Abstract:
A method including aluminization treatment preceded by a palladium predeposition treatment, the palladium being associated with at least one barrier metal selected from nickel, cobalt, and chromium, thereby avoiding hydrogen occlusion.

Description:
The invention relates to protecting metal materials against corrosion at high temperature, and more particularly materials based on nickel, cobalt, and/or iron, including steels. 
     BACKGROUND OF THE INVENTION 
     The invention relates more particularly to superalloys, in particular those based on nickel and used for making the hot parts of turbomachines, for example the fixed or moving turbine blades of gas turbines which must have excellent resistance to corrosion and oxidization at high temperature, particularly in the presence of molten sodium sulfate from spray and sulfur-containing impurities in the fuel. 
     One known protection method for this purpose consists in diffusing and/or depositing a layer containing nickel, cobalt, and/or iron aluminide on the surface of the substrate to be protected by using a treatment which includes putting the part to be treated into contact with an additional substance containing aluminium. 
     One such deposition treatment known as &#34;low activity aluminization&#34; or as &#34;chrome-aluminization&#34; is described in detail in French patent number FR-A-1 490 744, the content of which is incorporated into the present description by reference. In this prior treatment, the additional substance comprises a chromium-based alloy containing 5% to 25% by weight of aluminum and possibly containing 3% to 10% by weight of silicon, with the parts to be treated being put into contact with the additional substance in a finely divided state under a hydrogen-containing atmosphere at a temperature lying between 750° C. and 1200° C. 
     In order to improve the effectiveness of the aluminization treatment, proposals have been made to precede the aluminization treatment with a predeposition treatment comprising depositing and/or diffusing at least one platinum group metal on the surface of the substrate, with the aluminization treatment then being either low activity aluminization as mentioned above, or else a variant thereof referred to as &#34;high activity aluminization&#34;, or else some other type of treatment, e.g. vapor phase aluminization as designated under the reference RT22 by Chromalloy. High activity aluminization differs from low activity aluminization described above in that the metal fraction of the additional substance comprises 55% to 70% aluminum and 45% to 30% chromium, by weight, in that the treatment temperature lies in the range 650° C. to 750° C., and is preferably equal to about 700° C, and in that its duration lies in the range 7 hours to 8 hours and is preferably equal to about 71/2 hours. Such high activity deposition is followed by post diffusion treatment under a non-oxidizing atmosphere (argon or hydrogen) for a period of time and at a temperature which depend on the substrate. 
     Such a method is described, for example, in the following patents: FR-A-2 071 753, FR-A-2 333 055, GB-A-2 129 017, US-A-3 677 789, US-A-3 819 338, US-A-4 43-9 470, and US-A-3 692 554. 
     The most commonly used platinum group metal for predeposition treatment is platinum itself which provides a clear improvement in the protection provided by the aluminization treatment. However platinum suffers from the drawback of being very expensive. 
     Other platinum group metals are mentioned in the above-referenced prior documents, including palladium which is about four times cheaper than platinum. The Applicant has therefore attempted to use palladium instead of platinum, but great difficulties were encountered. It was observed that nickel-based superalloy samples subjected to a predeposition treatment using pure palladium followed by low or high activity aluminization treatment suffered from numerous blisters, leading to poor resistance to corrosion when hot and fragilizing the coating. The Applicant has determined that these blisters are due to a large quantity of occluded hydrogen in the palladium layer which is dissolved in the predeposit while the coating is being made, since hydrogen is very highly soluble in palladium. 
     The object of the invention is to obtain an aluminide coating modified by a predeposit containing a platinum group metal other than platinum itself, and in particular containing palladium, while avoiding the occlusion of hydrogen or any other gas that could give rise to the phenomenon of blistering. 
     SUMMARY OF THE INVENTION 
     The present invention provides a part comprising a metal substrate based on nickel, cobalt, and/or iron, and a protective coating constituted by a deposit and a predeposit between the substrate and the deposit, the predeposit containing at least one platinum group metal other than platinum itself and comprising one or more layers, and the deposit containing an aluminide of nickel and/or cobalt and/or iron modified by said platinum group metal, wherein the predeposit also contains at least one barrier metal selected from nickel, cobalt, and chromium, the barrier metal being present in any layer containing said platinum mine metal and/or in at least one overlying layer. 
     Nickel, cobalt, and chromium have the property of dissolving practically no gas even at high temperature, thereby protecting platinum group metals, and in particular palladium, against such dissolution. 
     The thickness of the predeposit preferably does not exceed about 100 μm, and may be about 10 μm. 
     In one implementation of the invention, at least one layer of predeposit is essentially constituted by an alloy of at least one platinum group metal with at least one barrier metal. The predeposit may then comprise one layer only. 
     In another implementation, a first layer of predeposit is formed essentially from at least one platinum group metal, and a second layer overlying the first is formed essentially from at least one barrier metal. The terms &#34;first layer&#34; and &#34;second layer&#34; do not refer herein to the absolute position of the layers within the predeposit, but serve simply to identify the two layers under consideration relative to each other. In this case, the first and second layers may be adjacent and they may constitute the only two layers in the predeposit. 
     The invention also provides a method of protecting the surface of a metal substrate based on nickel, cobalt, and/or iron, the method comprising predeposition treatment followed by deposition treatment, the deposition treatment comprising depositing and/or diffusing aluminum at the surface of the pretreated substrate, and the predeposition treatment comprising one or more successive stages during which a substance is added by being deposited and/or caused to diffuse at the surface of the substrate, the composition of the added substance optionally being different from one stage to another and said added substance containing, at least during one stage, at least one platinum group metal other than platinum itself, wherein during any stage for which said added substance contains said platinum group metal and/or during at least one subsequent pretreatment stage, said added substance includes at least one barrier metal selected from nickel, cobalt, and chromium. 
     During the deposition treatment, the pretreated substrate may be put into contact with a carrier substance containing aluminum and chromium. Such deposition treatment may be low or high activity aluminization as defined above, for example. 
     At least one predeposition treatment stage may comprise an operation of depositing an additional substance at low temperature followed by a diffusion operation at high temperature and under a vacuum. Said diffusion operation is preferably performed at a temperature of about 850° C under a pressure of air not greater than 10 -5  torr. 
     In an implementation of the invention, at least one predeposition treatment stage comprises depositing and/or diffusing an alloy of at least one platinum group metal and at least one barrier metal. The predeposition treatment may then comprise a single stage during which substance is added. 
     In a second implementation, the added substance in the first predeposition treatment stage is essentially constituted by at least one platinum group metal, and in a second stage, subsequent to the first, the added substance is constituted essentially by at least one barrier metal. The terms &#34;first stage&#34; and &#34;second stage&#34; call for the same comments as made above about the terms &#34;first layer&#34; and &#34;second layer&#34;. The first and second stages may follow one another directly and, for example, they may constitute the only two stages during which substance is added during the predeposition treatment. 
     The invention is more particularly applicable to a substrate based on nickel, with the aluminide being essentially nickel aluminide and with the barrier metal being likewise, preferably, essentially nickel. 
     The platinum group metal is preferably essentially palladium. 
     When depositing an alloy of palladium and nickel, the alloy advantageously comprises about 80% palladium and 20% nickel by weight and is, for example, deposited electrolytically. 
     When palladium and nick=1 are deposited in succession, palladium may be deposited by a self-catalyzing chemical technique while nickel is deposited by triode cathode sputtering. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are described by way of example with reference to the accompanying drawings, in which: 
     FIGS. 1 and 2 are photographs showing a sample treated using the prior art, respectively before and after a high temperature corrosion test; 
     FIG. 3 is a graph showing change in mass per unit area of various treated samples subjected to corrosion tests; 
     FIGS. 4, 6, 8, and 9 are photographs showing samples treated in accordance with the invention; and 
     FIGS. 5 and 7 are photographs showing the samples of FIGS. 4 and 6 respectively, after corrosion testing. 
    
    
     DETAILED DESCRIPTION 
     In order to illustrate the difficulties that stem from using palladium in the prior art method of depositing aluminide as modified by a predeposition stage, an 8 μm thick predeposit of pure palladium was formed on a substrate made of IN100 superalloy, with predeposition being performed by triode cathode sputtering. The sample was then subjected to two hours of heat treatment at 850° C in order to diffuse the predeposit, with the treatment being performed under a total air pressure of not more than 10 -5  torr. A high activity type of nickel aluminide, as defined above, was then coated onto said sample by pack activated cementation. After the final post diffusion operation, the surface of the sample was covered by a very large number of blisters (FIG. 1). Such a coating surface state makes the material unusable for turbomachine components. The sample was then subjected to high temperature corrosion testing by thermocycling in an oven open to the air and between temperatures of about 200° C. and 850° C., with one hour pauses at 850° C., periodically contaminating the sample with 0.5 mg/cm 2  of sodium sulfate every 50 cycles. This test is representative of the constraints to which components in the hot portions of turbomachines are subjected under hot corrosion conditions. The resistance of the coating to corrosion while hot was very low. After  150 one hour cycles the state of degradation of the sample became critical (FIG. 2) and major corrosion in the form of corrosion pits can be observed. The increase in mass was very high (FIG. 3, curve A). The behavior of this sample was comparable to the behavior of a sample subjected to conventional high activity aluminization in the absence of any predeposit, as can be seen in FIG. 3 where curve B relates to such a sample. 
     Entirely similar results were obtained when the high activity aluminization treatment was replaced by low activity type pack aluminization and/or when the palladium predeposit was obtained by a self-catalyzing chemical procedure. 
     Examples of the invention are described below. 
     EXAMPLE 1 
     About 10 μm of palladium-nickel alloy having 20% nickel by weight was electrolytically deposited on an IN100 nickel based superalloy substrate. The sample was then subjected to two hours of diffusion heat treatment at 850° C. under a total air pressure of not more than 10 -5  torr. A standard high activity type nickel aluminide coating was then made on this sample by pack activated cementation. After a final post diffusion operation, the surface of the sample was free from any blisters (FIG. 4). The sample was subjected to the hot corrosion test described above for samples treated using the prior technique. Excellent results were obtained. The curve showing change in weight is given in FIG. 3 (curve C). In addition, after 1000 one-hour cycles, no pitting corrosion and no internal corrosion of the part were observed (FIG. 5). By way of comparison, FIG. 3 also shows (curve D) the change in weight of a sample of identical shape constituted by a substrate of the same nature covered with an RT22 type platinum-modified aluminide coating. Pitting corrosion was observed along the edges of this sample after about 600 cycles. 
     EXAMPLE 2 
     The procedure was the same as in Example 1 except that high activity aluminization was replaced using standard low activity type aluminization. The same result was obtained (see FIG. 3, curve E), and FIGS. 6 and 7 show the sample respectively before and after corrosion testing. 
     EXAMPLE 3 
     This example differs from Example 1 solely in the predeposition treatment. In this case the predeposition treatment comprises two stages during which substance is added. In the first stage, about 8 μm of pure palladium were added by a self-catalytic chemical process. The sample was then subjected to two hours of diffusion heat treatment at 850° C under a total air pressure of not more than 10 -5  torr. During the second stage, about 3 μm of pure nickel was deposited by triode cathode sputtering. The sample was then subjected to a second diffusion heat treatment identical to the first. FIG. 8 shows that the resulting sample is free from any blistering and has an irreproachable surface state. 
     By way of comparison, a similar sample was treated in the same way except that the second predeposition stage was omitted, with both samples being subjected to the same aluminization charge and both being subjected to the same annealing treatment under hydrogen. Surface blistering was observed on the comparison sample at the end of treatment. 
     EXAMPLE 4 
     The procedure was the same as in Example 3 except that the high activity aluminide coating was replaced by standard low activity type nickel aluminide coating. A similar result was obtained (FIG. 9). Similarly, a comparison sample treated by omitting the second predeposition stage and subjected to the same charge of cement had surface blistering at the end of treatment. 
     EXAMPLE 5 
     The procedure was the same as in Example 4 except that diffusion heat treatment was omitted between depositing the palladium and depositing the nickel. Here again the surface of the sample was free from any blistering and its surface state was irreproachable. 
     EXAMPLE 6 
     The procedure was the same as in Example 5, except that the nickel deposit was replaced by a deposit of about 3 μm of cobalt by means of an electrolytic technique under the following conditions: 
     
         ______________________________________ Composition of the bath:______________________________________hydrated cobalt sulfate:              175 g/lcobalt chloride:   80 g/lboric acid:        20 g/lCurrent density lying in the range              2 A/dm.sup.2 to 4 A/dm.sup.2.Temperature:       45° C.______________________________________ 
    
     EXAMPLE 7 
     The procedure was the same as Example 6 except that the chemical deposition of palladium was replaced by electrolytical deposition under the following conditions: 
     
         ______________________________________ Composition of the bath:______________________________________tetramine palladium chloride:              50 g/lammonium hydroxide:              to make pH = 8.5current density:   2 A/dm.sup.2temperature:       50° C.______________________________________ 
    
     EXAMPLE 8 
     The procedure was the same as in Example 7 with the low activity aluminide coating being replaced by standard high activity type aluminide. 
     EXAMPLE 9 
     The procedure was the same as in Example 7, with the cobalt deposit being replaced by a deposit of about 3 μm of chromium applied electrolytically under the following conditions: 
     
         ______________________________________ Composition of the bath:______________________________________chromium anhydride:              250 g/lsulfuric acid (d = 1.82)               2.5 g/lCurrent density lying in the range              30 A/dm.sup.2 to 50 A/d.sup.2.Temperature lying in the range              45° C. to 55° C.______________________________________ 
    
     EXAMPLE 10 
     The procedure was the same as in Example 9, except that the low activity aluminide coating was replaced by a standard high activity type aluminide coating. 
     In all of these examples, samples were obtained that were free from surface blisters and in which the surface state was irreproachable. 
     Although the method of the invention has been described more particularly in implementations using palladium as the platinum group metal, platinum group metals other than platinum and palladium may also be used in the method. It is also possible to make use of two or more platinum group metals, including platinum, in the same stage or in successive stages of the predeposition treatment. 
     Naturally the invention is not limited to the methods of predeposition described. In particular, predeposition may be performed chemically, electrolytically, thermochemically, physically, or by sputtering. The aluminization may be performed by diffusion or chemically, electrolytically, thermochemically, or physically.