DE1269559B - Process for the anti-corrosion treatment of objects made of graphite and graphite objects treated according to this process - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. Cl .:

Number:
File number:
Registration date:
Display day:

C04b

German class: 80 b -8/10

1,269,559
P 12 69 559.9-45
June 7, 1966
May 30, 1968

The invention relates to a method for the treatment of objects made of graphite, the for this purpose serves to protect these objects against corrosion, especially in an oxidizing atmosphere. The invention also relates to graphite articles protected against corrosion by this method.

Graphite is known to have favorable properties that make its frequent use as a moderator in Establish nuclear reactors. In particular, graphite is practically the only solid moderator which has been used in power reactors to date. Indeed, the graphite has very interesting Properties: good thermal conductivity, high sublimation temperature, high compressive strength, which is maintained up to high temperatures, a very small neutron capture cross-section and excellent moderation skills. Its scope, however, is due to its low resistance of graphite against oxidation in air or carbon dioxide gas as well as by its Reaction with a large number of molten metals (especially alkali metals) is limited.

The first disadvantage so far has been the operating temperature moderated by graphite and by circulation limited by carbon dioxide gas under excess pressure cooled reactors. The second disadvantage has forced to use the graphite in those reactors in which the moderator is used as a coolant molten metal used comes into contact with a protective cover.

Numerous methods have already been proposed in order to provide the surface of the graphite with an impenetrable cover which is also as little oxidizable as possible. One can mention here in particular the impregnation of the graphite with phosphates, the covering or sheathing with oxides, metal carbides and silicon carbide; in order to produce this latter covering or sheathing, it has already been proposed to provide the graphite with an application of a silicon powder suspension and then to heat it up to the melting temperature of the silicon; immersing the graphite in a bath of molten silicon or, better still, of Si — Ti or Si — Mo alloys, which have better wetting properties than pure silicon; Finally, silicon vapor has been allowed to react with the graphite, this vapor being obtained either by _{heating the silicon or a mixture of SiO 2} + C to a high temperature; it is also no longer a process for anti-corrosion treatment

of objects made of graphite and graphite objects treated by this method

Applicant:

Commissariat a l'finergie Atomique, Paris

Representative:

Dipl.-Ing. R. Beetz

and Dipl.-Ing. K. Lamprecht, patent attorneys,

8000 Munich 22, Steinsdorfstr. 10

Named as inventor:

Michel Besson, Orsay, Essonne;

Jean Eiston, Palaiseau, Essonne;

Pierre Morize,

Bourg-la-Reine, Hauts-de-Seine (France)

Claimed priority:

France 9 June 1965 (20 088)

new, gases containing silicon and carbon in the presence of the graphite to be protected thermally to decompose.

In all of these cases the problem is the formation of a protective layer or deposit Protective cover that is sufficiently compact to prevent subsequent corrosion of the graphite it protects to prevent, but also the differences in the expansion coefficients of graphite and the Silicon carbide can adapt so that it does not crack during sudden temperature changes. This last-mentioned difficulty is increased by the anisotropy of graphite, its Expansion coefficient in the extrusion direction and in the directions perpendicular thereto different is.

The present invention relates to a new method by which deposits are made on the graphite of silicon carbide can be produced, this process being better than the known processes corresponds to the requirements of practice, in particular

809 557/425

3 4

in so far as one can produce a precipitate or one of SiC that is as dense as possible; Protective layer is obtained, which at high temperature- to do this, the reaction temperature is increased and Changes do not get cracked, even if it receives a deposit of uniform thickness Graphite is anisotropic, and therefore countered to the so treated on the entire surface of the graphite Graphite object has a high resistance 5 level by having the supply of gas-vapor capability against oxidation and enhances mixture.

is difficult to penetrate even for gases. If during the second procedural

The process according to the invention is essentially a mixture of vapors from hydrocarbons, characterized in that the graphite objects are used in a material and silicon halide and hydrogen first process stage from see the number of silicon atoms and the number of vapors of hydrocarbons and silicon halo of carbon atoms, which are supplied by the vapors of the genide as well as of hydrogen, the graphite on mixture, not too far from which a first temperature lower than 1100 ° C, is pre-value 1 is removed to the presence of free preferably in the order of 1050 ⁰ C to avoid ER- 15 silicon in the precipitate. When the silicon halide comes into contact with this second stage, it is namely the reduction with the graphite that is decomposed, and the graphite when the silicon halide is in contact with the hydrogen, this first temperature for a period in which prevails. A ratio of 1.4 is left, for example, of the gas-vapor mixture, which is sufficient to produce an anchoring layer wisely at a reaction temperature of the size that 20 order of 1400 ^° C. is suitable if the graphite hydrogen toluene is used,
brings objects into an atmosphere that either If, however, during the second process

a mixture of hydrogen and orgasm and silicon halide as well as hydrogen niche silicon compounds are used from a mixture of vapors of the hydrocarbon stage, these or a mixture of hydrogen and 25 in their molecule must have an equal number of carbon vapors of organic silicon compounds, material- and have silicon atoms,
wherein in this second stage the graphite object. The second process stage, during which one

strives for a second, higher than 1200 ° C temperature, a surface layer with the greatest possible size is heated. Licher density is obtained at a temperature

During the first process stage, in the 30 carried out ^{on the order of 1400 ° C.} Mixtures of hydrocarbons, silicon halide and hydrogen can be used among the mixtures used for this purpose, so those mixtures are cited that keep the temperature at a value that is sufficiently methyltrichlorosilane CH ₃ Cl ₃ Si and hydrogen to keep the speed low the upper ones.

surface reaction is slow and the silicon carbide 35 In general, it can be said that the second SiC in the pores of the graphite within a process stage - which forms a dense depth of several millimeters before the surface layer of SiC is used - known per se, clogs surface pores will. While this is and has already been used to treat graphite and time, the reduction of the silicon halide seems to be treated indirectly, although the genids are thought to be caused by the carbon, 40 silicon-yielding compound either a halo, in particular the carbon in statu nascendi, the used genid or a silane,
by the thermal decomposition of the hydrocarbons To explain the invention in more detail

Molecules of matter are created; a thermal decomposition, now some numerical examples are given, of which the first two basically result from contact with the graphite the known prior art. Under these conditions the ratio is 45, while the following examples show the invention between the number of silicon atoms and the proper method, but the number of carbon atoms, which is in no way restricted by the gases or the invention.
Vapors are delivered, of little importance; in any case, however, there should not be too high an excess of coal
Atoms of matter are present. The same atomic form 50

formation of the silicon carbide precipitate takes place example

always because the appearance of free carbon atoms on a tube made of isotropic graphite of 30 mm

causes the reduction of the silicon halide. With an outer diameter, which can have a total surface area of, for example, 76 cm ² at a reaction temperature and heated to a temperature of 1050 ° C. for the atomic ratio of silicon to 55 1400 ° C., a carbon gas mixture with a value of the order of magnitude is allowed of 5% by volume silicon tetrachloride, 0.5% oTo-0.4, when the hydrocarbon is about luene and 94.5% hydrogen, which is methane. Gas mixture with a flow rate of 101 / mn

The hydrocarbon used should be fed in for 20 minutes. In this case, being volatile, so that at the temperature at 60, a precipitate of 4.3 g SiC, ie at which the mixture is allowed to flow around the graphite, 56.5 mg / cm ^{2 was obtained} .

is gaseous; it is also said to be relatively stable, this pipe which is in the air at 100 ° C during

so that it would not be kept alone under the action of the 1600 hours, during which heat decomposed. These conditions make the time the tube was exposed to twenty cooling and rapid use of saturated hydrocarbons, especially heating, did not show any particular of the methane CH ₄ , beneficial. Weight loss, that is, it had a full

During the second stage of the process, protection is sought against oxidation, as the graphite in contrast, after that, a surface layer was isotropic.

Example 2

For comparison, a gaseous mixture of 5 percent by volume silicon tetrachloride, 0.5% toluene and 94.5% hydrogen with a throughput of 101 / min was allowed to act on a disk made of anisotropic graphite with a diameter of 30 mm, which was heated to 1400 ° C and had a total surface area of 18.6 cm ² . A precipitate of 912 mg SiC, ie 49 mg / cm ^{2, was obtained} .

This disk was completely through in about 20 hours when heated to 1000 ° C in air Oxidation consumed; after this period of time only the SiC precipitate remained.

These two examples show that a surface deposit of SiC, which in a single Process stage through direct thermal decomposition of corresponding starting materials at high Temperature is generated, protection is only guaranteed if the graphite is isotropic; if In the case of anisotropic graphite, the graphite becomes perfect very quickly through oxidation destroyed.

Examples according to the invention

The following examples relate to the implementation of the method according to the invention; they will show that the process according to the invention is satisfactory in all cases Protection is achievable.

Example 3

A disc of anisotropic graphite with a diameter of 30 mm, with a total surface area of 18.6 cm ² and a total porosity of the order of 15% (graphite of nuclear quality) was allowed to act in succession:

a) On the graphite brought to a temperature of 1050 ° C by heating with HF, a gaseous mixture with 25 volume percent silicon tetrachloride, 6% methane and 69% hydrogen in a feed rate of 425 cm ³ / min for 6 hours; a precipitate of 800 mg of silicon carbide is obtained, which is mainly stuck in the pores of the graphite.

b) contains on this pre-treated graphite, is heated at 1400 ⁰ C, a gas flow of 5 volume percent of silicon tetrachloride, 0.5% toluene and 94.5% hydrogen. The exposure time is 10 minutes, the amount of gas supplied is 101 / mn; in this way an additional precipitate of Ig is obtained, so that the total weight of the precipitate corresponds to ^{a top layer weight of 100 mg / cm 2.} This graphite disk treated according to the invention was kept in air for 175 hours at a temperature of 1000 ° C .; during this time it was subjected to fifteen coolings and rapid heating; the weight loss from this disc was only 170 mg.

Example 4

The following are allowed to act one after the other on the surface of a disk made of anisotropic graphite with a diameter of 30 mm and a total surface area of 18.6 cm ^2:

a) At 1050 ⁰ C a gaseous mixture which contains 25 percent by volume silicon tetrachloride, 6% methane and 69% hydrogen; the amount of the gas mixture was 425 cm ³ / mn for one hour. In this way a precipitate of approximately 600 mg is obtained, which is essentially located in the pores of the graphite.

b) At 1200 ⁰ C a gaseous stream with 11 volume percent silicon tetrachloride, 1% toluene and 88% hydrogen with a throughput of 900 cm ³ / min for 30 minutes; a surface precipitation of 600 mg SiC is obtained. The total precipitation is 63 mg / cm ² .

^{When heated to 1000 °} C. in air for 40 hours, this disk treated in this way had only a weight loss of 400 mg for nine cooling and rapid reheating processes.

Example 5

The same disk as in Examples 3 and 4 was allowed to act in succession:

a) On the pane heated to 1050 ° C, the same gaseous mixture as in Example 3 and during the same time.

b) A stream of the same gaseous form is applied to the disc pretreated in this way and heated to 1350 ° C Mix as during the first stage; an additional precipitate was obtained in this way made of practically stoichiometric SiC, which increases by about 0.3 mm per hour.

The resistance of this coated disc to oxidation is of the same order of magnitude as that of the two previously mentioned disks.

It should be readily understood that the invention does not affect the implementation of the invention Process is limited, which are described above in the examples, and that they are also extends to amendments to at least part of the procedural measures included in the framework of equivalents remain.

Claims (2)

Patent claims: 1. Process for treating objects made of graphite for the purpose of producing a die Corrosion, especially in an oxidizing atmosphere, inhibiting surface layer made of silicon carbide, characterized in that the graphite object is brought into an atmosphere in a first process stage, those from a mixture of vapors of hydrocarbon and silicon halide as well consists of hydrogen, and the graphite to a first temperature lower than 1100 ° C, preferably on the order of 1050 ° C., at which the silicon halide is in contact is decomposed with the graphite, and that the graphite article is at this first temperature holds for a period sufficient to produce a To form anchoring layer that you then in a second process stage the object in an atmosphere that is made up of either a mixture of vapors of hydrocarbon and silicon halide as well as hydrogen or a mixture of hydrogen and vapors of organic silicon compounds, and that at this stage the subject holds for a sufficiently long time at a second temperature higher than 1200 ° C is to obtain a corrosion-inhibiting surface layer made of silicon carbide. 2. The method of claim 1, wherein during the second stage a mixture of Fuming methane silicon halide and hydrogen is used, characterized in that that during this second stage the temperature is of the order of 1400 ° C. 3. The method according to claim 1, wherein a mixture of vapors of toluene and silicon halide and hydrogen is used during the second process stage, characterized in that the second temperature is of the order of 1400 ⁰ C and the atomic ratio between the silicon and the carbon of the gaseous mixture is at most 1.4. 4. The method according to claim 1, characterized in that the organic silicon compound which is used in the second process stage, at least partially consists of methyltrichlorosilane CH ₃ Cl ₃ Si. 5. Object made of graphite, the pores of which in a surface area of at least 1 mm Deep corrosion-inhibiting with silicon carbide are occupied or filled. 809 557/425 5.68 © Bundesdruckerei Berlin