Electroless plating Ni-P coatings on La(Fe, Si)13 hydride bulks for room-temperature magnetic-refrigeration application

La(Fe,Si)13 based magnetocaloric alloys have been implemented as regenerators in room-temperature cooling systems in search for environmental-friendly alternatives to conventional vapour-compression refrigeration. Unfortunately, their corrosion resistance in water related transfer fluids has as yet been insufficient, although various attempts were made, including optimizing elemental compositions, customizing heat transfer fluids, and surface coating. This work presents a simple electroless plating strategy to deposit continuous Ni-P layers on the La0.7Ce0.3Fe11.45Mn0.2Si1.35 hydride thin-plates sintered under a high pressure, affording super corrosion protection and simultaneous improvement of heat transfer ability. Upon deposition of 2–6 µm Ni-P layers possessing robust interfacial coupling with the substrate, a large magnetic entropy change of about 12 J/kg K under 0–2 T was retained at room temperature. More significantly, the charge transfer resistance in 3.5% NaCl solution was increased to 6–17 times and the corrosion current density was reduced to 5–7 times compared to the high-pressure sintered hydride bulk, accompanied by a simultaneous increase of thermal conductivity to about 2 times. The Ni-P layers consisting of columnar microstructure may facilitate relief of the local stress from the field-induced lattice expansion, thus improving cycling stability of the magnetic refrigerants.