THE EFFECT OF TITANIUM CHLORIDES ON CORROSION
RESISTANCE OF MATERIALS

Ivanov I. I., Dubovikov O.A., Kuzhaeva А.А.

Saint-Petersburg State Mining University

The corrosion stability of some pure metals and alloys in the titanium(II) and (III) chloride melts was under investigation. It was demonstrated that the rate and specific features of the process depend upon the chemical composition of alloys. Under these conditions chrome-containing alloys are most influenced to the corrosion. The relative corrosion stability data of some materials were obtained.

The stability of steels and metals in melts containing titanium (II) and (III) chlorides, is of considerable interest especially in obtaining high purity titanium. It was found that the corrosion rate of steel is mainly determined by the composition of the medium [1]: the most active component is titanium(IV) chloride , and then the titanium (III) chloride. The high content of titanium in molten titanium (II) chloride leads to its disproportionation on the steel surface to form the metallic and intermetallic coatings. Comparison of different types of steels (St.-3, 1H18N9T, 2H13) shows that Steel St.-3 in the melts containing titanium (III) chloride is more stable than other brands that exhibit selective dissolution of chromium. Authors found the decrease of corrosion resistance in a following series of the pure metals: Ni, Mo, Cr, V, Mn. S.I. Stepanov [2] noted that the corrosion of metals in molten chlorides has generally electrochemical nature, and selective dissolution of certain metal alloys components can be explained by the difference of electrochemical potentials of these elements. The effect of temperature and duration of the recovery process, as well as the content of titanium (III) chloride in the melt on the corrosion of steel is reported by S.V. Alexandrovsky [3]. Decrease in the relative content of titanium dichloride enhances corrosion of constructional steels.

All the studies mentioned above were made in static conditions, in which the diffusion processes are known to play a great role. Since the melts are mixed intensively during metallothermic production and purification from the impurities, it seems necessary to clarify the stirring effect on the materials corrosion resistance. Study of the materials corrosion in the melt under agitation was carried out using the technique and the installation described in [4] at 800 º C and the intensity of quartz ampoule shaking as much as 240 oscillations per minute with an exposure time 30 minutes.

It is evident (Table 1) that the most aggressive component of the considered melts is titanium (III) chloride: a melt of sodium chloride and titanium trichloride has a major corrosive effect on all the materials under investigation, except for molybdenum and nickel.

Table 1

Summary of materials corrosion resistance research

Sample material		The composition of the samples used, wt. %			Corrosion rate, mg/cm^2 ^.hr
Russian translation	English translation	TiCl3	TiCl2	NaCl
Ст.3	St. 3	40 - -	- - 50	60 100 50	110.6 2.1 0
1Х18Н9Т	1H18N9T	40 - -	- 50 -	60 50 100	164.2 0 1.3
ЭИ-868	EI-868	40 - -	- 50 -	60 50 100	28.4 5.2 2.9
Ti	Ti	40 -	- -	60 100	30 8.8
Ni	Ni	40	-	60	0.5
Mo	Mo	40	-	60	0

Fig. 1. The study of corrosion St.-3 and VT 1 in the NaCl melts containing 12.3% Ti³⁺, and 0.40% Ti²⁺

1 – VT 1 (high purity compact titanium), 2 – St-.3.

Despite the significant difference in the rate of destruction of titanium and St.-3 (Fig. 1), the corrosion of these materials in melts containing titanium trichloride in the initial period of interaction is characterized by the same order of magnitude and then approaches to the minimal positive value. It is evident from the following data that corrosional destroying of 1H18N9T and EI-868 is caused to a large extent by the selective dissolution of chromium:

Table 2

Sample material	Composition of the salt medium before the experiment, wt.%		The content^1 of impurities in the melt after the experiment, wt.%
	TiCl3	NaCl	Cr	Ni
1H18N9T	40	60	1.06	следы
EI-868	40	60	0.44	0,47[1]

Only in the tests with EI-868 nickel which is known to be the main component of this alloy was eventually found at the melt. It seems that the transition of nickel into the melt in this case occurs in conjunction with chromium due to the non-crystalline corrosion. The stainless steel corrosion study demonstrates the absence of nickel in the melts after exposition; the higher corrosion resistance can be caused by the cementation by chromium at the inox steel.

An additional data on the constructional materials (St.-3, 1H18N9T, EI-868) corrosion in melts containing titanium trichloride were obtained by the use of the rotating disk with equally accessible surface [4, 5]. The metal disk immersed in melt was rotated at a given rate within a certain period of time, after which the melt was raised, cooled, washed out of chloride salts, dried and weighted.

Dissolution rate was determined by weight loss:

where: - n - the dissolution rate, mg / cm² •min;
Δm - weight loss, mg;
- the surface of the sample contacting with the melt, cm²;
- time of exposure, min.

Duration of contact material with the melt was chosen empirically, for the obtaining a sufficient weight loss as a criteria, and to be possible ignore the amount of dissolved metal, corresponding to the conditions of the rotor method [4]. For all materials under investigation it was established the exposure time can be varied at the range of 1-10 min for the melt containing 4 wt.% Ti.

When the disc rotation speed changes from 240 to 830 rpm, the linear dependence of dissolution rate of titanium on the square root of rotation speed occurs (Fig. 2), which seems to be a consequence of laminar flow over the disc.

Fig.2 Dependence of the dissolution rate of titanium on the angular velocity of the sample.

A - dissolution rate, mg / cm²• min

B - angular velocity of the sample min

Fig.3. The kinetics of corrosion of structural materials

1– EI-868; 2 – 1H18N9T; 3 – ST.-3; 4– VT-I.

The summary data, which allow to characterize the materials corrosion resistance are shown at Fig.3. For comparison, curve 1 at this figure presents the destruction intensity of high purity compact titanium (VT-I) samples.

The destruction rates of the specimens VT-I, St.-3, EI-868 are reduced with increasing exposure time which is probably caused by disk surface passivation due to the accumulation of. low-reactivity substances thereon. These inactive surface substances are expected to form oxide and nitride films. The behavior of stainless steel pattern with respect to other materials is unusual; the destruction rate of the inox steel at the first time increases and afterwards demonstrates the trend to stabilize at the definite rather high level. This phenomenon may be caused by high-rate interactions of the melt with chrome, one of the main components of the stainless steel.

Thus, we conclude that the most durable construction material in the melts enriched with titanium trichloride, is EI-868. Due to its high thermal stability, this material can be used for the manufacturing of the multipurpose valves. The St.-3 and EI-868 are characterized by close values of the corrosion resistance at a significant exposure time. Destruction rate of titanium is 5-10 times higher than that of all other materials under the investigation.

The results of these studies are well-correlated with the data obtained by the study of the interaction of metal samples with melts in a sealed, intensive-shaking quartz cells.
One of the main results of the present study is that the usage of high-chromium steels which are known to have high chemical stability towards the major part of corrosion and surface oxydation processes is strictly undesirable in manufacturing the equipment for the melts enriched with titanium trichloride.

Literature

1. V.G. Gopienko, G.N. Gopienko, V.V. Timofeev, and D.I. Podushkin /Behavior of steels in melts containing titanium chlorides. // Journal of Applied Chemistry. 1966. V. 39, No.6. P.1249.

2. S.I. Stepanov. Physical chemistry of molten salts. Moscow: Metallurgiya, 1965. P. 340.

3. S. Alexandrovsky, Ph.D. Thesis. Leningrad, 1966. 15 p.

4. R. Sandler et al. / Izv. Vuzov (Russian-Soviet edition: "News of the High Schools"). "Non-ferrous metallurgy", 1976. № 5. P. 31-35.

5. V.G. Levich. Physico-chemical thermodynamics. Princeton, 1959.
699 p.

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[1] Duration 30 minutes of interaction.