Chromium-free inorganic coating system for hot corrosion protection of superalloy substrates

Abstract

This invention describes a hexavalent chromium-free slurry formulation suitable for producing inorganic overcoating systems. This formulation provides excellent hot corrosion protection and high-temperature oxidation protection for superalloy substrates. A base coat slurry and a top coat slurry are provided. The base coat slurry comprises an aluminum phosphate-based aqueous solution containing pigments with an Al:P molar ratio greater than about 1:3 and incorporating metal particles or metal oxide particles, or a combination of both. The top coat slurry comprises an aluminum phosphate-based aqueous solution with an Al:P molar ratio greater than about 1:3. The inorganic overcoating coating formed on a substrate using the slurry formulation of this invention is used for hot corrosion protection against hot corrosion on superalloy substrates. Furthermore, multilayer coatings comprising a metallic binder coating and an inorganic overcoating coating formed on a superalloy substrate further enhance high-temperature oxidation protection and hot corrosion protection.

Description

Technical Field

[0001] This invention relates to novel hexavalent chromium-free slurry formulations suitable for producing inorganic overcoatings and durable multilayer coating systems that provide excellent hot corrosion and high-temperature oxidation protection for superalloy substrates and can replace traditional hexavalent chromium (Cr(VI))-containing slurries and coatings. The slurry formulations and coatings are particularly suitable for turbine engine components. Background Technology

[0002] The surfaces of turbine engine components are exposed to hot gases from the turbine combustion process. Turbine engine superalloy materials are selected based on their high-temperature stability and corrosion resistance. Typically, gas turbine engine components are composed of nickel-based superalloys, which possess generally acceptable high-temperature mechanical properties, such as fatigue resistance, and generally acceptable resistance to oxidation and corrosion damage. Currently, various superalloys are commercially available, such as… and These and other superalloys have been extensively studied and discussed in the literature. For example, Pollock et al. reviewed and discussed the typical composition and progress of nickel-based superalloys for turbine disks in “Nickel-based Superalloys for Advanced Turbine Engines: Chemistry, Microstructure and Properties”, J. Propulsion and Power, v.22, 2, 2006. However, many current nickel-based superalloy materials are prone to degradation under harsh conditions at high operating temperatures. Furthermore, newer generations of disk superalloys contain lower concentrations of chromium, thus exhibiting poorer resistance to so-called hot corrosion (i.e., sulfidation-induced) erosion.

[0003] As used herein and throughout, “hot corrosion” is defined as the direct erosion of metallic components, particularly nickel-based alloys, by molten or solidified sulfates. Hot corrosion has been identified and combated in turbine engines for many years. There are two main types of hot corrosion. The first type is the so-called high-temperature type 1 hot corrosion, which is typically observed in the temperature range of approximately 825°C–950°C (1517°F–1742°F), and occurs most aggressively at approximately 885°C (1625°F). The second type is low-temperature type 2 hot corrosion, which is most aggressive at around 700°C (1292°F). The temperature of the final stage of a compressor can reach and exceed 1300°F. Components in the final stage of a compressor operating at temperatures of 1300°F or higher, including but not limited to nickel-based alloy blades, guide vanes, discs, and seals, are susceptible to pitting damage caused by sulfates accumulating on the surfaces of such components due to low-temperature type 2 hot corrosion. Different mixtures of alkali metal and alkaline earth metal sulfates (such as sodium, magnesium, potassium and calcium) can be ingested in marine environments along with incoming air or formed as a result of combustion processes.

[0004] Oxidation and corrosion reactions on component surfaces can lead to metal loss and wall thickness reduction. This metal loss rapidly increases stress on the corresponding component, ultimately causing failure. Therefore, protective coatings are applied to components to protect them from oxidation and sulfide corrosion degradation.

[0005] Coatings used to protect superalloys are generally classified into diffusion and overcoating groups. An example of a diffusion coating is an aluminide, where aluminum is enriched on the surface of an alloy (such as a nickel-based alloy) to form intermetallic compounds (such as nickel aluminum compounds).

[0006] Furthermore, the hot corrosion of superalloy components can be limited by a "chromizing process," in which chromium diffuses to create intermetallic compounds on the component surface. Methods have been developed to modify these diffusion-generated intermetallic compounds by adding metals such as platinum, palladium, and / or rhodium. Modified intermetallic coatings are more expensive due to the incorporation of precious metals but generally exhibit improved resistance to sulfide corrosion. For example, U.S. Patent No. 9,777,583 to Leggett discloses a protective coating applied to the shank and root of a gas turbine blade; this protective coating contains platinum and chromium, thereby creating a chromium-rich outer oxide layer on top of the coating, thus minimizing the diffusion path to the sulfur-corroded substrate.

[0007] While generally effective against type 2 hot corrosion, all types of diffusion-derived coatings share the common drawback of requiring high processing temperatures to initiate the diffusion process and form the desired intermetallic phase in the substrate surface layer. Specifically, heat treatment can involve temperatures ranging from 850°C to 1150°C (1562℉–2102℉). As another disadvantage, the formation of diffusion-derived coatings may require controlled atmosphere treatment, such as argon or vacuum.

[0008] Another option for protecting superalloys is a topcoat. Commercially available ceramic topcoats are typically formed by curing in air at 315°C–340°C (599°F–644°F), making them easier to process and less expensive than diffusion coatings.

[0009] Various multi-layered protective systems have been proposed and used to protect turbine engine components. However, evaluations of commercially available protective systems have revealed common deficiencies in their composition and functional characteristics, as well as several possible failure modes.

[0010] For example, commercially available multi-layered coverage systems (referred to as...) N3000 provides a generally acceptable smooth, stain-resistant surface, thus preventing the deposition of corrosive substances. The N3000 system comprises a chromate-phosphate binder filled with metal oxide pigments (particularly alumina Al2O3 and chromium oxide Cr2O3) and provides effective protection against type 2 hot corrosion caused by a less corrosive mixture of calcium sulfate and carbon black, as discussed by BGMcMordie in “Impact of Smooth Coatings on the Efficiency of Modern Turbomachinery”, Aerospace / Airline Plating & Metal Finishing Forum, Cincinnati, Ohio, March 27-29, 2000. However, The N3000 system was designed for lower operating temperatures and is therefore prone to cracking and delamination at the higher operating temperatures (≥1300℉, 704℃) that may be encountered in the final compressor stage of newer engines.

[0011] This drawback of the N3000 coating system was overcome by a multilayer coating system described and patented in U.S. Patent No. 9,598,775 to Belov. When exposed to the same corrosive mixture of calcium sulfate and carbon black, this patented multilayer coating system exhibits improved corrosion and thermal stability at temperatures up to 1400℉ (760°C). However, as will be shown in the detailed description and comparative examples below, the multilayer system of U.S. Patent No. 9,598,775 does not provide sufficient protection against more corrosive mixtures of molten alkali metal sulfates and alkaline earth metal sulfates.

[0012] In several U.S. patents granted to Hazel et al., such as U.S. Patent Nos. 7,314,674 and 7,754,342 (“Hazel Patents”), another composition of a well-established overcoating system for protecting turbine components from Type 2 thermal corrosion is described, wherein the corrosion-resistant coating composition comprises a glass-forming binder component and a particulate corrosion-resistant component. The glass-forming binder component forms a phosphate-containing or silica-based matrix. The particulate corrosion-resistant component comprises aluminum oxide (i.e., alumina) particles and non-alumina metal alloy particles (such as MCrAlY alloy particles, where M represents nickel or a nickel-cobalt alloy). According to Hazel et al., the coefficient of thermal expansion (CTE) of the non-alumina particles is greater than that of alumina and can comprise 5% to 100% of the total particulate content in the coating composition. The role of the non-alumina metal alloy particles in the coating composition of the Hazel Patents is to mitigate the CTE mismatch between the coating and the underlying metal substrate, which makes the coating prone to peeling when subjected to thermal cycling and / or cyclic mechanical strain at elevated temperatures of about 1200℉ (649°C) or higher.

[0013] When ceramic overlay systems are not used as a standalone protective layer, but rather in combination with diffusion-bonded coatings to increase the durability of the protective system and provide additional oxidation and sulfidation resistance to the substrate material, their functional performance for type 2 hot corrosion protection can be further enhanced. For example, U.S. Patent No. 8,596,985 to Walker et al. describes a method for protecting turbine components from hot corrosion by applying a combination of a chromium diffusion coating and an overlay coating of a ceramic material, wherein the ceramic coating comprises a chromate-phosphate binder matrix filled with metal oxide particles, such as alumina (Al₂O₃), titanium dioxide (TiO₂), or chromium oxide (Cr₂O₃).

[0014] Compared to commercially used coating systems for thermal corrosion protection (such as...) The main concern related to N3000 (the covering system disclosed in U.S. Patent Nos. 9,598,775, 8,596,985, 7,314,674, etc.) is the presence of hexavalent chromium (Cr(VI)) in the chromate-phosphate binders of these ceramic layers and precursors.

[0015] Ceramic coating compositions based on chromate-phosphate binder compositions are well-known and have been considered the industry standard for forming highly corrosion- and heat-resistant coatings for decades. For example, U.S. Patent No. 3,248,251, granted to Allen forty years ago, recognized and described the ability of aluminum-ceramic chromium (VI) coatings to exhibit corrosion resistance, heat resistance, and abrasion resistance while maintaining adhesion and flexibility. Additionally, U.S. Patent Nos. 4,537,632, 4,606,967, and 4,544,408, granted to Mosser et al., describe corrosion-resistant coating compositions comprising chromate-phosphate binder systems filled with aluminum metal or alumina pigments. Today, original equipment manufacturers (OEMs) in the aircraft and power generation industries rely on these aluminum-ceramic coatings to protect various engine components subjected to high temperatures and corrosive environments.

[0016] Despite the wide range of applications for the aforementioned ceramic coatings, hexavalent chromium (Cr(VI)) has been identified as a substance harmful to health and the environment. Therefore, in accordance with recent policy changes by the Department of Defense (DoD), the Air Force, and various OEMs, Cr(VI)-containing compounds have become targets for phase-out. The impact of these policy changes has created a demand for Cr(VI)-free coatings that can exhibit at least the same functional properties as ceramic coatings with Cr(VI)-based binders.

[0017] A hexavalent chromium-free, two-layer protective coating system is disclosed in U.S. Patent No. 6,444,332 to Bettridge. This Bettridge coating system comprises a chromium-plated coating diffused onto the surface of a metal component and a cover glass coating on the chromium-plated coating, wherein the cover glass coating preferably comprises borosilicate glass with chromium oxide filler. This coating system provides oxidation and sulfidation resistance to the shank and root of turbine blades. Although the cover glass coating is hexavalent chromium-free, both layers of the disclosed protective system require high processing temperatures of 1050°C–1100°C (1922°F–2012°F) to diffuse the chromium-plated layer and 1030°C (1886°F) to form the cover glass coating. Furthermore, fused glass is known to be mechanically unfavorable because it is prone to cracking and shattering upon impact.

[0018] Given the aforementioned shortcomings of current coating systems, there is a continuous need for improved coating systems that are based on hexavalent chromium-free binders and can provide effective hot corrosion protection for nickel-based superalloys against molten and / or solidified sulfate attack. Summary of the Invention

[0019] This invention relates in part to slurries for producing coating compositions with specific properties. It has been found that inorganic overcoatings exhibiting excellent resistance to cyclic thermal corrosion and capable of replacing conventional chromium-containing coating systems are produced using chromium-free aluminum phosphate-based coating systems, including a base coat sealed with a top coat.

[0020] In a first aspect, an aqueous slurry composition is provided for generating an inorganic overcoating system for thermal corrosion protection of a substrate, the aqueous slurry composition comprising: a base coat slurry comprising: a first binder comprising an aluminum phosphate-based aqueous solution with an Al:P molar ratio greater than about 1:3, the first binder being characterized by the absence of hexavalent chromium; and a pigment incorporated into the first binder, metal particles, metal oxide particles, or a combination thereof; and a top coat slurry comprising: a second binder comprising an aluminum phosphate-based aqueous solution with an Al:P molar ratio greater than about 1:3, the second binder being characterized by the absence of hexavalent chromium.

[0021] In a second aspect, an inorganic overcoating system for thermal corrosion protection of a substrate is provided, comprising: a substrate; a base coat on the substrate, the base coat comprising: a first aluminum phosphate-based binder with an Al:P molar ratio greater than about 1:3, and a pigment comprising metal particles, metal oxide particles, or a combination thereof embedded within the first aluminum phosphate binder; and a top coat comprising: a second aluminum phosphate-based binder with an Al:P molar ratio greater than about 1:3.

[0022] In a third aspect, a durable multilayer coating system for enhanced protection against high-temperature oxidation and thermal corrosion is provided, formed on a substrate by the aqueous slurry composition of the present invention. The multilayer coating comprises a metallic binder coating adjacent to the substrate and an inorganic overlay coating on the metallic binder coating. The metallic binder coating protects the substrate from high-temperature oxidative corrosion. The inorganic overlay coating comprises a base coat adjacent to the metallic binder coating and a top coat on the base coat. The base coat of the inorganic coating comprises a first aluminum phosphate-based binder with an Al:P molar ratio greater than about 1:3, and pigments, metal particles, metal oxide particles, or combinations thereof, embedded within the first aluminum phosphate binder. The top coat of the inorganic coating comprises a second aluminum phosphate-based binder with an Al:P molar ratio greater than about 1:3. Attached Figure Description

[0023] This specification contains at least one color photograph. A copy of this patent or a published patent with a color photograph will be provided by the Patent Office upon request and payment of the necessary fees.

[0024] The objectives and advantages of the invention will be better understood from the following detailed description of preferred embodiments of the invention, taken in conjunction with the accompanying drawings, in which similar numerals refer to the same features throughout the specification, and wherein:

[0025] Figure 1 A schematic diagram of the inorganic overcoating system of the present invention, including a base coat and a top coat.

[0026] Figures 2(a), 2(b), and 2(c) show the inorganic coatings of the present invention after thermal cycling tests in air at 1500℉ (815°C): after 140 cycles, 260 cycles, and 400 cycles, respectively.

[0027] Figures 3(a) and 3(b) show the surface morphology of the inorganic coating of the present invention in thermal cycling tests: optical micrographs (magnification x40, Figure 2(a)) and SEM micrographs (magnification x500, Figure 2(b)) were taken after 200 cycles in air at 1500℉ (815℃).

[0028] Figure 4 Differential thermal analysis (DTA) results for aluminum phosphate-based binders with different aluminum to phosphate molar ratios are provided to illustrate the phase transitions relative to the Al:P ratio: Al:P = 1:3 for stoichiometric aluminum phosphate Al(H2PO4)3; Al:P = approximately 1:2.7; Al:P = approximately 1:2.4.

[0029] Figures 5(a) and 5(b) show the inorganic coating of the present invention in thermal cycling tests in the absence of corrosive sulfate deposition, and show its visual appearance before testing (Figure 5(a)), and its visual appearance after 100 cycles in air at 1310℉ (710°C) (Figure 5(b)) and cross-sectional SEM (Figure 5(c)).

[0030] Figure 6 Baseline data on the corrosive erosion of bare superalloy substrate Rene 80 by sulfate mixtures A and B are provided, measured as metal loss during thermal cycling tests at 1310℉ (710℃). Figure 6 The figure in the diagram summarizes the results of two tests conducted using two different sulfate mixtures, A and B.

[0031] Figures 7(a) and 7(b) show magnified X1000 cross-sectional SEM images of bare superalloy substrate Rene 80, showing the formation of corrosion product scale during thermal cycling tests at 1310℉ (710℃) with sulfate deposition of mixture A after 100 cycles (Figure 7(a)) and mixture B after 500 cycles (Figure 7(b)).

[0032] Figures 8(a) and 8(b) show that, visually, after 100 cycles at 1310℉ (710℃) in the presence of the corrosive mixture A of corrosive sulfate deposits, the Cr(VI)-free inorganic coating of the present invention exhibits superior performance compared to commercially available Cr(VI)-containing coating systems.

[0033] Figure 9 The results show that at 1310℉ (710℃) in the presence of a mixture of corrosive sulfates A, the inorganic coating of the present invention effectively prevents weight loss during thermal cycling tests compared to the bare substrate.

[0034] Figures 10(a) and 10(b) present cross-sectional SEM data magnified 1000× after 100 cycles at 1310℉ (710°C) with the inorganic coating of the present invention as described in Example 2, without sulfate deposition and with a corrosive sulfate mixture A.

[0035] Figure 11 shows the visual appearance of the samples coated with the inorganic coating system of the present invention before testing (Figure 11(a)) and after thermal cycling tests at 1310℉ (710℃) with sulfate deposition of mixture B (Figure 11(b) after 100 cycles and Figure 11c after 500 cycles), thus demonstrating that the Cr(VI)-free undercoat was retained throughout the test.

[0036] Figures 12(a) and 12(b) show cross-sectional SEM data of the inorganic coating of the present invention after corresponding 100 and 500 cycles of deposition with a less corrosive sulfate mixture B at 1310℉ (710℃) as described in Example 3, magnified 1000×.

[0037] Figures 13(a), 13(b), and 13(c) show the visual appearance of samples coated with the Cr(VI)-containing system prepared according to U.S. Patent 7,314,674; the samples are shown before testing (Figure 13(a)) and after thermal cycling tests with sulfate deposition of mixture B at 1310℉ (710°C) (Figure 13(b) after 100 cycles and Figure 13(c) after 500 cycles), thus showing that this commercially available coating system performs worse than the Cr(VI)-free coating system of the present invention as described in Comparative Example 1.

[0038] Figure 14 The weight changes of the bare substrate were compared with those of the commercially available Cr(VI) coating of Comparative Example 1 and the Cr(VI)-free coating of the present invention during thermal cycling tests performed at 1310℉ (710℃) using sulfate deposition of mixture B.

[0039] Figure 15 The weight loss of the bare substrate after 150 thermal cycles of exposure to sulfate deposition of mixture A at 1310℉ (710℃) was compared with that of two different Cr(VI)-containing coating systems (samples I and J) and the Cr(VI)-free inorganic coating of the present invention (sample K), as described in Comparative Example 2.

[0040] Figure 16 illustrates the performance failure of the Cr(VI)-free coating system with a lithium-doped potassium silicate-based substrate coating as described in Comparative Example 3 after 50 thermal cycles at 1310℉ (710℃) in the presence of an erosive mixture A of corrosive sulfate deposits.

[0041] Figure 17 A schematic diagram of the durable multilayer coating system of the present invention, comprising a metallic bonding coating and an inorganic overcoating, is shown, wherein the inorganic overcoating comprises a base coat and a top coat.

[0042] Figure 18 The maximum pit depth of the multilayer coating and inorganic overlay coating of the present invention after 200 hours and 500 hours of hot corrosion testing at 1310℉ (710°C) is shown compared with the aluminide coating and the uncoated superalloy CMSX-4.

[0043] Figure 19 shows a cross-sectional image that shows zero pitting in the multilayer coating (a) and approximately 20 pitting in the inorganic overlay coating of the present invention (b) after a 500-hour corrosion test with mixture A, compared to approximately 60 micrometer pitting depths in the conventional aluminide coating (c) and uncoated CMSX-4 (d). Detailed Implementation

[0044] The relationship and function of the various elements of the invention will be better understood through the following detailed description. However, the embodiments of the invention described below are by way of example only. This detailed description contemplates features, aspects, and embodiments in various arrangements and combinations as within the scope of the detailed description. The detailed description can therefore be specified to include any such combination and arrangement of these specific features, aspects, and embodiments, or one or more of them selected, consisting of or substantially consisting of any such combination and arrangement of these specific features, aspects, and embodiments, or one or more of them selected.

[0045] This invention relates in part to slurry formulations for producing inorganic overcoatings and multilayer coating systems with specific properties. It has been found that the composition of the binder material and the specific combination of the binder material with pigment particles affect the morphology and microstructure of the resulting inorganic coating, thereby producing coated products with superior functional properties compared to conventional coating systems. Furthermore, the use of a metallic binder between the inorganic coating and the substrate further enhances performance. The coated products of this invention are advantageous for several applications, and are particularly advantageous in aerospace and power generation applications.

[0046] The aqueous slurry composition of the present invention can be used to produce protective coatings or coating systems on a variety of solid substrates, including (by way of example) iron-based alloys, nickel-based alloys, nickel-cobalt-based alloys, and other metallic alloys (such as aluminum-based alloys, cobalt-based alloys, etc.). It should be understood that the aqueous slurry composition of the present invention can also be used to produce protective coatings or coating systems on non-metallic thermally stable surfaces such as ceramics. While the aqueous slurry composition of the present invention is most advantageous for protecting nickel-based superalloys from the harmful effects of hot corrosion, any solid substrate is suitable for the application of the coatings of the present invention, provided that the solid substrate is preferably able to withstand a coating processing temperature of about 600℉-650℉ (315℃-343℃).

[0047] In one aspect of the invention, an aqueous slurry composition for producing an inorganic overcoating system is provided. The aqueous slurry composition comprises a base coat slurry. The base coat slurry comprises a first chromium-free aluminum phosphate-based binder. Preferably, the first aluminum phosphate-based binder has an Al:P molar ratio greater than about 1:3. The first aluminum phosphate-based binder incorporates pigments of metal particles or metal oxide particles, or a combination thereof. The metal particles or metal oxide particles or combination of said pigments are chemically resistant to molten alkali metal sulfates at high temperatures. Preferably, the first aluminum phosphate-based binder incorporates alumina pigment particles. The aqueous slurry composition further comprises a top coat slurry, which also comprises an aqueous solution of a second aluminum phosphate-based binder with an Al:P molar ratio greater than about 1:3. The second aluminum phosphate-based binder in the top coat slurry may be pigment-free. Preferably, the second aluminum phosphate-based binder in the top coat slurry incorporates pigments of metal oxide particles, such as chromium(III)Cr2O3.

[0048] In other aspects of the invention, an inorganic overcoating coating for thermal corrosion protection is formed on a substrate by an aqueous slurry composition of the invention. The inorganic overcoating coating comprises a base coat adjacent to the substrate and a top coat over the base coat. The base coat of the inorganic coating comprises a first aluminum phosphate-based binder with an Al:P molar ratio greater than about 1:3, and pigments of metal particles, metal oxide particles, or a combination thereof embedded within the first aluminum phosphate binder. The metal particles or metal oxide particles, or a combination thereof, of the pigment exhibit chemical resistance to molten alkali metal sulfates at high temperatures. Preferably, the first aluminum phosphate-based binder incorporates alumina particles. The first aluminum phosphate-based binder constitutes 5% to 50% by volume in the cured base coat. The pigment constitutes 50% to 95% by volume in the cured base coat. The top coat of the inorganic coating comprises a second aluminum phosphate-based binder with an Al:P molar ratio greater than about 1:3. The second aluminum phosphate-based binder in the top coat may be pigment-free. Preferably, the second aluminum phosphate-based binder in the top coating is incorporating pigments containing metal oxide particles such as chromium(III) oxide (Cr₂O₃). The aluminum phosphate-based binder in each of the undercoat and top coatings of the present invention is free of Cr(VI) substances, and therefore the aqueous slurry composition and the resulting inorganic overcoat system are free of hexavalent chromium.

[0049] In another aspect of the invention, a durable multilayer coating system for enhanced protection against high-temperature oxidation and thermal corrosion is provided, formed on a substrate by the aqueous slurry composition of the invention. The multilayer coating comprises a metallic binder coating adjacent to the substrate and an inorganic overlay coating on the metallic binder coating. The metallic binder coating protects the substrate from high-temperature oxidative corrosion. The inorganic overlay coating comprises a base coat adjacent to the metallic binder coating and a top coat on the base coat. The base coat of the inorganic coating comprises a first aluminum phosphate-based binder with an Al:P molar ratio greater than about 1:3, and pigments of metal particles, metal oxide particles, or combinations thereof embedded within the first aluminum phosphate binder. The metal particles or metal oxide particles, or a combination of both, of the pigments are chemically resistant to molten alkali metal sulfates. Preferably, the first aluminum phosphate-based binder incorporates alumina particles. The first aluminum phosphate-based binder constitutes 5% to 50% by volume in the cured base coat. And the pigment constitutes 50% to 95% by volume in the cured base coat. The top coat of the inorganic coating comprises a second aluminum phosphate-based binder with an Al:P molar ratio greater than about 1:3. The second aluminum phosphate-based binder in the top coating may be pigment-free. Preferably, the second aluminum phosphate-based binder in the top coating incorporates pigments such as metal oxide particles, such as chromium(III)Cr2O3.

[0050] In this invention, it has been discovered that an aluminum phosphate-based binder composition incorporating chemically resistant pigments, including metal particles, metal oxide particles, or a combination of both, wherein the binder comprises Al and P substances with an aluminum (Al):phosphorus (P) molar ratio greater than about 1:3, forms part of a topcoat system that maintains adhesion to the metal substrate under thermal cycling and effectively protects the superalloy substrate from corrosion by molten or solidified alkali metal sulfates and alkaline earth metal sulfates at elevated temperatures typical of type 2 hot corrosion. The molar ratio Al:P represents the ratio of the molar amount of aluminum-containing substances from all sources to the molar amount of phosphorus-containing substances (such as phosphates) from all sources. In each of the resulting top and bottom coatings, the Al:P molar ratio can be in the range of about 1:2.1 to 1:2.9, more preferably about 1:2.2 to 1:2.8, and most preferably about 1:2.4 to 1:2.7. As will be shown in the embodiments, the overcoating system derived from the aqueous slurry of the present invention exhibits excellent adhesion to the superalloy substrate and interlayer adhesion, enabling the coating system to withstand thermal cycling in a corrosive environment at elevated temperatures of approximately 1300℉, typical of type 2 hot corrosion, without peeling off, thus providing a stable and protective barrier against corrosion.

[0051] Furthermore, it was unexpectedly discovered that the hexavalent Cr(VI)-free coating of the present invention provides significantly better functional performance than commercial coating systems based on chromate-phosphate binders (such as SermaFlow N3000, disclosed in U.S. Patent 9,598,775), and has the same or better performance as the coating described in U.S. Patent No. 7,314,674. These findings by the applicant represent a significant departure from conventional systems. Prior to this invention, chromate-phosphate binder-based coating systems were generally accepted as the benchmark for heat corrosion resistance.

[0052] The applicant has surprisingly discovered, and as will be demonstrated in the examples, that when a first aluminum phosphate-based binder with an Al:P molar ratio increased from 1:3 to approximately 1:2.7 and approximately 1:2.4 is incorporated with micron-sized, chemically resistant pigments of metal particles, metal oxide particles, or a combination of both to form the undercoat of the present invention, the undercoat provides excellent adhesion to the superalloy substrate. The undercoat of the present invention is sealed with a topcoat comprising a second aluminum phosphate-based binder. The aluminum phosphate-based binder of the topcoat is preferably filled, embedded, or otherwise typically incorporated with micron-sized chromium(III)Cr2O3 pigment particles. The inorganic coating system is able to withstand thermal cycling in air at temperatures up to 1500℉ (816°C) without exhibiting any signs of coating peeling.

[0053] In one aspect of the invention, Figure 1The inorganic overcoating system shown includes a base coat having a first aluminum phosphate-based binder with an Al:P molar ratio of approximately 1:2.4, and the binder incorporating pigments containing alumina particles. A top coat comprises a second aluminum phosphate-based binder with an Al:P molar ratio of approximately 1:2.7, and the binder of the top coat incorporating Cr₂O₃ pigment particles. Figure 1 The resulting inorganic coating was applied to Inconel The sample was placed on a substrate and subjected to thermal cycling (defined as 50 minutes in a hot zone followed by 10 minutes at room temperature). No coating peeling was observed even after up to 400 cycles at a hot zone temperature of up to 1500℉ (710℃) (Fig. 2c). Furthermore, the applicant did not observe any increase in coating thickness. Additionally, as shown in Fig. 3(a) and Fig. 3(b), the coated sample remained smooth and continuous (i.e., R before and after the test). a <40 microinches), its shape is consistent and it did not deteriorate after testing.

[0054] This result is indeed surprising compared to the findings disclosed in U.S. Patent Nos. 7,314,674 and 7,754,342 (“Hazel Patents”) granted to Hazel et al. (which generally represent conventional wisdom). According to the teachings of the Hazel Patents, when subjected to elevated temperatures of typical Type 2 hot corrosion, such as 300 thermal cycles at 1400℉ (760°C), corrosion-resistant coatings on superalloy substrates containing only alumina particles in a phosphate- or silica-containing binder matrix will fail and peel off. The inventors of the Hazel Patents attributed the failure and peeling to a mismatch in the coefficient of thermal expansion (CTE) between the coating and the substrate.

[0055] Contrary to the teachings of the Hazel patent, the applicant has discovered that specific compositional changes in the aluminum phosphate-based binder of the present invention, independent of the coefficient of thermal expansion (CTE) of the pigment particles embedded in the aluminum phosphate-based binder, enable the coating of the present invention to remain unfailed during thermal cycling tests conducted at even higher temperatures and longer exposures. In other words, the compositional changes in the aluminum phosphate-based binder of the present invention eliminate any detrimental CTE mismatch effects observed in the Hazel patent, thus eliminating any need for specific pigment particles with a CTE greater than that of alumina particles. Alternatively, other metal oxide particles and metal particles resistant to molten sulfate corrosion at high temperatures can be used as pigments in the aluminum phosphate-based binder of the present invention. For example, the metal oxide particles of the pigment in the undercoat can be selected from the group consisting of: alumina, titanium oxide, zirconium oxide, and chromium oxide. The metal pigment particles in the undercoat can be selected from the group consisting of: MCr, MA1, MCrAl, MCrAlY (where M = Ni, Co, Fe, or combinations thereof). The metal particles have at least 15% by weight of chromium or at least 6% by weight of aluminum. Metal particles, metal oxide particles, or combinations thereof may have a certain particle size distribution, characterized in that the 50th percentile of the particle size distribution has a diameter between about 1 micrometer and 10 micrometers.

[0056] Unbound by any theory, the absence of any thermally induced transformation of the coating binder matrix at elevated operating temperatures above approximately 1300℉ is likely a contributing factor to the high resistance to thermal cycling stress demonstrated by the coating system of this invention.

[0057] Aluminum dihydrogen phosphate (Al(H2PO4)3) with a stoichiometric Al:P molar ratio of 1:3 is well known in the art as an effective binder for various ceramic materials (AS Wagh, Chemically Bonded Phosphate Ceramics, 2002) and is widely used in the refractory industry. When Al(H2PO4)3 is heated, it loses moisture and forms a number of complex hydrates; under further heat treatment, these complex hydrates decompose to form amorphous and crystalline aluminum phosphate phases with different stoichiometry; it is known in the art that temperatures close to 500 °C (930 °F) are required to produce a completely anhydrous aluminum phosphate phase, and that the subsequent transformation of the anhydrous aluminum phosphate phase occurs at temperatures up to 750 °C (1380 °F) (see, for example, M. Vipola et al., J. Eur. Ceram. Soc., 22, 2002, pp. 1937-1946). The completion of this high-temperature thermal transition renders stoichiometric aluminum dihydrogen phosphate (Al(H2PO4)3) with an Al:P molar ratio of 1:3 unsuitable as a binder for the slurry-derived coatings of the present invention, which have conventional curing temperatures in the range of 600℉ to 1000℉. Continued thermal transition above this curing temperature range would result in volume changes in the matrix and mechanical stresses in the coating under operating conditions, thus leading to a loss of coating integrity and its peeling from the underlying substrate.

[0058] As the applicant previously recognized and disclosed in U.S. Patent No. 9,394,448 to Belov & Copeland, the thermal transformation pathway of aluminum phosphate is strongly dependent on the Al:P molar ratio, the entire contents of which are incorporated herein by reference. Increasing the Al:P molar ratio from stoichiometric 1:3 of aluminum dihydrogen phosphate results in a lower temperature at which these transformations are completed, thus achieving complete curing at temperatures ranging from 600℉ to 1000℉. U.S. Patent No. 9,394,448 discloses that these aluminum phosphate-based binders with Al:P molar ratios greater than 1:3 have been used in topcoat compositions in combination with chromium-free undercoats that are filled, embedded, or incorporated into a silicate matrix with aluminum metal particles. However, the coating systems disclosed in U.S. Patent No. 9,394,448 are suitable and generally intended for corrosion protection of steel substrates and components at temperatures not exceeding about 1200℉ (i.e., below the melting temperature of aluminum metal, 1220℉ (660°C)). Therefore, the coating system of U.S. Patent No. 9,394,448 is not suitable for Type 2 hot corrosion protection above 1250℉, and cannot withstand the operating temperature conditions that the coating of the present invention is specifically designed to withstand.

[0059] The applicant acknowledges that a portion of the teachings in their prior U.S. Patent No. 9,394,448 to Belov & Copeland applies to this invention. Specifically, the compositional change of stoichiometric aluminum dihydrogen phosphate involves increasing the Al:P molar ratio from 1:3 of stoichiometric aluminum dihydrogen phosphate to make the aluminum phosphate-based binder suitable and beneficial for use as both a base coat and a top coat in the inorganic overcoating system of this invention. However, it should be understood that, unlike U.S. Patent No. 9,394,448, the coating system of this invention is suitable for entirely different applications with entirely different operating conditions, namely, for type 2 hot corrosion protection of superalloy substrates at higher temperatures.

[0060] For the slurry-derived coating to be stable at operating temperatures, any compositional and / or phase transformation of the binder must be completed during the coating curing process. Otherwise, the internal stresses associated with thermally induced transformations may damage the coating and potentially cause it to delaminate from the substrate under operating conditions. To determine the effect of the Al:P molar ratio on the curing process of the aluminum phosphate-based binder of this invention, the applicant has investigated the thermal transformation pathway using thermal analysis methods known in the art. The samples studied were prepared by preheating small amounts of aluminum phosphate at 400℉ for 1 hour, each with a different Al:P molar ratio. Thermal analysis was performed (using a Universal V4.5A TA thermal analyzer, heated from room temperature to 700℃ (1292℉) at a rate of 10℃ / min in air). The results of differential scanning calorimetry (DSC) used to determine the peak temperature of the thermal effect are shown below. Figure 4 As can be seen from the data, a strong endothermic effect was observed at approximately 250 °C (482 °F) for a stoichiometric aluminum dihydrogen phosphate molar ratio of Al:P = 1:3, and another endothermic effect was observed at approximately 522 °C (972 °F). Increasing the aluminum content in the aluminum phosphate-based binder solution from a stoichiometric Al:P = 1:3 molar ratio to Al:P = 1:2.7, and further to Al:P = 1:2.4, resulted in the disappearance of the endothermic effect. These results indicate that the aforementioned increase in aluminum content compared to a stoichiometric aluminum dihydrogen phosphate molar ratio of Al:P = 1:3 leads to a decrease in the thermally induced transformation completion temperature. In other words, the compositions of the present invention are capable of undergoing thermally induced transformation completion at temperatures far below the service temperature (i.e., the typical temperature for type 2 hot corrosion). This, in turn, means that coatings of the present invention cured within the conventional curing temperature range of 600 °F to 1000 °F do not undergo any stress-induced transformation during use.

[0061] The aluminum content required to increase the Al:P molar ratio can be provided by any suitable aluminum-containing raw material such as aluminum hydroxide or aluminum nitrate that is soluble in a concentrated (approximately 20%-50% by weight) acidic aqueous solution of aluminum dihydrogen phosphate.

[0062] Optionally, other additives known in the art may be incorporated into aluminum phosphate-based binders. By way of example, aluminum phosphate-based binders may contain pH-adjusting compounds (such as magnesium oxide, magnesium carbonate, etc.), small amounts (e.g., about 1%-2% by weight) of boron oxide (B₂O₃) and / or viscosity-adjusting components. Alternatively, or in addition, relatively small amounts of suitable organic solvents known in the art may be added to improve properties such as sprayability, substrate wettability, and film-forming properties.

[0063] The slurry composition of the present invention can be applied to a substrate by any number of conventional application techniques known in the art, such as by spraying, brushing, dipping, swirl, etc. The applied layer is dried and then cured. The binder solution polymerizes and cures under drying and curing cycles to form a continuous matrix with acceptable mechanical strength, flexibility and chemical resistance. Pigments of metal particles or metal oxide particles or combinations (preferably alumina Al2O3) are embedded or generally incorporated into the matrix of the undercoat, and where the topcoat used is not pigment-free, metal oxide particles, preferably chromium(III) Cr2O3, are embedded or otherwise generally incorporated into the matrix of the topcoat.

[0064] Therefore, in another aspect of the invention, an inorganic overcoating is disclosed, which is capable of withstanding thermal cycling at elevated temperatures typical of type 2 hot corrosion and provides protection to a metallic substrate against such type 2 hot corrosion. The undercoating layer of the inorganic coating of the present invention is typically applied to a thickness between 0.5 mils and 3.0 mils, preferably between 1.5 mils and 2.0 mils. Such coating thickness can be formed in a single layer (i.e., a single application-drying and curing cycle) or preferably in multiple curing cycles over multiple layers. The minimum thickness of the undercoating layer is determined by the need to provide a continuous layer over the substrate, and the maximum thickness of the undercoating layer is typically determined by the target or specified thickness of the entire multilayer overcoating system. Preferably, the coating thickness is avoided from exceeding the functional requirements of a particular application.

[0065] The topcoat slurry of the present invention is then applied onto the basecoat to form the resulting multilayer coating system of the present invention. In a preferred embodiment, the topcoat is applied to a thickness between 0.1 mil and 0.5 mil, more preferably between 0.1 mil and 0.3 mil.

[0066] The typical curing temperature for the coating of this invention is in the range of 600℉ to 1000℉. As will be apparent to those skilled in the art, suitable curing procedures may include a shorter period at a higher temperature or a longer period at a lower temperature.

[0067] The inorganic coatings of this invention exhibit the formation of dense, smooth, and defect-free layers, as demonstrated in the examples. The surface finish (smoothness) R of the coating, as measured by a Mitutoyo Surftest 301 (at a 5.1 mm transverse and 0.030" (0.76 mm" cutoff), is... a It is about 40 microinches (about 1 μm) or smaller.

[0068] Furthermore, a durable multilayer coating system for enhanced protection against high-temperature oxidation and hot corrosion is provided, formed on a substrate by the aqueous slurry composition of the present invention. This multilayer coating comprises a metallic bonding coating adjacent to the substrate and an inorganic overlay coating of the present invention on the metallic bonding coating. A schematic diagram of the durable multilayer coating system is illustrated in [illustration missing]. Figure 17 The metal bonding coating provides further protection against high-temperature oxidative corrosion and improves the durability of the substrate. The metal bonding coating is selected from the group consisting of: aluminides, chromium-rich coatings, platinum-aluminides, chromium-aluminides, platinum-modified chromium, and MCrAlY alloys (where M = Ni, Co, Fe, or a combination thereof). The metal bonding coating typically contains at least 15% by weight of chromium and / or at least 6% by weight of aluminum.

[0069] As will be shown and discussed in the examples below, the inventors have conducted extensive experiments to test the ability of the coating system of the present invention to withstand thermal cycling in a corrosive environment at elevated temperatures typical of Type 2 hot corrosion without peeling, and its ability to provide a stable protective barrier against sulfur-containing corrosion to a superalloy substrate. The selected test temperature was 1310℉ (710°C), which represents the temperature encountered in Type 2 hot corrosion, where thermal cycling was defined as a 50-minute stay in the hot zone at the test temperature followed by a 10-minute stay at room temperature.

[0070] First, the examples confirm that the inorganic coating of the present invention is completely stable under thermal cycling at a test temperature of 1310℉ (710℃), with no corrosive sulfate deposition and effectively protecting the superalloy substrate from thermal oxidation. In this regard, as shown in Figure 5, the change in the visual appearance of the coating is negligible, and after 100 thermal cycles, both the undercoat and topcoat of the system essentially maintain structural integrity. Furthermore, no change in sample weight was observed.

[0071] Next, two different corrosive sulfate mixtures were prepared, as shown in Table 1. Mixture A corresponds to the eutectic composition of a ternary sulfate system with a melting point of 1275℉ (690℃). Therefore, Mixture A exists in the liquid phase at a test temperature of 1310℉ (710℃). Compared to Mixture A, Mixture B contains a higher amount of calcium sulfate, with the amount of CaSO4 exceeding the eutectic composition of Mixture A by 50% by weight (i.e., at the test temperature, Mixture B exists in both the liquid and solid phases). Therefore, Mixture A is expected to be more corrosive than Mixture B in terms of sulfur-based corrosion erosion. Nevertheless, Mixture B is believed to represent conditions closer to actual operating conditions than Mixture A. However, using such a corrosive mixture A for sulfate thermal corrosion erosion testing provides accelerated test conditions. Therefore, the total number of thermal cycles in tests conducted with Mixture A is less than that in tests conducted with Mixture B.

[0072] Table 1. Composition of Corrosion Materials

[0073]

[0074] To collect baseline data on the effects of corrosion on superalloys, on a bare nickel-based disk superalloy substrate (16 wt% chromium, such as...) Tests were conducted on a nickel-based disk superalloy substrate. Mixture A was applied to the sample surface of the substrate at a rate of 2 mg per square centimeter of sample surface area every 50 cycles, for a total of 100 cycles. Mixture B was also applied to the sample surface of the nickel-based disk superalloy substrate at a rate of 2 mg per square centimeter of sample surface area, but was applied again every 100 cycles, extending the total test duration to 500 cycles. It was observed that the bare substrate deteriorated rapidly in both tests due to sulfate corrosion. Figure 6 The weight loss of the bare substrate samples during thermal cycling tests at 1310℉ (710℃) is presented. As can be seen from the data, the corrosion of both mixture A and mixture B leads to significant loss of substrate material, and as expected, the weight loss after 100 cycles of exposure to the corrosive mixture A (i.e., in a liquid state at the test temperature) is approximately twice as high as that after 500 cycles using mixture B. In contrast, thermal cycling at the same test temperature of 1310℉ (710℃) in the absence of sulfates from mixtures A and B (i) did not produce any significant weight change in the bare substrate (Table 2), and (ii) revealed very small hot skin growth on the superalloy surface.

[0075] Table 2. Bare substrates: Thermal cycling tests at 1310℉ (710℃) without sulfate exposure

[0076] bare substrate Number of thermal cycles Initial weight, grams Weight after testing, in grams Sample A 50 6.0122 6.0129 Sample B 100 13.2402 13.2397

[0077] Therefore, the weight loss of the substrate material is a result of corrosive erosion in the presence of sulfates, and is caused by the consumption of metal through the growth of corrosion product scale on the surface and the subsequent peeling off of this scale. As shown by the SEM data in Figure 7, the scale thickness after 100 cycles in the case of mixture A erosion (Figure 7(a)) is greater than the scale thickness after 500 cycles in the case of exposure to mixture B (Figure 7(b)). SEM data and Figure 6 The weight loss results shown are consistent.

[0078] Next, the protective capability of the chromium-free coating system of the present invention against sulfate attack will be compared with that of Cr(VI)-containing systems known in the art (such as commercially available systems). The N3000 system was compared. In this respect, a superalloy substrate coated with the coating system of the present invention was tested side-by-side with a substrate coated with a conventional coating system, which has a base coat containing a chromate-phosphate binder filled with alumina Al2O3 particles and a top coat containing a chromate-phosphate binder filled with chromium(III)Cr2O3 particles. Therefore, the only difference between the two coating systems is the type of binder matrix: the Cr(VI)-free aluminum phosphate-based binder of the present invention versus the Cr(VI)-containing chromate-phosphate binder of the conventional coating system. The test was conducted under conditions of exposure to a corrosive mixture A sulfate deposition. Thermal cycling tests were performed at 1310℉ (710°C); the results after one hundred cycles are presented in Figures 8(a) and 8(b). It can be seen that the Cr(VI)-free coating system of the present invention performs significantly better (Figure 8(a)). While the commercially available chromate-phosphate-based coating system deteriorates and partially peels off, exposing the bare substrate (Figure 8b), the coating system of the present invention maintains its integrity. These results will be described in more detail in Comparative Example 2. This side-by-side comparison demonstrates that the change in the binder matrix from a Cr(VI) chromate-phosphate binder to a Cr(VI)-free aluminum phosphate-based binder resulted in a significant and substantial performance improvement.

[0079] Finally, the performance of the multilayer coating system of the present invention, comprising a metallic binder coating and an inorganic overlay coating, was compared with that of an uncoated superalloy substrate and an aluminide-only coating. The multilayer coating of the present invention did not exhibit pitting depth on either the aluminide binder coating or the superalloy substrate. The external inorganic coating acts as an excellent environmental barrier against molten sulfate, while the aluminide binder coating provides protection against high-temperature oxidative corrosion to the superalloy substrate. In contrast, after the same 500-hour hot corrosion test, the conventional aluminide-only coating or the uncoated superalloy showed a maximum pitting depth of approximately 60 micrometers.

[0080] While preferred embodiments of the slurry formulations and coating systems of the present invention have been described above along with representative examples, the following additional examples are intended to provide a basis for better evaluating the properties and functions of the slurries and coating systems of the present invention. These examples are merely illustrative and should not be construed as limiting the scope of the invention.

[0081] It should be understood that all slurries of the present invention are characterized by their pH, viscosity, specific gravity, and solids content. These parameters, along with D... 50 and D 90 (Defined as the diameters of the 50th and 90th percentiles of the pigment particle size distribution, respectively), to test the stability and aging of the slurry. The slurry was found to exhibit acceptable stability and a shelf life of six months or longer.

[0082] Each of the coatings in the following examples and comparative examples was applied to a nickel-based superalloy substrate. The substrate was first surface-treated by sandblasting with 100-mesh abrasive.

[0083] Example 1

[0084] A base coat slurry was prepared by mixing 120 g of alumina (Al₂O₃) powder into 153 mL (187 g) of aluminum phosphate-based binder, wherein the Al:P molar ratio was approximately 2.4. The pH of the base coat slurry was measured to be approximately 1.4. The alumina pigment particles were characterized by a density of approximately 2.5 μm to 3.0 μm. 50 And D approximately 4.0 micrometers to 4.5 micrometers 90 The base coat slurry was then sprayed onto the substrate, dried at 175℉ for 15 minutes, and then cured at 650℉ for 30 minutes to form the cured base coat layer. The top coat slurry was prepared by mixing 24 g of chromium(III)Cr2O3 green pigment with 200 g of aluminum phosphate-based binder, wherein the Al:P molar ratio was approximately 1:2.4. The binder also contained 3.0 g of B2O3. The pH of the top coat slurry was measured to be approximately 1.5. The pigment particle size in the top coat slurry was characterized by a Di of approximately 1.3 μm–1.7 μm. 50 And D, approximately 2.2 micrometers to 2.7 micrometers 90 The topcoat slurry is sprayed onto the cured base coat and dried at 175℉ for 15 minutes, then cured at 650℉ for 30 minutes, followed by curing at 1000℉ for 60 minutes to form the cured topcoat layer. The total thickness of the base and topcoat coating systems is determined to be in the range of 2.1 mils to 2.3 mils.

[0085] The coated sample was subjected to a thermal cycling test, exposed to sulfate mixture A, which was applied to the surface of the coated sample at a rate of 2 mg per square centimeter of the coated sample surface area per 50 cycles.

[0086] Table 3. Thermal cycling test of sulfate mixture A exposed at 1310℉ (710℃)

[0087] coating Cycle number Initial W, g Ultimately W, g ΔW, g ΔW, mg / cm2 Sample A 50 6.8486 6.8504 0.0018 0.43 Sample B 50 4.7575 4.7582 0.0007 0.23 Sample C 100 5.0961 5.095 -0.0011 -0.35 Sample D 100 6.2305 6.2298 -0.0007 -0.18 bare substrate 50 5.2185 5.1857 -0.0328 -10.18 bare substrate 100 7.113 7.0579 -0.0551 -15.52 bare substrate 100 5.616 5.5547 -0.0613 -18.46

[0088] Four samples, A through D, were used. Samples A and B were tested for 50 cycles each, and samples C and D were tested for 100 cycles each. Weight change was measured after every 50 cycles, with sulfate residue on the coating surface removed with water before weight measurement. The data are presented in Table 3, expressed in milligrams as weight change ΔW per square centimeter of sample surface area.

[0089] As can be seen from the data, the weight change of the coated AD sample is not significant, especially compared to the very high weight loss of the bare substrate. The comparison results are also presented in graphical form. Figure 9 middle.

[0090] Example 2

[0091] Samples of the inorganic overcoating system of Example 1 were subjected to 100 cycles of thermal cycling at 1310℉ (710℃). One set of samples was tested without the application of a corrosive sulfate mixture, and the other set was tested with the application of an corrosive sulfate mixture A. After testing, the samples were cross-sectioned and examined by SEM analysis. As can be seen from the SEM results (Figures 10(a) and 10(b)), both sets of samples retained the coating—even the top coating was not lost due to sulfate erosion. Therefore, although the coated samples were exposed to corrosive test conditions designed to be more demanding than actual working conditions, only minor changes in the substrate were detected in some areas at the boundaries of the coating. The SEM results confirm that the Cr(VI)-free multilayer coating system of the present invention serves as an effective barrier against molten sulfate corrosive erosion.

[0092] Example 3

[0093] Samples E and F were prepared and coated with the inorganic overcoating system of the present invention to a total thickness of 1.6 to 1.8 mils for the undercoat and topcoat layers, as described in Example 1. The coated samples were subjected to a thermal cycling test using a less corrosive sulfate mixture B. Mixture B was applied to the sample surface at a rate of 2 mg per square centimeter of sample surface area per 100 cycles. The total test length was extended to 500 cycles. After 100 cycles and at the end of the test (i.e., after 500 cycles), the samples were washed with water and weighed. For baseline data, the bare substrate was also included in the test, and weight change data were collected after 200 and 500 cycles.

[0094] As observed by visual appearance (Figs. 11(a), 11(b) and 11(c)), the coating system of the present invention remained on the substrate throughout the test, and even the top coating was retained after 500 test cycles (see sample F in Fig. 11(c)).

[0095] As can be seen from the weight change data given in Table 4, the coating system of the present invention is also effective in protecting the substrate from the corrosive erosion of a mixture of molten sulfate and solid sulfate: the coated samples showed a smaller increase in weight, most likely due to the thermal oxidation process, while the bare substrate experienced a significant weight loss due to sulfate corrosion. As previously discussed, the substrate weight loss in this longer test with the less corrosive sulfate mixture B was less than the substrate weight loss observed with the more corrosive mixture A (see...). Figure 6 (as shown in Figure 7), but still shows harmful loss of the base metal.

[0096] Table 4. Thermal cycling test of sulfate mixture B exposed at 1310℉ (710℃)

[0097] coating Cycle number Initial W, g Ultimately W, g ΔW, g ΔW, mg / cm2 Sample E 100 7.4466 7.4490 0.0024 0.537 Sample F 500 8.1059 8.1100 0.0041 0.835 bare substrate 200 7.0026 6.9697 0.0329 -7.616 bare substrate 500 6.0786 6.0559 0.0227 -6.219

[0098] After testing, samples E and F were cross-sectioned and examined by SEM / EDS analysis. As can be seen from the SEM results (Figures 12(a) and 12(b)), both samples retained the integrity of the coating-base layer, and even the top layer showed no loss after 500 cycles of sulfate corrosion. EDS analysis data confirmed that the slight increase in sample weight observed after testing was mainly due to slight oxidation of the substrate in some localized areas at the coating boundaries, with no significant sulfide corrosion. Therefore, under these less corrosive but five-fold extended testing conditions, the applicant demonstrates that the Cr(VI)-free coating system of the present invention serves as an effective barrier against corrosive corrosion from mixtures of molten and solid sulfates.

[0099] Example 4

[0100] The substrate is made of single-crystal nickel-based superalloy It is manufactured under the trademark of CANNON-MUSKEGON CORPORATION. CMSX-4 has a nominal composition of approximately 9% Co, 6.5% Cr, 5.6% Al, 6% W, 6.5% Ta, 3% Re, 1% Ti, 0.6% Mo, 0.1% Hf, and the balance being nickel, by weight. A multilayer coating system of the present invention, comprising an aluminide binder coating and an inorganic coating, is applied to a CMSX-4 substrate for hot corrosion performance testing.

[0101] First, an aluminide coating was applied to the CMSX-4 substrate using a conventional vapor-phase aluminizing process. The resulting aluminide coating was approximately 60 micrometers thick.

[0102] Second, a base coat slurry was prepared by mixing 120 g of alumina (Al₂O₃) powder into 153 mL (187 g) of aluminum phosphate-based binder, wherein the Al:P molar ratio was approximately 1:2.4. The pH of the base coat slurry was measured to be approximately 1.4. The alumina pigment particle size was characterized by a density of approximately 2.5 μm to 3.0 μm. 50 And D approximately 4.0 micrometers to 4.5 micrometers 90 The primer slurry is then sprayed onto the substrate, dried at 175℉ for 15 minutes, and then cured at 650℉ for 30 minutes to form a cured primer layer. The resulting primer layer is approximately 50 micrometers thick.

[0103] Third, a topcoat slurry was prepared by mixing 24 g of chromium(III)Cr₂O₃ green pigment with 200 g of aluminum phosphate-based binder, wherein the Al:P molar ratio was approximately 1:2.4. The binder also contained 3.0 g of B₂O₃. The pH of the topcoat slurry was measured to be approximately 1.5. The pigment particle size in the topcoat slurry was characterized by a Di of approximately 1.3 μm–1.7 μm. 50 And D, approximately 2.2 micrometers to 2.7 micrometers 90 The topcoat slurry is sprayed onto the base curing layer, dried at 175℉ for 15 minutes, then cured at 650℉ for 30 minutes, followed by curing at 1000℉ for 60 minutes to form the cured topcoat layer. The resulting topcoat thickness is approximately 5 micrometers.

[0104] Hot corrosion tests were performed on the coated samples. The hot corrosion tests were conducted in air at a static exposure temperature of 710°C for a total exposure time of up to 500 hours. The maximum pit depth of the test samples was evaluated after 200 hours and 500 hours of laboratory testing. Approximately 2 mg / cm² was used as listed in the table before testing. 2 The synthetic sulfate mixture A was applied to the sample surface and replenished every 100 hours.

[0105] As from Figure 18As shown in Figure 19, after 200 hours of testing, neither the inorganic overcoat nor the multilayer coating of this invention exhibited corrosion erosion of the substrate, compared to the approximately 50 micrometer pit depth on both the base CMSX-4 substrate and the conventional aluminide coating. After 500 hours of testing, the pit depth on both the bare CMSX-4 and the conventional aluminide coating increased to approximately 60 micrometers. Some corrosion erosion with pit depths up to approximately 20 micrometers was observed on the samples with the inorganic overcoat. The multilayer coating with both a metal binder coating and an inorganic coating performed best, and no pit depth was observed on the aluminide binder coating and the superalloy substrate. The external inorganic coating acts as an excellent environmental barrier against molten sulfate, while the presence of the aluminide binder coating between the substrate and the inorganic overcoat coating provides further enhanced protection of the superalloy against high-temperature oxidative corrosion.

[0106] Comparative Example 1

[0107] To compare the functional performance of the Cr(VI)-free coating system of the present invention with that of another commercially available Cr(VI)-containing coating system in terms of thermal corrosion protection, coated samples G and H were prepared according to the teachings of U.S. Patent 7,314,674 to Hazel et al. Samples G and H were prepared as follows: The base coat slurry of the Cr(VI)-containing coating system contained a chromate-phosphate based binder filled with a metal alloy powder having the representative formula MCrAlY (where M is Ni or Co). The base coat slurry was sprayed onto a substrate, dried at 175℉ for 15 minutes, and then cured at 650℉ for 30 minutes to form a cured base coat. The cured base coat was then further coated with a top coat slurry containing a chromate-phosphate based binder filled with metal oxide pigment particles. SermaSeal, commercially available from Praxair Surface Technologies, Inc. (Indianapolis, IN), was used. TM 570A slurry was used as the topcoat slurry. The topcoat slurry was sprayed onto the cured base coat, dried at 175℉ for 15 minutes, and cured at 650℉ for 30 minutes. The total thickness of the Cr(VI)-containing coating system applied to samples G and H was 1.6 mil to 1.8 mil, the same as that of the Cr(VI)-free coating system samples E and F in Example 3. Coated samples G and H were subjected to thermal cycling tests with exposure to a less corrosive sulfate mixture B, according to the test protocol as described in Example 3.

[0108] Based on the results, unlike the Cr(VI)-free coating system of the present invention which retained its top coating after 500 cycles, the Cr(VI)-containing coating system samples showed that the top coating was mostly lost only after 100 cycles and completely disappeared after 500 cycles (see Figures 13(b) and 13(c) respectively). For sample G exposed to 100 cycles, the weight increase per square centimeter of coated sample surface area was measured to be 1.478 mg, and for sample H after 500 cycles, the weight increase per square centimeter of coated sample surface area was measured to be 3.417 mg, which is 3-4 times higher than the corresponding weight increase of the Cr(VI)-free coating system of the present invention. Furthermore, EDS analysis showed that sulfide erosion was more significant in the case of the Cr(VI)-containing coating: sulfur up to 6.5 atomic percent was detected at most analytical points at the substrate-coating boundary.

[0109] Compared with the data of Example 3, the data obtained in Comparative Example 1 are presented as follows: Figure 14 In summary, although both systems protect the substrate from corrosive sulfate attack, the coating system of the present invention performs significantly better. Therefore, the coating system of the present invention not only provides the benefits of environmental adaptability and being free of hexavalent chromium, but also exhibits superior functional performance compared to commercial Cr(VI)-containing coating systems that were generally accepted as the benchmark for heat corrosion resistance prior to the present invention.

[0110] Comparative Example 2

[0111] In this test, two commercially available coating systems were prepared to evaluate their ability to impart protective properties against the attack of an aggressive molten sulfate mixture A. Thermal cycling tests were conducted at 1310℉ (710°C) according to the protocol described in Example 1, but for a longer duration of 150 cycles. For this test, coated samples I to K were prepared as follows. Sample I was coated with a commercially available Cr(VI)-containing system, which is referred to as… N3000, purchased from Praxair Surface Technologies, Inc. (Indianapolis, IN), has a base coat containing a chromate-phosphate binder filled with alumina Al2O3 particles and a top coat containing a chromate-phosphate binder filled with chromium(III)Cr2O3 particles. Another Cr(VI)-containing coating system, Sample J, was prepared as described in Comparative Example 1 (i.e., using a base coat containing a chromate-phosphate-based binder filled with a metal alloy powder having the representative formula MCrAlY and SermaSeal). TM (Top coating of 570A). As described in Example 1, sample K was coated with the coating system of the present invention. A bare substrate sample was also included in this test to provide a baseline. (From...) Figure 15As can be seen, the results of this side-by-side comparison show that the Cr(VI)-free coating system of the present invention with an aluminum phosphate-based binder matrix is ​​significantly superior to commercially available systems and produces a significant improvement in protecting the superalloy substrate from hot corrosion caused by molten sulfate.

[0112] Comparative Example 3

[0113] In this test, a Cr(VI)-free coating system with a base coat of lithium-doped potassium silicate binder, as described in U.S. Patent No. 9,394,448 to Belov et al., was prepared to evaluate its performance against the erosion of an aggressive molten sulfate mixture A. The base coat slurry was prepared by mixing 230 g of alumina (Al₂O₃) powder into 307 g of a lithium-doped potassium silicate-based binder solution. The alumina pigment particle size was the same as in Example 1 (i.e., D). 50 It is approximately 2.5 micrometers to 3.0 micrometers and D 90 The thickness was approximately 4.0-4.5 micrometers. The pH of the primer slurry was measured to be approximately 10.9. The primer slurry was then sprayed onto the substrate, dried at 175℉ for 15 minutes, and then cured at 650℉ for 30 minutes to form the cured primer layer. A topcoat slurry as used in Example 1 was used (i.e., chromium(III)Cr2O3 mixed into an aluminum phosphate-based binder, wherein the Al:P molar ratio is approximately 1:2.4). The topcoat slurry was sprayed onto the cured primer layer, dried at 175℉ for 15 minutes, then cured at 650℉ for 30 minutes, and subsequently cured at 1000℉ for 60 minutes to form the cured topcoat layer. The total thickness of the primer and topcoat coating system was determined to be in the range of 2.5 mils to 2.7 mils.

[0114] The two coated samples were then exposed to sulfate mixture A for thermal cycling tests. The same thermal cycling test protocol as in Example 1 was used. The results are presented in Figure 16, which clearly shows that most of the coating was destroyed and peeled off after only 50 cycles. These results indicate that Cr(VI)-free coating systems with lithium-doped potassium silicate-based substrate coating binders, as described in U.S. Patent No. 9,394,448 to Belov et al., are unsuitable for use under sulfate corrosion.

[0115] Comparative Example 4

[0116] A conventional aluminide coating was applied to the CMSX-4 substrate using a standard vapor-phase aluminizing process. The resulting aluminide coating thickness was approximately 60 micrometers. This aluminide-only coated sample and an uncoated CMSX-4 sample were included in the hot corrosion test described in Example 4. The hot corrosion test was conducted in air at a static exposure temperature of 710°C for a total exposure time of 500 hours. Approximately 2 mg / cm³ of the aluminide coating listed in the table was applied prior to the test. 2 The synthetic sulfate mixture A was applied to the sample surface and replenished every 100 hours.

[0117] like Figure 18 As can be seen in Figure 19(a), after 500 hours of hot corrosion testing, both the aluminide-coated sample and the uncoated superalloy showed a corrosion pit depth of about 60 micrometers, resulting in significant material loss and performance degradation.

Claims

1. An aqueous slurry composition for generating a multilayer coating system for thermal corrosion protection of a substrate, the aqueous slurry composition comprising: The primer coating slurry comprises: A first binder comprising an aluminum phosphate-based aqueous solution with an Al:P molar ratio greater than 1:3, characterized in that the first binder is free of hexavalent chromium; and Metal particles or metal oxide particles or combinations thereof incorporated into the first binder, wherein the metal oxide particles do not include trivalent chromium oxide Cr2O3 particles. Topcoat slurry, the topcoat slurry comprising: The second binder comprises an aluminum phosphate-based aqueous solution with an Al:P molar ratio greater than 1:3, and is characterized by the absence of hexavalent chromium. Trivalent chromium oxide (Cr2O3) particles are incorporated into the second binder.

2. The aqueous slurry composition according to claim 1, wherein the metal oxide particles are selected from the group consisting of: alumina, titanium oxide, and zirconium oxide.

3. The aqueous slurry composition according to claim 1, wherein the metal particles are selected from the group consisting of: MCr, MA1, MCrAl or MCrAlY alloy, wherein M = Ni, Co, Fe or a combination thereof.

4. The aqueous slurry composition according to claim 1, wherein the metal oxide particles of the primer slurry comprise alumina (Al2O3) particles incorporated into the first binder.

5. The aqueous slurry composition according to claim 1, wherein the molar ratio of Al:P in the second binder of the topcoat slurry is in the range of 1:2.1 to 1:2.9, and the molar ratio of Al:P in the first binder of the bottomcoat slurry is in the range of 1:2.1 to 1:2.

9.

6. The aqueous slurry composition according to claim 1, wherein the molar ratio of Al:P in the second binder of the topcoat slurry is in the range of 1:2.4 to 1:2.7, and the molar ratio of Al:P in the first binder of the bottomcoat slurry is in the range of 1:2.4 to 1:2.

7.

7. The aqueous slurry composition of claim 1, wherein the metal particles or the metal oxide particles or a combination of both in the primer slurry have a certain particle size distribution, characterized in that The 50th percentile of the particle size distribution has a diameter between 1 micrometer and 10 micrometers.

8. The aqueous slurry composition according to claim 1, wherein the trivalent chromium oxide (Cr2O3) particles in the topcoat slurry have a certain particle size distribution, characterized in that... The 50th percentile of the particle size distribution has a diameter between 1.0 micrometer and 2.0 micrometer, and the 90th percentile of the particle size distribution has a diameter less than or equal to 3.0 micrometer.

9. An inorganic overcoating system for thermal corrosion protection of a substrate, the inorganic overcoating system comprising: The base coating comprises a first hexavalent chromium-free aluminum phosphate-based binder with an Al:P molar ratio greater than 1:3, and metal particles or metal oxide particles, or a combination of both, embedded in the first hexavalent chromium-free aluminum phosphate-based binder, wherein the metal oxide particles do not include trivalent chromium oxide (Cr2O3) particles. The top coating comprises a second hexavalent chromium-free aluminum phosphate-based binder with an Al:P molar ratio greater than 1:3 and trivalent chromium oxide (Cr2O3) particles incorporated into the second hexavalent chromium-free aluminum phosphate-based binder.

10. The inorganic overcoating system according to claim 9, wherein the Al:P molar ratio in the first hexavalent chromium-free aluminum phosphate-based binder is in the range of 1:2.1 to 1:2.9, and the Al:P molar ratio in the second hexavalent chromium-free aluminum phosphate-based binder is in the range of 1:2.1 to 1:2.

9.

11. The inorganic overcoating system according to claim 9, wherein the Al:P molar ratio in the first hexavalent chromium-free aluminum phosphate-based binder is in the range of 1:2.4 to 1:2.7, and the Al:P molar ratio in the second hexavalent chromium-free aluminum phosphate-based binder is in the range of 1:2.4 to 1:2.

7.

12. The inorganic coating system according to claim 9, wherein the metal oxide particles are selected from the group consisting of: alumina, titanium oxide, and zirconium oxide.

13. The inorganic overcoating system according to claim 9, wherein the metal particles are selected from the group consisting of: MCr, MA1, MCrAl or MCrAlY alloy, wherein M = Ni, Co, Fe or a combination thereof.

14. The inorganic coating system according to claim 9, wherein the metal oxide particles or metal particles or a combination of both have a certain particle size distribution, characterized in that... The 50th percentile of the particle size distribution has a diameter between 1 and 10 micrometers.

15. The inorganic overcoating system of claim 9, wherein the metal oxide particles are embedded or otherwise typically incorporated into the first ceramic matrix of the undercoating layer, the metal oxide particles comprising alumina (Al2O3), and further wherein trivalent chromium oxide (Cr2O3) particles are embedded or otherwise typically incorporated into the second ceramic matrix of the topcoating layer.

16. The inorganic overcoating system according to claim 9, wherein the top coating comprises chromium oxide (Cr2O3) particles with a certain particle size distribution, characterized in that... The 50th percentile of the particle size distribution has a diameter between 1.0 micrometer and 2.0 micrometer, and the 90th percentile of the particle size distribution has a diameter less than or equal to 3.0 micrometer.

17. A durable multilayer coating system for enhancing high-temperature oxidation and hot corrosion protection of a substrate, the durable multilayer coating system comprising: Base; Metal bonding coating on the substrate; An inorganic coating on the metal bonding coating, the inorganic coating comprising: The base coating comprises a first hexavalent chromium-free aluminum phosphate-based binder with an Al:P molar ratio greater than 1:3, and metal particles or metal oxide particles, or a combination of both, embedded in the first hexavalent chromium-free aluminum phosphate-based binder, wherein the metal oxide particles do not include trivalent chromium oxide (Cr2O3) particles. The top coating comprises a second hexavalent chromium-free aluminum phosphate-based binder with an Al:P molar ratio greater than 1:3 and trivalent chromium oxide (Cr2O3) particles incorporated into the second hexavalent chromium-free aluminum phosphate-based binder.

18. The multilayer coating system of claim 17, wherein the Al:P molar ratio in the first hexavalent chromium-free aluminum phosphate binder is in the range of 1:2.1 to 1:2.9, and the Al:P molar ratio in the second hexavalent chromium-free aluminum phosphate binder is in the range of 1:2.1 to 1:2.

9.

19. The multilayer coating system of claim 17, wherein the Al:P molar ratio in the first hexavalent chromium-free aluminum phosphate-based binder is in the range of 1:2.4 to 1:2.7, and the Al:P molar ratio in the second hexavalent chromium-free aluminum phosphate-based binder is in the range of 1:2.4 to 1:2.

7.

20. The multilayer coating system of claim 17, wherein the metal oxide particles are selected from the group consisting of: alumina, titanium oxide, and zirconium oxide.

21. The multilayer coating system of claim 17, wherein the metal particles are selected from the group consisting of MCr, MA1, MCrAl or MCrAlY alloys, wherein M = Ni, Co, Fe or a combination thereof.

22. The multilayer coating system according to claim 17, wherein the metal oxide particles or metal particles or a combination of both have a certain particle size distribution, characterized in that... The 50th percentile of the particle size distribution has a diameter between 1 and 10 micrometers.

23. The multilayer coating system of claim 17, wherein the metal oxide particles are embedded or otherwise typically incorporated into the first ceramic matrix of the undercoat, the metal oxide particles comprising alumina (Al2O3), and further wherein trivalent chromium oxide (Cr2O3) particles are embedded or otherwise typically incorporated into the second ceramic matrix of the topcoat.

24. The multilayer coating system according to claim 17, wherein the chromium oxide (Cr2O3) particles have a certain particle size distribution, characterized in that... The 50th percentile of the particle size distribution has a diameter between 1.0 micrometer and 2.0 micrometer, and the 90th percentile of the particle size distribution has a diameter less than or equal to 3.0 micrometer.

25. The multilayer coating system according to claim 17, wherein the metal bonding coating is selected from the group consisting of aluminides and chromium-rich coatings.

26. The multilayer coating system according to claim 17, wherein the metal bonding coating is selected from the group consisting of: platinum aluminate, chromium aluminate, platinum-modified chromium, and MCrAlY alloy, wherein M = Ni, Co, Fe, or a combination thereof.

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