Electrodeposited Fe-Ni Alloy Powders

Electrodeposition of Fe-Ni powders was performed in ammonium chloride-sodium citrate containing electrolyte of the composition 1 M NH4CI -f 0.2 M Na3C6H507. Fe(in) and Fe(II) chloride and nickel chloride were used as sources of Fe or Fe and Ni ions. Total concentration of cations was kept at 0.1 M. Four different 

It should be mentioned here that an attempt was made to deposit Fe-Ni powders from the electrolytes of the same composition but of low pH 2. The powder agglomerates were successfully produced on the cathode, but immediately after detaching from the cathode surface, they started dissolving with gas evolution in aU investigated electrolytes. Only in the case of very short time of electrolysis and removal of remained powder (which had not yet been dissolved) it was possible to obtain small amount of powder for further analysis. That was the reason why aU experiments were performed in the solutions of pH 4.0 or 4.5, since in this solution powders were stable after electrodeposition. 

From all solutions, Fe-Ni alloy powders were electrodeposited at a constant current density corresponding to the slightly lower value (/p i) than the inflection point B (marked with ( )) on the polarization curves (see Fig. 8.14) [1, 8]. 

1 Polarization Curves for Fe-Ni Alloy Powder Electrodeposition from Chloride Citrate-Supporting Electrolyte 

The polarization curves corrected for IR drop for the processes of Fe, Ni, and Fe-Ni alloy powder electrodeposition from ammonium chloride-sodium citrate containing supporting electrolyte in the presence of Fe(ll) and Ni(II) species are shown in Fig. 8.14. In the case of Fe(II) salts, polarization curve for iron electrodeposition (Fe) was placed at more positive potentials than that for nickel (Ni) as it is expected from the values of their reversible potentials. The polarization curves for Fe-Ni alloy powder electrodeposition are placed in between, and all of them were placed at more positive potentials than expected from the Ni/Fe ratio, indicating anomalous codeposition. 

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A low current efficiency for the Fe-Ni alloy powder electrodeposition from the solution containing Fe(III) salt is the consequence of the first step in the overall reaction being reduction of Fe(III) species into Fe(II) species, taking place at all potentials more negative than —0.2 V versus AglAgCl. Nevertheless, some experiments were performed in the solution of Fe(III) ions and the morphology of electrodeposited Fe-Ni alloy powders was investigated. 

Lacnjevac U, Jovic BM, Maksimovic VM, Jovic VD (2010) An attempt to produce NiFe204 powder from electrodeposited Fe-Ni alloy powders by subsequent recrystallization in air. J Appl Electrochem 40 701-708... 

The morphology and composition of electrodeposited Fe-Ni alloy powders depend on the Ni/Fe ions concentration ratio in both electrolytes. Anomalous codeposition of Fe and Ni has been... 

Fig. 8.14 Polarization curves for the electrodeposition of iron (Fe), nickel (Ni), and Fe-Ni alloy powders after JR drop correction recorded for different Ni/Fe ions ratios (9/1, 3/1, 1/1, and 1/3 (marked in the figure)), in the solution of Fe(II) and Ni(II) species (Reprinted from Ref. [1] with kind permission from Springer)...

Fig. 8.15 Polarization curve for the Fe-Ni alloy powder electrodeposition (/tot) (n), polarization curve for hydrogen evolution (/h) (O), and polarization curve for Fe-Ni powder electrodeposition after subtraction of hydrogen evolution current (/pe-Ni) ( )- The values of Ni/Fe ions ratios are marked in the figure (Reprinted from Ref. [1] with kind permission fi-om Springer)...

Fig. 8.17 Morphology of the Fe-Ni alloy powder electrodeposited at the Ni/Fe = 9/1. (a) Typical agglomerates with the large cone-shaped cavities and much smaller cylindrical cavities, (b) Pagoda-like crystals on nodular surfaces, (c) Crystals of the shape of elongated prism, (d) (c) at higher magnification (Reprinted from Ref. [1] with kind permission from Springer)...

Fig. 8.18 Morphology of the Fe-Ni alloy powder electrodeposited at the Ni/Fe = 3/1 (Reprinted from Ref. [1] with kind permission from Springer)...

In Fig. 8.17 are presented typical agglomerates for Fe-Ni alloy powders electrodeposited at the Ni/Fe ratio 9/1. Large cone-shaped cavities and much smaller cylindrical cavities, corresponding to the places were hydrogen bubbles were formed, are clearly seen in Fig. 8.17a. Nodular surfaces are covered with two types of crystals pagoda-like crystals (b) and crystals of the shape of elongated prism (c, d), indicating possible presence of different phases. 

In the case of the solutions for Fe-Ni alloy powder electrodeposition, among the species presented above for Fe, the following species could be formed with Ni in the investigated solutions [124]... 

Fig. 5.28 Diffractograms of Fe-Ni alloy powders electrodeposited at different Ni/Fe ions ratios (marked in the figure) (Reprinted from [62] with the permission of Springer-Verlag.)...

The diffractograms of Fe-Ni alloy powders electrodeposited at different Ni/Fe ions ratios from the solution of Fe(II) species are presented in Fig. 5.28. It should be stated that the dimensions of crystallites were 7 nm for the Ni/Fe ratio 9/1 and that they increased with the increase of iron content in the powder to 20 nm (for Ni/Fe = 1/3). Because of very small dimensions of crystallites, only phases with the highest intensity were determined with high certainty and these were peaks of the a-Fe phase (A). Taking into account that the peaks of FeNi (37-0474), a-Fe (06-0696), and Ni (45-1027) practically overlap, a wide peak at about 44.5 for Ni/Fe = 9/1, 3/1, and 1/1 could be ascribed to any of these phases. The appearance of a small peak at about 44.3° for Ni/Fe = 9/1 and 3/1 indicates the presence of FeNis 111 (38-0419), since pagoda-like crystals, typical for this phase, have been detected on the surface of powder agglomerates obtained from these solutions (see Sect. 5.3.4). 

The morphology of the Fe-Ni alloy powders electrodeposited at different Ni/Fe ratios are presented in Figs. 5.29-5.32. A common characteristic of all investigated Fe-Ni powders is the formation of... 

Fig. 5.35 Crystals formed on the surface of the Fe-Ni alloy powder agglomerates electrodeposited from the solution with Ni/Fe = 9/1 (Reprinted from [61] with the permission of Elsevier.)...

With the increase of iron concentration (as well as with the increase of the amount of iron in the powder - decrease of Ni/Fe ratio), the current efficiency for powder electrodeposition decreased, which is in accordance with the data obtained for compact Fe-Ni alloy electrodeposits [1, 18]. 

In Fig. 5.1 are shown polarization curves corrected for IR drop (see Chap. 2), for the processes of Co, Ni, and Co-Ni alloy powders electrodeposition from ammonium sulfate-ammonium hydroxide containing supporting electrolyte (1M (NH4)2S04 -1- 0.7 M NH4OH). As can be seen their shape is identical to that for pure Co, Fe, and Ni powders electrodeposition, characterized with two inflection points, A and B. For Co electrodeposition, sharp increase of current occurs at... 

It was found that for the Fe(III) salts electrolytes the current efficiency was very low, 1-2% (the polarization curves for powder deposition (ytot) and for hydrogen evolution (yn) practically overlapped), and it was necessary to deposit powders at least for 2 h to obtain the amount of powder that could be used for the morphology and composition analysis (SEM, EDS). In the case of Fe(II) salt electrolytes, current efficiency at the potentials more negative than the second inflection point ( in Fig. 5.21) varied between 8% and 15% depending on the Ni/Fe ratio, as shown in Fig. 5.22. The average values for the diffusion limiting current densities for alloy powder electrodeposition were ype-Ni = —0.26 A cm for the ratio 1/3 and /pe-Ni = —0.49 A cm for the ratio 9/1. 

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