Magnetocaloric La(Fe,Si)13 based alloys are essential building blocks for magnetic cooling systems. Unfortunately, their insufficient corrosion resistance in water related transfer fluids is very critical. Solid state magnetocaloric elements have been implemented as regenerators in magnetic cooling systems in quest for environmental-friendly alternatives to typical gas compression cooling methods. These materials experience reversible magnetic phase transition which can be induced by an external magnetic field. The magnetic state change causes temperature changes under adiabatic condition, or entropy change under an isothermal condition.
The synthesis of magnetocaloric La(Fe,Si)13 phase from as-cast pre-alloys necessitates longer heat treatment at high temperature for a number of days or weeks. Lanthanum melt, alpha-iron(silicon) as well as La(Fe,Si)13 phases coexist at 1500-1650K, conventional microstructures of as-cast alloys indicate alpha-iron(silicon) dendrites with a few micrometers length and width surrounded by lanthanum rich phases.
Optimum annealing conditions have been analyzed in a bid to obtain nearly single phase materials. Rapid cooling methods, for instance melt spinning, have been identified to minimize annealing time through refined phase distribution. However, single phase materials are yet to be achieved. This is because, diffusion driven phase reaction residual amounts of lanthanum rich phases and alpha-iron(silicon) are inevitable. Annett Gebert, Maria Krautz, and Anja Waske at the Leibniz Institute for Solid State and Materials Research (IFW) in Germany studied the corrosion behavior of annealed and as-cast La-Fe-Si alloys in order to evaluate the effect of different microstructural states. Their work is now published in Intermetallics.
The authors performed base studies for pre-investigation of corrosion in distilled water on homogenized sample prepared by arc-melting and subsequently annealed to establish magnetocaloric relevant phase. They also performed corrosion analysis under forced flow conditions of the electrolyte on copper mold cast and induction molten and on annealed alloy samples.
The researchers employed 2mm thick samples for electrochemical studies. The samples were prepared from as-cast and annealed pieces. Also, high purity iron and lanthanum materials were used as reference materials. After the open circuit potential exposure, the authors examined the alloy sample surfaces with scanning electron microscopy to analyze the corrosion modes.
They observed that the exposure of the magnetocaloric material in distilled water applied as a heat transfer medium was damaging under stagnant conditions. Acidification of the near-surface electrolyte enhanced local surface phenomena. These were based on galvanic coupling between the phases based on their corrosion activity. Continuous laminar flow was important for the alloy surface protection and passivation. Nevertheless, anion contaminants such as hydrogen phosphate ions and sulfate ions counteracted the weak passivity in flowing water. A pH value control of the fluid towards weak alkaline condition was efficient in establishing stable passive states at the samples’ surface owing to the formation of lanthanum and iron hydroxides and oxides that were relatively insoluble.
The researchers also noticed that attempting to realize corrosion protection via thin phosphate conversion coatings was unfruitful. The reactive characteristics of the alloying elements as well as their concentrations affected the general corrosion activity along with passivation ability of the magnetocaloric alloy. Microstructural optimization towards the single phase La(Fe,Si)13 state was impossible through the existing reaction paths. Enhancement of the corrosion resistance could only be achieved through compositional modifications. This would mean addition of elements which would trigger self-passivation and re-passivation ability of the sample under near neutral to acidic conditions.
Annett Gebert, Maria Krautz, and Anja Waske. Exploring corrosion protection of La-Fe-Si magnetocaloric alloys by passivation. Intermetallics, volume 75 (2016), pages 88-95.Go To Intermetallics