CC BY
40УДК 544.651.13: 544.621.22 Vladimir L. Krasikov1, Aleksey V. Krasikov2
MECHANISM FOR INDUCED CODEPOSITION OF ALLOYS AND SOME SINGLE REFRACTORY METALS
JSC Russian Institute of Radionavigation and Time RIRT, 120 Obukhovskoy Oborony pr., St. Petersburg, 192012, Russia Central Research Institute of Structural Materials "Prometey", 49 Shpalernaya str., St. Petersburg, 191015, Russia e-mail: elchkras@yandex.ru
It is shown that the new model of induced codeposition, based on experimental data for nickel-tungsten alloys electrodeposition from pyrophosphate electrolyte, may be applied for the explanation of electrodeposition regularities of other alloys of refractory metals as well as some single refractory metals, in particular, chromium and rhenium. The reduction of refractory metals to the metal state is caused by the formation of cluster heterometal compounds with the direct chemical bond between refractory metal and inducing metal. The length of the formed bond is shorter than the one in metallic crystal. During chromium deposition, Cr (III) ions act as the inducing metal while Cr (VI) ions behave as refractory metal. Any transition metal including electropositive ones may play the role of inducing metal. The special role of small clusters of inducing metal in the formation of refractory metals alloys is also shown. The suggested mechanism is assumed to be responsible for the occurrence of non-metals, such as phosphorus, selenium, boron, carbon in alloys, obtained from the electrolytes containing the appropriate additives.
Keywords: induced codeposition, electrodeposition mechanism, nickel-tungsten alloy, chromium, rhenium, cluster heterometal compound, metal-metal bond, refractory metal, inducing metal
В.Л. Красиков, А.В. Красиков
МЕХАНИЗМ ИНДУЦИРОВАННОГО СООСАЖДЕНИЯ СПЛАВОВ И НЕКОТОРЫХ ТУГОПЛАВКИХ МЕТАЛЛОВ
АО «Российский институт радионавигации и времени», пр. Обуховской Обороны, д.120, Санкт-Петербург, 192012, Россия
ФГУП «Центральный научно-исследовательский институт конструкционных материалов «Прометей», Шпалерная ул., 49, Санкт-Петербург, 101015, Россия e-mail: krasikov.av@mail.ru
Показано, что модель индуцированного соосаждения, построенная на основании полученных ранее экспериментальных данных по электроосаждению сплава никель-вольфрам из пирофосфатного электролита, применима для объяснения закономерностей осаждения других сплавов тугоплавких металлов, а также некоторых индивидуальных металлов, в частности, рения и хрома. Причиной восстановления ионов тугоплавких металлов в сплав является образование кластерных гетерометаллических соединений с металлом-осадителем с непосредственной связью металл-металл, причём более короткой, чем в кристалле металла. При осаждении хрома роль металла-осадителя играет ион Cr(III), а роль тугоплавкого металла - ион Cr(VI). Металлом-осадителем может являться любой переходный металл, в том числе, электроположительный. Показана роль малых кластеров металла-осадителя в восстановлении сплавов тугоплавкого металла. Предложенный механизм также может быть применен для объяснения появления в сплавах неметаллов: фосфора, селена, бора, углерода и т.д. из электролитов, содержащих соответствующие добавки.
Ключевые слова: индуцированное соосаждение, сплав никель-вольфрам, механизм электроосаждения, хром, рений, кластерное гетерометаллическое соединение, связь металл-металл, тугоплавкий металл, металлы группы железа
DOI 10.15217/issn1998984-9.2016.37.8
The study of kinetics and mechanism of nickel and tungsten induced codeposition to the alloy obtained from the alkaline pyrophosphate electrolyte (using traditional experimental technique for recording potentiostatic polarization curves in hydrogen atmosphere) has shown, that the partial rate of tungsten reduction grows in direct proportion with increase in the concentration of both nickel and tungstate ions [1]. At the same time, the partial rate of nickel reduction to the alloy is directly proportional to nickel and hydroxyl ions concentration, inversely proportional to pyrophosphate anions concentration and does not depend on tungstate concentration. The carried out research allows us to make a conclusion that the tungsten reduction to metal is preceded through the stage of nickel ions Ni (II) reduction to intermediate particle NiOHads. The subsequent chemical interaction of this particle and tungstate in its initial form WO42- results in formation of heterometal cluster compound with nickel-tungsten di-
rect chemical bond [HO-Ni-WO4]2-ads. This very particle is the source of tungsten. The achievement of conditions favouring to its formation leads to induced codeposition of alloys. The formation of such particle may be energetically profitable [1], since the length of metal-metal bond in cluster compounds is significantly shorter than in metallic crystals. According to our kinetic data, the cluster compound reduction to metal proceeds with slow transfer of the first electron in compliance with the reaction (1):
(1)
1 Vladimir L. Krasikov, Ph.D. (Chem.), Deputy chief of department, Russian Institute of Radionavigation and Time RIRT, e-mail: elchkras@yandex.ru Красиков Владимир Леонидович, канд. хим. наук, зам. начальника отдела, АО «Российский институт радионавигации и времени»
2 Aleksei V. Krasikov, Ph.D. (Chem.), Deputy chief of research department, FSUE CRISM "Prometey", е-mail: krasikov.av@gmail.com Красиков Алексей Владимирович. канд. хим. наук, зам. начальника научно-исследовательского отдела, ФГУП ЦНИ КМ «Прометей»
Received December 20, 2016
The obtained compound [NiWO4]2- with nickel-tungsten multiple bond then reduces quickly to the alloy by the following overall reaction:
[NiWOJ2" + 4 H2O + 6 e" ^ Ni0 + W0 + 8 OH"
(2)
In accordance with the mechanism [1], if all the intermediate particles NiOHads, which are formed, react with tungstate and further discharge with formation of alloy, maximal possible tungsten content in alloy amounts to 50 at.%. In practice we obtained the alloys with tungsten content up to 23 at.% even if there was a significant excess of tungstate in the electrolyte. Hence, no more than a half of reducing nickel ions can interact with tungstate and then discharge forming the alloy. The other part of nickel ions is reduced to metallic nickel according to the mechanism, presented in article [2], without interacting with tungstate. Our kinetic data showed that in the limiting stage of tungsten reduction there participated a particle which contained one nickel atom and one tungsten atom, in the meantime the maximal tungsten content in the cathode coating was formally close to Ni2W composition. Thus, based on this, one can suppose that after the limiting stage of discharge the subsequent reduction of generated [NiWO4]2- particle may proceed with participation of another nickel-containing particle, probably as following:
[Ni=W04]2- + NiOHads
NiOH
I
; Ni=wo4]2-
(3)
[NiWO4(NiOH)]2-ads + 4 H2O + 7 e- ^ 2 Ni0 + W0 + 9 OH- (4)
So, consecutive assembly and reduction of hetero-metal cluster compound occurs. Tungstate reduction begins at the same potential at which NiOHads intermediates start to form. Tungstate reduction to metal is impossible without meeting this condition. If nickel (II) ions are not present in the electrolyte, tungstate reduction does not take place even to intermediate compounds, and the only process taking place on the cathode in tungstate solution is hydrogen evolution [1].
The analysis of results of numerous papers on various alloys induced codeposition allows one to draw a conclusion that NiOHads intermediate particle is an initiator of various cathodic processes. Some ways of its transformation are shown in simplified form in the following scheme:
After the generation of the NiOHads intemediate particle in compliance with the reaction (5), it may be subjected to various transformation. The most probable way of such transformation on the cathode is the reduction to the metal adatom in accordance with the reaction (6). If there is a refractory metal anion in the solution, then chemical interaction with adsorbed anion is likely to occur with producing cluster compound (reaction 7), which can further be reduced to metal state in compliance with reaction (8). If there is a sulphur-containing compound in the electrolyte, for example, thiourea, SO2 or saccharin [3-5], then sulphur occurs in the coating through the stage of NiOHads particle chemical interaction with saccharin or other component according to the reaction (9) with the formation of unstable intermediate. Then the intermediate particle is reduced to elementary sulphur (10). Similarly, the short-lived NiOHads intermediate particle, interacting with phosphorous-containing component [6-10] according to reaction (11) is able to generate cluster compound, which is reduced in compliance with the overall reaction (12) to phosphorous and is included in the composition of cathodic coating, etc. Due to NiOHads intermediate, selenium, tellurium, boron, carbon and other elements may be incorporated in a similar way into the cathodic coating and can exist in the alloy not only in the elementary state, but also forming compounds such as phosphides, carbides and others [11-12]. Finally, as it is shown in [13], delay in NiOHads particles reduction results in occurrence of oxygen-containing impurities and absorbed hydrogen in the metal coating (reaction 13 in the scheme), besides there proceeds an oxidation of NiOHads particles during cathode process. If no nickel ions (or manganese, tin, zinc, cobalt and so on) present in the electrolyte, then codeposition of tungsten (or molybdenum, titanium, thorium, zirconium, phosphorus and so on) does not take place, as the conditions are not appropriate for heterometal cluster compound formation according to reactions (7), (9) or (11). In the presence of several alloying components, which can interact with Me(I) particle, competition for Me(I) particle may arise in the electrolyte. Thus, addition of sodium hypophosphite into the electrolyte for Ni-Fe-W alloy deposition may lead to including phosphorus in the alloy while tungsten content decreases [14]. Increase in sodium hypophosphite concentration causes much greater decrease in tungsten content in the alloy, apart from that iron content also decreases, and the coating mostly transforms to Ni-P alloy. This may be caused by the replacement of tungstate and iron ions from the electrode surface and preferential adsorption of hypophosphite and nickel ions. For the first time we presented a new mechanism of induced codeposition of alloys in [15], and later we found lots of confirmations of our ideas in experimental works of other authors.
It should be noted, that in here alloy means cathodic coating that contains only metal atoms (and absorbed hydrogen), but does not contain oxygen atoms. Real alloys of refractory metals deposit if there is an excess of reducing metal concentration in the electrolyte. It is known that from aqueous solution of tungstate, molybdate or some other refractory metals compounds, in the absence of other transition metals ions, it is possible to deposit coatings which are actually just nonstoichiometric oxides [16-20]. In intermediate variant, at low concentration of inducing metal, a composite coating, which contains both metal phase and metal oxides, can be deposited on the cathode. The conditions for deposition of alloys, oxides and composite coating are similar, but the mechanisms of their deposition are quite different. So, if scientists do not pay sufficient attention to the oxygen occurrence in the coating or to the fact that its content varies depending on deposition conditions and don't take into account the cause of changing the partial current density of hydrogen evolution, they quite often can make wrong conclusions concerning the mechanism of alloys deposition.
It should be also mentioned that publications on induced codeposition mostly refer to the alloys of molybdenum with iron-group metals. From our point of view in the presence of excess of inducing metal, molybdenum is reduced to
the alloy in compliance with the same mechanism as tungsten does, but the researchers often conclude, that the first stage of molybdenum alloy deposition is molybdate reduction to intermediate oxidation state, which is followed by the interaction with iron-group metal ions [21-28]. Generally it is not true, as the first stage of molybdate reduction and the first stage of the alloy deposition are not the same. This error in reasoning is caused by the fact that molybdates, unlike tungstates, can be reduced to intermediate oxides at less negative cathodic potentials than the potentials necessary for iron-group metal reduction and they can be deposited on cathode as special independent coating, however such coating is not an alloy, but a composite material with significant molar concentration of oxygen. Besides that, refractory metal oxides are often elec-trochemically indifferent and, after being generated, are not reduced further, for example, molybdenum blue [19] or chromium anion НСгОз2- [29]. The true alloy begins to deposit if the rate of iron-group metal reduction considerably exceeds the rate of molybdate reduction. If such a requirement is met, then, in the absence of oxygen-containing impurities in the coating it is possible to make conclusions about the kinetics and mechanism of alloy deposition.
In contrast to tungsten alloys, molybdenum alloys electrodeposition has an important peculiarity. Paper [30] showed that in pyrophosphate electrolyte, containing 0.18 M [Ni(P2O7)2]6-, which was used for of nickel-molybdenum alloy deposition (40oC), when concentration of Na2MoO^2H2O increased in the solution from 1 to 6.5 g/l (from 0.004 to 0.0027 М), molybdenum content in the alloy increased from 4 to 35 mass.% (from 2.5 to 25 at.%), and the rest part of the alloy content was nickel. Oxygen was absent in the alloy. Current efficiency of the alloy with highest molybdenum content amounted to 75%. That is, at relatively low sodium molybdate concentration and rather high cathodic current density 5 A/ dm2, there is quite high partial current density of molybdenum reduction, more than 1 A/dm2, and no limiting current for molybdenum occurs. It means, that molybdate-ion is present on the cathode surface in adsorbed state and the adsorption rate is quite high. Molybdate locates mostly on the electrode surface, not within the electrolyte. Paper [31] determined that under the similar conditions in pyrophosphate electrolyte (20oC) with insignificant increase in sodium molybdate concentration from 0.020 to 0.023 М, molybdenum content in the alloy increased in a leap from 26 to 38 mass.%. Current efficiency also changed in a leap, it decreased from 55 to 12% and up to 10 mass.% of oxygen occurred in the coating. The coating became composite, containing besides metal phase significant content of nonstoichiometric molybdenum oxides. Hence, the electrodeposition of nickel-molybdenum alloy with relatively low concentrations of sodium molybdate in the electrolyte follows the mechanism of induced codeposition suggested in our paper [1] that includes the stage of heterometal cluster compound [HO-Ni-MoO4]2-ads formation. If sodium molybdate concentration exceeds 0.02 M, a vast site occupation of the cathode's surface with adsorbed molybdate is certain to take place; as a result, the replacement of NiOHads particles from the surface occurs. Thus, in case of molybdenum alloys elec-trodeposition one can observe the effect of high site occupation [32]. The rate of molybdenum reduction (i) depends upon the surface concentration of molybdenum ГМо and nickel ГМ:
i = КГмсГы exp(-aFE/RT) (14)
The rate i is maximal if the surface concentrations of ГМо and Гм are almost equal. In the case of the shortage of inducing metal, the formation rate of heterometal compound [HO-Ni-MoO4]2-ads and its further reduction becomes too low (see reactions 3 and 4), and hence, only small part of mo-lybdate is reduced to metal state, the rest part, that does not react with nickel, is reduced just to intermediate nonstoichio-metric oxide, which has very low hydrogen overvoltage. That is why the prevailing or even the only cathode process becomes hydrogen evolution [19, 33].
Tungsten under similar conditions (with the significant excess of tungstate ions and shortage of nickel ions in the electrolyte) is not reduced to intermediate compounds, and tungstate-ions adsorption does not lead to NiOHads particles replacement. So, despite many similar properties, molybdenum and tungsten show- rather different electrochemical behaviour, which is manifested by the difference in their adsorption rates on the cathode surface.
In paper [1] we have made a conclusion, that to begin refractory metal reduction on foreign substrate it is necessary that at first small clusters of inducing metal, consisting of not more than 12 atoms [34-36], be formed, and only after that the adsorption and reduction of refractory metal starts on these clusters. In [1] we designated conventionally nickel clusters as {Ni12}. The inducing metal performs two functions: firstly, it forms small clusters, on which refractory metal anions adsorption takes place in strictly determined configuration, secondly, the intermediate particle of inducing metal reduction (for example, NiOHads) interacts with refractory metal adsorbed anion (for example, with WO42-ads), forming heterometal cluster compound [HO-Ni-WO4-{Ni12}]2-, in which chemical bond of refractory metal and oxygen is loosened significantly, and the oxidation states of nickel and tungsten have any intermediate value. All this makes it possible to carry out easy electrochemical reduction of this complex cluster compound to the alloy. We think, that induced codeposition of other refractory metals, which are in the solution in the form of anions in high oxidation state, also proceeds through the stage of heterometal cluster compound generation with the direct chemical bond of metal(1)-metal(2), moreover both these metal atoms are simultaneously connected with the cluster atoms [1], forming stable closed metal core. Electrochemical behaviour of intermediate cluster compounds is still poorly investigated (in the review [37] they are not even mentioned) but is a very prospective direction both in theoretical and applied aspect.
However several questions arise. Why does chromium deposit from chromic acid solution and why does rhenium deposit from perrhenate solution as compact metals, though they belong to refractory metals and they present in the solution in the highest oxidation state? Why is there no need for an inducing metal to deposit these refractory metals?
To find the answer may be quite easy, if to take into consideration the following well-known facts. Let us deal with chromium as an example.
• From the electrolyte, containing only chromic acid, chromium does not deposit, and the only cathodic process is hydrogen evolution [38].
• To deposit chromium with acceptable current efficiency the presence of catalysing anions in the electrolyte is necessary, usually SO42-, F-, SiF62- [38-39]. The addition of sulfuric acid is most frequently applied. It is discovered experimentally, that chromium current efficiency extremely depends on H2SO4 concentration, the optimal value is 1 mol.% with reference to CrO3 concentration.
• For chromium electrodeposition the presence of Cr(III) ions in the solution is necessary in some optimal quantity, usually 1-2 mass.% in relation to CrO3 concentration. In the absence of Cr(III) ions chromium practically does not deposit [39]. From freshly prepared H2CrO4+H2SO4 solution chromium deposition begins not immediately, but after the pretreatment of the electrolyte, that is, when it accumulates some quantity of Cr(III) ions. Cr(III) ions generation takes place owing to partial Cr(VI) ions reduction.
• In the standard pretreated electrolyte H2CrO4+H2SO4 at low cathode potentials -0.5...-0.9 V (SHE), only Cr(VI) reduction to Cr(III) and then the hydrogen evolution proceed, SO42- ions do not participate in these processes [40-42]. At cathode potentials -1.1 V and higher metallic chromium electrodeposition, hydrogen evolution and Cr(VI) reduction to Cr(III) proceed simultaneously [38, 39, 41]. Metallic chromium deposition starts only on reaching certain minimal current density (1.2-3.0 A/dm2 depending on conditions). At the lower
current density on cathode, there is a hydrogen evolution and Cr(VI) to Cr(III) transformation proceeds [38].
• In the range of potentials (or cathodic current densities), appropriate for Cr(VI) reduction to metal, Tafel slope of partial polarization curves for chromium reduction equals 120-130mV [29, 40, 42], the order of chromium reduction reaction with respect to chromium acid equals to +1. Hence, we can conclude, that chromium reduction takes place in several stages, follows electrochemical kinetics and is limited by the transfer of the first electron to the electroactive particle. Sulfate ion also participates in the reaction, as the reduction order of chromium reaction in respect with sulphate ions also equals +1 [43] (at low concentration of sulfuric acid).
• As for the nature of discharging particle, there are various opinions: some authors think, that Cr(III) particle reduces, some authors think, that Cr(VI) particle does so, the others - that both ions are reduced at the same time [29, 38].
• It is possible to deposit chromium from acid electrolyte on the basis of trivalent chromium Cr(III) salts. It is known, that the process proceeds in two stages: at first, one electron transfer takes place:
Cr(III) + e- - Cr(II)
then two electrons transfer simultaneously: Cr(II) + 2e" - Cr0
(15)
(16)
Cr(II) ions, being the products of reaction (14), partially go to the solution, hence, chromium current efficiency from electrolytes on the basis of Cr(III) is always considerably less than 100%. The question, what stage of the process is a limiting one - (15) or (16) - is still has no exact answer, but opinion frequently expressed in the literature assumes that slow stage of the process is a reaction (16) [44-47].
Taking into account all the phenomena listed above and in compliance with our model of induced codeposition, it is possible to make a conclusion that chromium reduction from pretreated electrolytes on the basis of Cr(VI), for example, H2CrO4+H2SO4, containing certain amount of Cr(III) ions, proceeds as follows.
In the initial moment after switching on cathodic current, chromium clusters generate on indifferent foreign surface of cathode, and the source for them are Cr(III) ions, reducing to metal stage-by-stage, most probably, with slow transfer of the first electron.
CrSO4+ + e" ^ CrSO40ads (15a)
CrSO40ads + 2e" ^ Cr0 + SO42" (16a)
Chromium is reduced through the stage of adsorbed intermediate particle CrSO40ads formation. As a result of the processes (15a, 16a) small chromium clusters {Cr} generate on the cathode. These clusters consist of the limited number of atoms, besides that CrO42" anions do not take part in their formation. After clusters formation the process becomes stable and follows the next scenario: CrO42" anions adsorb on {Cr} clusters in a proper configuration and interact with adsorbed intermediate particles of Cr(III) reduction, that is with CrSO40ads particles:
(17)
We think, that such interaction is energetically profitable, as the distance Cr-Cr between atoms in cluster compounds can be very small. In particular, in chromium acetate Cr2(CH3COO)4, in which two chromium atoms are direct-
ly connected, the distance between the atoms amounts to just 2.30 A, while the minimal distance in chromium crystal amounts to 2.50 A [48].
As the process develops with participation of chromium cluster {Cr}, the forming intermediate particle has the following structure:
~ S04Cr — Cr04
\/
{Cr}
In such structure all chromium atoms compose stable cluster core; oxygen atoms bound with Cr(VI) atom, and sulfate-anion bound with Cr(III) atom play the role of stabilizing ligands. The similar role can be played by F- or SiF62- anions. Similarly to the model of Ni-W alloy reduction [1], chromium reduction from this cluster polynuclear particle with direct bond of Cr-Cr-Cr occurs in steps with the slow first electron transfer, since according to the most reliable experimental data [29, 40, 42-43] the Tafel slope of partial polarization curves of chromium reduction is equal to 120-130mV.
(18)
Most probably, in the formed cluster particle chromium atoms are equalized by oxidation state, and also oxygen is distributed evenly among chromium atoms:
O2O — C1-O2 \ / {Cr}
Then the unstable formed cluster particle is quickly reduced to metal in compliance with the following overall reaction:
[O2Cr-CrO2-{Cr}]- + 8H+ + 7e- --> 2Cr + {Cr} + 4H2O (19)
It should be emphasized that such type of the mechanism for individual chromium reduction is significantly different from the mechanisms suggested by other researchers, although it practically fully agrees with the mechanism of Ni-W alloy induced codeposition [1]. In compliance with our model, Cr(III) ions perform the function of iron-group metal, and Cr(VI) ions plays the role of refractory metal. Thus, both types of chromium ions participate in forming the cathode plating. Cr(III) ions play the crucial role: their concentration, the nature of ligand and the rate of reduction to Cr(II) will define the rate of the whole process and the structure of metal. Cr(VI) reduction to metal starts at the potential, at which generation of CrSO40ads intermediate particles begins, and at the time, when chromium clusters are formed. Dual role of chromium ions in various state of oxidation is responsible for the fact, that the mechanism of chromium reduction has not been revealed till now. Also our model allows us to explain the numerous complicated and controversial data of radioactivity of chromium deposits obtained from the electrolyte with addition of radioactive either Cr(VI) or Cr(III) compounds.
It is appropriate to note, that we know hundreds of publications considering various aspects of chromium electrodeposition from electrolytes on the basis of Cr(VI). We don't know any paper where the reaction order of chromium reduction with respect to Cr(III) ions is correctly
determined. We hope that the researchers, studying chromium electrodeposition, will succeed in solving this task and obtaining more exact data concerning this mechanism, than the model that is presented in here a priori.
The impossibility of chromium deposition from freshly prepared standard electrolyte, not containing Cr(III) ions, is due to the absence of "inducing metal" - Cr(III) ions, and respectively, by the impossibility of forming CrSOAds particle and cluster compound [Cr(III)-Cr(VI)-{Cr}], in which the strong bond of Cr(VI) with oxygen is half-destroyed. To generate CrSO40ads particle (or CrSiF6°ads, CrF20ads), the presence of the ion SO42- (or SiFe2-, F-) is necessary, because of that the reaction order of chromium reduction with respect to sulfate-ions equals to +1 [43]. Hence, in chroming electrolyte it should be definite correspondence between Cr (III) and SO42-concentration.
Considering chromium reduction, they often mention in the literature the cathodic film, that exists just as long as the current is turned on, and chromium reduces from the material of that film. In our opinion, the term "cathodic film" should be attributed to the cluster compounds on the cathode, considered above, and to the intermediate compounds of chromium reduction. Besides we have briefly mentioned the compounds generated on the basis of chromate CrO42-, however the electrolyte may also contain bichromates and trichromates, which in their turn are also able to generate more complex cluster compounds and more dense and viscous the near-cathode liquid film.
We think, that rhenium from perrhenate solution is reduced according to the mechanism of induced codeposition similar to the one in the case of chromium reduction. In its compounds rhenium can have an oxidation state from -1 to +7, perrhenate reduction proceeds step-by-step forming lots of intermediate oxides [49-50]. Obviously, at the certain stage of perrhenate reduction, intermediate particles with low oxidation state generate, and they can interact with original perrhenate ReO4- ion, forming cluster particle, which is further reduced to metal. It is a fact of common observation, that rhenium current efficiency can be favourably influenced by fluoride, ammonium ions and some other additives. They do not chemically interact with original perrhenate. We have not focused on rhenium electrodeposition kinetics, so we can just suppose, that the above-mentioned ions interact with intermediate particles of rhenium reduction, facilitate formation and stabilize cluster compounds with rhenium-rhenium direct bond, (similarly to chromium cluster compounds). All these factors cause the redistribution of electron density in the adsorbed particle and make it possible to reduce rhenium to metal in the absence of transition inducing metal of another chemical nature. But the process of rhenium electrodeposition has been studied so insufficiently, that we can only guess, what kind of particles can participate in the formation of cluster polynuclear compounds with rhenium-rhenium bond. We hope that the scientists, dealing with this metal and its alloys deposition, will pay more attention to short-lived intermediate particles of perrhenate reduction.
In the light of our model the electrodeposition mechanism of such refractory metal alloys as chromium-molybdenum [51], chromium-tungsten [52], chromium-rhenium [53] becomes clear. In these cases the role of inducing metal (the role of iron-group metal) is performed by intermediate particles of chromium and rhenium reduction with low oxidation state, which form cluster compounds with each other and with molybdenum (VI) or tungsten (VI) compounds.
Thus, the experimental data obtained by studying the kinetics of electrochemical reduction of nickel-tungsten alloy from pyrophosphate electrolyte and the conclusions about the mechanism of this alloy electroreduction [1] with proper correction may be applied for explaining the deposition regularities of other alloys and some individual metals. Our model significantly differs from the previous theories of induced codeposition and allows one to explain the occurrence of sulfur, carbon, phosphorus and other elements in alloys, and fi-
nally it explains the occurrence of hydrogen and oxygen-containing admixtures [13]. The common reason for these effects is the generation of transition metal intermediate particle and the variety of ways for its further transformation. Finally, our model is confirmed with the fact that the role of inducing metal may be played not only by the iron-group electronegative transition metals, but also by electropositive metal, for example, copper. So, we obtained Cu-W alloys with tungsten content of about 3 at.% by adding sodium tungstate to pyrophosphate electrolyte for copper plating. The authors of paper [54] succeeded in obtaining Cu-W alloy with tungsten content up to 1° at.% from the citrate electrolyte. It confirms once again, that the possibility for alloys induced electrodeposition is not limited with the application of iron-group metals as inducing metals. Any transition metal may play the role of inducing metal if it is reduced through the stage of intermediate particle generation, which is able to form cluster compound with refractory metal. Again we should emphasize the role of the small inducing metal clusters by the following simple, but illustrative experiment. From pyrophosphate electrolyte, containing nickel and tungsten compounds, Ni-W alloy deposits on copper cathode [1]. But on abundantly amalgamated copper only pure nickel deposits, that can be easily controlled by the method of X-ray fluorescence spectrometry. Only after several minutes, when liquid mercury layer is covered with solid crust of nickel and small nickel clusters are formed, the layer of Ni-W alloy begins to grow. This fact proves one more time the validity of our model for metals and alloys induced codeposition.
Conclusions
Experimental data obtained in paper [1, 15] allowed us to determine the reaction orders of nickel and tungsten reduction with respect to all components of pyrophosphate electrolyte and to elucidate the nature of discharging particle. It was established that pyrophosphate anion was not included in tungsten-containing particle. Previously the ionic equilibria in pyrophosphate electrolyte as well as mechanism and kinetics of hydrogen, nickel and cobalt electroreduction were determined in detail [2, 55].
At the same time the analysis of a great amount of papers on refractory metals and alloys deposition from citrate electrolytes is not still allowed to draw a definite conclusion about the nature of discharging particle and whether it contains a citrate anion. Moreover, in overwhelming majority of papers the mechanism and kinetics of hydrogen and iron-group metals reduction were not previously determined. Apart from that the authors often did not control the oxygen content in the cathode deposit. Most of authors paid attention to the fact that the reduction of refractory metals occurs in stages but missed the fact that the iron-group metals also reduce step-by-step. Majority of authors took into account that the forced electrolyte stirring or change in rotation rate of cylinder electrode affected considerably the regularities of alloys deposition but did not take into consideration an extremely essential role of stirring the electrolyte by liberated hydrogen bubbles, and hence, the influence of hydrogen evolution rate on the partial rates of alloy components reduction.
In our work we followed the experimental technique that excluded all the possible distorting impacts. The present paper has briefly considered some common peculiarities for refractory metals codeposition with transition metals. The obtained results were compared with experimental data concerning molybdenum and tungsten alloys electrodeposition. It was shown that the main cause for refractory metals codeposition was the interaction of intermediate particle of iron-group metal reduction with adsorbed refractory metal anion resulting to the formation of heterometal cluster compound with chemical bond metal-metal and its further reduction to metal state. The presented model is also allowed to explain the mechanism of chromium and rhenium electrodeposition and the occurrence of sulphur, phosphorus and other non-metals in the cathode coating.
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