CA1045916A - Corrosion protection methods for refractory metal pieces - Google Patents

Abstract

Method of protecting refractory metal parts against corrosion. According to this process, a layer of nickel and / or cobalt alloy with boron B is formed on the parts, and this boron is then removed. Application: protection of superalloys.

Description

1 ~) 45916 The invention relates to treatments for protecting parts made of metallic materials against corrosion by formation of metallic deposits incorruptible constituting protective sheathing. It concerns more particularly protection against corrosion at high temperature in the atmosphere oxidizing redox or sulfurizing, of parts made of metallic materials fractionals of the kind sometimes called "superalloys" based on metals of the group iron, cobalt, or nickel and whose composition by weight comprises at least 50% of metals from this group. It aims in particular to protect those of the said materials which have a high carbon content, for example at least equal to 0.4%, to form refractory carbides with elements of addition of group of chromium, molybdenum, tungsten, tantalum, niobium, etc ...
It can however be applied to the protection of non-refractory materials.
The invention generally implements a method of treatment of the kind sometimes called "duplex", during which one realizes first a first simple or composite cladding which will be called pre-cladding in the remainder of the present application then a second simple or com- plete sheathing posite which will be called simply cladding, said cladding ensuring the function desired protective action and pre-cladding having, among other functions possible-them, that of facilitating the connection between the cladding and the base material and that of standardizing the behavior of the different surface regions of parts during cladding treatment. In the process of the invention, the cladding treatment is generally a thermo-chemical treatment of dep ~ t-diffusion of the known genre, that is to say that the pieces are, after the operation sheathing, heated in an atmosphere containing the sheathing elements as gaseous compounds at the processing temperature. Preferably, the-said cladding treatment is carried out in a hydrogenated-halogenated atmosphere and aims to form refractory sheathing on the parts by deposit and diffusion incorrodible with appreciably aluminum and / or chromium contents ~) 4S916 higher than those of the material.
The metal parts to which the invention applies can 8tre made by all known methods, such as molding, wrought, f ~ ttage, electro-forming, etc. They can be compact or porous and monolithic or composite that is to say, in the latter case, constituted by soldering or welding of pi ~ these elementary materials which can 8tre different.
Before approaching the description of the invention and explaining its advantages, it is advisable to return to the circumstances which can lead to form a pre-cladding before the cladding when the latter is formed ~ per channel thermochemical. Generally, it is possible to achieve by thermochemical and in particular by deposit-diffusion in a halogenated atmosphere of cladding giving very good resistance to hot corrosion to parts of refractory metallic materials, provided however that the surface said parts do not present before treatment of significant heterogeneities.
As such, alloys refractory to more than 0.4% carbon pose special problems. Indeed, those of these alloys which are used by conventional molding or sintering methods and for example cobalt alloys of type X40 ~ cobalt base; additions: chrome, nickel, tungsten, iron and 0.5% carbon) or of type MAR 509 (cobalt base; addi-tions; chromium, nickel, tungsten, tantalum, titanium, zirconium and 0.6% of carbon ~ include ~ large batches of carbides, some of which open out on the surface. After cladding and in use with exposure to oxidizing or sulfurizing hot gases, local pitting, initiation of corrosion severe, may occur in the vicinity of these islets. In addition, these local surface heterogeneities can be multiplied by a structure intentionally conferred on the parts by a method of solidification special tion. We know that we can achieve by solidification ~ `
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11) 45916 directed exploiting phenomena of eutectic segregation of parts to be high mechanical resistance which include, in a nickel or co-balt with various addition elements including chromium, oriented fibers of reinforcement in tantalum carbide, niobium, etc. Each end of fiber emerging at the surface constitutes a heterogeneity which can cause a pitting of the aforementioned kind and a corrosion initiation.
We have already tried to eliminate the effects of these structural accidents on the cladding obtained thermochemically.
In a first category of known methods, it is sought to reduce surface heterogeneity gradients by a softening treatment superficial, that is to say by local decarburization or dissolution partial ~ surface lots by subjecting the parts to heating prolonged at high temperature within a pulverulent mass containing by example, in addition to a refractory thinner, nickel powder and sulfur. This This type of process is expensive because of the duration of the heating and the pollution.
fast mass and especially the need to polish the parts as well treated to restore a suitable surface finish before the operation cladding.
In a second category of known processes, sometimes called treat-Secondly, "duplex", before the cladding thermochemical treatment, a electrolytic pre-cladding deposit. We notably carried out pre-nickel or platinum cladding. But, in general, the coatings electrolytic have a low penetration power and the deposited layers are all the more irregular in thickness and composition as the shape parts to be treated is more complex. This kind of process therefore does not give satisfactory results when applied to engine components of complex shape such as sectors grouping fixed or mobile blades of turbo-machines, blades cooled by convection, by fluid projection, by films .
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1 ~) 45916 liquids, or by blowing into multiple channels, etc. In addition, nickel electrolytic cells resist poorly even after diffusion to the thermal traces undergone by the parts during thermochemical treatment cladding or during service and cladding ends up being affected blisters or detachments leading to rapid destruction. As to platinum pre-cladding, if they prove ineffective on base materials of cobalt, they often give satisfaction on nickel-based materials but their cost is prohibitive for many applications because the thickness deposited must reach ten to twenty microns.
The invention relates to a method for forming a coating based on of nickel and / or cobalt on parts made of metallic materials including the overall weight position contains at least 50% of iron group metals, cobalt and nickel, characterized in that one first forms on said parts at least one layer of a nickel and / or cobalt alloy with boron and in that the boron is then removed; the metallic material being a refractory material; the deposition of the alloy layer being carried out by immersion sion of the parts in an aqueous bath containing at least one nickel salt and / or cobalt and a boron salt; and characterized in that the boron is removed by heating of the coated parts in a halogenated reducing atmosphere to a temperature between 800 and 1250C, the elimination of boron being accom-supported by diffusion of cobalt and / or nickel in the metallic material.
An object of the invention is to provide a pre-cladding method economical Easy to apply, applicable to parts made of materials of complex shapes or to composite parts and parts anyway having significant surface heterogeneities due to particular to the presence of islets or fibers of metallic carbides or of composed of heavy elements emerging on the surface of the material.
Another object of the invention is to provide a protection method ~.,. ~

1 ~) 4S916 parts made of refractory metallic materials, said method using works the dep8t of a coating ~ pre-cladding and the filing of a coating of cladding and which gives said parts a high and lasting resistance to corrosion ~ high temperature, in particular by oxidation and by sulfurization, particularly during abruptly varying thermal cycles.
A subsidiary object of the invention is to provide a method of corrosion protection of composite parts produced by assembly of elementary parts by brazing using solders containing boron.
The invention relates not only to the methods of the invention but also on the one hand the parts treated by said processes, and on the other hand the parts presenting the particular surface structures conferred, between other means, by the methods of the invention.
The method according to the invention of pre-cladding of parts made of materials metallic consists of forming on said parts, preferably by chemical, at least one layer of an alloy containing boron as well as at least a metal from the group of cobalt and nickel, then to subject said parts to thermochemical treatment to substantially eliminate boron and provo-quer the partial diffusion of the pre-cladding metal (s) in the material.
The proc ~ d ~ according to the protection of parts in materials metallic consists in carrying out on said parts a pre-sheathing of at least a metal layer containing at least one metal from the group of cobalt and nickel in accordance with the aforementioned pre-cladding process and then to carry out a alloy cladding containing chromium and / or aluminum in sensitive content-ment sup ~ superior to that of the material, preferably by heating the parts in a hydrogen-halogenated atomsphere containing in the vapor state at minus a chromium and / or aluminum halide.
The metal parts according to the invention consist of a metallic material surrounded by a first metallic cladding constituted by at B ~

at least one layer of at least one metal from the group of cobalt and nickel, said cladding further containing particles of carbide and / or boron carbonitride and a second metallic cladding containing chromium and / or aluminum in a content substantially higher than that of the material.
The invention will be better understood on reading the comments which follow and examples of implementation given below with reference to attached drawings including:
- Figure 1 is a set of diagrams illustrating chronologically the phenomena that occur during the production of a pre-lQ cladding according to the invention, - Figure 2 is a set of diagrams illustrating examples of implementation of the pre-cladding method of the invention, and which make appear other arrangements and benefits.
It seems appropriate, at this stage of the description, to set out for what the invention plans to deposit at the same time as the metal or metals of cladding of boron which is then removed by a special treatment in order to eliminate its harmful effects on the resistance of the refractory sheathing while bene-ficiant of its properties during the formation of the deposit.
On the other hand, current techniques of chemical deposition that allow nickel-boron or cobalt-boron or nickel- layers to be formed cobalt-boron which give satisfaction. Chemical deposition using aqueous baths containing, in addition to these metals, reducing compounds based on boron, are easy to make, commonly used in industry, very suitable for h ~ rents and very regular thickness whatever the shape complexity parts to be treated. It is also easy in case one wishes not to sheathing certain areas of the rooms to preserve reserves of dep ~ t by means masks in varnish, in collodion, etc. which allows, if necessary, make pre-sheaths of different thicknesses on the same piece by eliminating ,., la4ssl6 masks at the right time.
For example, an aqueous bath of chemical deposition of nickel-boron can have the following composition:
- nickel chloride 30 g / liter - ethylene diamine 60 g / liter - caustic soda 40 g / liter - sodium boranate 0.5 g / liter.
Such a bath has a pH of 12 to 14 and, at a temperature of 90 to 92C, the deposition rate is 10 to 20 microns per hour.
The deposited nickel-boron alloy contains 5 to 7% of boron, its temperature of fusion is from 1070 to 1080C and its density of 8.2 g per cm3.
The alloy layer thus formed cannot be used as a layer.
pre-cladding, a high boron content being, as we know, extremely detrimental to the hot corrosion resistance of the cladding layer which must then be deposited, due to the high chemical activity of the boron and in particular its affinity for oxygen. So it is necessary, before the cladding thermochemical treatment, remove the boron from the pre-sheathing by a particular heat treatment which also makes it possible to make partially diffuse the pre-cladding metal (nickel and / or cobalt) in the material substrate. This need to eliminate boron by treatment particular thermal, which could be considered a disadvantage, on the contrary has great advantages. It should first be noted that this elimination cannot be effected by simple heating of the pre-coated parts in a vacuum or in an atmosphere of argon or hydrogen. However, it is practically total except for a fraction which plays, as we will see, a r81e beneficial, if the parts are heated in a halogenated reducing atmosphere for a fraction of an hour to a few hours in a range of temperatures between 850 and 1300C.

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This poses, we can describe briefly, with reference to the diagrams in Figure 1, the elimination mechanism and the role of boron. The diagram represents the section of an uncoated part P and comprises, in a matrix of alloy A of cobalt-chromium, nickel-chromium, etc. of fibers F of carbide tantalum, niobium, etc. opening onto the surface of the room. Said on-face is inevitably covered with a thin layer O of various oxides or various impurities which can negate the attachment of a pre-cladding layer or cladding. Diagram lb represents the same section after filing of the metallic pre-cladding layer M 10 to 20 microns thick (nickel or cobalt) in which the boron B atoms are distributed.
It is recalled that boron is a non-metal whose compounds, in par-especially with oxygen and halides, have an enthalpy free of forma-very high tion. Boron halides are very volatile and very stable.
Despite the presence of the O layer and because of their high activity chemical, there is an intimate bond between the boron atoms and the substrate as shown in diagram lb.
In diagram lc, the part P is represented in the presence of a halogenated atmosphere H of suitable composition, examples of which will be given below. As the temperature rises, the atoms of boron, due to their great affinity for halogens, escape by diffusion through the pre-cladding layer M and those which were attached to the substrate give way to metallic atoms. There is thus the formation of a diffusion layer D between the metals of the substrate and those of the pre-cladding. Said metals are absolutely not attacked by halogens of the atmosphere.
Because the pre-cladding alloy has a melting temperature the higher it becomes depleted in boron, there is no fusion at any time pre-cladding.

} - 8 -, ~ ,, ~ 1, ~, In addition, electron microprobe examinations show that very small part of the boron initially contained in the pre-cladding remains fixed in the form of a very fine dispersion of C particles of very compound stable, such as carbides or carbonitrides, by carbon or nitrogen exchange with less stable compounds initially planned in the surfaces of treated parts, such as chromium carbides, tungsten, etc. These parts ticles will later form a barrier preventing the re-diffusion of the protective sheathing then deposited and significantly extend the service life said cladding in use.
We will now give indications on the choice of conditions of the various phases of the process of the invention according to the initial characteristics of the parts and the results to be obtained.
Regarding the chemical deposition phase of the alloy layer nickel-boron or cobalt-boron, or nickel-cobalt-boron, the adaptation of the composition tion of the bath to the composition of the pre-cladding to be obtained does not pose any problems individuals. An example of a nickel bath composition was given above.
boron. Cobalt-boron or nickel-cobalt-boron baths differ only in replacing more or less of the nickel ions with ions cobalt. The thickness of the chemical deposit can be controlled by the immersion time.
the parts in the bath. Although we may adopt, as appropriate, deposit thicknesses of 5 to 70 microns for example, the preferred values are generally between 10 and 40 microns. The optimal thickness is the most often around 20 microns. It is obtained by an immersion time about 2 hours in a bath having the above composition. An agitation mechanics of the bath, for example by ultrasonic excitation or insufflation of gas, is favorable to the regularity of the thicknesses. It is all the more useful that the parts to be treated have more complex shapes. The invention per-furthermore, using the masking methods mentioned above, B- g 1 ~) 45916 adjust the coating thicknesses of different areas of the same room according to the constraints of use to which they must be submitted.
Boron removal and pre-cladding diffusion treatment can be carried out according to different procedures depending on the desired results.
Diagram 2a of FIG. 2 relates to a mode of processing in fluorine reducing atmosphere at the equilibrium reduction of chromium fluoride and therefore non-chromizing. The treatment chamber 21 intended for ~ be placed in an oven not shown is closed by a cover 22 ensuring a closure of the so-called semi-sealed type, that is to say allowing limited exchanges between the atmosphere of the enclosure and the outside atmosphere.
On the bottom of the enclosure 21 is arranged a layer 23 of a mixture of chromium granules and chromium fluoride which can incorporate fluoride ammonium. The pieces 24 are placed in a basket 25, for example in nickel sheet, with perforated cylindrical walls and the closure of which is provided by a cover 26. A tube 27 provided with a valve 28 ensures the arrival of argon, then of a hydrogenated atmosphere in enclosure 21 to effect it purge and form a reducing atmosphere. During heating, the mixture 23 gives off chromous fluoride which reacts with hydrogen. Boron pre-cladding is attacked by hydrofluoric acid vapors resulting from the combination of fluorine released by the decomposition of chromium fluoride with hydrogen present in the atmosphere. This is evacuated by the tube 29 in causing boron fluoride vapors. This process is perfectly suitable when the parts have been rev ~ killed completely by a boron pre-cladding.
In diagram 2b, the enclosure 21 closed by the cover 22 contains in addition to the parts 24 of the cups 31 containing powder or chips of alloy age of composition similar to that of the mass of the parts and added with . ~, - 10 -1 ~ 4S916 chromous fluoride or a mixture of ammonium halides ~ bromide, fluorideJ
etc.) A tube 32 provided with a 3-way valve 33 is used as the tube.
purge at the start of treatment and as a source of hydrogenated atmosphere, the-said atmosphere being evacuated by the interstices between the enclosure 21 and its cover 22. This process, by which the halogenated atmosphere of the enclosure is maintained at the reduction equilibrium of the material constituting the parts particularly suitable when said parts include serves as pre-cladding (movable blade roots of reactors for example) because it avoids any risk of modification of the surface composition of the areas served.
In the diagram of FIG. 2c, the parts 24 are immersed in the enclosure 21 within a reactive mass 41 constituted by a mixture of powder of a refractory diluent (alumina or magnesia for example), powders or nickel chips, cobalt or refractory alloys, and powders halides such as ammonium chloride or fluoride, chromium fluoride, etc ... This process is significantly slower than the previous one due to the low thermal conductivity of the inert diluent and the treated parts have a less smooth and less shiny surface finish. But it has the advantage allow along with boron removal and pre-diffusion to adjust the chemical composition of the pre-cladding by adjusting the composition metallic constituents of the reactive mass. If for example this one is similar to the composition of the substrate, the boron will be eliminated from the pre-cladding which will broadcast without input; if the reagent contains chromium in a higher content than that of the substrate, the pre-cladding will be enriched with chromium. It is anyway, we prefer to use powders or shavings alloy; thus, on parts with nickel-boron pre-cladding, a reduced mass active with homogeneous alloy powder with 30% nickel and 70% cobalt, easily prepared by known techniques of diffusion in atmosphere ~ ,; ,.

~ 04591 ~;
halogenated, will enrich the pre-cladding in cobalt.
Whatever the procedure adopted and the composition of the pre-cladding, the boron removal and pre-diffusion heat treatment is generally carried out in a few hours at a temperature close to 1050C.
s these values can vary very significantly depending on the desired results and the means available, the duration of treatment being in principle all the more longer than the temperature is lower, for example a fraction hour at 1250C or around 20 hours at 800C. The structure of the layers Obviously obtained depends on the operating conditions. We can indicate, in slightly anticipating the description of the cladding treatment, that is to say formation of protective layers, that a deposit of nickel-boron on super-cobalt-based alloys followed by short treatment at moderate temperature boron removal and pre-diffusion and thermochemical treatment of sheathing by chromaluminization in a halogenated atmosphere finally gives a cladding, the external part of which consists essentially of aluminum nickel miniatures containing chromium while an elimination treatment prolonged at high temperature gives a cladding whose external part includes in addition to nickel aluminides, cobalt aluminides also containing chromium.
The semi-tight seals of diagrams 2a and 2c can obviously be replaced by other known arrangements such as sand joints, seals glassy, tight closures with valves, etc.
An advantage of the process of the invention is that the treatment for eliminating tion of boron can be carried out even if the boron pre-cladding is covered with additional pre-cladding in noble metals. This is how the regions of pi ~ these most fragile and most exposed to corrosion, such as trailing and leading edges of moving blades of reactor turbines can be covered after the pre-cladding treatment of platinum deposits and / or - 12 _ of palladium by electrolysis or by coating with diffusion layers thermochemical, then subjected to the elimination treatment. Boron is easy-ment eliminated through additional deposits.
The pre-cladding treatment of the invention does not impose any restrictions choice of cladding treatment, no special precautions. We can use any thermochemical treatment of the known type, and comprising by example the dep8t of chromium, aluminum, or chromium and aluminum and preceded if there is a treatment to form tantalum diffusion barriers, with yttrium, etc. No special surface preparation by polishing, lQ blage, etc ..... is not necessary. In general, the keeping in service of cladding thus deposited is very significantly improved by pre-cladding.
But the results are, as will be seen in the description of the examples of implementation which follow, particularly spectacular when the sheathing is carried out by chromaluminization within a mass active in which the metals to be deposited are pre-alloyed in the form of grains powder with a very fine particle size of the order of a micron.
Example 1a: protection of turbine rectifier vane sectors multiple-hole (multi-hole) cooling reactors, cobalt X 40 refractory superalloy.
2Q It is recalled that the weight composition of the alloy X 40 is, in percent:
C: 0.5 Cr: 25 Ni: 10 W: 17.5 Fe: 1.5 Co: remainder.
The parts have undergone, as well as witness test tubes, the following:
- pre-cladding with a nickel-boron coating of uniform thickness 25 microns by immersion at 90C for 2 hours in an aqueous bath having the composition already given, - elimination of boron and pre-diffusion by annealing in atmosphere ~ re re-B

~ 045916 fluorinated ductrix at equilibrium for 4 hours at 1040C, - chromaluminization for 20 hours at 1080C in a mass re-active consisting of a mixture of very fine powder of chromium-aluminum alloy (average particle size one micron) with an inert diluent and with 0.5% by weight ammonium chloride.
The parts and test pieces thus treated have a light gray tint and their protective coating has a total thickness ~ sheathing plus pre-sheathing) 85 micron uniform. It is frankly ductile even when cold. The review of electron microprobe test tubes show that it consists of a mixture of nickel and cobalt aluminides containing chromium and that the substrate bonding layer has a homogeneous dispersion of fine particles of defined compounds containing boron which appear to be carbon boron bonitride. The cobalt and nickel contents of the aluminides are in excess over stoichiometric values. The aluminum content crolt very gradually from the surface to the junction area while the chromium content is increasing. The cladding is free of any disconti-overnight or sting.
During air oxidation tests at 1100C with thermal cycles mics with abrupt variations, the test pieces remain intact after 300 hours of testing. During tests of resistance to sulfurization at 1000 C in the combustion gas from a fuel containing 1% sulfur, combustion state controlled so that the gases are alternately oxidative in nature and reducing nature, the coating of the test pieces withstands more than 1000 hours.
Example lb: comparative tests on constitution test pieces identical to those of example la and chromaluminized in an identical manner, but not having undergone the pre-cladding treatment of the invention.
The parts thus treated have a light gray tint. The thick-The total coating value is 45 microns. It is regular but the coating .

104 ~ 59iti is not very ductile. Examination with a micro-probe shows that the cladding is essentially consisting of cobalt aluminide containing islets coalesced with carbides and defined compounds rich in chromium. During oxidation and sulfurization tests identical to those of Example la, we notes an onset of oxidation deterioration after approximately 65 hours and deterioration to sulfurization after 300 hours.
EXAMPLE 1c: Comparative tests on constitution test pieces identical to those of examples la and lb, pre-sheathed like the test pieces of example la and chromaluminized in an identical way but not having undergone boron removal treatment.
During the oxidation and sulfurization tests the test pieces thus coated show flaking and pitting after only a few tens of hours.
EXAMPLE 2 Protection of Blades of Nickel-Based Alloys carrying heavy metals (MAR 200). The weight composition of the alloy MAR 200 in percent is as follows:
C: 0.15 Cr: 9 Co: 10 W: 12.5 Nb: 1 Ti: 2 Al: 5.
The structure of this alloy, similar to that of other alloys based on cobalt or nickel with low carbon content but relatively rich in so-called heavy elements ~ W, Nb, Mo, Ta, etc ...) presents ~ lots of intermetallic compounds whose disadvantages are the same, albeit at a weaker, than those of alloys rich in carbon. The pieces are treated in the following way:
- chemical deposit of cobalt-boron 10 microns thick, - chemical deposit of nickel-boron 10 microns thick, - elimination of boron by a treatment identical to that of the example the, - electrolytic dep ~ t of platinum of 15 microns on the trailing edges, 1 ~ 4S916 - chromaluminization under conditions identical to those of example 1a.
The results are similar to those of Example 1a.
Example 3 Comparative Tests on Blades of Matrix Alloys nickel-chromium and oriented reinforcing phase of niobium carbide fibers. The overall weight composition of this alloy is in percent:
C: 0.8 Cr: 10 Nb: 7.7 Ni: residue.
After oriented solidification involving ph ~ nomenes of segregated-eutectic cation, it comprises, in a matrix of solid nickel-chromium, niobium carbide fibers.
The treatment includes, with reservations on the blade feet, - 10 micron cobalt-boron chemical deposit, - chemical deposit of nickel-boron of 10 microns, - elimination of boron and pre-diffusion by annealing of parts in atmospheres fluorinated reducing sphere at equilibrium for 6 hours at 1050C, - chromaluminization for 12 hours at 1065C.
The resistance of the parts thus treated to oxidation and sulfurization hot is practically identical ~ that of the parts of Example la. The results are all the more convincing here since the protective coatings of parts coated by chromaluminization which have not been coated with a pre-coating according to the invention have very poor corrosion resistance due to the numerous surface heterogeneities caused by the emergence of fibers of carbon.
Example 4: Protection of fuel injectors in superalloy nickel base (PD 16) or Nimomic 105 used in power gas turbines sance. The weight composition of the PD 16 alloy, in percent, is the next :
C: 0.13; Cr: 6; Al: 6; W: 11; Mo: 2; Nb: 1.5; Ni: stay.

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The weight composition of Nimomic 105 is:
C: Q, 2; Cr: 15; A1: 45; Co: 20; Mo: 5; Ti: 1.2; Ni: stay.
The injectors are subject in operation to corrosion by impurities in industrial fuels used, in particular sulfur and vanadium and salt spray. The operating temperature is normally low but quickly reaches 900 to 950C at the time of periodic shutdowns of installations, due to the effects of radiation.
The injectors therefore undergo abruptly varying thermal cycles.
evoking very significant corrosions at the level of the fine injection channels.
The parts are treated in the following way:
- chemical deposit of nickel-boron 30 microns thick, - elimination of boron and pre-diffusion by annealing in a re-atmosphere fluorinated ductrix at equilibrium for 6 hours at 1040C, - chromisation for 20 hours at 1065C in a pre-reactive mass ally initially constituted by a mixture of very fine powder (granulo-average metric 5 microns of Cr: 40%; If: 10% mischmetal: 5%; alumina calcined: 45% with 0.5% by weight of ammonium chloride.
The sillicon present with the chromium in the cladding formed strengthens the corrosion resistance caused by the oxide derivatives of vanadium contained in the fuel. Mischmetal removes any island of oxide ~ especially oxide of Cr) in the cladding obtained.
The protective coating obtained on the PD 16 has a uniform thickness 65 microns. The coating obtained on the Nimomic 105 has a uniform thickness 70 micron shape.
During comparative real operation tests including abruptly varying thermal cycles, we observe from a hundred hours of significant corrosion on untreated parts while the coating of the treated parts resists more than 1000 hours.

, 1, 1 (~) 45916 The above examples show that the process of the invention is particularly effective in improving corrosion resistance hot parts of refractory alloys strongly loaded with carbon or heavy elements. Additional tests have shown that said process can be applied to iron or stainless steel parts complexes subjected to severe corrosion conditions and whose duration of service, that their protective external sheathing be carried out by chrom-aluminization or by simple chromization, is very appreciably increased.
In addition, the boron elimination treatment which constitutes one of the phases of the process of the invention proves to be very advantageous for facilitate the protection of composite parts made up of elements assembled by nickel-boron soldering. Nickel-boron solders, melting around 1050 to 1100C, are easy to use and allow make assemblies resistant to high temperature. Unfortunately, as shown in example lc, boron significantly reduces the resistance corrosion of protective sheaths. The aforementioned processing allows to eliminate this drawback and to raise the melting temperature in addition brazed zones as shown in Example 5 below.
Example 5: Protection of Cooled Blades Using Tubes internal brazed.
The blades are made of MAR 509 cobalt alloys, the composition of which by weight is, in percent:
C: 0.6 Cr: 21.5 Ni: 10 W: 7 Ta: 3.5 Ti: 0.2 Zr: 0.5 rest: Co The tubes are brazed with a brazing composition:
Cr: 15 B: 3.5 C: 0.1 remains nickel with a melting temperature of 1055C.
- When the parts are treated by chromaluminization without pre-~ ' 1 {) 45916 special precautions in a reactive mass or cement with alloy powder chrome-aluminum, there are cement adhesions to the brazed areas.
By cons, blades ~ ras ~ es submitted before chromaluminization ~ annealing from 6 hours at 1050C in a reducing atmosphere chlor ~ e at ~ equilibrium, the source of the atmosphere being a mixture of MAR 509 alloy chips and chloride of ammonium, do not show any attachment to you cement or any sagging of tubes, although the chrome plating process was continued at 1120 for 16 hours.
To conclude this description, it will be noted that it is pos-in principle likely to form the pre-cladding of nickel and / or cobalt alloy other than chemically. Boron being an energetic fontant of oxides metallic, molten nickel or cobalt alloys with boron wet metal surfaces perfectly. As little ~ -on for example sprinkle the parts to be pre-coated with a powder of such an alloy, for example a powder of boron solder, and heat the pieces ~ these to a temp ~ rature sufficient in atmos-oxidizing sphere or in a vacuum, either in the oven or by surface heating, by a known method, for example by induction or by electron beam.
But the deposits thus obtained have ~ greater thicknesses and less re-regular, especially if the pieces have complex shapes, chemically required which are therefore generally preferable.

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Claims (10)

ACHIEVEMENTS OF THE INVENTION, ABOUT WHICH ONE
EXCLUSIVE RIGHT OF OWNERSHIP OR PRIVILEGE IS CLAIMED, ARE DEFINED AS FOLLOWS 1. Process for forming a nickel-based coating and / or cobalt on parts made of metallic materials, the overall weight composition contains at least 50% of metals from the group of iron, cobalt and nickel, characterized in that at least one layer is first formed on said parts of a nickel and / or cobalt alloy with boron and in this that the boron is then removed; the metallic material being refractory material; the deposition of the alloy layer being made by immersing the pieces in an aqueous bath containing at least one nickel and / or cobalt salt and one boron salt;
and characterized in that the boron is removed by heating the parts coated in a reducing atmosphere halogenated at a temperature between 800 and 1250 ° C, elimination of boron being accompanied by diffusion of cobalt and / or nickel in the metallic material. 2. Method according to claim 1, characterized in that that the metallic material has a weight content of at least minus 0.4% carbon, and characterizes in that the material metallic comprises, in a matrix based on at least one iron group metal, cobalt, nickel and chromium, refractory metal carbide fibers. 3. Method according to claim 1, characterized in that that the thickness of the alloy layer is between 5 and 70 microns. 4. Method according to claim 1, characterized in that that the thickness of the alloy layer is between 10 and 40 microns. 5. Method according to claim 1, characterized in that that the rooms are heated in the presence of a source of chromous fluoride. 6. Method according to claim 1, characterized in that that the rooms are heated in the presence of a halo-genes and a divided mass of a composition alloy analogous to that of the material. 7. Method according to claim 1, characterized in that that the parts are heated within a reactive mass consisting of a mixture of a chemically inert powder, metal powder shavings or grains and a source of helogens. 8. Method for forming on parts made of materials metallic protective coating against corrosion to high temperature, said process comprising depositing a pre-cladding coating then depositing a coating of cladding and being characterized in that the coating of pre-cladding is formed by a method according to claim 1. 9. Method according to claim 8, characterized in that that the cladding is made of an alloy containing chromium and / or aluminum in substantially contents greater than that of the material. 10. Method according to claim 8, characterized in that that the sheathing coating is deposited by heating the parts within a reactive mass constituted by a mixture of a chemically inert powder, chrome alloy powder aluminum whose particle size is preferably at most equal to one micron and a halogenated compound.