| Rare earth permanent magnet NdFeB has been widely used in many fields, but its poor resistance to oxidation and corrosion greatly limits its application. Therefore, the research in the protection of NdFeB from corrosion has currently become the urgent needs to solve the problem. The pollution-free and non-toxic magnetic refrigeration has the characteristics of harmlessness to the environment, high reliability, high efficiency and low energy consumption, thus making a kind of new refrigeration method with large future potential. The corrosion and corrosion protection of NdFeB magnets in various refrigerant system have lately become research hot spot, but the related reports are yet rarely seen. This paper, based on the electrochemical technology and surface energy spectrum and surface analysis methods, studies the oxidation characteristics and corrosion of NdFeB materials in natural environment and various kinds of refrigerants. After NdFeB electrode surface is treated by the protection methods of preparing membrane, the surface passivation, and the adsorption, the corrosion inhibition effects are tested and compared. Polyaspartic acid is added into the medium and the efficiency of corrosion inhibitors and corrosion inhibition mechanism are discussed. The information analysis of the experimental data and the parameter calculation are conducted, and the main results and conclusions are as follows:1. The polarization resistances of the magnet in CaCl2, NH4Cl, NaCl, ethanol, ethylene glycol and propylene glycol solutions are tested by linear polarization method. The results show that the polarization resistances are reduced with the increase of the solution concentration in three kinds of inorganic solutions, while they are increased with the increase of the solution concentration in the organic solutions.2. According to the Ea calculation, it can be seen that the activation energies of the magnet in inorganic Secondary refrigerants are less than the organic refrigerants. The minimum is Ea=14.65 KJ/mol in 8%CaCl2+2%NaCl, and the maximum is Ea=48.86 KJ/mol in the ethanol solution of 10%.3. The corrosion inhibition of P ASP for the NdFeB magnets in ethanol and CaCl2 solutions are tested by the Tafel polarization curve method. By using PASP for the magnet surface modification in 10% ethanol solution, the inhibition efficiency reaches 98.4% at the largest at 20℃.It can reach 63.9% in 20% CaCl2 solution.4. The circulation voltampere is tested after the surface modification of magnet, and the results show that, in CaCl2 solution before and after modification the CV graphs there do not indicate obvious difference,and the current range change of the CV graph in the ethanol solution is obviously less than before modification.5. AC impedance test technology is adopted on NdFeB in ethanol and CaCl2 solutions at the test temperature range, and the corrosion inhibition of the solutions for the PASP. The solution concentration, the concentration of PASP and the membrane time on the influence of corrosion inhibition are studied. In 20% CaCl2 solution, the best membrane time is 60 minute, the corrosion rate can reach 73.9%. In the 10% ethanol solution which add 0.5 g/L PASP the corrosion rate can reach 62.2%. The experimental data is fitted by the EQU software, the fitting equivalent circuit:R([R(RQ)]C).6. The corrosion products of the magnet are characterized by the test methods of SEM, EDS spectrum analysis, and specific surface area. In a natural environment corrosion products are mainly FeO and Nd2O3.7. In the 10% ethanol solution, the adsorption of PASP in the surface of NdFeB conforms to the Langmuir adsorption isotherm：C/θ=0.01843+1.53605×C, the adsorption of equilibrium constant K= 5.4×104, the free energy of Gibbs △Gθ=-36.3 KJ/mol, which shows that the adsorption process of PASP at the surface of NdFeB magnet is spontaneous. PASP acts as the absorption type corrosion inhibitor on NdFeB electrode in solution system. |
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