Intergranular corrosion testing method suitable for high-chromium ultra-pure ferritic stainless steel

Abstract

The invention discloses a double-ring electrochemical potential reactivation (DL-EPR) intergranular corrosion testing method suitable for high-chromium ultra-pure ferritic stainless steel, and belongs to the technical field of corrosion. The DL-EPR intergranular corrosion testing method which can accurately reflect the sensitization degree of a material and ensure that testing data has good reliability and reproducibility and has good grain boundary corrosion selectivity is developed by preparing a testing solution and adjusting the potential scanning rate and the testing temperature. According to the method, rapid, nondestructive and quantitative evaluation on the intergranular corrosion sensitivity of the high-chromium ultra-pure ferritic stainless steel can be realized.

Description

Technical Field

[0001] This invention relates to the field of corrosion technology for high-chromium ultra-pure ferritic stainless steel, and specifically to a test method for intergranular corrosion of high-chromium ultra-pure ferritic stainless steel. Background Technology

[0002] High-chromium ultra-pure ferritic stainless steel was developed in the 1970s. Its chromium content is above 25%, molybdenum content is 1-4%, and (C+N) content is ≤300ppm. Some grades contain small amounts of nickel, and stabilizing elements such as Nb and Ti are added. High-chromium ultra-pure ferritic stainless steel achieves its superior corrosion resistance through high alloying, ultra-purity, and stabilization. It not only possesses excellent resistance to pitting corrosion, crevice corrosion, and stress corrosion cracking, but also exhibits high yield strength ratio, surface hardness, coefficient of thermal expansion, and thermal conductivity. It has been widely used in nuclear power plant cooling systems and seawater heat exchange pipelines.

[0003] However, a common problem with ferritic stainless steel during use is its susceptibility to intergranular corrosion under corrosive conditions. The bcc structure of ferritic stainless steel makes it more sensitive to the presence of interstitial elements than austenitic stainless steel. Furthermore, carbon and nitrogen, as interstitial atoms, have extremely low solubility in ferrite and diffuse much faster than in austenite. Chromium-rich precipitates are also more likely to form in ferritic stainless steel. Therefore, ferritic stainless steel is more susceptible to intergranular corrosion than austenitic stainless steel. Due to its rapid, non-destructive, and quantitative detection capabilities, DL-EPR is widely used to detect the intergranular corrosion susceptibility of stainless steel. For stainless steel materials with different compositions, the DL-EPR evaluation method largely depends on the test operating conditions, such as the nature and concentration of the electrolyte, the type of passivating agent, the potential scan rate, and the solution temperature. Therefore, developing a DL-EPR intergranular corrosion testing method suitable for high-chromium ultrapure ferritic stainless steel is of great significance for improving the performance, optimizing the process, and promoting the application of high-chromium ultrapure ferritic stainless steel.

[0004] The purpose of this invention is to develop a DL-EPR intergranular corrosion testing method suitable for high-chromium ultrapure ferritic stainless steel by optimizing the concentration of depassivating agent, potential scan rate, and solution temperature, so as to conduct rapid, sensitive, and accurate intergranular corrosion sensitivity testing. Summary of the Invention

[0005] The purpose of this invention is to address the above-mentioned problems by providing a method for testing intergranular corrosion of high-chromium ultrapure ferritic stainless steel.

[0006] The objective of this invention is achieved as follows: A method for testing intergranular corrosion of high-chromium ultrapure ferritic stainless steel, comprising the following steps: Step 1: Sample preparation: Mechanical polishing, the high-chromium ultrapure ferritic stainless steel sample is sequentially polished with SiC sandpaper of grades 180, 500, 800, and 1000. Each time the sandpaper is changed, all scratches from the previous grade of sandpaper must be removed; Step 2: DL-EPR experimental testing: The test system is a three-electrode system, the auxiliary electrode is a platinum electrode, the reference electrode is a saturated calomel electrode, and the high-chromium ultrapure ferritic stainless steel sample is used as the working electrode embedded in epoxy resin with a test area of ​​1 cm². 2 The test solution consisted of a 2 mol / L H₂SO₄ solution and a passivating agent; the maximum activation peak current density (I₀) during the forward scan was measured. a ) and the maximum reactivation peak current density during reverse scan (I r The ratio Ra = (I) r / I a The value of 100% was used as a quantitative parameter to evaluate the IGC susceptibility of high-chromium ultrapure ferritic stainless steel. During the test, the working electrode was immersed in the test solution. First, it was cathodically polarized for 300 seconds at -1000mV vs. SCE. Then, after the open circuit potential Eocp stabilized, it was subjected to a voltage of 0.05–0.3V / min. -1 The potential is scanned towards the anode at a rate of 0.05–0.3 V / min until the potential reaches +300 mV vs. SCE, and then the same scan rate is maintained at 0.05–0.3 V / min. -1 Reverse until Eocp; (1) Prepare the test solution, which consists of 2 mol / L H2SO4 and different concentrations of passivating agent HCl, namely 1, 2, 3 and 4 mol / L of passivating agent HCl. Perform the test according to the DL-EPR test procedure and determine the optimal concentration of passivating agent HCl as 3 mol / L; (2) At different scan rates, i.e. dE / dt = 0.05~0.3Vmin -1 The test was conducted according to the DL-EPR test procedure, and the optimal potential scan rate was determined to be 0.1 Vmin. -1 (3) Select different temperatures, namely 20, 25, 30, and 35℃, for testing. Follow the DL-EPR test procedure to determine the test temperature as 30℃ and the potential scan rate as 0.1Vmin. -1 Based on a comparison of the effects of passivating agent concentration, scan rate, and electrolyte temperature on DL-EPR test results, the suitable DL-EPR test conditions for high-chromium ultrapure ferritic stainless steel were determined as follows: electrolyte: 2 mol / L H₂SO₄ + 3 mol / L HCl; potential scan rate: 0.1 V / min. -1 The test temperature was 30℃.

[0007] The high-chromium ultrapure ferritic stainless steel described in this method contains the following chemical composition in mass percentage: C: ≤0.02%; Si: ≤1.00%; Mn: ≤1.00%; S: ≤0.03%; Cr: 25.0~30.0%; Mo: 1.0~4.0%; Ni: 0~3.0%; N: ≤0.04%; Ti+Nb: 0.20~1.0 and ≥6(C+N); balance Fe.

[0008] The beneficial effects of this invention are: the method of this invention provides a DL-EPR intergranular corrosion test method suitable for high-chromium ultra-pure ferritic stainless steel, so as to achieve a rapid, sensitive and accurate test method for the intergranular corrosion sensitivity of high-chromium ultra-pure ferritic stainless steel. Attached Figure Description

[0009] The present invention will now be further described with reference to the accompanying drawings.

[0010] Figure 1 This is a schematic diagram of the DL-EPR curve for high-chromium ultrapure ferritic stainless steel.

[0011] Figure 2 The DL-EPR curve of high-chromium ultrapure ferritic stainless steel S44660 in the sensitized state was obtained by using 2 mol / L H2SO4 + 3 mol / L HCl, a potential scan rate of 0.1 V min-1, and a test temperature of 30℃.

[0012] Figure 3 For Comparative Example 1, DL-EPR curves of high-chromium ultrapure ferritic stainless steel S44660 in a sensitized state were obtained by using 2 mol / L H2SO4 + 4 mol / L HCl, a potential scan rate of 0.1 V min-1, and a test temperature of 30℃.

[0013] Figure 4 The image shows the surface morphology of the sample after the DL-EPR test in Example 1(a) using a scanning electron microscope (SEM).

[0014] Figure 5 The image shows the surface morphology of the sample in Comparative Example 1(b) after DL-EPR testing, obtained using a scanning electron microscope (SEM). Detailed Implementation

[0015] This invention discloses a dual-ring electrochemical potentiodynamic reactivation (DL-EPR) intergranular corrosion testing method suitable for high-chromium ultrapure ferritic stainless steel, belonging to the field of corrosion technology. The DL-EPR intergranular corrosion testing method is characterized by its speed, non-destructive nature, and quantitative detection, and is widely used to detect the intergranular corrosion susceptibility of stainless steel. For stainless steel materials with different composition systems, the DL-EPR evaluation method largely depends on the test conditions, such as the nature and concentration of the electrolyte, the type of passivating agent, the potential scan rate, and the solution temperature. This invention develops a DL-EPR intergranular corrosion testing method suitable for high-chromium ultrapure ferritic stainless steel by setting the passivating agent concentration, potential scan rate, and electrolyte temperature as variables. This method exhibits good selectivity for grain boundary corrosion and avoids uniform corrosion; it also features extremely high testing sensitivity for samples with low sensitization levels (low Cr depletion in intergranular areas).

[0016] This invention designs an experimental scheme using the DL-EPR testing conditions—passivator concentration, potential scan rate, and electrolyte temperature—as variables to study the optimal DL-EPR testing conditions suitable for high-chromium ultra-pure ferritic stainless steel. By changing the DL-EPR experimental parameters and comparing the results with the microstructure after testing, the optimal DL-EPR testing conditions for high-chromium ultra-pure ferritic stainless steel are obtained.

[0017] In DL-EPR experiments, the purpose of adding the depassivating agent is to disrupt the original, intact passivation film on the material surface, causing the metal surface to transition from a passivated state to an activated state. This results in preferential corrosion of the depassivated areas (intergranular Cr-depleted regions) in the corrosive medium. Precise control of the HCl concentration is crucial for the accuracy of the experimental results. Insufficient HCl concentration leads to... r The R value is too low, resulting in a A low HCl value can lead to the misjudgment that high-chromium ultrapure ferritic stainless steel remains in an "unsensitized" state under different cooling rates. Conversely, an excessively high HCl concentration can cause R... a The abnormal increase in size and accelerated internal corrosion of grains led to an overestimation of the intergranular corrosion susceptibility of high-chromium ultrapure ferritic stainless steel.

[0018] In DL-EPR experiments, the potential scan rate, as a key kinetic parameter, affects the morphology of the potential-current density curve, activation peak parameters, and the assessment of intergranular corrosion susceptibility. If the scan rate is too low, the Re of the three samples will be significantly affected. a The values ​​will all increase significantly, which is due to I r The increase in the value is due to the corrosive medium having sufficient time to react with the high-chromium ultrapure ferritic stainless steel matrix, resulting not only in the dissolution of chromium-depleted intergranular regions but also in uniform corrosion within the grains. Conversely, under high scan rates, R... aThe values ​​did not show any sensitivity for different sensitized samples. This is because the electrochemical reaction kinetics are very slow compared to the potential scanning kinetics, resulting in incomplete dissolution of the passivation film on the Cr-depleted region, thus reducing the sensitivity of the DL-EPR test.

[0019] The effect of test temperature (i.e., electrolyte temperature) on the DL-EPR experiment is mainly reflected in the damage to the passivation film. According to the Arrhenius equation:

[0020]

[0021] Where A is the exponential factor, ΔG is the activation energy, and T is the absolute temperature. This formula reveals the relationship between the passivation film dissolution rate and the test temperature: the dissolution rate i(T) of the passivation film increases exponentially with increasing test temperature T. This formula indicates that the dissolution rate i(T) of the passivation film increases exponentially with increasing test temperature T. Increased temperature enhances the corrosiveness of the test solution, significantly increasing the reaction rate constant and accelerating the reaction process. Simultaneously, increased temperature also leads to increased activation and reactivation current densities, thereby inducing uniform corrosion.

[0022] The technical solution adopted in this invention is as follows: The high-chromium ultrapure ferritic stainless steel of this invention contains the following chemical composition, in mass percentage: C: ≤0.02%; Si: ≤1.00%; Mn: ≤1.00%; S: ≤0.03%; Cr: 25.0~30.0%; Mo: 1.0~4.0%; Ni: 0~3.0%; N: ≤0.04%; Ti+Nb: 0.20~1.0 and ≥6(C+N); the balance is Fe.

[0023] Determining the optimal DL-EPR experimental method includes the following steps:

[0024] Step 1: Sample preparation: Mechanical polishing. The high-chromium ultra-pure ferritic stainless steel sample is polished sequentially with 180-1000# SiC sandpaper. Each time the sandpaper is changed, all the scratches of the previous grade of sandpaper need to be polished away.

[0025] Step 2: DL-EPR Experimental Test: The DL-EPR test was conducted using a potentiostat workstation (CHI 660D electrochemical workstation). The test system was a three-electrode system, with a platinum electrode (Pt) as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. A high-chromium ultrapure ferritic stainless steel sample was embedded in epoxy resin as the working electrode (test area 1 cm²). 2 The test solution consists of a 2 mol / L H₂SO₄ solution and a passivating agent. Figure 1 This is a schematic diagram of the DL-EPR test procedure. The maximum activation peak current density (I) during the forward scan is shown. a) and the maximum reactivation peak current density during reverse scan (I r The ratio R) a =(I r / I a The value of 100% is used as a quantitative parameter to evaluate the IGC susceptibility of high-chromium ultrapure ferritic stainless steel. During testing, the working electrode is immersed in the test solution, first cathodically polarized for 300 seconds at -1000mV vs. SCE; then, at an open-circuit potential E... ocp After stabilization, scan the potential towards the anode at a certain rate until the potential reaches +300mV vs. SCE. Then reverse the scan rate at the same speed until E... ocp .

[0026] (1) Prepare the test solution. The test solution consists of 2 mol / L H2SO4 and different concentrations of passivating agent HCl (1, 2, 3 and 4 mol / L). The test is carried out according to the DL-EPR test procedure to determine the optimal concentration of passivating agent HCl as 3 mol / L.

[0027] (2) At different scan rates (dE / dt = 0.05–0.3 Vmin) -1 The test was conducted according to the DL-EPR test procedure, and the optimal potential scan rate was determined to be 0.1 Vmin. -1 .

[0028] (3) Tests were conducted at different temperatures (20, 25, 30, and 35°C). Following the DL-EPR test procedure, the optimal test temperature was determined to be 30°C, and the potential scan rate was 0.1 Vmin. -1 .

[0029] Based on a comparison of the effects of test conditions (passivating agent concentration, scan rate, and electrolyte temperature) on DL-EPR test results, the optimal DL-EPR test conditions suitable for high-chromium ultrapure ferritic stainless steel were determined: electrolyte of 2 mol / L H₂SO₄ + 3 mol / L HCl, and potential scan rate of 0.1 V / min. -1 The test temperature was 30℃.

[0030] The specific implementation of this method is described in detail below with reference to the embodiments, but the specific implementation of this invention is not limited to the following embodiments.

[0031] Example 1

[0032] Taking a 1mm solution-treated cold-rolled plate of sensitized S44660 high-chromium ultra-pure ferritic stainless steel as an example, the chemical composition is: C: 0.013%; Si: 0.15%; Mn: 0.10%; S: 0.001%; ​​Cr: 27.5%; Mo: 3.69%; N: 0.015%; Ni: 1.96%; Ti: 0.15%; Nb: 0.25%; with the balance being Fe. The specific implementation steps are as follows: Step 1: Sample preparation: Mechanical polishing. The high-chromium ultra-pure ferritic stainless steel sample is polished sequentially with 180-1000# SiC sandpaper. Each time the sandpaper is changed, all scratches from the previous grade of sandpaper must be removed.

[0033] Step 2: The DL-EPR test was conducted using a potentiostat workstation. The test system was a three-electrode system, with a platinum electrode (Pt) as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. A high-chromium ultrapure ferritic stainless steel sample was embedded in epoxy resin as the working electrode (test area 1 cm²). 2 During testing, the working electrode was immersed in the test solution (2 mol / L H₂SO₄ + 3 mol / L HCl) at a temperature of 30°C. First, cathodic polarization was applied for 300 seconds at -1000 mV vs. SCE; second, the electrode was applied at an open-circuit potential E. ocp After stabilization, use 0.1Vmin -1 The potential is scanned towards the anode at the same scan rate until the potential reaches +300mV vs. SCE. Then the scan rate is reversed until E... ocp The DL-EPR curve obtained from the test is as follows: Figure 2 Intergranular corrosion sensitivity calculation results R a =I r / I a =8.56%. Surface morphology photographs taken using SEM after the test are shown below. Figure 4 .

[0034] Comparative Example 1

[0035] Taking a 1mm solution-treated cold-rolled plate of sensitized S44660 high-chromium ultra-pure ferritic stainless steel as an example, the chemical composition is: C: 0.013%; Si: 0.15%; Mn: 0.10%; S: 0.001%; ​​Cr: 27.5%; Mo: 3.69%; N: 0.015%; Ni: 1.96%; Ti: 0.15%; Nb: 0.25%; with the balance being Fe. The specific implementation steps are as follows: Step 1: Sample preparation: Mechanical polishing. The high-chromium ultra-pure ferritic stainless steel sample is polished sequentially with 180-1000# SiC sandpaper. Each time the sandpaper is changed, all scratches from the previous grade of sandpaper must be removed.

[0036] Step 2: The DL-EPR test was conducted using a potentiostat workstation. The test system was a three-electrode system, with a platinum electrode (Pt) as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. A high-chromium ultrapure ferritic stainless steel sample was embedded in epoxy resin as the working electrode (test area 1 cm²). 2 The test solution consists of [components]. During the test, the working electrode is immersed in the test solution (2 mol / L H₂SO₄ + 4 mol / L HCl) at a temperature of 30°C. First, cathodic polarization is applied for 300 seconds at -1000 mV vs. SCE; second, [the electrode is then subjected to] open-circuit potential E [electrode]. ocp After stabilization, use 0.1Vmin -1 The potential is scanned towards the anode at the same scan rate until the potential reaches +300mV vs. SCE. Then the scan rate is reversed until E... ocp The DL-EPR curve obtained from the test is as follows: Figure 3 Intergranular corrosion sensitivity calculation results R a =I r / I a =17.56%. Surface morphology photographs taken using SEM after the test are shown below. Figure 5 .

[0037] Example 1 and Comparative Example 1 both used S44660 high-chromium ultrapure ferritic stainless steel in the same sensitized state and followed the same DL-EPR procedure, differing only in the concentration of the passivating agent. Comparisons are made using SEM morphology images after the tests (e.g., ...). Figure 4 It can be seen that the test parameters in Example 1 are reasonable, giving the test good selectivity for grain boundary corrosion and avoiding uniform corrosion (e.g.) Figure 4 In Comparative Example 1, the passivating agent concentration was too high, using 4 mol / L HCl, making the test solution too corrosive, which would cause I in the DL-EPR curve. r Value too large Figure 3 And accelerates severe internal corrosion of grains, such as Figure 5 This led to an overestimation of the intergranular corrosion susceptibility of S44660.

[0038] The above description is only a specific embodiment of the present invention, but the structural features protected by the present invention are not limited thereto. Any changes or modifications made by those skilled in the art within the scope of the present invention are covered by the patent scope of the present invention.

Claims

1. A method for testing intergranular corrosion of high-chromium ultrapure ferritic stainless steel, characterized in that: Includes the following steps: Step 1: Sample preparation: Mechanical polishing. The high-chromium ultrapure ferritic stainless steel sample is polished in sequence with SiC sandpaper of grades 180, 500, 800, and 1000. Each time the sandpaper is changed, all the scratches of the previous grade of sandpaper need to be polished away. Step Two: DL-EPR Experimental Test: The test system is a three-electrode system, with a platinum electrode as the auxiliary electrode, a saturated calomel electrode as the reference electrode, and a high-chromium ultrapure ferritic stainless steel sample embedded in epoxy resin as the working electrode, with a test area of ​​1 cm². 2 The test solution consisted of a 2 mol / L H₂SO₄ solution and a passivating agent; the maximum activation peak current density (I₀) during the forward scan was measured. a ) and the maximum reactivation peak current density during reverse scan (I r The ratio Ra = (I) r / I a The value of 100% is used as a quantitative parameter to evaluate the IGC susceptibility of high-chromium ultrapure ferritic stainless steel. During the test, the working electrode is immersed in the test solution. First, it is cathodically polarized for 300 seconds at -1000mV vs. SCE. Then, after the open circuit potential Eocp stabilizes, it is subjected to a voltage of 0.05~0.3V / min. −1 The potential is scanned towards the anode at a rate of 0.05~0.3V / min until the potential reaches +300mV vs. SCE, and then the same scan rate is maintained at 0.05~0.3V / min. −1 Reverse until Eocp; (1) Prepare the test solution, which consists of 2 mol / L H2SO4 and different concentrations of passivating agent HCl, namely 1, 2, 3 and 4 mol / L of passivating agent HCl. Perform the test according to the DL-EPR test procedure and determine the optimal concentration of passivating agent HCl as 3 mol / L; (2) At different scan rates, i.e. dE / dt = 0.05~0.3Vmin −1 The test was conducted according to the DL-EPR test procedure, and the optimal potential scan rate was determined to be 0.1 Vmin. −1 (3) Different temperatures, namely 20, 25, 30, and 35°C, were selected for testing. The DL-EPR test procedure was followed, and the test temperature was determined to be 30°C. Based on the comparison of the effects of the passivating agent concentration, scan rate, and electrolyte temperature on the DL-EPR test results, the suitable DL-EPR test conditions for high-chromium ultrapure ferritic stainless steel were determined: the electrolyte was 2 mol / L H2SO4 + 3 mol / L HCl, and the potential scan rate was 0.1 Vmin. -1 The test temperature was 30℃.

2. The intergranular corrosion test method for high-chromium ultrapure ferritic stainless steel according to claim 1, characterized in that: The high-chromium ultrapure ferritic stainless steel described in this method contains the following chemical composition in mass percentage: C: ≤0.02%; Si: ≤1.00%; Mn: ≤1.00%; S: ≤0.03%; Cr: 25.0~30.0%; Mo: 1.0~4.0%; Ni: 0~3.0%; N: ≤0.04%; Ti+Nb: 0.20~1.0 and ≥6 (C+N); the balance is Fe.

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