M E T H O D S OF I N V E S T I G A T I O N

AND T H E P R O P E R T I E S

OF POWDER M A T E R I A L S

CORROSION RESISTANCE OF CHROMIUM CARBIDES IN CHLORIDECONTAINING NONAQUEOUS MEDIA UDC 620.193.01

S. G. Babich, V. M. Knyazheva, V. B. Kozhevnikov, O. S. Yurchenko, and Yu. P. Kolosvetov

Industry is extremely in need of materials suitable for operation in highly-aggressive acid media, above all else chloride-containing ones, both aqueous and nonaqueous. Judging from the data of [1-4], chromium carbides may be promising under these conditions. In this work an investigation was made of the possibility of use of chromium carbides as corrosionresistant materials in media of this type. In addition, the reasons were revealed for the uniquely high tendency of chromium carbides toward self-passivation, which is responsible for their resistance in a large number of media, in comparison with chromium and chromenickel steels. The investigations were made on single-phase (content of the basic phase, according to the data of x-ray diffraction analysis, was not less than 95%) compact specimens of the carbides Cr23C6, CrTCs, and Cr3C 2 obtained by hot pressing from previously synthesized powder of these compounds [ 4 ] . The corrosion and electrochemical measurements were made using the method of [4]. The methanol was dehydrated in accordance with the recommendations of [5]. The residual water content in the dried methanol was less than 0.1%. The character and directivity of the Cr-C bonds, the structure of the valence zone of the carbides, and also the passive films formed on them were investigated by the method of x-ray electron spectrometry on an Eskalab-5 spectrometer. From Fig. 1 it follows that in concentrated HCI at 295 K the corrosion potentials Eco r of the carbides CrTC 3 and Cr3C 2 are established in the area of the passive, and of the carbide Cr23C 6 of the transition states.* An increase in the acid temperature to 345 K has little influence on the Eco r of Cr3C 2 carbide but shifts the corrosion potential of CrTC s and Cr~3C 6 toward more negative values, into the transition and active areas, respectively, which is caused by acceleration of the corrosion process. Under the highly aggressive conditions considered, especially at 345 K, a clear relationship to composition of the corrosion properties of the chromium carbides, which improve in the order Cr23C6-CrTC3-Cr~C2, is observed in the area of potentials of the active-passive transition. The departure of the carbide from the passive state as the result of the activating action of chloride ions and temperature is also made more difficult in the same order with an increase in the share of the Cr-C bond~ The high tendency toward passivation and stability of the passive state of the chromium carbides advantageously distinguishes these compounds from chromium and stainless steels, which are extremely sensitive to these factors. For example, in 5 N HCI solution at 345 K~, in contrast to chromium carbides, they are not able to shift into the passive state (Fig. I, curve 4). Let us turn our attention to the fact that in the passive state the solution currents of the CrvC3 and Cr2~C 6 carbides both at 295 and at 345 K are close to each other while for Cr3C 2 they are substantially higher (Fig. i, curves 3 and 3'). As the result of special tests it was established that one of the reasons for this is oxidation of the carbon component of Cr3C 2 carbide. The high tendency of chromium carbides toward passivation in aqueous HCI solutions made it possible to assume that this extremely valuable property of them may also appear in

*In Figs. 1 and 2 Eco r is designated by horizontal broken lines. tials are given relative to a normal hydrogen electrode.

The values of the poten-

Scientific-Research Institute for Physical Chemistry. Kiev Teaching Institute. Translated from Poroshkovaya Metallurgiya, No. 5(305), pp. 45-48, May, 1988. Original article submitted March 30, 1987.

0038-5735/88/2705-0373 $12.50

9 1988 Plenum Publishing Corporation

373

_

E, V

-0,5 -

/

-'-'-~

E,V --

~5 I,

3 7,0

1,5 -2

1 --1

I 0

Fig. 1

I I I ~f~A/M 2

_

--

7

0,0

6~ -2

i I -7

I 0

I

!

I 2 Zgi,AIm 2

Fig. 2

Fig. i. Anodic potentiodynamic curves (1.44 V/h) recorded on the carbides Cr23C e (I, i'), CrvC 3 (2, 2'), and Cr3C 2 (3, 3') in ii N HCI solution and also on chromium (4) in 5 N HCI solution: T: 1-3) 295 K; 1'-3', 4) 345 K. Fig. 2. Anodic potentiodynamic curves (1.44 V/h) recorded in a 1 N methanol solution of HCI on 08KhI8NIOT steel (i), chromium (2), and the carbides Cr=3C 6 (3), CrvC 3 (4), and Cr3C 2 (5). nonaqueous halogen-containing media, for which the range of sufficiently effective corrosion-resistant materials is very limited. The expensive and scarce Ni-Mo and Ni-Cr-Mo alloys used at present do not completely satisfy the needs of industry. Actually, as follows from Fig. 2, in contrast to stainless steel (curve i) and chromium (curve 2) chromium carbides in a dried methanol solution of HCI are in the passive state. This is indicated by the values of Eco r and also by the corrosion rates of the carbides, which are 0.0014 mg/yr for Cr23C6, 0.0013 n~n/yr for Cr7C3, and 0.0016 mm/yr for Cr3C 2. Close results were also obtained in a water-free acetonitrile i N solution of HCI. Two reasons for the self-passivation of chromium carbides in nonaqueous chloride-containing solutions are possible, chemosorption interaction of the carbides with the oxygen of the water present in the methanol as an impurity and also chemosorption interaction with the oxygen of the methanol. In this work the problem of identification of the particles responsible for passivation of the chromium carbides was not raised. However, taking into consideration that the water content in the solutions is about 30 times less than the chloride ion content, the second reason is more probable. The chemosorption of the oxygencontaining components of the solution is very effective, apparently because in nonaqueous media, as in aqueous solutions [4], the chromium carbides have low values of the critical points of passivation as the result of the presence in these compounds of strong Cr-C bonds. Regardless of the specific mechanism of passivation of chromium carbides it was established (Fig. 2) that these materials possess a very high chemosorption capacity in relation to the oxygen-containing components of the solution, which increases in order given above. Obviously the acceptor properties of the carbides increase in the same order. For a proof of this assumption the x-ray electron spectra of the valence zone of the investigated compounds were obtained. From them it may be seen (Fig. 3a) that the valence bands of the chromium carbides (spectra 2-4) differ significantly in structure from the valence bands of both metallic chromium (spectrum i) and of pure carbon (spectrum 5). The x-ray electron spectra of the valence bands of the carbides are also not simple superpositions of the corresponding x-ray electron spectra of chromium and carbon. An analysis showed that the relative intensity of the first band of the valence bands (0-2 eV energy interval) decreases while the intensities of the second band, which lies in the 2-6 eV interval, increases (the ratio of the intensities of the bands to the intensity of the 2p3/2 level of chromium was used) in the order Cr-~r23C6-CrvC3-Cr3C2. It is natural to assume that the increase in intensity of the second band of the valence band in t h e o r d e r of carbides given above is caused by transfer to this band of a portion of the valence electrons of chromium and carbon participating in the chemical bond of these elements to one another. A similar conclusion in relation to the carbides was drawn on the basis of the configuration approach [6] and from x-ray spectral data [7]. The third bands of the valence band of the chromium car-

374

a

b

I, u n i t s ~

6 "%.'9u

_..,..'

,"%'." 77 r

,...I ::Z

,9

""

,

9

" I '.5 0

5

IO

9r

,-P" 580

,

I 59"9

,eV Fig. 3. X-ray electron spectra of the 6c

valence bands of Cr (i), Cr23C 6 (2), Cr7C s (3), Cr3C 2 (4), and carbon (5) and also of the 2p3/2 level of chromium recorded on metallic Cr (6, 6'), Cr23C 6 (7, 7'), CrvC 3 (8), and Cr~C 2 (9, 9'): 6-9) air oxidized specimens; 6', 7', 9') anodically passivated at a potential of 0.7 V for 7 h in 1 N H2SO 4 solution at 295 K. bides (Fig. 3a, spectra 3 and 4), which lie in the 10-12 eV range, are formed by electrons not significantly participating in the chemical bond according to the same works. It is also important that in changing from chromium to its carbides the thickness of the passivating films decreases both on the air oxidized specimens and on the anodically passivated ones. On the anodically passivated it is 2-2.5 nm for the chromium and 1-1.5 um for the chromium carbides. The thickness of the oxide films on chromium and its carbides was determined from the ratio of the intensities of the peaks on the x-ray electron spectra of the 2p3/2 level of chromium (Fig. 3b) responsible for the oxidized (right peak) and metallic (left peak) states of chromium [8]. The results of [9] were also taken into consideration. Probably the lower intensity of oxidation of the surface atoms of chromium in its carbides than in pure chromium is caused by transfer of a portion of the valence electrons of chromium to the Cr-C bonds. Therefore, chromium carbides are promising corrosion-resistant materials in aqueous and nonaqueous chloride-containing media. In particular, their use as protective coatings is of great interest. The high corrosion resistance of the carbides may be explained by the presence in them of strong Cr-C bonds and also by the increased acceptor capacity of their chromium component and the high stability of the passive state of the carbides. LITERATURE CITED i.

2. 3.

4.

5.

6. 7.

T. Ya. Kosolapova and G. V. Samsonov, "The chemical resistance of chromium carbides," Ukr. Khim. Zh., 28, No. 8, 931-933 (1962). G. V. Samsonov and I. M. Vinitskii, Refractory Compounds (Handbook) [in Russian], Metallurgiya, Moscow (1976). Ya. M. Kolotyrkin and V. M. Knyazheva, "The properties of the carbide phases and the corrosion resistance of stainless steels," in: The Results of Science and Technology. Corrosion and Protection from Corrosion [in Russian], Vol. 3, Vsesoyuz. I n s t Nauch. i Tekh. Inf., Moscow (1974), pp. 5-83. O. S. Yurchenko, V. M. Knyazheva, Yu. P. Kolosvetov, and S. G. Babich, "Obtaining and investigations of the corrosion and electrochemical properties of specimens of highdensity chromium carbides," Poroshk. Metall., No. 4, 72-74 (1983). A. Gordon and R. Ford, The Chemists' Companion (Handbook) [Russian translation], Mir, Moscow (1976). G. V. Samsonov, I. F. Pryadko, and L. F. Pryadko, Electron Localization in a Solid [in Russian], Nauka, Moscow (1976). V. V. Nemoshkalenko, M. M. Kindrat, V. P. Krivitskii, et al., "The x-ray emission and photoelectron spectra of carbon and chromium in its carbides," Izv. Akad. Nauk SSSR, Neorg. Mater., 17, No. 6, 996-999 (1981).

375

.

.

V. I. Nefedov, "The use of x-ray electron spectroscopy for investigation of chemical compounds and materials," Zh. Vsesoyuz. Khim. O-va im. D. I. Mendeleeva, 30, No. 2, 159-165 (1985). A. M. Sukhotin, N. K. Koreva, Yu. P. Kostikov, et al., "The passivity of chromium. Electron spectroscopic investigation of the formation of CrH on the surface of a chromium anode," Elektrokhimiya, No. 9, 1403-1405 (1980).

ELECTROPHYSICAL PROPERTIESOF

RARE-EARTH ELEMENT

BORIDE-BASE GRANULAR FILMS II.

VOLT-AMPERE AND GALVANOMAGNETIC CHARACTER-

ISTICS UDC 539.2:621.316.8

R. K. Islamgaliev, A. V. Zyrin, O. I. Shulishova, and I. A. Shcherbak

In Report I it was noted that granular films containing an equimolar solid solution of europium and praseodymium hexaborides as the conducting phase and an aluminomagnesium fluorosilicate-base glass ceramic binder as the glassy phase possess an S-shaped relationship of the temperature resistance of the films. I t is of interest to investigate their volt-ampere and galvanomagnetic characteristics. In granular films there are a multitude of dielectric layers separating the grains of the conducting phase. As a first approximation the tunnel conductivity through these layers will have an ohmic character , as follows from the expression for the density of the tunnel current [i]: j =

3 , 1 6 . 1 ~ ~l/2Vexp(_ 10,25s~1~) '

(1)

$

where s i s t h e w i d t h o f t h e p o t e n t i a l

barrier

i n um and ~ i s ~ t s h e i g h t

i n eV.

I n t h i s work t h e v o l t - a m p e r e c h a r a c t e r i s t i c s were i n v e s t i g a t e d f o r a d i r e c t c u r r e n t w i t h V7:21 and F-30 d i g i t a l i n s t r u m e n t s a t t e m p e r a t u r e s o f 300, 78, and 4 . 2 K. At room temperature the specimens for the investigations had a s p e c i f i c f i l m r e s i s t a n c e o f 3"10 z, 104 , and 10 ~ ft. I n t h e r a n g e o f s t r e n g t h s o f t h e e l e c t r i c a l f i e l d o f 10 -4 t o 1 0 1 V / c m t h e relationship of current to the applied electrical f i e l d was l i n e a r ( F i g . 1 ) . From t h e p h y s i c a l p o i n t o f view t h e m o s t i n t e r e s t i n g area of the values of the strength of the electrical field is its very low values, where deviations from Ohm's Law may be observed. For the investigated specimens this area lies below values of 10 -4 V/cm (Fig. I). If it is assumed that the grains of the conducting phase are distributed in the volume of the film in the form of a cubic quasilattice and have an average size of i Dm, then a voltage of 10 -4 V applied to the film with a length of i cm strikes an average of 104 barriers or taking into consideration the coefficient of tortuosity [2] 106 barriers, which leads to a voltage of 10 -I~ V on each of them. With such small voltages applied to a potential barrier with a height of several electron volts deviations from ideal tunneling may appear. Therefore, at 4.2 K in electrical fields with a strength of less than 10 -4 V/cm the current occurring decreases more rapidly than is predicted by the tunnel mechanism of conductivity. As is known [3], in a metal-dielectric-metal structure a tunnel current predominates over a thermoelectronic one with low temperatures, high strength electrical fields, and a narrow width of the potential barrier. In this work at temperatures of 78 and 300 K a decrease in strength of the electrical field below i0 -4 V/cm is accompanied by a slower change in current than predicted by Eq. (i), that iS, the situation is opposite to that which was observed at 4.2 K. This may be ex-

Institute of Problems of Material Science, Academy of Sciences of the Ukrainian SSR. Translated from Poroshkovaya Metallurgiya, No. 5(305), pp. 48-51, May, 1988. Original article submitted February 12, 1987.

376

0038-5735/88/2705-0376

$12.50

9 1988 Plenum Publishing Corporation