Perspectives on Chemical Stability and Corrosion of Magnetocaloric Materials

Active magnetic regenerative (AMR) refrigeration is an energy-efficient and environmentally-friendly alternative to conventional vapor-compression refrigeration technology associated with harmful chemical refrigerants and high carbon emissions having high ozone-depleting potential that encourages planetary warming. To address this challenge and in response to strategic global treaties such as the Montreal and Kyoto Protocol, many countries worldwide, including the European Union (EU), Japan, USA, and China, have begun to unveil new rules to phase out GWP gases. These policies have inspired collaborative research efforts worldwide to find possible alternative cooling methods. Magnetic cooling devices enabled with the "magnetocaloric" class of functional materials are attractive as they have the potential for efficiency improvements of up to 50% over conventional vapor compression systems, which is equivalent to 60% of Carnot efficiency. The core component of AMR is a porous magnetocaloric material (MCM) that undergoes millions of thermal and magnetic field cycles throughout the device's lifetime while immersed in a heat transfer fluid. Despite significant research on MCMs spanning three decades, an overlooked and less frequently studied but critical engineering challenge to be considered is the interaction between the magnetic refrigerant material and the heat exchange fluid, namely, the aging of the material when exposed to prolonged corrosive action. Research on the corrosion and chemical stability of room-temperature MCMs is discussed with particular attention given to Gd, Gd₅Si₂Ge₂, La(Fe,Si)₁₃ and its compositional variants. Following a brief overview of the wide variety of corrosion monitoring methods used to evaluate magnetocaloric regenerator structures, corrosion inhibition mechanisms are discussed in the context of metallurgical, processing, and environmental factors. Challenges associated with corrosion testing of magnetocaloric alloys prepared by additive manufacturing, corrosive-dependent magnetic data on 3D printed La(Fe,Si)₁₃ magnetocaloric structures, and insights into corrosion resistance of 3D printed magnetocaloric structures are presented.