Protection of Metals, Vol. 41, No. 3, 2005, pp. 234–242. Translated from Zashchita Metallov, Vol. 41, No. 3, 2005, pp. 258–266. Original Russian Text Copyright © 2005 by Shein, Zubova.
Electrochemical Behavior of Manganese Silicides in Sulfuric Acid Solution A. B. Shein and E. N. Zubova Perm State University, ul. Bukireva 15, Perm, GSP, 614990 Russia e-mail:
[email protected] Received December 23, 2002
Abstract—The corrosion-electrochemical behavior of Mn5Si3 and MnSi silicides in the sulfuric acid electrolyte is studied by using the steady-state and cyclic voltammetry in combination with atomic absorption analysis. The following main regularities of the anodic dissolution of silicides are revealed: Mn dissolves selectively from the sublattice in silicide and, then, the process is controlled by diffusion of Mn in the compound and oxidized Mn through a layer of hydrated SiO2 oxide formed at the surface.
Metal silicides are promising electrode materials, because they are characterized by a high chemical resistance [1–3] and a low hydrogen overpotential [4, 5] in acidic electrolytes. These properties strongly depend both on the nature of the metal component, as well as its amount in the silicide, and on the structural and physical parameters of the silicide itself. Earlier [6–9], we comprehensively studied silicides of iron family metals. It is of great interest to investigate other silicides, which have not yet been studied in electrochemical aspects, in particular, manganese silicides. Here, the corrosion-electrochemical behavior of manganese silicides MnSi and Mn5Si3 in a sulfuric acid electrolyte is extensively studied. The procedures of making the specimens and preparing them prior to investigations, as well as devices, methods of electrochemical measurements, and procedure of electrolyte preparation were similar to those we have already used for iron, cobalt, and nickel silicides [6, 7]. Figure 1 gives potentiostatic voltammograms of manganese, silicon, and manganese silicides, which were measured in 0.5 M H2SO4; Table 1 lists the data, obtained from the voltammograms. It is seen that, under these conditions, Mn is not passivated in a potential (E) range studied. The free-corrosion potential of manganese Ecor is approximately –0.7 V (hereafter, the potentials are referred to the standard hydrogen scale). The manganese anodic dissolution rate is high: in a ∆E range of –0.8 to 1.8 V, the anodic current density rate is 1.3 × 103 A/m2 . The anodic dissolution rate of silicon is by ~3 orders of magnitude lower than that of manganese, and Ecor of Si is approximately –0.1 V. The manganese silicides show still lower ia. As is seen from Fig. 1, the anodic dissolution rates of Mn5Si3 and MnSi are approximately 5 orders of magnitude lower than that of Mn and by nearly 3 orders lower than that of Si.
That is, with an increase in the silicon content in silicide, the silicide’s anodic dissolution rate in sulfuric acid solution decreases. It should be noted that the silicon behavior in the above electrolyte is complicated by its inherent semiconductive properties and the presence of SiO2 film at the electrode surface. It is virtually impossible to determine the mechanism of hydrogen cathodic evolution on silicon based only solely kinetic parameters of voltammograms. Supposedly, the pathway of hydrogen evolution (HE) corresponds to discharge-recombination processes, with the desorption as the limiting stage. On manganese, the limiting stage is the discharge of H3O+ ions. This is supported by a Tafel slope (bc) of 0.146 V and a reaction order n H+ of 0.9 determined in this work. Silicon belongs to elements that exceed manganese in the affinity to hydrogen (the breakage energies of Si–H E, V –2 –1 0 4
3
2
1
1 2 3 –7
–3
1
5 log i [A/m2]
Fig. 1. Voltammograms of (1) Mn, (2) Si, (3) Mn5Si3 , and (4) MnSi in 0.5 M H2SO4 .
0033-1732/05/4103-0234 © 2005 Pleiades Publishing, Inc.
ELECTROCHEMICAL BEHAVIOR OF MANGANESE SILICIDES
and Mn–H bonds are 301.38 and 233.66 kJ/mol, respectively [10]), which makes Mn–H bond rupture in the presence of silicon atoms [11]. Obviously, the higher the silicon content in a silicide, the more similar should become the cathodic processes on the silicide and silicon. The parameters of potentiostatic voltammograms of Si and MnSi are scarcely affected by the concentration of acid solution. With an increase in the pH value from 0.15 to 1.32, the cathodic currents increase only slightly ( n H+ ≈ 0), while bc and Ecor on Si and MnSi become equal. These results, probably, point to a common essence of HE on MnSi and Si. An increase in the manganese content in a silicide leads to an increase in the corrosion current icor. The free-corrosion potential of Mn5Si3 is lower than that of MnSi, and the cathodic reaction order ( n H+ = 0.61) is higher for Mn5Si3 . Probably, n H+ is a fraction because of proceeding HE on the electronegative and electropositive components on the manganese silicide surface. Considering the cathodic process on silicides, it should be noted that, probably, the mechanism and kinetics of hydrogen evolution are determined by different components at different potentials. At E higher than Ecor (Mn), the kinetics of cathodic process on silicides is probably determined by Si. However, at E < –1.0 V, the HE rate is determined by the second component (Mn). In manganese monosilicide (MnSi), atoms of Mn tend to surround themselves with silicon atoms, which form no couples, chains, and nets, but are surrounded by metal atoms. Probably, this results from the fact that the energy of interatomic interaction of Mn–Si is higher than that of Mn–Mn and Si–Si [12]. At the same time, it is unlikely that substantially different anodic behavior of manganese and its silicides is caused by a simple increase in the strength of Mn–Si bond compared with Mn–Mn. Probably, it is also associated with different mechanisms of their dissolution. The standard potential of manganese E0 (Mn2+/Mn) is –1.18 V [13], whereas its passivation, which is used to be related to the following possible reactions: 2Mn2+ + 3H2O = Mn2O3 + 6H+ + 2e,
E0 = 1.48 V;
Mn2O3 + H2O = 2MnO2 + 2H+ + 2e,
E0 = 0.98 V;
Mn2+ + 2H2O = MnO2 + 4H+ + 2e,
E0 = 1.23 V
is not observed in a potential range of –1.18 to 1.40 V at pH 0–1. According to [14], in the latter potential range, manganese dissolves yielding Mn2+. In the same potential range, silicon is oxidized according to the following possible reactions: Si + 2H2O = SiO2 + 4H+ + 4e,
E0 = –0.857 V;
Si + 3H2O = H2SiO3 + 4H+ + 4e, PROTECTION OF METALS
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Table 1. Kinetic parameters of electrochemical processes for Si, Mn, MnSi, and Mn5Si3 in sulfuric acid electrolyte Electrode Si MnSi Mn5Si3 Mn
Ecor , V
icor , A/m2
bc , V
ba , V
–0.077 –0.072 –0.297 –0.677
5.62 × 10–2
0.151 0.131 0.127 0.146
0.110 0.182 0.153 0.068
1.41 × 10–3 1.57 × 10–2 34.7
Consequently, transferring manganese atoms into solution under the anodic dissolution of manganese silicides will be complicated by the presence of SiO2 film on their surface. In addition, the presence of strong Mn–Si bonds should be taken into account. This raises the question of the ratio between these factors affecting the anodic dissolution rate of silicides of various composition. The authors of [15], by the example of the dissolution of cobalt silicides in H2SO4 , concluded that the strength of Me–Si bonds plays a crucial role. In the anodic potentiostatic voltammograms of manganese silicides MnSi and Mn5Si3 in the above sulfuric acid solution (Fig. 1), peaks of active dissolution are not pronounced. The electrochemical behavior of manganese monosilicide in an anodic potential range is in many ways similar to that of silicon. This is evidenced by the close magnitudes of the anodic reaction orders n OH_ and free-corrosion potentials. According to the literature data, Ecor ≈ –0.1 V corresponds to the oxidized silicon surface. Thus, we may suppose that the surface layer of MnSi alloy also contains a considerable amount of silicon dioxide. All silicon atoms at the surface can form very strong Si–O bonds; therefore, the oxidizing agent should penetrate through the surface layer and enter into the reaction in those sites, where much weaker Si–Si bonds are located [16]. A SiO2 film, which has a high ohmic resistance, can passivate the electrode in the acidic solutions to such an extent that it almost ceases to dissolve [17]. Actually, the anodic growth of SiO2 is accompanied by its slow dissolution, so that the film in time becomes more porous [18]. In a case of metal silicides, of interest is the composition of the surface oxide film upon the anodic polarization. According to [19], the lower the silicon content in the alloys, the more probable is the formation of mixed oxide films of Mex–Siy–Oz type. Such compounds were found, for instance, at the surface of TiSi alloy [20]. The Mn5Si3 alloy, which contains a larger amount of manganese than MnSi, has a more complex structure. Therefore, these silicides essentially differ in the electrochemical behavior (Fig. 1). An anodic reaction order n OH_ of –0.2 is intermediate between the reaction orders of Si and Mn anodic dissolution. The Ecor of Mn5Si3 alloy lies in a range between the free-corrosion potentials of Si and Mn. According to the Sato theory [21], the passivation of AB binary alloys comes when
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the surface concentration of passivated component B (which is accumulated as the less resistant A is removed) reaches a certain critical value. If component B virtually does not dissolve, only component A dissolves at the electrode boundary: it passes into the solution and is compensated for via solid-phase diffusion from the bulk of the alloy. The data obtained make us conclude that the onset of Mn5Si3 active dissolution comprises selective dissolution of manganese and the accumulation of silicon atoms at the surface. Upon 0 reaching the standard potential E (Si4+/Si), ionization and oxidation of Si atoms to SiO2 proceed almost concurrently to decrease the current density with an increase in the potential and thus cause the first peak in the anodic voltammogram. The second peak, no more pronounced than the first one, is probably associated with the structural peculiarities of silicide. According to the data on the interatomic distances in the manganese silicide lattice, it is MnII atoms that will selectively dissolve first (MnI and MnII are two types of manganese atoms, which differ in the magnetic moments). This is also evidenced by a slight redistribution of the electron density from lattice sites occupied by MnI and Si, to the sites occupied by MnII [12]. The MnI–MnI and Mn–Si bonds are stronger. Most probably, breakage of MnII– Si, MnII–MnII, and MnII–MnI bonds in the bulk alloy and subsequent diffusion of manganese atoms through oxide film correspond to the second peak. In a potential range ∆E of 1.0 to 1.8 V, Mn5Si3 is passivated, and the anodic currents coincide with those for MnSi. In this potential range, the surface of manganese silicides is considerably screened by SiO2 film. Cyclic voltammetry with a great number of cycles yields more detailed information on the anodic dissolution of silicides [22]. Figure 2 gives the cyclic voltammograms for Si, MnSi, and Mn5Si3 in 0.5 M H2SO4 (the number of a curve corresponds to the number of the cycle). The curves are obtained by alternately increasing the anodic polarization from the free-corrosion potential and backward decreasing it. Comparing the electrochemical behavior of manganese silicides with the behavior of individual components under potentiodynamic conditions, we can see that the silicides and silicon have much in common. The curves recorded with a high polarization rate, have more pronounced peaks of the active alloy dissolution. For instance, in the first cycle, one peak is observed in the cyclic voltammogram of MnSi, and two peaks are observed for Mn5Si3 . The first anodic peaks of manganese silicides and silicon are observed at close potentials, whereas the second peak, which is typical of only Mn5Si3 and vanishes in subsequent cycling, shifts to higher potentials. The current densities of the first and second peaks at Mn5Si3 are close. With cycling, both at MnSi and Mn5Si3 , the anodic peak decreases most significantly in the first two cycles (by 2.3 and 2.7 times, respectively), and the peak potential shifts to the anodic range. For
manganese silicides, the anodic peak almost degenerates even in the forth or fifth cycle. The anodic currents are higher for Mn5Si3 rather than MnSi. These results are in agreement with the supposition that Mn dissolves selectively and the surface layers of alloy are depleted of it. Under potentiodynamic voltammetric measurements, the variation in the current with an increase in E depends on two factors. An increase in the current is associated with the increase in the dissolution rates of each of components, while a decrease in the current is caused by enriching the surface (as a result of selective dissolution of components) in the electrochemically positive component. In a return branch of the cyclic voltammogram, the absence of peaks identical to those in the direct branches proves the irreversibility of the process. A necessary condition for the surface development in a voltammetric study is a certain critical charge passed through the alloy/solution interface [23, 24]: E cr
∫ I dE,
Q = 1/v
(1)
E cor
where Q is the critical charge (C), calculated from the area under the peak in the current (I, A) vs. potential (E, V) plot; v is the polarization rate (V/s); Ecor is the freecorrosion potential; and Ecr is the critical potential. At v = 5 mV/s and Ecr (MnSi) = 1.2 V, the critical charge for MnSi is 2.48 × 10–3 C/cm2 , and for Mn5Si3 , Q = 3.38 × 10–3 C/cm2 at Ecr (Mn5Si3) = 1.2 V. Thus, up to reaching equal critical potentials (the onset of surface development due to phase conversions of Si), Mn passes into solution from Mn5Si3 in a larger amount than from MnSi, which is reasonable. Figure 3 gives the first cycles of multicyclic voltammograms of Si and manganese silicides, which were measured with various polarization rates v (v, mV/s is shown by the numbers at the curves). It is seen that the peak current ip increases with an increase in v, and the current densities at Mn5Si3 are higher than at MnSi, while at v > 5 mV/s, at Mn5Si3 , ip1 becomes higher than ip2. Therewith, the peak potential Ep of MnSi shifts in the positive direction, whereas Ep1 and Ep2 at Mn5Si3 vary slightly. The initial segments of the above voltammograms measured with different v, differ. However, after repeatedly cycling, all curves gradually come to a common potentiodynamic curve. Its shape depends only slightly on the components ratio and polarization rate; it is determined by the strength of Mn–Si bonds (in Mn5Si3 , MnI–MnI bonds as well) and the diffusion of Mn through protective SiO2 film. At a low polarization rate (1 mV/s), a greater number of Mn atoms has time to diffuse to the surface layer from the bulk MnSi compound; this explains the fact PROTECTION OF METALS
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i, A/m2 0.16
i, A/m2 60 (a)
(b) 0.12
40
1 2 0.08
20
0
–20
10
0
0.4
0.8
1.2
1 2 3 5
5 10 15
0.04
0
1.6 E, V
–0.04 –0.4
0
0.4
0.8
1.2
1.6 E, V
i, A/m2 0.4 (c) 1 0.3
0.2 2
0.1
10 15
5 0
–0.1 –0.4
1
0
0.4
0.8
2 1.2
1.6 E, V
Fig. 2. Cyclic voltammograms of (a) Si, (b) MnSi, and (c) Mn5Si3 measured in 0.5 M H2SO4 with v = 5 mV/s. The curve numbers correspond to the cycle numbers.
that the peaks in the CVAs are retained longer (up to the 10th cycle). In this case, the electrode is for a longer time in the anodic range, and considerable amounts of silicon dioxide form on its surface. Selective dissolution of manganese from the sublattice of manganese silicides is accompanied by the silicon oxidation to SiO2 . Therewith, the dissolution of Mn can be controlled both by its diffusion in the alloy and the diffusion of its oxidized form through the pores of surface layer consisting of SiO2 [25], so that the higher the Si content in the alloys, the more compact films form at the surface. Therefore, the contribution of diffusion processes increases with an increase in the nonmetal component (Si) content in the alloy. This concluPROTECTION OF METALS
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sion is supported if one treats the plot of the peak current density vs. the polarization rate with the use of the equation ip = k1v + k2v1/2,
(2)
where k1v corresponds to the surface processes and k2v1/2, to the diffusion processes. This way, the mechanism of anodic dissolution of compounds can be estimated [26]. For manganese silicides, in a v range of 1 to 50 mV/s, the mechanism is mixed. From the dependence of peak current density ip on the polarization rate v in the logarithmic frame of axes log ip = f(logv), was determined the x-value in the equation ip = kvx. The
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SHEIN, ZUBOVA i, A/m2 1.2
i, A/m2 300
(a)
(b) 0.8
200
50 50 0.4
100
20 10 5
20 10 5
0
–100
0
1
0
0.4
0.8
1.2
1.6
2.0 E, V
1
–0.4
0.4
0
i, A/m2 1.6
0.8
1.2
1.6 E, V
(c)
1.2
50
0.8
20
0.4
10 5 1
0 –0.4 –0.4
0
0.4
0.8
1.2
1.6 E, V
Fig. 3. First cycles of cyclic voltammograms of (a) Si, (b) MnSi, and (c) Mn5Si3 measured in 0.5 M H2SO4 . The curve numbers correspond to the polarization rates v, mV/s.
equation is the limiting case of Eq. (2). When k1 = 0, ip = k2v1/2, and the anodic dissolution of compound proceeds by the diffusion mechanism; when k2 = 0, ip = k1v, and the anodic dissolution of compound is determined by the kinetic limitations. As is seen from Fig. 4, x is 0.719 for MnSi and 0.859 for Mn5Si3 . It was reported [27] that Mn dissolution in the acidic solutions proceeds concurrently by electrochemical and chemical mechanisms: in the anodic potential range prevails the first mechanism, whereas in the cathodic range, the second one. By the use of atomic absorption analysis of solutions (Perkin-Elmer 3110 device, in the mode of flame ionization), the content of manganese ions in 0.5 M H2SO4 solution was determined after preliminary cathodic polarization for 20 min (with i of 100 to
202 A/m2), subsequent exposure of the electrode to the solution at Ecor for 15 min, and its anodic polarization in the active anodic dissolution range (at E = 0.328 V) for two hours. The amounts of manganese were found to be (24.39 ± 0.02) × 10–6 and (50.81 ± 0.04) × 10−6 g/(cm2 h) for MnSi and Mn5Si3 , respectively (the values are referred to unit electrode surface area). Upon integrating the data of chronoammetric measurements, the charge Q was calculated by the equation 7200
Q =
∫
i dt,
where t is the time (s).
0
It was found that Q was 2.52 × 10–3 C/cm2 for MnSi and 13.21 × 10–3 C/cm2 for Mn5Si3 . During the anodic PROTECTION OF METALS
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dissolution of manganese silicides, silicon almost does not pass into the solution; this makes difficult its determination and, consequently, the separation of manganese selective dissolution and Si oxidation. Judging from the reaction order n OH– , manganese silicides dissolve abnormally in the anodic potential range, i.e. the chemical process proceeds along with electrochemical one. To more precisely determining the composition of the alloy surface layer from the results of atomic absorption analysis and chronoammetric data, we can use a boundary condition. Assume that Q, which is calculated from the i vs. t curve at E = 0.328 V, is consumed only by manganese selective dissolution. Then, by using an equation m = Q/NMn, where NMn is the electrochemical equivalent of Mn (g-equiv/(A h)), the amounts of Mn equal 19.8 × 10–6 and 103.4 × 10−6 g/(cm2 h) for MnSi and Mn5Si3 , respectively. Thus, it becomes possible to calculate the minimum amount of manganese that dissolves chemically from MnSi: 4.59 × 10–6 g/(cm2 h). As regards Mn5Si3 , two processes proceed obviously: the dissolution of Mn and the oxidation of Si to SiO2 . From the data on the anodic dissolution of manganese silicides with omitted preliminary cathodic polarization and reaching Ecor (Fig. 5), one can suppose that, in sulfuric acid solutions, a considerable amount of Mn dissolves chemically even in the course of cathodic polarization when preparing the electrode surface prior to the experiment. In the absence of cathodic polarization, the current density ia is higher in the initial period of dissolution both for MnSi and Mn5Si3 , (especially in the first seconds). With time, when the steady-state ia is reached (i.e. in a range of predominant phase conversion of electrochemically positive component) the anodic dissolution currents of the cathodically activated and unactivated specimens become equal. As to the experimentally observed high anodic resistance of manganese silicides, it can probably be explained not only by strong Mn–Si bonds and the formation of SiO2 film at the surface, but also by Mn dissolution even at Ecor and the cathodic potentials. In such a case, the surface layer of a silicide is enriched in Si atoms by already the onset of an experiment. As a result, the processes proceed more slowly, probably, by the way of a predominant oxidation of Si atoms and diffusion of Mn from the bulk of compound to the surface layer. This is supported by the results of chronoammetric and chronopotentiometric measurements on the test systems at various E and i, respectively (Fig. 6). As was noted in [28], in a case that the anodic dissolution of an alloy (manganese silicides in our case) is controlled by mixed kinetics (the rate is mainly determined by slow charge transfer, but transport limitations are also possible), the processes are irreversible, and electrochemically positive component in the test PROTECTION OF METALS
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logip [A/m2] 0.6 2 0.2 1 –0.2 –0.6 –1.0 –1.4
0
0.4
0.8
1.2
1.6 2.0 logv[mV/s]
Fig. 4. Plots of peak current density in the cyclic voltammograms vs. the polarization rate for (1) MnSi and (2) Mn5Si3 .
i, A/m2
0.8
0.4 3
4
1
2 0
200
400
600 t, s
Fig. 5. Chronoammograms of (1 and 2) MnSi and (3 and 4) Mn5Si3 measured in 0.5 M H2SO4 at E = –0.328 V (1 and 3) after preliminary cathodic polarization of specimens and (2 and 4) without it.
medium is stable, the chronoammogram obeys equation i a ( t ) = i 0 exp [ ( 1 – α )z A F∆E/ ( RT ) ] × exp [ β 0 ( t ) ]erfc [ β 0 ( t ) ], 2
(3)
where β0(t) = i0t1/2/(zAF D A C A )exp[(1 – α)zAF∆E/(RT)]. 1/2
0
If β0 1, Eq. (3) is reduced to the Cottrell equation [28]: i a ( t ) = n A FC A D 0
1/2
/ ( πt )
1/2
,
(4)
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SHEIN, ZUBOVA i, A/m2 1200
i, A/m2 5000
(a)
(b) 3
3
4000
800
3000 2
400
2 2000 1
1 i*100
1000
0
0.2
0.4
0.6
0.8 1.0 t –1/2, s –1/2
0
0.2
0.4
0.6
0.8 1.0 t –1/2, s–1/2
i, A/m2 2.0 (c) 1.5 2 3 1.0
0.5
0
1
0.2
0.4
0.6
0.8 1.0 t –1/2, s –1/2
Fig. 6. Chronoammograms of (a) Mn, (b) Si, and (c) Mn5Si3 measured in 0.5 M H2SO4 at E, V: (1) 0.328, (2) 0.728, and (3) 1.328.
The β0(t), which is calculated at zA = 1, α = 0.5,
0
where C A is the bulk concentration of component A, mol/cm3; D is its diffusivity in the alloy, cm2 /s; ia is the anodic current density; t is the time of specimen polarization, s; nA is the number of electrons involved in the electrochemical dissolution of A. Table 2. Diffusivity (D) of Mn and effective thickness (δ) of surface layer depleted of manganese (at t = 1 h) in manganese silicides; calculated from the chronoammetric data at various potentials Electrode
D, cm2/s, at E (V) 0.328
0.728
1.328
δ, cm
MnSi 2.22 × 10–17 8.41 × 10–17 8.41 × 10–17 0.98 × 10–6 Mn5Si3 1.24 × 10–14 1.26 × 10–14 1.25 × 10–14 11.89 × 10–6
0 CA
= 0.1 mol/cm3 , and i0 = 10–3 A/cm2 , shows that the lesser the magnitude and duration of anodic polarization and the higher the diffusivities in the alloy, the stronger the effect of kinetic peculiarities of the dissolution of electrochemically negative component A. As was noted in [29], the Cottrell equation was obtained provided that the electrochemically negative component concentration at the alloy surface is zero from the very beginning of dissolution. However, at the earliest stages of dissolution, concentration of this component at the surface, generally, may differ from zero. As a result, the dissolution of the negative component can be limited by one of subsequent stages of the overall dissolution process: the ionization or the removal of the ions formed from the electrode surface to the bulk PROTECTION OF METALS
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solution. In this case, at the beginning of the dissolution, in the frame of axes straightening the plot of Eq. (4), the real plot deviates from the straight line, and at a rather short t, under potentiostatic, i(A) can be independent of t indicating that the initial surface concentration of A does not change for these t. At sufficiently long t, the Mn dissolution rate from metal (Fig. 6a) or silicides starts to decrease. At the individual metal, the variations are caused by the diffusion of Mn ions through the surface layer of corrosion products, whereas at the alloys, the concentration decreases in the surface layers of the alloy as itself. In this case, for silicides (especially for MnSi), the i vs. t curves are linearized in the i vs. t–1/2 frame of axes, and linear segments of the curves coincide at different E values; at some E, the curves extrapolated to t = ∞ pass through the origin (Fig. 6c). As was shown in [30], this means that at long t, dissolution of Mn is limited by the diffusion of its atoms from the bulk of the alloy according to the following mechanism: in the initial period, the process kinetics is determined by the electrochemical properties of the negative component, but then, the solid-phase diffusion of Mn atoms becomes the limiting stage. The diffusivities (Table 2) were calculated from the slopes of i vs. t–1/2 curves at various potentials by the Cottrell equation. Under these experimental conditions, for each electrode, the bulk concentration C0 is assumed to be constant at different E, because the electrodes were of equal purity. In the curve for Mn5Si3 (Fig. 6), at E = 1.328 V, Si oxidation contributes significantly to the recorded current. Therefore, the corresponding i vs. t–1/2 straight line extrapolated to t = ∞ does not pass through the origin, but cuts off an Y-intercept. Knowing the diffusivity, the thickness of surface layer, which is depleted of manganese as a result of selective dissolution, can be estimated by the equation δ = (πDt)1/2, where t is the duration of the alloy dissolution (s). As is seen from Table 2, with an increase in the Mn content in the compound, the diffusivity increases by more than two exponents and correspondingly increases the thickness of layer enriched with Si. Comparing the diffusivities D obtained chronoammetrically (Table 2), with those calculated from the results of chronopotentiometric measurements, we see that they are quite comparable. Thus, the initial anodic currents are higher for manganese silicide Mn5Si3 than for monosilicide MnSi; however, with time, they approach each other due to the selective dissolution of Mn atoms proceeding according to the mechanism of transient bulk diffusion and enriching the surface layer in silicon with its subsequent oxidation to SiO2 . PROTECTION OF METALS
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CONCLUSIONS (1) Anodic resistance of manganese silicides is higher than that of their components, and it increases with an increase in the silicon content in the compounds. A high chemical resistance of silicides is associated with strong Mn–Si and Si–Si bonds and the protective properties of surface oxide SiO2 . (2) In the anodic polarization of silicides, Mn dissolves selectively, while Si remains at the surface and oxidizes to SiO2 . Thereafter, the process is controlled by the diffusion of Mn in silicide and oxidized Mn through the layer of hydrated SiO2 . (3) The contribution of diffusion processes increases with an increase in the silicon content of silicides, probably, due to the formation of more dense and compact surface oxide layer. (4) Anodic dissolution of manganese silicides, in many ways, obeys general regularities that were determined earlier for silicides of iron-family metals. REFERENCES 1. Aitov, R.G. and Shein, A.B., Zh. Prikl. Khim., 1991, vol. 64, no. 3, p. 667. 2. Aitov, R.G. and Shein, A.B., Zashch. Met., 1993, vol. 29, no. 6, p. 895. 3. Aitov, R.G. and Shein, A.B., Elektrokhimiya, 1993, vol. 29, no. 5, p. 611. 4. Shein, A.B. and Kichigin, V.I., Elektrokhimiya, 1986, vol. 22, no. 12, p. 1670. 5. Shein, A.B., Elektrokhimiya, 1988, vol. 24, no. 10, p. 1335. 6. Shein, A.B. and Kanaeva, O.V., Elektrokhimiya, 2000, vol. 36, no. 8, p. 1034. 7. Shein, A.B. and Kanaeva, O.V., Elektrokhimiya, 2000, vol. 36, no. 9, p. 1155. 8. Shein, A.B., Zashch. Met., 2001, vol. 37, no. 3, p. 315. 9. Shein, A.B. and Kanaeva, O.V., Zashch. Met., 2001, vol. 37, no. 4, p. 430. 10. Gurvich, L.V., Karachentsev, G.V., Kondrat’ev, V.N., et al., Energii razryva khimicheskoi svyazi. Potentsialy ionizatsii i srodstvo k elektronu (Energies of Chemical Bond Rupture. Ionization Potentials and Electron Affinity), Moscow: Nauka, 1974. 11. Agladze, R.I., Kabdinadze, E.V., and Gofman, N.G., Elektokhimiya margantsa (Electrochemistry of Manganese), Tbilisi: Metsniereba, 1988, vol. 9, p. 60. 12. Gel’d, P.V. and Sidorenko, F.A., Silitsidy perekhodnykh metallov chetvertogo perioda (Silicides of Transient Metals of Fourth Period), Moscow: Metallurgiya, 1981. 13. Efimov, A.I., Belorukova, L.P., Vasil’kova, I.V., and Chechev, B.P., Svoistva neorganicheskikh soedinenii: Spravochnik (Properties of Inorganic Compounds: Handbook), Leningrad: Khimiya, 1983. 14. Spravochnik khimika (Handbook of Chemist), Nikol’skii, B.P., Ed., Leningrad: Khimiya, 1964, vol. 3. 15. Shein, A.B., Zashch. Met., 2000, vol. 36, no. 2, p. 190.
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16. Mott, N.F., Passivity of Metals and Semiconductors, Amsterdam, 1983, p. 1. 17. Efimov, E.A. and Erusalimchik, I.G., Elektrokhimiya germaniya i kremniya (Electrochemistry of Germanium and Silicon), Moscow: Goskhimizdat, 1963. 18. Parkhutik, V.P., Makushok, Yu.E., Kudryavtsev, V.I., et al., Elektrokhimiya, 1987, vol. 23, no. 2, p. 192. 19. Saldanha, B.J. and Streicher, M.A., Mater. Perform., 1986, vol. 25, no. 1, p. 37. 20. Kolotyrkin, V.I., Knyazheva, V.M., Yurchenko, O.S., et al., Zashch. Met., 1992, vol. 28, no. 4, p. 545. 21. Sato, N. and Seo, M., Abstracts of Papers, Kongress “Zashchita-92” (Congress “Protection-92”), Moscow, 1992, vol. 1, part 1, p. 46. 22. Aitov, R.G. and Shein, A.B., Elektrokhimiya, 1991, vol. 27, no. 1, p. 74.
23. Zartsyn, I.D., Vvedenskii, A.V., and Marshakov, I.K., Elektrokhimiya, 1994, vol. 30, no. 4, p. 544. 24. Vvedenskii, A.V., Bobrinskaya, E.V., Marshakov, I.K., et al., Zashch. Met., 1993, vol. 29, no. 4, p. 560. 25. Shein, A.B., Elektrokhimiya, 1998, vol. 34, no. 8, p. 900. 26. Saidman, S.B., Bellocq, E.C., and Bessone, J.B., Electrochim. Acta, 1990, vol. 35, no. 2, p. 329. 27. Kolotyrkin, Ya.M. and Agladze, T.R., Zashch. Met., 1968, vol. 4, no. 6, p. 721. 28. Marshakov, I.K., Vvedenskii, A.V., Kondrashin, V.Yu., and Bokov, G.A., Anodnoe rastvorenie i selektivnaya korroziya splavov (Anodic Dissolution and Selective Corrosion of Alloys), Voronezh: Vor. Gos. Univ., 1988. 29. Marshakov, A.I., Pchel’nikov, A.P., Losev, V.V., et al., Elektrokhimiya, 1981, vol. 17, no. 5, p. 725. 30. Marshakov, A.I., Serdyuk, T.M., Pchel’nikov, A.P., et al., Elektrokhimiya, 1982, vol. 18, no. 9, p. 1285.
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