Phase equilibria in the system Fe-Mo-Cl-H-O

PHASE EQUILIBRIA IN THE SYSTEM Fe-Mo-CI-H-O

UDC 621.762.242

Yu. G. Gurevich, N. V. Germanyuk, and N. R. Frage

To alloy the surface of structural components, processes of diffusion transfer of alloying elements through a chloride phase are widely used; these processes are based on the chlorination of metallic solutions or metal oxides. In the present work we determine the thermodynamic conditions of chlorinating an alloy in the system Fe-Mo-CI-H-O at 1273-1373"K. According to [I], the oxides FeO, MoO 3 are stable in the system Fe-Mo-O at 1273-1373"K. We determined the conditions of equilibrium of oxide phases with the solid solution [Fe-Mo]s dealing with the chemical interactions:

1 O2fg)= FeO~; [Fe]s + -~3 [Mols+ -~ O~g) =

b l o 0 3 s);

[blo]s+ O ~ = MoO~;

Kz= K~ =

1 _ttFe.ro, nil"1

aMo. P ~

(i)

;

.

'

1 Ks =

aMo.Po, "

(2)

(3)

We calculated the temperature dependences of the constants of equilibrium of the reactions according to the data of [I], and also by taking the following balance equation into account: xw+X~o=

1.

(4)

The activity of molybdenum in the solid solution was determined with the aid of the experimental data of [2, 3]:

lgaM~ lgxM~

13o5 T (1 -- XMo)~.

(5)

With T - const, no more than two oxide phases can be simultaneously in equilibrium wlth the solid solution and with the gas phase (number of degrees of freedom C - 0). The parameters of four-phase equilibria are unambiguously determined. If one oxide phase is in equilibrium with the solid solution and the gas phase, the composition of the solid solution is determined by the partial pressure of oxygen. We calculated the composition of the equilibriumphases by solving Eqs. (1)-(5). The results of the calculation are presented in the form of a diagram of the phase equilibrium of the system Fe-Mo-O (Fig. i). The llne ABC is the geometric locus of the points of equilibrium [Fe-Mo],-FeO(s)-MoO2(,)-Oz(s) at different te mperatures; to the right of the llne are the isotherms of equilibrium . . . . . . [Fe-Mo]s-MoO2(,)02(s), to the left [Fe-Mo]~-FeO(,)-O2(s). Equilibrium of the solid solution with the oxide MoO 3 in the presence of iron is not attained. It follows from the calculations that with all compositions of the alloy x ~ < 0.1455 (1373*K) the stable oxide phase is wQstite. Under the conditions we deal w i t h , the molybdenum oxides are reduced, and Mo passes into the solution. The same conclusions were arrived at in [2, 4]. According to the data of [5], in the system Fe-Mo-CI at 1273-1373~ the stable gaseous chlorides Feel2, MoCl 5 are in equillbrlumwith the metallic solution. Having analyzed the chemical reactions of the formation of chlorides and the corresponding balance equations, and

Kurgan Engineering Institute. Translated from Poroshkovaya Metallurglya, No. I(301), pp. 43-46, January, 1988. Original article submitted November 26, 1986.

0038-5735/88/2701-0041512.50

9 1988 Plenum Publishing Corporation

41

I~ C/%\

873 ~

i/

~ac, ' kPa ziO I41

/ j'zJ

12

I 2

O

~K_ I f z~-Zg xMa

2

20

I 0

I

Fig. 1 Fig. i.

I

O,00Z o, oo~

o, oo5

Jl~,

f

A...

Fig. 2

Equilibrium of phases in the system Fe-Mo-O.

Fig. 2. Effect of the composition of the gas phase on the suitability of the alloy iron-molybdenum for chlorination. PHz = I0 (i), 30 (2), 50 kPa (3).

TABLE 1. Phase

Parameters of Equilibrium of the System [Fe-Mo],-Gas Partial pressure of chlorides~ kPa

T~ OK 1273

1373

XMo

aMo

0,0020 0,0600 0,9999 1,0000 0,0020 0,0600 0,9999 1,0000

0,0210 0,4830 1,0000 1,0000 0,0177 0,4148 1,0000 1,0000

MoCI~ 1,3754. I0 -a~ 3,6742.10 -15 6,5170.10 -5 98,4600 2,4810-10 -m 6,7529.10 -15 1,3947.10 -4 96,6500

l

FeC1,

CI,

100,00 I00,00 100,00 -100,00 i00,00 i00,00 --

5,2188.10 -7 5,5408.10 -7 5,2083. I0 -11 1,5430 1,5400.10 -8 1,6351-10 -8 1,5370.10 -I~ 3,3340

with a view to the results of [6] we determined the parameters of equilibrium solid solutiongas phase. It follows from Table 1 that when the alloy iron-molybdenum is chlorinated, iron chloride predominates in the gas phase. Pure molybdenum interacts with chlorine but when even a small amount of Fe is introduced into the system, chlorination of Mo ceases in fact. Introduction of hydrogen into the system has a greater effect on the process of chlorination of the alloy. Figure 2 shows the dependence of the partial pressure PFeCl2 on the composition of the alloy and on the hydrogen content of the gas phase. We can see t h a t a n addition of hydrogen reduces the pressure of the iron chloride and impairs the conditions of chlorination of iron from the alloy Fe-Mo. When molybdenum interacts with chlorine in the presence of oxygen, it is posslble that in addition to chlorides, oxychlorides [6] form, too. However, there are insufficient thermodynamic data available for taking these compounds into account. Since molybdenum is practically not chlorinated in the presence of iron, and the stable oxide phase is wflstite, we dealt with the equilibrium of the solid solution, of wfistite, and of the gas phase in the system Fe-Mo-CI-H-O. We calculated the parameters of equilibrium for P~t - I00 kPa and T 1373"K. The composition of the solid solution and the partial hydrogen pressure were determined by the equation

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TABLE 2. Parameters of Equilibrium [Fe-Mo],-FdO(,)-Gas Phase in the System Fe-Mo-CI-H-O at 13730K I

Partial pressures of components of the gas phase, kPa

XMo

aMo

0,001O

0,0089

0,0050

0,0436

0,0100

0,0854

H,

HzO

10,0O0 30,000 50,000 10,000 30,000 50,000 10,000 30,000 50,000

8,2600 24,7700 41,2900 8,2900 24,8700 41,4600 8,3300 25,0000 41,6700

HCI 24,6500 25,2500 7,6200 24,6800 25,2400 7,4900 24,7300 25,2300 7,3300

FeCIs 57.0900 19,9800 1,0900 57,0300 19,8900 1,0500 56,3500 19,7700 1,0000

MoCI~,IO*' 306,900 22,204 0,0154 1512,6315 108,5567 6,9700 2992,9618 212,2120 0,1225

PFeCI.+I 3504"10 -16" aMo ps/2 (I,049"PH,~ u2 D1/2 , Z~/: " FeCIs+ . XFe ] "--FeCI,+ + 0 , 8 2 5 P ~ ' + PH, = 1. XFe

(6)

The results of the calculation are presented in Table 2; it follows from it that with hydrogen content of more than 50 kPa and P~t - I00 kPa wQstlte is practically completely reduced, and the iron passes into the solution. At the same time, in consequence of the high partial pressure of the water, chlorlnatlon of the alloy also ceases. Thus, with PH2 " 50 kPa and xMo -- 0.010 PFeCI2 -- i kPa. Conclusions. Under the conditions we dealt with, only wQstlte is stable in the system Fe-Mo-O while molybdenum is reduced and passes into the solution. Practically with all compositions of the solid solution iron is predominantly chlorinated, and it passes into the molybdenum. Therefore the alloy iron-molybdenum can be obtained through the chloride phase only by chlorination of pure molybdenum. Chlorination of iron oxides and molybdenum oxides in the presence of hydrogen is inefficient because it is difficult to ensure the necessary composition of the gas phase. The results of the present work can be used for working out technological regimes of alloying the surface of iron objects through the chloride phase and for obtaining powdered alloys Iron-molybdenum.

LITERATURE CITED 1. 2.

3. 4.

I.S. Kulikov, Reduction of Metals [in Russian], Metallurglya, Moscow (1975). A . S . Grlntsov, N. R. Frage, Yu. G. Gurevlch, et al., "Joint reduction of molybdenum and iron from their oxides at low temperatures," Izv. Vyssh. Uchebn. Zaved., Chern. Metall., No. 7, 5-7 (1977). T. Nishlzawa, "Thermodynamic study of the Fe-Mo-C system at 1000~ Scand. J. Met., i, 41-48 (1972). A . S . Grlntsov, I. V. Uvarova, and V. V. Skorokhod, "Investigation of the process of reduction and alloy formation in the system Fe-Mo-O," Izv. Akad. Nauk SSSR, Met., No. 2, 7-11

5. 6.

(1976).

V . P . Glushko (ed.), in: Thermal Constants of Substances (Handbook) [in Russian], Issue 7, VINITI, Moscow (1974), p. 130. U . D . Veryatin, V. P. Mashirev, N. G. Ryabtsev, et al., Thermodynamic Properties of Inorganic Substances (Handbook) [in Russian], Atomlzdat, Moscow (1965).

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