CA1038114A - Corrosion-resistant coating for superalloys - Google Patents

Abstract

Abstract An oxidation and hot corrosion resistant alloy for coating a superalloy substrate. The surface coating com-position comprises a nickel base alloy containing chromium and silicon.

Description

CORROSI N-RESISTANT COATING FOR S~PERALEOYS
Background of the Invention This invention relates to the coating of a superalloy substrate with an oxidation and hot-corrosion resistant sur-face coating alloy composition comprising a nickel base alloy containing chromium and silicon which alloy will be referred to hereinafter as AMB-2.
Hot-corrosion resistance is required for applications in turbines burning natural gas or uncontaminated light distil-lates or in contaminated environmentR involving combusted diesel, heavy distillates or residual oils.
Many protective alloy coatings for superalloy substrates are Rhown by the prior art. These coatings include the conven-tional aluminide coatings which have technical limitations which restrict the coating thickness resulting in an early deterior- ¦
ation of the coatings and further do not exhibit the superior hot corrosion resistance properties of the sub~ect alloy coat-! ing, especially in a hot-corrosion operating environment. The sub~ect coating alloy is also more economical in its initial ~ 20 processing than the alloys of the prior art. The field repair ¦ of components removed from service is simple and more economical since selected areas can be recoated without need to mask or protect areas not requiring additional coating, as is the case of pack-aluminide or electron beam vapor deposited coatings.
In considering alloy coating compositions for super-alloy substrates it was discovered that the alloy must exhibit the following characteristics:
1) Must be oxidation and hot-corrosion resistant over the temperature range 1400 to about 2000F.

lO~B114

2) Must melt, wet and flow uniformly at some temper-ature below the inclpient melting point of the superalloy substrate. Ideally, the vacuum brazing time/temperature cycle used to apply the coating alloy to the substrate should be com-patible with the normal heat treatment cycle for the substrate.

3) Must be metallurgically stable, and compatible with the substrate alloy.
Summary of the Invention It is therefore, an ob~ect of my invention to provide a superalloy with a coating alloy which is oxidation and hot-cor-rosion resistant over a temperature range.
Another ob~ect is to provide an alloy coating that melts, wets and flows uniformly at some temperature below the incipient melting point of the superalloy substrate.
A further ob~ect is to provide an alloy which is metal-lurgically compatible with the substrate alloy and may be applied as a thick coating.
Briefly stated, the present invention relates to a coating alloy for a superalloy substrate having the following composition:
Chromium 45-65%, Silicon 5-12Z, Nickel-balance. All compositions are given in weight percent. A nominal coating composition as proposed by applicant may comprise 45Z Chromium, lOZ Silicon with the balance Nickel.
It is known that high Cr levels are the most effective deterrent to hot corrosion caused by Na and S- bearing atmospheres in the temperature range 1400-1800F. At these temperatures, greater than about 36Z Cr is required to generate a-Cr precipi-tate. ~owever, it should be noted that the content of chromium should not be too high since the coating may become brittle over the range recited. The embrittlement is caused by the formation of excessive amounts of hard, body centered cubic ~-Cr precipitate.

10;~8114 Similarly, silicon ls beneflcial for oxldatlon nnd hot corrosion resistance through the formation of SiO2. Silicon is used to control the melting and solidification behavior of the coating since the eutectic temperature in the pure ~i-Cr binary system occurs at 2450F which is too high for most nlckel based superalloy substrates.
The coating may be applied to the substrate by various methods including vacuum brazing, which is an established indus-trial technique. However, other conventional methods of applying the coating alloy to the superalloy substrate could be used such as the slurry, aerosol spray or plasma spray plus heat treatment and transfer tape methods. However, some methods should be avoided such as the vapor deposition method which yields a coatlng mlcro-structure orlented normal to the substrate surface, thus establlsh-ing potentlal short-clrcult dlffuslon paths e.g., graln boundaries, and growth defects for the introduction of corrodents such as sul-phur and oxygen to the substrate. The re-solldlfication struc-ture of the subject vacuum bra~ed alloy is non-orlented, preclud-ing this potential failure mode. Since the subject process .- ~ 20 involves the liquid and not the vapor state, greater segregation of the coatlng elements occurs. The present alloy takes advantage of this fact since the high chromium content produces ~-Cr pre-cipitate particles diqpersed ln Y-qolld .Yolutlon nickel mntrix containing a high chromium level. The lower melting point ~ Si eutectic phase is equally well dispersed throughout the coating during solidification.
Those parts of the present invention which are considered to be new are set forth in detail in the claims appended hereto.
The invention, however, may be better understood and the advantage appreciated from a detailed description as follows:

10;3B114 Detalled Descriptlon A coatlng alloy was prepared using a nominal ~lloy com-posltion comprised of 45% Chromium, 10~ Silicon and the balance Nickel. Other compositions may be used falling within the range, supra. The substrate was prepared by mechanical abrading or by chemical cleaning plus electroplating a 0.2 to l.O mil layer of nickel thereon. The alloy used is in the form of a powder and is fabricated into a brazing transfer tape. The tape is com-prised of -200 + 325 mesh powder, held together with about 5~0 of an organic binder on a plastic backing sheet. A template of the desired shape to fit the substrate is cut from the transfer tape. The plastic backing is removed and the tape applied to the substrate.
Additional liquid brazing cement may be needed to firmly adhere the tape in place. The coated part is then subjected to the vacuum brazing cycle. The vacuum brazing cycle is controlled to permit outgas~ng of the binder at about 700 to 1000F, to min-imize contamination of the coating and substrate. The optimum vacuum brazing cycle consists of heating the alloy to a temperature of about 2075F for about five minutes followed by argon gas cool-ing. Generally no finishing treatments are required, since the as-brazed coated surface yields a surface finish in the 35 to 60 microinch (RMS) range. The part may receive a final heat treatment to develop the mechanical properties of the substrate. This step j 25 should be well below the alloy brazing temperature, to prevent re-melting of the coating or excessive interdiffusion of critical elements across the interface between the coating and substrate.
~ence, the coatlng cycle must be included in the appropriate heat , trea~ment quence for the =peclfic substrate alloy.

~; 103~114 In the as-solidifled condition, the microstrttct~lre of the subJect alloy contains a mlxture of y-Ni matrix, a-Cr precipitate particles, and Ni:Si eutectic. The precise com-position and morphology of these phases depend both on the starting compositions of the subJect alloy powder and substrate alloy, as well as the subsequent brazing and heat treatment cycles. The corrosion resistance of the subject alloy is derived from the high bulk Cr content (i.e. 45~) of the coating, but more specifically, it is due to the ~-Cr particles and the high Cr, Y-Ni matrix, which constitute a very significant portion of the coated structure. Since the coating is applied in the liquid state and resolidified, elements from the substrate are easily incorporated into the coating. Hence, the brazing cycle (time and temperature) can be utilized to control the morphology and composltion of the coating to some extent. Nickel base super- ¦
alloys have been coated in the temperature range 2060 to 2130F, with time-at-temperature between 2 and 20 minutes. Lower braz-ing temperatures are not preferred due to ~B-2's melting char-acteristics. Higher brazing temperatures can be used depending upon the heating and cooling rates, the equipment used and other considerations. Specific superalloy substrates may even require . ~ ~ higher temperatures; however, the optimum parameters for the .- . reference alloys is 2075F for 5 minutes. In general, the higher the temperature, the short~r the time,to prevent excessive flow, reaction, and interdiffusion with the substrate. High temperatures and/or longer brazing times promote large a -Cr particles, less Ni:Si eutectic but greater interdiffusion with the substrate.
Lower temperatures and time produce smaller, better dispersed ~ -Cr particles and Ni:Si eutectic, with little substrate interdif-fusion. One feature of the subject alloy coating in the as-coated conditlon ls lts lack of a complex "dlffuslon zone" between the coatlng and substrate. By contrast, conventlonal alumi-nide coatlngs are characterized by a finger-like dlffusion zone containing brittle intermetalllc compounds, such as S sigma and carbides.
The nominal alloy compositions of the superalloy substrates to which our coating is applied are listed in Table A a8 follo=a:
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_7_ ~0381~4 The non-oricnted structure Or the sub~ect nlloy ts due to the nature of the melting and re-solldiflcation process.
Segregation of the elements and resulting precipitates ls related to composition, heat input during brazing, and cooling conditions.
Line-of-sight vapor-deposited coatings, such as the MCrAlY's deposited by electron beam evaporation, generally grow normal to the substrate surface. Grain growth is, therefore, normal to the substrate, hence, growth defects are also oriented. Growth defects, when they occur in the sub~ect alloy, are non-oriented solidif~-cation defects.
As previously stated, the sub~ect alloy coating for a super-alloy substrate has exhibited superior hot corrosion resistant prop-erties over other prlor art coated superalloys. Oxldatlonthot-cor-roslon testing have been conducted under simulated gas turbine conditions in a small combustion burner rig. A controlled atmos-phere was produced by combustlng doped diesel oll containing 1~ S, to whlch artiflcial sea salt was mixed to produce 8 ppm Na in the combustion products. The rigs were run at 1600F, at an air:fuel ratio of 60:1 with a gas velocity of 70 fps. The speclmens were removed and alr-blasted to room temperature every 50 hours to simulate turblne shutdown and to promote oxide and/or coating spallation under severe thermal cycling conditions. This is the most severe test condition utllized to simulate a hot-corrosion operating environment.
In a first test AMB-2 was braze-coated on IN-738 using techniques previously described, and compared to available commercial aluminide coatings applied to IN-738. Results were obtained by sectioning the specimens, and metallographically determining at 100 times magnification the maximum depth of corrosion penetration through the coating and substrate, the average bulk coating surface loss, as well as an approximation of the area percent coating remain-ing. The results, listed in Table I below show the clear superiority¦
of AMB-2 over conventional aluminide coatings.

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~ 1()3t~114 It Is lm~ortunt t~ note th~t the comm~rclal pro~s~ used for applylng conventional aluminide coatings, known as p~ck concen- ;
tration, has technical and economic limitations which restrict aluminide thickness to approximately 3 mils and somewhat less on Co-base superalloys. Since pack cementation is basically a vapor deposition process, applied thickness is time-dependent.
AMB-2, however, can be applied up to about 10 mils thickness, with no change in the timeltemperature vacuum brazing cycle.
These data in Table I show that the aluminide coatings tested were essentially fully penetrated after ~ust 600 to 1000 hours, with virtually no coating remaining. In many cases, significant corrosion of the IN-738 substrate resulted from the destruction of the coatlng.
The data in Table I further shows that a substantial por-tion of AMB-2 remains after 2000 hours of testing. This is due, in part, to the fact that ~MB-2 can be applied in thicknesses up to about 10 mils, as stated above, with no change required in the technique or time/temperature parameters used in its appli-cation. Hence, AMB-2 offers both a more corrosion resistant alloy composition and increased coating thickness, both of which result in a longer life.
Another series of burner rig tests were conducted on ~`~-2 coated Rene-77, (See Table II below) and compared to conventional aluminide coatings. Some of the tests were run in an undoped natural gas atmosphere, which produces a normal oxldizing environ-ment. Aluminide coatings generally offer excellent resistance in this case, but suffer severe attack in the presence of S and Na contaminated atmospheres. This data in Table II, post, confirms the above information that AMB-2~s at least as protective as aluminide coatings in undoped environments, and is superior in con-taminated environments.

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l :~0;~E~114 Another measure of coating behavior is its effect on the mechanical properties of the substrate. (See Table III below).
Two different effects are possible: (1) the coating alloy ¦
reacts metallurgically with the substrate producing harmful phases which embrittle the structure, or (2) the thermal treat-ment required to apply and stabilize the coating is incompatible with the substrate's heat treatment, hence degrading its mechan-ical properties.
In Table III, standard 0.252" diameter as-cast test bars of Rene-77 aDd IN-738 were bra~e coated with AMB-2, and tensile and rupture tested. The results are compared in Table III with aluminide-coated bars of these two alloys. Each coating/alloy combinatlon was given the full heat treatment previously found optimal for that system. The data shows that the room temperature tensile properties of both IN-738 and Rene-77 are least affected by AMB-2. Yield strengths are 10 to 15% higher than the aluminide coatings, with only slightly less ductility. The latter is due in part to the greater thickness of ~B-2.

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Claims (4)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. An oxidation and corrosion resistant composite comprising a superalloy substrate and a coating alloy bonded thereto consisting essentially of the following ele-ments in weight percent;
Chromium - 40-65%
Silicon - 5-12%
Nickel - Balance

2. The coating alloy of Claim 1 consisting essen-tially of 45% chromium 10% silicon and the balance nickel.

3. The alloy of Claim 1 wherein the microstructure contains a mixture of ?-Ni matrix, .alpha.-Cr precipitate parti-cles and Ni:Si eutectic.

4. The coating alloy of Claim 1 wherein the thick-ness of the alloy coating composition for the superalloy may range up to 10 mils.