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21Journal of Siberian Federal University. Chemistry 2 (2015 8) 160-180
УДК 546.30
Thermodynamic Evaluation of Electrochemical Stability of Me - Si Systems (Me = 4th Row Transition Metal)
Pavel A. Nikolaychuk*
Chelyabinsk State University 129 Bratyev Kashirinykh Str., Chelyabinsk, 454001, Russia
Received 22.04.2015, received in revised form 16.05.2015, accepted 30.05.2015
This paper aims to review the studies of electrochemical properties of Me - Si binary systems for scandium, titanium, molybdenum, manganese, iron, cobalt, nickel, copper and zinc from the point of view of chemical thermodynamics. The binary phase diagrams of Me - Si and Me - O systems were considered and thermodynamic properties of metals silicides and oxides at standard conditions were collected. The potential - pH diagrams of Me - Si - H2O systems at 25 °C, air pressure of 1 bar and activities of ions in solution equal to 1 mol l-1were plotted. The corrosion-electrochemical behavior of studying systems was discussed.
Keywords: transition metals, silicides, oxides, chemical and electrochemical stability, potential - pH diagram, corrosion-electrochemical behavior.
DOI: 10.17516/1998-2836-2015-8-2-160-180.
© Siberian Federal University. All rights reserved
* Corresponding author E-mail address: [email protected]
Термодинамическая оценка
электрохимической устойчивости систем Ме - Si (Ме = переходный металл 4-го периода)
П.А. Николайчук
Челябинский государственный университет Россия, 454001, Челябинск, ул. Братьев Кашириных, 129
Статья представляет собой обзор исследований электрохимических свойств двойных систем Ме - Si для скандия, титана, молибдена, марганца, железа, кобальта, никеля, меди и цинка с точки зрения химической термодинамики. Рассмотрены двойные фазовые диаграммы систем Ме - Si и Ме - О, собраны термодинамические свойства силицидов и оксидов металлов в стандартных условиях. Построены диаграммы потенциал - рН систем Ме - Si - Н2О при 25 °С, давлении воздуха 1 бар и активностях ионов в растворе, равных 1 моль/л. Обсуждается коррозионно-электрохимическое поведение исследуемых систем.
Ключевые слова: переходные металлы, силициды, оксиды, химическая и электрохимическая устойчивость, диаграммы потенциал - рН, коррозионно-электрохимическое поведение.
Introduction
The silicides of transition 3d-metals are characterized by a very wide variety of physical and chemical properties and an exceptionally wide range of applications in technology. There are conductors with high electroconductivity, semi- and superconductors, metalloids among them. In addition to other things, these compounds have high corrosion resistance; their thermodynamic and electrochemical properties are quite different. The experimental investigations of electrochemical properties of 3d-metals silicides are widely performed [1]. However along with it, a theoretical studying of corrosion-electrochemical behaviour of these systems is also important because it allows complementing and extending experimental data. This paper aims to review the studies of electrochemical properties of Me - Si binary systems for some 4th row d-metals from the point of view of chemical thermodynamics.
All the methods, techniques and approaches used in solving this problem were detailed in monograph [2]. The best carrier of thermodynamic information about possible chemical and electrochemical reactions proceeding in water environments is the diagram of electrochemical equilibrium (electrode potential vs. pH relationship), which allows one to clearly depict the domains of thermodynamic stability of all possible phases and determine the type of system corrosion-electrochemical behavior in certain environmental conditions.
The potential - pH diagrams for pure elements at 25 °C were introduced by M. Pourbaix [3] and are now well known. The procedure of plotting potential - pH diagrams for Me - Si - H2O systems consists from three steps. At the first step one should study all phase and chemical equilibria in binary Me - Si system at 25 °C basing on its phase diagram, look for reference thermodynamic
data, check they are matched to each other and calculate thermodynamic properties of system components. At the second step one should collect information about all possible oxides and silicates that can be formed during system oxidation, develop the consequence on oxidation processes in oxygen environments and plot the state diagram for Me - Si - O system [2]. At the last step one should consider the ions that can be formed during system oxidation in solutions, possible chemical and electrochemical reactions involving these ions, calculate their electrode potentials and plot the potential - pH diagram. For all systems under consideration the calculations were performed exactly in this order.
The area of electrochemical stability of water has the most importance in analysis of the corrosion behavior of studying systems. The equilibria corresponding to the hydrogen and the oxygen electrodes in atmospheric conditions, which border the area of electrochemical stability of water, are denoted at Figures 1 through 9 by dotted lines. The equations for electrode potentials of these electrodes are presented in Table 1.
In highly reducing conditions metals can form the hydrides. Moreover, the possibility of electrochemical formation of metals nitrides from atmospheric nitrogen was considered in thermodynamic predictions. However, only titanium, nickel and copper hydrides and only a titanium nitride have the domains of thermodynamic stability on potential - pH diagrams. The hydrides and nitrides of other metals either have no thermodynamic data or found tn be thermodynamically not stable in aqueous solution.
E, V(5- h-«.) 0U0
40
-1.0
-1,5
■i.O
'Î.5 -1.0
0 Î 4 S 0 12 14 14 PH
Fig. 1. The potential - pH diagram of Sc — Sbi — H2O system at 2998 K, air pressure of 1 bar and at= 1 mol l-1
Table 1. Electrode potentials of the hydrogen and the oxygen electrodes at atmospheric conditions
Electrode reaction E, "V (s. h. e)
2H+(aq) + 2e- ^ H2(g); PH2 « 5 • 10"7 bar 0.186 - 0.0591 • pH
O2(g) + 4H+(aq) He 4e" ^ H2O(l); Pq2 « 0.21 bar 1.219 - 0.0591 • pH
IX
X VIII XI XII
IV
ii
—i—1—■—1— -î—|—i—î—i—î—.—|—t— H—
It is to be noticed that there are no reliable thermodynamic data on the standard Gibbs energies of formation of some transition metals oxides and silicides. In this case some empirical relationships were used to estimate them.
Sc - Si system
According to Sc - Si system phase diagram [4], two scandium silicides - Sc5Si3 and ScSi - exist is system at standard conditions. The third silicide, Sc2Si3, exists only at elevated temperatures. All silicides are stoichiometric compounds without any noticeable homogeneity ranges. There are no data on solid solubility of Si in (hcp-Sc) and Sc in (diamond-Si); it is vanishingly small and the components can be treated as insoluble in each other. The standard Gibbs energies of formation of scandium silicides were taken from [5].
Scandium can form only one oxide, Sc2O3. Its standard Gibbs energy of formation was obtained from [6]. Some scandium silicides exist at elevated temperatures [7] but only thorvertite (Sc2Si2O7) remains stable at 298 K. The standard Gibbs energy of formation of this compound was estimated according to ДО2- method, proposed by the authors of [8, 9]. It seems that chemical affinity of scandium to oxygen is greater, than that of silicon, and it oxidizes more easily. The oxidation of Sc - Si system in air environments ends with formation of Sc2O3 and Sc2Si2O7 in scandium-rich region and of Sc2Si2O7 and SiO2 in silicon-rich region.
In water environments, scandium can form only Sc3+cations [10], and silicon can simultaneously exist in form of SiO2, H2SiO3 and H4SiO4. However, only orthosilicic acid was considered by means of convenience.
The potential - pH diagram of Sc - Si - H2O system is plotted at temperature of 298 K, air pressure of 1 bar and activities of ions in solutions, equal to 1 mol l-1 and presented at Fig. 1. 12 domains of prevalence of certain phases can be depicted in it: I - Sc (hcp) + Sc5Si3 + ScSi + Si (dia); II - Sc3+ + Sc5Si3 + ScSi + Si (dia); III - Sc2O3 + Sc5Si3 + ScSi + Si (dia); IV - Sc3+ + ScSi + Si (dia); V - Sc2O3 + ScSi + Si (dia); VI - Sc3+ + Si (dia); VII - Sc2O3 + Si (dia); VIII - Sc2Si2O7 + Si (dia); IX -Sc3+ + Sc2Si2O7 + Si (dia); X - Sc2Si2O7 + H4SiO4; XI - Sc2Si2O7 + H2SiO42-; XII - Sc2O3 + Sc2Si2O7 + Si (dia).
All equilibria in system are realized below the potential of hydrogen electrode. In the area of electrochemical stability of water thermodynamically stable products are Sc3+(in acidic solutions), Sc2Si2O7 and H4SiO4. Depending on silicon content in the sample, the passivation film on Sc - Si alloys may consist of thorvertite or orthosilicic acid.
Ti - Si system
Titanium silicides are used as cathodes in vacuum arc during the synthesis of super hard nanocrystal coatings [11]. They reveal high corrosion resistance in both reducing and oxidizing environments and can be characterized by stability of the passive state [12].
The following silicides exist in Ti - Si system at 25 °C [4]: Ti3Si, Ti5Si3, Ti5Si4, TiSi and TiSi2. It was reported [13] about the compounds Ti3Si4 and Ti6Si5, however, there are no confirmations of their existence in other literature. The silicide Ti5Si3 has wide homogeneity range at elevated temperature, but it can be considered as stoichiometric compound at 25 °C; other silicides do not have any homogeneity ranges. The maximum solid solubility of Si in (hcp-Ti) equals ~3.3 atomic percent
E.V(s.Ji.e.}
-2 « 2 4 S 3 10 11 i* 1i pH
Fig. 2a. The potential - pH diagram of Ti - Si - H2O system at 298 K, air pressure of 1 bar and nf = 1 mol l-1 without consideration of titanium hy dride and nitride
at 1000 °C and only ~0.7 at. % at 800 °C [4], therefore it is vanishingly small at standard temperature and thus can be neglected. The solubility of Ti in (diamond-Si) is about 10-17 at. % at 1100 °C [14]. The values of standard Gibbs energies of formation of titanium silicides were taken from [15].
The phase diagram of Ti - O system [4] reveals a big number of titanium oxides and suboxides at 298 K - Ti6O, Ti3O, Ti2O, Ti3O2, TiO, Ti2O3, Ti3O5, Magnelli phases TinO2n-1 (4 < n < 10), TiO2. There are no compounds in binary TiO2 - SiO2 system [16]. The standard Gibbs energies of formation of titanium oxides were taken from [17]. Titanium can form anions Ti3+ and TiO2+ in water solution. No titanate-anions can be formed [18].
The potential - pH diagram of Ti - Si - H2O system plotted in accordance with Ti - Si - O state diagram [19] is presented at Fig. 2a. It determines 43 domains of thermodynamic stability of certain phases: I - Ti (hcp) + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); II - Ti6O + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); III - Ti3O + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); IV - Ti2O + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); V - Ti3O2 + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); VI - Ti3+ + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); VII - TiO + ^Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); VIII - Ti3+ + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); IX - TiO + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); X - Ti2O3 + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); XI - Ti3+ + Ti5Si4 + TiSi + TiSi2 + Si (dia); XII - Ti2O3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); XIII - Ti2O3 + Ti5Si4 + TiSi + TiSi2 +H2SiO42-; XIV - Ti3+ + TiSi + TiSi2 + Si (dia); XV - TC2O3 + TiSi + TiSi2 + Si (dia); XVI - ^3 + TiSi + TiSi2 + H2SiO2-; XVII - Ti3+ + TiSi2 + Si (dia); XVIII - Ti2O3 + TiSi2 + Si (dia); XIX - Ti3+ + TiSi2 + H4SiO4; XX - Ti2O3 + TiSi2 + H4SiO4; XXI - Ti2O3 + TiSi2 +H2SiO2-; XXII - Ti3+ + H4SiO4; XXIII - Ti2O3 + H4SiO4; XXIV - Ti2O3 +H2SiO2-; XXV - Ti3O5 + H4SiO4; XXVI - Ti3O5 +H2SiO2-; XXVII - TiO2+ + H4SiO4; XXVIII - Ti4O7 + H4SiO4; XXIX - Ti4O7 +H2SiO2-; XXX - Ti5O9 + H4SiO4; XXXI - Ti5O9 + H2SiO2-; XXXII - Ti6O11 + H4SiO4; XXXIII - Ti6O11 +H2SiO2-; XXXIV - Ti7O13 + H4SiO4; XXXV -Ti7O13 +H2SiO2-; XXXVI - Ti8O15 + H4SiO4; XXXVII - Ti8O15 +H2SiO2-; XXXVIII - Ti9O17 + H4SiO4; XXXIX - Ti9O17 +H2SiO2-; XL - Ti10O19 + H4SiO4; XLI - Ti10O19 + H2SiO2-; XLII - TiO2 + H4SiO4; XLIII - TiO2 +H2SiO2-.
Fig. 2b. The potential - pH diagram of Ti - Si - H2 O sy stem at 229)8 K, air pressure of 1 bae and ft = 1 mol l-1 with consideration of titanium h=dride and nitride
However, titanium has high chemical affinity not only to oxygen but also to hydrogen and nitrogen; therefore electrochemical formation of titanium hydride TiH2 and nitride TiN has to be considered. The diagram taking into account these compounds is presented at Fig. 2b, there are 15 domains of stability of certain phases: I - TiH2 + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + Si (dia); II -TiN + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi 2 + Si (dia); III - TiN + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 + H4SiO4; IV - TiN + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + TiSi2 +H2SiO2-; V - TiN + ^Si + Ti5Si3 + Ti5Si4 + TiSi + H4SiO4; VI - TiN + Ti3Si + Ti5Si3 + Ti5Si4 + TiSi + H2SiO2-; VII - TiN + Ti3Si + Ti5Si3 + Ti5Si4 4SiO4; VIII - TiN + Ti3Si + Ti5Si3 + Ti5Si4 +H2SiO2-; IX - TiN + ^Si + Ti5Si3 + H4SiO4;
3S
+ H4SiO4; VIII - TiN + Ti3Si + Ti5Si3 + Ti5Si4 +H2SiO42-; IX - TiN + Ti3Si + Ti5Si3 + H4SiO4; X - TiN + Ti3Si + Ti5Si3 + H2SiO4-; XI - TiN + Ti3Si + H4SiO4; XII - TiN + Ti3Si + H2SiO4-; XIII - TiO2+ +
Ti3Si + H4SiO4; XIV - TiO2 + Ti3Si + H4SiO4; XV - TiO2 + Ti3Si + H2SiO2-.
In presence of titanium hydride and nitride all its oxides except TiO2 become thermodynamically unstable. There is no domain of stability of metallic titanium. Moreover, presence of hydride and nitride changes the mechanism and order of titanium silicides oxidation. As can be seen from Fig. 2b, full decomposition of titanium silicides to components is thermodynamically unprofitable process. The presence of silicon in system can stabilize titanium and prevent it from formation of hydride and further oxidation. This can be explained by high strength of Ti - Si covalent bonds [12].
Mo - Si system
Molybdenum does not belong to 3d-metals. However, it is the only one metal amongst all 4d-metals that has big importance in metallurgy. Molybdenum silicides are used for making high-temperature thermocouples for measuring temperatures in air up to 1700 °C, electric resistance heaters, working without protective atmosphere, for creating materials to be used in oxidizing environments. Therefore, this system was included into consideration.
The phase diagram of Mo - Si system [4, 20] assumes the existence of molybdenum silicides Mo3Si, Mo5Si3 and MoSi2. All these compounds do not have any ranges of nonstoichiometry. The maximum solid solubility of Si in (fcc-Mo) equals ~2.5 atomic percent at 1500 °C and does not
Fig. 3. The potential - pH diagram ofMo - Si - H2O system at 2988 K, air pressure of 1 bar and b = 1 mol l-1
exceed 1 at. % at 1300 °C [4], therefore it is vanishingly small at standard temperature and thus can be neglected. The values of standard Gibbs energies of formation of molybdenum silicides were taken from [15, 20].
Molybdenum forms stable oxides MoO2, Mo4O11, Mo8Or3, Mo9O26, MoO3 and a variety of metastable phases (Moi7O47, Mo5O14, Mo1sO52) [4]. Metastable compounds were not taken into account in thermodynamic calculations. There is no data about the existence of molybdenum silicates. In water solution molybdenum forms the cations Mo3+ and MoO^ and can be oxidized to various isopolymolybdate-anions with oxidation degree +6 [18, 21 - 23]. According to [24], the following anions are most stable in solution: H2Mo8O2-, HMosO2-, Mo8O4-, H2Mo7O4- , Mo7O2- and MoO^-. The values of standard Gibbs energies of formation for molybdenum oxides are taken from [25] and the values for molybdate anions are calculated according to data from Pourbaix diagram for molybdenum [26, 27].
The potential - pH diagram of Mo - Si - H2O system [28] is presented at Fig. 3; 24 domains of predominance of certain phases can be depicted in it: I - Mo (bcc) + Mo3Si + Mo5Si3 + MoSi2 + Si (dia) ; II - Mo (bcc) + Mo3Si + Mo5Si3 + MoSi2 + ^SiO4 ; III - Mo (bcc) + Mo3Si + Mo5Si3 + MoSi2 + H2SiO42-; IV - Mo (bcc) + Mo3Si + Mo5Si3 + H4SiO4 ; V - Mo (bcc) + Mo3Si + Mo5Si3 + H2SiO42- ; VI - Mo (bcc) + Mo3Si + ^SiO4 ; VII - Mo (bcc) + Mo3Si + H2SiO2- ; VIII - Mo (bcc) + H4SiO4 ; IX - Mo (bcc) + H^iO^ ; X - Mo3+ + H4SiO4 ; XI - MoO2 + H4SiO4 ; XII - MoO2 +HrSiO4-; XIII - MoO2+ + H4SiO4 ; XIV - Mo4Oii + H4SiO4 ; XV - MosO^ + H4SiO4 ; XVI - Mo9O26 + H,SiO4 ; XVII - MoO3 + H4SiO4 ; XVIII - H2MosO2- + H4SiO4 ; XIX - HMosO2- + H4SiO4 ; XX - Mo^- + H4SiO4 ; XXI - H2Mo7O2- + H4SiO4 ; XXII - Mo7O2- + H4SiO4 ; XXIII - MoO^ + H4SiO4 ; XXIV -MoO2~, H2SiO^.
As can be seen from Fig. 3, molybdenum has very narrow domain of active dissolution, because the cations Mo3+ and MoO^ exist only in strongly acidic environments. However, the domain of the oxide passivation of molybdenum is also quite small, and if the environment becomes more and more alkaline, then the domain of transpassivity (which contains all possible isopolymolybdate forms) becomes more and more broad. Generally, electrochemical stability of molybdenum-silicon alloys is
E, V (t h, e.) 2,0 -t-
-2 0 2 * 6 0 10 12 14 16 pH
Fig. 4. Title potential - pH diagram ofMn - Si - H2O system at 298 Km, air pre ssure of 1 bar and at = 1 moll-1
completely determined by silicon content in them. If it is enough to form a continuous passivation film from H4SiO4 on alloy surface, then the active oxidation of molybdenum stops.
Mn - Si system
Manganese doped by silicon is used in manufacturing of rail and structural steels, in metallurgy as the deoxidizer, as the dopant to alloys based on aluminum, magnesium and copper.
There are seven intermetallic phases in Mn - Si system [4]: Mn11Si19, MnSi, Mn5Si3, Mn5Si2, Mn3Si, v-phase (Mn9Si2) and R-phase (Mn6Si). The first five silicides are stoichiometric phases. v-phase has a narrow homogeneity range at elevated temperatures but can be considered as stoichiometric compound at standard conditions. R-phase has a noticeable homogeneity range at 25 °C. Moreover, a solid solution of Si in (cubic-Mn) can be formed (a-phase). According to [29], R-phase in equilibrium with a-phase has the composition of Mn085Si015.
The equation for the excess Gibbs energy of a-phase was taken from [30]. Calculated solid solubility of Si in (cubic-Mn) at 25 °C equals 4.56 at. % [30]. Thermodynamic activities of solid solution components are the following: aMn(a ) = 0.83, aSi(a ) = 3.8 10-28. The solid solubility of Mn in (diamond-Si) is about 10-14 at. % at 1100 °C [14]. The standard Gibbs energies of formation of manganese silicides were collected from various sources [17, 29, 31 - 33].
Manganese forms the oxides MnO, Mn3O4, Mn2O3, MnO2 and Mn2O7 though the last one is not stable in water solutions. There are two manganese silicates at 25 °C - MnSiO3 and Mn2SiO4 [34]. The compound Mn7SiO12 exist only at elevated temperatures [35]. The ions Mn2+, Mn3+, MnO4- and MnO-exist in water solutions. The standard Gibbs energies of formation of manganese oxides, silicates and ions were taken from [17].
The potential - pH diagram of Mn - Si - H2O system [30] is presented at Fig. 4. 42 domains of thermodynamic stability of certain phases can be depicted in the diagram: I - a-phase (Mn) + R-phase (Mn0 85Si015) + v-phase (Mn9Si2) + Mn3Si + Mn5Si2 + Mn5Si3 + MnSi + Mn11Si19 + Si (dia); II - Mn2+ + Mn085Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + MnSi + Mn11Si19 + Si (dia); III - Mn2+ + Mn0 85Si015
+ Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + MnSi + Mn11Si19 + H4SiO4; IV - Mn2+ + Mn085Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + MnSi + H4SiO4; V - Mn2+ + Mn0.85Si0.15 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + H4SiO4; VI - Mn2+ + Mn085Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + H4SiO4; VII - Mn2+ + Mn085Si015 + Mn9Si2 + Mn3Si + H4SiO4; VIII - Mn2+ + Mn0.85Si0.15 + M^Si2 + ^SiO4; IX - Mn2+ + Mn0.85Si0.15 + H4SiO4; X - Mn3+ + Mn0.85Si0.15 + H4SiO4; XI - Mn2+ + Mn085Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + MnSiO3; XII - Mn2+ + Mno.85Si0.15 + Mn9Si2 + Mn3Si + Mn5Si2 + MnSiO3; XIII - Mn2+ + Mn085Si0.15 + Mn9Si2 + Mn3Si + MnSiO3; XIV - Mn2+ + Mn0 85Si015 + Mn9Si2 + MnSiO3; XV - Mn2+ + Mn0.85Si0.15 + MnSiO3; XVI - Mn2+ + Mn0 85Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn2SiO4; XVII - Mn2+ + Mn085Si015 + Mn9Si2 + Mn3Si + Mn2SiO4; XVIII - Mn2+ + Mn0 85Si015 + Mn9Si2 + Mn2SiO4; XIX - Mn2+ + Mn085Si0.15 + Mn2SiO4; XX - a + Mn0 85Si0.15 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + MnSi + Mn11Si19 + H4SiO4; XXI - a + Mn085Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + MnSi + Mn11Si19 + H2SiO4- ; XXII - a + Mn0.85Si0.15 + Mn9Si2 + Mn3Si + M^Si2 + Mn5Si3 + MnSi + ^SiO4; XXIII - a + Mn0.85Si0.15 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + MnSi + H2SiO4- ; XXIV - a + Mn085Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + H4SiO4; XXV - a + Mn085Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + H2SiO4-; XXVI - a + Mn0 85Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn5Si3 + MnSiO3; XXVII - a + Mn085Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + MnSiO3; XXVIII - a + Mn0 85Si015 + Mn9Si2 + Mn3Si + Mn5Si2 + Mn2SiO4; XXIX - a + Mn0 85Si015 + Mn9Si2 + Mn3Si + Mn2SiO4; XXX - a + Mn0 85Si015 + Mn9Si2 + Mn3Si + Mn2SiO4 + MnO; XXXI - a + Mn085Si015 + Mn9Si2 + Mn2SiO4 + MnO; XXXII - a + Mn085Si015 + Mn2SiO4 + MnO; XXXIII - Mn0 85Si015 + Mn2SiO4 + MnO; XXXIV - Mn085Si015 + Mn2SiO4 + Mn3O4; XXXV - Mn085Si015 + MnSiO3 + Mn3O4; XXXVI - Mn085Si015 + MnSiO3 + Mn2O3; XXXVII -
Mn0.85Si0.15 + MnSiO3 + MnO2; XXXVIII - Mn0.85Si0.15 + MnO2 + H4SiO4; XXXIX - Mn0.85Si0.15 + MnO2 + H2SiO42- ; XL - Mn0. 85Si0 .15 + H4SiO4 + MnO- ; XLI - Mn0 . 85Si> .15 + MnO42-, H2SiO42- ; XLII -Mn0.85Si0.15 + MnO4, H2SiO4".
The corrosion-electrochemical behavior of manganese-silicon alloys is determined by the acidity of environment and the value of equilibrium potential. In acidic environments the selective corrosion of manganese takes place and it forms the cations Mn2+ and silicon from alloy is oxidized to silicic acid. In neutral and alkaline environments oxidation can end with formation of passivation film consisting of silicates Mn2SiO4 or MnSiO3.
Fe - Si system
Iron-silicon is a very important binary system for metallurgy. Iron silicides are perspective materials; they are well-known for their unusual magnetic, optical and thermodynamic properties [36]. Iron-silicon alloys can be found in Earth's core [37], they are using in technology as thin films [38] and nanowires [39]. Moreover, many important ternary and multicomponent systems, including Fe - Si binary system are of great interest to the researcher.
According to the Fe - Si phase diagram [4], several intermetallic phases exist in system: Fe3Si7, FeSi2, FeSi, Fe5Si3 and Fe2Si. But only FeSi2 and FeSi are thermodynamically stable at standard conditions. FeSi has a narrow homogeneity range [4, 40], from FeSi0961 to FeSi1 033, which does not depend on temperature. The solid solubility of Si in (bcc-Fe) is equal to about 25 atomic percent at ambient temperatures and silicon can form three types of solutions. The first one is the disordered phase, in which only short range ordering exists (a-phase), two others have long range atomic ordering - a4-phase and a1-phase. Additionally, a miscibility gap exists between a2 and a1 phases. Moreover, a fully ordered a1-
ErV(s.h. e.)
■z.G-I—t—«—I—«—I—•—I—«—I—*—i—«—I—«—i—
0 2 4 6 & 10 12 14 pH
Fig. 5. The potential - pH diagram ofFe - Si - H2O system at 298 K , air pressure of 1 bar and at = 1 mol l1
phase is often treated as independent compound Fe3Si [41]. The expression for the excess Gibbs energy of a-phase was taken from [42]. The activities of the components of solid solution corresponding to maximum silicon solubility in arphase are the following: a Fe(a )= 0.473 and aSi(a ) = 5.5 • 10-20 meaning strong negative deviations from ideal behaviour. The solid solubility of silicon in disordered a-phase at 25 °C is about ~11 at. %. The activities of the components in this solution are equal to a Fe(a )= 0.13, aSi(a) = 5.0 • 10-20. The solubility of Fe in (diamond-Si) is about 10 17 at. % at 1100 °C [14]. The standard Gibbs energies of formation of iron silicides were collected from [17, 42 - 45].
Iron can form the following oxides: FeO, Fe3O4 and Fe2O3. However, FeO is not thermodynamically stable at temperatures below 570 °C [9]. Fe3O4 and Fe2O3 have noticeable homogeneity ranges at elevated temperatures but they are stoichiometric compounds at 25 °C. Only one ternary compound exists in Fe - Si - O system, it is Fe2SiO4 [46]. Another compound, FeSiO3, exists only at elevated temperatures and pressures [47]. A solid solution of Fe3O4 in Fe2SiO4 can be formed [48] and its maximum solubility at 25 °C equals 6.85 10-5 at. %. In addition, up to 1.54 10-3at. % of SiO2 can be dissolved in Fe2O3 [48]. The values of standard Gibbs energies of formation of iron oxides were obtained from [17, 25, 49], the values for Fe2SiO4 - from [46, 50, 51].
The following ions of iron can be formed in water solution: Fe2+, Fe3+, FeO;;-. The potential - pH diagram of Fe - Si - H2O is presented at Fig. 5, and 17 domains of stability of certain phases can be depicted in it: I - a-phase (Fe) + FeSix + FeSi; + Si (dia); II - a + FeSix + FeSi; + H4SiO4; III - a + FeSix + FeSi2 +H2SiO2-; IV - a + FeSix + H4SiO4; V - a + FeSix + H2SiO2-; VI - a + H4SiO4; VII - a +H2SiO2-; VIII - a + Fe;SiO4; IX - Fe2+ + HtS^; X - Fe2+ + Fe;S^; XI - [Fe;(Fe, SOO4] + Fe;S^; XII - Fe;O3 + Fe2SiO4; XIII - Fe3+ + H4SiO4; XIV - Fe2O3 + H4SiO4; XV - Fe2O3 + H2SiO2-; XVI - FeO2- + H4SiO4; XVII - FeO2- ,H2SiO2-.
In contrast to other systems, silicon plays a minor role in corrosion properties of iron silicides. The domain of thermodynamic stability of Fe2SiO4 lies almost below the potential of hydrogen electrode. The passivation film consists of Fe2O3 or H2SiO4 depending on silicon concentration.
E. V£9.h.e.) 2.0 T-
0 2 fl 0 fl 10 12 14 16 16 pH
Fig. 6. The potential - pH diagram of Co - Si - H2O system at 298 K, air pressure of 1 bar and at = 1 mol F1
Co - Si system
Cobalt and silicon are the basic components of cobalt-based amorphous alloys [52], which are of interest to the researchers due to their unusual magnetic, mechanical, electric properties as well as high corrosion resistance.
The compounds Co2Si, CoSi and CoSi2 exist in Co - Si system at 25 °C [4, 53]. Moreover, a solid solution of Si in (hcp-Co) (e-phase) exists in cobalt-rich region of Co - Si phase diagram. The expression for the excess Gibbs energy of this phase was provided in paper [42]. Calculated maximum solid solubility of silicon at 25 °C [54] equals 10.7 at. %, thermodynamic activities of this saturated solid solution are the following: aCo(e) = 0.27, aSi(e) = 4 • 10-16. The solubility of Co in (diamond-Si) is about 10-17 at. % at 1000 °C [14]. The values of standard Gibbs energies of formation of cobalt silicides were collected from [17, 25, 42, 53].
Cobalt can form a variety of oxides - CoO, Co3O4, Co2O3 and CoO2; the first two ones have a noticeable homogeneity range, which covers the compositions from CoO to CoOL07 for "CoO" phase and from CoOL3i to CoO141 for "Co3O4" phase [4, 55]. Only one ternary compound, Co2SiO4, exists in Co - Si - O system [56]. The values of Gibbs energies of formation of cobalt oxides and silicides were obtained from [17, 25, 55].
Cobalt can form the cation Co2+ and the anions HCoO- and CoO^ in water solution. The potential - pH diagram of Co - Si - H2O system [54] is presented at Fig. 6.
24 domains of stability of certain phases can be depicted at the diagram: I - e-phase (Co) + Co2Si + CoSi + CoSi2 + Si (dia); II - e + Co2Si + CoSi + CoSi2 + H4SiO4; III - e-#3a(Co) + Co2Si + CoSi + CoSi2 + H2SiO42- ; IV - e + Co2Si + CoSi + H4SiO4; V - e + Co2Si + CoSi + H2SiO42- ; VI - e + Co2Si + H4SiO4; VII - e + Co2Si + H2SiO2" ; VIII - e + HSO4; IX - e + H2SiO2" ; X - e + Co2SiO4; XI - ^SiO4 + Co2+; XII - Co2SiO4 + Co2+; XIII - Co2SiO4 + "CoO"; XIV - "CoO" + H2SiO2- ; XV - HCoO-, H2SiO2" ; XVI - Co2SiO4 + HCoO- ; XVII - Co2SiO4 + "^O/'; XVIII - "Co3O4" + HSO4; XIX -"Co3O4" + H2SiO2" ; XX - Co2O3 + H4SiO4; XXI - Co2O3 + H2SiO2" ; XXII - CoO2 + H4SiO4; XXIII -H4SiO4 + CoO2- ; XXIV - CoO;;-, H2SiO2-.
If the content of silicon in system is sufficient to form a continuous layer of cobalt silicate, it becomes a primarily passivation film. Otherwise, the protecting film consisting of cobalt oxides "CoO" and "Co3O4" would be formed. In strongly acidic environments, where cobalt dissolves to form Co2+, the oxidation film consists of orthosilicic acid.
Ni - Si system
According to the Ni - Si phase diagram [4, 57], several phases exist in system at standard conditions. There are six intermediate phases: p1-phase (Ni3Si), y-phase (Ni5Si2 or Ni31Si12 [58]), 5-phase (Ni2Si), e-phase (Ni3Si2), NiSi and NiSi2. Although almost all nickel silicides have a narrow homogeneity ranges at 800 °C [57], they can be treated as stoichiometric phases at standard temperature. The solubility of Ni in (diamond-Si) does not exceed 10-16 at. % at 800 °C [14]. At the nickel-rich part of the system, the solid solution of Si in (fcc-Ni) can be formed (a-phase). There is no published information about silicon maximum solid solubility at 25 °C, but it is relatively high (more that 10 atomic percent) at 1000 °C [4]. Most CALPHAD assessments of Ni - Si system [57, 59 - 61] consider prphase as an ordered solid solution and use an order-disorder contribution model [62] for its Gibbs energy function. Moreover, they use sublattice model [63] for Gibbs energy function of a-phase. But, since p1-phase can be assumed as stoichiometric and composition invariant at standard temperature, there is no necessity to pay attention on order-disorder transformations in it and use such complicated thermodynamic models. Therefore, simpler thermodynamics model was used to describe the excess Gibbs energy of a-phase [64]. Calculated solid solubility of Si in (fcc-Ni) at 25 °C equals 2.5 at. %. The activities of solid solution components are the following: aNi(a ) = 0.927, aSi(a ) = 9 • 10-36.
It is well known, that nickel monoxide (NiO) and higher nickel oxides (Ni2O3, NiO2) can form a continuous series of solid solutions, which corresponds to a variable composition phase NiOx [65].
A single ternary compound exist in Ni - Si - O system, it is nickel orthosilicate Ni2SiO4 [46]. The values of standard Gibbs energies of formation of nickel silicides and nickel silicate were collected from various sources and analyzed by the authors of [64]. The expression for the standard Gibbs energy of formation of phase NiOx was also proposed in [64].
Nickel can form the following ions in water solutions: Ni2+, HNiO- and NiO4- [2]. The potential -pH diagram of Ni - Si - H2O system [64] is presented at Fig. 7a. 24 domains of thermodynamic stability of certain phases can be depicted at the diagram: I - a-phase (Ni) + Ni3Si + Ni5Si2 + Ni2Si + Ni3Si2 + NiSi + NiSi2 + Si (dia); II - a + Ni3Si + Ni5Si2 + Ni2Si + Ni3Si2 + NiSi + NiSi2 + ^SiO4; III -a + Ni3Si + Ni5Si2 + Ni2Si + Ni3Si2 + NiSi + NiSi2 +H2SiO2- ; IV - a + Ni3Si + Ni5Si2 + Ni2Si + Ni3Si2 + NiSi + H4SiO4; V - a + Ni3Si + Ni5Si2 + Ni2Si + Ni3Si2 + NiSi + H2SiO2- ; VI - a + Ni3Si + Ni5Si2 + Ni2Si + Ni3Si2 + H4SiO4; VII - a + Ni3Si + Ni5Si2 + Ni2Si + Ni3Si2 + H2SiO2- ; VIII - a + Ni3Si + Ni5Si2 + Ni2Si + H4SiO4; IX - a + Ni3Si + Ni5Si2 + Ni2Si + H2SiO2- ; X - a + Ni3Si + Ni5Si2 + ^SiO4; XI - a + Ni3Si + Ni5Si2 + H2SiO2- ; XII - a + Ni3Si + H4SiO4; XIII - a + Ni3Si + H2SiO2- ; XIV - a + H4SiO4; XV - a + H2SiO2- ; XVI - a + Ni2SiO4; XVII - Ni2+ + H4SiO4; XVIII - Ni2+ + Ni2SiO4; XIX - NiOx + Ni2SiO4; XX - HNiO- + Ni2SiO4; XXI - NiOx + H4SiO4; XXII - NiOx + H2SiO2- ; XXIII - NiO4- + H4SiO4; XXIV - NiO2-, H2SiO2-.
In addition nickel can form the hydride Ni2H [66]. The possibilities of electrochemical reduction of nickel to its hydride are discussed in [67]. The information about the Gibbs energy of formation for
E. V (s. h. O.)
-
0 2 i 6 fl 10 12 14 IS 16 pH
Fig. 7a. The potential - pH diagram of Ni - Si - H2O system at 298 K, air pressure of 1 ber and at =1 mol l-1 without consideration of nickel hydriOe
E.Vfs.n.e.)
0 1 4 fi 8 10 12 14 16 18 pH
Fig. 7b. The potential - pH diagasm of NTi m Si - H2O sy stem at 2y8 K, air pressure of" 1 bar and af = 1 mol l-1 with consideration of nickel hydride
nickel hydride Ni2H was taken from paper [68]. The potential - pH diagram of Ni - Si - H2O system with consideration of nickel hydride is presented at Fig. 7b.
17 domains of thermodynamic stability of certain phases can be depicted at the diagram: I - Ni2H + Ni3Si + Ni5Si2 + Ni2Si +Ni3Si2 + NiSi + NiSi2 + Si (dia); II - Ni2H + Ni3Si + Ni5Si2 + Ni2Si + Ni3Si2 + NiSi + NiSi2;III - Ni2H + Ni3Si + Ni5Si2 + Ni2Si + Ni3Si2 + NiSi; IV - Ni2H + Ni3Si + Ni5Si2 + Ni2Si + Ni3Si2; V - Ni2H + Ni3Si + Ni5Si2 + Ni2Si; VI - Ni2H + Ni3Si + Ni5Si2; VII - Ni2H + Ni3Si; VIII - Ni2H + Ni2SiO4; IX - Ni (a) + Ni2SiO4; X - Nia+ + H4SiO4; XI - Nia+ + Ni2SiO4; XII - NiOx + Ni2SiO4; XIII -
HNiO- + Ni 2SiO4; XIV - NiOx + ^SiO,,; XV - NiOx + H2SiO42-; XVI - NiO42- + H.SiO,,; XVII - NiO42-+ H2siO4-.
In presence of nickel hydride orthosilicic acid becomes thermodynamically unstable and the mechanism of nickel silicides decomposition slightly changes.
Generally, the corrosion-electrochemical behaviour of iron, cobalt and nickel silicides is very similar. In acidic environments the passivation layer on Fe - Si, Co - Si h Ni - Si system alloys will consist of pure H4SiO4, if there will be no F- ions in solution, and metal from alloys will form the cations Fe2+, Co2+ and Ni2+, respectively. In neutral and alkaline environments three cases can be considered. If silicon content in alloys is high, it will be sufficient to form a persistent passivation layer of SiO4 • 2H4Oon the surface. At lower silicon content in alloy it will be sufficient only to form a persistent layer of silicates Fe4SiO4, Co4SiO4 or Ni4SiO4. If silicon content in alloy is very low, the protective film will consist from corresponding metal oxides, and silicon in form of local inclusions of silicates will be present in its inner layer.
Cu - Si system
Alloying of copper alloys with silicon allows increasing their durability, plasticity, improving their mechanical, foundry and anticorrosive properties. Siliceous bronzes and brasses are very cheap substitutes for tin bronzes. In addition, silicon can be included in other copper-based alloys with aluminum, nickel, manganese. Therefore, Cu - Si system is a very important binary system for the metallurgy and chemical technology.
In Cu - Si system 25 °C the existence of the following copper silicides was approved [4, 69]: Cu 56Sin (y-phase), Cu^Si4 (e-phase) and Cui9Si6 (n'-phase). Despite all copper silicides have, in greater or lesser degree, the homogeneity range, it is negligibly small at the standard temperature, and the silicides can be assumed as daltonides. Additionally, a solid solution of Si in (fcc-Cu) (a-phase) exists in copper-rich copper of system. The expression for the excess Gibbs energy of a-phase was taken from [69]. The calculated silicon maximum solid solubility in a-phase at standard temperature slightly exceeds 4 at. %. Thermodynamic activities of the components of saturated solid solution are the following: aCu(a) = 0.94, aSi(a, = 5.6 10-8. The standard Gibbs energies of formation of copper silicides were estimated in [70].
In addition to well known oxides Cu4O and CuO copper can also form a Cu4O3 compound [71]. Only one ternary compound exists in Cu - Si - O ternary system, it is CuSiO3 [46]. The standard Gibbs energies of formation of copper oxides and silicate were collected from [17, 25, 71].
Copper in water solution can form the the cation Cu2+ and the anions HCuO- and CuO4-, however HCuO- is stable only at low concentrations of copper species in solution [70]. Despite copper hydride CuH08 is not stable in air, it was shown [72 - 74], that it can be obtained during copper electrochemical reduction. The value of standard Gibbs energy of formation for copper hydride was determined by the authors of [75, 76].
The potential - pH diagram of Cu - Si - H4O system [70] is presented at Fig. 8 and it determines 22 domains of prevalence of certain phases: I - CuH08 + y-phase (Cu56 Siii) + e-phase (Cu15Si4) + n''-phase (Cu19Si6) + Si (dia); II - CuH08 + y + e + n'' + H4SiO4; III - CuH08 + y + e + n'' + H2SiO4-; IV - a-phase (Cu) + y + e + n'' + H4SiO4; V - a + y + e + n'' + H2SiO4-;
0 2 J 6 ft 10 12 14 ie pH
Fig. 8. "TTlie; potential - pH diagram of Cu - Si - H2O sy stem at 298 K, air pressure of 1 bar and a =1 mol a1
VI - a + y + e + H4SiÛ4; VII - a + y + e + H2SiO42- ; VIII - a + y + ^SiO,,; IX - a + y + H2SiO42- ; X - a + H,SiO,; XI - a + H2SiO;;- ; XII - Cu2+ + H,SiO,; XIII - CU4O + H,SiO,; XIV - CU4O + H2SiO;;- ; XV - Cu2+ + CuSiO3; XVI - Cu4O + CuSiO3; XVII - CuO + CuSiO3; XVIII - CuO +
H2SiO4- ; XIX - CuO4- + H2SiO4- ; XX - CU4O3 + CuSiO3; XXI - CU4O3 + H,SiO,; XXII - CU4O3 + H2sio4-.
The electrochemical behaviour of Cu - Si system strongly differs depending on the composition of system and solution. Many compounds including copper and silicon oxides, copper silicate and various ionic species have the domains of prevalence in the area of electrochemical stability of water. The composition of passivation layer on Cu - Si alloys will vary depending on environmental conditions.
Zn - Si system
There are no intermediate compounds in Zn - Si system. Moreover, hcp-Zn and diamond-Si are insoluble in each other and form only mechanical mixture at ambient temperature [4].
Zinc can form two oxides, ZnO and ZnO4 [77]. One zinc silicate, ZnSiO3 can be formed in Zn -Si - O system [78]. Another compound, willemite (Zn4SiO4) can be formed only at high temperatures or high pressures [46]. The standard Gibbs energies of formation of zinc oxides and silicate were collected from [17, 25, 79]. Zinc in water solution can form the cation Zn2+ and the anions HZnO- and ZnO4-, however HZnO- is stable only at low concentrations of zinc species in solution [80].
The potential - pH diagram of Zn - Si - H4O system is presented at Fig. 9. There are 12 domains corresponding to stability of certain phases: I - Zn (hcp) + Si (dia); II - Zn (hcp) + H4SiO4; III - Zn (hcp) +H2SiO4- ; IV - Zn (hcp) + ZnSiO3; V - Zn2+ + ^SiO4; VI - Zn2+ + ZnSiO3; VII - ZnO + ZnSiO3; VIII -ZnO4- + ZnSiO3; XI -ZnO4-, H2SiO;;- ; X - ZnO4 + ZnSiO3; XI - ZnO4 + H4SiO4; XII - ZnO4 + H2SiO4-.
E, V (S. h. e.) 3,0-,-
0 2 4 6 В 10 12 14 16 19 PH
Fig. 9. The potential - pH diagram of Zn - Si - H2O system at 298 K, air pressure of 1 bar and ai = 1 mol l-1
Zinc silicate has a very wide domain of thermodynamic stability and determines the corrosion behaviour of Zn - Si system. If the silicon content in system is high enough, the protective film consisting of ZnSiO3 would be formed, otherwise zinc oxide ZnO would form the passivity layer.
Conclusions
The dotted lines on the diagrams (Figures 1 through 9) mark the potentials of hydrogen and oxygen electrodes. The area between these lines depicts the domain of electrochemical stability of water, which is of most interest to researchers. Thermodynamic calculations show that high corrosion resistance of transition metals silicides and metal-silicon alloys in this area is related with formation of the protective films consisting of metals silicates on their surface. These films are much stable in chemical and electrochemical term than films from simple metal oxides. Therefore, silicon reveals good influence on the corrosion resistance of 3d-metals.
The usage of thermodynamic modelling of corrosion properties is not limited to binary systems. It is the powerful instrument that can complement the results of experiments and broaden the knowledge about corrosion-electrochemical behaviour of multicomponent silicon-containing alloys like siliceous bronzes and brasses.
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