A gradient detection method for uniform corrosion performance of ferritic stainless steel

Abstract

The application discloses a gradient detection method for uniform corrosion performance of ferritic stainless steel and belongs to the field of stainless steel corrosion performance detection. The method provides a gradient detection method for the problems that the standard method of GB4334.6-2015 cannot measure the uniform corrosion rate of 409, 430 and other ferritic stainless steel and the test time of the prior art is too long. The method comprises sample preparation, preparation of a basic corrosion medium, selection and adjustment of the gradient of the corrosion medium, corrosion test, treatment of the sample after corrosion and calculation of the uniform corrosion rate. By controlling the hydrogen ion concentration [H + ] to be 0.5-2.0 mol / L, the chloride ion concentration [Cl ‑ ] to be 0.5-2.0 mol / L and the molar ratio of [H + ] / [Cl ‑ ] to be 0.3-0.9, the uniform corrosion is ensured; a three-level gradient detection system of 'basic medium-concentration optimization-corrosion inhibitor assistance' is established, and universal detection of ferritic stainless steel with different corrosion resistance is realized. The test time of the method is only 0.5-1 h, which is 6-12 times more efficient than the prior art of 6 h, the technical blank of the uniform corrosion detection of ferritic stainless steel is filled, and the method has good reproducibility and stability.

Description

Technical Field

[0001] This invention relates to the field of stainless steel corrosion performance testing technology, specifically to a gradient testing method for the uniform corrosion performance of ferritic stainless steel. Background Technology

[0002] Uniform corrosion of stainless steel refers to the phenomenon where corrosion occurs uniformly across the entire surface of the stainless steel in contact with a corrosive medium, resulting in thinning. Resistance to uniform corrosion is one of the important performance indicators of stainless steel's corrosion resistance. Currently, the standard method for testing the uniform corrosion performance of stainless steel is GB4334.6-2015, "Test Method for 5% Sulfuric Acid Corrosion of Stainless Steel," which is applicable to the evaluation of the uniform corrosion performance of most austenitic stainless steels.

[0003] However, in practical applications, the corrosion resistance of stainless steel varies significantly among different steel grades. Ferritic stainless steels such as 409 (10.5-11.7% Cr) and 430 (16-18% Cr), due to their lower alloy element content and the absence or presence of Ni, exhibit relatively poor resistance to uniform corrosion. In actual testing, it was found that when testing ferritic stainless steels such as 409 and 430 using the GB4334.6-2015 standard method, the samples reacted violently in a boiling 5% sulfuric acid corrosion solution, dissolving completely within a short time (approximately 3-10 minutes), making it impossible to determine their uniform corrosion rate. While JB / T7901-2001, "Laboratory Method for Uniform Corrosion Full Immersion Test of Metallic Materials," can be used to evaluate the uniform corrosion performance of metallic materials in full immersion tests, it does not provide specific parameters such as the test solution and testing time for stainless steel. Therefore, it is difficult to evaluate the uniform corrosion resistance of stainless steel using this standard method. This technological gap leaves a lack of effective testing methods for evaluating the uniform corrosion resistance of this type of stainless steel, hindering its performance assessment and quality control in practical engineering applications.

[0004] In the existing technology, although there are some detection methods for the corrosion performance of stainless steel, such as corrosion testing using sodium chloride or ferric chloride solutions, or rapid detection using electrochemical methods, these methods are mainly for pitting corrosion evaluation or austenitic stainless steel, and have failed to solve the special problems of ferritic stainless steel in uniform corrosion testing.

[0005] Therefore, developing a rapid detection method for the uniform corrosion performance of ferritic stainless steel with poor corrosion resistance is of great practical significance and application value.

[0006] This invention aims to solve the technical problem that the existing GB4334.6-2015 standard method for uniform corrosion testing of ferritic stainless steels such as 409 and 430 results in violent reaction and complete dissolution of the samples, making it impossible to determine the corrosion rate. At the same time, it overcomes the shortcomings of the existing technology, such as excessively long testing time and lack of optimization for ferritic stainless steel, and provides a rapid detection method for the uniform corrosion performance gradient of this type of stainless steel. Summary of the Invention

[0007] The purpose of this invention is to address the above-mentioned problems by providing a gradient detection method for the uniform corrosion performance of ferritic stainless steel.

[0008] The objective of this invention is achieved as follows: a gradient detection method for the uniform corrosion performance of ferritic stainless steel, comprising the following steps: Step 1: Sample preparation and pretreatment: Select the ferritic stainless steel to be tested, process it into a sample with a regular geometric shape, and sequentially polish the sample surface with 180-grit, 600-grit, 1000-grit, and 2000-grit sandpaper to remove the surface oxide layer and processing marks, so that the sample surface exhibits a uniform metallic luster. Measure and record the geometric dimensions of the sample, and calculate the initial surface area S of the sample, in m². 2 After polishing, the sample is degreased, rinsed with deionized water, and ultrasonically cleaned with anhydrous ethanol for 5-10 minutes at an ultrasonic frequency of 25kHz~40kHz. Then, it is dried with cold air and placed in a desiccator until constant weight is achieved, which takes at least 30 minutes. The initial mass W0 of the sample is then weighed using an analytical balance with an accuracy of 0.1mg (in grams). Step Two: Preparation of the basic corrosive medium: Prepare the basic corrosive medium, which is a mixed aqueous solution of sodium chloride and hydrochloric acid, wherein the hydrogen ion concentration [H+] is... + The concentration of chloride ions [Cl] is 0.5-2.0 mol / L. - The concentration is 0.5-2.0 mol / L, and the molar ratio of hydrogen ions to chloride ions [H] is... + ] / [Cl - The corrosion medium is controlled within the range of 0.3 to 0.9. The basic corrosion medium is prepared using deionized water. During preparation, first weigh solid sodium chloride and dissolve it in deionized water, then add concentrated hydrochloric acid to the sodium chloride solution, stir evenly, and bring the volume to the predetermined level. Step 3: Gradient selection and adjustment of the corrosion medium: Based on the alloy composition and predicted corrosion resistance of the ferritic stainless steel to be tested, the corrosion medium is selected from the following gradients: First gradient: For most ferritic stainless steels, the basic corrosion medium prepared in step 2 is directly used for testing; Second gradient: For stainless steels where the corrosion rate exceeds the preset detectable window of 0.5-50 g / (m²) under the first gradient condition... 2 Steel grades within the range of ·h) while maintaining [H + ] / [Cl -With the molar ratio within the range of 0.3 to 0.9, adjust the total concentration of the base corrosive medium to bring the corrosion rate into the detectable window, i.e., 0.5-50 g / (m³). 2 •h); Third gradient: For extremely non-corrosion-resistant steel grades or samples under special working conditions where the corrosion rate still exceeds the upper limit of the detectable window under the second gradient conditions, a corrosion inhibitor is added to the basic corrosive medium as an auxiliary regulator. The corrosion inhibitor is selected from one or more of molybdates, benzotriazole compounds, and gluconates. By adjusting the type and concentration of the corrosion inhibitor, the corrosion rate is finely controlled to 0.5-50 g / (m). 2 •h) within the detectable range; Step 4: Corrosion test: Place the ferritic stainless steel corrosion test sample prepared in Step 1 into the corrosion test solution determined in Step 3 at a constant temperature of 30-50℃, and conduct the corrosion test. The corrosion test time is 0.5-1 hour. The sample should be completely immersed below the liquid surface and should not be in contact with the container wall or bottom. The samples should not be in contact with each other; Step 5: Sample treatment after corrosion: After the predetermined corrosion test time is reached, take out the ferritic stainless steel corrosion test sample, brush off the corrosion products adhering to the sample surface, and use deionized water to remove the corrosion products. Rinse thoroughly with water, place the sample in a beaker containing anhydrous ethanol, and ultrasonically clean for 5-10 minutes at a frequency of 25kHz-40kHz. Remove the sample and dry it with cold air, then place it in a desiccator for at least 30 minutes until constant weight is achieved. Weigh the sample after corrosion using an analytical balance with an accuracy of 0.1mg, and the mass W1 is in g. Step Six: Calculation of Uniform Corrosion Rate: Calculate the uniform corrosion weight loss rate V of ferritic stainless steel according to the following formula: V=(W0-W1) / (S×t), where: V—uniform corrosion rate, in g / (m 2 •h); W0—Initial mass of the sample, in g; W1—Mass of the sample after corrosion, in g; S—Initial surface area of ​​the sample, in m² 2 t—corrosion test time, in hours.

[0009] In step one, the ferritic stainless steel to be tested is selected. Ferritic stainless steel is a type of stainless steel for which the uniform corrosion rate cannot be determined under the 5% sulfuric acid corrosion test conditions specified in GB4334.6-2015 standard, or which requires rapid testing.

[0010] The beneficial effects of this invention are: 1. Filling a technical gap: This invention is the first to establish a dedicated rapid detection method for uniform corrosion performance of ferritic stainless steels such as 409 and 430 that cannot be tested by the standard method of GB4334.6-2015, thus filling the technical gap in the evaluation of uniform corrosion of this type of stainless steel material.

[0011] 2. Significantly improved testing efficiency: The method of this invention shortens the corrosion test time to 0.5-1 hour. Compared with the GB4334.6-2015 standard method, which cannot be tested and the standard test time of 6 hours, the testing efficiency is significantly improved, which can meet the needs of rapid detection in industrial production.

[0012] 3. Dual-parameter control ensures corrosion uniformity: through precise control of [H] + ] / [Cl - A molar ratio of 0.3-0.9 ensures uniform corrosion on the sample surface rather than pitting or localized over-corrosion, and the test results can truly reflect the uniform corrosion performance of the material.

[0013] 4. Gradient system achieves wide applicability: The three-level gradient detection system of "basic medium-concentration optimization-corrosion inhibitor assistance" established in this invention can be applied to ferritic stainless steels with different corrosion resistance from 409 to 446, and has wide applicability.

[0014] 5. The test results are accurate and reliable: The results of repeated experiments show that the relative standard deviation of the method of the present invention is less than 1%, and it has good reproducibility and stability.

[0015] 6. Simple operation and low cost: The method of this invention does not require special equipment, can be implemented under normal laboratory conditions, the reagents are readily available, the operation is simple, and it is easy to promote and apply. Detailed Implementation

[0016] This invention addresses the technical gap in the current national standard GB / T 4334.6-2015 "Test Method for Corrosion of Stainless Steel with 5% Sulfuric Acid" regarding the detection of uniform corrosion performance of ferritic stainless steel. Ferritic stainless steels such as 409 and 430 react violently in the boiling 5% sulfuric acid solution specified in the standard and dissolve completely in a short time, making it impossible to determine their uniform corrosion rate. It also addresses the problem of excessively long testing time in existing technologies by providing a gradient detection method.

[0017] The core innovations of this invention include: revealing for the first time the hydrogen ion [H + ] with chloride ions [Cl - The synergistic effect of [H] in the corrosion of ferritic stainless steel was determined through numerous experiments, and the molar ratio of the two was determined [H]. + ] / [Cl - To achieve controllable and uniform corrosion, the corrosion rate must be controlled within the range of 0.3 to 0.9, transforming the corrosion medium from a "fixed formula" to an "adjustable system." A pioneering three-level gradient detection system of "basic medium - concentration optimization - corrosion inhibitor assistance" enables universal detection of steels with different corrosion resistance properties, ranging from 409 to 446. The system also introduces for the first time a "detectable window" for corrosion rate (0.5-50 g / (m³)). 2The concept of ·h) is used as the design goal of the detection method.

[0018] This invention successfully solves the technical challenge of determining the uniform corrosion rate of ferritic stainless steel, shortening the testing time to 0.5-1 hour, improving efficiency by 6-12 times compared to existing methods. Verified results show a relative standard deviation of <1%, demonstrating good reproducibility and stability. This method is simple to operate and low in cost, and can be widely applied in steel enterprises and testing institutions for material screening, quality control, and service performance evaluation, possessing significant value for widespread application.

[0019] This invention provides the following technical solution: a gradient detection method for the uniform corrosion performance of ferritic stainless steel, comprising the following steps: Step 1, sample preparation and pretreatment: select the ferritic stainless steel corrosion test sample to be tested, grind and clean the sample surface, measure the sample size and calculate the initial surface area S, and weigh the initial mass W0 of the sample: the ferritic stainless steel is a variety that cannot be tested for uniform corrosion rate or requires rapid testing under the 5% sulfuric acid corrosion test conditions specified in GB4334.6-2015 standard, including 409 stainless steel, 430 stainless steel, etc.

[0020] The stainless steel sample to be tested is processed into a regular geometric shape (the geometric shape is not limited, as long as it is convenient for calculating the area; the commonly used sample is rectangular, 30-50mm long, 20-30mm wide, and 1-3mm thick). The sample surface is then successively polished with 180-grit, 600-grit, 1000-grit, and 2000-grit sandpaper to remove the surface oxide layer and processing marks, so that the sample surface has a uniform metallic luster. The geometric dimensions of the sample (30-50mm long, 20-30mm wide, and 1-3mm thick) are measured and recorded, and the initial surface area S of the sample is calculated in m². 2 .

[0021] After polishing, the sample is degreased, rinsed with deionized water, and ultrasonically cleaned with anhydrous ethanol for 5-10 minutes at an ultrasonic frequency of 25kHz~40kHz. Then, it is dried with cold air. The cleaned and dried sample is placed in a desiccator until it reaches constant weight (not less than 30 minutes). The initial mass W0 of the sample is weighed using an analytical balance with an accuracy of 0.1mg. The unit is g.

[0022] Step 2: Preparation of the basic corrosive medium: Prepare the basic corrosive medium, which is a mixed aqueous solution of sodium chloride and hydrochloric acid, wherein the hydrogen ion concentration [H+] is... + The concentration of chloride ions [Cl] is 0.5-2.0 mol / L. - The concentration is 0.5-2.0 mol / L, and the molar ratio of hydrogen ions to chloride ions [H] is... + ] / [Cl -The concentration should be controlled within the range of 0.3 to 0.9. The basic corrosive medium is prepared using deionized water. During preparation, first weigh solid sodium chloride and dissolve it in deionized water, then add concentrated hydrochloric acid to the sodium chloride solution. After stirring evenly, bring the volume to a predetermined level (the volume is determined based on the actual sample surface area; generally, the immersion solution volume should not be less than 20 ml / cm²). 2 If the actual sample surface area is 20 cm² 2 (Then the volume of the immersion solution should not be less than 400 ml). Background techniques use a single sulfuric acid medium and do not involve dual-parameter synergy. This feature, for the first time, uses the "ratio relationship" of the corrosive medium as a controllable parameter in the detection method and determines the critical range to ensure uniform corrosion.

[0023] Step 3: Gradient Selection and Adjustment of Corrosion Media: Construct a three-level gradient detection system of "basic media - concentration optimization - corrosion inhibitor assistance". Based on the alloy composition and predicted corrosion resistance of the ferritic stainless steel to be tested, select the corrosion media from the following gradients: First gradient: For most ferritic stainless steels, directly use the basic corrosion media prepared in Step 2 for detection; Second gradient: For stainless steels where the corrosion rate exceeds the preset detectable window (0.5-50 g / (m²)) under the first gradient conditions. 2 Steel grades within the range of ·h)) while maintaining [H + ] / [Cl - Provided the molar ratio is within the range of 0.3 to 0.9, adjust the total concentration of the base corrosive medium (adjusting the total concentration of the corrosive medium is to make its corrosion rate fall within 0.5-50 g / (m³)). 2 • h) range, thus facilitating detection), allowing the corrosion rate to enter the detectable window; Third gradient: For extremely non-corrosion-resistant steel grades or special working condition samples where the corrosion rate still exceeds the upper limit of the detectable window under the second gradient conditions, a corrosion inhibitor is added to the basic corrosive medium as an auxiliary regulator. The corrosion inhibitor is selected from one or more of molybdates, benzotriazole compounds, and gluconates. By adjusting the type and concentration of the corrosion inhibitor, the corrosion rate is finely controlled to 0.5-50 g / (m 2 Within the detectable range of h). Applicable to various ferritic stainless steels such as 409, 430, 439, 425, and 446. Background technologies use single test conditions, which cannot cover steel grades with different corrosion resistance properties. This feature upgrades the detection method from a "single condition" to a "systematic solution".

[0024] This invention specifically regulates the corrosion rate to 0.5-50 g / (m²). 2 The detection target is defined within the "detectable window" of h). Previous techniques lacked the concept of a "window," making it ineffective in handling situations with excessively fast or slow corrosion rates. This feature establishes clear technical objectives and control criteria for the detection method.

[0025] Step 4, Corrosion Test: Place the ferritic stainless steel corrosion test sample prepared in Step 1 into the corrosion test solution determined in Step 3 at a constant temperature of 30-50℃, and conduct the corrosion test for 0.5-1 hour. The sample should be completely submerged below the liquid surface and should not be in contact with the container wall or bottom. The samples should not be in contact with each other.

[0026] Step 5: Post-corrosion sample treatment: After the predetermined corrosion test time has elapsed, remove the ferritic stainless steel corrosion test sample; gently brush off the corrosion products adhering to the sample surface with a soft brush, and rinse thoroughly with deionized water; place the sample in a beaker containing anhydrous ethanol and ultrasonically clean for 5-10 minutes at an ultrasonic frequency of 25kHz~40kHz; remove the sample and dry it with cold air, then place it in a desiccator until it reaches constant weight (for no less than 30 minutes); weigh the sample after corrosion, W1, using an analytical balance with an accuracy of 0.1mg, in grams.

[0027] Step Six: Calculation of Uniform Corrosion Rate: Calculate the uniform corrosion weight loss rate V of ferritic stainless steel using the following formula: V = (W0 - W1) / (S × t), where: V—uniform corrosion rate, unit is g / (m³). 2 •h); W0—Initial mass of the sample, in g; W1—Mass of the sample after corrosion, in g; S—Initial surface area of ​​the sample, in m² 2 t—corrosion test time, in hours.

[0028] The ferritic stainless steel mentioned in step one includes at least one of the ferritic stainless steels such as 409, 430, 439, 425, and 446.

[0029] Preferably, the hydrogen ion concentration [H] in step two is... + The concentration of chloride ions [Cl] is 0.8-1.5 mol / L. - The concentration is 0.8-1.5 mol / L, [H + ] / [Cl - The molar ratio is 0.5:1 to 0.8:1.

[0030] Preferably, the corrosion inhibitor in step three is sodium molybdate, with an addition concentration of 0.05-0.5 g / L.

[0031] The gradient selection in step three is based on adjusting the corrosion rate to 0.5-50 g / (m²). 2 Within the detectable window of ·h).

[0032] 2. Technical Principle: This invention addresses the excessively rapid corrosion rate of ferritic stainless steels such as 409 and 430 in strong acid media by systematically studying the hydrogen ion concentration [H]. + ] and chloride ion concentration [Cl -The synergistic mechanism of [missing information] was investigated, and a "dual-parameter control model" was proposed for the first time for the detection of uniform corrosion in ferritic stainless steel.

[0033] 2.1 Dual-Parameter Synergistic Effect: Studies have found that the corrosion behavior of ferritic stainless steel in acidic sodium chloride media is not a simple superposition of the effects of hydrogen ions and chloride ions, but rather exhibits a significant synergistic effect. Hydrogen ions provide the driving force for acidic corrosion, while chloride ions promote activation dissolution by destroying the passivation film. When [H... + ] / [Cl - When the molar ratio is too low (<0.3), chloride ions are relatively excessive, which easily leads to pitting corrosion; when [H] + ] / [Cl - When the molar ratio is too high (>0.9), the hydrogen ion drive is too strong, the corrosion rate is too fast, and even complete dissolution occurs. Only within the suitable ratio range of 0.3-0.9 does the corrosion process exhibit uniform corrosion characteristics, and the corrosion rate has a good correlation with the alloy composition of the material.

[0034] 2.2 Detectable Window Concept: This invention introduces the concept of a "detectable window" (0.5-50 g / (m³)) for the first time. 2 The concept of ·h). Corrosion rate too low (<0.5g / (m)). 2 When the weight loss per unit time is too small (·h), the weighing error will affect the accuracy of the test; if the corrosion rate is too high (>50g / (m)), the weight loss per unit time will be too small (·h)). 2 When the corrosion rate is within a certain range (e.g., h), the sample may completely dissolve or its surface condition may change drastically within a short time, making it impossible to obtain a stable and uniform corrosion rate. By adjusting the corrosion rate to within a detectable window, the accuracy and repeatability of the test results can be ensured.

[0035] 2.3 Gradient Detection System: This invention constructs a three-level gradient detection system of "basic medium - concentration optimization - corrosion inhibitor assistance", which can flexibly select detection conditions according to the corrosion resistance of different steel grades, and realize universal detection of different ferritic stainless steels from 409 (extremely poor corrosion resistance) to 446 (good corrosion resistance).

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0037] I. Experimental Materials and General Methods: 1. Experimental Materials: Ferritic stainless steel, including 409 stainless steel, 430 stainless steel, etc.; Chemical reagents: NaCl, HCl (mass fraction 36-38%), sodium molybdate (Na2MoO4·2H2O), benzotriazole (BTA), etc., all of which were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd.

[0038] 2. General test steps: (1) Sample preparation: The stainless steel to be tested is processed into a sample with regular geometric shape. The sample surface is polished step by step with 180 grit, 600 grit, 1000 grit and 2000 grit sandpaper until the surface shows a uniform metallic luster. The length, width and height of the sample are measured with vernier calipers. Each dimension is measured three times and the average value is taken. The initial surface area of ​​the sample is calculated as S = 2 × (length × width + length × height + width × height). The polished sample is degreased, rinsed with deionized water, ultrasonically cleaned with anhydrous ethanol for 5 minutes, and then dried with cold air. It is placed in a desiccator for 30 minutes until constant weight. The initial mass W0 of the sample is weighed using an analytical balance with an accuracy of 0.1 mg.

[0039] (2) Preparation of corrosive medium: Prepare the corrosive solution according to the specific requirements of the example. Use deionized water. First, weigh the sodium chloride solid and dissolve it in deionized water. Then, measure the concentrated hydrochloric acid and add it to the sodium chloride solution. Stir well and then make up to the predetermined volume. If corrosion inhibitor is required, add it before making up to the volume and stir to dissolve.

[0040] (3) Corrosion test: Transfer the corrosive solution to a glass beaker, place it in a constant temperature water bath, set the required temperature, and wait for the solution temperature to stabilize to the set value ±0.5℃; suspend the sample in the beaker with a polytetrafluoroethylene bracket to ensure that the sample is completely submerged below the liquid surface and does not contact the beaker wall and bottom, and record the start time.

[0041] (4) Post-corrosion treatment: After the predetermined time is reached, take out the sample; gently brush off the corrosion products attached to the sample surface with a soft brush, rinse with deionized water; place in anhydrous ethanol for ultrasonic cleaning for 5 minutes; take out and dry with cold air, place in a desiccator for 30 minutes until constant weight; weigh the sample after corrosion, W1.

[0042] (5) Corrosion rate calculation: The uniform corrosion rate is calculated using the formula V=(W0-W1) / (S×t), with units of g / (m³). 2 ·h).

[0043] (6) Surface morphology observation: The surface morphology of the sample after corrosion is observed by scanning electron microscopy to determine whether the corrosion is uniform.

[0044] II. The implementation examples are as follows. Example 1: Basic Scheme (First Gradient) Test of 430 Stainless Steel

[0045] The test was conducted according to the general testing procedures, with the following specific conditions: Corrosive medium: 1.0 mol / L NaCl + 1.0 mol / L HCl, [H + ]=1.0M, [Cl - ]=2.0M, [H + ] / [Cl - =0.5; Temperature: 35℃; Time: 1 hour; Test results: Initial sample mass W0 = 2.4189g, surface area S = 0.00131m² 2 The mass after corrosion is W1 = 2.3558 g, and the corrosion rate is V = (2.4189 – 2.3558) / (0.00131 × 1) = 48.2 g / (m³). 2 •h), Surface morphology after corrosion: SEM observation shows uniform corrosion on the surface, with no pitting or local over-corrosion. Example 2: Basic Scheme (First Gradient) Test of 439 Stainless Steel

[0046] The test shall be conducted in accordance with the general test procedure, and the specific conditions shall be the same as those in Example 1.

[0047] Test results: W0 = 2.5977g, S = 0.00124m 2 , W1=2.5453g, V=(2.5977-2.5453) / (0.00124×1)=42.3g / (m 2 ·h), the surface is uniform after corrosion. Example 3: Different [H] + ] / [Cl - Proportioning test (430LX stainless steel, 35℃, 1h)

[0048] Maintain the total chloride ion concentration [Cl] - [ ] = 2.0M remains unchanged, [H ] changes + [H] concentration, to investigate different [H] + ] / [Cl - The effect of molar ratio on corrosion behavior. Test results are shown in Table 1.

[0049]

[0050] The results show that when [H] + ] / [Cl - The molar ratio is in the range of 0.3 to 0.9 (numbers #2 to #7), and the corrosion rate is moderate (8.12-35.67 g / (m³)). 2 The surface is uniform; below 0.3, the corrosion rate is low and pitting occurs; above 0.9, the corrosion rate is too high and there is obvious corrosion product coverage. This verifies the rationality and criticality of the proportion range defined in this invention. Example 4: Corrosion Inhibitor-Assisted Adjustment (Third Gradient) Test on 409 Stainless Steel

[0051] The corrosion rate of 409 stainless steel in the base medium (1.0M NaCl + 1.0M HCl) is 181.32 g / (m²). 2 The corrosion rate (·h) exceeds the detectable window. After adjusting the corrosive medium to 0.5M NaCl + 0.5M HCl, the corrosion rate is 118.01 g / (m²). 2 To verify the regulating function of the corrosion inhibitor, sodium molybdate of different concentrations was added to the corrosive medium to investigate its effect on the corrosion rate. The results are shown in Table 2.

[0052]

[0053] It is evident that by adjusting the concentration of the corrosion inhibitor, the corrosion rate can be reduced from 118.01 g / (m²). 2 •h) lowered to 45.41g / (m 2 The corrosion inhibitor (·h) was introduced into a more stable and measurable range, verifying its effectiveness as an auxiliary adjustment tool. Example 5: Comparison of the adjustment effects of different corrosion inhibitors

[0054] Different types of corrosion inhibitors were added to the base medium (1.0M NaCl + 1.0M HCl) to test their effect on the corrosion rate of 430LX stainless steel. The results are shown in Table 3.

[0055]

[0056] The results show that different corrosion inhibitors all have a regulating effect, and the combined use has a better effect, verifying the feasibility of corrosion inhibitors as an auxiliary regulating module.

[0057] III. Comparison examples are as follows. Comparative Example 1: Test of 430 Stainless Steel according to GB4334.6-2015 Standard Method

[0058] According to the standard method of GB4334.6-2015, when a 430 stainless steel sample is placed in a boiling 5% H2SO4 solution, it completely dissolves within 10 minutes, making it impossible to determine the corrosion rate. This proves that the standard method is ineffective for ferritic stainless steel. Comparative Example 2: Tests exceeding the formulation range of this invention

[0059] Reference to Example 3-1 ([H) + ] / [Cl - [H] = 0.25) and Examples 3-9 ([H + ] / [Cl -The results of [1] show that at low molar ratios, corrosion is slow and pitting occurs, making it impossible to accurately evaluate uniform corrosion performance; at high molar ratios, the sample reacts violently and is covered with obvious corrosion products, affecting the determination of its uniform corrosion rate. This demonstrates that the mixing ratio range defined by this invention is critical and non-obvious. Comparative Example 3: Adding corrosion inhibitors alone without optimizing the formulation

[0060] Prepare a 1M NaCl + 1M HCl solution ([H + ] / [Cl - [=1.0]), experimental temperature 35℃, immersion for 1 hour to test 409 stainless steel, without corrosion inhibitor, corrosion rate = 181.32 g / (m²). 2 •h); After adding 0.5 g / L of sodium molybdate, the corrosion rate decreased to 96.42 g / (m). 2 ·h).

[0061] Compared to Example 4 (1M NaCl + 1M HCl + 0.5 g / L sodium molybdate), the corrosion inhibitor showed limited adjustment effect and excessively high corrosion rate, with significant coverage of corrosion products, which was not conducive to experimental detection. This indicates that corrosion inhibition adjustment needs to be carried out at a lower total concentration to achieve both rapid and controllable corrosion.

[0062] IV. The reproducibility test is as follows.

[0063] Take the same batch of 430LX stainless steel and repeat the test 10 times under the conditions of Example 1. The results are shown in Table 4.

[0064]

[0065] The results show that the method of the present invention has good reproducibility and stability, with a relative standard deviation of <1%, which meets the requirements of the detection method.

[0066] V. Verification of Comprehensive Results: To verify the applicability of the gradient detection system of this invention, samples with different corrosion resistance were tested under different gradient conditions, and the results are shown in Table 5.

[0067]

[0068] The results show that the gradient detection system of the present invention can cover ferritic stainless steels with different corrosion resistance properties and has good versatility.

[0069] 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 concept 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 gradient detection method for the uniform corrosion performance of ferritic stainless steel, characterized in that: Includes the following steps: Step 1: Sample Preparation and Pretreatment: Select the ferritic stainless steel to be tested and process it into a sample with a regular geometric shape. Use 180-grit, 600-grit, 1000-grit, and 2000-grit sandpaper to progressively polish the sample surface, removing the oxide layer and processing marks, until the sample surface exhibits a uniform metallic luster. Measure and record the geometric dimensions of the sample, and calculate the initial surface area S of the sample (in m²). 2 After grinding, the sample is degreased, rinsed with deionized water, and ultrasonically cleaned with anhydrous ethanol for 5-10 minutes at an ultrasonic frequency of 25kHz~40kHz. Then, it is dried with cold air. The cleaned and dried sample is placed in a desiccator until it reaches constant weight for no less than 30 minutes. The initial mass W0 of the sample is weighed using an analytical balance with an accuracy of 0.1mg. The unit is g. Step Two: Preparation of Basic Corrosion Medium: Prepare the basic corrosion medium, which is a mixed aqueous solution of sodium chloride and hydrochloric acid, wherein the hydrogen ion concentration [H+] is... + The concentration of chloride ions [Cl] is 0.5-2.0 mol / L. - The concentration is 0.5-2.0 mol / L, and the molar ratio of hydrogen ions to chloride ions [H] is... + ] / [Cl - The concentration is controlled within the range of 0.3 to 0.

9. The basic corrosive medium is prepared with deionized water. When preparing the medium, first weigh the sodium chloride solid and dissolve it in deionized water, then measure the concentrated hydrochloric acid and add it to the sodium chloride solution. After stirring evenly, the volume is adjusted to the predetermined volume. Step 3: Gradient Selection and Adjustment of Corrosion Media: Based on the alloy composition and predicted corrosion resistance of the ferritic stainless steel to be tested, select the corrosion media from the following gradients: First gradient: For most ferritic stainless steels, directly use the basic corrosion media prepared in Step 2 for testing; Second gradient: For stainless steels where the corrosion rate exceeds the preset detectable window of 0.5-50 g / (m²) under the first gradient condition. 2 Steel grades within the range of ·h) while maintaining [H + ] / [Cl - With the molar ratio within the range of 0.3 to 0.9, adjust the total concentration of the base corrosive medium to bring the corrosion rate into the detectable window, i.e., 0.5-50 g / (m³). 2 •h); Third gradient: For extremely non-corrosion-resistant steel grades or samples under special working conditions where the corrosion rate still exceeds the upper limit of the detectable window under the second gradient conditions, a corrosion inhibitor is added to the basic corrosive medium as an auxiliary regulator. The corrosion inhibitor is selected from one or more of molybdates, benzotriazole compounds, and gluconates. By adjusting the type and concentration of the corrosion inhibitor, the corrosion rate is finely controlled to 0.5-50 g / (m). 2 Within the detectable range of h); Step 4: Corrosion test: Place the ferritic stainless steel corrosion test sample prepared in Step 1 into the corrosion test solution determined in Step 3 at a constant temperature of 30-50℃ and conduct the corrosion test. The corrosion test time is 0.5-1 hour. The sample should be completely submerged below the liquid surface and should not be in contact with the container wall or bottom. The samples should not be in contact with each other. Step 5: Post-corrosion sample treatment: After the predetermined corrosion test time has elapsed, remove the ferritic stainless steel corrosion test sample, brush off the corrosion products adhering to the sample surface, rinse it clean with deionized water, place the sample in a beaker containing anhydrous ethanol, and ultrasonically clean it for 5-10 minutes at an ultrasonic frequency of 25kHz~40kHz. Remove the sample and dry it with cold air, then place it in a desiccator for at least 30 minutes until constant weight is achieved. Weigh the sample after corrosion, W1, using an analytical balance with an accuracy of 0.1mg. The unit is g. Step Six: Calculation of Uniform Corrosion Rate: Calculate the uniform corrosion weight loss rate V of ferritic stainless steel using the following formula: V=(W0-W1) / (S×t), Where: V—uniform corrosion rate, in g / (m²) 2 •h); W0—Initial mass of the sample, in g; W1—Mass of the sample after corrosion, in g; S—Initial surface area of ​​the sample, in m² 2 t—corrosion test time, in hours.

2. The gradient detection method for the uniform corrosion performance of ferritic stainless steel according to claim 1, characterized in that: In step one, the ferritic stainless steel to be tested is selected. Ferritic stainless steel is a type of stainless steel for which the uniform corrosion rate cannot be determined under the 5% sulfuric acid corrosion test conditions specified in GB4334.6-2015 standard, or which requires rapid testing.

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