EP0370838B1 - Process for the surface protection of metallic articles against high-temperature corrosion, and article treated by this process - Google Patents

Description

The invention relates to protection against corrosion at high temperature of metallic materials, and more particularly of materials based on nickel, cobalt and / or iron, including steels.

The invention relates more particularly to superalloys, in particular nickel-based, used for the manufacture of hot parts of a turbomachine, for example fixed or mobile turbine blades and distributors, which must have excellent resistance to oxidation and to corrosion at high temperature, in particular in the presence of molten sodium sulphate from the spray and sulfur impurities contained in the fuel.

A known protection method for this purpose consists in forming, by deposition and / or diffusion on the surface of the substrate to be protected, a layer containing a nickel, cobalt and / or iron aluminide, by a treatment comprising bringing into contact. of the part to be treated with a filler material containing aluminum.

Such a deposition treatment, known in the art under the name of low activity aluminization or chromaluminization, is described in detail in patent FR-A-1 490 744. In this known treatment, the filler material comprises an alloy with chromium base comprising aluminum in a proportion by weight of 5 to 25% and, optionally, silicon in a proportion by weight of 3 to 10%, and the parts to be treated are brought into contact with the finely divided filler material, under a hydrogenated atmosphere, at a temperature between 750 and 1200 ° C.

To improve the efficiency of the aluminization treatment, it has been proposed to precede it with a pre-deposition treatment comprising the deposition and / or diffusion on the substrate surface of at least one metal of the platinum mine, the aluminization treatment can be either the low activity aluminization already mentioned, or a variant thereof called high activity aluminization, or another type of treatment, for example a vapor phase aluminization such as that designated under the reference RT22 by the Chromalloy Company. High activity aluminization differs from low activity aluminization described above in that the metallic part of the filler material is composed by weight of 55 to 70% aluminum and 45 to 30% chromium, in that that the treatment temperature is between 650 and 750 ° C, and preferably equal to approximately 700 ° C, its duration being between 7 and 8 hours and preferably equal to approximately 7 hours 30. This high activity deposit is followed by a post-diffusion treatment in a non-oxidizing atmosphere (argon or hydrogen), the duration and temperature of which vary depending on the substrate.

Such a process is described for example in patents FR-A-2,071,753, FR-A-2,072,284, FR-A-2,333,055, GB-A-2,129,017, US-A-3,677,789, US-A-3,819,338, US-A-4,439,470 and US-A-3,692,554.

In some cases, a nickel or chromium pre-deposit precedes the platinum mine metal pre-deposit or accompanies the aluminization treatment.

The most commonly used platinum mine metal in the pre-deposit treatment is platinum itself, which significantly improves the protection provided by the aluminization treatment. However, platinum has the disadvantage of being very expensive.

Other metals of the platinum mine are mentioned in the prior documents cited above, including palladium, whose price is about four times lower than that of platinum. The Applicant therefore attempted to use palladium to replace platinum, but encountered great difficulties. It has been found that samples of nickel-based superalloy subjected to a pure palladium pre-deposit treatment and to a high activity or low activity aluminization treatment exhibit numerous blisters, leading to poor resistance to hot corrosion as well. weakening of the coating. The Applicant has determined that these blisters are to be attributed to the presence in the palladium layer of a large amount of occluded hydrogen, dissolved in the pre-deposit during the preparation of the coating, the solubility of hydrogen in palladium. being very high.

The object of the invention is to obtain an aluminide coating modified by a pre-deposit containing palladium, by avoiding the occlusion of hydrogen or any other gas capable of causing the phenomenon of blistering.

The invention relates to a method of protecting the surface of a metallic substrate based on nickel, cobalt and / or iron, comprising a pre-deposition treatment followed by a deposition treatment, the deposition treatment comprising the deposition and / or the diffusion of aluminum on the surface of the pretreated substrate and the pre-deposit treatment comprising one or more successive phases during which are provided with material which is deposited and / or which is diffused on the surface of the substrate, the composition of the filler material being able to be different from one phase to another and said filler material containing , during at least one phase, palladium, characterized in that palladium is the only metal of the platinum mine present in the filler material and that this contains, during any phase for which it contains palladium and / or at least one subsequent phase of the pre-deposition treatment, at least one barrier metal chosen from nickel, cobalt and chromium.

During the deposition treatment, the pretreated substrate can in particular be brought into contact with a filler material containing aluminum and chromium. Such a deposition treatment is, for example, a high activity or low activity aluminization as defined above.

At least one phase of the pre-deposition treatment can comprise a deposition operation of filler material at low temperature followed by a diffusion operation at high temperature under vacuum, this diffusion operation being carried out, preferably, at a temperature of about 850 ° C under an air pressure at most equal to 1.3 mPa (10⁻⁵ torr).

According to one embodiment of the invention, at least one phase of the pre-deposition treatment comprises the deposition and / or diffusion of a palladium alloy and at least one barrier metal. The pre-deposit treatment can then comprise a single material supply phase.

According to another embodiment, in a first phase of the pre-deposition treatment, the filler material is formed essentially of palladium, and in a second phase subsequent to the first, the filler material is formed essentially of at least one barrier metal. The terms "first phase" and "second phase" do not refer here to the absolute position of the phases in the course of the pre-deposit processing but are simply used to identify one compared to the other the two phases considered. The first and second phases can then follow each other directly and constitute, for example, the only two phases of supply of material from the pre-deposit treatment.

The invention applies more particularly to a nickel-based substrate, the aluminide being essentially a nickel aluminide and the barrier metal preferably also being essentially nickel.

In the case where an alloy of palladium and nickel is deposited, this advantageously comprises approximately 80% of palladium and 20% of nickel by weight and can be deposited for example by electrolytic means.

In the case where palladium and nickel are successively deposited, the first can be deposited chemically autocatalytically and the second by triode sputtering.

• les figures 1 et 2 sont des photographies représentant un échantillon traité selon la technique antérieure, respectivement avant et après un essai de corrosion à haute température ;
• la figure 3 est un graphique montrant les variations de masse par unité de surface de différents échantillons traités soumis à des essais de corrosion ;
• les figures 4, 6, 8 et 9 sont des photographies montrant des échantillons traités selon l'invention ; et
• les figures 5 et 7 sont des photographies montrant les échantillons des figures 4 et 6 respectivement après essai de corrosion.

Other advantages of the invention will emerge from the detailed description below and the attached drawings, in which:

• Figures 1 and 2 are photographs showing a sample treated according to the prior art, respectively before and after a high temperature corrosion test;
• Figure 3 is a graph showing the variations mass per unit area of different treated samples subjected to corrosion tests;
• Figures 4, 6, 8 and 9 are photographs showing samples treated according to the invention; and
• Figures 5 and 7 are photographs showing the samples of Figures 4 and 6 respectively after the corrosion test.

To illustrate the difficulties arising from the use of palladium in the known process of deposition of aluminide modified by a pre-deposit, a pre-deposit of pure palladium 8 ”m thick was sprayed on an IN100 superalloy substrate. cathode triode. The sample was then subjected to a 2-hour heat treatment to diffuse the pre-deposit at 850 ° C, under a total air pressure at most equal to 1.3 mPa (10⁻⁵ torr). A high activity type nickel aluminide coating as defined above was then produced on this sample by activated case hardening in a pack. At the end of the final post-diffusion operation, the surface of the sample was covered with a very large number of blisters (Figure 1). Such a coating surface condition makes the material unusable for turbomachine components. The sample was then subjected to a high temperature corrosion test consisting of thermal cycling in an air oven between temperatures of about 850 ° C and 200 ° C, with one hour increments at 850 ° C and with periodic contamination of the sample with sodium sulfate at a rate of 0.5 mg / cm² every 50 cycles. This test is representative of the stresses on the components of the hot parts of the turbomachinery under so-called hot corrosion conditions. The resistance of the coating to hot corrosion has been found to be very low. After 150 one hour cycles, the degraded state of the sample is critical (Figure 2) and there is significant pitting corrosion. The mass gain is very high (Figure 3, curve A). The behavior of this sample is comparable to that of a sample having undergone a conventional high activity aluminization in the absence of any pre-deposit, as can be seen in Figure 3 where curve B relates to this last sample.

Quite similar results have been obtained by replacing the high-activity aluminization treatment with a low-activity pack aluminization treatment and / or by performing the pre-deposition of palladium by autocatalytic chemistry.

Examples of embodiments of the invention are described below.

Example 1

On an IN100 nickel-based superalloy substrate, approximately 10 μm of a palladium-nickel alloy containing 20% nickel by mass was electrolytically deposited. The sample was then subjected to a 2 h diffusion diffusion treatment at 850 ° C under a total air pressure at most equal to 1.3 mPa (10⁻⁵ torr). A coating of nickel aluminide of high standard activity type was then carried out on this sample by case hardening in a pack. At the end of the final post-diffusion operation, the surface of the sample was free from the slightest blister (Figure 4). The sample was subjected to the hot corrosion test described above with respect to the samples treated according to the prior art. Excellent results have been obtained. The weight variation curve is given in Figure 3 (curve C). In addition, after 1000 one hour cycles, neither pitting corrosion nor internal corrosion of the part is observed (FIG. 5). To compare, the variation in weight of a sample of identical shape composed of a substrate of the same kind, covered with a coating of aluminide modified by platinum, of RT22 type, has been plotted on FIG. 3 (curve D). Pitting corrosion is observed on the edges of this sample after approximately 600 cycles.

Example 2

The procedure was as in Example 1, replacing the high activity aluminization by an aluminization of the standard low activity type. The same result was obtained (see FIG. 3, curve E), FIGS. 6 and 7 showing the sample respectively before and after the corrosion test.

Example 3

This example differs from Example 1 only in the pre-filing processing. The latter here comprises two phases of material input. In the first phase, approximately 8 µm of pure palladium was deposited by autocatalytic chemistry. The sample was then subjected to a 2 hour diffusion heat treatment at 850 ° C under a total air pressure at most equal to 1.3 mPa (10⁻⁵ torr). In the second phase, a deposit of approximately 3 m of pure nickel was applied by triode sputtering. The sample then underwent a second diffusion heat treatment identical to the first. FIG. 8 shows that the sample obtained is free from all blisters and has an impeccable surface condition.

For comparison, a similar sample was treated in the same way, but omitting the second phase of the pre-deposit, the two samples being placed in the same aluminizing charge and having undergone the same treatment. annealing under hydrogen. It is found that the comparative sample had superficial blisters at the end of treatment.

Example 4

The procedure was as in Example 3, but replacing the high activity aluminide coating with a nickel aluminide coating of standard low activity type. A similar result was obtained (Figure 9). In the same way, a comparison sample treated by omitting the second phase of pre-deposition and in the same charge of cement exhibited at the end of treatment superficial blisters.

Example 5

The procedure was as in Example 4, but omitting the diffusion heat treatment between the palladium deposit and the nickel deposit. Here again, the surface of the sample was free of any blistering and its surface condition was impeccable.

Example 6

The procedure was as in Example 5, replacing the deposit of nickel with a deposit of approximately 3 m of cobalt by electrolytic route under the conditions below: Composition of the bath: - hydrated cobalt sulfate: 175 g / l - cobalt chloride: 80 g / l - boric acid : 20 g / l
Current density between 2 and 4 A / dm²
Temperature: 45 ° C.

Example 7

The procedure was as in Example 6, replacing the chemical deposition of palladium by an electrolytic deposition carried out under the following conditions: Composition of the bath: - pallet chloride tetrammine: 50 g / l - ammonium hydroxide qs pH = 8.5 Current density: 2 A / dm² Temperature : 50 ° C.

Example 8

The procedure was as in Example 7, replacing the low activity aluminide coating with a standard high activity aluminide coating.

Example 9

The procedure was as in Example 7, replacing the deposit of cobalt with a deposit of approximately 3 m of chromium applied electrolytically under the conditions below: Composition of the bath: - chromic anhydride: 250 g / l - sulfuric acid (d = 1.82): 2.5 g / l
Current density between 30 and 50 A / dm²
Temperature between 45 and 55 ° C.

Example 10

The procedure was as in Example 9, replacing the low activity aluminide coating with a standard high activity type aluminide coating.

In all these examples, samples were obtained which were free from surface blisters and which had an irreproachable surface condition.

Of course, the invention is not limited to the deposition methods described. The pre-deposit can be carried out in particular by chemical, electrolytic, thermochemical, physical or by spraying. The aluminization can be carried out in particular by diffusion or by chemical, electrolytic, thermochemical or physical way.

Claims (18)

1. Process for the surface protection of a nickel-, cobalt- and/or iron-based metallic substrate, comprising a preplating treatment followed by a plating treatment, the plating treatment comprising the plating and/or the diffusion of aluminium on the surface of the pretreated substrate and the preplating treatment comprising one or more successive phases during which the material is applied by plating and/or diffusion on the surface of the substrate, where the composition of the added material can vary from phase to phase and the said material contains palladium during at least one phase, characterised in that palladium is the only platinum-ore metal present in the material and that this material contains, during all phases for which it contains palladium and/or at least one subsequent phase of the preplating treatment, at least one of the barrier metals nickel, cobalt or chromium.
2. Process in accordance with Claim 1, characterised in that during the plating treatment, the pretreated substrate is brought into contact with a plating material containing aluminium and chromium.
3. Process in accordance with Claim 2, characterised in that the plating treatment is a high-activity aluminisation.
4. Process in accordance with Claim 2, characterised in that the plating treatment is a low-activity aluminisation.
5. Process in accordance with any of the preceding claims, characterised in that at least one phase of the preplating treatment includes an operation for the low-temperature plating of the added material followed by a high-temperature diffusion operation under vacuum.
6. Process in accordance with Claim 5, characterised in that the diffusion operation is carried out at a temperature of approximately 850°C under an air pressure at most equal to 1.3 mPa (10⁻⁵ torr).
7. Process in accordance with any of the preceding claims, characterised in that at least one phase of the preplating treatment includes the plating and/or the diffusion of an alloy of palladium and at least one barrier metal.
8. Process in accordance with Claim 7, characterised in that the preplating treatment includes a single plating phase.
9. Process in accordance with any of Claims 1 to 6, characterised in that in a first phase of the preplating treatment, the plating material is formed essentially of palladium and that in a second phase subsequent to the first, the plating material is formed essentially of at least one barrier metal.
10. Process in accordance with Claim 9, characterised in that the first and second phases are immediately consecutive.
11. Process in accordance with Claim 10, characterised in that the preplating treatment comprises two plating phases.
12. Process in accordance with any of the preceding claims, characterised in that the substrate is nickel-based, that the aluminiure is essentially a nickel aluminiure and the barrier metal is essentially nickel.
13. Process in accordance with one of Claims 7 and 8, characterised in that the said alloy consists of a weight proportion of approximately 80% palladium and 20% nickel.
14. Process in accordance with Claim 13, characterised in that the said alloy is applied by electrolysis.
15. Process in accordance with one of Claims 9 to 11, characterised in that the plating material of the first phase is essentially palladium and that of the second phase is essentially nickel.
16. Process in accordance with Claim 15, characterised in that the palladium is applied by chemical autocatalysis and the nickel by triode cathodic evaporation.
17. Nickel-, cobalt- and/or iron-based metallic component, protected by the process in accordance with one of the preceding claims.
18. Component in accordance with Claim 17, characterised in that it is designed to form one of the hot parts of a turbo-engine.

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