Corrosion test method for bimetal composite pipe

Abstract

The invention discloses a corrosion test method of a bimetal composite pipe, the bimetal composite pipe comprises a carbon steel material base layer and a corrosion-resistant alloy material inner coating, and the method comprises the following steps: determining the size of a sample and the shape and size of the inner coating or base layer surface defect; cutting a flat sample containing a weld joint, a base layer and an inner coating from the bimetal composite pipe; prefabricating defects, so that the interface of the inner coating and the base layer is exposed and can be in contact with a corrosive medium; wrapping other surfaces, except the defect surface, of the prefabricated defective sample; carrying out a corrosion test on the wrapped sample; the corrosion conditions of a welding seam and a body after the inner coating of the bimetal composite pipe is damaged to different degrees are simulated, and an important technical means is provided for corrosion behavior research and adaptability evaluation of the bimetal composite pipe.

Description

Technical Field

[0001] The invention relates to the technical field of metal material performance testing, and in particular to a corrosion testing method for a bimetallic composite pipe. Background Art

[0002] During the development of highly sulfuric acid gas fields, oil casing perforations and surface gathering and transportation pipelines frequently leak due to corrosion. The corrosion of surface gathering and transportation pipelines in acidic gas fields not only brings huge economic losses to oil and gas development, but also brings about a large number of safety and environmental protection issues. In order to prevent and control the corrosion of oil and gas field pipes, the most reliable method at present is to use corrosion-resistant alloy pipes. The inner-coated corrosion-resistant alloy pipe includes a base layer and an inner coating layer, wherein the base layer is generally high-strength, low-cost carbon steel or alloy steel, and the inner coating layer is generally a corrosion-resistant alloy such as stainless steel or nickel-based alloy with good corrosion resistance. The corrosion resistance of the bimetallic composite pipe is mainly determined by the corrosion resistance of the corrosion-resistant alloy of the inner coating layer.

[0003] The inner coating has good corrosion resistance and usually does not cause uniform corrosion and induce failure. However, in actual service, since the inner coating is about 3mm thin, it is easily damaged by mechanical damage during cleaning. In addition, high H2S and high Cl - Under harsh working conditions with multiple medium coupling such as elemental sulfur deposition, the inner coating may be at risk of pitting and stress corrosion cracking. Once the inner coating is perforated or cracked, the medium enters the inner coating / base interface, and the inner coating and base have galvanic corrosion due to dissimilar metal contact, which will accelerate the corrosion of carbon steel and cause the pipe body itself to fail, causing huge economic losses to the efficient development of oil and gas fields. In addition, bimetallic composite pipe welding is dissimilar metal welding, and it is usually a combination of outer carbon steel + inner corrosion-resistant alloy. The melting point, density, thermal expansion coefficient, crystallographic characteristics, mechanical properties, etc. of the inner and outer layers are quite different, and the welding quality is difficult to control and the welding is difficult. As a result, the weld is the weakest part of the composite pipe performance. It is subject to load during the service of the pipeline and is prone to corrosion cracking. For the base carbon steel, the outer protective layer will age and break under long-term service, and external corrosion of the pipe wall will occur.

[0004] In summary, it is particularly important to comprehensively evaluate pitting pits, crack damage and their accelerated corrosion behavior on the entire pipe in the laboratory. At present, the corrosion resistance of bimetallic composite pipes is evaluated by sampling the inner coating corrosion-resistant alloy body and welds, and conducting uniform weight loss corrosion, sulfide stress corrosion cracking and stress corrosion cracking tests. The existing technology still has great limitations, mainly manifested in the following ways: conventional sampling is only for the inner coating of bimetallic composite pipes, and the test evaluation can only represent the corrosion resistance of the inner coating corrosion-resistant alloy material, and cannot accurately and comprehensively reflect the corrosion of the entire composite pipe pipeline and welds after the inner coating is damaged under actual working conditions, and the corrosive medium penetrates into the contact base carbon steel. Summary of the invention

[0005] The technical problem to be solved by the present invention is that the conventional sampling for evaluating the corrosion resistance of bimetallic composite pipes is only for the inner coating of the bimetallic composite pipes, and the test evaluation can only represent the corrosion resistance of the corrosion-resistant alloy material of the inner coating, and cannot accurately and comprehensively reflect the corrosion of the entire composite pipe pipeline and welds caused by the corrosive medium penetrating and contacting the base carbon steel after the inner coating is damaged under actual working conditions.

[0006] The object of the present invention is to provide a corrosion test method for a bimetallic composite pipe, wherein the bimetallic composite pipe comprises a carbon steel material base layer and a corrosion-resistant alloy material inner coating layer, and is characterized in that the method comprises:

[0007] Determine the size of the specimen, shape and size of defects in the inner coating or base surface;

[0008] Taking the inner wall surface of the bimetallic composite pipe weld joint as the reference and perpendicular to the girth weld direction, a flat rectangular parallelepiped specimen including a weld area, a base layer and an inner coating is cut, the weld area is located in the middle of the specimen, and the surface roughness of the specimen is less than Ra 0.8 μm;

[0009] Prefabricate defects on the sample obtained by cutting so that the interface between the inner coating and the base layer is exposed to contact with the corrosive medium;

[0010] Wrapping the surfaces of the prefabricated defective specimen except the defective surface;

[0011] The wrapped samples were subjected to corrosion tests simulating the on-site working environment.

[0012] As a possible design, defects are prefabricated on the sample obtained by cutting so that the interface between the inner cover and the base layer is exposed, specifically:

[0013] The inner covering layer is provided with a prefabricated elongated groove, the bottom of which is the bonding surface between the inner covering layer and the base layer, and the elongated groove covers the weld zone, the heat affected zone and the body zone;

[0014] or;

[0015] Prefabricate at least two pits, at least one of which is opened on the inner covering layer and its bottom is the bonding surface between the inner covering layer and the base layer, and at least one of which is opened in the weld area;

[0016] or;

[0017] At least two pits are prefabricated, at least one pit is opened on the base layer and its bottom is the bonding surface of the inner covering layer and the base layer, and at least one pit is opened in the welding seam area.

[0018] As a possible design, the inner wall roughness of the dimples and the elongated grooves is less than Ra 0.8 μm.

[0019] As a possible design, the pit is circular with a diameter of 3 to 15 mm.

[0020] As a possible design, the material used for the wrapping is epoxy resin, which is temperature resistant above 80°C and resistant to H2S corrosion.

[0021] As a possible design, the samples can be divided into multiple groups according to different defect forms and defect sizes. The simulation of the on-site working environment should consider at least five environmental parameters: total pressure, hydrogen sulfide partial pressure, carbon dioxide partial pressure, temperature and liquid medium components.

[0022] As a possible design, the samples after corrosion are cut and observed in the corrosion test, including the metallographic structure, corrosion morphology and elemental composition of the corrosion products of the inner coating and base layer of the bimetallic composite tube after corrosion.

[0023] As a possible design, the determination of the sample size, the shape and size of the inner coating or base surface defect specifically includes:

[0024] Determine the area ratio of the base layer that needs to be exposed in the sample, the area and size of the base layer that needs to be exposed, and the area and size of the remaining inner covering layer.

[0025] As a possible design, the temperature during the wrapping process is 50-100°C, preferably 80-100°C.

[0026] The beneficial effects of the present invention are:

[0027] 1. Accurately simulate the risk and weak points of the bimetallic composite pipe on site, close to the actual working environment, and avoid the problem that the existing evaluation technology cannot directly restore the corrosion situation caused by the damage of the inner coating of the bimetallic composite pipe.

[0028] 2. According to actual needs, corrosion research can be carried out on the inner coating with different degrees of damage, providing technical means for scientific evaluation research.

[0029] 3. The extreme working condition simulation evaluation of the weld performance of bimetallic composite pipes can be carried out to provide support for the comprehensive evaluation of its adaptability and service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

[0031] Figure 1is a flow chart of a corrosion test method in an embodiment of the present invention;

[0032] Figure 2 Schematic diagram of the sampling position of the bimetallic composite tube in the embodiment of the present invention;

[0033] Figure 3 Schematic diagram of a prefabricated defective sample of a bimetallic composite pipe according to an embodiment of the present invention;

[0034] Figure 4 This is the macroscopic morphology of the sample after the corrosion test in the embodiment of the present invention;

[0035] Figure 5 This is the microscopic morphology of the corrosion of the inner coating / base layer cross section after the sample is cut open in the embodiment of the present invention.

[0036] Marks and corresponding parts names in the attached drawings:

[0037] In the figure, 1-inner covering layer, 2-base layer, 3-weld, 4-sampling position, 5-groove, 6-inner covering layer pit, 7-inner weld pit, 8-base layer pit, 9-outer weld pit, 10-inner covering layer / base layer interface. DETAILED DESCRIPTION

[0038] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0039] At present, the corrosion resistance of bimetallic composite pipes is mainly evaluated by evaluating the corrosion resistance of the inner coating corrosion-resistant alloy. In the existing technology, when evaluating the corrosion resistance of the corrosion-resistant alloy body and weld, first, a section of the pipe is sampled and various tests such as weight loss of hanging pieces and stress corrosion are carried out. Second, the samples are mainly placed in the corrosive liquid at room temperature, which cannot accurately obtain the corrosion resistance after the inner coating is damaged under actual corrosion conditions. Third, the existing evaluation methods have not yet evaluated the corrosion of the entire bimetallic composite pipe pipeline when the inner coating is damaged due to internal corrosion of the bimetallic composite pipe or mechanical reasons.

[0040] In response to the above problems, the inventors directly sampled the bimetallic composite pipe. The samples obtained not only included the inner coating and the base layer, as well as the bonding parts between the two, but also included the welds, thereby being able to accurately reflect the corrosion resistance of the bimetallic composite pipe, as follows.

[0041] A corrosion test method for a bimetallic composite pipe, the bimetallic composite pipe comprising a carbon steel base layer and a corrosion-resistant alloy inner coating layer, the method comprising the following steps:

[0042] S1. Determine the sample size, the shape and size of the inner coating or base surface defect, that is, the size of the groove opened in step S3;

[0043] like Figure 1 and Figure 2 As shown, S2. A flat sample including a weld, a base layer and an inner coating is cut from the bimetallic composite pipe, 1 is the inner coating, 2 is the base layer, 3 is the weld, and 4 is the sampling position;

[0044] like Figure 1 and Figure 3 As shown, S3. Prefabricate defects on the sample obtained by cutting so that the interface between the inner coating and the base layer is exposed to contact the corrosive medium;

[0045] Among them, the prefabrication defects are as follows:

[0046] like Figure 3 As shown in Figure A, a prefabricated elongated groove is provided, and the bottom of the elongated groove is the bonding surface between the inner covering layer and the base layer. The elongated groove covers the weld area, the heat affected zone and the body area. 5 is a prefabricated elongated groove, specifically a groove.

[0047] or;

[0048] At least two pits are prefabricated, at least one pit is opened on the inner covering layer and its bottom is the bonding surface of the inner covering layer and the base layer, and at least one pit is opened on the weld. Figure 3 Figure B is a schematic diagram of the prefabricated pits of the inner covering layer. There are two pits, one of which is on the inner covering layer and the other is on the weld. 6 is the inner covering pit and 7 is the inner weld pit. The diameter of the inner covering pit and the inner weld pit is generally 5 to 15 mm, preferably 10 to 15 mm, and more preferably 13 to 15 mm. The depth of the inner weld pit is generally about 6.5 mm.

[0049] or;

[0050] At least two pits are prefabricated, at least one pit is opened on the base layer and its bottom is the bonding surface of the inner covering layer and the base layer, and at least one pit is opened on the weld. Figure 3 Figure C is a schematic diagram of prefabricated pits in the base layer. There are two pits, one on the base layer and the other on the weld. 8 is the base layer pit and 9 is the outer weld pit. The diameter of the base layer pit and the outer weld pit is generally 5 to 15 mm, preferably 10 to 15 mm, and more preferably 13 to 15 mm. The depth of the outer weld pit is generally about 6.5 mm.

[0051] It should be noted that the aforementioned “inside” and “outside” refer to the inner covering layer as the inside and the base layer as the outside; therefore, an external weld pit refers to a pit made from one side of the base layer to the weld, with the pit opening facing one side of the base layer; an internal weld pit refers to a pit made from one side of the inner covering layer to the weld, with the pit opening facing one side of the inner covering layer.

[0052] like Figure 1 As shown, S4. Wrap the remaining surfaces of the cut specimen except the prefabricated defective surface;

[0053] S5. Conduct corrosion test on the wrapped samples.

[0054] Step S5 specifically includes:

[0055] like Figure 1 As shown, S5-1: prepare the corrosion liquid, NaCl 10000~20000mg / L, Na2CO3 is 1500~2000mg / L, Na2SO4 is 2000-3000mg / L, and the solvent is water; S5-2: the sample can be divided into multiple groups according to different defect forms and different defect sizes; the sample is placed in a high temperature and high pressure corrosion test device for 600~750h; S5-3: after the corrosion is completed, take out the sample and dry it; S5-4: observe the corrosion products and morphology after the sample is cut by wire cutting, including but not limited to the metallographic structure, corrosion morphology and elemental composition of the corrosion products after the inner coating and base layer of the bimetallic composite pipe are corroded. In the corrosion test, the total gas phase pressure P≥5MPa, the hydrogen sulfide partial pressure is 1~2MPa, the carbon dioxide partial pressure is 1.5~2MPa, and the inert gas.

[0056] When determining the size of the sample and the size of the groove in step S1, it should be noted that the size of the sample must ensure that the position of the weld (body, heat-affected zone, parent material) can be removed.

[0057] The sample acquisition in step S2 must comply with the provisions of GB-T13298 metallographic sample. In step S2, the "flatness" means that the sample has no curvature. Generally, the weld is located in the middle of the sample, which is close to the actual situation.

[0058] In step S3, the specific process of cutting to form the groove is as follows: first, two holes are drilled on the inner coating, and then the inner coating of the sample is cut off by wire cutting to form the groove, and the cutting starts from one hole and ends at the other hole. After the cutting is completed, if there are burrs on the cut surface, it needs to be polished to ensure that the roughness of the groove wall is less than Ra 0.8μm. It should be noted that during the cutting process, the inner coating in the groove must be completely removed.

[0059] In step S4, except for the surface with the groove, the rest of the surface of the sample needs to be wrapped. The material used for wrapping must have heat-resistant and corrosion-resistant properties, and the heat-resistant temperature is about 80 degrees Celsius. Among them, corrosion is mainly aimed at hydrogen sulfide corrosion. In general, the material used for wrapping is epoxy resin. Since epoxy resin is AB glue, when using it, mix component A and component B and apply them to the surface of the sample to be coated, and then keep it at room temperature for more than 24 hours to observe whether the epoxy resin is cured to form a dense protective film, so as to avoid contact between the base layer and the corrosive liquid when conducting the sample corrosion test, which affects the judgment of the corrosion test results.

[0060] Example

[0061] The parameters of the bimetallic composite pipe used in this example are: model L360QS / 825, pipe outer diameter: 323.9 mm, inner coating thickness: 3 mm, base thickness: 12 mm.

[0062] A corrosion test method for a bimetallic composite pipe, the bimetallic composite pipe comprising a carbon steel base layer and a corrosion-resistant alloy inner coating layer, the method comprising the following steps:

[0063] S1. Determine the sample size to be 8.0mm-12mm (length, ground surface) × 2.0mm (width) × 3.0mm (height, i.e. thickness direction). The inner coating defect is a long groove with a size of 40 to 80mm, a width of 2 to 5mm, and a depth of 3mm.

[0064] S2. The wire cutting method is used to sample the flat surface of the corrosion-resistant alloy, including the weld of the bimetallic composite pipe, which is located in the middle of the sample;

[0065] S3. Use wire cutting to cut a long specimen along the inner coating of the bimetallic composite pipe. There is no burr after cutting. Use a drilling machine to drill holes on both sides of the inner coating specimen, and then use wire cutting to cut a slice from the inner coating of the specimen to form a long groove.

[0066] S4. Except for the grooved surface, the other five surfaces of the cut rectangular specimen are wrapped with high temperature resistant and H2S corrosion resistant epoxy resin. The raw material of epoxy resin is 6420 epoxy resin AB glue. After wrapping, place it for 25 hours to form a sealing protective film after the epoxy resin is completely cured.

[0067] S5. Carry out corrosion evaluation test on step S4. The test requires that the total gas pressure P≥5MPa, the hydrogen sulfide partial pressure is 1-2MPa, the carbon dioxide partial pressure is 1.5-2MPa, the inert gas is 1.5-2.5MPa, and the liquid phase composition is: NaCl 20000mg / L, Na2CO3 is 1500mg / L, Na2SO4 is 2500mg / L and distilled water. After soaking in the high temperature and high pressure corrosion test device for 720 hours, take out, wash the sample, and dry it with a hair dryer.

[0068] S6. Cut the corrosion-completed sample obtained in step S5 by wire cutting, and observe the corrosion of the inner coating and the base layer. Figure 4 As shown, 10 is the interface between the inner covering layer and the base layer, Figure 4 In the figure, Figure A shows the corrosion morphology of the inner covering layer with grooves, and Figure B is a schematic diagram of the morphology observation after Figure A is cut open.

[0069] like Figure 5 As shown in the figure, the interface between the base layer and the inner coating is peeled and cracked, the base layer is severely corroded, a large amount of corrosion products are attached, and the inner coating has obvious pitting corrosion. It should be noted that: Figure 5 The line scan distribution diagram of each element on the right side of the middle, where the horizontal axis is the X-ray energy and the vertical axis is the signal intensity.

[0070] The present invention breaks through the existing corrosion evaluation method for bimetallic composite pipes, that is, only the inner coating is taken for corresponding corrosion evaluation, which cannot accurately and comprehensively reflect the corrosion of the entire composite pipe and weld after the inner coating is damaged under actual working conditions, and the corrosive medium penetrates and contacts the base carbon steel to the entire composite pipe pipeline and weld; the present invention uses a bimetallic composite pipe as a material, directly takes a sample from it (the sample includes not only the base and the inner coating, but also the weld), and also grooves the sample to simulate the corrosion of the base after the inner coating is damaged in actual application. In order to avoid the influence of the single exposed base on the evaluation test, the remaining surface except the groove is wrapped before the test. It can be seen that the corrosion test method for bimetallic composite pipes disclosed in the present invention can truly reflect the situation of bimetallic composite pipes in actual application, so it provides reliable technical support for practical application, and can also carry out corrosion research under different degrees of damage to the inner coating, providing technical means for scientific evaluation research.

[0071] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A corrosion test method for a bimetallic composite pipe, the bimetallic composite pipe comprising a carbon steel base layer and a corrosion-resistant alloy inner layer, characterized in that: The method comprises: Determine the size of the specimen, shape and size of defects in the inner coating or base surface; Taking the inner wall surface of the bimetallic composite pipe weld joint as the reference and perpendicular to the girth weld direction, a flat rectangular specimen including the weld area, base layer and inner coating is cut, and the surface roughness of the specimen is less than Ra 0.8μm; Prefabricate defects on the sample obtained by cutting so that the interface between the inner coating and the base layer is exposed to contact with the corrosive medium; Wrapping the surfaces of the prefabricated defective specimen except the defective surface; The wrapped samples were subjected to corrosion tests simulating the on-site working environment.

2. The corrosion test method according to claim 1, characterized in that: The weld zone is located in the middle of the specimen.

3. The corrosion testing method according to claim 1, characterized in that: The sample obtained by cutting is prefabricated with defects, so that the interface between the inner cover and the base layer is exposed, specifically: The inner covering layer has a prefabricated elongated groove, the bottom of which is the bonding surface between the inner covering layer and the base layer, and the elongated groove covers the weld zone, the heat affected zone and the body zone.

4. The corrosion testing method according to claim 1, characterized in that: The sample obtained by cutting is prefabricated with defects, so that the interface between the inner cover and the base layer is exposed, specifically: At least two pits are prefabricated, at least one pit is opened on the inner covering layer and its bottom is the bonding surface of the inner covering layer and the base layer, and at least one pit is opened on the weld area.

5. The corrosion testing method according to claim 1, characterized in that: The sample obtained by cutting is prefabricated with defects, so that the interface between the inner cover and the base layer is exposed, specifically: At least two pits are prefabricated, at least one pit is opened on the base layer and its bottom is the bonding surface of the inner covering layer and the base layer, and at least one pit is opened in the welding seam area.

6. The corrosion testing method according to claim 3, characterized in that: The inner wall roughness of the elongated groove is less than Ra0.8 μm.

7. The corrosion testing method according to claim 4 or 5, characterized in that: The inner wall roughness of the pit is less than Ra 0.8 μm, and the pit is circular with a diameter of 3 to 15 mm.

8. The corrosion testing method according to claim 1, characterized in that: The material used for wrapping is epoxy resin, which is temperature resistant above 80°C and resistant to H2S corrosion.

9. The corrosion testing method according to claim 1, characterized in that: The samples can be divided into multiple groups according to different defect forms and different defect sizes. The simulated field working environment should consider at least five environmental parameters, namely, total pressure, hydrogen sulfide partial pressure, carbon dioxide partial pressure, temperature and liquid medium components.

10. The corrosion testing method according to claim 1, characterized in that: In the corrosion test, the samples after corrosion were cut and observed, including the metallographic structure, corrosion morphology and elemental composition of the corrosion products of the inner coating and base layer of the bimetallic composite tube after corrosion.

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