Method for prefabricating pitting corrosion on metal surface

Abstract

The invention relates to a method for prefabricating pitting corrosion on a metal surface, which comprises the following steps of: soaking metal in a corrosion solution, regularly taking out the metal, recording time and pitting corrosion depth, counting the pitting corrosion depth, and calculating the cumulative probability of the maximum pitting corrosion depth according to the definition of Gumbel extreme value distribution to obtain the central parameter of the maximum pitting corrosion depth under different soaking time conditions; fitting and establishing a pitting growth curve according to the center parameter of the maximum pitting depth; and the pitting depth is adjusted by controlling the soaking time. Compared with an acceleration method based on an electrochemical means, the method provided by the invention is not limited by experimental equipment, and can perform pitting corrosion on the prefabricated surface of a sample with a larger size under different conditions.

Description

Technical Field

[0001] The present invention belongs to the field of prefabricating pitting corrosion on a metal surface, and particularly relates to a method for prefabricating pitting corrosion on a metal surface with adjustable pitting depth. Background Art

[0002] Pitting corrosion is one form of metal corrosion. According to the growth process of pitting corrosion, the pitting corrosion process can be divided into pitting induction time and pitting growth time. It is more common to test stainless steel materials. The standard test method for testing and evaluating the pitting corrosion sensitivity of stainless steel materials is generally ASTM G48. However, the standard method is mainly for evaluating the pitting corrosion sensitivity of stainless steel. When pitting corrosion has occurred on the surface of stainless steel materials before or during service, or when the corrosion environment changes, the existing pitting pits will have a significant impact on the subsequent pitting growth. In order to prefabricate similar pitting corrosion on the specimen surface, it is necessary to accelerate the corrosion of the metal surface to shorten the experimental period and ensure the uniformity of the pitting pit depth to ensure the subsequent experimental effect.

[0003] Currently, the publicly available technical means can provide some references. For example, the pitting corrosion prefabrication method disclosed in patent (CN106918545B) accelerates the prefabrication of pitting corrosion through an electrochemical test method. However, the electrochemical method has certain requirements for the specimen size and experimental device. Summary of the Invention

[0004] In view of the foregoing problems, the present invention uses an immersion method to prefabricate pitting corrosion and adjusts the pitting depth by controlling the immersion time. And for the problem that the electrochemical method involved in the background art has limitations on the specimen size, the prefabrication of pitting corrosion by the immersion method is not limited by the size of the experimental device. Specifically, the present invention discloses a method for prefabricating pitting corrosion on a metal surface. The metal is immersed in a corrosion solution and regularly taken out to record the time and pitting depth. The pitting depth is statistically analyzed. According to the definition of the Gumbel extreme value distribution, the cumulative probability of the maximum pitting depth is calculated to obtain the central parameter of the maximum pitting depth under different immersion time conditions;

[0005] A pitting corrosion growth curve is fitted and established according to the central parameter of the maximum pitting depth;

[0006] The pitting depth is adjusted by controlling the immersion time.

[0007] Further, the metal is stainless steel, and it is immersed in an FeCl3 solution to ensure that the immersion temperature is 5°C above the critical pitting temperature.

[0008] Further, the stainless steel is 316L stainless steel, and the solution is an FeCl3 solution with a mass fraction of 6%.

[0009] Further, the metal is a Ni-based alloy, and the immersion liquid is a mixed solution of FeCl3 and HCl.

[0010] Further, the metal is an 825 nickel-based alloy, and the immersion liquid is a mixed solution of 6% FeCl3 and 1% HCl solution.

[0011] Further, the metal is brass, and the immersion liquid is an H2O2 solution.

[0012] Further, the immersion liquid is a 1 mol / L H2O2 solution.

[0013] Further, the cumulative probability F(Y) of the maximum pitting depth is:

[0014]

[0015] where Dmax is the maximum pitting depth, in μm; μ is the central parameter in the Gumbel extreme value distribution, in μm; and λ is the scale parameter in the extreme value distribution.

[0016] Further, the pitting depth statistics are performed by laser confocal or ultra-deep well microscopy to statistically analyze the pitting depth, or by comparing the depths of the bottom and the opening of the pit with a microscope to test the pitting depth.

[0017] Further, it is applied to simulate the pitting process of service pipelines in oil and gas fields.

[0018] The present invention accelerates the corrosion of the metal surface by means of immersion. Compared with the acceleration method based on electrochemical means, the method of the present invention is not restricted by experimental equipment and can prefabricate surface pitting for samples of larger sizes in different situations. On the other hand, the present invention uses different immersion liquids for different metal materials for acceleration. For stainless steel, an FeCl3 solution is used and the immersion temperature is ensured to be 5°C above the critical pitting temperature to increase the speed. The results show that its acceleration effect is better than the electrochemical acceleration method in the prior art.

[0019] In other cases, different immersion liquids are used for nickel alloys and brass, and excellent acceleration effects are also obtained. In addition, the present invention also considers the random effect in the pitting generation process, and fits the time-depth relationship according to the property that the pitting distribution conforms to the Gumbel extreme value distribution characteristics.

[0020] Other features and advantages of the present invention will be described in the subsequent specification, and part of them will be obvious from the specification, or will be understood by implementing the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the structures pointed out in the specification, the claims and the drawings. Brief Description of the Drawings

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0022] Figure 1 Shows the cumulative probability distribution diagram of the maximum pitting depth according to an embodiment of the present invention;

[0023] Figure 2 Shows the Gumbel extreme value distribution diagram of the maximum pitting depth according to an embodiment of the present invention;

[0024] Figure 3 Shows the curve diagram of the pitting growth kinetic equation according to an embodiment of the present invention;

[0025] Figure 4 Shows the curve diagram of the pitting growth kinetic equation of 825 according to an embodiment of the present invention;

[0026] Figure 5 Shows the curve diagram of the pitting growth kinetic equation of brass according to an embodiment of the present invention. Detailed implementation manners

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0028] Embodiment 1

[0029] The pitting prefabrication method disclosed in this embodiment mainly includes the following steps:

[0030] (1) Prepare a stainless steel prefabricated pitting FeCl3 solution. For stainless steel with poor corrosion resistance, the solution ratio is different in different embodiments.

[0031] In one embodiment, the steel is 304 stainless steel. The main process for preparing the FeCl3 solution is to dissolve 100 g of FeCl3·6H2O in 900 mL of deionized water to obtain a 6% mass fraction FeCl3 solution.

[0032] In some other embodiments, for stainless steels with good corrosion resistance, such as 2205, the prefabricated solution used is to dissolve 68.72 g of FeCl3·6H2O in 600 mL of deionized water and add 16 mL of analytical pure (36.5% - 38%) HCl to prepare a 6% FeCl3 and 1% HCl solution.

[0033] (2) Determination of the immersion test temperature. Usually, the prefabricated pitting temperature is selected as room temperature (25 °C).

[0034] In the embodiment of prefabricated pitting for super corrosion-resistant stainless steel, when conducting the immersion test, an electrochemical testing method is used to determine the critical pitting potential of the material in a 6% FeCl3 and 1% HCl solution, ensuring that the immersion temperature is 5 °C above the critical pitting temperature.

[0035] (3) Statistics of the maximum pitting depth. In order to precisely control the change in pitting depth of stainless steel in the prefabricated pitting solution, first, the immersion tests are selected as 72 h, 36 h, and 24 h. Five specimens (with a recommended size of 50 * 10 * 3 mm) are immersed in each test section. After soaking for 72 h, the corrosion products on the surface of the stainless steel are cleaned. A super-well-depth 3D microscope or a laser confocal microscope is used to statistically analyze the pitting depth. During the measurement process, note that pitting at the edges can be ignored to determine the maximum pitting depth on the surface of the specimen.

[0036] (4) Plotting of the pitting growth curve. The control of prefabricated pitting has the following relationship with the length of the immersion time:

[0037] D max = a(t - t ind ) b (Formula 2)

[0038] where D max is the maximum pitting depth, both a and b are constants, t ind is the pitting induction time, and t is the immersion time. Usually, the maximum pitting depth follows the Gumbel extreme value distribution. The specific operation is as follows:

[0039] Arrange the maximum pitting depths on the surface of the sample from smallest to largest. The cumulative probability distribution F(Y) can be expressed as:

[0040]

[0041] M is the serial number of the pitting depth arranged from smallest to largest; N is the number of maximum pitting depths.

[0042] According to the definition of the Gumbel extreme value distribution, the cumulative probability F(Y) of the maximum pitting depth can be expressed as:

[0043]

[0044] Where Dmax is the maximum pitting depth, in μm; μ is the central parameter in the Gumbel extreme value distribution, in μm; λ is the scale parameter in the extreme value distribution. According to the Gumbel extreme value distribution, the central parameter of the maximum pitting depth under different immersion times can be obtained, a relationship diagram between the immersion time and the pitting depth is plotted, and the key parameters of the pitting growth curve are determined by fitting with Equation 2.

[0045] (5) Plot the growth curve of the stainless steel in the prefabricated pitting solution. According to the pitting growth curve parameters obtained in step 3, the pitting growth formula is obtained, and then the immersion time is determined by inputting the target pitting depth.

[0046] (6) Verification experiment. According to the prefabricated pitting time obtained in step (5), the pitting of 5 specimens is prefabricated respectively. The maximum pitting depth on the surface of the sample is statistically analyzed by a super well depth 3D microscope or a laser confocal microscope and compared with the target value.

[0047] In a specific embodiment, the metal is 316L stainless steel commonly used in oil fields as an example for prefabricating pitting. The prepared solution is a 6% mass fraction FeCl3 solution. After soaking at 25 °C for 72 h, 48 h, 36 h, and 24 h, the maximum pitting depth on the surface of the sample is as Figure 1 shown, and the Gumbel extreme value distribution is used to determine the maximum pitting depth at different immersion times. The fitting results are as Figure 2 shown. The growth curve of the maximum pitting depth obtained according to different immersion times is as Figure 3 shown. At the same time, the fitting of the maximum pitting depth growth kinetic equation is:

[0048] D = 14.42 * (t - 4.93) 0.4 ;

[0049] Based on the pitting growth kinetic equation and the target pitting depth, the required immersion time t is derived.

[0050] In an embodiment, the soaking environment is heated. To prefabricate a pitting depth of 30 μm, only an immersion time of 11.16 h is required. As disclosed in the prior art (CN106918545B), it also takes nearly 48 h to accelerate by an electrochemical method. It can be seen that the method in this embodiment can effectively achieve corrosion acceleration and shorten the total experimental duration.

[0051] Example 2

[0052] The material of this embodiment is 825 nickel-based alloy commonly used in oil fields. The prefabricated pitting solution used in the experiment is a 6% FeCl3 and 1% HCl solution. After soaking for 24 h, 48 h, 72 h, and 96 h respectively, the Gumbel extreme value distribution is used to calculate the pitting depth. The final pitting growth kinetic equation is as Figure 4As shown

[0053] D = 4.47 * (t - 2.87)^0.33;

[0054] If a pitting depth of 30 μm needs to be prefabricated, the immersion time required is 119.56 h.

[0055] Example 3

[0056] The material of this example is the commonly used brass in oil fields. The prefabricated pitting solution used in the experiment is 1 mol / L H2O2 solution. After soaking for 24 h, 48 h, 72 h, and 96 h respectively, the Gumbel extreme value distribution is used to calculate the pitting depth. The final pitting growth kinetic equation is as Figure 5 As shown

[0057] D = 10.30 * (t - 14.31)^0.27;

[0058] If a pitting depth of 30 μm needs to be prefabricated, the immersion time required is 27.79 h.

[0059] As in the content of the foregoing three examples, the method is applied to the preparation of pipeline specimens in oil fields. In some cases, the overall situation of the pipeline needs to be considered. For example, simulating only locally the cracking of stainless steel caused by pitting during the service process of oil and gas fields may not have a good effect. The immersion method used in the foregoing examples can overcome the problem that various acceleration means by electrochemical methods in the prior art are limited by experimental equipment, and realize the pitting prefabrication of large samples.

[0060] Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for prefabricating pitting corrosion on a metal surface, characterized in that: Immerse the metal in the corrosion solution and take it out regularly to record the time and pitting depth. Statistically analyze the pitting depth. According to the definition of the Gumbel extreme value distribution, calculate the cumulative probability of the maximum pitting depth to obtain the central parameter of the maximum pitting depth under different immersion time conditions; Fit and establish a pitting growth curve based on the central parameter of the maximum pitting depth; Adjust the pitting depth by controlling the immersion time.

2. The method for prefabricating pitting corrosion on a metal surface according to claim 1, characterized in that: The metal is stainless steel, and it is immersed in an FeCl3 solution to ensure that the immersion temperature is 5 °C above the critical pitting temperature.

3. The method for prefabricating pitting corrosion on a metal surface according to claim 2, characterized in that: The stainless steel is 316L stainless steel, and the solution is an FeCl3 solution with a mass fraction of 6%.

4. The method for prefabricating pitting corrosion on a metal surface according to claim 1, wherein: The metal is a Ni-based alloy, and the immersion liquid is a mixed solution of FeCl3 and HCl.

5. The method for prefabricating pitting corrosion on a metal surface according to claim 4, characterized in that: The metal is 825 nickel-based alloy, and the immersion liquid is a mixed solution of 6% FeCl3 and 1% HCl solution.

6. The method for prefabricating pitting corrosion on a metal surface according to claim 1, wherein: The metal is brass, and the immersion liquid is an H2O2 solution.

7. The method for prefabricating pitting corrosion on a metal surface according to claim 6, wherein: The immersion liquid is a 1 mol / L H2O2 solution.

8. The method for prefabricating pitting corrosion on a metal surface according to claim 1, characterized in that, The cumulative probability F(Y) of the maximum pitting depth is: where Dmax is the maximum pitting depth, in μm; μ is the central parameter in the Gumbel extreme value distribution, in μm; λ is the scale parameter in the extreme value distribution.

9. The method for prefabricating pitting corrosion on a metal surface according to claim 1, characterized in that: For pitting depth statistics, laser confocal or ultra-deep well microscopy is used to statistically analyze the pitting depth, or a microscope is used to compare the depth changes at the bottom and the opening of the pit to test the pitting depth.

10. The method for prefabricating pitting corrosion on a metal surface according to any one of claims 1-9, characterized in that: It is applied to simulate the pitting process of service pipelines in oil and gas fields.

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