at PV equal to full size commercial pumps. The pin on disk PV was set substantially higher by increasing the load on the pin to accelerate the test and estimate the material limits. The coefficient of friction is approximately 0.6. The measured wear rate for NiWC3b™ is 20% that of H230. For NiWC3b™, there is evidence that suggests the formation of a lubricating layer between the bearing surfaces.
The photos above show a HybriMet™ NiWC3b™ pin and disk before and after a 250 hour run at 720C in chloride salt.
A Tribology Test Bed (TTB) was constructed to evaluate materials as plain bearings, shaft sleeves and vanes in a laboratory setting. The TTB can test multiple specimens simultaneously at different surface speeds and loads in extended tests to maximize the data output. Results are obtained by measuring changes in weight, dimensions and surface finish. The pressure-velocity (PV) factor for the tests run on the TTB was about the same as for the validation pump. A 720⁰C 500 hour test on the Tribology Test Bed resulted in no wear on any of three bushings and sleeves and no erosion on the vanes.
A pump was built and tested to confirm the design and performance of the bearing materials. The pump is a vertically suspended 2-stage pump with submerged bearings designed to pump salt at 35 gpm but was operated at over 75 gpm. While this pump and test loop have no features to accelerate a life test, it is an evolution of a commercial design as would be scaled up to be used in a commercial system.
The test plan was designed to acquire pump performance data at two temperatures (550⁰C and 720⁰C) and three pump speeds (1300 RPM, 1450 RPM and 2000 RPM). At each temperature / speed setting the flow control valve was throttled from fully open to almost closed to generate pump curves at flow rates from zero to 18 m3/h (80 gpm), over twice the design flow rate of 8 m3/h (35 gpm). The valve was adjusted only after the system became stable for 15 or 20 seconds. At each point mass flow and pressure measurements were recorded. The valve was never fully closed so we did not deadhead the pump.
The pump ran successfully throughout a 91.1-hour test. Five shutdown and re-start cycles were completed from 1450 RPM, three at 720⁰C and two at 550⁰C in addition to the initial startup and final shut down. These cycles were completed successfully including a long shut down of 22 minutes. Temperatures, flow rates and salt pressure were recorded throughout the campaign and results show good correlation with model predictions and the factory quality confirmation water test.
Confirming the veracity of the described test results, over the last four years there have been initial field installations by major valve manufacturers of HybriMet™ NiWC3b™ and NiWC3b2™ molded valve seats and coated valve stems in a variety of salt applications with no reported performance issues.
Erosion
Erosion tests were completed on the Tribology Test Bed (TTB). In place of actual impellers, erosion data was generated on test specimens shaped as pins mounted radially on a hub. Stator fins were employed to prevent the salt rotating with the pins. Erosion was measured radially around the pin and along the length of the pin, providing erosion data versus angle of impingement and versus speed. There was flow turbulence that we expect provided a more aggressive environment than a well-designed impeller.
The test ran for 500 hours in 720⁰C chloride salt at 1100 RPM. The tip velocity was 6.4 m/sec. HybriMet™ NiWC3b™ pin specimens had no measurable erosion within the accuracy of the micrometer along the pin from the tip to the hub or around the pin from the leading edge to the trailing edge.
These tests were supplemented using available computational erosion models from UW-Madison, ORNL, MIT and INL. A key finding by the developers of these models is that in salt, mechanical wear and erosion on structural components accelerates chemical dissolution exponentially by continuously removing protective, chromium-depleted surface layers, causing increased localized degradation.
The significant parameters effecting erosion fall into two categories, the salt properties (flow rate, particulate concentration, particulate hardness and the impact angle) and the material properties (fracture toughness, hardness, corrosion resistance, and boundary layer properties).
These models all gave high weight to the impact of hardness and toughness of the structural material over other parameters. They ranked the materials of interest differently but similarly. Because of its high hardness and fracture toughness the service life of HybriMet™ NiWC3b™ and NiWC3b2™ greatly exceeds that of superalloys. We used 650⁰C FLiBe, 2.5 m/sec flow rate and 0.05% particulate concentration in each of the models for our calculations. The average of the predictions from these models is provided in this chart.
