NO326210B1 - Corrosion resistant alloy, its preparation and use of the alloy - Google Patents

Description

The present invention relates to an alloy which is resistant to oxidation and/or corrosion, particularly at high temperature. More particularly, the invention relates to an alloy that is resistant to oxidation in a corrosive environment, for example molten glass or another similar material, which can be used to produce objects that are brought into contact with molten glass or said material when it is produced or hot converted.

In the glass industry, elements or tools in contact with the molten glass are made of refractory materials and especially ceramics. For certain applications it is preferred to use metal tools, generally made of alloys and especially when the tool must have electrical conduction properties or when the tool must have a certain ductility and mechanical strength at the temperature at which the molten glass is processed.

This applies, for example, to the production of glass wool by the technique known as internal centrifugation, more particularly with regard to the final phase of the process where the glass, when it leaves the melting furnace, is cast continuously in a set of axisymmetric elements that rotate at a very high rotational speed on their vertical axis. When arrested in its first fall by the bottom in an inner part or "basket," the glass is spread out under the action of centrifugal force against the cylindrical wall of the same part, this wall being perforated. These holes allow glass to pass through and, still under the influence of centrifugal force, the glass is pressed against the wall or "ribbon" of an outer part or "spinner" which is also pierced, these holes being smaller than those previously mentioned. Still under the influence of the centrifugal force, the glass passes through the belt on the spinner from all sides in the form of molten glass filaments. A ring burner located above the outside of the spinner provides a downwardly directed gas flow that runs along the outer wall of the ribbon, bending the filaments downwards and stretching them at the same time. These then "solidify" in the form of glass wool.

The parts here called "basket" and "spinner" are fiber processing tools that are subjected to very high levels of thermal stress (thermal shocks at start-up and stop), mechanical impact (centrifugal force, erosion due to the passage of glass) and further chemical stress (oxidation and corrosion due to molten glass and due to the hot gases leaving the burner, in the case of the spinner). As an indication, the working temperature is in the order of at least 1000°C for the glass to have a suitable viscosity.

The lifetime of these components generally depends on the corrosion resistance of the material from which they are made. In this connection, a nickel-based alloy is generally used containing around 30% chromium and which is reinforced with carbide precipitation as described in particular in FR-A-2 536 385.

The oxidation and corrosion resistance of this alloy at the temperature at which it is used is ensured by the high proportion of chromium which forms a protective layer of chromium oxide, Cr2C>3, on the surface of the part in contact with the oxidizing environment. Continuous diffusion of chromium towards the corrosion front makes it possible to keep a reserve of chromium behind the layer of CføOa oxides.

The working temperatures at which this alloy can be successfully used are, however, limited to a maximum value in the order of 1050 to 1100°C. Above this temperature, the material is quickly broken down by corrosion and due to shrinkage.

This material is therefore not able to satisfy the need for techniques where wool is produced from glass that is more viscous than the usual glasses of the borosilicate type and whose application requires temperatures in excess of 1100°C.

In particular to satisfy this need, the object of the present invention is to provide an alloy with improved resistance to corrosion and more particularly oxidation at high temperature, especially up to temperatures of the order of 1300°C.

Certain alloys based on metals that are more refractory than nickel are known to have good resistance to corrosion from glass at elevated temperatures. Particular mention should be made of a cobalt-based alloy according to FR-A-2 273 075.

Other special alloys, for example alloys reinforced by oxide dispersion (ODS), superalloys generally based on nickel and/or iron which in dispersion in their matrix contain fine particles of oxides and generally yttrium oxide, have also been used to produce fiber processing spinners which have good mechanical properties at elevated temperature. An alloy of this type is particularly described in US-A-5 328 499.

WO 88/058830 describes a sintered, shrinkage-resistant alloy comprising at least one refractory metal selected from Mo, W, Ta, V and Cr and further comprising at least one compound selected from oxides, nitrides, carbides, borides, silicates and aluminates. The oxide may be selected from various metal oxides, including chromium oxide.

However, with known alloys it is difficult to achieve temperatures above 1200 to 1250°C, while at the same time having an oxidation strength that is compatible with industrial production requirements.

As an alternative, it is known to use highly refractory metals such as molybdenum or tungsten which are given high resistance to corrosion due to glass at high temperatures when they are fully immersed in the glass. The problem these refractory metals have in common, however, is still their sensitivity to atmospheres containing oxygen. This is because their reactivity is in reality very high and leads to the formation of oxides which are either poor protectors, as is the case with tantalum or niobium, or highly volatile, as is the case with molybdenum and tungsten, and responsible for rapid breakdown by corrosion at high temperatures.

Another purpose of the invention is to improve the oxidation and/or corrosion strength of a metal or metal alloy at high temperature.

These objects have been obtained according to the invention by providing a dispersed phase of chromium oxide - Cr2O3 - (chromium oxide) particles in the matrix of the material. It has been observed that the presence of Cr 2 O 3 really inside the material promotes the formation and above all the maintenance of a protective chromium oxide layer on the surface of the material. The dispersed phase of chromium oxide - Cr203 - can either originate from a dispersion of Cr203 which is introduced in this form into the material, or from an in situ reaction which starts with a suitable precursor which is dispersed in the matrix and which is able to react in and possibly with the matrix to form inner 0^03.

The present invention therefore relates to a refractory alloy comprising metallic chromium, capable of forming on its surface a layer of chromium oxide resistant to oxidation and/or corrosion, particularly due to glass, particularly at high temperatures, characterized by the fact that it contains, in dispersion in its matrix, chromium oxide Q2O3 and/or at least one precursor of this oxide, where said precursor, in case of oxidation, is able to react in the medium of the matrix to form chromium oxide, 0203.

In the present description, the term alloy is used in a conventional way to denote a metallic product which is obtained by incorporating one or more other elements into a metal. As applied to the present invention, this definition includes both materials whose matrix consists of the combination of chromium alloyed with at least one other metal and materials whose matrix consists of pure chromium, the alloy constituents in the latter case being chromium oxide, C^Ch, and/or its special precursor.

The term "chromium oxide-forming" or "Cr 2 O 3 -forming" is intended to mean an alloy which, in the presence of an oxidizing or corrosive environment such as oxygen in air or molten glass, can form a protective layer of chromium oxide on its surface.

According to the invention, this ability is improved by the presence of particles of chromium oxide, CføOs and/or at least one precursor of Cr2O3 in the internal dispersion.

The term "precursor of Cr 2 O 3 oxide" is intended to mean any compound which, in the matrix environment, can react to form chromium oxide, Cr 2 O 3 , where appropriate by means of a suitable treatment, particularly a heat treatment. Preferably, a precursor of this type is an oxygenated compound of a metal which can be reduced by the neighboring chromate atoms in the matrix. Suitable oxygenated compounds are oxides of metals which are more oxidizing than chromium, such as Fe2C>3, FeO, NiO or CoO in particular. As a chromium oxide precursor according to the invention, one can particularly mention the mixed oxides of chromium and another metal and in particular chromites of iron, nickel or cobalt.

Depending on the reaction kinetics of the precursor compounds, the alloy as a dispersoid may, over periods of varying length, contain either essentially the precursor or both chromium oxide and its precursor or essentially chromium oxide.

Preferably, the particles in internal dispersion, namely the particles of Cr 2 O 3 oxide and/or one or more precursors, occupy at least 0.1% and in particular at least 0.5%, most preferably 1% of the volume of the alloy. As an indication, the proportion is generally at most 10% by volume, in particular at most 5% by volume. The proportion of CtøCh on a volume basis is very advantageous in the order of magnitude from 1 to 10% and especially from 1 to 5%. Preferably, the proportion of dispersed chromium oxide particles is in the order of 2 to 3%.

The internal chromium oxide particles are advantageously distributed in the matrix in the form of a nanometric or micrometric dispersion, that is to say that the particles preferably have a size of around 1 nanometer to around 10 micrometers, and in particular from 1 nm to 10 nm, in particular from 0.1 to 5 µm.

Preferably, the alloy owes its Cr 2 O 3 -forming character to a matrix containing chromium in a proportion of at least 10% by weight, preferably at least 15% by weight and in particular at least 20% of the weight of the alloy.

While high proportions of chromium are not generally recommended in the prior art, due to the paradoxical higher rate at which pure chromium is oxidized, the internal dispersion of G^Ch oxide in the alloys according to the invention also has a beneficial effect in terms of chromium- rich alloys whose wear in an oxidizing environment is found to be less rapid.

In a particular embodiment of the invention, the alloy matrix consists of chromium or comprises chromium in combination with at least one other refractory metal such as molybdenum, tungsten, niobium or tantalum in particular.

The very good high-temperature stability of these refractory metals, which all have melting points above 1700°C, makes them advantageous for the production of tools that are subjected to very high temperatures, particularly in the order of 1300°C.

In this connection, matrices based on molybdenum and chromium, based on molybdenum, chromium and tungsten or based on tungsten and chromium are preferred.

Chromium provides as a base element or as an alloying element material with the ductility required according to the loads to which the tool is subjected

use. It also acts as a chromium reserve for the formation of the surface chromium oxide layer.

In general, the proportions of the constituent elements of the alloy can be chosen in a manner that is known per se, in particular on the basis of the corresponding phase diagrams, in particular to adjust the material's mechanical properties.

As a non-limiting illustration, the matrices can be mentioned:

- molybdenum-chromium containing from 15 to 50% by weight of chromium and from 50 to 85% by weight of molybdenum; - tungsten-chromium containing from 20 to 99% by weight of chromium and from 1 to 80% by weight of tungsten.

Molybdenum-tungsten-chromium matrices containing:

from 10 to 60 wt% Cr

preferably from 20 to 40 wt% Cr, from 10 to 50 wt% Mo

in particular from 20 to 40% by weight Mo

from 10 to 70 wt% W

preferably from 10 to 50% by weight W, in particular from 20 to 40% by weight W.

In particularly advantageous matrices, the molybdenum:tungsten weight ratio is in favor of tungsten, particularly in the order of 0.3 to 0.6.

The above-mentioned refractory metal alloys are preferably produced by sintering and in this connection may additionally comprise a sintering agent such as palladium or another platinoid, preferably a proportion of 0.1 to 3% by weight of the alloy.

It is generally preferred with regard to the proportion of palladium that this is not too high in connection with the proportion of chromium in order to thereby avoid the appearance of an intergranular phase of Pda saturated with Cr in solid solution which, due to its low melting point, tends to to reduce the mechanical properties of the alloy at high temperature.

A range for a preferred refractory alloy is as follows:

A preferred refractory alloy composition is as follows:

Return units < 0.5%

Another preferred refractory alloy composition is particularly as follows:

However, the improvements in oxidation strength are not limited only to the refractory alloys mentioned above, but also concern other alloys comprising chromium. In another particular embodiment of the invention, the matrix is based on iron, nickel and/or cobalt, in combination with chromium. In particular, matrices based on nickel-chromium, cobalt-chromium, nickel-cobalt-chromium, nickel-iron-chromium, cobalt-iron-chromium and cobalt-nickel-jem-chromium can be mentioned, in which the proportions of these elements are preferably chosen within the following areas, based on % by weight:

Mention may be made, for example, of nickel-chromium matrices with around 20 to 30 wt% Cr, iron-chromium matrices with around 15 to 25 wt% Cr or cobalt-chromium matrices with around 25 to 35 wt% Cr.

Matrices of the ODS type are also conceivable, particularly based on nickel-chromium or nickel-chromium-cobalt, which are reinforced by oxide dispersion. The oxide in dispersion can in particular be chosen from oxides of yttrium, zirconium, lanthanum, cerium, hafnium, thorium and other elements which are capable of forming stable oxides, which do not oxidize the metal or metals in the matrix. In this type of alloy with the combined presence of dispersoids of chromium oxide and at least one oxide of an active element above, the material can simultaneously be given excellent mechanical properties at high temperature and improved resistance to corrosion and/or oxidation.

With all the previous chromium alloys that have been studied, it appears that internal C^CV oxide dispersions lead to a better anchoring of the protective CføOs layer on the surface of the material in contact with the oxidizing or corrosive atmosphere. The "mechanical" stability of the oxide layer achieved in this way is a contributing factor to improving oxidation resistance because it reduces the risk of the material being stripped by erosion.

Even under particularly severe conditions where the material is subjected to very strong mechanical loads which lead to local failure of the oxide layer due to cracking or erosion, it has further been shown that the alloys according to the invention have a remarkable ability to quickly restore the chromium oxide layer if this is destroyed .

It also seems that the presence of internal chromium oxide promotes the formation of a more compact and denser chromium oxide surface layer where the diffusion of the reactive species is retarded. This appears to be the case with oxygen, because the oxidation rate for the alloys with internal Cr 2 O 3 according to the invention is lower than that of alloys which are similar but do not include internal Cr 2 O 3 . An improvement of the same type is observed in the case of nitriding.

In general, it can be stated that the presence of particles of chromium oxide, Cr 2 O 3 or one of its precursors promotes the maintenance of effective protection against oxidation or other types of corrosion.

The invention also relates to a method for the production of an alloy as indicated above and which is characterized by the fact that it comprises one or more steps of hot consolidation of metallic powders, and in particular sintering, pressing, forging or welding.

The different types of consolidation can be complementary.

When it comes to sintering, one can imagine free sintering, but also sintering under load using techniques with hot uniaxial pressing or hot isostatic pressing.

The elements that make up the matrix of the alloy, individually in powder form, are mixed to obtain a particulate material of homogeneous composition in a preliminary step, of the grinding type, where the metallic powders are mixed.

Particularly according to the original particle size distribution for the powders, this operation can be carried out using traditional grinding agents or stronger ones.

In certain cases, the process may thus include a preliminary, mechanical alloying step.

This powder grinding technique, using a grinding material generally in the form of balls, makes it possible to reduce the particle size distribution of the powders and this can possibly be accompanied by a chemical reaction. This can in particular lead to an alloy synthesis that starts with elementary metallic powders to thereby form intermetallic compounds or solid solutions or alternatively a redox reaction between the powders.

This technique is advantageous, especially for the dispersion of a brittle phase in a metallic matrix, this is the case in the production of ODS alloys.

There are several options available for introducing Cr2O3 oxide particles into the material and these can be used as alternatives or in combination: - addition of chromium oxide in divided form to the mixture of metallic powders; - formation of oxide particles in situ from metallic chromium in divided form by exposure of chromium to an oxidizing atmosphere. In this connection, the oxidation of chromium is preferably carried out before or during the mixing of the metallic powders; - addition of a chromium oxide precursor, in particular an oxygenated compound of a metal that can be reduced with chromium, for example an oxide, in finely divided form, to the mixture of metallic powders.

Preferably, the heat consolidation step is carried out under vacuum or under an inert atmosphere to prevent unwanted contamination. In the presence of elements such as chromium which have a high vapor pressure, it is preferred to work under inert gas pressures, particularly of the order of at least 5 x IO"<5> Pa, rather than under vacuum, at least under some of the pressing. Argon can for example is used as an inert gas.

The consolidation temperature of course depends on the composition of the alloy matrix as well as any infill or loading that may be applied. These parameters can be selected in a simple manner by the person skilled in the art.

As indicated above, the alloys according to the invention make it possible to produce objects that can be used in an oxidizing or corrosive environment at high temperature. In this connection, the invention further relates to the use of alloy, as mentioned above, for the production of an object which can be used for the production and/or heat conversion of glass.

Among other applications, use of this type can be in particular with a fiber processing spinner for the production of glass wool, a nozzle element for the production of textile glass fibers, a refractory container for melting glass-forming material, a component for stirring molten glass, an element or a carrier for a probe immersed in molten glass, a melting electrode, and so on.

In general, the alloy according to the invention can be used to produce furnace outlets, nozzles and feeders for the production of materials from a glass-forming mixture. As mentioned above, this can involve the production of so-called textile glass fibers which are used for reinforcement, as well as the production of mineral fibers such as glass fibers or stone fibers which are usually used for thermal or acoustic insulation.

This also includes packaging glass, referring to a hollow product such as bottles or flasks.

Outside the glass industry, the alloys according to the invention can be used in the production of a wide range of objects when they must have a high degree of resistance in an oxidizing and/or corrosive environment, particularly at high temperature, in particular for the production of furnace parts for heat treatment, electric heating resistors or alternatively in aircraft for turbine elements.

In general, these alloys can be used to produce any refractory alloy part used for operation or work with heat treatment furnaces at high temperatures (above 1100°C). They can thus be, for example, hot fan blades, heat carriers, feeding equipment, and so on. They can also be used to make any type of heating resistor intended to operate in a hot, oxidizing atmosphere, and to make turbine elements that form part of engines for land, sea or aircraft or any other application that does not concern vessels or vehicles, for example power stations.

Other characteristics and advantages of the invention will be apparent from the following illustrative examples, with simultaneous reference to the attached figures, where: Figure 1 compares the isothermal oxidation properties of a first alloy according to the invention and a comparison alloy which does not contain chromium oxide in internal dispersion;

figure 2 compares the isothermal oxidation properties of a second alloy according to the invention and a comparison alloy which does not contain chromium oxide as internal dispersion;

figure 3 shows the isothermal oxidation properties for four other alloys according to

the invention; and

Figure 4 compares the isothermal oxidation properties for three other alloys according to the invention and a comparison alloy that does not contain chromium oxide in internal dispersion.

Example 1

An alloy based on chromium-molybdenum-tungsten according to the invention is produced by modifying a Cr-Mo-W matrix with the following composition in % by weight:

by adding Cr203 particles in a proportion of around 5% by volume.

The weight composition of the alloy is as follows in %:

Molybdenum in the form of a powder with a particle size of 2-4 µm and tungsten in the form of a powder with a particle size of 1-2 nm are first annealed under hydrogen for 1 hour at 900°C to reduce the oxide on the surface of the metal grains. They are then mixed with chromium in the form of a powder with a particle size of 1 µm, palladium with a particle size of 1-1.5 nm and finely divided chromium oxide and the whole is finally ground in an agate mortar to obtain a homogeneous mixture with a fine and uniform distribution of the chromium particles.

Finally, the powders are sintered under load by uniaxial hot pressing in a LILLIPUT press furnace from the company ECM, comprising a graphite heating element, a hydraulic unit that makes it possible to apply a maximum load of 10,000 daN at the center of the furnace by means of a piston, so as well as a primary vacuum station and a feed station for inert gas (argon).

The mixture of powders is first pressed into a pellet with a diameter of 15 mm in the graphite cavity mold by pressing the powders at ambient temperature under a load of around 100 kg.

This is followed by the actual sintering under the following conditions:

- first the temperature is raised slowly at a rate of 10 to 15°C/min. and in tirnn up to 1200°C under a dynamic primary vacuum and under a load of 5 MPa; - then an argon flow is set up and the temperature is further raised slowly with a long stay of around 60 minutes at 1370°C under a load of 27 MPa and another long stay of around 120 minutes at 1420°C, still under a load of 27 MPa; - then a temperature jump from 1420°C to ambient temperature under zero load and argon flow.

After sintering, annealing must be carried out, on the one hand to reduce residual stresses that may have arisen during cooling and on the other hand to improve the homogeneity of the alloy if this is necessary. An advantageous annealing takes place as follows: - the temperature is raised to 1370°C and this temperature is maintained for 2 hours under a stream of hydrogen;

- the temperature is raised to 1420°C and held for 24 hours at 1420°C under argon; and

- everything cools down to room temperature.

The obtained alloy is observed under optical microscopy, scanning electron microscopy (SEM) and Castaing microprobe analysis, using common techniques.

The results of the synthesis are a homogeneous alloy with a grain size of the order of 50 nm, in which the grains of chromium oxide CføOa, with a size of a few micrometres, were uniformly distributed at the grain boundaries in the alloy. It is verified that the volume occupied by the CføCh grains constitutes around 5% of the volume of the alloy.

The properties of the alloy, expressed by oxidation at high temperature, are assessed by thermogravimetry, where the mass uptake of an alloy sample (indicative of the amount of oxide formed) is measured as a function of time in a chamber heated to 1300°C. The general conditions are as follows: - ascent with isothermal stay in steps of 10°C/min.; - duration of isothermal holding for 100 hours at 1300°C; - reduction to ambient temperature at 10°C/min., as the oven is then flushed with 1.5 l/hour of dry, synthetic air.

The results are shown in the diagram in figure 1, expressed by mass uptake (in mg/cm<2>) per time unit (in hours).

Comparative example 1

An alloy with the same matrix as the alloy in example 1 was produced, but without the addition of chromium oxide to the elemental metal powders.

The weight composition is therefore as follows:

The manufacturing conditions are the same as in example 1, and the oxidation properties are also shown in Figure 1.

Figure 1 shows that the mass uptake of the alloy in example 1 (curve 1) is less than that of the comparison alloy without the addition of chromium oxide (curve 1 comp.), and that therefore the addition of internal CT2O3 improves the resistance to oxidation.

Example 2

In the same way as in example 1, an alloy based on chromium-molybdenum-tungsten according to the invention and with a higher proportion of chromium is produced. The weight composition is as follows:

The manufacturing conditions are the same as in example 1. They may, for example, comprise a preliminary mechanical alloy: after molybdenum and tungsten have been annealed under hydrogen, the metallic powders are introduced into a container containing tempered steel balls, under a dehumidified argon atmosphere. The hermetically sealed container is then mounted on a planetary mill and the powders are ground in three sequences of 1 hour with a quarter of an hour between each to avoid excessive heating.

The results of the synthesis are an alloy whose grain size is in the order of 15 to 20 Hm, in which chromium oxide grains are distributed uniformly at the grain boundaries and occupy around 5% of the volume of the alloy.

As in example 1, this alloy is characterized by thermogravimetry at 1300°C. Thermogram, curve 2a, as shown in figure 2, shows that the oxidation kinetics are interrupted by sequences of rapid mass loss, followed by a resumption of the usual kinetic behavior within a relatively short time (with the exception of these interruptions, curve 2b, the kinetic behavior of same type as in example 1).

The rapid mass losses seem to be due to local breaks in the oxide layer, mainly at the edges of the oxidized disc. The resumption of normal behavior suggests that the alloy is capable of self-healing its own chromium oxide surface layer when defects occur.

The alloy in example 2 is thus highly resistant because it resists oxidation at 1300°C for 325 hours.

Comparative example 2

An alloy with the same matrix as the alloy in example 2 was produced, but without the addition of chromium oxide to the elemental metal powders.

The weight composition was as follows:

The oxidation properties of this comparison alloy are also shown in the form of a thermogram (curve 2 comp.) in Figure 2.

Comparative alloy 2 has a very different oxidation behavior compared to alloy 2: it starts with oxidation with moderate mass uptake, which represents a limited formation of the protective surface chromium oxide layer, then the mass uptake drops sharply, which suggests an irreparable breakdown of the surface layer and total degradation of the material.

It seems in particular that the surface oxide layer in the comparison alloy is very heavily stressed and therefore cracks and loses its impermeability to nitrogen. The nitriding of the exposed chromium would accelerate the corrosion of the alloy which then quickly becomes complete.

The presence of chromium oxide particles in the internal dispersion in alloy 2 makes it possible to remedy this problem and allows the material to resist oxidation by surface formation of a permanent, protective layer of C₂C₂.

Examples 3 to 6

These examples still concern alloys based on chromium-molybdenum-tungsten, where the vector ratio molybdenum:tungsten is now in favor of tungsten.

The manufacturing conditions are essentially the same as in Example 1, except that the last heat treatment step is:

- omitted in examples 3 and 5,

- carried out in examples 4 and 6 with a temperature of 1600°C, held for 24 hours.

The essential characteristics of these alloys are indicated in the table.

The results of thermogravimetric tests as shown in figure 3 (curves 3, 4, 5 and 6 are the thermograms for the alloys in examples 3, 4, 5 and 6 respectively) show that these four alloys perfectly resist oxidation for at least 100 hours at 1300°C and that the resistance can easily exceed 150 hours.

Example 7

An alloy based on nickel-chromium according to the invention is produced by modifying a Ni-Cr matrix with the following composition in % by weight:

by adding Cr203 particles in a proportion of around 5% by volume.

The weight composition for the alloy is as follows:

The metals are mixed in powder form with the finely divided chromium oxide and ground up for a long time in an agate mortar to obtain a homogeneous mixture with a fine and uniform chromium particle distribution.

The powders are then sintered under load by uniaxial hot pressing in the press furnace in example 1 into a powder pellet with a diameter of 15 mm, pressed at ambient temperature under a load of around 100 kg.

The actual sintering is carried out with a long stay (approx. 60 minutes) at 1000°C under a load of 27 MPa and a further long stay of around 120 minutes at 1100°C, still under a load of 27 MPa.

As in the previous examples, microscopy is used to examine that the alloy obtained is homogeneous and that the Cr2O3 chromium oxide grains with a size of a few micrometers are uniformly distributed at the grain boundaries of the alloy. A check is then made that the volume taken up by the Cr203 grains amounts to around 5% by volume of the alloy.

The properties of the alloy expressed by high temperature oxidation are assessed by thermogravimetry at 1300°C under the conditions described above, for around 100 hours and are graphically shown by thermogram 7 in figure 4.

The constant Kp for the parabolic oxidation kinetics is of the order of 2.6 x 10"<10>g^.cm^.s"<1>.

An observation of the oxidized material by scanning electron microscope in backscattering electro mode shows the presence of a very regular and perfect, compact surface layer of chromium oxide with a thickness of around 50 nm, which adheres to the alloy. The properties of this alloy in the presence of a corrosive environment consisting of molten glass are characterized in an electrochemical test, consisting of measuring the passivation potential of the alloy when it is immersed in a bath of molten glass at a temperature of 950°C. The measurements are made using a 3-electrode arrangement: a working electrode of alloy 7, a platinum counter electrode and a zirconia reference electrode. The working electrode is a rotating electrode that makes it possible to improve the replenishment of glass around the electrode and to simulate the erosion that is exerted on the material in use.

Under the conditions of the test, the current/voltage curve for alloy 7 shows a passivation peak for a voltage of the order of -1.1 mV with a passivation current around 5.0 mA/cm<2>. The intensity of the passivation peak is indicative of the inverse of the capacity of the alloy to form and to maintain its protective surface layer of chromium oxide.

Comparative example 7

Under the conditions of Example 7, a comparison alloy 7 is prepared which does not include chromium oxide in internal dispersion. The composition is that of the matrix used above, namely:

Nine 70%

Cr 30%

This alloy is subjected to the same thermogravimetry test at 1300°C, and the performance is shown by thermogram 7 comp. in Figure 4.

The mass uptake for the comparative alloy 7 is clearly greater than that of alloy 7, which suggests a lower resistance to oxidation. The constant Kp for the parabolic oxidation kinetics for comparative alloy 7 is of the order of 3.9 x 10-V.cm-V.

Observation of the oxidized material by scanning electron microscopy shows structural differences on the Cr203 surface layer compared to that observed in alloy 7, the layer being less compact and showing a significant number of cracks.

The properties of this alloy, in the presence of a corrosive environment consisting of molten glass, are characterized by the electrochemical measurements as described under example 7.

Under the same test conditions, the current/voltage curve for the comparative alloy 7 shows a passivation peak for the same voltage of the order of -1.1 mV, with a passivation current around 6.3 mA/cm<2>. The intensity of the passivation peak indicates the inverse of the capacity of the alloy to form and to maintain its protective surface layer of chromium oxide.

Example 8

A further alloy based on nickel-chromium according to the invention is produced as in example 7 by modifying the Ni-Cr matrix by adding Cr 2 O 3 particles in a proportion of around 1% by volume.

The weight composition for the alloy is as follows:

The oxidation properties of alloy 8 are assessed by thermogravimetry at 1300°C under the same conditions, and are shown graphically by thermogram 8 in figure 4.

A comparison of thermograms 7 and 8 shows that alloy 8 has better resistance to oxidation than alloy 7, the constant Kp for the parabolic oxidation kinetics for alloy 8 being of the order of 2.0 x 10"10 g^.cm^.s"<1 >.

Example 9

A further alloy based on nickel-chromium according to the invention is produced as in example 7, by modifying the same Ni-Cr matrix by adding Q2O3 particles in a proportion of around 2.5% by volume.

The weight composition of the alloy in % is as follows:

The oxidation properties of alloy 9 are assessed by thermogravimetry at 1300°C under the same conditions, and are shown graphically by thermogram 9 in Figure 4.

A comparison of thermograms 7, 8 and 9 shows that alloy 9 shows better resistance to oxidation than alloys 7 and 8, which suggests an optimum for the internal chromium oxide concentration at around 2.5% by volume. The constant Kp for the parabolic oxidation kinetics for alloy 9 is of the order of 1.80 x 10"10 g^cm^.s"<1>.

Observation of oxidized material with a scanning electron microscope in backscattering electro mode gives results that are similar to those in example 7, but with better adhesion between oxide layer and alloy.

Claims (16)

1. Refractory alloy comprising metallic chromium, capable of forming on its surface a layer of chromium oxide resistant to oxidation and/or corrosion, particularly due to glass, particularly at high temperatures, characterized in that it contains, in dispersion in its matrix, chromium oxide Cr2C«3 and/or at least one precursor of this oxide, said precursor, in case of oxidation, is able to react in the medium of the matrix to form chromium oxide, Cr2O3. 2. Alloy according to claim 1, characterized in that the particles in the dispersion occupy from 0.1 to 10% by volume of the alloy, preferably from 1 to 5% by volume<.> 3. Alloy according to claim 1 or 2, characterized in that at least one chromium oxide precursor is an oxygenated compound of a metal that can be reduced with chromium. 4. Alloy according to any one of the preceding claims, characterized in that the chromium oxide particles have a size in the order of 1 nm to 10 (im. 5. Alloy according to any one of the preceding claims, characterized in that the alloy matrix is a matrix of chromium or comprises chromium in combination with at least one other refractory metal such as in particular molybdenum, tungsten, niobium or tantalum, selected in particular from a matrix based on molybdenum chromium, molybdenum-chromium-tungsten and tungsten-chromium. 6. Alloy according to claim 5, characterized in that the matrix on a weight basis comprises: from 10 to 60% Cr from 10 to 50% W from 10 to 70% Mo 7. Alloy according to claim 5 or 6, characterized in that it further comprises a sintering aid such as palladium or another platinoid, in particular in a proportion of 0.1 to 3% by weight. 8. Alloy according to any one of claims 1 to 4, characterized in that the matrix is a matrix based on iron, nickel and/or cobalt in combination with chromium. 9. Alloy according to claim 8, characterized in that the matrix is selected from matrices based on nickel-chromium, cobalt-chromium, nickel-cobalt-chromium, nickel-iron-chromium, cobalt-iron-chromium and cobalt-nickel-iron-chromium, and that the proportions of these elements are chosen within the following weight ranges: 10. Alloy according to any one of the preceding claims, characterized in that it further contains in dispersion in its matrix at least one oxide selected from oxides of yttrium, lanthanum, cerium, zirconium, hafnium and thorium. 11. Method for producing an alloy according to any one of the preceding claims, characterized in that it comprises one or more steps of hot consolidation of metallic powders, and in particular sintering, pressing, forging or welding. 12. Method according to claim 11, characterized in that it comprises a preliminary step in which the metal powders are mixed, in particular by grinding, for example by mechanical alloying. 13. Method according to any one of claims 11 or 12, characterized in that chromium oxide CT2O3 is introduced in finely divided form to the metallic powders. 14. Method according to any one of claims 11 to 13, characterized in that the chromium oxide Cr 2 O 3 particles are formed in situ from metallic chromium, by exposure of chromium to an oxidizing atmosphere. 15. Method according to any one of claims 11 to 14, characterized in that at least one precursor for chromium oxide C^O^ is introduced in finely divided form to the metallic powders. 16. Use of alloy according to any one of claims 1 to 10 for the production of an object which can be used for the production and/or heat conversion of glass.