Magnetocaloric La(Fe,Mn,Si)13Hz particles and their chemical stability in heat transfer fluids employed in magnetic refrigeration

La(Fe,Mn,Si)(13)H-z alloys are prominent solid-state refrigerants for magnetic refrigeration, given their outstanding magnetocaloric properties. However, their chemical stability is yet an issue to be further investigated, especially under the working conditions of magnetic refrigerators. These alloys should not corrode nor lose their magnetocaloric capabilities when in contact with the heat transfer fluids used in magnetic refrigeration. Such topics are addressed in this work by performing immersion tests with La(Fe,Mn,Si)(13)H-z particles in two kinds of media: deionized water and deionized water containing a corrosion inhibitor (Entek), the latter medium being an usual heat transfer fluid for magnetic refrigeration. Corrosion products were detected at the surface of the particles after just 15 min of exposure in water in the absence of the inhibitor, and corrosion progressed over time, increasing the pH in the surroundings of the particles. After 10 weeks, there were clear signs of surface damage, and the fraction of oxygen at the surface increased from about 2% wt. up to 12% wt. Consequently, the thermomagnetic properties changed as well: the Curie temperature of the particles was shifted to higher values (increased about 1.5%), and the magnetic transition was smeared out. A significant decrease in the variation of specific entropy was also observed: depending on the initial T-C of the particles, such decrease was as high as 44,5% (from 8.1 J/kg & BULL;K before the immersion test to 4.5 J/kg & BULL;K after 10 weeks of immersion). In contrast, no changes were observed in the particles exposed to water containing the corrosion inhibitor for the same period: their microstructure remained stable, the pH of the exposure media did not vary significantly, and their thermomagnetic behaviour remained unchanged. Therefore, the presence of the inhibitor effectively avoided the occurrence of aqueous corrosion and preserved the microstructure and thermomagnetic properties of the La(Fe, Mn,Si)(13)H-z particles.