CHARACTERISTICS OF FERRITE ELECTRODES

Transcription

1 CHARACTERISTICS OF FERRITE ELECTRODES S. Wakabayashi, T. Aoki To cite this version: S. Wakabayashi, T. Aoki. CHARACTERISTICS OF FERRITE ELECTRODES. Journal de Physique Colloques, 1977, 38 (C1), pp.c1-241-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1977 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque Cl, supplkment au no 4, Tome 38, Avril 1977, page Cl-241 CHARACTERISTICS OF FERRITE ELECTRODES S. WAKABAYASHI and T. AOKT Ferrite Division, TDK Electronics Company, Ltd., Nikaho-cho, Akita-ken, Japan R6sum6. - Les materiaux utilises comrne anode pour electrolyse en solution doivent presenter une resistivite faible, une tenue &levee la corrosion et une resistance mtcanique importante. On montre que ces proprietes peuvent etre reunies dans les ferrites, obtenus en utilisant des materiaux de depart de haute pureti, en choisissant la composition chimique adequate et en frittant dans des conditions appropriks. Les proprietb typiques de difftrents ferrites ainsi que de la magnttite moulke sont presentees et discutees. Abstract. - Anode materials used in solution electrolysis should have low electric resistivity, high corrosion resistance and high mechanical strength. It is shown that such properties can be found in ferrites and achieved by using high purity raw materials, choosing an appropriate chemical composition and sintering under proper conditions. Typical properties of different ferrites and cast magnetite are given and discussed. 1. Introduction. - Generally, anode materials in solution electrolysis should have low electric resistivity, high corrosion resistance and high mechanical strength. Catalytic activity is required in some cases. Noble metal-coated titanium, graphite, lead preoxide, and magnetite have been used as anode materials in the electrochemical industry. Magnetite electrodes have been used for the production of chlorate. Usually, magnetite electrodes have been produced by casting. This method has the advantage that no atmospheric control is required. However, cast magnetite electrodes have disadvantages such as chemical inhomogeneity, high porosity, poor mechanical strength, and difficulty of dimensional control. Ferrites are produced usually by powder metallurgy techniques in which phase constitution, microstructure and density may be controlled adequately. Based on such backgrounds, we have developed ferrite electrodes of high performance by powder metallurgy techniques. The electric conduction in ferrites is caused mainly by the electron hopping between Fe2+ and Fe3+ [l, 21- Therefore, it is necessary to choose a chemical composition containing ~ e and ~ Fe3+ + ions. Further, it is supposed that the spine1 solid solutions are stable over wide ranges of oxygen partial pressure and temperature by incorporating divalent metal ions such as Mn2+, Zn2+ and Niz+. It is also expected that the incorporation of divalent metal ions should imprqve the corrosion resistance and mechanical strength. Generally, impurities such as SiO, and CaO have the tendency to accumulate at the grain boundaries causing an increase in resistivity. We found that it was possible to obtain ferrite electrodes which have higher corrosion resistance, mechanical strength and density than magnetite electrodes. Further, the ferrite electrode have the additional features of high corrosion resistance at high voltages and in fields of changing polarity, less virulence of dissolved metal ions and economy in manufacturing. 2. Experimental. - The optimum chemical composition was determined by changing the ratio of Fe203 to one of the divalent metal oxides such as MnO, NiO, COO, MgO and ZnO. Mixtures of constituent oxides of high purity were calcined, ground, pressed, heated at 1400OC for 2 hours, and then cooled to room temperature at an oxygen partial pressure between 10-3 and 10-' atm. The electrode characteristics of the obtained specimens were compared with conventional anode materials. The electric resistivity was determined by the D. C. four-point-compensation method The anodic dissolution rate expressed in mg/a. day, was calculated from the amount of dissolved metal ions which was determined by flame absorption photometry after 24-hour performance under the anodic current density of 5 A/dm2 in 3 % NaCl solution at 30 OC. 3. Results and Discussion PROPERTIES OF THE FERRITES. - Typical properties of ferrous ferrites containing 90 m01 % Fe203 and 10 m01 % divalent metal oxide and cast magnetite are listed in table I. All the ferrites are superior to cast magnetite in corrosion resistance and mechanical strength. Ni-, Mn-, CO- and Zn-ferrites have lower electric resistivity than cast magnetite. Still, Ni-ferrite has the highest corrosion resistance, mechanical strength and heat shock resistance, though the resistivity is nearly the same as that of Mu-, CO- and Zn-ferrites. Therefore, Article published online by EDP Sciences and available at

3 p C l -242 S. WAKABAYASHI AND T. AOKI Typical properties of ferrites and cast magnetite Material Mg-ferrite (* *) Zn-ferrite Mn-ferrite CO-ferrite Ni-fcrrite Cast magnetite Electric resistivity (Q-cm) Anodic dissolution rate (*) (mg/a. day) Bending strenght (kglmm2) Heat shock resistance fairly good poor good good fairly good - (*) 3 % NaCI, Anodic current : 5 A/dm?. (**) Nominal ferrite composition : 0.1 MO-0.9 Fe20,. we chose NiO as a divalent metal oxide and examined characteristics of Ni-ferrous ferrites of various compositions. 3.2 EFFECT OF COMPOSITION ON THE ELECTRODE BEHAVIOUR. - Figure 1 shows the effect of composition on the anodic dissolution rate and the electric resistivity. The anodic dissolution rate decreases with increasing NiO content. From table I and figure 1, we can 01 I I I, Composition, NiO mol % FIG Effect of composition in the system Ni0-Fez03 on the anodic dissolution rate in 3 % NaCl at 5 A/dmz and on the electric resistivity. see that the anodic dissolution rate of ferrites is governed by the kind and the content of the divalent metals. The resistivity increases with increasing NiO content. This is explained by the decrease in Fe2+ content. Figure 2 shows the effect of composition on the poro- 10 Composition. NiO mol% FIG ELTect of composition on the porosity and the bending strength in the system NiO-FezO3. sity and the bending strength. The porosity decreases and the bending strength increases with NiO content. We found that the density and the mechanical strength of Ni-ferrous ferrites, important properties of the electrode, were improved by increasing the NiO content. From these results, it is supposed that the microstructure of Ni-ferrous ferrite is improved by an increase in NiO content, and that the anodic dissolution rate of Ni-ferrous ferrite is closely associated with the microstructure. Hence, we chose Ni-ferrous ferrite containing 60 m01 O/; Fe2O3 and 40 m01 O/; NiO which ensured high corrosion resistance, density and mechanical strength, though the resistivity was rather high. But high resistivity of Ni-ferrite containing 60 m01 % F203 and 40 m01 O/; NiO is not a critical problem when the electrode is applied to actual electrolytic equipments, because the apparent resistivity could be reduced by combination with conductive materials. 3.3 EVALUATION OF THE FERRITE ELECTRODE Anodic dissolution rate. - Figure 3 shows the effect of NaCl concentration on the anodic dissolution rate of the ferrite electrode and cast magnetite at the current density of 5 A/dm2. The anodic dissolution rate of ferrite and magnetite decreases with increasing NaCl concentration. The result shown in figure 3 is possibly explained by the fact that the anode potential decreases with increasing NaCl concentration in neutral solution [3, 41. So, it is considered that the anodic dissolution rate of ferrites in neutral solutions depends on the anode potential. Figure 4 shows the effect of ph on the anodic dissolution rate. The anodic dissolution rate is almost constant in the ph range between 10 and 1, but increases markedly

4 CHARACTERISTICS OF FERRITE ELECTRODES Anodic dissolution rate of different electrode materials Electrode material Ferrite (0.4 Ni0-0.6 Fe203). Cast magnetite... Stainless steel (l8 Cr-8 Ni).. Lead-silver alloy (1.5 Ag)... Pt-coated titanium.... Graphite... Anodic dissolution rate * (g/a Year) , l Ferr~te electrode NaCl concentration (%) FIG Effect of NaCl concentration on the anodic dissolution rate of the ferrite electrode and cast magnetite at 5 A/dm2. (*) 3 % NaCI, Anodic current ; 5 A/dmZ. dissolution rate of ferrite is appreciably lower than that of other anodic materials, but higher than that of Pt-coated titanium. The anodic dissolution rate of ferrite is about 1/100 of cast magnetite. Furthermore, it is possible to use ferrite electrodes for the equipment in which the electrode is subjected to a polarity change, because the ferrite is stable under the reductive condition where hydrogen evolves. This is one of the important features of ferrite electrode which cannot be met by Pt-coated titanium, because Ptcoated titanium has pin-holes and titanium is dissolved through the pin-holes under changing polarity. Another feature of the ferrite electrode which is different from Pt-coated titanium is that the ferrite is applicable to the region of higher voltages, where the Pt-coating peels off from the base material. The problem about our ferrite electrodes is that the catalytic activity is low in the production of chlorine. Figure 5 shows the chlorine over-voltage of ferrite and A ; Pt-TI 30 - B ; Graphite C ; Magnetite D; Ferrite FIG Effect of ph on the anodic dissolution rate of the ferrite electrode at 5 A/dmz. below l. Thus, the anodic dissolution rate of ferrite increases with acid concentration, contrary to the case of NaCl solutions. However, the anode potential decreases with increasing acid concentration, as in the case of NaCl solutions. Hence, it is concluded that the mechanism of dissolution in acidic solutions differs from that in neutral ones Comparison of the ferrite electrode with other electrodes. - Table I1 shows the anodic dissolution rate of different electrode materials. The anodic n 20 a E W I.O 1.5 E ( vs SCE*) FIG Polarization curves of the ferrite electrode and others in saturated NaCl solution (ph = 1) at 30 OC. * SCE ; Saturated Calomel Electrode. 18

5 C S. WAKABAYASHI AND T. AOKI other electrodes. The chlorine over-voltage of ferrite is higher than those of the graphite and Pt-coated titanium, though it is equal to that of magnetite. Thus, it is not advantageous to use ferrites for the production of chlorine. But it is possible to decrease the chlorine over-voltage of the ferrite electrode by applying some material having low chlorine overvoltage on the ferrite surface. 4. Summary. - It is shown that high corrosion resistance, high mechanical strength, and low resistivity of ferrites are achieved by using high purity raw materials, choosing an appropriate chemical composition and by sintering under the proper atmospheric conditions. We found that one might choose the chemical composition having the proper resistivity and the corrosion resistance according to the performance. Ferrites have been used mainly as anode materials, and have been applied widely to the electrolytic floatation and electric protection. It is expected that the application of ferrites may be extended to various fields in industry and especially to the water treatment. References [l] YAMASHITA, J., KUROSAWA, T,, J. Phys. Soc. Japan 15 (1960) [3] NAGAI, T. and TAKEI, T., Japan 1. Electrochern. Soc (1956) 557. [2] MIYATA, N.. J. Phys. Soc. Japan 16 (1961) 206. [4] NAGAI, T. and TAKEI, T., ibid. 25 (1957) 55.