Test Device and Test Method for Crevice Corrosion of Bimetallic Composite Materials

Abstract

The present application discloses a test device for crevice corrosion of a bimetallic composite material, which comprises an electrolytic cell and a crevice corrosion model arranged in the electrolytic cell. The crevice corrosion model includes a specimen and a crevice forming part arranged on the specimen. The crevice forming part includes a clamping cylinder and a crevice forming rod. A jack is arranged in the cylinder body of the clamping cylinder. One end of the crevice forming rod is inserted into the jack, and the other end forms a rough surface and presses against the specimen to form a detection crevice. The clamping cylinder is made of a non-metallic insulating material. The specimen is a bimetallic composite material plate. The present application also discloses a test method for crevice corrosion of a bimetallic composite material by using the above test device. By using the present application, a plurality of detection crevices can be formed at one time, thereby improving the uniformity of the widths of the respective crevices, improving the identity of the same batch of detection data, and reducing the influence caused by random errors.

Description

Technical Field

[0001] The present invention relates to a test device and a test method for crevice corrosion of a bimetallic composite material. Background Art

[0002] For a carbon steel / stainless steel bimetallic composite material, which uses a carbon steel base layer and a stainless steel cladding layer, this composite material combines the corrosion resistance of stainless steel and the high strength and high toughness of carbon steel. Moreover, the production process mainly includes a heating process, a plastic deformation process, and a machining process, resulting in less environmental pollution and belonging to a green and environmentally friendly production process. At the same time, since the thickness of the stainless steel layer of the composite material is generally 0.2 - 0.4 mm, compared with the integral stainless steel material, the cost of the corrosion-resistant material is greatly reduced, and its comprehensive cost performance is very high.

[0003] However, due to the existence of crevice corrosion, crevice corrosion is likely to occur between metals and between metals and non-metals, and crevice corrosion can occur in almost all media. The carbon steel / stainless steel bimetallic composite material cannot avoid the occurrence of crevice corrosion either.

[0004] To study crevice corrosion, numerous crevice corrosion models have been built according to different corrosion scenarios, but they mainly focus on the crevice corrosion of single-metal materials. In the crevice corrosion model, a large component and a specimen are used to form a detection crevice, and then they are immersed in a solution to corrode the specimen and conduct research. Since multiple surfaces of the specimen participate in the reaction simultaneously during the corrosion process, the reproducibility of the corrosion degree of the specimen is relatively low, and the results of almost every corrosion are different. Therefore, a large number of crevice corrosion tests need to be carried out and the test results need to be comprehensively processed, which not only increases the test cost and prolongs the test time, but also easily causes deviations in the test data.

[0005] In addition, when using the existing crevice corrosion model to conduct crevice corrosion tests on bimetallic composite materials, the uncontrollability of the detection data increases. During the detection, the electrochemical data fluctuates greatly and normal detection cannot be carried out, making the existing crevice detection model inapplicable to the crevice corrosion test of bimetallic materials. Summary of the Invention

[0006] To solve the above problems, the present invention first proposes a test device for crevice corrosion of a bimetallic composite material, which includes an electrolytic cell and a crevice corrosion model disposed within the electrolytic cell. The electrolytic cell is used to hold a test solution. The crevice corrosion model includes a specimen and a crevice forming portion disposed on one side in the thickness direction of the specimen. The crevice forming portion includes a clamping cylinder and a crevice forming rod. The clamping cylinder extends along a first axis direction. The two ends of the clamping cylinder in the first axis direction are respectively formed as a clamping end and a crevice end, and the crevice end faces the specimen. At least three jacks are provided in the cylinder body of the clamping cylinder. Each jack extends along the first axis direction, and each jack is a blind hole with an opening on the end face of the crevice end. Each jack is formed by concave from the end face of the crevice end towards the clamping end. One end of the crevice forming rod is inserted into the jack, and the other end of the crevice forming rod extends out of the jack. The end face of the crevice forming rod extending out of the jack is formed as a crevice forming surface, and the crevice forming surface is polished into a rough surface with sandpaper.

[0007] The clamping cylinder is made of a non-metallic insulating material; the specimen is a bimetallic composite material plate;

[0008] A bolt and a nut press the crevice forming portion against one side in the thickness direction of the specimen, so that the crevice forming surface presses against the surface of the specimen, and a detection crevice is formed between the crevice forming surface and the specimen. Specifically, the crevice forming rod includes a stainless steel rod, a carbon steel rod or a composite material rod, and the composite material rod is prepared from a carbon steel / stainless steel bimetallic material. In the same test, the materials of all the crevice forming rods participating in the test are the same.

[0009] To facilitate clamping the crevice forming portion on one side of the specimen, a bolt hole in the shape of a through hole can be provided on the specimen. After the bolt passes through the bolt hole and the central hole of the clamping cylinder, it is screwed onto the nut to press the crevice forming portion against one side of the specimen.

[0010] When polishing the crevice forming surface, sandpaper with a mesh size of 320 - 800 is used. In this application, no gasket is used to form the detection crevice.

[0011] At present, when conducting crevice corrosion tests, an annular gap forming component is generally used, and sometimes a nut is directly used as the gap forming component. After the bolt passes through the center hole of the gap forming component, the gap forming component is pressed on the sample, and a gasket is arranged between the gap forming component and the sample to form a detection gap with a set width. The inventor of the present application found in the study that due to the limitation of processing accuracy, a sealed contact cannot be formed between the gasket and the sample, and a tiny non-ideal gap will be formed between the gasket and the sample. The non-ideal gap is connected to the detection gap and affects each other, resulting in a large deviation in the obtained detection results. In addition, during the detection process of the existing test, during the clamping process of the bolt, since both the test and the gasket will be deformed during the compression process, the actual width of the detection gap formed is smaller than the set width.

[0012] In addition, existing theories believe that crevice corrosion mainly occurs in crevice with a width of 25-100 μm, and gaskets are used to facilitate the formation of detection crevice with corresponding width. However, the inventors of the present application found in their research that crevice corrosion can also occur in crevice with smaller width, and sometimes the crevice corrosion in crevice with smaller width is more serious. In order to form a detection crevice with smaller width, in the present application, a crevice forming rod is used to directly press against the sample, and during the detection, multiple detection crevice are formed at one time to obtain multiple corrosion samples, thereby eliminating the randomness brought by a single sample during the test.

[0013] In addition, in the existing test device, a gasket is used to form a detection gap. However, in actual industrial products, various seals are generally installed on the sealing surface. The seal will only form a gap in the end area of the outer peripheral surface, and the gap is irregular. On the non-sealing surface, welding, riveting or bolts are generally used to connect the two parts together. The gap formed between the two parts is actually caused by the uneven or rough contact surface of the two parts. There are contact points or contact surfaces of different sizes and shapes between the contact surfaces of the two parts, and the width of the gap between the two parts changes irregularly. The simulated gap formed by the gasket in the existing test device is a regular space, which leads to a large deviation between the test data and the actual corrosion situation.

[0014] In this application, in order to better simulate the gaps existing in actual industrial products, the gasket used to form the detection gap in the test device is cancelled, and the gap forming surface of the gap forming rod is polished with sandpaper to form a rough surface. Since there are criss-cross and different-depth micro grooves on the gap forming surface, a detection gap that is more in line with the actual situation can be formed between the gap forming surface and the sample. The use of sandpaper with different mesh counts can form gap forming surfaces with different roughness, and combined with the different locking forces generated by different torques when the nut is screwed, a detection gap with a width of 10-100μm can be formed.

[0015] With the existing test method, although multiple specimens can be put into the test liquid for testing at one time, due to the random error in the width of the detection gap formed by each specimen, the same batch of detection data obtained also has a certain random error, which also affects the accuracy of the corrosion data to a certain extent. In this application, multiple detection gaps are formed at one time. Since the same clamping cylinder can form multiple detection gaps at the same time, the uniformity of the width of each gap is improved, the identity of the same batch of detection data is improved, and the influence caused by random error is reduced.

[0016] When measuring the width of the detection gap, an optical microscope is used to take pictures of the detection gap, and then the width of the detection gap is measured. Since after grinding, the gap forming surface forms micro-grooves with different depths, the detection gap is actually a gap roughly in a sawtooth shape. Therefore, in this application, the average width of the detection gap is taken as the width of the detection gap. In this application, the arithmetic mean of the distances between the bottoms of 5-15 micro-grooves and the bimetallic composite plate is taken as the width of the detection gap.

[0017] To avoid direct contact between the bolt and the specimen, so that the electrochemical data generated during the corrosion of the bolt interfere with the electrochemical data generated during the corrosion of the specimen, a plastic gasket needs to be placed between the bolt and the specimen, and a plastic sheath with an isolation function is sleeved on the screw rod. The plastic sheath can be made of a heat shrinkable tube.

[0018] Specifically, when screwing the nut onto the bolt, the torque is 1-10 N·M. Different torques generate different pressures on the gap forming rod. Since the gap forming surface is a rough surface, under the extrusion of the pressure, the rough surface will deform, so as to form detection gaps with different widths between the rough surface and the specimen.

[0019] Furthermore, the gap forming rods are uniformly arranged along the barrel of the clamping cylinder. The gap forming rod is a solid rod with a circular radial cross-section. The diameter of the gap forming rod is 2.5-4 mm. The center distance between adjacent gap forming rods is 2-4 times the diameter of the gap forming rod, and the net distance between the gap end of the clamping cylinder and the specimen is ≥2 mm. Preferably, each group of gap forming rods includes 4-12 gap forming rods. To avoid the deformation of the jack caused by the extrusion of the gap forming rod, which affects the width of the detection gap, the gap forming rod is adhesively bonded in the jack to increase the connection area between the gap forming rod and the jack, reduce or eliminate the deformation of the jack, so as to ensure the consistency of the widths of the detection gaps.

[0020] In order to simulate the influence of liquid flow on the detection of crevice corrosion degree under realistic conditions, it is necessary to stir the test liquid to form flowing liquid. When the center distance between adjacent crevice-forming rods is too small, it will affect the infiltration amount of the test liquid into the detection crevice. When the center distance between adjacent crevice-forming rods is too large, it will affect the assembly amount of the crevice-forming rods. When the center distance between adjacent crevice-forming rods is 2-4 times the diameter of the crevice-forming rod, the assembly quantity of the crevice-forming rods on a single clamping cylinder can be increased while ensuring the effective infiltration of the test liquid into the detection crevice.

[0021] To avoid the influence of too small net distance between the crevice end of the clamping cylinder and the specimen on the smooth infiltration of the test liquid into the detection crevice, the net distance between the crevice end of the clamping cylinder and the specimen is at least 2 mm.

[0022] If the diameter of the crevice-forming rod is too large, although a larger area of detection crevice can be formed, when the area of the crevice-forming surface is too large, due to the machining accuracy error, the uniformity of the width of the formed detection crevice will become poor. If the diameter of the crevice-forming rod is too small, a sufficient area of detection crevice cannot be formed. When the diameter of the crevice-forming rod is 2.5-4 mm, it can ensure the uniformity of the width of the detection crevice to the greatest extent while ensuring the formation of a sufficient area of detection crevice.

[0023] Furthermore, the surface of the specimen is divided into a detection surface and a non-detection surface. One side, and only one side, in the thickness direction of the specimen is formed as the detection surface. An insulating layer is provided on the non-detection surface of the specimen, and the crevice-forming part is pressed against the detection surface. In the present application, except for one side in the thickness direction of the specimen as the detection surface, the remaining inner and outer surfaces of the specimen are all used as non-detection surfaces.

[0024] The inventor of the present application found in the experiment that when directly testing the bimetallic composite material, the electrochemical detection data is inaccurate. After a large number of experiments, it is found that during the corrosion test, electrochemical reactions will also occur between the two materials in the bimetallic composite material, and the carbon steel material itself will also react with the test liquid. The current generated by these reactions will be superimposed on the current generated during crevice corrosion, resulting in fluctuations in the electrochemical data. After shielding the side surface of the specimen with an insulating layer, the reaction between the bonding surface of the two materials in the specimen and the test liquid is avoided, ensuring the stability and accuracy of the detection data.

[0025] Furthermore, in order to simulate the influence of the test liquid in a flowing state on crevice corrosion under realistic conditions, a stirrer is installed in the electrolytic cell, and the stirrer is used to stir the test liquid forward and backward.

[0026] Secondly, the present application also provides a test method for crevice corrosion of a bimetallic composite material, which is carried out by using any one of the above test devices. The test method includes the following steps:

[0027] (1) First detection:

[0028] Use sandpaper to polish the gap-forming surface of the gap-forming rod, assemble the crevice corrosion model, and measure the width of the detection gap between the gap-forming surface and the specimen surface;

[0029] Immerse the crevice corrosion model, auxiliary electrode and reference electrode in the test solution in the electrolytic cell. The test solution submerges all the gap-forming rods. Connect the specimen, auxiliary electrode and reference electrode to the corresponding electrodes of the electrochemical tester respectively; then conduct the first electrochemical test; after the first electrochemical test is completed, take out the crevice corrosion model, observe the macroscopic and microscopic corrosion morphologies of the specimen, and process the crevice corrosion current and potential data measured by the electrochemical tester;

[0030] (2) Second detection: Conduct the first grinding on the detection surface of the specimen to grind off the first set thickness to form the first ground specimen, and immerse the crevice corrosion model carrying the first ground specimen in the test solution. The test solution submerges all the gap-forming rods. Connect the first ground specimen, auxiliary electrode and reference electrode to the corresponding electrodes of the electrochemical tester respectively, and then conduct the second electrochemical test; after the second electrochemical test is completed, take out the crevice corrosion model, observe the macroscopic and microscopic corrosion morphologies of the first ground specimen, and process the crevice corrosion current and potential data measured by the electrochemical tester;

[0031] (3) Repeat step (2) until the outer stainless steel layer of the bimetallic composite material is completely ground off.

[0032] In this test method, by continuously grinding, the stainless steel layer of the bimetallic composite material is gradually thinned to detect the crevice corrosion situation of the stainless steel layer under the penetration of the inner layer material, provide test data for the research and production of the bimetallic composite material, determine the appropriate thickness of the stainless steel layer in the bimetallic composite material under different environments, and reduce the corrosion aggravation of the stainless steel layer caused by the inner layer material.

[0033] Specifically, the test solution is a simulated seawater solution, which contains 22 - 25 g / L of NaCl, 4 - 6 g / L of MgCl2, 3 - 5 g / L of Na2SO4, 1 - 3 g / L of CaCl2, 0 - 1 g / L of KCl, 0 - 1 g / L of NaHCO3, and 0 - 1 g / L of KBr. Using this simulated solution, the test when seawater is used as the corrosion solution can be successfully completed.

[0034] Specifically, in order to study the influence of temperature change on crevice corrosion, the temperature of the test solution is 0 - 50 °C.

[0035] Further, to simulate the influence of seawater flow on crevice corrosion, during the test, the test solution was stirred following the following procedure:

[0036] (2.1) Start the stirrer, make the stirrer rotate forward, and continue for 5 - 10 minutes;

[0037] (2.2) Then make the stirrer rotate backward, and continue for 5 - 10 minutes;

[0038] (2.3) Make the stirrer rotate forward again, and continue for 5 - 10 minutes;

[0039] (2.4) Repeat steps (2.2) and (2.3) until the test is completed. Brief Description of the Drawings

[0040] Figure 1 is a schematic structural diagram of the test device for crevice corrosion of bimetallic composite materials.

[0041] Figure 2 is a schematic structural diagram of the crevice corrosion model.

[0042] Figure 3 Schematic structural diagram of the clamping cylinder.

[0043] Figure 4 is Figure 3 the top view of

[0044] Figure 5 is a schematic structural diagram of the sample.

[0045] Figure 6 is a schematic diagram of the cutting area when the specimen is cut during the measurement of the width of the detection crevice at different torques.

[0046] Figure 7 is Figure 6 the schematic diagram of the specimen after cutting shown in

[0047] Figure 8 is the optical microscope photograph of the detection crevice at a torque of 2 N·M.

[0048] Figure 9 is the optical microscope photograph of the detection crevice at a torque of 4 N·M.

[0049] Figure 10 is the optical microscope photograph of the detection crevice at a torque of 8 N·M.

[0050] Figure 11 is the photograph of the specimen after corrosion, where Figure 11 (a) is the corrosion morphology diagram of a crevice former forming eight corrosion areas on the sample at one time, Figure 11 (b) is Figure 11(a) Enlarged view of a corroded area. For clarity, a circle is used in Figure 11 (a) to identify the area to be enlarged.

[0051] Figure 12 is the cross-sectional view of the sample after corrosion is completed.

[0052] Figure 13 is the potentiodynamic cyclic polarization curve of the bimetallic specimen in the seawater solution. Detailed implementation manners

[0053] Example 1

[0054] First, the test device for crevice corrosion of the bimetallic composite material will be described below. Please refer to Figure 1 . This test device includes an electrolytic cell 50 and a crevice corrosion model 100 disposed in the electrolytic cell. Specifically, in this embodiment, the electrolytic cell 50 includes a cylindrical side wall 51, a bottom plate 52 installed at the bottom of the side wall, and a top plate 53 installed at the top of the side wall. An inlet / outlet 54 is provided on the top plate, and a clamp 55 is installed on the top plate. A stirrer 57 is rotatably installed at the bottom inside the electrolytic cell. The stirring shaft of the stirrer passes through the bottom plate in a sealed manner vertically downward and is installed with a motor 56. The motor is fixedly installed on the lower surface of the bottom plate. Driven by the motor, the stirrer can rotate forward or backward to perform forward stirring and reverse stirring on the test liquid. The motor in this embodiment uses a servo motor. To facilitate the installation of the motor, legs 59 are installed on the lower side of the bottom plate. This electrolytic cell is used to hold the test liquid.

[0055] Please refer to Figures 2 - 5 , in Figure 2 , the arrow S represents the first axis direction. The crevice corrosion model 100 includes a specimen 40 and a crevice forming part 110 disposed on one side in the thickness direction of the specimen 40. The thickness of the specimen 40 extends along the first axis direction. The structure of the crevice forming part 110 will be described below.

[0056] The crevice forming part 110 includes a clamping cylinder 10 and eight crevice forming rods 16. The clamping cylinder extends along the first axis direction. The radial cross-section of the cylinder body 11 of the clamping cylinder 10 is circular. The two ends of the clamping cylinder 10 in the first axis direction are respectively formed as a clamping end 101 and a crevice end 102, and the crevice end faces the specimen.

[0057] Eight jacks 12 are provided inside the cylinder body 11 of the clamping cylinder 10. The jacks 12 are blind holes with openings on the end faces at the gap ends, and each jack 12 is formed by concave downward from the end face at the gap end towards the clamping end. Eight gap forming rods 16 respectively correspond to the eight jacks 12. One end of each gap forming rod 16 is inserted into the jack, and the other end of the gap forming rod 16 extends out of the jack. The end face of the end of the gap forming rod extending out of the jack forms a gap forming surface, and this gap forming surface is polished into a rough surface with sandpaper. In this embodiment, 600-mesh sandpaper is used to polish the gap forming surface.

[0058] In this embodiment, the clamping cylinder is cast by mixing acrylic powder and curing agent with a mass ratio of 5:4. Eight jacks are provided on the cylinder body of the clamping cylinder, and the eight jacks are evenly arranged along the cylinder body of the clamping cylinder. The eight gap forming rods 16 are all made of 304 stainless steel rods. After the clamping cylinder is made, the eight gap forming rods 16 are respectively inserted into the eight jacks, and then the gap forming rods 16 are glued in the jacks with modified acrylate adhesive. In this embodiment, each gap forming rod 16 is a solid rod with a circular radial cross-section. The diameter of the gap forming rod is 3 mm, and the length is 5 mm. The length of the gap forming rod exposed out of the jack is 2 mm.

[0059] The eight gap forming rods 16 are evenly arranged along a virtual circle with a diameter of 16 mm, so that the center distance between adjacent gap forming rods is twice the diameter of the gap forming rod.

[0060] It can be understood that in other embodiments, the center distance between adjacent gap forming rods can also be 2.5 times, 3 times, 3.5 times or 4 times the diameter of the gap forming rod, or other multiples between 2 and 4 times.

[0061] In this embodiment, the specimen 40 is a carbon steel / stainless steel bimetal composite plate. A bolt hole 41 and a wire connection hole 42 in the shape of a through hole are formed in the specimen along the thickness direction. For easy clamping, an insulating rubber sleeve 45 is wrapped around one end of the specimen. The inner diameter of the bolt hole 41 is 10 mm. After the crevice corrosion model is placed in the electrolytic cell, the clamping jaw 55 is clamped on the specimen through the rubber sleeve 45, so that the crevice corrosion model is suspended in the electrolytic cell. It can be understood that in another embodiment, insulating paint can also be applied to the clamping part of the specimen, and the clamping jaw is clamped on the area coated with insulating paint.

[0062] To avoid corrosion of the intermediate layer and the remaining areas of the bimetallic composite material plate during the test, which may interfere with the stability of the electrochemical data, in this embodiment, only one side in the thickness direction of the specimen is used as the detection surface, and all other surfaces of the specimen except the detection surface are used as non-detection surfaces. That is, the non-detection surfaces include the other side in the thickness direction, the outer peripheral surface, and the inner peripheral surfaces of the bolt holes and wire connection holes. Epoxy resin AB glue is used to coat all non-detection surfaces to avoid interfering with the detection of electrochemical data and affecting the stability of the electrochemical data.

[0063] The bolt 31 passes through the central hole 13 of the clamping cylinder 10 and the bolt hole 41 in sequence and is then screwed with a nut 32. That is, the bolt and the nut press the gap forming part on one side of the specimen, so that the gap forming surface presses against the surface of the specimen, and a detection gap is formed between the gap forming surface and the specimen. In this embodiment, when screwing the nut, the torque is set to 4 N·M. It can be understood that in other embodiments, the torque can also be 1 N·M, 2 N·M, 5 N·M, 8 N·M or 10 N·M. Of course, it can also be other torque values between 1 - 10 N·M. In this embodiment, the bolt 31 is specifically an M8 stainless steel bolt, and a plastic sheath is sleeved on the screw rod of the bolt 31, and a polytetrafluoroethylene gasket is placed between the bolt and the specimen to avoid direct contact between the bolt 31 and the specimen. The plastic sheath is made of polyvinyl chloride heat shrinkable tube.

[0064] The auxiliary electrode 61 and the reference electrode 62 are fixed on the top plate 53, and the detection ends of the auxiliary electrode 61 and the reference electrode 62 both extend into the electrolytic cell 50. The compression bolt 421 is inserted into the wire connection hole 42 to connect the conductive copper wire 422 to the specimen 40, and then the conductive copper wire 422, the auxiliary electrode 61 and the reference electrode 62 are respectively connected to the corresponding electrodes of the electrochemical tester 300.

[0065] Please refer to Figure 6 and Figure 7 , the following describes the measurement method of the width of the detection gap: Use sandpaper with a set mesh number to polish the gap forming surface to form a rough surface, then complete the assembly of the gap corrosion model and make the torque of the nut at the set value, and then cut or polish the specimen to form a measurement end surface on the specimen, so that the measurement end surface is tangent to the outer peripheral surface of the gap forming rod. Figure 6 In, the mark 46 represents the measurement end surface of the specimen, and this measurement end surface is tangent to the outer peripheral surfaces of the two gap forming rods to be able to measure the widths of the two detection gaps. The mark 47 represents the cutting or polishing area of the specimen. After cutting or polishing the specimen, specifically as Figure 7 shown. Of course, the same specimen can also be cut and polished separately.

[0066] After forming the measurement end face, then use an optical microscope to take a photo of the contact surface between the specimen and the gap-forming rod along the direction perpendicular to the measurement end face, that is, take a photo of Figure 7 the area pointed by arrow B in

[0067] to obtain a photo of the detection gap, measure the distance between the bottom of the micro-groove in the detection gap and the specimen, calculate the arithmetic mean of the distances between the bottoms of each micro-groove and the specimen, and this arithmetic mean is used as the width of the detection gap formed by polishing with a certain set mesh number of sandpaper and at a certain set torque. In this embodiment, specifically, the arithmetic mean of the distances between the bottoms of 5 micro-grooves and the specimen is used as the width of a certain detection gap.

[0068] In this embodiment, use 600-mesh sandpaper to polish the gap-forming surface and at a torque of 4 N·M to obtain a photo of the detection gap as shown in Figure 9 where the gap-forming rod is on the left, the specimen is on the right, and the middle shaded area is the detection gap. Take the distances between the bottoms of 5 micro-grooves and the specimen on the photo, which are 30.8 μm, 23.4 μm, 23.4 μm, 33.0 μm, and 40.4 μm respectively. The arithmetic mean of the 5 distances is 30.2 μm, that is, when polished with 600-mesh sandpaper and the torque is 4 N·M, the width of the detection gap is 30.2 μm. Figure 9 Take photos of the obtained detection gaps at torques of 2 N·M and 8 N·M respectively, and obtain photos of the detection gaps as shown in

[0069] where the width of the detection gap obtained when the torque is 2 N·M is 47.0 μm, and the width of the detection gap obtained when the torque is 8 N·M is 18.2 μm. Figure 8 and Figure 10 respectively.

[0070] Example 2

[0071] The test method for crevice corrosion of the bimetallic composite material will be described below. This test method is carried out with the above test device and includes the following steps:

[0072] (1) First detection:

[0073] The gap-forming surface of the gap-forming rod is polished with sandpaper, the crevice corrosion model is assembled, and the width of the detected gap between the gap-forming surface and the specimen surface is measured; in this embodiment, the crevice corrosion model in Embodiment 1 is directly used, the gap-forming rod is polished with 600-mesh sandpaper, and the torque of the nut is 4 N·M.

[0074] The crevice corrosion model, the auxiliary electrode and the reference electrode are immersed in the test solution in the electrolytic cell, the test solution submerges all the gap-forming rods, and the specimen, the auxiliary electrode and the reference electrode are respectively connected to the corresponding electrodes of the electrochemical tester; after the first test is completed, the crevice corrosion model is taken out, the macroscopic and microscopic corrosion morphologies of the specimen are observed, and the crevice corrosion current and potential data measured by the electrochemical tester are processed.

[0075] (2) Second detection: The detection surface of the specimen is ground for the first time to grind off the first set thickness to form a first ground specimen, and the crevice corrosion model carrying the first ground specimen is immersed in the test solution, the test solution submerges all the gap-forming rods, and the first ground specimen, the auxiliary electrode and the reference electrode are respectively connected to the corresponding electrodes of the electrochemical tester; after the second test is completed, the crevice corrosion model is taken out, the macroscopic and microscopic corrosion morphologies of the first ground specimen are observed, and the crevice corrosion current and potential data measured by the electrochemical tester are processed.

[0076] (3) Repeat step (2) until the outer stainless steel layer is completely ground off.

[0077] In this embodiment, the test solution is a simulated seawater solution, and the simulated seawater solution includes 22 - 25 g / L NaCl, 4 - 6 g / L MgCl2, 3 - 5 g / L Na2SO4, 1 - 3 g / L CaCl2, 0 - 1 g / L KCl, 0 - 1 g / L NaHCO3, 0 - 1 g / L KBr; the temperature of the test solution is 0 - 50 °C.

[0078] During the test, the test solution is stirred, and the stirring is carried out according to the following process:

[0079] (2.1) Start the stirrer to make the stirrer rotate forward for 8 minutes.

[0080] (2.2) Then make the stirrer rotate backward for 8 minutes.

[0081] (2.3) Make the stirrer rotate forward again for 8 minutes.

[0082] (2.4) Repeat steps (2.2) and (2.3) until the test is completed.

[0083] After the test is completed, the corrosion area of the specimen is detected macroscopically and microscopically to obtain Figure 11and Figure 12 the corrosion coupon diagram shown in the figure, where Figure 11 is the top view photo of the sample corrosion area, Figure 11 in the attached drawing (a) of Figure 11 is the corrosion morphology diagram of forming eight corrosion areas on the sample at one time by using a single crevice former in this embodiment, Figure 11 in the attached drawing (b) of Figure 11 is the enlarged view of one of the corrosion areas in the attached drawing (a), and a circle is used to represent the enlarged corrosion area in the attached drawing (a). Figure 12 is the cross-sectional view of one of the corrosion areas.

[0084] During the test, at the first detection, a potentiodynamic cyclic polarization curve diagram as shown in Figure 13 is obtained. It can be seen from the figure that the repassivation potential of the bimetallic composite plate is around -0.5V. The repassivation potential is an electrochemical parameter for measuring crevice corrosion. The more positive the repassivation potential is, the better the repassivation ability of the bimetallic composite plate is, and the better the crevice corrosion resistance is. In each subsequent detection, a similar potentiodynamic cyclic polarization curve diagram can be obtained.

[0085] It shows that the test device and test method in this application can successfully complete the crevice corrosion test of the bimetallic composite material.

Claims

1. A test device for crevice corrosion of a bimetallic composite material, characterized in that, It includes an electrolytic cell and a crevice corrosion model disposed inside the electrolytic cell. The electrolytic cell is used to hold a test solution. The crevice corrosion model includes a specimen and a crevice forming part provided on one side in the thickness direction of the specimen. The crevice forming part includes a clamping cylinder and a crevice forming rod. The clamping cylinder extends along the first axis direction. The two ends of the clamping cylinder in the first axis direction are respectively formed as a clamping end and a crevice end, and the crevice end faces the specimen. At least three jacks are provided inside the cylinder body of the clamping cylinder. Each jack extends along the first axis direction, and each jack is a blind hole with an opening on the end face of the crevice end. Each jack is formed by being recessed from the end face of the crevice end towards the clamping end. One end of the crevice forming rod is inserted into the jack, and the other end of the crevice forming rod extends out of the jack. The end face of the end of the crevice forming rod extending out of the jack is formed as a crevice forming surface, and the crevice forming surface is polished with sandpaper into a rough surface. The crevice forming rods are uniformly arranged along the cylinder body of the clamping cylinder. The crevice forming rods are solid rods with a circular radial cross-section. The diameter of the crevice forming rods is 2.5 - 4 mm. The center distance between adjacent crevice forming rods is 2 - 4 times the diameter of the crevice forming rods, and the net distance between the crevice end of the clamping cylinder and the specimen is ≥ 2 mm. The clamping cylinder is made of a non-metallic insulating material; the specimen is a bimetallic composite plate. Bolts and nuts press the crevice forming part on one side in the thickness direction of the specimen, so that the crevice forming surface presses against the surface of the specimen, and a detection crevice is formed between the crevice forming surface and the specimen; the width of the detection crevice is 10 - 100 μm.

2. The test device according to claim 1, wherein When screwing the nut onto the bolt, the torque is 1 - 10 N·M.

3. The test device according to claim 1, wherein The surface of the specimen is divided into a detection surface and a non-detection surface. One side, and only one side, in the thickness direction of the specimen is formed as the detection surface. An insulating layer is provided on the non-detection surface of the specimen, and the crevice forming part presses against the detection surface.

4. The test device according to claim 1, characterized in that, A stirrer is installed inside the electrolytic cell, and the stirrer is used to perform forward stirring and reverse stirring on the test solution.

5. The test device according to claim 1, wherein The crevice forming rod includes a stainless steel rod, a carbon steel rod or a composite material rod, and the composite material rod is prepared from a carbon steel / stainless steel bimetallic material.

6. A test method for crevice corrosion of a bimetallic composite material, characterized in that, Using the test device according to any one of claims 1 - 5, the test method includes the following steps: (1) First detection: Use sandpaper to polish the crevice forming surface of the crevice forming rod, assemble the crevice corrosion model, and measure the width of the detection crevice between the crevice forming surface and the specimen surface. Immerse the crevice corrosion model, the auxiliary electrode and the reference electrode in the test solution inside the electrolytic cell. The test solution submerges all the crevice forming rods. Connect the specimen, the auxiliary electrode and the reference electrode to the corresponding electrodes of the electrochemical tester respectively, and then perform the first electrochemical test. After the first electrochemical test is completed, take out the crevice corrosion model, observe the macroscopic and microscopic corrosion morphologies of the specimen, and process the crevice corrosion current and potential data measured by the electrochemical tester. (2) Second detection: Grind the detection surface of the specimen for the first time to grind off the first set thickness to form a first ground specimen. Immerse the crevice corrosion model carrying the first ground specimen in the test solution. The test solution submerges all the crevice-forming rods. Connect the first ground specimen, the auxiliary electrode, and the reference electrode to the corresponding electrodes of the electrochemical tester respectively, and then conduct the second electrochemical test. After the second electrochemical test is completed, take out the crevice corrosion model, observe the macroscopic and microscopic corrosion morphologies of the first ground specimen, and process the crevice corrosion current and potential data measured by the electrochemical tester. (3) Repeat step (2) until all the outer stainless steel layers of the bimetallic composite material are ground off.

7. The test method according to claim 6, wherein the test solution is a simulated seawater solution, and the simulated seawater solution contains 22 - 25 g / L of NaCl, 4 - 6 g / L of MgCl2, 3 - 5 g / L of Na2SO4, 1 - 3 g / L of CaCl2, 0 - 1 g / L of KCl, 0 - 1 g / L of NaHCO3, and 0 - 1 g / L of KBr.

8. The test method according to claim 6, characterized in that, The temperature of the test solution is 0 - 50 °C.

9. The test method according to claim 6, characterized in that, During the test process, stir the test solution, and the stirring is carried out according to the following process: (2.1) Start the stirrer to make the stirrer rotate forward for 5 - 10 minutes; (2.2) Then make the stirrer rotate backward for 5 - 10 minutes; (2.3) Make the stirrer rotate forward again for 5 - 10 minutes; (2.4) Repeat steps (2.2) and (2.3) until the test is completed.

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