A method for controlling the life of a powder-pack aluminized coating

Abstract

A method for controlling the life of a powder-pack aluminized coating, belonging to the technical field of coating life control, overcomes the defect in the prior art that the life control of the aluminized coating cannot be achieved. The method for controlling the life of the powder-pack aluminized coating of the present invention includes the following steps: Step 1, prepare aluminized specimens with initial coating thicknesses of H1 and H2; Step 2, fit to obtain the total amounts S01 and S02 of Al in the two aluminized specimens; Step 3, conduct a high-temperature steam oxidation experiment at T1; Step 4, fit to obtain the diffusion coefficient D1 and η1 of the aluminized specimen with an initial coating thickness of H1 at T1; and / or, fit to obtain the diffusion coefficient D2 and η2 of the aluminized specimen with an initial coating thickness of H2 at T1; Step 5, calculate the values of a and b; Step 6, substitute D1 or D2 into Equation (1) to calculate S0; Step 7, calculate the initial thickness H0 of the aluminized coating required for the coating to reach the target life at the temperature of T1; Step 8, prepare an aluminized specimen with an initial coating thickness ≥ H0.

Description

Technical Field

[0001] The present invention belongs to the technical field of coating life control, and particularly relates to a method for controlling the life of a powder-pack aluminized coating. Background Art

[0002] The boiler heating surfaces (water walls, superheaters, reheaters and economizers, also known as the "four tubes" of the boiler) are key components in the boiler responsible for recovering the energy of the coal combustion flue gas, heating steam and realizing energy conversion. Among the failure causes of the "four tubes" of high-parameter power station boilers, flue gas corrosion caused by pulverized coal combustion outside the furnace tubes and high-temperature steam oxidation corrosion inside the furnace tubes play an important role, and are also one of the essential reasons for boiler explosion and leakage accidents. With the development of high steam parameters and high efficiency of thermal power plant units, the service conditions of boiler tubes have become more complex and demanding, and the service performance of the materials used also needs to be further improved under higher requirements.

[0003] Surface modification technology is a technology that strengthens the surface of parts or materials by changing the chemical composition or organizational structure of the surface of materials or workpieces, and can improve the high-temperature oxidation and corrosion resistance of superalloys. Compared with developing higher-grade heating surface materials, surface modification technology can significantly improve the antioxidant and corrosion resistance of boiler tubes without reducing the mechanical properties of the alloy matrix, not only solving the oxidation / corrosion problems of existing boiler tube alloys, but also providing technical support for the existing materials to continue to serve in higher-parameter units.

[0004] Aluminum coating technology is an economical and effective way to significantly improve the high-temperature oxidation resistance of the substrate without changing the mechanical properties of the substrate. At present, it has been widely applied in the aerospace field. After using aluminum coatings on key components of gas turbines, they have excellent oxidation resistance and can operate for thousands of hours above 900°C. The excellent protection of aluminide coatings comes from the slow growth of the oxide film, and the growth of alumina depends on temperature. When the temperature is reduced by 200°C, the growth of alumina can be reduced by several orders of magnitude. Therefore, when aluminide coatings are applied to power station boiler tubes with lower temperatures (<700°C), their service life will be greatly extended. Low-temperature aluminizing has a lower diffusion rate, can obtain a thinner coating, and no brittle phases precipitate. Therefore, it can significantly change the steam oxidation performance without changing the structure and mechanical properties of the substrate material.

[0005] The P92-Al coating has good oxidation resistance. This is mainly because, in the initial stage of oxidation, water vapor reacts with Al elements in the coating to undergo selective oxidation, forming a single-molecule oxide film. Subsequently, the growth of the film is achieved through an electrochemical reaction, and the growth rate is relatively slow, and the Al elements are not completely oxidized to Al2O3. On the one hand, the formed Al2O3 film can reduce the diffusion coefficient; on the other hand, the Al-rich layer that is not completely oxidized can also provide an abundant Al source for the alloy to continue to form the Al2O3 film, making the P92-Al coating have excellent steam oxidation resistance.

[0006] Generally, it is considered that the failure mechanisms of aluminide coatings under high-temperature conditions mainly include the following: a. The oxide film exfoliates, and Al elements diffuse outward to form a new Al2O3 film; b. Interdiffusion with the substrate. When the P92-Al coating is oxidized in steam at 650 °C, a protective Al2O3 film is rapidly formed on the surface. Although the temperature is relatively low, the Al2O3 film still has good adhesion and does not exfoliate during the test. This makes the degradation mechanism of the aluminide coating mainly manifested as the inward diffusion of Al. Among the current numerous aluminizing coating preparation processes, it is impossible to control the life of the aluminizing coating. And the invention research in this aspect plays a crucial and decisive role in the coating preparation process. Summary of the Invention

[0007] Therefore, the technical problem to be solved by the present invention is to overcome the defect in the prior art that the life of the aluminizing coating cannot be controlled, so as to provide a method for controlling the life of a powder-pack aluminizing coating.

[0008] For this reason, the present invention provides the following technical solutions.

[0009] A method for controlling the life of a powder-pack aluminizing coating, comprising the following steps:

[0010] Step 1, prepare aluminized specimens with initial coating thicknesses of H1 and H2;

[0011] Step 2, the coating satisfies the following relational expression:

[0012]

[0013] In the formula, x is the depth from a certain position in the coating to the coating surface, C is the mass percentage of aluminum at x in the coating, D is the diffusion coefficient, t is the oxidation time, S is the total amount of Al in the coating, and η is the calibration displacement of the Gaussian distribution center;

[0014] Respectively obtain the mass percentages of aluminum at different x positions of the aluminized specimens with initial coating thicknesses of H1 and H2 prepared in Step 1; substitute the data obtained in this step into formula (1) to fit and obtain the total amounts of Al, S01 and S02, of the two aluminized specimens;

[0015] Step 3: Subject aluminized specimens with initial coating thicknesses of H1 and H2 to high-temperature steam oxidation experiments at temperature T1;

[0016] Step 4: Obtain the mass percentage of aluminum at different x positions in the coating of the aluminized specimens at oxidation time t at temperature T1 according to Step 3; Substitute the data obtained in the above step into Equation (1) to fit and obtain the diffusion coefficient D1 and η1 of the aluminized specimen with an initial coating thickness of H1 at T1; and / or, fit and obtain the diffusion coefficient D2 and η2 of the aluminized specimen with an initial coating thickness of H2 at T1;

[0017] Step 4: The total amount S of Al in the coating satisfies:

[0018] S = aH + b Equation (2)

[0019] where a and b are constants related to the aluminized coating;

[0020] Substitute H1, S01 and H2, S02 into Equation (2) to calculate the values of a and b;

[0021] Step 5: Substitute D1 or D2 into Equation (1), where C takes the critical content to form alumina, η takes the value corresponding to D1 or D2, x takes 0, and t is the target life of the coating, and calculate S0;

[0022] Step 6: Substitute S0 into Equation (2) to calculate the initial thickness H0 of the aluminized coating required for the coating to reach the target life at temperature T1;

[0023] Step 7: Prepare aluminized specimens with an initial coating thickness ≥ H0.

[0024] Further, Step 1 includes:

[0025] S101: Degrease and remove rust from the aluminized specimen to be treated, and then bury the aluminized specimen to be treated in a powder can filled with the prepared aluminized powder and compact it;

[0026] S102: Place the compacted powder can in a heat treatment furnace and sinter it under a protective gas atmosphere;

[0027] S103: Cool it to room temperature with the furnace to obtain an aluminized specimen.

[0028] Further, in S101, the aluminized powder includes FeAl powder and a promoter.

[0029] Further, the promoter is NH4Cl.

[0030] Further, the mass ratio of iron to aluminum in the FeAl powder is 1:1.

[0031] Further, the mass ratio of the FeAl powder to the infiltration aid is 99:1.

[0032] Further, in S102, the sintering conditions are sintering at 640 - 780 °C for 4 - 8 h.

[0033] Further, the protective gas is argon.

[0034] Further, the high-temperature steam oxidation experimental conditions in step 2 are: dynamic 100% saturated water vapor, pressure 0.1 - 30 MPa, and steam flow rate 100 - 120 ml / s.

[0035] The original thickness of the aluminized coating and the diffusion depth of the aluminized coating at different oxidation times are analyzed by a scanning electron microscope (SEM), and the composition analysis of different depth positions of the aluminized coating is carried out by an energy spectrometer.

[0036] The technical solution of the present invention has the following advantages:

[0037] The method for controlling the life of the powder-pack aluminized coating provided by the present invention can predict the thickness of the coating when the target life is reached, so as to control the coating preparation process, so that the prepared material meets the use requirements, and avoid the situation that the entire system cannot operate normally due to the insufficient service life of the pipe system after use. At the same time, the aluminizing process can be improved, production waste can be avoided, production costs can be reduced, and the production efficiency and economic benefits of the enterprise can be improved. Description of the Drawings

[0038] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0039] Figure 1 It is a scanning electron microscope image of an aluminized specimen with a coating thickness of 5 μm prepared in Example 1.

[0040] Figure 2 It is a scanning electron microscope image of an aluminized specimen with a coating thickness of 30 μm prepared in Example 1. Specific Embodiments

[0041] The following embodiments are provided to better understand the present invention further. They are not limited to the best embodiments, and do not limit the content and protection scope of the present invention. Any product obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with those of other prior arts and being the same as or similar to the present invention falls within the protection scope of the present invention.

[0042] For those without specific experimental steps or conditions noted in the examples, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For reagents or instruments without the manufacturer noted, they are all conventional reagent products that can be obtained through commercial purchase.

[0043] Example 1

[0044] A method for controlling the life of a powder-pack aluminized coating includes the following steps:

[0045] Step 1. Prepare aluminized specimens:

[0046] S101. Degrease and derust the specimens to be aluminized. Prepare the pack aluminizing powder by mixing 99 wt% FeAl powder + 1 wt% infiltration promoter, and layer the pretreated specimens into a stainless-steel can and compact them.

[0047] S102. Place the aluminized specimens after embedding in a heat treatment furnace at 750 °C and sinter for 4 h in an argon atmosphere to prepare aluminized specimens with a coating thickness H1 of 30 μm.

[0048] Place the aluminized specimens after embedding in a heat treatment furnace at 640 °C and sinter for 6 h in an argon atmosphere to prepare aluminized specimens with a coating thickness H2 of 5 μm.

[0049] S103. Cool in the furnace to room temperature and take out the specimens to complete the preparation of the aluminized coating. As Figure 1 、 Figure 2 shown.

[0050] Step 2. The coating satisfies the following relationship:

[0051]

[0052] In the formula, x is the depth of a certain position in the coating from the coating surface, C is the mass percentage of aluminum at x in the coating, D is the diffusion coefficient, t is the oxidation time, S is the total amount of Al in the coating, and η is the calibration displacement of the center of the Gaussian distribution;

[0053] Respectively obtain the mass percentages of aluminum at different x in the aluminized specimens with initial coating thicknesses H1 and H2 prepared in Step 1 before the oxidation test; substitute the data obtained in this step into Equation (1) to fit and obtain the total amounts S01 of Al in the aluminized specimens with H1 and H2 as 1418.50138 and S02 as 243.23653;

[0054] Step 3. Conduct a high-temperature steam oxidation experiment on the aluminized specimens at a temperature of 650 °C. The steam oxidation experiment is a dynamic 100% saturated water vapor test, with a pressure of 0.1 MPa and a steam flow rate of 100 ml / s, simulating the service environment on the steam side of a power plant boiler tube.

[0055] Step 4: Obtain the mass percentage of aluminum corresponding to different x values after aluminized specimens are oxidized at 650°C for 1000 h according to Step 3.

[0056] Substitute the data obtained above into Equation (1), and the diffusion coefficients D1, S1, and η1 of the aluminized specimens with H1 coating at 650°C are respectively obtained by fitting as: 0.06332, 897.09465, 7.84419.

[0057] Step 5: The total amount S of Al in the coating satisfies:

[0058] S = aH + b Equation (2)

[0059] where a and b are constants related to the aluminized coating;

[0060] Substitute H1, S01 and H2, S02 into Equation (2), and calculate the values of a and b, which are 47.02 and 7.9 respectively;

[0061] Step 6: Substitute D1 into Equation (1), C takes the critical content of 3% that can form aluminum oxide, η takes the value corresponding to D1, x takes 0, and t is the target life of the coating of 70000 h, and calculate to obtain S0 as 713.2;

[0062] Step 7: Substitute S into Equation (2), and calculate the initial diffusion depth H0 of the aluminized coating required for the target life of the coating as 15 μm;

[0063] Step 8: Prepare an aluminized specimen with a coating thickness of 15 μm.

[0064] Example 2

[0065] A method for controlling the life of a powder-pack aluminized coating, comprising the following steps:

[0066] Step 1: Prepare an aluminized specimen:

[0067] S101: Degrease and rust-remove the specimen to be aluminized, prepare a pack aluminizing powder by mixing 99 wt% FeAl powder + 1 wt% infiltration promoter, and layer the pretreated specimen into a stainless steel can and compact it.

[0068] S102: Place the aluminized specimen to be embedded in a heat treatment furnace at 750°C and sinter it in an argon atmosphere for 4 h to prepare an aluminized specimen with a coating thickness H1 of 30 μm.

[0069] Place the aluminized specimen to be embedded in a heat treatment furnace at 640°C and sinter it in an argon atmosphere for 6 h to prepare an aluminized specimen with a coating thickness H2 of 5 μm.

[0070] S103. Cool it in the furnace to room temperature, take out the test piece to complete the preparation of the aluminized coating. As Figure 1 , Figure 2 shown.

[0071] Step 2. The coating satisfies the following relationship:

[0072]

[0073] In the formula, x is the depth of a certain position in the coating from the coating surface, C is the mass percentage of aluminum at x in the coating, D is the diffusion coefficient, t is the oxidation time, S is the total amount of Al in the coating, and η is the calibration displacement of the Gaussian distribution center;

[0074] Before the oxidation test, obtain the mass percentages of aluminum at different x for the aluminized test pieces with the initial coating thicknesses of H1 and H2 prepared in Step 1 respectively; substitute the data obtained in this step into Equation (1), and by fitting, the total amounts of Al, S01 and S02, for the aluminized test pieces with H1 and H2 are 1418.50138 and 243.23653 respectively;

[0075] Step 3. Conduct a high-temperature steam oxidation experiment on the aluminized test piece at a temperature of 650 °C. The steam oxidation experiment is a dynamic 100% saturated water vapor test, with a pressure of 0.1 MPa and a steam flow rate of 100 ml / s, simulating the service environment on the steam side of a power plant boiler tube.

[0076] Step 4. According to Step 3, obtain the mass percentages of aluminum corresponding to different x for the aluminized test piece after oxidation at 650 °C for 1000 h.

[0077] Substitute the data obtained above into Equation (1), and by fitting, the diffusion coefficient D1, S1, and η1 for the aluminized test piece with the H1 coating at 650 °C are 0.06332, 897.09465, and 7.84419 respectively.

[0078] Step 5. The total amount of Al, S, in the coating satisfies:

[0079] S = aH + b Equation (2)

[0080] where a and b are constants related to the aluminized coating;

[0081] Substitute H1, S01 and H2, S02 into Equation (2), and calculate the values of a and b, which are 47.02 and 7.9 respectively.

[0082] Step 6. Substitute D1 into Equation (1), take C as the critical content of 3% that can form aluminum oxide, take η as the value corresponding to D1, take x as 0, and t as the target life of the coating, 125000 h, and calculate to obtain S0 as 948.3;

[0083] Step 7: Substitute S into Equation (2) to calculate the initial diffusion depth H0 of the aluminized coating required for the target coating life, which is 20 μm.

[0084] Step 8: Prepare an aluminized specimen with a coating thickness of 20 μm.

[0085] Example 3

[0086] A method for controlling the life of a pack cementation aluminized coating includes the following steps:

[0087] Step 1: Prepare an aluminized specimen:

[0088] S101: Degrease and remove rust from the specimen to be aluminized. Prepare the pack cementation aluminized powder by mixing 99 wt% FeAl powder + 1 wt% accelerating agent, and layer the pretreated specimen and the powder into a stainless - steel can and compact it.

[0089] S102: Place the aluminized specimen after embedding in a heat - treatment furnace at 750 °C and sinter it for 4 h in an argon atmosphere to prepare an aluminized specimen with a coating thickness H1 of 30 μm.

[0090] Place the aluminized specimen after embedding in a heat - treatment furnace at 640 °C and sinter it for 6 h in an argon atmosphere to prepare an aluminized specimen with a coating thickness H2 of 5 μm.

[0091] S103: Cool it in the furnace to room temperature and take out the specimen to complete the preparation of the aluminized coating. As Figure 1 、 Figure 2 shown.

[0092] Step 2: The coating satisfies the following relationship:

[0093]

[0094] In the formula, x is the depth from a certain position in the coating to the coating surface, C is the mass percentage of aluminum at position x in the coating, D is the diffusion coefficient, t is the oxidation time, S is the total amount of Al in the coating, and η is the calibration displacement of the Gaussian distribution center;

[0095] Respectively obtain the mass percentages of aluminum at different x positions in the aluminized specimens with initial coating thicknesses H1 and H2 prepared in Step 1 before the oxidation test. Substitute the data obtained in this step into Equation (1) to fit and obtain the total amounts of Al, S01, of the aluminized specimens with H1 and H2, which are 1418.50138 and S02, which is 243.23653;

[0096] Step 3: Conduct a high - temperature steam oxidation experiment on the aluminized specimen at a temperature of 650 °C. The steam oxidation experiment is a dynamic 100% saturated water - steam test, with a pressure of 0.1 MPa and a steam flow rate of 100 ml / s, simulating the service environment on the steam side of a power - plant boiler tube.

[0097] Step 4: Obtain the mass percentage of aluminum corresponding to different x values after aluminized specimens are oxidized at 650°C for 1000 h according to Step 3.

[0098] Substitute the data obtained above into Equation (1), and the diffusion coefficients D1, S1, and η1 of the aluminized specimens with H1 coating at 650°C are respectively obtained by fitting as: 0.06332, 897.09465, 7.84419.

[0099] Step 5: The total amount S of Al in the coating satisfies:

[0100] S = aH + b Equation (2)

[0101] where a and b are constants related to the aluminized coating;

[0102] Substitute H1, S01 and H2, S02 into Equation (2), and calculate the values of a and b, which are 47.02 and 7.9 respectively.

[0103] Step 6: Substitute D1 into Equation (1), C takes the critical content of 3% that can form alumina, η takes the value corresponding to D1, x takes 0, and t is the target life of the coating of 497000 h, and calculate to obtain S0 as 1888.7;

[0104] Step 7: Substitute S into Equation (2), and calculate the initial diffusion depth H0 of the aluminized coating required for the target life of the coating = 40 μm;

[0105] Step 8: Prepare an aluminized specimen with a coating thickness of 40 μm.

[0106] Obviously, the above embodiments are only examples clearly described and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or alterations can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. And the obvious changes or alterations derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for controlling the life of a powder-pack aluminized coating, characterized in that, It includes the following steps: Step 1: Prepare aluminized specimens with initial coating thicknesses of H1 and H2; Step 2: The coating satisfies the following relational expression: Formula (1) Wherein, x is the depth of a certain position in the coating from the coating surface, C is the mass percentage of aluminum at x in the coating, D is the diffusion coefficient, t is the oxidation time, S is the total amount of Al in the coating, η is the calibration displacement of the center of the Gaussian distribution; Respectively obtain the mass percentages of aluminum at different x positions for the aluminized specimens with initial coating thicknesses of H1 and H2 prepared in Step 1; substitute the data obtained in this step into Equation (1) to fit and obtain the total amounts S01 and S02 of Al for the two aluminized specimens; Step 3: Subject the prepared aluminized specimens with initial coating thicknesses of H1 and H2 to high-temperature steam oxidation experiments at temperature T1; Step 4. Obtain the mass percentage of aluminum at different x in the coating of the aluminized specimen at oxidation time t at temperature T1 according to Step 3; substitute the data obtained in the above steps into Equation (1), and fit to obtain the diffusion coefficient D1 of the aluminized specimen with an initial coating thickness of H1 at T1 and η 1; and / or, fit to obtain the diffusion coefficient D2 of the aluminized specimen with an initial coating thickness of H2 at T1 and η 2; Step 5, total amount of Al in the coating S Satisfy: S = aH + b Formula (2) where a and b are constants related to the aluminized coating; Substitute H1, S01 and H2, S02 into Equation (2) to calculate the values of a and b; Step 6: Substitute D1 or D2 into Equation (1), where C takes the critical content that can form aluminum oxide, η Take the corresponding value of D1 or D2, x Take 0, where t is the target life of the coating, and calculate S0; Step 7: Substitute S0 into Equation (2) to calculate the initial thickness of the aluminized coating required for the coating to reach the target life at temperature T1 H0 ; Step 8. Prepare aluminized specimens with an initial coating thickness ≥ H0 .

2. The method for controlling the service life of the powder-pack aluminized coating according to claim 1, wherein The said Step 1 includes: S101: Degrease and remove rust from the specimens to be aluminized, and then bury the specimens to be aluminized in a powder can filled with the prepared aluminized powder and compact it; S102: Place the compacted powder can in a heat treatment furnace and sinter it in a protective gas atmosphere; S103: Cool it to room temperature in the furnace to obtain aluminized specimens.

3. The method for controlling the service life of the powder-pack aluminized coating according to claim 2, characterized in that, In S101, the said aluminized powder includes FeAl powder and a promoting agent.

4. The method for controlling the lifespan of the powder-pack aluminized coating according to claim 3, wherein The promoting agent is NH4Cl.

5. The method for controlling the service life of the pack aluminized coating according to claim 3, characterized in that, The mass ratio of iron to aluminum in the FeAl powder is 1:

1.

6. The method for controlling the life of the powder-pack aluminized coating according to claim 3, characterized in that, The mass ratio of the FeAl powder to the promoting agent is 99:

1.

7. The method for controlling the life of the powder-pack aluminized coating according to claim 2, wherein In S102, the said sintering conditions are sintering at 640 - 780 °C for 4 - 8 h.

8. The method for controlling the service life of the powder-pack aluminized coating according to claim 2, wherein The said protective gas is argon.

9. The method for controlling the life of the powder-pack aluminized coating according to claim 1, characterized in that, The conditions for the high-temperature steam oxidation experiment in the said Step 2 are: dynamic 100% saturated water vapor, pressure 0.1 - 30 MPa, and steam flow rate 100 - 120 ml / s.

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