Anomalous codeposition

In the normal deposition of alloys, the more positive (noble) metal deposits preferentially. The result of this is that 

In anomalous deposition of alloys the opposite equation is valid 

A very interesting and technologically important case of anomalous deposition is the electrodeposition of Permalloy, Fe—Ni alloy (see Sect. 3.8). Glasstone and Symes [65] found that in the deposition of Fe—Ni alloys from solutions containing more than about 20% of the total metal as Fe, the initial deposit contains relatively more Ni than the electrolyte. 

Most theories of anomalous deposition in this case are based on the studies on kinetics and mechanism of electrodeposition of iron by Bockris et al. [66]. They proposed the following mechanism for the electrodeposition of Fe 

According to this mechanism, the elec-trochemically active species are [FeOH]+ and FeOH. Iron-hydroxide ions result from the hydrolysis of Fe2+ ions in the solution, Eq. (83). 

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This mechanism is based on the known importance of hydroxides in other deposition reactions, such as the anomalous codeposition of ferrous metal alloys [38-39], Salvago and Cavallotti claim an analogy with the mechanism of Ni2 + reduction from colloids in support of their proposed mechanism. There is no direct evidence for the hydrolyzed species, however. Furthermore, the mechanism does not explain two experimentally observed facts Ni deposition will proceed if the Ni2 + and the reducing agent are in separate compartments of a cell [36, 37] and P is not deposited in the absence of Ni2 +. The chemical mechanism does not take adequate account of the role of the surface state in catalysis of the reaction. It has no doubt been the extreme oversimplification, by some, of the electrochemical mechanism that has led other investigators to reject it. 

A hydroxide suppression model first proposed by Dahms and Croll (2) explains anomalous codeposition behavior of zinc-iron group alloys. This explanation was later supported by a number of workers (3) who measured a rise in pH near the cathode surface during the deposition of Zn-Co alloy. In this model it was assumed that the Zn(OH)2 was formed during deposition as a consequence of hydrogen evolution, thus raising pH in the vicinity of the cathode. Zinc would deposit via the Zn(OH)2 layer, while cobalt deposition took place by discharge of Co2+ ions... 

The electrodeposition of Zn-Co and Zn-Fe alloys in an aqueous bath is classified as an anomalous codeposition [44] because the less noble Zn is preferentially deposited with respect to the more noble metal. This anomaly was attributed to the formation of Zn(OH)+ which adsorbs preferentially on the electrode surface and inhibits the effective deposition of the more noble metal. This anomaly was circumvented by using zinc chloride-n-butylpyridinium chloride ([BP]+C1 / ZnCf ) [27] or [EMIMJ+Ch/ZnCh [28] ionic liquids containing Co(II). The Zn-Co deposits can be varied from Co-rich to Zn-rich by decreasing the deposition potential or increasing the deposition current. XRD measurement reveals the presence of CosZ i in the deposited Zn-Co alloys and that the Co-rich alloys are amorphous and the crystalline nature of the electrodeposits increases as the Zn content of the alloys increases. Addition of propylene carbonate cosolvent to the ionic liquid decreases the melting temperature of the solution and allows the electrodeposition to be performed at a lower temperature. The presence of CoZn alloy is evidenced by the XRD patterns shown in Figure 5.2. 

The term anomalous codeposition (ACD) was first introduced by Abner Brenner/ to describe an electrochemical deposition process m which the less noble metal is deposited preferentially under most plating conditions. This behavior is typically observed in codeposition of iron-group metals (i.e. Fe, Co and Ni) or in codeposition of an iron-group metal with Zn or Cd, with either inhibition or acceleration of the rate of deposition of one of the alloying elements by the other. ... 

It would seem that mixing the two ions creates some interaction between them, which slows down the rate of deposition of the component having a more positive standard potential. It was shown by Landolt et that adding 0.025 M FeSO4 to a solution of 0.2 M NiSO4 could reduce the partial current density for deposition of nickel by as much as a factor of ten. On the other hand, the same authors found that addition of 0.025 M NiSO4 to a solution of 0.25 M FeSO4 could increase the partial current density for deposition of iron by a factor of two or more. This is evidently a case of anomalous codeposition of two metals. 

Dahms H, CroU IM (1965) The anomalous codeposition of iron-nickel alloys. J Electrochem Soc 112 771-775... 

Hessami S, Tobias CW (1989) A mathematical-model for anomalous codeposition of nickel-iron on a rotating-disk electrode. J Electrochem Soc 136 3611-3616... 

This system shows the effect of anomalous codeposition (see Sect. 5.5.1.2). The deposits have a light-gray appearance. 

Zinc-nickel Zn-Ni alloys with 5 to 15 wt% Ni offer excellent corrosion resistance and are mainly used in the automotive, aerospace, and electronics industries. Above 15% Ni, the alloy coating becomes more noble than steel, and the corrosion-protection mechanism changes from a sacrificial to a pure physical one (comparable to pure Ni coatings, see Sect. 5.5.4.2.2). They can be electrode-posited from acid or alkaline baths. The acid baths are usually based on sulfate, chloride, sulfate-chloride, pyrophosphate, or acetate (Table 15). The system shows anomalous codeposition (see Sect. 5.5.1.2), which has been explained by a hydroxide suppression mechanism [47]. As in the case of Ni-Fe, the alkaline baths must contain complexing agents (see Sect. 5.5.4.6.2). The alloys electroplated from add haths contain approximately 10 to 14% Ni, whereas the alkaline Zn-Ni... 

Figure 83 Different combinations of partial current potential diagrams to explain anomalous codeposition with preferential deposition of the less noble component in the gray areas the less noble component is preferentially deposited, (a) A and B kinetically controlled, different Tafel slopes, similar exchange current densities, (b) A and B kinetically controlled, different Tafel slopes and different exchange current densities (i q > Jq ), (c) A kinetically controlled, B diffusion controlled, different exchange current densities (Iq > Jq ), (d) A diffusion controlled, B kinetically controlled, similar exchange current densities. (Reproduaxl with permission from Ref. [6], 1994, Elsevier.)...

According to Brenner s classification [3], anomalous codeposition is characterized by the fact that the less noble metal electrodeposits before the more noble one as the potential is driven cathodic. As a consequence, the content of the less noble metal in... 

Based on the chemical analysis, the content of Co in the electrodeposit increases within the range 8 at. % to 80 at. % with increasing concentration of Co " " ions in the solution. Brenner s diagram presented in Fig. 7.18b clearly shows pronounced anomalous codeposition. 

Different explanations for such behavior are offered in the literature [3]. The most likely one appears to be the hydroxide suppression mechanism [20- 23]. According to this concept, coevolution of hydrogen during the electrodeposition causes an increase of pH at the electrode/solution interface, producing hydrolysis of less noble metal species and their precipitation as a layer of solid hydroxide. Formed hydroxide layer provides a good supply of ions of the less noble metal for their discharge and electrodeposition but suppresses the transport of species of the more noble metal to the cathode surface, causing anomalous codeposition. 

Anomalous Codeposition of Alloy Powders 8.2.1 Electrodeposited Co-Ni Powders... 

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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