Method for speedily and comprehensively evaluating pitting corrosion resistance of weld zone of stainless steel pipe

Abstract

To provide a method that belongs to the technical field of metal performance tests and, more specifically, speedily and comprehensively evaluates the pitting corrosion resistance of a weld zone of a stainless steel pipe.SOLUTION: The method according to the present invention can evaluate pitting corrosion resistance of a weld zone of a stainless steel pipe to be tested, by: obtaining a plurality of weld zone samples from different positions of one or more weld zones on the stainless steel pipe; acquiring a surface oxidation inclusion density and critical pitting corrosion temperature of each weld zone sample; acquiring a fitting function relational expression of the surface oxidation inclusion density and critical pitting corrosion temperature, including the surface oxidation inclusion density of each weld zone sample as an independent variable, and the critical pitting corrosion temperature of each weld zone sample as a dependent variable; and obtaining a standard critical pitting corrosion temperature of a weld zone to be tested by substituting the surface oxidation inclusion density of the weld zone to be tested which is obtained by welding in the same welding process on the stainless steel pipe to be tested into the fitting function relational expression.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention belongs to the technical field of metal performance testing, and more specifically, relates to a method for quickly and comprehensively evaluating the pitting corrosion resistance of the welded part of a stainless steel pipe.

Background Art

[0002] AISI 316L stainless steel has good corrosion resistance and is therefore widely used in the marine industry. The added Cr can form Fe / Cr oxides on the steel surface, which is beneficial for improving the stability of the passivation film. Gas Metal Arc Welding (GMAW) is usually used for the lining welding of 316L steel pipes. Although the efficiency of GMAW is very high, welding defects in the lining weld metal often cause a decrease in the pitting corrosion resistance of the welded part. In the international standard for measuring the critical pitting temperature under a constant potential (GB / T 32550-2016 Standard for Measuring the Critical Pitting Temperature of Metals and Alloys under the Control of a Constant Potential), it is required that the surface area of the test sample is 1 cm 2 , and the size of the generally used sample is 10×10×B mm (B is the thickness of the sample). In the process of welding austenitic stainless steel pipes by GMAW, due to reasons such as the welding process being unqualified, insufficient protection of the welded part, and incomplete slag removal after welding in the welding process, the risk of pitting corrosion occurring in the welded part of austenitic stainless steel increases significantly, affecting the pitting corrosion resistance of the welded part of austenitic stainless steel itself. In the actual use process of austenitic stainless steel, the local pitting corrosion rate of the welded part with a relatively low critical pitting temperature is faster, so local pitting corrosion develops rapidly to the inside of the welded part, causing defects in the entire welded part.

[0003] Therefore, there is an urgent need for a method to quickly and comprehensively evaluate the pitting corrosion resistance of the welded part of a stainless steel pipe. It is beneficial for an accurate and comprehensive evaluation of the pitting corrosion resistance of the entire welded part and has extremely great value for accurately predicting the service life of austenitic stainless steel as an oil and gas pipeline in the marine industry.

Summary of the Invention

[0004] In view of the deficiencies of the prior art, the object of the present invention is to provide a method for quickly and comprehensively evaluating the pitting corrosion resistance of the welded part of a stainless steel pipe. By this method, the accuracy and reliability of evaluating the pitting corrosion resistance of the welded part of a stainless steel pipe by the prior art are low, the experimental cost of evaluating the influence of different welding processes or different heat input weldings on the pitting corrosion resistance of the welded part of a stainless steel pipe is high, and there is no method that can accurately predict the service life of a stainless steel pipe, especially the service life as an offshore industrial oil and gas pipeline, so as to solve the above technical problems.

[0005] In order to achieve the above object, according to the present invention, there is provided a method for quickly and comprehensively evaluating the pitting corrosion resistance of the welded part of a stainless steel pipe, including the following steps S1 to S5: S1: Collect a plurality of welded part samples from different positions of one or more welded parts on the stainless steel pipe. The sampling points of the plurality of welded part samples cover at least from the root to the top of at least one welded part. Obtain the number of oxide inclusions per unit area in each welded part sample, that is, the surface oxide inclusion density. S2: Obtain the critical pitting temperature of each welded part sample by a critical pitting temperature test. S3: Taking the surface oxide inclusion density of each welded part sample as an independent variable and the critical pitting temperature of each welded part sample as a dependent variable, obtain the fitting function relational expression between the surface oxide inclusion density and the critical pitting temperature: y = f(x) (In the formula, y is the critical pitting temperature, x is the surface oxide inclusion density, and f is the fitting function.) S4: Collect a test welded part sample from the test stainless steel pipe. The welding process of the test welded part sample is the same as that of the welded part sample. Obtain the number of oxide inclusions per unit area in the test welded part sample, that is, the surface oxide inclusion density. S5: Substitute the surface oxide inclusion density of the test welded part sample into the fitting function relational expression to obtain the standard critical pitting temperature of the test welded part sample, thereby providing a method that can quickly and comprehensively evaluate the pitting corrosion resistance of the welded part of the test stainless steel pipe.

[0006] Preferably, in step S1, the stainless steel includes any one of austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, duplex stainless steel, and age-hardening stainless steel.

[0007] Preferably, in step S1, the number of the plurality of welded joint samples is ≥5.

[0008] More preferably, in step S1, the number of the plurality of welded joint samples is ≥8.

[0009] Preferably, in step S1, the size of the welded joint sample is 10×10×B mm, where B≥5.

[0010] More preferably, in step S1, the size of the welded joint sample is 10×10×B mm, and B is 8 - 15.

[0011] Preferably, in step S1, before obtaining the number of oxide inclusions per unit area in each welded joint sample, an electrolytic polishing treatment is performed.

[0012] Preferably, in step S2, the critical pitting temperature test is a potentiostatic critical pitting temperature test.

[0013] More preferably, before performing the potentiostatic critical pitting temperature test on the welded joint sample, a pretreatment is performed, and the pretreatment includes steps of polishing the welded joint sample, immersing it in an acetone solution, further rinsing it with an alcohol solution, and drying it.

[0014] Preferably, in step S3, the analysis is any one of linear regression analysis and polynomial regression analysis.

[0015] Preferably, in step S3, the fitting function relational expression is y = ax 2+bx + c where y is the critical pitting temperature, x is the surface oxide inclusion density, and a, b, and c are fitting parameters.

[0016] The technical means of the present invention has the following beneficial effects compared with the prior art. (1) In the method for rapidly and comprehensively evaluating the pitting corrosion resistance of the welded part of a stainless steel pipe provided by the present invention, a plurality of welded part samples are taken from different positions of one or more welded parts on the stainless steel pipe, and the sampling points of the plurality of welded part samples cover at least from the root to the top of any one of the welded parts. The number of oxide inclusions per unit area in each welded part sample, that is, the surface oxide inclusion density, is obtained, and the critical pitting temperature of each welded part sample is obtained by a critical pitting temperature test. Then, taking the surface oxide inclusion density of each welded part sample as an independent variable and the critical pitting temperature of each welded part sample as a dependent variable, a fitting function relational expression between the surface oxide inclusion density and the critical pitting temperature is obtained. By substituting the surface oxide inclusion density of the test welded part obtained by welding with the same welding process in the test stainless steel pipe into the above fitting function relational expression, the standard critical pitting temperature of the test welded part can be rapidly obtained without performing a potentiostatic critical pitting temperature test on the test welded part. As a result, the test time and test cost are further saved, and the pitting corrosion resistance of the welded part of the test stainless steel pipe is accurately and comprehensively evaluated. The method provided by the present invention has extremely high value for the service life of stainless steel pipes, especially for the service life as offshore industrial oil and gas pipelines.

[0017] (2) In actual use, by the method provided in the present invention, it is possible to determine the height of the standard critical pitting temperature value of the welded joint welded by different welding methods or different heat input welding methods, and it is possible to quickly, accurately and comprehensively evaluate the pitting corrosion resistance of the welded joint welded by different welding methods or different heat input welding methods. The cost of evaluating the influence of different welding processes or different heat input welding methods on the pitting corrosion resistance of the welded joint of austenitic stainless steel is low, the critical pitting temperature is relatively high due to the evaluation effect, and a welding method with relatively excellent pitting corrosion resistance can be selected for mass welding, thereby improving the production efficiency.

[0018] (3) In the present invention, a potentiostatic critical pitting temperature test is performed on the welded joint sample for which the surface oxide inclusion density has been obtained. By taking the surface oxide inclusion density of each welded joint sample as an independent variable and the critical pitting temperature of each welded joint sample as a dependent variable to obtain the fitting function relational expression between the surface oxide inclusion density and the critical pitting temperature, the pitting corrosion resistance of the welded joint of the test stainless steel pipe with the same welding process can be accurately evaluated, avoiding the problem of inaccurate evaluation results by immersion tests, without wasting more samples and test time, the pitting corrosion resistance of the welded joint of the test stainless steel pipe is evaluated more quickly, comprehensively and accurately, and the reliability of the evaluation of the critical pitting temperature is improved.

[0019] (4) In the present invention, the distribution of oxide inclusions of each welded joint sample is observed using an optical microscope, the surface oxide inclusion density of each welded joint sample is statistically analyzed by software, and by obtaining the surface oxide inclusion density of each welded joint sample, the damage caused by inaccurate oxygen content measurement due to low oxygen content is avoided.

Brief Description of the Drawings

[0020]

Figure 1

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Figure 5

Embodiments for Carrying Out the Invention

[0021] In order to make the object, technical means and advantages of the present invention clearer, the present invention will be described in more detail below with reference to the drawings and embodiments. It should be noted that the specific embodiments in this specification are only for explaining the present invention and do not limit the present invention.

[0022] The pitting corrosion resistance of austenitic stainless steel has a complex relationship with the austenite structure, ferrite structure, etc. in the stainless steel. Due to reasons such as an unqualified welding process during welding, insufficient protection for the welded part, and incomplete removal of post-weld residues, the risk of pitting corrosion occurring in the welded part of austenitic stainless steel increases significantly, affecting the pitting corrosion resistance of the austenitic stainless steel welded part itself. Due to reasons such as insufficient welding protection, there are significant differences in the pitting corrosion resistance of austenitic stainless steel. How to quickly and comprehensively evaluate the pitting corrosion resistance of the welded part of austenitic stainless steel pipes, evaluate the influence of different welding processes or different heat input welding methods on the pitting corrosion resistance of the welded part of austenitic stainless steel at low cost, and accurately evaluate the service life of austenitic stainless steel are problems that need to be addressed urgently. Therefore, according to the present invention, there is provided a method for quickly and comprehensively evaluating the pitting corrosion resistance of the welded part of a stainless steel pipe, including the following steps S1 to S5: S1: Collect a plurality of weld sample from different positions of one or more welds on the stainless steel pipe. The sampling points of the plurality of weld samples cover at least from the root to the top of any one weld. Obtain the number of oxide inclusions per unit area in each weld sample, that is, the surface oxide inclusion density. S2: Obtain the critical pitting temperature of each weld sample by a critical pitting temperature test. S3: Using the surface oxide inclusion density of each weld sample as an independent variable and the critical pitting temperature of each weld sample as a dependent variable, obtain the fitting function relational expression between the surface oxide inclusion density and the critical pitting temperature: y = f(x) (where y is the critical pitting temperature, x is the oxide inclusion density, and f is the fitting function.) S4: Collect a test weld sample from the stainless steel pipe to be tested. The welding process of the test weld sample is the same as that of the weld sample. Obtain the number of oxide inclusions per unit area in the test weld sample, that is, the surface oxide inclusion density. S5: Substitute the surface oxide inclusion density of the test weld sample into the fitting function relational expression to obtain the standard critical pitting temperature of the test weld sample, thereby providing a method for quickly and comprehensively evaluating the pitting corrosion resistance of the weld of the stainless steel pipe to be tested.

[0023] In the present invention, the type of the above stainless steel is not limited, and any stainless steel in the prior art may be used. In some embodiments, the stainless steel may be, but is not limited to, austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, duplex stainless steel, and age-hardening stainless steel.

[0024] In some embodiments, in step S1, the specific operation of obtaining a plurality of weld joint samples is as follows. That is, a plurality of weld joint samples are collected from different positions of one or more weld joints on the stainless steel pipe. The sampling points of the weld joint samples include at least from the root to the upper part of any one of the above weld joints (the root of the weld joint, the middle part of the weld joint, the upper part of the weld joint, the root of the heat affected zone, the middle part of the heat affected zone, the upper part of the heat affected zone). The purpose is to make the surface oxide inclusion density of the obtained weld joint samples more complete and accurate later, to more accurately evaluate the pitting corrosion resistance of the weld joints, and to improve the reliability of the critical pitting temperature evaluation. When collecting weld joint samples from a plurality of weld joints of the stainless steel pipe, the welding processes of different weld joints are the same.

[0025] In some embodiments, in step S1, the number of the plurality of weld joint samples is ≥5.

[0026] In a preferred embodiment, in step S1, the number of the plurality of weld joint samples is ≥8.

[0027] In some embodiments, the size of the weld joint sample is 10×10×B mm, where B≥5.

[0028] In a preferred embodiment, the size of the weld joint sample is 10×10×B mm, where B is 5 to 15.

[0029] Those skilled in the art can obtain the number of oxide inclusions per unit area, i.e., the surface oxide inclusion density, in each weld joint sample of the stainless steel pipe by different methods. In some embodiments, in step S1, after observing the oxide inclusions on the surface of each weld joint sample by an optical microscope and obtaining the distribution image of the oxide inclusions, the surface oxide inclusion density of each weld joint sample is obtained by software such as Image-pro plus, Image J, Fiji.

[0030] In some embodiments, in step S1, before obtaining the number of oxide inclusions per unit area in each welded joint sample, an electrolytic polishing treatment is performed. The purpose is to expose the insoluble oxide inclusions and obtain a clear distribution photo of the oxide inclusions.

[0031] In some embodiments, in step S2, the critical pitting temperature test is a potentiostatic critical pitting temperature test.

[0032] In some embodiments, in step S2, before performing the potentiostatic critical pitting temperature test on the welded joint sample, a pretreatment is performed. In a preferred embodiment, the pretreatment includes the steps of polishing the welded joint sample, immersing it in an acetone solution, further rinsing it with an alcohol solution, and drying it.

[0033] In some embodiments, in step S2, the specific operation of the potentiostatic critical pitting temperature test is as follows. That is, (1) Place the sample, auxiliary electrode, and salt bridge in a solution (cool the initial temperature of the solution to 3°C or lower), connect the sample to a potentiostat and a data recording device, and connect a temperature measurement and temperature control device. (2) Apply a predetermined anodic potential of 0.7 V SCE to the sample. After applying the potential for 60 s or more, raise the temperature of the solution at a controllable rate of 1°C / min. (3) Record the monitoring current and solution temperature throughout the test process, and define the critical pitting temperature (CPT) as the temperature corresponding to the case where the current density reaches 100 μA×cm -2 and remains at that level for at least 60 s. In some embodiments, the above test solution is one or more selected from an NaCl solution, an NaCl + Na2SO4 solution, an NaCl + NaNO3 solution, and an NaCl + Na2S2O3 solution.

[0034] In some embodiments, the concentration of the test solution is 0.1 mol / L to 1.5 mol / L.

[0035] In the present invention, the above analysis method is not limited. For example, it may be linear regression analysis or polynomial regression analysis, but it is not limited thereto. In actual use, an appropriate analysis method can be selected and analyzed according to factors such as the sample amount of the welded joint sample and the fitting effect of the mathematical model obtained by fitting with different analysis methods.

[0036] In some embodiments, in step S3, the fitting function relational expression is y = ax 2 + bx + c is. In the formula, y is the critical pitting temperature, x is the surface oxide inclusion density, and a, b, and c are fitting parameters.

[0037] Hereinafter, the above technical means will be described in detail with specific examples.

[0038] Example 1 In this example, the solder used for the welded part of the pipeline is 316L austenitic stainless steel welding wire. The welding method used is gas metal arc welding (GMAW), and the shielding gas of the welding gun is 98% Ar + 2% CO2. The inside of the pipeline is filled with pure Ar for protection.

[0039] 1) Samples were obtained from different positions of the welded part of the above austenitic stainless steel pipe. The sampling interval is preferably such that the samples taken can cover all positions of the pipeline welded part so that the density of the surface oxide inclusions of the welded joint samples obtained later is more complete. Also, the size of each welded joint sample is preferably 10×10×10 mm. In this example, welded joint samples (including the root of the welded part, the middle of the welded part, the upper part of the welded part, the root of the heat affected zone, the middle of the heat affected zone, and the upper part of the heat affected zone) were taken from different positions of the welded part of one pipeline. Here, two welded joint samples were taken from both the middle of the welded part and the middle of the heat affected zone, and one welded joint sample was taken from each of the other positions, for a total of 8 welded joint samples.

[0040] 2) The obtained welded joint sample was pretreated. The pretreatment procedure is as follows. That is, the surface of the welded joint sample was polished with sandpaper of 240 mesh to 1000 mesh to remove surface burrs, and then immersed in an acetone solution to remove surface oil stains, and finally rinsed with an alcohol solution to obtain a welded joint sample with a clean surface.

[0041] 3) After electrolytically polishing the clean welded joint sample, the oxide inclusions were observed under an optical microscope, and the surface oxide inclusion density was obtained by software such as Image-pro plus, Image J, and Fiji. Here, the oxide inclusion density is the number of surface oxide inclusions of the welded joint sample per unit area. In this example, the specific procedure of the electrolytic polishing is as follows. That is, (1) the electrolytic solution was put into an electrolytic polishing container, and the rotation speed was selected, (2) a DC voltage of 30 V was applied to the surface of the sample, and the sample was put into the electrolytic solution, (3) after the electrolytic polishing was continued for 10 s to 15 s, the current was turned off and the surface of the sample was cleaned. Then, the distribution status of the surface oxide inclusions of each welded joint sample was observed under an optical microscope, and an image of the oxide inclusions at 500x was obtained using the optical microscope. The image was processed by Image-pro plus software. Specifically, the image was opened with Image-pro plus software, and then the reference length was set in sequence, the oxide inclusions were selected as the calculation target, and parameters such as area and diameter were selected as the calculation items to obtain the area of the image and the number of oxide inclusions. Finally, the surface oxide inclusion density of the welded joint sample was obtained. Figure 2 is a schematic diagram of the surface oxide inclusions of one welded joint sample obtained from the welded joint of the austenitic stainless steel pipe according to this example.

[0042] 4) The critical pitting temperature in the potentiostatic critical pitting temperature test of each welded joint sample was obtained. In this example, the potentiostatic critical pitting temperature test was carried out according to the standard GB / T 32550-2016. Specifically, (1) the sample, auxiliary electrode, and salt bridge were placed in a solution (the initial temperature of the solution was cooled to 3°C or below), the sample was connected to a potentiostat and a data recording device, and a temperature measuring device and a temperature control device were connected, (2) a predetermined 0.7 V was applied to the sample SCEAn anodic potential was applied, and after applying the potential for 60 s or more, the temperature of the solution was raised at a controllable rate of 1 °C / min, and (3) the monitoring current and the solution temperature throughout the test process were recorded. The test solution is preferably an NaCl solution with a concentration of 1 mol / L, and the critical pitting temperature (CPT) is the temperature corresponding to when the current density reaches 100 μA×cm -2 and is maintained for at least 60 s.

[0043] The critical pitting temperatures of the welded joint samples according to this example are shown in Fig. 3. The critical pitting temperature of the welded joint with a surface oxide inclusion density of 1.83×10 4 pieces / mm 2 is 16.81 °C, and the critical pitting temperature of the welded joint with a surface oxide inclusion density of 1.56×10 4 pieces / mm 2 is 20.51 °C. The critical pitting temperature of the welded joint with a surface oxide inclusion density of 1.24×10 4 pieces / mm 2 is 22.01 °C. The critical pitting temperature of the welded joint with a surface oxide inclusion density of 1.15×10 4 pieces / mm 2 is 23.32 °C. The critical pitting temperature of the welded joint with a surface oxide inclusion density of 0.99×10 4 pieces / mm 2 is 23.92 °C. The critical pitting temperature of the welded joint with a surface oxide inclusion density of 0.97×10 4 pieces / mm 2 is 24.01 °C. The critical pitting temperature of the welded joint with a surface oxide inclusion density of 0.79×10 4 pieces / mm 2 is 24.81 °C. The critical pitting temperature of the welded joint with a surface oxide inclusion density of 0.43×10 4 pieces / mm 2 was 25.19 °C.

[0044] 5) The critical pitting temperature and the surface oxide inclusion density of the above-mentioned welded joint sample were fitted. A function relational expression may be fitted using mathematical fitting software (such as Mathematica, Maple, or Origin, etc.), or may be fitted using other software capable of realizing other curve fittings. In this embodiment, the critical pitting temperature and the oxide inclusion density obtained using Origin software were fitted by polynomial regression analysis. Specifically, taking the surface oxide inclusion density of each welded joint sample as the independent variable and the critical pitting temperature of each welded joint sample as the dependent variable, the above data was input into Origin software, the data was selected from the data list, the analysis in the Origin menu bar was clicked, the polynomial fitting in the fitting options was selected to perform polynomial fitting analysis, and finally, the fitting function relational expression between the surface oxide inclusion density and the critical pitting temperature was obtained. In the change curve shown by the dashed line in Figure 3, the fitting function relational expression between the critical pitting temperature (y) of the welded joint of the pipeline and the surface oxide inclusion density (x) of the welded joint sample is y = 24.25 + 4.03x - 4.35x 2 (1) where y is the critical pitting temperature of the welded joint of the austenitic stainless steel pipe, with the unit of °C, and x is the surface oxide inclusion density of the welded joint sample, with the unit of ×10 4 pieces / mm 2 .

[0045] 6) By collecting a test welded joint from the test austenitic stainless steel pipe, obtaining the surface oxide inclusion density of the test welded joint and substituting it into the above fitting function relational expression to obtain the standard critical pitting temperature of the test welded joint, the pitting corrosion resistance of the welded joint of the test austenitic stainless steel pipe can be evaluated quickly and comprehensively, and furthermore, the service life of the test austenitic stainless steel pipe can be predicted.

[0046] Example 2 In this embodiment, the solder material used for the welded part of the pipeline is 316L austenitic stainless steel welding wire, the welding method used is gas metal arc welding (GMAW), the shielding gas of the welding gun is 98% Ar + 2% CO2, and the pipeline is not filled with gas inside.

[0047] 1) Samples were obtained from different positions of the welded part of the above austenitic stainless steel pipe. The sampling interval is preferably such that the sampled samples can cover all positions of the pipeline welded part so that the density of surface oxide inclusions of the welded part samples obtained later is more complete. Also, the size of each welded part sample is preferably 10×10×10 mm. In this embodiment, welded part samples (including the root of the welded part, the middle of the welded part, the upper part of the welded part, the root of the heat-affected zone, the middle of the heat-affected zone, and the upper part of the heat-affected zone) were taken from different positions of the welded part of one pipeline. Here, two welded part samples were taken from both the middle of the welded part and the middle of the heat-affected zone, and one welded part sample was taken from each of the other positions, for a total of eight welded part samples.

[0048] 2) The above-obtained welded part samples were pretreated. The pretreatment procedure is as follows. That is, the surface of the welded part samples was polished with 240-mesh to 1000-mesh sandpaper to remove surface burrs, then immersed in an acetone solution to remove surface oil stains, and finally rinsed with an alcohol solution to obtain clean welded part samples.

[0049] 3) After electrolytically polishing the clean welded part samples, the oxide inclusions were observed under an optical microscope, and the surface oxide inclusion density was obtained by software such as Image-pro plus, Image J, and Fiji. Here, the oxide inclusion density is the number of surface oxide inclusions per unit area of the welded part sample. In this embodiment, the operation procedure of performing the electrolytic polishing process on each welded part sample and obtaining the surface oxide inclusion density of each welded part sample was the same as that in Example 1. Figure 4 is a schematic diagram of the oxide inclusions of one welded part sample obtained from the welded part of the austenitic pipeline according to this embodiment.

[0050] 4) The critical pitting temperature in the potentiostatic critical pitting temperature test of each welded joint sample was obtained. In this example, the potentiostatic critical pitting temperature test was carried out in accordance with the standard GB / T 32550-2016. Specifically, (1) the sample, auxiliary electrode, and salt bridge were placed in a solution (the initial temperature of the solution was cooled to 3 °C or lower), the sample was connected to a potentiostat and a data recording device, a temperature measuring device and a temperature control device were connected, (2) a predetermined anodic potential of 0.7 V was applied to the sample, and after applying the potential for 60 s or more, the solution was heated at a controllable rate of 1 °C / min, (3) the monitoring current and solution temperature throughout the test process were recorded. The test solution was preferably an NaCl solution with a concentration of 1 mol / L, and the critical pitting temperature (CPT) was the temperature corresponding to when the current density reached 100 μA×cm SCE and was maintained for at least 60 s. -2

[0051] The critical pitting temperature of each welded joint sample in this example is shown in Fig. 5. The critical pitting temperature of the welded joint with a surface oxide inclusion density of 2.79×10 4 particles / mm 2 is 10.56 °C, the critical pitting temperature of the welded joint with a surface oxide inclusion density of 2.43×10 4 particles / mm 2 is 13.51 °C, the critical pitting temperature of the welded joint with a surface oxide inclusion density of 1.93×10 4 particles / mm 2 is 15.32 °C, the critical pitting temperature of the welded joint with a surface oxide inclusion density of 1.57×10 4 particles / mm 2 is 17.63 °C, the critical pitting temperature of the welded joint with a surface oxide inclusion density of 1.13×10 4 particles / mm 2 is 19.91 °C, the critical pitting temperature of the welded joint with a surface oxide inclusion density of 0.87×10 4 particles / mm 2 is 22.43 °C, the critical pitting temperature of the welded joint with a surface oxide inclusion density of 0.59×10 4 particles / mm 2 is 23.74 °C, the critical pitting temperature of the welded joint with a surface oxide inclusion density of 0.55×10 4 particles / mm 2The critical pitting temperature of the welded part was 24.01 °C.

[0052] 5) The critical pitting temperature and the surface oxide inclusion density of the above-mentioned welded part sample were fitted. A functional relational expression may be fitted using mathematical fitting software (such as Mathematica, maple, or origin, etc.), or may be fitted using other software capable of realizing other curve fittings. In this example, the critical pitting temperature and the oxide inclusion density obtained using origin software were fitted by polynomial regression analysis. Specifically, taking the surface oxide inclusion density of each welded part sample as the independent variable and the critical pitting temperature of each welded part sample as the dependent variable, the above data was input into origin software, the data was selected from the data list, the analysis in the origin menu bar was clicked, the polynomial fitting in the fitting options was selected to perform polynomial fitting analysis, and finally the fitting functional relational expression between the surface oxide inclusion density and the critical pitting temperature was obtained. In the change curve shown by the dashed line in Fig. 5, the fitting functional relational expression between the critical pitting temperature (y) of the welded part of the pipeline and the surface oxide inclusion density (x) of the welded part sample is y = 27.77 - 6.95x + 0.33x 2 (2) It was as follows. In the formula, y is the critical pitting temperature of the welded part of the austenitic stainless steel pipe, the unit is °C, x is the surface oxide inclusion density of the welded part, and the unit is ×10 4 pieces / mm 2 It is.

[0053] 6) By collecting a test welded part from the test austenitic stainless steel pipe, obtaining the surface oxide inclusion density of the test welded part and substituting it into the above fitting functional relational expression to obtain the standard critical pitting temperature of the test welded part, the pitting corrosion resistance of the welded part of the test austenitic stainless steel pipe can be rapidly and comprehensively evaluated, and furthermore, the service life of the test austenitic stainless steel pipe can be predicted.

[0054] As can be seen from Example 1 and Example 2, there is a great correlation between the pitting corrosion resistance of the welded joints of austenitic stainless steel in the same welding process and the content of oxide inclusions. Austenitic stainless steel with a relatively high oxide inclusion density has a low critical pitting temperature and poor pitting corrosion resistance. Also, for the welded joints of austenitic stainless steel formed by different heat inputs with the same process, the standard critical pitting temperature is obtained by calculating the surface oxide inclusion density of the welded joint samples, and further, the pitting corrosion resistances of the welded joints obtained by welding with different heat input methods are compared, and mass welding is carried out by the heat input welding method with a relatively high critical pitting temperature and relatively high pitting corrosion resistance.

[0055] The present invention collects a plurality of welded joint samples from different positions of one or more welded joints on a stainless steel pipe, the sampling points of the plurality of welded joint samples cover at least from the root to the top of any one of the welded joints, obtains the number of oxide inclusions per unit area in each welded joint sample, that is, the surface oxide inclusion density, obtains the critical pitting temperature of each welded joint sample by a critical pitting temperature test, and then, with the surface oxide inclusion density of each welded joint sample as an independent variable and the critical pitting temperature of each welded joint sample as a dependent variable, obtains a fitting function relational expression between the surface oxide inclusion density and the critical pitting temperature. By substituting the surface oxide inclusion density of the test welded joint obtained by welding with the same welding process in the test stainless steel pipe into the above fitting function relational expression, the standard critical pitting temperature of the test welded joint can be quickly obtained without performing a potentiostatic critical pitting temperature test on the test welded joint, thereby further saving the test time and test cost, and accurately and comprehensively evaluating the pitting corrosion resistance of the welded joints of the test stainless steel pipe. The method provided by the present invention has extremely high value for the service life of stainless steel pipes, especially for the service life as an offshore industrial oil and gas pipeline.

[0056] In actual use, by determining the height of the standard critical pitting temperature value of the welded joints welded by different welding methods or different heat input welding methods according to the method provided by the present invention, the pitting corrosion resistance of the welded joints welded by different welding methods or different heat input welding methods can be accurately evaluated. Furthermore, a welding method with a relatively high critical pitting temperature and relatively excellent pitting corrosion resistance can be selected for mass welding, thereby improving production efficiency.

[0057] In the present invention, a potentiostatic critical pitting temperature test is performed on the welded joint samples for which the surface oxide inclusion density has been obtained. By taking the surface oxide inclusion density of each welded joint sample as an independent variable and the critical pitting temperature of each welded joint sample as a dependent variable to obtain the fitting function relational expression between the surface oxide inclusion density and the critical pitting temperature, the pitting corrosion resistance of the welded joints of the test stainless steel pipes with the same welding process can be accurately evaluated, avoiding the problem of inaccurate evaluation results by immersion tests, without wasting more samples and test time, the pitting corrosion resistance of the welded joints of the test stainless steel pipes can be evaluated more quickly, comprehensively and accurately, and the reliability of the evaluation of the critical pitting temperature is improved.

[0058] As can be understood by those skilled in the art, the above description is only a preferred embodiment of the present invention and does not limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should all be included within the protection scope of the present invention.

Claims

1. A method for rapidly and comprehensively evaluating the pitting corrosion resistance of the welded joints of stainless steel pipes, comprising the following steps S1 to S5: S1: Collect a plurality of welded joint samples from different positions of one or more welded joints on the stainless steel pipe. The sampling points of the plurality of welded joint samples cover at least from the root to the upper part of at least one welded joint. Obtain the number of oxide inclusions per unit area in each welded joint sample, that is, the surface oxide inclusion density. S2: Obtain the critical pitting temperature of each welded joint sample by a critical pitting temperature test. S3: Taking the surface oxide inclusion density of each welded joint sample as an independent variable and the critical pitting temperature of each welded joint sample as a dependent variable, the fitting function relational expression between the surface oxide inclusion density and the critical pitting temperature: y = f(x) (where y is the critical pitting temperature, x is the surface oxide inclusion density, and f is the fitting function.) is obtained. S4: Collect a test welded joint sample from the test stainless steel pipe. The welding process of the test welded joint sample is the same as that of the welded joint sample. Obtain the number of oxide inclusions per unit area in the test welded joint sample, that is, the surface oxide inclusion density. S5: Substitute the surface oxide inclusion density of the test welded joint sample into the fitting function relational expression to obtain the standard critical pitting temperature of the test welded joint sample, thereby rapidly and comprehensively evaluating the pitting corrosion resistance of the welded joints of the test stainless steel pipe. A method characterized by this.

2. In step S1, the stainless steel includes any one of austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, duplex stainless steel, and age-hardening stainless steel. The method according to claim 1, characterized by this.

3. In step S1, the number of the plurality of welded joint samples is ≥5. The method according to claim 1, characterized by this.

4. In step S1, the number of the plurality of welded joint samples is ≥8. The method according to claim 3, characterized by this.

5. In step S1, the size of the welded joint sample is 10×10×B mm, where B≥5. The method according to claim 1, characterized by this.

6. In step S1, before obtaining the number of oxide inclusions per unit area in each welded joint sample, an electrolytic polishing treatment is performed. The method according to claim 1, characterized by this.

7. The method according to claim 1, wherein in step S2, the critical pitting temperature test is a potentiostatic critical pitting temperature test. **Claim 8** Before performing the potentiostatic critical pitting temperature test on the welded joint sample, pretreatment is performed, and the pretreatment The method according to claim 7, characterized by including the steps of polishing the welded joint sample, immersing it in an acetone solution, further rinsing it with an alcohol solution, and drying it. **Claim 9** The method according to claim 1, wherein in step S3, the analysis method of the fitting function relational expression includes any one of linear regression analysis and polynomial regression analysis. **Claim 10** In step S3, the fitting function relational expression is y = ax ２ + bx + c where y is the critical pitting temperature, x is the surface oxide inclusion density, and a, b, and c are fitting parameters. The method according to claim 9.

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