A new, collaborative study by the Southwest Research Institute (SwRI) (San Antonio, Texas, USA) and Sandia National Laboratories (Albuquerque, New Mexico, USA) was designed to examine the differences in oxide film growth on additively manufactured (AM) metals and wrought stainless steel in supercritical carbon dioxide (sCO2) environments.
The researchers describe sCO2 as carbon dioxide held above a critical temperature and pressure, which causes it to combine gas and liquid properties.
Current power plants in the United States typically use water as a thermal medium in power cycles. Replacing water with sCO2 increases efficiency by up to 10 percent, allowing for considerably smaller turbomachinery and a smaller footprint. The supercritical state makes sCO2 an efficient fluid to generate power, because small changes in temperature or pressure cause significant shifts in density.
SwRI describes itself as a leader in sCO2 power cycles. The Institute has received numerous U.S. Department of Energy and industry-funded projects to implement pilot-scale sCO2 power cycle components and system-level equipment, in addition to a supercritical transformational electric power pilot plant that is under construction at the Institute.
Florent Bocher, senior research engineer, began exploring how oxidation affects AM materials as part of a collaborative effort with Sandia. “The smaller, more complex machinery necessary for the small turbines that sCO2 power cycles utilize makes additive manufacturing an attractive resource,” Bocher says.
The additive manufacturing process uses three-dimensional printing, or rapid prototyping, to build items by layering plastic, metal, and other materials for a custom, computer-generated design. Because AM metals are sturdy components with intricate design qualities, the process appeals to many 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 explains. “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, Bocher and his collaborators exposed samples of both to a simulated sCO2 power cycle environment, including a temperature of 450 °C and pressure of 76 bar, for two weeks. The AM materials were built and analyzed by Sandia.
“Both types of metals showed oxide growth,” Bocher says. “But the oxide covered about 72 percent of the wrought stainless steel and 54 percent 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 jointly funded study is accessible online through ScienceDirect after being published in the June 2022 edition of Corrosion Science. Further research background and information is available on the “Corrosion Failure Analysis” and “Advanced Power Systems” page tabs at the SwRI web site.
Source: SwRI, www.swri.org.