Oxide growth in additively manufactured metals examined in sCO2 environment

sCO2 is carbon dioxide held above a critical temperature and pressure, which causes it to combine the properties of gas and liquid. Current power plants typically use water as a thermal medium in power cycles. Replacing water with sCO2 increases efficiency by as much as 10 percent, which also allows for considerably smaller turbomachinery and a smaller footprint. Its supercritical state makes sCO2 a highly efficient fluid to generate power because small changes in temperature or pressure cause significant shifts in its density.

SwRI is a leader in sCO2 power cycles. The Institute has received numerous Department of Energy and industry-funded projects to implement pilot-scale sCO2 power cycle components and system-level equipment in addition to the 10 MWe Supercritical Transformational Electric Power (STEP) Pilot Plant under construction at SwRI.

Senior Research Engineer Dr. Florent Bocher began examining how oxidation affects AM materials as part of an existing sCO2 collaborative effort with Sandia National Laboratories.

"The smaller, more complex machinery necessary for the small turbines that sCO2 power cycles utilize makes additive manufacturing an attractive resource," Bocher said.

Additive manufacturing is a novel process that uses 3D printing or rapid prototyping to build an item by layering plastic, metal and other materials for a custom, computer-generated design. Because AM creates sturdy components with intricate design qualities, it appeals to a wide range of users, including the aerospace, medical and manufacturing industries.

"The high temperatures and pressures of the sCO2 environment make oxidation a concern for metal components," Bocher explained. "As these two industries move forward, it's important to understand how oxidation affects them."

To test the durability of AM metals versus traditional wrought stainless steel in the sCO2 environment, Bocher and his collaborators exposed samples of both to a simulated sCO2 power cycle environment, including a temperature of 450 degrees Celsius and pressure of 76 bar, for two weeks. The AM materials were built and analyzed by Sandia National Laboratory.

"Both types of metals showed oxide growth," Bocher said. "But the oxide covered about 72% of the wrought stainless steel and 54% of the AM material, with the grain size and thickness of the oxide layer being statistically larger and thicker for the wrought material. Ultimately, though, this doesn't prove that one is more reliable than the other. More data is needed, but this certainly suggests that AM processes should be optimized going forward for these types of conditions."

The study was funded jointly by Southwest Research Institute and Sandia National Laboratories.