A method for predicting the life of aluminized coating

Abstract

A method for predicting the life of an aluminized coating belongs to the technical field of life prediction and overcomes the defect that the prior art cannot predict the life of an aluminized coating. The method for predicting the life of an aluminized coating of the present invention comprises the following steps: step 1, obtaining an aluminized specimen; step 2, taking two groups of aluminized specimens and conducting high-temperature steam oxidation experiments at temperatures T1 and T2, respectively; step 3, obtaining the mass percentage of aluminum at different x points in the coating when the aluminized specimen is oxidized at temperature T1 and time t1 according to step 2; the mass percentage of aluminum at different x points in the coating when the aluminized specimen is oxidized at temperature T2 and time t2; fitting to obtain diffusion coefficients D1, D2, S1, S2 and η1, η2 at T1 and T2; step 4, obtaining constants D0, Q0; step 5, obtaining the diffusion coefficient D at any temperature T3; step 6, substituting D3 into formula (1) to calculate the time corresponding to T3, which is the predicted life of the aluminized specimen coating at temperature T3.

Description

Technical Field

[0001] The invention belongs to the technical field of life prediction, and in particular relates to a method for predicting the life of an aluminized coating. Background Art

[0002] The boiler heating surface (water-cooled wall, superheater, reheater and economizer, also known as the "four tubes" of the boiler) is the key component in the boiler responsible for recovering the energy of coal-fired flue gas, heating steam, and realizing energy conversion. Among the failure causes of the "four tubes" of high-parameter power station boilers, the flue gas corrosion caused by coal powder combustion on the outside of the furnace tube and the high-temperature steam oxidation corrosion inside the furnace tube play an important role, and are also one of the essential causes of boiler explosion accidents. The high steam parameters and high efficiency development of thermal power plant units have made the service conditions of boiler tubes more complex and demanding, and the performance of the materials used under higher requirements also needs to be further improved.

[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. It can improve the high-temperature oxidation corrosion performance of high-temperature alloys. Compared with the development of higher-grade heating surface materials, surface modification technology can significantly improve the oxidation resistance and corrosion performance of boiler tubes without reducing the mechanical properties of the alloy matrix. It not only solves the oxidation / corrosion problems of existing boiler tube alloys, but also provides 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. It has been widely used in the aerospace field. The key components of gas turbines have excellent oxidation resistance after using aluminum coating, and can run for thousands of hours above 900°C. The excellent protection of aluminide coating comes from the slow growth of the oxide film, and the growth of alumina depends on the temperature. When the temperature is reduced by 200°C, the growth of alumina can be reduced by several orders of magnitude. Therefore, the application of aluminide coating to power station boiler tubes with lower temperatures (<700°C) will greatly extend its service life. Low-temperature aluminizing has a lower diffusion rate, can obtain a thinner coating, and there is no brittle phase precipitation. Therefore, the steam oxidation performance can be significantly changed without changing the organization and mechanical properties of the substrate material.

[0005] The P92-Al coating has good anti-oxidation properties. This is mainly because, in the initial stage of oxidation, water vapor selectively oxidizes the Al element in the coating to form a single-molecule oxide film, followed by electrochemical reaction to achieve film growth, and the growth rate is slow, and the Al element is not completely oxidized to Al2O3. On the one hand, the formed Al2O3 film can reduce the diffusion coefficient; on the other hand, the aluminum-rich layer that is not completely oxidized can also provide sufficient Al sources for the alloy to continue to form Al2O3 film, making the P92-Al coating have excellent anti-steam oxidation properties.

[0006] The failure mechanism of aluminized coating under high temperature conditions mainly includes the following: a. The oxide film peels off, and the Al element diffuses outward to form a new Al2O3 film; b. Mutual diffusion with the substrate. When the P92-Al coating is steam oxidized at 650°C, a protective Al2O3 film is quickly formed on the surface. Although the temperature is low, the Al2O3 film still has good adhesion and no peeling occurs during the test. This makes the degradation mechanism of the aluminide coating mainly manifested as the internal diffusion of Al. However, the existing technology cannot accurately predict the coating life of aluminized coatings. Summary of the invention

[0007] Therefore, the technical problem to be solved by the present invention is to overcome the defect that the prior art cannot predict the life of an aluminized coating, thereby providing a method for predicting the life of an aluminized coating.

[0008] To this end, the present invention provides the following technical solutions.

[0009] The present invention provides a method for predicting the life of an aluminized coating, comprising the following steps:

[0010] Step 1, obtaining an aluminized specimen;

[0011] Step 2, take two groups of aluminized specimens and conduct high temperature steam oxidation experiments at temperatures T1 and T2, respectively, where T2 is the highest temperature used for the aluminized specimens, T1 = T2-(20-80°C);

[0012] Step 3: The relationship between aluminum concentration in the coating and depth and oxidation time is in accordance with:

[0013]

[0014] Where 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;

[0015] According to step 2, the mass percentage of aluminum at different x positions in the coating of the aluminized specimen at temperature T1 and oxidation time t1 is obtained; the mass percentage of aluminum at different x positions in the coating at temperature T2 and oxidation time t2 is obtained; the data obtained in step 2 is substituted into formula (1), and the diffusion coefficients D1, D2, S1, S2, η1, η2 at T1 and T2 are obtained by fitting;

[0016] Step 4: Get constants Diffusion activation energy

[0017] Step 5, obtain the diffusion coefficient D3 at any temperature T3 according to lnD3=lnD0-Q0 / (RT3); R is a thermodynamic constant;

[0018] Step 6, substitute D3 into formula (1), where C is the critical content that can form aluminum oxide, S and η are the values ​​corresponding to T1 or T2 that are not less than T3, x is 0, and the time corresponding to T3 is calculated, which is the predicted life of the aluminized specimen coating at temperature T3.

[0019] Furthermore, the step 1 comprises:

[0020] S101, pre-treating the specimen to be aluminized, and then burying the specimen to be aluminized in a powder tank prepared with aluminizing powder, and compacting it;

[0021] S102, placing the compacted powder can in a heat treatment furnace and sintering it under a protective gas atmosphere;

[0022] S103, cooling to room temperature in the furnace to obtain an aluminized specimen.

[0023] Furthermore, in S101, the pretreatment includes: degreasing and derusting the aluminized specimen, and then placing it in a resistance furnace at 100-200°C for preheating for 20-60 minutes.

[0024] Furthermore, the aluminized specimens are degreased and derusted by pickling liquid, which is a mixture of sulfuric acid, hydrochloric acid and an emulsifier, and the temperature of the pickling liquid is 80-95°C.

[0025] Furthermore, in S101, the aluminized powder includes 98-99 wt.% FeAl powder and 1-2 wt.% NH4Cl.

[0026] Furthermore, S102 is to place the compacted powder can in a heat treatment furnace, close the valve, introduce protective gas for 30-60 minutes, exhaust the air, and heat up to the sintering temperature for sintering in the protective gas atmosphere.

[0027] Furthermore, the sintering condition is sintering at 750-780° C. for 4-8 hours.

[0028] Furthermore, the protective gas is argon.

[0029] Furthermore, the high temperature steam oxidation experimental conditions in step 2 are: dynamic 100% saturated water vapor, pressure 0.1-30 MPa, steam flow rate 100-120 ml / s.

[0030] Furthermore, the diffusion depth of the coating was analyzed by scanning electron microscopy on the aluminized specimens;

[0031] Energy spectrometer was used to analyze the composition of the coating at different depths.

[0032] Exemplarily, the emulsifier is an OP emulsifier, such as OP-3-50 series.

[0033] For example, in S101, the aluminized powder ratio is: 99wt.% FeAl powder, 1wt.% NH4Cl. The mass ratio of iron to aluminum in the FeAl powder is 1:1.

[0034] The original thickness of the aluminized coating and the diffusion depth of the aluminized coating at different oxidation times can be analyzed by a scanning electron microscope (SEM), and the composition of the aluminized coating at different depths can be analyzed by an energy dispersant.

[0035] The technical solution of the present invention has the following advantages:

[0036] The design life of boiler pipes can generally reach 100kh. Whether the aluminized coating can maintain the growth of Al2O3 oxide film and provide good protection for the base steel is an important issue in the application of the coating. The coating life prediction method provided by the present invention can predict whether the aluminized test piece meets the use requirements, and avoid the entire system from being unable to operate normally due to insufficient service life of the pipe system after use. At the same time, the present invention can also predict the concentration change of Al in the coating after different oxidation times, thereby improving the aluminizing process, avoiding production waste, reducing production costs, and improving the production efficiency and economic benefits of the enterprise. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0038] Figure 1 This is a scanning electron microscope image of the aluminized specimen prepared in step 1 of Example 1. DETAILED DESCRIPTION

[0039] The following examples are provided for a better understanding of the present invention, but are not intended to limit the best mode of implementation, nor to limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the guidance of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

[0040] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be purchased commercially.

[0041] Example 1

[0042] This embodiment provides a method for predicting the life of an aluminized coating, comprising the following steps:

[0043] Step 1: Obtain aluminized specimens:

[0044] S101. The aluminized specimen (T92 steel) was degreased and rusted using a pickling solution, which included sulfuric acid (98% by mass), hydrochloric acid (38% by mass) and OP emulsifier (model OP-10), with the mass fractions being 15%, 10% and 1% respectively, and the balance being water, and the temperature of the pickling solution was 90°C. The aluminized specimen was then placed in a resistance furnace at 200°C for preheating for 20 minutes.

[0045] The aluminized powder was embedded in the prepared material according to 99wt.% FeAl powder (iron:aluminum mass ratio 1:1) + 1wt.% NH4Cl, and the pretreated specimens were layered into a stainless steel tank and compacted.

[0046] S102. Place the embedded aluminized specimen in a heat treatment furnace, close the valve, introduce argon for 30 minutes to exhaust the air, and heat to 750°C in an argon atmosphere for sintering for 4 hours.

[0047] S103, cool down to room temperature in the furnace, take out the test piece and make the aluminized test piece. Figure 1 shown.

[0048] Step 2: Take two groups of aluminized specimens, each group contains 10 aluminized specimens, and carry out high-temperature steam oxidation experiments on the two groups of aluminized specimens at 650°C and 700°C, respectively. 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 of the steam side of the boiler tube in a power plant.

[0049] The mass percentage values ​​of aluminum at different depths of the aluminized specimen after oxidation at 700°C for 1000 hours are obtained; and the mass percentage values ​​of aluminum at different depths of the aluminized specimen after oxidation at 650°C for 1000 hours are obtained.

[0050] Step 3: The relationship between aluminum concentration in the coating and depth and oxidation time is in accordance with:

[0051]

[0052] Where 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] Substituting the data obtained in step 2 into equation (1), the diffusion coefficients at T1 = 650 °C and T2 = 700 °C are fitted as D1: 0.06332, D2: 0.33724, S1: 897.09465, S2: 698.54861, and η1: 7.84419, η2: 19.53143;

[0054] Step 4: Get Diffusion activation energy When calculating, T1 and T2 use the open-type temperatures of 923.15K and 973.15K.

[0055] Step 5. This embodiment predicts the coating life at 600°C, that is, T3 is 600°C. According to lnD3=lnD0-Q0 / (RT3), the diffusion coefficient D3 at any temperature is 0.009816334; R is a thermodynamic constant;

[0056] Step 6, substitute D3 into formula (1), where C takes the critical content of 3% that can form aluminum oxide, S and η take the values ​​corresponding to T1, x is 0, and the oxidation time corresponding to 600°C is calculated to be 1809000h, that is, the service life of the aluminized specimen coating under 600°C steam oxidation conditions predicted in this embodiment is 1809000h.

[0057] Example 2

[0058] This embodiment provides a method for predicting the life of an aluminized coating. Steps 1 to 4 are substantially the same as those of Embodiment 1, except that this embodiment predicts the life at 622°C.

[0059] Step 5, according to lnD3=lnD0-Q0 / (RT3), the diffusion coefficient D3 at any temperature is 0.022872411; R is a thermodynamic constant; in this embodiment, T3 is 622°C.

[0060] Step 6, substitute D3 into formula (1), where C takes the critical content of 3% that can form aluminum oxide, S and η take the values ​​corresponding to T1, x is 0, and the oxidation time corresponding to 622°C is calculated to be 776500h, that is, the service life of the aluminized specimen coating under 622°C steam oxidation conditions predicted in this embodiment is 776500h.

[0061] Example 3

[0062] This embodiment provides a method for predicting the life of an aluminized coating. Steps 1 to 4 are substantially the same as those of embodiments 1 and 2, except that this embodiment predicts the life at 650°C.

[0063] Step 5: According to lnD3=lnD0-Q0 / (RT3), the diffusion coefficient D3 at any temperature is 0.06332; R is a thermodynamic constant; in this embodiment, T3 is 650°C.

[0064] Step 6, substitute D3 into formula (1), where C takes the critical content of 3% that can form aluminum oxide, S and η take the values ​​corresponding to T1, x is 0, and the oxidation time corresponding to 650°C is calculated to be 280,000 h, that is, the service life of the aluminized specimen coating under steam oxidation conditions at 650°C predicted in this embodiment is 280,000 h.

[0065] Example 4

[0066] This embodiment provides a method for predicting the life of an aluminized coating. Steps 1 to 4 are substantially the same as those of Embodiment 1, except that this embodiment predicts the life at 700°C.

[0067] Step 5: According to lnD3=lnD0-Q0 / (RT3), the diffusion coefficient D3 at any temperature is 0.33724; R is a thermodynamic constant; in this embodiment, T3 is 700°C.

[0068] Step 6, substitute D3 into formula (1), where C takes the critical content of 3% that can form aluminum oxide, S and η take the values ​​corresponding to T2, x is 0, and the calculated oxidation time corresponding to 700°C is 52000h, that is, the service life of the aluminized specimen coating under steam oxidation conditions at 700°C predicted in this embodiment is 52000h.

[0069] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the scope of protection of the invention.

Claims

1. A method for predicting the life of an aluminized coating, characterized in that: The following steps are involved: Step 1, obtaining an aluminized specimen; Step 2, take two groups of aluminized specimens and conduct high temperature steam oxidation experiments at temperatures T1 and T2, respectively, where T2 is the highest temperature used for the aluminized specimens, T1 = T2-(20-80°C); Step 3: The coating satisfies the following relationship: Where 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; According to step 2, the mass percentage of aluminum at different x positions in the coating of the aluminized specimen at temperature T1 and oxidation time t1 is obtained; the mass percentage of aluminum at different x positions in the coating at temperature T2 and oxidation time t2 is obtained; the data obtained in step 2 is substituted into formula (1), and the diffusion coefficients D1, D2, S1, S2, η1, η2 at T1 and T2 are obtained by fitting; Step 4: Get constants Diffusion activation energy Step 5, obtain the diffusion coefficient D3 at any temperature T3 according to lnD3=lnD0-Q0 / (RT3); R is a thermodynamic constant; Step 6, substitute D3 into formula (1), where C is the critical content that can form aluminum oxide, S and η are the values ​​corresponding to T1 or T2 that are not less than T3, x is 0, and the time corresponding to T3 is calculated, which is the predicted life of the aluminized specimen coating at temperature T3.

2. The method for predicting the life of an aluminized coating according to claim 1, characterized in that: The step 1 comprises: S101, pre-treating the specimen to be aluminized, and then burying the specimen to be aluminized in a powder tank prepared with aluminizing powder, and compacting it; S102, placing the compacted powder can in a heat treatment furnace and sintering it under a protective gas atmosphere; S103, cooling to room temperature in the furnace to obtain an aluminized specimen.

3. The method for predicting the life of an aluminized coating according to claim 2, characterized in that: In S101, the pretreatment includes: degreasing and derusting the aluminized specimen, and then placing it in a resistance furnace at 100-200°C for preheating for 20-60 minutes.

4. The method for predicting the life of an aluminized coating according to claim 3, characterized in that: The aluminized specimens are degreased and derusted by pickling liquid, which is a mixture of sulfuric acid, hydrochloric acid and an emulsifier. The temperature of the pickling liquid is 80-95°C.

5. The method for predicting the life of aluminized coating according to claim 2, characterized in that: In S101, the aluminized powder includes 98-99 wt.% FeAl powder and 1-2 wt.% NH4Cl.

6. The method for predicting the life of aluminized coating according to claim 2, characterized in that: S102 is to place the compacted powder can in a heat treatment furnace, close the valve, introduce protective gas for 30-60 minutes, exhaust the air, and heat up to the sintering temperature for sintering in the protective gas atmosphere.

7. The method for predicting the life of an aluminized coating according to claim 6, characterized in that: The sintering condition is sintering at 750-780° C. for 4-8 hours.

8. The method for predicting the life of an aluminized coating according to claim 6, characterized in that: The protective gas is argon.

9. The method for predicting the life of an aluminized coating according to claim 1, characterized in that: The high temperature steam oxidation experimental conditions in step 2 are: dynamic 100% saturated water vapor, pressure 0.1-30 MPa, steam flow rate 100-120 ml / s.

10. The method for predicting the life of an aluminized coating according to claim 1, characterized in that: The diffusion depth of the coating was analyzed by scanning electron microscopy on the aluminized specimens; Energy spectrometer was used to analyze the composition of the coating at different depths.

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