A high-throughput method for pitting corrosion evaluation of low alloy steels for oil and gas equipment

Abstract

The present application relates to a kind of oil and gas equipment with low alloy steel pitting high-throughput evaluation method, it includes the following steps: S1.on the basis of the composition of carbon steel, different metal elements are added to prepare a plurality of low alloy steel samples with different composition and record the overall exposed area of each sample;S2.Each low alloy steel sample is respectively immersed in autoclave simulating corrosive environment, and different time of immersion corrosion is carried out respectively;S3.Take out low alloy steel sample, remove the corrosion product on the surface of low alloy steel sample and obtain the local corrosion area of each low alloy steel sample, calculate the local corrosion percentage of each low alloy steel sample.The present application changes the sequential iteration of traditional material high-throughput characterization technology to parallel processing high-throughput characterization, can obtain the corrosion surface morphology of multiple groups of parallel samples in one experiment and obtain the local corrosion rate data of multiple groups of parallel samples once, improve the calculation efficiency of low alloy steel local corrosion rate.

Description

Technical Field

[0001] The present invention relates to the technical field of local corrosion evaluation methods for oil and gas equipment, and in particular to a high-throughput evaluation method for pitting corrosion of low-alloy steel used in oil and gas equipment. Background Art

[0002] A major technical challenge hindering the development of low-alloy steels is understanding their corrosion resistance mechanisms and evaluating their corrosion resistance. In the industrial sector, localized corrosion is a key issue causing equipment perforation. Localized corrosion is a type of corrosion that concentrates on a small area on the metal surface and penetrates deep into the metal. It has a long gestation period before nucleation, sometimes lasting several months or even years.

[0003] The pipeline steel used in traditional oil and gas production is primarily carbon steel, whose primary components are C, Si, Mn, and Fe. Currently, in the research and development of low-alloy steel for oil and gas equipment, the main focus of corrosion resistance evaluation is on uniform corrosion, while there are few methods for evaluating localized corrosion. Current research methods for localized corrosion primarily use static coupon weight loss methods and electrochemical corrosion depth methods to calculate localized corrosion rates on test samples. These methods have long test cycles, large errors, and cannot accurately determine the impact of corrosive media on localized corrosion.

[0004] Applying high-throughput concepts and methods to the field of material corrosion has become a widely discussed topic. Current research into improving the localized corrosion resistance of traditional low-alloy steel by adding small amounts of metallic elements to carbon steel primarily relies on a "one-at-a-time" characterization approach, resulting in long test cycles and inefficient calculation of localized corrosion rates. Therefore, developing a low-cost, easily scalable method for rapidly calculating the localized corrosion rate of low-alloy steel is an urgent technical challenge in this field. Summary of the Invention

[0005] Based on this, it is necessary to provide a high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment, targeting the current technical problems in the research and development of low-alloy steel for oil and gas equipment, in which a small amount of metal elements are appropriately added to the low-alloy steel composition to improve the low-alloy steel's resistance to local corrosion. The high-throughput characterization method mainly adopts a "one at a time" characterization method, which has a long test cycle and low efficiency in calculating the local corrosion rate.

[0006] The present invention provides a high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment, comprising the following steps:

[0007] S1. Prepare a variety of low alloy steel samples with different compositions and record the overall exposure area S of each sample;

[0008] S2. Each low alloy steel sample was immersed in an autoclave simulating a corrosive environment and subjected to immersion corrosion for different periods of time;

[0009] S3. Take out the low alloy steel samples, remove the corrosion products on the surface of the low alloy steel samples and obtain the local corrosion area S of each low alloy steel sample l , calculate the local corrosion percentage of each low alloy steel sample

[0010] In a preferred embodiment of the present invention, in step S1, the low alloy steel sample preparation includes the following process:

[0011] Cr element was added to the composition of carbon steel to prepare low alloy steel sample 1;

[0012] Cr and Ni elements were added to the composition of carbon steel to prepare low alloy steel sample 2;

[0013] Cr, Ni and Mo elements were added to the composition of carbon steel to prepare low alloy steel sample three.

[0014] In a preferred embodiment of the present invention, the low alloy steel sample 1 includes the following components by mass fraction: 0.18%-0.23% C, 0.15%-0.35% Si, 0.3%-0.6% Mn, 3.11% Cr, and the balance Fe;

[0015] and / or, the low alloy steel sample 2 includes the following components by mass: 0.18%-0.23% C, 0.15%-0.35% Si, 0.3%-0.6% Mn, 2.95% Cr, 1.14% Ni, and the balance Fe;

[0016] And / or, the low alloy steel sample 3 includes the following components in mass fractions: 0.18%-0.23% C, 0.15%-0.35% Si, 0.3%-0.6% Mn, 3.06% Cr, 1.05% Ni, 0.49% Mo, and the balance Fe.

[0017] In a preferred embodiment of the present invention, the low alloy steel sample 1 includes the following components by mass fraction: 0.21% C, 0.22% Si, 0.35% Mn, 3.11% Cr, and the balance Fe;

[0018] and / or, low alloy steel sample 2 includes the following components by mass: 0.19% C, 0.22% Si, 0.44% Mn, 2.95% Cr, 1.14% Ni, and the balance Fe;

[0019] And / or, the low alloy steel sample 3 includes the following components in mass fractions: 0.23% C, 0.21% Si, 0.52% Mn, 3.06% Cr, 1.05% Ni, 0.49% Mo, and the balance Fe.

[0020] In a preferred embodiment of the present invention, in step S1 , the overall exposed area S of each sample is the same, and / or the overall exposed area S is 40 mm×15 mm×5 mm.

[0021] In a preferred embodiment of the present invention, in step S2, each sample is subjected to immersion corrosion for a time period of t1, t2, t3, and t4, respectively, and t1, t2, t3, and t4 increase in sequence.

[0022] In a preferred embodiment of the present invention, t1, t2, t3, and t4 are 6 hours, 24 hours, 72 hours, and 168 hours, respectively.

[0023] In a preferred embodiment of the present invention, in step S2, the process of simulating a corrosive environment is as follows: adding a corrosive medium into a reactor and continuously introducing carbon dioxide gas into the corrosive medium to maintain a saturated state, and heating the medium to a temperature T.

[0024] In a preferred embodiment of the present invention, the temperature T is 90° C.-180° C., and / or the corrosive medium is a NaCl solution with a concentration of 0.2 mol / L.

[0025] In a preferred embodiment of the present invention, step S3 includes the following process:

[0026] S31. Take out the low alloy steel samples and use scanning electron microscope to capture the surface morphology of the corrosion products of each sample at the same magnification. Use Image J software to process the surface morphology of the sample after corrosion into a binary image and calculate the corrosion product coverage area S of each low alloy steel sample. c , calculate the surface coverage percentage of corrosion products of each low alloy steel sample

[0027] S32. Remove corrosion products from the surface of the low-alloy steel samples and use a scanning electron microscope to capture the surface morphology of each sample at the same magnification after the corrosion products are removed. Use Image J software to process the surface morphology of each sample after the corrosion products are removed into a binary image, and calculate the local corrosion area S of each low-alloy steel sample. l , calculate the local corrosion percentage of each low alloy steel sample

[0028] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0029] 1. The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment of the present invention evaluates and studies the effect of adding a small amount of metal elements on the local corrosion resistance of low-alloy steel. The traditional high-throughput material characterization technology that adopts "one at a time" sequential iteration is changed to high-throughput characterization with parallel processing. The corrosion surface morphology of multiple groups of parallel samples can be obtained in one experiment, and the local corrosion rate data of multiple groups of parallel samples can be obtained at one time. The characterization data of a large number of samples can be collected in parallel and integrated analysis can be performed in a single time by multiple devices, which greatly saves experimental time and improves the calculation efficiency of the local corrosion rate of low-alloy steel. At the same time, the effect of the addition of different metal elements on the local corrosion resistance of low-alloy steel can be quickly obtained in one experiment.

[0030] 2. The method of the present invention takes into account the accelerated anodic dissolution and membrane destruction under high temperature and / or high pressure environments, and that low alloy steel materials are still susceptible to CO2 corrosion under high temperature and / or high pressure environments. Cr is added to carbon steel, and then some more noble alloying elements (such as Ni, Mo, etc.) are appropriately added. The low alloy steel samples are subjected to immersion corrosion experiments for different times to determine the evolution and development of the corrosion process over time under static conditions, revealing the actual corrosion process of low alloy steel; based on the high-throughput screening method, the corrosion pit area is compared with the overall exposed area, and the local corrosion percentage is calculated to obtain the local corrosion degree of the low alloy steel, which confirms that the addition of Ni and Mo elements helps to improve the corrosion resistance of low chromium steel, especially the pitting corrosion resistance.

[0031] 3. The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment of the present invention uses a scanning electron microscope to capture the sample surface morphology at the same magnification, and uses Image J software to process the sample surface morphology into a binary image, thereby calculating the corrosion product coverage area and the local corrosion zone area of ​​each low-alloy steel sample. The corrosion product coverage percentage and local corrosion percentage of the low-alloy steel sample are then calculated based on the corrosion product coverage area and the local corrosion zone area, thereby analogically obtaining the local corrosion rate of the sample, thereby improving the accuracy of the experimental results and reducing experimental errors. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a flow chart of a high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment according to Example 1 of the present invention;

[0033] Figure 2 Surface morphology of the low alloy steel samples covered with corrosion products formed in Example 2 of the present invention after being immersed in a corrosive environment at 90°C for different times;

[0034] Figure 3 This is a graph showing the percentage of surface coverage of corrosion products formed on each low alloy steel sample in Example 2 of the present invention when immersed in a corrosive environment at 90°C;

[0035] Figure 4 Surface morphology of corrosion pits formed when each low alloy steel sample is immersed in a 90°C corrosive environment for different times in Example 2 of the present invention;

[0036] Figure 5 This is a graph showing the percentage of localized corrosion formed when each low alloy steel sample is immersed in a corrosive environment at 90° C. in Example 2 of the present invention;

[0037] Figure 6 Surface morphology of the low alloy steel samples covered with corrosion products formed in Example 3 of the present invention after being immersed in a corrosive environment at 180°C for different times;

[0038] Figure 7 This is a graph showing the percentage of surface coverage of corrosion products formed on each low alloy steel sample in Example 3 of the present invention when immersed in a corrosive environment at 180°C;

[0039] Figure 8 Surface morphology of corrosion pits formed on low alloy steel samples in Example 3 of the present invention after being immersed in a 180° C. corrosive environment for different times;

[0040] Figure 9 This is a graph showing the percentage of local corrosion formed when each low alloy steel sample is immersed in a corrosive environment at 180°C in Example 3 of the present invention. DETAILED DESCRIPTION

[0041] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0042] Example 1

[0043] Please refer to Figure 1 This embodiment provides a high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment, comprising the following steps:

[0044] S1. Prepare a variety of low alloy steel samples with different compositions and record the overall exposure area S of each sample.

[0045] The low alloy steel sample preparation includes the following processes:

[0046] Cr was added to the carbon steel composition to prepare low alloy steel sample 1. Low alloy steel sample 1 included the following components by mass: 0.18%-0.23% C, 0.15%-0.35% Si, 0.3%-0.6% Mn, 3.11% Cr, and the balance Fe.

[0047] Cr and Ni elements were added to the carbon steel composition to prepare low alloy steel sample 2. Low alloy steel sample 2 included the following composition by mass: 0.18%-0.23% C, 0.15%-0.35% Si, 0.3%-0.6% Mn, 2.95% Cr, 1.14% Ni, and the balance Fe.

[0048] Cr, Ni, and Mo elements were added to the carbon steel composition to prepare low alloy steel sample 3. Low alloy steel sample 3 contains 0.18%-0.23% C, 0.15%-0.35% Si, 0.3%-0.6% Mn, 3.06% Cr, 1.05% Ni, 0.49% Mo, and the balance is Fe.

[0049] The overall exposure area S of the low alloy steel sample 1, the low alloy steel sample 2, and the low alloy steel sample 3 is the same.

[0050] S2. Immerse each low alloy steel sample in an autoclave simulating a corrosive environment and perform immersion corrosion for different periods of time.

[0051] The low alloy steel sample 1, the low alloy steel sample 2 and the low alloy steel sample 3 were subjected to immersion corrosion for time periods of t1, t2, t3 and t4, respectively, with t1, t2, t3 and t4 increasing in sequence.

[0052] The process of simulating a corrosive environment is as follows: adding a corrosive medium into a reactor, continuously introducing carbon dioxide gas into the corrosive medium to maintain a saturated state, and heating the medium to a temperature T. The temperature T is 90° C. to 180° C., and / or the corrosive medium is a NaCl solution with a concentration of 0.2 mol / L.

[0053] S3. Take out the low alloy steel samples, remove the corrosion products on the surface of the low alloy steel samples and obtain the local corrosion area S of each low alloy steel sample l , calculate the local corrosion percentage of each low alloy steel sample

[0054] This embodiment changes the traditional high-throughput material characterization technology that adopts "one at a time" sequential iteration to high-throughput characterization with parallel processing. In one experiment, the corrosion surface morphology of 12 groups of parallel samples can be obtained and the local corrosion data of 12 groups of parallel samples can be obtained at one time. The characterization data of a large number of samples can be collected in parallel and integrated for analysis in a single time by multiple devices, which greatly saves the experimental time and improves the calculation efficiency of the local corrosion rate of low alloy steel. At the same time, in one experiment, the effect of the addition of Ni and Mo on the local corrosion resistance of low Cr alloy steel can be quickly obtained.

[0055] Example 2

[0056] The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment of this embodiment is substantially the same as the high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment of Example 1, except that the high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment of this embodiment specifically comprises the following steps:

[0057] S1. Prepare three low-alloy steel samples with different compositions and record the overall exposed area S of each sample. The three samples are:

[0058] Low alloy steel sample 1 includes the following components by mass fraction: 0.21% C, 0.22% Si, 0.35% Mn, 3.11% Cr, and the balance Fe.

[0059] Low alloy steel sample 2, low alloy steel sample 2 includes the following components in mass fractions: 0.19% C, 0.22% Si, 0.44% Mn, 2.95% Cr, 1.14% Ni, and the balance is Fe.

[0060] Low alloy steel sample three, low alloy steel sample three includes the following components in mass fractions: 0.23% C, 0.21% Si, 0.52% Mn, 3.06% Cr, 1.05% Ni, 0.49% Mo, and the balance Fe.

[0061] The overall exposure area S of the low alloy steel sample 1, the low alloy steel sample 2, and the low alloy steel sample 3 is 40 mm × 15 mm × 5 mm.

[0062] S2. Immerse low alloy steel sample 1, low alloy steel sample 2, and low alloy steel sample 3 in an autoclave simulating a corrosive environment, and perform immersion corrosion for different time periods.

[0063] The process of simulating the corrosion environment is as follows: adding the corrosive medium into the reactor and continuously introducing carbon dioxide gas into the corrosive medium to maintain a saturated state during the experiment, and heating to a temperature T, T is 90°C, wherein the corrosive medium can be a NaCl solution with a concentration of 0.2 mol / L.

[0064] Low alloy steel sample 1, low alloy steel sample 2, and low alloy steel sample 3 were all subjected to immersion corrosion for times t1, t2, t3, and t4, respectively, and t1, t2, t3, and t4 were 6h, 24h, 72h, and 168h, respectively.

[0065] S3. Take out the low alloy steel samples, remove the corrosion products on the surface of the low alloy steel samples and obtain the local corrosion area S of each low alloy steel sample l , calculate the local corrosion percentage of each low alloy steel sample The specific process includes the following:

[0066] S31. Take out the low alloy steel samples and use scanning electron microscope to capture the surface morphology of the corrosion products of each sample at the same magnification. Use Image J software to process the surface morphology of the corrosion products of the sample into a binary image to obtain clearer corrosion areas and uncorroded areas on the test surface. Calculate the corrosion product coverage area S of each low alloy steel sample. c , calculate the surface coverage percentage of corrosion products of each low alloy steel sample

[0067] The surface morphology of each sample covered by corrosion products after corrosion is shown in the figure below. Figure 2 As shown in the figure, the surface morphologies of the low alloy steel sample after immersion corrosion for time periods of t1, t2, t3, and t4 are respectively Figure 2 a1, a2, a3, a4; the surface morphologies of low alloy steel sample 1 after immersion corrosion for time t1, t2, t3, t4 are respectively Figure 2 b1, b2, b3, b4; the surface morphologies of low alloy steel sample 3 after immersion corrosion for time t1, t2, t3, t4 are respectively Figure 2 c1, c2, c3, c4. Figure 2 It can be seen that after adding Ni and Ni-Mo elements, the precipitation of FeCO3 crystals on the surface of low alloy steel sample 2 and low alloy steel sample 3 is significantly restricted in the early stage of corrosion. Figure 2 It can be seen from a1-a3, b1-b3 and c1-c3 that after immersion in a corrosive environment at 90℃ for 72h, a relatively complete crystal film is obtained, and the FeCO3 crystals formed have a smooth morphology and no obvious damage. Figure 2 As can be seen from c1 and c2, the inner film of low alloy steel sample 3 has obvious cracks after immersion corrosion. This phenomenon shows that the Cr content in the inner film of low alloy steel sample 3 is higher than that of low alloy steel sample 1 and low alloy steel sample 2, indicating that under the action of Mo alloying, the enrichment process of Cr element in the inner film is accelerated.

[0068] The surface coverage percentage of corrosion products of each low alloy steel sample calculated in this example is as follows: Figure 3 As shown, the surface coverage percentages of corrosion products of low alloy steel sample 1 after immersion corrosion for time periods of t1, t2, t3, and t4 were 47.23±2.10%, 83.85±2.11%, 96.71±1.22%, and 98.86±0.37%, respectively. The surface coverage percentages of corrosion products of low alloy steel sample 2 after immersion corrosion for time periods of t1, t2, t3, and t4 were 39.82±1.37%, 75.37±1.63%, 88.92±0.76%, and 91.12±0.47%, respectively. The surface coverage percentages of corrosion products of low alloy steel sample 3 after immersion corrosion for time periods of t1, t2, t3, and t4 were 33.88±1.26%, 51.89±1.74%, 66.42±1.41%, and 91.26±1.22%, respectively. Figure 3 It can be seen that with the addition of Ni and Ni-Mo, the surface coverage percentage of corrosion products decreases significantly.

[0069] S32. Remove corrosion products from the surface of the low-alloy steel samples and use a scanning electron microscope to capture the surface morphology of each sample at the same magnification after the corrosion products are removed. Use Image J software to process the surface morphology of each sample after the corrosion products are removed into a binary image, and calculate the local corrosion area S of each low-alloy steel sample. l , calculate the local corrosion percentage of each low alloy steel sample

[0070] The surface morphology of each sample after removing the corrosion products in this embodiment is shown in the figure below. Figure 4 As shown in the figure, the surface morphologies of the low alloy steel sample after immersion corrosion for time periods of t1, t2, t3, and t4 are respectively Figure 4 a1, a2, a3, a4; the surface morphologies of low alloy steel sample 1 after immersion corrosion for time t1, t2, t3, t4 are respectively Figure 4 b1, b2, b3, b4; the surface morphologies of low alloy steel sample 3 after immersion corrosion for time t1, t2, t3, t4 are respectively Figure 4 c1, c2, c3, c4. Figure 4 From a1, a2, a3, and a4, it can be clearly seen that several obvious corrosion pits appeared on the surface of the low alloy steel sample 1 after being immersed in a 90℃ corrosive environment for different times. Figure 4 It can be clearly seen from b1, b2, b3, b4, c1, c2, c3, and c4 that with the addition of Ni and Ni-Mo, the local corrosion behavior of low alloy steel is significantly restricted in a 90°C corrosion environment.

[0071] The local corrosion percentage of each low alloy steel sample calculated in this embodiment is as follows: Figure 5 As shown, the local corrosion percentages of low alloy steel sample 1 after immersion corrosion for time periods of t1, t2, t3, and t4 were 8.50±2.51%, 9.04±2.50%, 7.81±1.96%, and 3.77±1.34%, respectively. The local corrosion percentages of low alloy steel sample 2 after immersion corrosion for time periods of t1, t2, t3, and t4 were 3.68±2.26%, 5.47±0.50%, 7.88±1.55%, and 0.57±0.07%, respectively. The local corrosion percentages of low alloy steel sample 3 after immersion corrosion for time periods of t1, t2, t3, and t4 were 1.43±0.54%, 1.74±0.42%, 1.47±0.57%, and 0.43±0.11%, respectively. Figure 5 It can be seen that in a 90°C corrosive environment, the localized corrosion percentages of low alloy steel sample 1, low alloy steel sample 2, and low alloy steel sample 3 first increased and then decreased during the immersion time from 6h to 168h. Although several prominent locations appeared, the final localized corrosion percentage values ​​all showed a downward trend. The increase in localized corrosion percentage mainly occurred within 24h of the onset of corrosion, reflecting the generation and development of corrosion pits at the beginning of corrosion. After 72h of exposure, the values ​​of corrosion pits all decreased, which was mainly related to the gradual disappearance of localized corrosion pits over time on the basis of uniform thickness loss.

[0072] This embodiment presents a high-throughput evaluation method for pitting corrosion of low-alloy steel used in oil and gas equipment. Considering that anodic dissolution and membrane damage are accelerated in high-temperature and / or high-pressure environments, low-alloy steel materials are still susceptible to CO2 corrosion in high-temperature and / or high-pressure environments. Cr is added to carbon steel, and then some nobler alloying elements (such as Ni, Mo, etc.) are appropriately added. Immersion corrosion experiments are conducted on low-alloy steel samples for different periods of time to determine the evolution and development of the corrosion process over time under static conditions, revealing the actual corrosion process of low-alloy steel. Based on the high-throughput screening method, the area of ​​the corrosion pit is compared with the overall exposed area, and the percentage of local corrosion is calculated, confirming that the addition of Ni and Mo helps to improve the corrosion resistance of low-chromium steel, especially its pitting resistance.

[0073] This embodiment conducts 12 sets of parallel experiments simultaneously, which solves the problem that traditional experiments to study the corrosion characteristics of metals must be compared and verified through a large number of highly repetitive experiments. The corrosion surface morphology of 12 sets of parallel samples can be obtained in one experiment, and the data of 12 sets of parallel samples can be obtained in one experiment, so that the effect of the addition of different metal elements on the local corrosion resistance of low-alloy steel can be quickly obtained.

[0074] In this embodiment, a desktop scanning electron microscope DSEM is selected. The DSEM adopts Hitachi TM4000Ⅱ. This scanning electron microscope can directly observe the structure of the sample surface. The sample can be translated and rotated in three dimensions in the sample chamber. Therefore, the sample can be observed from various angles. In addition, it is small and light and can quickly scan the corrosion morphology of the sample surface. The surface morphology of the sample is captured by a scanning electron microscope at the same magnification, and the surface morphology of the sample is processed into a binary image using ImageJ software to calculate the corrosion product coverage area and local corrosion area area of ​​each low-alloy steel sample. Then, the corrosion product coverage percentage and local corrosion percentage of the low-alloy steel sample are calculated according to the corrosion product coverage area and local corrosion area, thereby analogically obtaining the local corrosion rate of the sample, thereby improving the accuracy of the experimental results and reducing the experimental error.

[0075] Example 3

[0076] The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment in this embodiment is basically the same as the high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment in Example 2. The only difference is that in the high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment in this embodiment, the temperature T of the simulated corrosion environment is 180°C.

[0077] The surface morphology of each sample covered by corrosion products after corrosion is shown in the figure below. Figure 6 As shown in the figure, the surface morphologies of the low alloy steel sample after immersion corrosion for time periods of t1, t2, t3, and t4 are respectively Figure 6 a1, a2, a3, a4; the surface morphologies of low alloy steel sample 1 after immersion corrosion for time t1, t2, t3, t4 are respectively Figure 6 b1, b2, b3, b4; the surface morphologies of low alloy steel sample 3 after immersion corrosion for time t1, t2, t3, t4 are respectively Figure 6 c1, c2, c3, c4. Figure 6 As can be seen in Figure a1, a large number of FeCO3 crystals were observed on the surface of the low alloy steel sample after only 6 hours of exposure to CO2-saturated NaCl solution at 180℃, indicating that the steel matrix has undergone accelerated dissolution and the saturation solubility of FeCO3 crystals has decreased with increasing temperature. However, some gaps can be clearly seen between these crystals, and corrosive ions can easily enter the steel matrix. In addition, Figure 6 It can be seen from a2 (area 1) that under the conditions of 180℃ high temperature and CO2-saturated NaCl corrosion electrolyte, the internal layered film of low alloy steel sample 1 is almost completely destroyed. Figure 6As can be seen from b1-b2 and c1-c2, after the addition of Ni and Ni-Mo elements, the number of FeCO3 crystals in the outer layer of low alloy steel sample 2 and low alloy steel sample 3 decreased significantly within the first 24 hours of exposure, and the structure of the inner film became dense (regions 2, 3, and 4). Although the shielding effect of the outer layer of FeCO3 crystals on corrosion has been weakened to a certain extent, the corrosion rate of low alloy steel sample 2 and low alloy steel sample 3 is still lower than that of low alloy steel sample 1. Therefore, the protective role of the inner film should also be given considerable attention.

[0078] The surface coverage percentage of corrosion products of each low alloy steel sample calculated in this example is as follows: Figure 7 As shown, the surface coverage percentages of corrosion products of low alloy steel sample 1 after immersion corrosion for time periods of t1, t2, t3, and t4 were 91.22±0.63%, 92.78±0.64%, 95.18±1.47%, and 98.59±0.35%, respectively. The surface coverage percentages of corrosion products of low alloy steel sample 2 after immersion corrosion for time periods of t1, t2, t3, and t4 were 41.54±1.68%, 52.73±0.58%, 95.46±1.46%, and 98.88±0.92%, respectively. The surface coverage percentages of corrosion products of low alloy steel sample 3 after immersion corrosion for time periods of t1, t2, t3, and t4 were 31.12±1.54%, 39.55±0.86%, 81.86±1.58%, and 94.53±1.34%, respectively. Figure 7 It can be seen that with the addition of Ni and Ni-Mo, the surface coverage percentage of corrosion products decreases significantly.

[0079] The surface morphology of each sample after removing the corrosion products in this embodiment is shown in the figure below. Figure 8 As shown in the figure, the surface morphologies of the low alloy steel sample after immersion corrosion for time periods of t1, t2, t3, and t4 are respectively Figure 8 a1, a2, a3, a4; the surface morphologies of low alloy steel sample 1 after immersion corrosion for time t1, t2, t3, t4 are respectively Figure 8 b1, b2, b3, b4; the surface morphologies of low alloy steel sample 3 after immersion corrosion for time t1, t2, t3, t4 are respectively Figure 8 c1, c2, c3, c4. Figure 8 From a1, a2, a3, and a4, it can be clearly seen that several obvious corrosion pits appeared on the surface of the low alloy steel sample 1 after being immersed in a 180℃ corrosive environment for different times. Figure 8 It can be seen from b1, b2, b3, b4, c1, c2, c3, and c4 that with the addition of Ni and Ni-Mo, this local corrosion behavior is significantly restricted.

[0080] The local corrosion percentage of each low alloy steel sample calculated in this embodiment is as follows: Figure 9 As shown, the local corrosion percentages of low alloy steel sample 1 after immersion corrosion for time periods of t1, t2, t3, and t4 were 29.18±5.99%, 22.33±6.98%, 16.02±2.03%, and 15.89±2.52%, respectively. The local corrosion percentages of low alloy steel sample 2 after immersion corrosion for time periods of t1, t2, t3, and t4 were 5.44±3.59%, 8.80±2.43%, 7.53±2.20%, and 2.55±0.89%, respectively. The local corrosion percentages of low alloy steel sample 3 after immersion corrosion for time periods of t1, t2, t3, and t4 were 2.09±0.28%, 1.37±0.53%, 1.14±0.50%, and 0.25±0.10%, respectively. Figure 9 It can be seen that during the immersion period of 72 to 168 hours in a corrosive environment at 180°C, the local corrosion percentage of low alloy steel sample 1 did not decrease significantly, which means that the loss in the depth direction of the corrosion pit is very serious. However, with the addition of Ni, the local corrosion percentage of low alloy steel sample 2 dropped to below 10%. With the addition of Ni-Mo, the local corrosion percentage of low alloy steel sample 3 even approached 0. This proves that the addition of Ni and Mo has a significant limiting effect on the local corrosion behavior of low Cr steel.

[0081] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0082] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment, characterized in that: The following steps are involved: S1. Add different metal elements to the composition of carbon steel to prepare a variety of low alloy steel samples with different compositions, and record the overall exposure area of ​​each sample S The low alloy steel sample preparation includes the following processes: adding Cr elements to the composition of carbon steel to prepare low alloy steel sample 1; adding Cr and Ni elements to the composition of carbon steel to prepare low alloy steel sample 2; adding Cr, Ni, and Mo elements to the composition of carbon steel to prepare low alloy steel sample 3; S2. Each low-alloy steel sample was immersed in an autoclave simulating a corrosive environment for different periods of time. The corrosive environment simulation process was as follows: a corrosive medium was added to the autoclave, carbon dioxide gas was continuously introduced into the medium to maintain saturation, and the medium was heated to temperature T. S3. Take out the low alloy steel samples, remove the corrosion products on the surface of the low alloy steel samples and obtain the local corrosion area of ​​each low alloy steel sample S l , calculate the local corrosion percentage of each low alloy steel sample .

2. The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment according to claim 1, characterized in that: Low alloy steel sample 1 includes the following components by mass fraction: 0.18%-0.23% C, 0.15%-0.35% Si, 0.3%-0.6% Mn, 3.11% Cr, and the balance Fe; Low alloy steel sample 2 includes the following components by mass fraction: 0.18%-0.23% C, 0.15%-0.35% Si, 0.3%-0.6% Mn, 2.95% Cr, 1.14% Ni, and the balance Fe; Low alloy steel sample 3 includes the following components by mass: 0.18%-0.23% C, 0.15%-0.35% Si, 0.3%-0.6% Mn, 3.06% Cr, 1.05% Ni, 0.49% Mo, and the balance Fe.

3. The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment according to claim 2, characterized in that: Low alloy steel sample 1 includes the following components by mass fraction: 0.21% C, 0.22% Si, 0.35% Mn, 3.11% Cr, and the balance Fe; Low alloy steel sample 2 includes the following components by mass fraction: 0.19% C, 0.22% Si, 0.44% Mn, 2.95% Cr, 1.14% Ni, and the balance is Fe; Low alloy steel sample 3 includes the following components by mass: 0.23% C, 0.21% Si, 0.52% Mn, 3.06% Cr, 1.05% Ni, 0.49% Mo, and the balance Fe.

4. The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment according to claim 1, characterized in that: In step S1 , the overall exposed area of ​​each sample is the same, and / or the overall exposed area is 40 mm×15 mm×5 mm.

5. The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment according to claim 1, characterized in that: In step S2, each sample is subjected to immersion corrosion for a time period of t1, t2, t3, and t4, respectively, where t1, t2, t3, and t4 increase in sequence.

6. The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment according to claim 5, characterized in that: t1, t2, t3 and t4 are 6h, 24h, 72h and 168h respectively.

7. The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment according to claim 1, characterized in that: The temperature T is 90° C.-180° C., and / or the corrosive medium is a NaCl solution with a concentration of 0.2 mol / L.

8. The high-throughput evaluation method for pitting corrosion of low-alloy steel for oil and gas equipment according to claim 1, characterized in that: Step S3 The following processes are included: S31. Take out the low alloy steel samples and use scanning electron microscope to capture the surface morphology of the corrosion product coverage of each sample at the same magnification. Use Image J software to process the surface morphology of the sample after corrosion into a binary image and calculate the corrosion product coverage area of ​​each low alloy steel sample. S c , calculate the surface coverage percentage of corrosion products of each low alloy steel sample ; S32. Corrosion products were removed from the surface of the low-alloy steel samples. Scanning electron microscopy was used to capture the surface morphology of each sample at the same magnification after the corrosion products were removed. The surface morphology after the corrosion products were removed was processed into a binary image using Image J software, and the local corrosion area of ​​each low-alloy steel sample was calculated. S l , calculate the local corrosion percentage of each low alloy steel sample .

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