Electrochemical behaviour of eutectic metalgermanium alloys in sulphuric acid

Ekctrochimica Acta, Vol. 36, No. 8, pp. 1247-1251,1991 Printed in Great Britain.

ELECTROCHEMICAL METAL-GERMANIUM

OW3-4686/9183.00+ 0.00 0 1991.Pergamon Press pk.

BEHAVIOUR OF EUTECTIC ALLOYS IN SULPHURIC ACID

A. B. SHEIN and R. G. AITOV Laboratory of Electrochemistry and Protection of Metals, Perm State University, Bukireva 15, 614600 Perm, U.S.S.R. (Received I June 1990; in revised form 11 September 1990) Abstrac-The electrochemical behaviour, passive properties and structural changes of eutectic Me&e alloys in 1 N H,SO, have been investigated by polarization measurements and scanning electron microscopy. It is shown that the mechanism and kinetics of cathodic process are mainly determined by the metallic component of the alloys and anodic process is influenced by the presence of large amounts of germanium. Investigated electrodes such as Cu,Ge-Ge, CoGe&e, NiGe-Ge, FeGer-Ge are not passivated in sulphuric acid. In the case of an FeGe,-Ge alloy the polarization characteristics are compared with those for pure Fe, Ge and intermetallic FeGe,. It has been concluded that the principle of independence of electrochemical reactions for each phase, and for each component of any phase, cannot be used for the analysis of the electrochemical behaviour of two-phase Me-Ge alloy. Key words: eutectic alloy, germanium, anodic dissolution, passivation, hydrogen evolution.

INTRODUCTION

New electrode materials with unique corrosion and electrochemical behaviour are of great importance to applied electrochemistry. In this aspect the “metal-non-metal” alloys, intermetallic compounds and composition materials are of special interest. Such prospective materials as carbides, nitrides, silitides of metals, etc. are well investigated. The electrochemical behaviour of metal germanides, due to their rare use, appear to be among the less studied of these materials. In this work, the electrochemical behaviour of eutectic alloys, consisting of two phases, intermetallic (MeGe) and non-metallic (Ge), has been investigated. In addition, the pure intermetallic compound FeGe, has been used. The results for the alloys are presented in comparison with the results for pure Ge and Fe. The work continues the investigations, made earlier on A,B,, alloys, where A = Fe, Co or Ni, and B = Si, and described in [14].

EXPERIMENTAL The eutectic alloys CoGer-Ge, NiGe-Ge, Cu,G& and FeGer-Ge were prepared from pure materials: semiconducted zone-refined germanium (99.99% Ge), electrolytic cobalt (99.98% Co), nickel (99.98% Ni) and copper (99.99% Cu) and iron (99.98% Fe) with the help of special equipment, made on the basis of a furnace for the industrial preparation of single crystals (OKB-8093). Single crystal seeds were not used because the preparation of highly perfect structures was not the aim of this work. Crystal pulling was performed on an alundum rod with a rate of 0.05 mm/s (or greater).

Before the electrochemical measurements, the investigated electrodes were pressed into Teflon tubes and only the working surface was not insulated. The test electrodes were prepolished mechanically, washed with triply distilled water and then placed in the three-compartment electrochemical cell. Electrolyte (the solution of H,SO, in triply distilled water) was deaerated before and during measurements by bubbling pure N, through it. All experiments were performed at 20°C. After stationary potential installation, potentiostatic polarization curves were obtained. The polarization time was 5 min for each step of potential. The counterelectrode was a platinum sheet and a standard hydrogen electrode was used as a reference electrode. The surface morphology of electrodes was examined by scanning electron microscopy (SEM) (JEOL 35 SM) and by metallography (Neophot-21). RESULTS

AND DISCUSSION

Electrochemical behaviour of Cu,Ge.-Ge, and CoGe,-Ge electrodes

NiGe-Ge

The system Me,Ge, may be, probably, described as a complex heterogeneous electrode, consisting of two phases, for which the total rate of a process is composed of the algebraic sum of the partial reaction rates on each phase. Electrochemical reactions on each phase have their own mechanism and kinetic parameters. But we must not exclude the fact that the electrochemical situation for one phase (for example, pH and adsorption conditions, etc.) may be changed as the result of the electrochemical reactions on the other phase. The situation becomes more complex for eutectic alloys containing germanium because pure Ge is a semiconductor with a complex 1247

A. B. SHEIN and R. G.

1248

-0.6

-

c -0.4 lu

oI -6

I log i (i/A

Fig. 1. Cathodic for CoGe,-Ge

I

-4

-2

cm?

polarization curves obtained in 1N H,SO, (O), NiGe-Ge (0) and Cu,Ge-Ge (0).

electrochemical behaviour. Electrochemical reactions on semiconductors have peculiarities when compared with reactions on pure metals. In our case, the two-phase system Me,Ge,Ge is described to a first approximation without taking into consideration the properties of Ge as a semiconductor. It is very difficult to show unambiguously the role of Ge in eutectic alloys by comparing the results of experiments obtained in equivalent conditions for pure Ge and for MeGe-Ge electrodes. The situation becomes more complex because it is impossible to obtain the separate pure intermetallic phases CoGe, and NiGe in the case of CoGe,-Ge and NiGe-Ge alloys. That is why the data of polarization experiments obtained for MeGeGe electrodes, in comparison with the data for Ge-electrodes (with the same procedure of surface preparation), are not sufficient for the quantitative conclusions and they allow us to distinguish only qualitatively the influence of each phase. The cathodic polarization curves for the eutectic alloys of Ge with Co, Ni and Cu are presented in Fig. 1. As can be seen, the curves for all materials are rather complex. The mechanism of the hydrogen evolution reaction (HER) in the case of pure Ge is well described in Ref.[S]. For eutectic alloys the main role in the kinetics of the cathodic process is apparently played by the MeGe phase, the rate of HER on which is much greater than on Ge-phase (the same result has been obtained for MeSi[3]). But, on the other hand, MeSi having in the cathodic process properties that are largely determined by the metallic component of the alloy, retains the role of non-metallic phase, and on polarization curves one can see the initial plot with high overvoltage. The latter is characteristic, first of all, for Cu,Ge-Ge in spite of the fact that here the percentage of Ge is less than, for example, in NiGe-Ge alloy. The maximal rate of HER in the region BE = -0.25 to -0.60 V is recorded for CoGe,Ge. On the polarization curve for this electrode at AE = -0.25 to -0.35V one can see a Tafel plot with 6, = aE,/a log i, = 0.09 V, and this b, is much less than for pure Co (according to Ref.[6] b, for Co is placed in interval from 0.140 to 0.170 V). Earlier the effect of the acceleration of HER on the surface of intermetallic compound has been noted by us for CoSi[3]. When EC< -0.8 V, the polarization curves for the investigated alloys differ insignificantly, but i,

AITOV

for these alloys is _ 100 times higher, than i, for Ge. So the results of the polarization experiments show, that in spite of the presence of high amounts of Ge phase in alloys, Ge has an insignificant effect on the kinetics of the cathodic process. But an intermetallic phase has a considerable influence. The important factor is not only the quantitative amount of a phase, but also the nature of the metallic component of an alloy. The highest rate of HER is realized for CoGe,-Ge. Figure 2 shows the anodic behaviour of Cu,GeGe, NiGe-Ge and GoGe,Ge in 1 N H, SO,, . In an opposite way to MeSi[2], the dissolution of MeGe-Ge occurs at a higher rate, and the passivation of the electrode does not occur (in the investigated region of potentials). Tafel slope 6, of the anodic polarization curve for Ge is 0.12OV (that is in accordance with Ref.[5]). A limiting current is recorded, in accordance with the results obtained for Ge-electrode of n-type[5]. The anodic dissolution of Ge at AE = O-O.4 V occurs with a lower rate than that of eutectic alloys, but at E > 0.4 V the rates of dissolution become close to each other. But in this region of potentials, i, for CoGe,-Ge is less than for Ge. In Ref.[S] it is noted that on the surface of a Ge-electrode at pH = &4, a layer can be formed that consists mainly of GeO (Ge+H,O= GeO + 2H+ + 2e-). But as distinct from SiO,, the protective properties of GeO seem weaker. Weak influence of the nature of the metallic component of alloys on the anodic dissolution indicates the determinative role of Ge in anodic process. The analysis of the surface morphology changes after anodic dissolution allows qualitative comparison of the partial rates of dissolution of Ge and MeGe phases in alloys. Figure 3 shows the surface structure of alloys after 60 s anodic dissolution at E = 0.8 V. It can be seen that the presence of two phases (MeGe and Ge) is clear because of the different dissolution rates of the phases (the identification of the phases was made by X-ray microanalysis). In

-6

I

log ((i/A

Fig. 2. Anodic for CoGe,-Ge

b

I -4

rj;

-2

cm-‘)

polarization curves obtained in 1 N H,SO, (O), NiGe-Ge (0) and Cu,Ge-Ge (0).

Eutectic metal-germanium alloys

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From SEM photographs it can be seen in CU,~ that the dissolution rate of the intermetallic phase is higher than that of Ge. The difference in the rates of dissolution of phases for NiGe-Ge is less, and the CoGer-Ge intermetallic phase is dissolved more slowly than Ge. In conclusion we can say that during the anodic dissolution of the eutectic alloys of Ge with Co, Ni and Cu the non-metallic component of the alloy is the dominant factor. The quantitative determination of the contribution of each phase in the total kinetics is probably made only by the separate investigations of the electrode processes on pure intermetallic and non-metallic phases. The preparation of the pure intermetallic phase in the case of NiGe-Ge and CoGe,-Ge is impossible because these alloys are usually crystallized as eutectic alloys, but we can obtain the separate pure FeGeZ. Electrochemical behaviour of FeGe,-Ge

Fig. 3. SEM observation of the electrode surface after anodic polarization in 1 N H,SO, at E = 0.8 V, r = 60 s: (a) CoGe,-Ge, (b) NiGe-Ge, (c) Cu,Ge-Ge. CoGer-Ge alloys (Fig. 3a) the Ge phase occupies one-third of the total area, and in the bulk a columntype structure is formed. The eutectic columns in NiGe-Ge have a more complex fibrous structure (Fig. 3b) and the volumes of the phases are equal. The cross-sections of the fibres have a complex winding form. In the case of CuJGe-Ge alloys, an irregular structure is formed. The ratio of the volumes of MeGe to Ge is 1.85 for CoGe,-Ge and 1.05 for NiGe-Ge.

electrode

Results obtained for the eutectic alloy FeGe,-Ge in comparison with the results for the intermetallic phase FeGe, and pure Ge are reported. The electron microscopy analysis of the FeGe,-Ge structure (Fig. 4a) shows that a bladed structure is formed. By stereometric metallography the percentage of phases in the alloy has been determined (FeGe,:Ge = 1.41). Figure 5 shows cathodic and anodic curves for FeGe,-Ge as well as for FeGe,, Fe and Ge. The mechanisms of the partial electrochemical processes on Fe and Ge are not discussed in this paper, because many other papers are devoted to this problem. Here we must note that cathodic curves for FeGe, and FeGe,-Ge differ insignificantly. Cathodic currents i, for FeGe, and FeGer-Ge are much higher than for Ge, and slightly less, than for pure Fe. On the basis of the data, presented above, it may be concluded that the cathodic behaviour of these electrodes is determined by the metallic component of the alloy. The other reason for this conclusion is that i, (at E = constant) for FeGe, is slightly higher than for FeGe,Ge, which contains more Ge. This effect only takes place beginning from the potential of hydrogen evolution for pure Fe. At more positive potentials, i, for FeGe,-Ge is higher than for FeGe,. From the results in Fig. 5a we can also see that large amounts of Ge influence the form of the cathodic curve (we can observe a small inflection of a curve at the same E as for pure Ge). The anodic dissolution of FeGe, and FeGe,-Ge at AE = 0.1-0.3 V is controlled, probably, by the presence of large amounts of Ge in alloys. The initial plots of anodic polarization curves for these materials differ insignificantly. Tafel slopes b, are equal to 0.1 V, that is to say, they coincide with b, for pure Ge, and they are much higher, than for Fe (b,, = 0.04 V). The most interesting results were obtained at AE = 0.348 V (see Fig. 5b). It is well known that pure iron is passivated in acids at these potentials. The passivation process for Fe can be described by the following reaction: 2Fe,O, + H,O = 3y-Fe,O,+2H++2ewith E”=0.58-0.059pH. The forming passive layer, as a rule, consists of Fe,O, (inner layer) and Y -Fe,O, (outer layer)[7]. Water

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A. B. SHEINand R. G. AITOV

-0.8

-4

-6

> 2

log i (i/A

cme2)

i (i/A

cme2)

-2

0.8

1.6

2.0

log

Fig. 5. Cathodic (a) and anodic (b) polarization curves, obtained in 1 N H,SO, for Fe (O), FeGe, (O), FeGerGe (0) and Ge (W

Fig. 4. SEM observation of the electrode surface of FeGe,-Ge before (a) and after (b) anodic polarization at E=0.4V,r=60minin lNH,SO,.

molecules are also present in the layer (Fe,O,. 1.4 H*OPl). On the surface of pure germanium at these potentials an oxide film has also been formed according to the reactions described in Ref.[S]. From the results presented in Fig. 5b, it can be seen that minimal i, at AE = 0.3-0.7 V is obtained for FeGe,-electrode. Anodic dissolution of pure Ge occurs with higher rate. The results of the structural analysis show, that after exposing for 60 min at E = 0.4 V, the dissolution of Ge phase occurs and FeGe, phase possesses a greater corrosion resistance (see Fig. 4). The difference in dissolution rates of FeGe, and FeGe,-Ge electrodes reaches a maximum at E e 0.6 V, that is to say, at the Flade-potential of pure iron. It is known that in the common case the stability of the passive film depends on the ratio of the numbers of atoms leaving the crystal lattice and the atoms that are fixed and stay in the film, and it

is determined by the balance of the binding energies of metal-metal, metal-solution, metal-oxygen, etc. The dissociative adsorption of water occurs by interaction of Fe-Ge alloys with solutions, first of all, on the atoms with greater affinity for oxygen (in our case, on Ge atoms). So one must expect the weakening of the bond of solution components with other atoms in the alloy (Fe atoms). This weakening of the bond of water molecules with Fe atoms makes the passivation process difficult; this is observed in the case of FeGe, and FeGe,-Ge. We must take into consideration also that GeO, is not stable in the solution and it can be dissolved even in H20. The results of polarization measurements in combination with the surface observation for the dissolving electrodes by SEM show that unlike pure iron, Fe-Ge alloys containing high amounts of germanium, either as FeGe, phase or as Ge-phase, have no tendency to passivation. The absence of the passive state region, that is conditioned in the case of Fe by the protective properties of y-Fe, OJ, makes it possible to assume that y-Fe,O, is not formed in significant amounts on the surface of the dissolving FeGe, phase. As the result of germanium oxidation, anodic films are formed that have weak protective properties.

Eutcctic metal-germanium alloys They can be dissolved in acids, and they do not prevent the transition of Fe and Ge ions into the solution. According to SEM observations in such eutectic alloys as FeGe&e and CoGe,-Ge, the Ge phase is dissolved preferentially and the inter-metallic phase has a higher corrosion resistance. The electrochemical properties of a pure FeGe, electrode differ from the properties of an FeGel phase in the eutectic alloy. The latter fact, together with the results of electrochemical experiments for Fe and Ge, allow us to conclude that the dissolution of such complex electrodes as two-phase eutectic alloys cannot be described with the help of the principle of independence of the electrochemical reactions for each phase and for each component of any phase. The situation becomes more complex if during crystallization of an

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eutectic alloy, germanium dissolves some Fe atoms, which changes its semiconduction properties. REFERENCES 1. A. B. Shein and V. I. Kichigin, Elektrokhimiya 22, 1670 (1986). 2. A. B. Shein, Zhur. Prikl. k%im. 59, 2548 (1986). 3. A. B. Shein, Korrosion (DDR) 19, 171 (1988). 4. A. B. Shein, Elektrokhitniya 24, 1335 (1988). 5. E. A. Efhnov and I. G. Erusalimchik, Elektrokhimiya germaniya i kremniya. Goskhimizdat, Moskva (1963).

6. C. Peraldo Bicelli, C. Romagnani

and M. Rosania,

J. electroanal. Chem. 63, 238 (1975). 7. N. Sato, 4th Int. Symp. Passivity, p. 29, Warrenton, VA,

Ott 17-21, 1977, Princeton, NY (1978). 8. N. Sato, Surface Electrochem. Adv. Tokyo, 65 (1978).