Study On Corrosion Behavior And Magnetic Properties Of La（Fe,Si）₁₃-based Magnetic Refrigerants

With low raw materials price, nontoxic, large magnetic entropy change and adjustable Curie temperature, La(Fe,Si)13-based magnetic refrigeration materials are regarded as potential solid-state refrigerant candidate for magnetic refrigeration prototype. But moving solid-state refrigerant between the heat-end and cold-end periodically is too difficult. So liquid or gas is used as heat exchange medium to achieve heat transfer. However, La(Fe,Si)13-based material with multiphase is easily corroded, especially in water-based solution. Corrosion can decrease the efficiency of magnetic refrigerator and shorten its lifetime. In this paper, corrosion behavior and magnetic properties of La(Fe,Si)13-based magnetic refrigerants have been studied. The influence of secondary phases and elements doping on the magnetic properties and corrosion resistance are discussed. Furthermore, a compound inhibitor with high inhibition efficiency was obtained through comprehensive comparison analysis.a-Fe phase and La-rich phase as secondary phases can always be observed in annealed La(Fe,Si)13-based materials. The secondary phases have impact on both corrosion resistance and magnetic properties. The decrease of secondary phases, especially the elimination of a-Fe phase worked as cathode could efficiently improve the corrosion resistance of the alloy. But if the amount of La-rich phase worked as sacrificial anode decreased below a certain value, corrosion would occur on the main phase. And the corrosion resistance of the alloy decreased. With the elimination of secondary phases, the magnetic entropy change increased first, and then it slightly decreased.Co, Si or C doping is the main methods to adjust the Curie temperature to room temperature for La(Fe,Si)13-based materials with secondary order phase change. But they influenced the magnetic properties and corrosion resistance very differently. According to the△S-Tc at room temperature (275-315K) with different Co, Si, C content,△S decreased fastest by the substitution of Si (1.3≤y≤1.5) for Fe, while it decreased slowly by adding C (0.10≤z≤0.20). It might be attributed to the variation of saturation magnetization. The addition of Co and C elevated the membrane resistance of the alloy and the corrosion resistance was improved. But high Si content would lead to a larger corrosion potential difference, which is harmful to corrosion resistivity. Therefore, to get magnetic refrigerant materials with better corrosion resistance and magnetic properties. adding some C should be the priority choice to increase the TC, and Si should be reduced as far as possible.Generally. Mn doping is used to control the Curie temperature of hydrogenated La(Fe,Si)13-based materials with first order magnetic phase change. However, Mn substitution for Fe could hinder the formation of1:13phase and led to the increase of α-Fe phase after heat treatment. Corrosion occurred simultaneously in the grains and between the grains. The weight loss test and electrochemical experiment show that Mn doping can improve the corrosion resistance of LaFe11.5-xMnxSi1.5compounds. The latent heat decreased with the increase of the number of heat cycles, and Mn doping accelerated the decrease tendency. After being placed for7days, Mn doping was proved to be harmful to the recovery of latent heat. The analysis of specific heat for LaFe11.5-xMnxSi1.5in low magnetic field indicated that the addition of Mn reduce both magnetic entropy change and temperature change.Finally, corrosion behavior of LaFe10.8Co0.7Si1.5C0.2alloy in common inhibitor has been investigated. The compound inhibitor K2CrO7+Na2CO3in acidic atmosphere (pH=6.5) and Na2Mo04·2H2O+Na2HPO4·12H2O compound inhibitor in alkaline atmosphere (pH=8) have excellent corrosion effect. The applicability and stability of the corrosion inhibitor for a long time were investigated for La(Fe,Si)13materials with different composition by weight loss experiment. The results shows that Na2MoO4·2H2O+Na2HPO4·12H2O compound inhibitor has excellent inhibition effect on La(Fe.Si)i3-based magnetic refrigeration material. The metallic luster of LaFe10.8Co0.7Si1.5C0.2and LaFe11.4Mn0.1Si1.5samples remained after a3886-hour-long immersion in Na2MoO4·2H2O and Na2HPO4·12H2O compound inhibitor solution. The average corrosion rate was just6.74×10-5g·m-2·h and2.10×10-4g·m-2·h for LaFe10.8Co0.7Si1.5C0.2and LaFe11.4Mn0.1Si1.5l, respectively.