Methods of applying chromium diffusion coatings onto selective regions of a component

Abstract

Unique and improved chromizing processes are disclosed. The processes involve forming localized chromizing coatings onto selected regions of a substrate. The chromium diffusion coatings are locally applied to selected regions of substrates in a controlled manner, in comparison to conventional chromizing processes, and further in a manner that produces less material waste and does not require masking. A second coating can be selectively applied onto other regions of the substrate.

Claims

1. A method for producing a chromium diffusion coating onto selected regions of a substrate, comprising the steps of: providing a chromium-containing slurry; applying the chromium-containing slurry onto localized surfaces of the substrate; curing the slurry; heating the slurry in a protective atmosphere to a predetermined temperature for a predetermined duration; generating chromium-containing vapors; diffusing the chromium into said localized surfaces to form the coating, said coating having a microstructure characterized by a substantial reduction in nitride and oxide inclusions and reduced levels of -Cr phase in comparison to conventional chromizing processes.

2. The method of claim 1, wherein said method comprises locally applying a second coating onto the substrate.

3. The method of claim 2, wherein said second coating comprises an aluminide coating.

4. The method of claim 2, wherein said second coating comprises a MCrAlY coating.

5. The method of claim 1, wherein the step of applying the slurry comprises brushing, spraying, dipping, dip-spinning, injection, or any combination thereof.

6. The method of claim 1, wherein said localized surfaces are subject to corrosion attack.

7. A method for producing a localized chromium diffusion coating and a localized aluminide diffusion coating onto selected regions of a substrate simultaneously, comprising the steps of: providing a chromium-containing slurry; applying the chromium-containing slurry onto a first region of the substrate, characterized by an absence of masking; providing an aluminide-containing material; heating the chromium-containing slurry and the aluminide-containing material in a protective atmosphere to a predetermined temperature for a predetermined duration; diffusing chromium into the first region; diffusing aluminum into a second region in the absence of masking; forming the localized chromium diffusion coating along the first region, said chromium diffusion coating having a microstructure characterized by a substantial reduction in nitride and oxide inclusions and reduced levels of -Cr phase in comparison to conventional chromizing processes; and forming a localized aluminide-diffusion coating along the second region.

8. The method of claim 7, wherein said first region is subject to corrosion attack.

9. The method of claim 7, wherein said second region is subject to oxidation attack.

10. The method of claim 7, wherein the step of applying the chromium slurry comprises brushing, spraying, dipping, dip-spinning or injection or any combination thereof.

11. The method of claim 7, wherein said chromium diffusion coating and aluminide diffusion coating are formed simultaneously during diffusion treatment.

12. The method of claim 7, wherein the substrate is a gas turbine blade and said first region comprises a shank.

13. The method of claim 12, wherein said second region comprises an airfoil.

14. A method for producing a localized chromium diffusion coating and a localized aluminide diffusion coating onto selected regions of a blade simultaneously, comprising the steps of: providing a chromium-containing slurry; applying the chromium-containing slurry onto a shank of the blade, characterized by an absence of masking; providing aluminide-containing material within the retort; loading the partially coated blade into the retort; heating the partially coated blade; generating aluminum containing vapors and chromium containing vapors; diffusing chromium from the chromium-containing vapors into an external region of the shank of the blade; diffusing aluminum from the aluminum-containing vapors into an airfoil of the blade; forming the localized chromium diffusion coating along the shank, said chromium diffusion coating having a microstructure characterized by a substantial reduction in nitride and oxide inclusions and reduced levels of -Cr phase in comparison to conventional chromizing processes; and forming the localized aluminide-diffusion coating along the airfoil.

15. The method of claim 14, wherein said chromium-containing slurry is locally applied onto the shank.

16. The method of claim 14, wherein said localized diffusion coating forms in the absence of masking.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The objectives and advantages of the invention will be better understood from the following detailed description of the preferred embodiments thereof in connection with the accompanying figures wherein like numbers denote same features throughout and wherein:

(2) FIG. 1 shows a conventional turbine blade;

(3) FIG. 2 shows a schematic of selectively applying a local aluminide coating and a local chromizing coating onto selective regions of a substrate;

(4) FIG. 3 shows a block flow diagram, in accordance with principles of the present invention, for an approach of simultaneously forming a chromium diffusion coating on the surface of selected regions of a turbine blade while forming an aluminide coating on the surface of other regions of the turbine blade;

(5) FIG. 4 shows a block flow diagram, in accordance with principles of the present invention, of a 2-step approach that initially forms a chromium diffusion coating on the surface of selected regions of a turbine component and thereafter forms an aluminide coating on the surface of other regions of the component;

(6) FIG. 5 shows a block flow diagram of a 2-step approach for applying chromium diffusion coating on the surface of selected regions of a turbine component and then applying a MCrAlY overlay coating onto the surfaces of other selected regions of the component; and

(7) FIG. 6a shows a cross-sectional microstructure of an aluminide coating locally applied on an airfoil, and FIG. 6b shows a cross-sectional microstructure of a chromium diffusion coating locally applied on the shank, whereby both coatings were produced by the method described in Example 1 utilizing the inventive approach shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

(8) The objectives and advantages of the invention will be better understood from the following detailed description of the preferred embodiments thereof in connection. The present disclosure relates to novel and improved methods for applying chromium diffusion coatings onto selective regions of a component. The disclosure is set out herein in various embodiments and with reference to various aspects and features of the invention.

(9) The relationship and functioning of the various elements of this invention are better understood by the following detailed description. The detailed description contemplates the features, aspects and embodiments in various permutations and combinations, as being within the scope of the disclosure. The disclosure may therefore be specified as comprising, consisting or consisting essentially of, any of such combinations and permutations of these specific features, aspects, and embodiments, or a selected one or ones thereof.

(10) In all of the embodiments of the present invention, the terms chromizing slurry and chromizing coating will refer to those chromium-containing compositions as more fully described in U.S. Provisional Patent Application 13603-US-P1, Application Ser. No. 61/927,180, filed concurrently on Jan. 14, 2014, and which is hereby incorporated by reference in its entirety. As more fully described therein, the chromizing coatings produced from such a chromizing slurry composition are unique and characterized by significantly reduced levels of nitride and oxide inclusions, along with lower -chromium phases, compared to those chromizing coatings produced by conventional chromizing processes. As a result, the coatings have superior resistance to corrosion, erosion and fatigue in comparison to chromizing coatings produced by conventional pack, vapor or slurry processes.

(11) The improved formulation is based, at least in part, upon the selected combination of specific halide activators and buffer materials within the slurry formulation. The slurry composition comprises a chromium source, a specific class of halide activator, a specific buffer material, a binder material and a solvent. The slurry composition comprises a chromium source in a range from about 10% to about 90% of the slurry; a halide activator in a range from about 0.5% to about 50% of the chromium source, a buffer material ranging from about 0.5% to about 100% of the chromium source; a binder solution in a range from about 5% to about 50% of the slurry in which the binder solution includes a binder and a solvent. An optional inert filler material may be provided that ranges from about 0% to about 50% of the slurry weight. In a preferred embodiment, the chromium source is in a range from about 30% to about 70%; the halide activator is in a range from about 2% to about 30% of the chromium source, the buffer material is in a range from about 3% to about 50% of the chromium source; the binder solution in a range from about 15% to about 40% of the slurry weight; and the optional inert filler material is in a range from about 5% to about 30% of the slurry.

(12) Generally speaking, the chromium slurry comprises a chromium source, a specific halide activator and a binder solution. The chromium slurry further comprises a specific metallic powder or powder mixture which can lower the chemical activity of chromium in the slurry and getter residual nitrogen and oxygen during coating process. Further details of the chromizing slurry and chromizing coating compositions are described in U.S. Provisional Patent Application 13603-US-P1, Application Ser. No. 61/927,180, filed concurrently on Jan. 14, 2014.

(13) In accordance with the principles of the present invention, the chromium diffusion coatings of the present invention are locally applied to selected regions of metallic substrates in a controlled manner, in comparison to conventional chromizing processes, and further in a manner that produces less material waste and does not require masking. Unless indicated otherwise, it should be understood that all compositions are expressed as weight percentages (wt %).

(14) The slurry chromizing process is considered to be a chemical vapor deposition process. Upon heating to elevated temperature, the chromium source and the halide activator in the slurry mixture react to form volatile chromium halide vapor. Transport of the chromium halide vapor from the slurry to the surface of the alloy to be coated takes place primarily by the gaseous diffusion under the influence of chemical potential gradient between the slurry and the alloy surface. Upon reaching the alloy surface, these chromium halide vapors react at the surface and deposit chromium, which diffuses into the alloy to form the coating.

(15) One embodiment of the present invention utilizes locally applying the chromium slurry composition onto a gas turbine blade (as shown in FIG. 1). Suitable methods include brushing, spraying, dipping, dip-spinning or injection. The specific method of application depends, at least in part, on the viscosity of the slurry composition as well as the geometry of the components. The chromizing slurry composition is applied onto any one or more of the regions of the blade susceptible to type II corrosion attack, such as, a surface of the shank, root, under platform and internal cooling passages. Complex and customized tooling and masking, as typical and known to be utilized for many pack processes, are not required, thereby simplifying the overall chromizing process. In general, application of approximately 0.02-0.1 inches of chromizing slurry ensures adequate coverage without the use of excessive amounts of slurry compositions, thereby minimizing the use of raw materials. Having applied the chromizing slurry, the slurry is subject to a heat cycle in a protective atmosphere for a predetermined temperature and duration to allow the chromium to diffuse into the localized regions of the component. After diffusion treatment, any remaining slurry residues along the localized regions can be removed by various methods, including wire blush, oxide grit burnishing, glass bead, high-pressure water jet or other conventional methods. Slurry residues typically comprise unreacted slurry compositional materials. The removal of any slurry residue is conducted in such a way as to prevent damage to the underlying chromizing surface layer. The resultant chromizing coating contains insubstantial amounts of oxide and nitride inclusions along with lower levels of alpha-chromium phase, compared to a conventional pack chromizing process. The average chromium content in the chromium diffusion coating is about 15-50 wt %, and more preferably 25-40 wt %.

(16) Compared to pack chromizing, the slurry method of the present invention allows the slurry to be locally applied only onto only those regions where chromizing coating is required. Furthermore, unlike pack chromizing, no complex and customized tooling and masking is necessary.

(17) Another embodiment of the present invention provides for application of different coatings onto selective regions of a component. Specifically, an aluminide coating can be locally applied in conjunction with the chromizing coating. FIG. 2 shows the resultant coating system that is produced by the methods of the present invention. A chromizing coating is located on the bottom region of the substrate where corrosion resistance is required, and an aluminide coating is located on the top region where oxidation resistance is needed. Any conventional aluminide coating process such as vapor phase, slurry or chemical vapor deposition aluminization processes may be employed to produce the aluminde diffusion coating. As an example, an aluminide slurry coating process may be utilized with a conventional aluminide slurry such as SermAlcote 2525, which is commercially made and sold by Praxair Surface Technologies, Inc. (Indianapolis, Ind.). The aluminde slurry can be applied in a manner as known in the art, and as described in U.S. Pat. No. 6,110,262, which is hereby incorporated by reference in its entirety.

(18) In a preferred embodiment of the present invention, FIG. 3 shows a block flow diagram for simultaneously forming in a single step a localized chromium diffusion coating on the surface of selected regions of a turbine blade while forming a localized aluminide coating on the surface of other regions of the turbine blade. One or more chromium slurry layers are applied onto selected regions of the blade which are susceptible to type II corrosion attack, such as the surface of the shank, root, under platform and internal cooling passages. Brushing, spraying, dipping, dip-spinning or injection may be used to apply the chromizing slurry at a thickness sufficient to ensure adequate coverage of the surfaces. Masking is not required by virtue of the ability to selectively apply the chromizing slurry onto only the desired surfaces of the blade.

(19) After applying the chromizing slurry, a conventional vapor phase, slurry or chemical vapor deposition aluminizing process may be utilized with suitable aluminide source materials as known in the art. Diffusion treatment may occur under an elevated temperature ranging from about 1000-1150 C. in a protective atmosphere for up to 24 hours, and more preferably about 2-16 hours. Upon heating to the elevated temperature, aluminum halide vapors are generated from aluminide source materials, transport to the surface of the alloy, and form aluminide coatings where no chromizing slurry is applied. These aluminum halide vapors can also reach the region of the outer surface of the chromizing slurry. However, these aluminum halide vapors react with chromium source in the slurry mixture to form chromium halide vapors, thereby leading to a substantial decrease in the partial pressure of aluminum halide vapor through the slurry thickness towards the alloy surface. Meanwhile, within the chromizing slurry, chromium halide vapors were partially generated via chemical reactions of the chromium source and the halide activator in the slurry mixture. As a result, chromium halide vapors, as opposed to aluminum halide vapors, tend to prevail and preferentially occupy the localized regions where the chromizing slurry has been applied. The existence of chromium halide vapors in such regions enables formation of a chromide coating that is thermodynamically favored over an aluminide coating. Consequently, the localized aluminide diffusion coating is locally produced in a controlled manner along those surfaces where no chromizing slurry had been applied, while a localized chromium diffusion coating is simultaneously produced along other regions.

(20) In a preferred embodiment, the chromium slurry is provided and applied onto a region of the turbine blade susceptible to type II corrosion (i.e., shank). No special tooling for masking is required. The partially slurry-coated blade is then loaded into a vapor phase aluminizing retort and heated in a protective atmosphere to carry out a vapor-phase aluminzation process. The chromium and aluminizing coatings are simultaneously formed during the heat cycle. The aluminizing coating forms along regions susceptible to oxidation (i.e., airfoil) while the chromizing coating forms along relatively cooler regions susceptible to corrosion (i.e., shank) without employing masking Any excess residue may be removed from the coated regions.

(21) Other variations are contemplated. For example, the aluminide coating can be applied separately after formation of the chromizing coating. Prior to the aluminizing process, an aluminizing mask is applied to the chromizing region that was previously produced by the localized slurry chromizing process of the present invention. This mask prevents the deposition of aluminide coating over the chromizing coating during the aluminizing process, as inadvertent deposition of the aluminide coating over the chromizing coating can weaken the corrosion resistance of the chromizing coating. In this regard, FIG. 4 shows a 2-step approach of a block flow diagram in accordance with principles of the present invention. Alternatively, the aluminide coating can be applied before formation of the chromizing coating.

(22) Still further, other types of coatings may be utilized in the present invention. As an example, after diffusion treatment of the chromium slurry-coated part onto those selected regions of the turbine blade susceptible to corrosion attack and removal of any residual coating, a second MCrAlY overlay coating can be applied to the airfoil by any conventional processes, such as air plasma spray, LPPS or HVOF. Prior to applying the MCrAlY coating, a mask is applied to the chromizing region that was previously produced by the localized slurry chromizing process of the present invention. FIG. 5 shows a block flow diagram of such a 2-step approach for the coating process.

EXAMPLE 1

(23) A turbine blade as shown in FIG. 1 was selectively coated with a chromizing slurry composition and an aluminide coating utilizing the one-step approach shown in FIG. 3. The chromizing slurry composition was prepared comprising an aluminum fluoride activator, chromium powder, nickel powder, and an organic binder solution. The slurry was prepared by mixing the following: 75 g chromium powder, 325 mesh; 20 g aluminum fluoride; 4 g klucel hydroxypropylcellulose; 51 g deionized water; 25 g nickel powder and 25 g alumina powder.

(24) The chromizing slurry composition was applied to selected surfaces of a shank as shown in FIG. 1 by dipping the blade into the slurry. The turbine blade was made of a single-crystal nickel-based superalloy which has a nominal composition of, by weight, about 7.5%Co, 7.0%Cr, 6.5%Ta, 6.2%Al, 5.0%W, 3.0%Re, 1.5%Mo, 0015%Hf, 0.05%C, 0.004%B, 0.01%Y and the balance nickel. The slurry coating was then allowed to dry in an oven at 80 C. for 30 minutes followed by curing at 135 C. for 30 minutes.

(25) The slurry coated part was loaded into a typical vapor phase aluminizing retort which contained a source of CrAl nuggets and aluminum fluoride powder. The CrAl nugget and aluminum fluoride powder were located in the bottom of coating retort. The slurry coated part was placed out of contact with both CrAl nugget and aluminum fluoride. After purging the retort with flowing argon for 1 hour, the retort was heated to 2010 F. in an argon atmosphere and held for 4 hours to allow the chromium and aluminum to selectively diffuse into the airfoil of the specimen, respectively. Upon the completed diffusion treatment, the specimen was cooled to ambient temperature under argon atmosphere and the slurry residues were removed from the specimen surface by a light grit-blasting operation.

(26) Results of the coating are shown in FIGS. 6a and 6b. The specimen had its upper half or airfoil region coated with the aluminide coating, as shown in FIG. 6a, to resist high-temperature oxidation and its bottom half or shank region coated with a chromium enriched layer, as shown in FIG. 6b, to resist low-temperature hot corrosion. The chromium diffusion coating had an insignificant amount of oxide and nitride inclusions compared to conventional pack or vapor phase chromizing processes. The coating was observed to substantially free of -Cr phase and the average chromium concentration in chromium diffusion coating was greater than 25 wt. %.

(27) The chromizing methods of the present invention represent a substantial improvement over conventional Cr diffusion coatings produced from pack, vapor or slurry processes. As has been shown, the present invention offers a unique method for locally applying chromizing slurry formulations with an optional second coating along other selected regions. The slurries of the present invention are advantageous in that they can be selectively applied with control and accuracy onto localized regions of the substrate by simple application methods, including brushing, spraying, dipping or injecting. Further, the control and accuracy of applying the chromizing and other coating can occur in a single step without masking On the contrary, conventional pack and vapor phase processes cannot locally generate chromium coatings along selected regions of a substrate. As a result, these conventional coatings require difficult masking techniques which typically are not effective in concealing those regions along the metallic substrate not desired to be coated.

(28) The ability for the present invention to locally apply slurry formulations to form coatings has the added benefit of significantly lower material waste. As such, the present invention can conserve overall slurry material and reduce waste disposal, thereby creating higher utilization of the slurry constituents. The reduction in the raw materials required for coating minimizes exposure of hazardous materials in the workplace.

(29) Still further, unlike pack and vapor phase processes, the modified slurry formulations of the present invention can be used to form the improved chromium coatings onto various parts having complex geometries and intricate internals. Pack processes have limited versatility, as they can only be applied to parts having a certain size and simplified geometry.

(30) It should be understood that in addition to gas blades, the principles of the present invention may be utilized to coat any suitable substrate requiring controlled application of chromizing coatings. In this regard, the methods of the present invention can protect a variety of different substrates that are utilized in other applications. For example, the chromizing coatings as used herein may be locally applied in accordance with the principles of the present invention onto stainless steel substrates which do not contain sufficient chromium for oxidation resistance. The chromizing coatings form a protective oxide scale along the stainless steel substrate. Additionally, the present invention, unlike conventional processes, is effective in locally coating selected regions of substrates having internal sections with complex geometries.

(31) While it has been shown and described what is considered to be certain embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail can readily be made without departing from the spirit and scope of the invention. It is, therefore, intended that this invention not be limited to the exact form and detail herein shown and described, nor to anything less than the whole of the invention herein disclosed and hereinafter claimed.