High-temperature molten salt corrosion simulation test method for thermal barrier coating

Abstract

The invention relates to the technical field of coating material testing, in particular to a thermal barrier coating high-temperature molten salt corrosion simulation testing method. According to the method, a circulating salt coating and thermal circulation synergistic mechanism is adopted, turbid liquid coated with molten salt is supplemented in each cooling stage, and the dynamic process of continuous generation and deposition of corrosion salt in the actual working condition is simulated; a heating-cooling thermal cycle system is adopted, thermal stress of the component in the actual start-stop / cycle process is simulated, meanwhile, salt coating and the cooling process are bound, and a synergistic effect test of thermal stress and corrosion is achieved; according to the method, the real salt coating amount is reflected to a great extent by coating salt each time; after each time of salt coating, the distance between a salt layer covering area and the edge of the sample is kept to be larger than 2mm, and the problem that local rapid corrosion caused by the edge effect can cover the real protection performance of the coating can be solved. In conclusion, the high-temperature molten salt corrosion environment of the thermal barrier coating in actual service can be truly simulated, the corrosion resistance of the coating is accurately evaluated, and a failure mechanism is revealed.

Description

Technical Field

[0001] This invention relates to the field of coating material testing technology, and in particular to a method for simulating high-temperature molten salt corrosion of thermal barrier coatings. Background Technology

[0002] The high-temperature corrosion resistance of thermal barrier coatings directly determines the service safety and lifespan of aero-engines and gas turbines, and their performance under actual operating conditions needs to be evaluated through simulation testing methods. High-temperature molten salt corrosion is one of the main failure causes of thermal barrier coatings, therefore, it is necessary to establish testing methods that can reproduce this operating condition.

[0003] The actual failure process of thermal barrier coatings is a process of thermo-salt coupling, in which the main corrosive media originate from S and V in the fuel and Na in the air. + O2 reacts to form Na2SO4 + V2O5 molten salt. When the gas turbine is working, the compressor continuously draws in air from the outside, compresses it, mixes it with fuel, and burns it. The resulting high-temperature gas stream continuously drives the turbine to do work. Under high-temperature conditions, the Na in the air... + O2 readily reacts with impurities such as sulfur (S) and volatile organic compounds (V) in fuel, generating a molten salt mixture of Na2SO4 and V2O5 that deposits as a eutectic on the surface of components such as turbine blades. This erosion of the protective film on the substrate surface leads to failure. Taking YSZ coatings as an example, when thermal barrier coatings operate under high-temperature conditions, these molten salts and oxides corrode the coating surface and penetrate into the coating interior through pores and microcracks. This reduces the coating's toughness, and the molten salt impurities react chemically with the stabilizers in the zirconium oxide (ZrO2) thermal barrier coating material, causing ZrO2 to destabilize and undergo a phase transition, generating internal stress and ultimately leading to coating failure.

[0004] Laboratory experiments for studying hot corrosion must provide corrosion conditions similar to those in actual working environments. However, due to economic and time constraints, most hot corrosion studies employ accelerated testing methods. The main experimental methods include combustion device methods, immersion methods, salt coating methods, and electrochemical measurement methods. These high-temperature molten salt corrosion testing methods generally suffer from the following drawbacks: ① Excessive salt coating: In most corrosion tests, the corrosion concentration was greater than 1 mg / cm³. 2 Even higher than 10 mg / cm 2 This is excessive corrosion, and the corrosion results are far from the actual corrosion situation.

[0005] ②The salt coating process is a one-time coating, but in reality, new corrosive salts are constantly generated and participate in corrosion during the cycle, resulting in a significant difference from the actual corrosion situation. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a method for simulating high-temperature molten salt corrosion of thermal barrier coatings. The method provided by this invention belongs to the near-operational condition experimental method, which can realistically simulate the high-temperature molten salt corrosion environment of thermal barrier coatings in actual service, accurately evaluate the corrosion resistance of the coating, and reveal the failure mechanism.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for simulating high-temperature molten salt corrosion of thermal barrier coatings, comprising the following steps: A suspension of molten salt is coated onto the surface of the thermal barrier coating sample to be tested to obtain a coated sample; the molten salt is Na2SO4 and / or V2O5, and the specific ratio is selected according to the service conditions of the thermal barrier coating. The coated sample was cyclically heated and cooled, with an equal amount of molten salt suspension applied during each cooling stage, until the thermal expansion coating peeling area of ​​the sample reached the target value after the last heating. The heating temperature for each heating cycle is the same as the service temperature of the thermal barrier coating. The coating amount of molten salt suspension for each application is 0.1~0.4 mg / cm³, based on the amount of molten salt. 2 ; After each application of the molten salt suspension, maintain a distance of >2 mm between the salt layer coverage area and the edge of the sample.

[0008] Preferably, the coating is applied by spray gun.

[0009] Preferably, the spray gun is used for vertical spraying.

[0010] Preferably, the heating time for each heating cycle is 30 to 60 minutes.

[0011] Preferably, the cooling time is 10 to 30 minutes each time.

[0012] Preferably, the suspension of the molten salt consists of molten salt and a dispersant, wherein the dispersant includes ethanol.

[0013] Preferably, the particle size of the molten salt is less than 500 mesh.

[0014] Preferably, the method for preparing the suspension of the molten salt includes: ball milling and mixing the molten salt with ethanol, drying and then pulverizing and sieving to obtain salt powder; mixing the salt powder with a dispersant to obtain the suspension of the molten salt.

[0015] Preferably, the thermal barrier coating sample to be tested is circular with a diameter > 8 mm.

[0016] Preferably, the mass content of the molten salt in the suspension of the molten salt is 10-30%.

[0017] This invention employs a synergistic mechanism of cyclic salt coating and thermal cycling (i.e., a heating-cooling thermal cycling system). During each cooling phase, a suspension of molten salt is replenished to simulate the dynamic process of continuous salt formation and deposition under actual operating conditions. Furthermore, this invention clearly designs the heating-cooling thermal cycling system per unit time to simulate the thermal stress during the actual start-up / shutdown / cycle process of the component. Simultaneously, the salt coating and cooling processes are linked to achieve a synergistic effect test of thermal stress and corrosion. Moreover, the amount of salt applied in each step of this invention is 0.1~0.4 mg / cm³. 2 This invention largely reflects the actual amount of salt applied. Finally, by maintaining a distance of >2mm between the salt-covered area and the sample edge after each salt application, it avoids the problem of localized rapid corrosion caused by edge effects masking the true protective performance of the coating itself. In summary, this invention can realistically simulate the high-temperature molten salt corrosion environment of thermal barrier coatings in actual service, accurately evaluate the coating's corrosion resistance, and reveal the failure mechanism.

[0018] Furthermore, this invention employs a spray gun coating process, which offers the advantage of precise spraying. It allows for accurate control of the salt concentration through the amount of salt applied, ensuring consistent salt application in each cycle and avoiding the uneven coating problem common in existing methods. Existing methods often involve directly sprinkling salt powder onto the coating surface, resulting in uneven salt concentration distribution, inconsistent corrosion levels, and low reliability of test data. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the high-temperature molten salt corrosion simulation test method for thermal barrier coatings of the present invention; Figure 2 The images show actual photographs of the thermal barrier coatings after high-temperature molten salt corrosion in the examples and comparative examples. From left to right, they are Comparative Example 1, Comparative Example 2, and Example 1. Figure 3 XRD patterns of the corrosion products of the examples and comparative examples; Figure 4 The images shown are SEM images of the thermal barrier coatings after high-temperature molten salt corrosion in the examples and comparative examples. In the examples, a is comparative example 1, b is comparative example 2, and c is example 1. Detailed Implementation

[0020] This invention provides a method for simulating high-temperature molten salt corrosion of thermal barrier coatings, comprising the following steps: A suspension of molten salt is coated onto the surface of the thermal barrier coating sample to be tested to obtain a coated sample; the molten salt is Na2SO4 and / or V2O5, and the specific ratio is selected according to the service conditions of the thermal barrier coating. The coated sample was cyclically heated and cooled, with an equal amount of molten salt suspension applied during each cooling stage, until the thermal expansion coating peeling area of ​​the sample reached the target value after the last heating. The heating temperature for each heating cycle is the same as the service temperature of the thermal barrier coating. The coating amount of molten salt suspension for each application is 0.1~0.4 mg / cm³, based on the amount of molten salt. 2 ; After each application of the molten salt suspension, maintain a distance of >2 mm between the salt layer coverage area and the edge of the sample.

[0021] The present invention involves coating the surface of a thermal barrier coating sample with a suspension of molten salt to obtain a coated sample.

[0022] In this invention, the molten salt is Na2SO4 and / or V2O5, with the specific ratio selected according to the service conditions of the thermal barrier coating. Generally, Na2SO4 and V2O5 are widely present in various fuels, and the specific ratio depends on the fuel's origin, type, and operating conditions. To comprehensively characterize the corrosion behavior of the Na2SO4 and V2O5 mixed salt, they are mixed in a 1:1 ratio in the embodiments of this invention.

[0023] In this invention, the molten salt suspension preferably consists of molten salt and a dispersant, wherein the dispersant preferably includes ethanol, more preferably anhydrous ethanol. Ethanol is used as the dispersant in this invention because it is volatile and, after evaporation, does not easily form hydrogen bonds or other substances that could cause new corrosion. This invention does not impose special requirements on the concentration of the molten salt suspension, as long as it can disperse the molten salt uniformly. In embodiments of this invention, the mass content of the molten salt in the suspension can be 10-30%, specifically 10%, 20%, or 30%. In this invention, the particle size of the molten salt is preferably below 500 mesh.

[0024] In this invention, the preferred method for preparing the suspension of molten salt includes: ball milling and mixing molten salt with ethanol, drying and then pulverizing and sieving to obtain salt powder; mixing the salt powder with a dispersant to obtain a suspension of molten salt.

[0025] This invention does not impose special requirements on the ball milling conditions, as long as the molten salt is finely ground. In an embodiment of this invention, the ball milling is specifically carried out in a planetary ball mill at a speed of 600 rpm for 12 hours. When the molten salt is Na2SO4 and V2O5, it also achieves the purpose of uniformly mixing the two, which is more in line with actual conditions.

[0026] In this invention, the drying temperature is preferably 80°C and the drying time is preferably 12 hours.

[0027] In this invention, the sieving is preferably performed through a 500-mesh sieve, and the material passing through the sieve is collected.

[0028] In this invention, the thermal barrier coating sample to be tested is preferably circular, with a diameter preferably >8 mm. Controlling the sample diameter to >8 mm ensures greater observability. In an embodiment of this invention, the diameter of the thermal barrier coating sample to be tested is 25 mm. This invention does not have special requirements regarding the type of thermal barrier coating; any thermally expanding coating well-known in the art is acceptable, specifically such as YSZ coatings and X / YSZ double ceramic layer coatings.

[0029] In this invention, the coating preferably includes spray gun application; the spray gun application is carried out using vertical spraying.

[0030] This invention uses a spray gun to achieve precise spraying, control the size of the salt coating area and the distance between the salt coating area and the edge of the sample, and precisely control the salt concentration by spraying amount to ensure consistent salt amount in each cycle.

[0031] In this invention, after coating the suspension of molten salt, the salt layer coverage area is kept at a distance of >2 mm from the edge of the sample.

[0032] In this invention, after each application of the molten salt suspension, a distance >2 mm is maintained between the salt-covered area and the edge of the sample. During thermal cycling corrosion, the coating at the edge is more prone to peeling due to the edge effect. The edge effect refers to the phenomenon where the corrosive medium penetrates more easily and the coating fails faster at the edges and corners of the coated sample due to its geometry, stress state, or interface characteristics. This results in a corrosion rate in the edge region that is much higher than in the flat area at the center of the sample, constituting a type of non-uniform corrosion. If the salt coating directly contacts the edge, the localized rapid corrosion caused by the edge effect will mask the true protective performance of the coating itself, leading to distorted test results. The measured corrosion rate will be too high, failing to reflect the actual protective effect of the coating on a flat surface. This may cause premature blistering, peeling, and other failure phenomena in the coating, misjudging its service life.

[0033] In this invention, the coating amount of the molten salt suspension is 0.1~0.4 mg / cm³, based on the amount of molten salt. 2 In specific embodiments, the concentration can be 0.1, 0.15, 0.2, 0.25, 0.3, 0.32, 0.35, or 0.4 mg / cm³. 2 In a specific embodiment of the present invention, the concentration is 0.32 mg / cm³. 2 This value represents the actual deposition amount of a certain stage rotor blade of the X-type aero-engine after 1 hour of operation. This invention controls the salt coating amount within the above range, thus largely reflecting the actual amount of molten salt generated on the surface of the thermally expanding coating.

[0034] After obtaining the coated sample, the present invention cyclically heats and cools the coated sample, and coats it with an equal amount of molten salt suspension in each cooling stage, until the thermal expansion coating of the sample peels off by more than 10% after the last heating.

[0035] In this invention, the heating temperature for each heating cycle is the same as the service temperature of the thermal barrier coating; generally above 800°C, and specifically 1000°C in the embodiments of this invention. The heating time for each cycle is preferably 30-60 minutes, and in specific embodiments, it can be 30, 40, 50, or 60 minutes; the cooling time for each cycle is preferably 10-30 minutes, and in specific embodiments, it can be 10, 20, or 30 minutes. This invention preferably uses a circulating furnace for heating. Specifically, the sample is placed into the furnace body only after the circulating furnace reaches the target temperature. In this invention, the cooling is natural cooling outside the furnace.

[0036] In this invention, an equal amount of molten salt suspension (based on the amount of molten salt) is applied during each cooling stage. After each application of the molten salt suspension, the salt layer coverage area remains consistent, and a distance of >2 mm is maintained from the sample edge. This invention, by replenishing the molten salt suspension during each cooling stage, simulates the dynamic process of continuous corrosion salt generation and deposition under actual operating conditions, ensuring the consistency of the corrosion process with the real service environment, thereby ensuring the reliability of the data.

[0037] Furthermore, this invention employs a cyclic heating-cooling thermal cycle system, which can simulate the thermal stress during the actual start-up / cycle of components. At the same time, it binds the salt coating process with the cooling process, thereby achieving a synergistic effect test of thermal stress and corrosion.

[0038] This invention simulates the synergistic effect of thermal cycling and molten salt corrosion, which better reflects the actual service conditions of the coating, and the test results are more valuable for reference.

[0039] This invention involves a repeated cycle of heating, cooling, and coating until the area of ​​thermal expansion coating peeling off the sample reaches the target value after the final heating. The coating failure life is conventionally determined by the area of ​​coating peeling off; generally, coating failure occurs when the area of ​​thermal expansion coating peeling off exceeds 10%. Therefore, this invention preferably stops the test when the area of ​​thermal expansion coating peeling off the sample reaches 10% or more after the final heating. This invention preferably combines XRD and SEM-EDS methods to analyze the phase composition, elemental distribution, and penetration pathway of corrosion products, clarifying the mechanism of thermal expansion coating failure.

[0040] The following detailed description of the high-temperature molten salt corrosion simulation test method for thermal barrier coatings provided by the present invention, with reference to the embodiments, should not be construed as limiting the scope of protection of the present invention.

[0041] Example 1: Figure 1 This is a schematic diagram of the high-temperature molten salt corrosion simulation test method for thermal barrier coatings of the present invention, according to... Figure 1 Conduct the test.

[0042] Sample parameters: M247 nickel-based alloy substrate (diameter 25mm, thickness 3.5mm), thermal barrier coating is 450μm thick YSZ coating, surface is free of oil and impurities.

[0043] Corrosive medium: 50wt.% Na2SO4 + 50wt.% V2O5 mixed salt, mixed with anhydrous ethanol at a mass ratio of 1:4 to form a suspension. When spraying, keep the salt layer coverage area 2.5mm away from the edge of the sample.

[0044] Test conditions: The cycle consisted of 50 minutes of constant temperature heating at 1000℃ followed by 10 minutes of natural cooling outside the furnace. During each cooling phase, 1 mL of salt solution was sprayed (salt concentration: 0.32 mg / cm³). 2 Repeat 100 times.

[0045] Comparative Example 1: Comparative Test of YSZ Coatings without Cycling after a Single Salt Application Sample parameters: Same as in Example 1.

[0046] Corrosive medium: Same as in Example 1. The salt coating amount should reach 3 mg / cm² before the isothermal period begins. 2 During spraying, maintain a distance of 2.5 mm between the salt layer coverage area and the edge of the sample.

[0047] Test conditions: Heating at a constant temperature of 1000℃ for 100 hours without circulation.

[0048] Comparative Example 2: Comparative Test of YSZ Coatings After a Single Salt Coating Cycle Sample parameters: Same as the YSZ coating sample in Example 1.

[0049] Corrosive medium: Same as in Example 1. The salt coating amount should reach 3 mg / cm² before the cycle begins. 2 Maintain an edge distance of 2.5mm during spraying.

[0050] Test conditions: 50 min constant temperature heating at 1000℃ + 10 min natural cooling outside the furnace, 100 cycles.

[0051] Characterization: The surface morphology of the sample was captured by a camera, and the area of ​​coating peeling was quantified. The results are shown in [Figure number missing]. Figure 2 The examples, from left to right, are: single salt coating without circulation (Comparative Example 1), single salt coating with circulation (Comparative Example 2), and the method of the present invention (Example 1). The phase composition of the corrosion products was analyzed using XRD equipment, and the results are shown in [Figure 1]. Figure 3 , Figure 3 The right image is a magnified view of the left image, showing the region including the main peak t'(111) of the tetragonal ZrO2 phase and the main peaks m(-111) and m(111) of the monoclinic zirconium oxide phase. Figure 3It can be seen that after corrosion, the diffraction peak intensities of t'-ZrO2 in the YSZ coating decreased significantly, indicating the formation of a new m-ZrO2 phase. The content of the m-ZrO2 phase can serve as an important reference for coating degradation. The content of m-ZrO2 (Cm) in the coating was obtained using the Miller equation based on the XRD diffraction peak intensities. The relative volume fractions of the m-ZrO2 phase in the YSZ coating after thermal cycling corrosion were calculated to be 24.6% (Comparative Example 1), 43.3% (Comparative Example 2), and 36.1% (Example 1). This demonstrates that under cyclic corrosion conditions at 1000°C, the m-ZrO2 phase content of YSZ increases with the number of corrosion cycles and the concentration of corrosion salt, and overall, the corrosion salt concentration has a greater impact on the corrosion process than the number of corrosion cycles.

[0052] comprehensive Figure 2 and Figure 3 As can be seen, in Example 1, the coating surface showed brownish-yellow corrosion products, mainly composed of m-ZrO2 and YVO4, with a peeling area of ​​approximately 4%. XRD analysis showed that the m-ZrO2 phase content was 36.1%, and no large-area peeling was observed. Comparative Example 1: Only cracks appeared on the surface, the coating surface peeling area was 0%, XRD analysis showed that the m-ZrO2 phase content was 24.6%, and a large number of clustered NaVO3 intermediate products were found in the corrosion products. Comparative Example 2: The coating surface peeling area reached 13%, with through cracks appearing. XRD analysis showed that the m-ZrO2 phase content was 43.3%, and a large number of clustered NaVO3 intermediate products were found in the corrosion products.

[0053] The cross-sectional morphology and elemental distribution of the coatings in the examples and comparative examples were observed using SEM combined with EDS to analyze the penetration path of the corrosive medium and the interface damage. SEM images are shown below. Figure 4 a) is a single salt coating without recycling (Comparative Example 1), b) is a single salt coating cycle (Comparative Example 2), and c) is the method of the present invention (Example 1). Figure 4 The EDS results of corrosion products at different marked locations are shown in Table 1.

[0054] Table 1. EDS results of corrosion products from the examples and comparative examples.

[0055] The test results above show that both the surface products and internal corrosion differ significantly under different cyclic conditions, indicating that the coating life obtained using the non-cyclic coating salt method cannot accurately reflect the coating performance. This application, however, simulates the thermal expansion coating corrosion process under conditions closer to real-world operating conditions, and therefore reflects the actual situation more accurately than traditional methods.

[0056] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method of high temperature molten salt corrosion simulation testing of thermal barrier coatings, characterized in that, The method comprises the following steps: coating a suspension of molten salt on the surface of the sample to be tested to obtain a coated sample; the molten salt is Na2SO4 and / or V2O5, and the ratio is selected according to the service condition of the thermal barrier coating; cycling the coated sample to heat and cool, coating an equal amount of the suspension of molten salt in each cooling stage until the peeling area of the thermal expansion coating of the sample reaches a target value after the last heating; the temperature of each heating is the same as the service temperature of the thermal barrier coating; The amount of the molten salt suspension coated each time was 0.1 to 0.4 mg / cm in terms of the amount of the molten salt 2 ; after coating the suspension of molten salt each time, the salt layer covering area is kept at a distance of >2mm from the edge of the sample.

2. The method of claim 1, wherein, The coating comprises spray coating.

3. The method of high temperature molten salt corrosion simulation testing of a thermal barrier coating according to claim 2, characterized in that, The spray coating adopts vertical spray coating.

4. The method of high-temperature molten salt corrosion simulation testing of a thermal barrier coating according to claim 1, characterized in that, The time of each heating is 30-60min.

5. The method of high-temperature molten salt corrosion simulation testing of a thermal barrier coating according to claim 1, characterized in that, The time of each cooling is 10-30min.

6. The method of high-temperature molten salt corrosion simulation testing of a thermal barrier coating of claim 1, wherein, The suspension of molten salt is composed of molten salt and a dispersant, and the dispersant comprises ethanol.

7. The method of high-temperature molten salt corrosion simulation testing of a thermal barrier coating according to claim 1, characterized in that, The particle size of the molten salt is below 500 mesh.

8. The method of high-temperature molten salt corrosion simulation testing of a thermal barrier coating according to claim 1, 6 or 7, characterized in that, The preparation method of the suspension of molten salt comprises: ball-milling the molten salt and ethanol, drying, and then crushing and screening in sequence to obtain salt powder; mixing the salt powder and a dispersant to obtain the suspension of molten salt.

9. The method of high-temperature molten salt corrosion simulation testing of a thermal barrier coating according to claim 1, wherein, The sample to be tested is circular, and the diameter is >8mm.

10. The method of high-temperature molten salt corrosion simulation testing of a thermal barrier coating according to claim 1, 6 or 7, characterized in that, The mass content of the molten salt in the suspension of molten salt is 10-30%.

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