Design method for evaluating corrosion resistance of metal in frozen soil environment

Abstract

The invention relates to a design method for evaluating corrosion resistance of metal in a frozen soil environment, belongs to the technical field of metal corrosion simulation evaluation, and solves the problems that in the prior art, no reliable evaluation and preliminary screening technology for the corrosion resistance of the metal in the frozen soil environment exists and no guidance / design method for guiding establishment of the technology exists. The invention provides a design method for evaluating the corrosion resistance of metal in a frozen soil environment. The design method comprises the steps of information collection, equipment design, simulated corrosion solution and temperature curve design, full immersion corrosion experiment, corrosion rate calculation and the like. According to the design method, a thought for evaluating the corrosion resistance of the metal in the frozen soil environment is provided, specific implementation steps and a parameter design method are pointed out, and the corrosion resistance of different kinds of metal materials in the frozen soil environment can be preliminarily evaluated and compared through the simulation method established according to the design method; and a support is provided for selection and research and development of metal materials for equipment or construction in the frozen soil environment.

Description

Technical Field

[0001] The invention relates to the technical field of metal corrosion simulation evaluation, and in particular to a design method for evaluating the corrosion resistance of metals in a frozen soil environment. Background Art

[0002] With the development of my country's economy, the development of the western region has become the focus of the next stage. Economic development is inseparable from infrastructure construction, so highway construction in the western high-altitude and cold regions has also been put on the agenda. Compared with the eastern region of my country, the western region has a high altitude and low temperature, and there is a large amount of frozen soil, which makes the road foundation construction conditions extremely poor, and the construction and maintenance of high-grade highways are extremely difficult.

[0003] In order to solve the problem of highway construction and life span in frozen soil areas, the stability of road foundation can be increased by changing the construction method and optimizing the structure to improve the strength, reduce and avoid uneven road settlement, extend the service life, reduce the maintenance, and reduce the comprehensive cost of the whole life cycle of the road. This will inevitably increase the amount of steel used in the road structure, mainly in the following categories: guardrails, road signs, and other auxiliary facilities used to maintain road driving safety; steel bars, steel pipes, steel plates, etc. used in road foundation construction; steel plates, profiles, steel cables, etc. used in bridge construction.

[0004] In addition to steel for frozen soil roads, the number of equipment installed in frozen soil environments for scientific research or other special needs is also gradually increasing. For example, high-end equipment used for environmental monitoring, seismic wave monitoring, geological exploration or other special purposes, especially high-end equipment that needs to be buried or semi-buried underground, also requires specific material selection according to environmental conditions.

[0005] Corrosion is an important factor leading to steel failure. The corrosion resistance of steel determines the amount of material used and the maintenance and replacement cycle. Generally speaking, materials with poor corrosion resistance will be designed with sufficient margins, which will increase the amount of material used and the construction cost. For replaceable parts, materials with poor corrosion resistance will shorten the service life and increase the number of replacements, thereby increasing maintenance costs. Therefore, it is an effective way to reduce the comprehensive cost of the entire life cycle of metal materials in frozen soil environments by selecting suitable materials according to regional conditions, correctly evaluating the corrosion resistance of the materials, predicting the service life, and combining the material manufacturing cost, construction cost and maintenance cost.

[0006] At present, there is still a blank in how to simulate and evaluate the corrosion behavior of metal materials in frozen soil environments at home and abroad. Therefore, it is particularly necessary to develop a design method for evaluating the corrosion resistance of metals in frozen soil environments. Summary of the invention

[0007] In view of the above analysis, an embodiment of the present invention aims to provide a design method for evaluating the corrosion resistance of metals in a frozen soil environment, so as to solve the problem that there is no reliable evaluation and preliminary screening technology for the corrosion resistance of metals in a frozen soil environment in the prior art, and there is a lack of guidance / design methods to establish the above technology.

[0008] The present invention provides a design method for evaluating the corrosion resistance of metals in a frozen soil environment, which specifically comprises the following steps:

[0009] S1: Collect monthly temperature curves, air pressure range, atmospheric composition, hydrology and soil composition information of the area to be simulated;

[0010] S2: Design and prepare a test device according to the upper and lower limits of the temperature curve, determine the composition of the simulated corrosion solution according to the hydrological and soil composition information, and prepare the simulated corrosion solution;

[0011] S3: Preset the temperature curve according to the annual and monthly temperature curves, and adjust the gas atmosphere inside the testing device according to the air pressure variation range and the atmospheric composition information;

[0012] S4: The metal to be tested is made into a metal sample of a certain specification, and a full immersion corrosion simulation experiment of at least one temperature curve cycle is carried out using the test device and the simulated corrosion solution, and the corrosion behavior and corrosion resistance of the corresponding metal in the frozen soil environment are estimated according to the corrosion simulation results; wherein one temperature curve cycle is 24 hours.

[0013] Specifically, the maximum temperature of the monthly temperature curve is not higher than the local freezing point, and the difference between the upper and lower limits of the monthly temperature curve is T max -T min ≤10℃, it is determined that the simulated area is a constant temperature frozen soil environment in the corresponding time period.

[0014] Specifically, the difference between the upper and lower limits of the monthly temperature curve is T max -T min >10℃ and the maximum temperature T max ≤ local freezing point T0, it is determined that the simulated area is in an alternating freezing environment during the corresponding time period;

[0015] The difference between the upper and lower limits of the monthly temperature curve T max -T min >10℃ and the maximum temperature T max >Local freezing point T0, and T max -T0<T0-T min When , it is determined that the area to be simulated is an alternating and gradual melting environment in the corresponding time period;

[0016] The alternating freezing environment and the alternating gradual thawing environment are collectively referred to as alternating low-temperature frozen soil environment.

[0017] Specifically, the maximum temperature T of the monthly temperature curve max >Local freezing point T0 and minimum temperature T min < local freezing point T0, and T max -T0≥T0-T min When the simulated area is in a short-term frozen soil environment in the corresponding time period.

[0018] Specifically, different months in the area to be simulated may belong to different permafrost environment types, and the actual simulation evaluation can be implemented by combining different types of temperature curve cycles.

[0019] Specifically, an alternating low-temperature frozen soil environment temperature curve used to simulate the environment of the northeastern Mohe basin in December is:

[0020] The temperature curve range is between -35°C and -20°C, and the preset initial temperature is -20°C;

[0021] Phase 1: Target temperature -35±1℃, cooling rate 0.5~10℃ / hour, keep warm after reaching the target temperature, a total of 12~16h;

[0022] The second stage: target temperature -20±1℃, heating rate 0.5~5℃ / hour, keep warm after reaching the target temperature, a total of 8~12h;

[0023] With a cycle of 24 hours, the total detection time is 1 to 30 days.

[0024] Specifically, the internal gas atmosphere includes nitrogen and oxygen, Vnitrogen:Voxygen=7-8:3-2, and the internal gas atmosphere pressure is 50-100 kPa.

[0025] Specifically, the specific operations of step S4 are:

[0026] S4-1: The metal to be tested is made into metal samples of certain specifications, and the metal samples are weighed and recorded;

[0027] S4-2: The metal sample is completely immersed in the simulated corrosion solution, and the container containing the simulated corrosion solution is placed in a closed simulation chamber and pre-temperatured;

[0028] S4-3: According to a preset temperature curve, a temperature-controllable circulating gas is introduced into the closed simulation chamber to adjust the temperature in the simulation chamber and start timing, so that the temperature in the chamber changes according to the preset temperature curve;

[0029] S4-4: After reaching the preset detection time, the corroded metal sample is taken out and cleaned to remove the surface corrosion part, and the corrosion amount is calculated by the weight difference before and after.

[0030] Specifically, the simulated corrosion solution comprises, by mass percentage, 0.06-1% Na2SO4, 0.05-0.1% MgCl2, 3-5% NaCl, and the remainder is deionized water; the pH value of the simulated corrosion solution is 6-8.

[0031] The present invention also provides a device for implementing the simulation evaluation method designed by the design method: the device comprises two parts: a temperature control module (1) and a simulation test module, the temperature control module is used to control the temperature change inside the simulation test module, and the simulation test module is used to implement a full immersion corrosion simulation corrosion test of a metal sample.

[0032] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0033] 1. The design method disclosed in the present invention provides for the first time a way of evaluating the corrosion resistance of metals in a frozen soil environment, and specifies specific implementation steps and parameter design methods.

[0034] First, design and prepare simulation equipment that can meet the implementation requirements according to the temperature change range of the area to be simulated, and then design the temperature change curve according to the actual local all-day change curve, so that the simulated temperature conditions are as close to reality as possible. It is worth noting that, in general, the change of the local temperature curve is related to the sunshine time, so the time of a single temperature cycle is set to 24h, and the heating and cooling time and speed are adjusted according to the actual curve. According to the local hydrology and soil composition of the area to be simulated, confirm the ions that play a major role in metal corrosion and prepare the simulated corrosion solution.

[0035] In addition, since metal materials are actually buried or semi-buried underground, the impact of atmospheric corrosion is relatively small, and its impact is ignored in the present invention; however, oxygen in the atmosphere has an important impact on metal corrosion in the soil. It not only participates in the metal oxidation reaction, but also acts as an electron acceptor in the corrosion process, accelerating the occurrence and development of corrosion. The corrosion rate of oxygen on metals increases with increasing temperature and concentration, and the presence of water will also accelerate the corrosion rate of metals. In order to restore the air pressure conditions of the frozen soil environment as much as possible (generally speaking, frozen soil environments are mostly plateaus with lower air pressure) and atmospheric components, the present invention can adjust the composition and air pressure of the circulating gas to make the simulated corrosion process as close to the actual situation as possible;

[0036] It is worth emphasizing that existing full immersion corrosion simulation experiments generally ignore the regulation of gas atmosphere. However, since frozen areas are often located at high altitudes or high latitudes, the air pressure and air composition ratio are significantly different from those in plains or low altitude areas. Therefore, the influence of air / atmosphere must be considered when simulating the corrosion resistance of metal materials in frozen soil environments.

[0037] The present invention comprehensively considers specific environmental factors and restores the real environmental corrosion mechanism as much as possible, appropriately simplifies some minor influencing factors, and strives to simulate and evaluate the metal corrosion behavior in a frozen soil environment through simplified simulation conditions and operating procedures.

[0038] 2. According to the environmental characteristics of the specific area being simulated, the temperature curve can be further optimized during the design process to achieve targeted simulation of the constant temperature frozen soil environment, alternating low temperature frozen soil environment, and short-term frozen soil environment; the composition of the simulated corrosion solution can be adjusted according to the hydrological information of the specific area, the concentration of each component can be adjusted, and appropriate amounts of other components or even sand and gravel can be added, so as to achieve targeted restoration and simulation of the metal corrosion behavior in the specific area, and the evaluation results will be more accurate and targeted.

[0039] 3. The present invention provides a supporting device for the above-mentioned evaluation method, wherein the device comprises a temperature control module and a simulation test module, and the two parts are independent of each other and have no physical connection relationship;

[0040] The temperature control module adopts the reverse Carnot cycle to control the temperature of the simulation test module by changing the direction of heat flow. The first heat exchanger is placed in the environment to exchange heat with the atmosphere; the first heat exchanger adopts an isolation design to exchange heat with the simulation test module (mainly the second heat exchanger), thereby achieving temperature control of the simulation test module under the premise of avoiding corrosive gas temperature control module.

[0041] When cooling is required, the heat pump transfers heat from the second heat exchanger to the first heat exchanger; after the temperature of the second heat exchanger decreases, it absorbs heat from the closed simulation chamber circulation system, causing its temperature to decrease; after the first heat exchanger obtains heat, its temperature increases and it releases the heat to the surrounding atmosphere. When heating is required, the heat pump reverses, and heat flows from the first heat exchanger to the second heat exchanger. At this time, the temperature of the first heat exchanger decreases and absorbs heat from the surrounding atmosphere; after the second heat exchanger obtains heat, its temperature increases and it releases heat to the simulation chamber circulation system to increase its temperature. The second heat exchanger itself is equipped with an electric heater. When the surrounding environment is insufficient in heat, the heater can be used for auxiliary heating to quickly increase the temperature of the simulation chamber.

[0042] Since corrosive reagents are volatile to a certain extent, the circulating gas contains certain corrosive components. The closed simulation chamber in the simulation test module adopts a closed design, is made of corrosion-resistant materials, and has a protective layer attached to the outer surface. In order to protect the internal equipment and the surrounding environment, the temperature control module and the simulation test module adopt an isolation design. The two exchange heat through a heat exchange system to avoid the impact of the simulated gas on the temperature control module.

[0043] The above-mentioned device can perfectly adapt to and meet the implementation requirements of the evaluation method provided by the present invention, thereby realizing a preliminary simulation of the corrosion behavior of metals in a frozen soil environment in a laboratory scenario, and can make a preliminary comparison and screening of the corrosion resistance of different metal materials in a frozen soil environment, and can also provide support for the subsequent development of metal materials.

[0044] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The drawings are only for the purpose of illustrating specific embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0046] Figure 1 A schematic diagram of a device (refrigeration mode) for implementing the simulation evaluation method;

[0047] Figure 2 This is a schematic diagram of the temperature control module (heating mode);

[0048] Figure 3 is a schematic diagram of the reagent tank;

[0049] Figure 4 This is the average temperature curve of Mohe in December.

[0050] Reference numerals:

[0051] 1. Temperature control module; 11. Four-way reversing valve; 12. Compressor; 13. Radiator; 141. First expansion valve; 142. First non-return valve; 151. Second expansion valve; 152. Second non-return valve; 16. First heat exchanger; 2. Closed simulation chamber; 21. Second heat exchanger; 22. Circulation fan; 23. Sample cell; 24. Negative pressure filter; 25. Sensor. DETAILED DESCRIPTION

[0052] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0053] In geology, frozen soil refers to various rocks and soils below 0℃ and containing ice. It can generally be divided into short-term frozen soil (several hours, days or even half a month), seasonal frozen soil (half a month to several months) and perennial frozen soil (more than several years). Frozen soil is a soil medium that is extremely sensitive to temperature. It contains abundant underground ice. There is a certain amount of unfrozen water in the frozen soil. Therefore, the frozen soil has rheological properties, and its long-term strength is much lower than the instantaneous strength characteristics. Moreover, with the change of temperature, the frozen soil constantly changes between frost heave and thaw settlement, which has a great impact on the safety of structures in the frozen soil area.

[0054] The temperature conditions in permafrost areas vary greatly. Take the Qinghai-Tibet Plateau as an example: the average temperature in the hottest month is 6-18°C; nearly 1 / 4 of the area in the northwest is the plateau cold zone, with an altitude of more than 5,000 meters, an annual average temperature of -10--4°C, and an average temperature of less than 6°C in the hottest month; nearly 1 / 4 of the area in the central and western regions is the plateau sub-cold zone, with an altitude of 4,500-5,000 meters. The annual average temperature is below 0°C, and the average temperature in the hottest month is 6-10°C. The average temperature in Genhe City, Hulunbuir, Inner Mongolia, known as the "China Cold Pole", is -5.3°C. The highest temperature of 34°C in 2024 occurred in July, and the lowest temperature of -45°C occurred in January. The historical record of the extreme lowest temperature is -58°C. Due to the large temperature span, the surface permafrost is in a state of alternating freeze-thaw, and the depth of permafrost changes with the seasons, which poses a great problem to road safety.

[0055] The frozen soil state is unstable due to the influence of the surrounding temperature and can be roughly divided into the following three types:

[0056] When the temperature is low, the water in the frozen soil solidifies into ice, and other components in the frozen soil combine with the ice to form a relatively stable solid. At this time, the environment around the steel is relatively stable, but the pressure changes caused by expansion during the solidification process have a certain impact on the steel, and the friction changes caused by expansion have a negative impact on the metal's external protective layer; when the temperature rises, the strength of the frozen ice in the frozen soil decreases with the increase in temperature, and melting will occur when the temperature is higher than the freezing point of water. Since the thermal resistance of metal is much smaller than that of soil, the frozen soil around the metal will change first during the warming process, and the water and the substances contained in the water will transform from solid to ionic state. At this time, the impact of corrosion on the metal increases with the increase in ion concentration; when the ambient temperature increases further, the frozen soil completely melts and becomes a flowing state, and the corrosion caused by the environment should be the strongest at this time.

[0057] Since the corrosion situation changes with the environment, the corrosion rate at different stages is difficult to obtain through simple calculations. Therefore, a set of devices and methods that can simulate the changes in the permafrost environment are needed to conduct simulation evaluation and quantify the amount of corrosion.

[0058] The present invention provides a design method for evaluating the corrosion resistance of metals in a frozen soil environment, which specifically comprises the following steps:

[0059] S1: Collect monthly temperature curves, air pressure range, atmospheric composition, hydrology and soil composition information of the area to be simulated;

[0060] S2: Design and prepare a test device according to the upper and lower limits of the temperature curve, determine the composition of the simulated corrosion solution according to the hydrological and soil composition information, and prepare the simulated corrosion solution;

[0061] S3: Preset the temperature curve according to the annual and monthly temperature curves, and adjust the gas atmosphere inside the testing device according to the air pressure variation range and the atmospheric composition information;

[0062] S4: The metal to be tested is made into a metal sample of a certain specification, and a full immersion corrosion simulation experiment of at least one temperature curve cycle is carried out using the test device and the simulated corrosion solution, and the corrosion behavior and corrosion resistance of the corresponding metal in the frozen soil environment are estimated according to the corrosion simulation results; wherein one temperature curve cycle is 24 hours.

[0063] The simulation evaluation method can preliminarily simulate the corrosion behavior of metals in a frozen soil environment in a laboratory scenario, can make a preliminary comparison and screening of the corrosion resistance of different metal materials in a frozen soil environment, and can also provide support for the subsequent development of metal materials;

[0064] There are many factors that affect the corrosion resistance of metals. It is generally believed that the corrosion system is composed of materials and the environment. The factors that affect corrosion can be basically divided into internal factors related to the material and external factors related to the medium environment. By simulating external environmental factors and using accelerated or simulated corrosion methods to study the corrosion of materials in specific environments, it is helpful to evaluate the corrosion resistance of materials and is helpful for the final application of the product. Under the premise of restoring the corrosion mechanism of the real environment as much as possible, the present invention appropriately simplifies some minor influencing factors, and strives to simulate and evaluate the corrosion behavior of metals in frozen soil environments through simplified simulation conditions and operating procedures. The simulated corrosion process is close to the real corrosion process and has strong reference value.

[0065] Specifically, since the method of the present invention is mainly aimed at metal materials buried or semi-buried under frozen soil, a simulated corrosion solution and full immersion method are selected to perform corrosion simulation.

[0066] Specifically, by setting the temperature range and temperature curve, the temperature change in the real environment is simulated. Since in the real environment, the temperature change generally changes in a "natural day" cycle and is closely related to the sunshine time, the temperature curve has a cycle of 24 hours.

[0067] Specifically, the temperature range is -70°C to 80°C, and changes according to a preset temperature curve. The temperature curve has a cycle of 24 hours, and a single detection time is 1 to 30 days, for example 1, 2, 3, 5, 10, 15, 20, 25, 30 days.

[0068] Specifically, the maximum temperature of the monthly temperature curve is not higher than the local freezing point, and the difference between the upper and lower limits of the monthly temperature curve is T max -T min ≤10℃, it is determined that the simulated area is a constant temperature frozen soil environment in the corresponding time period.

[0069] Optionally, when simulating a constant temperature frozen soil environment, the temperature curve is set to a constant temperature, and the constant temperature is the same as the preset initial temperature; the temperature range is -70°C to -10°C, for example, -70°C, -60°C, -50°C, -40°C, -30°C, -20°C, and -10°C. The temperature remains constant during the simulation, and the temperature fluctuation is ≤1°C; the detection time is 1 to 30 days, for example, 1, 2, 3, 5, 10, 15, 20, 25, and 30 days.

[0070] Specifically, the difference between the upper and lower limits of the monthly temperature curve is T max -T min >10℃ and the maximum temperature T max ≤ local freezing point T0, it is determined that the simulated area is in an alternating freezing environment during the corresponding time period;

[0071] The difference between the upper and lower limits of the monthly temperature curve T max -T min >10℃ and the maximum temperature T max >Local freezing point T0, and T max -T0<T0-T min When , it is determined that the area to be simulated is an alternating and gradual melting environment in the corresponding time period;

[0072] The alternating freezing environment and the alternating gradual thawing environment are collectively referred to as alternating low-temperature frozen soil environment.

[0073] Furthermore, when simulating an alternating low-temperature frozen soil environment, the temperature curve range is between -40°C and 5°C; the temperature change cycle is carried out in parallel with the temperature change cycle of the simulated environment, and the heating and cooling rates are consistent with the actual temperature curve of the simulated environment; the preset initial temperature is the highest temperature in the temperature curve range; the alternating low-temperature frozen soil environment is divided into freezing environment simulation and gradual melting environment simulation. When simulating a gradual melting environment, the highest temperature is higher than the corresponding real environment freezing point.

[0074] Specifically, the above-mentioned temperature range, heating and cooling rates, and heating and cooling times are determined according to the specific local meteorological and hydrological conditions, and are generally taken as monthly averages or extreme values, such as the monthly maximum temperature (obtained from the daily average), the average daily sunshine time of the month (obtained from the daily average), and the monthly minimum temperature (obtained from the daily average).

[0075] It is worth noting that the above simulation mode is suitable for areas with large annual / monthly temperature changes. The ambient temperature may be below freezing all year round or above freezing for a short time. This simulation mode is mainly aimed at frozen soil in a frozen state. With the large changes in the temperature range, the chemical corrosion and physical corrosion (expansion or contraction) properties will also change, thereby causing a comprehensive and unpredictable corrosion to metal materials.

[0076] Specifically, an alternating low-temperature frozen soil environment temperature curve used to simulate the environment of the northeastern Mohe basin in December is:

[0077] The temperature curve range is between -35°C and -20°C, and the preset initial temperature is -20°C;

[0078] Phase 1: Target temperature -35±1℃, cooling rate 0.5~10℃ / hour, keep warm after reaching the target temperature, a total of 12~16h;

[0079] The second stage: target temperature -20±1℃, heating rate 0.5~5℃ / hour, keep warm after reaching the target temperature, a total of 8~12h;

[0080] With a cycle of 24 hours, the total detection time is 1 to 30 days.

[0081] Specifically, the maximum temperature T of the monthly temperature curve max >Local freezing point T0 and minimum temperature T min < local freezing point T0, and T max -T0≥T0-T min When the simulated area is in a short-term frozen soil environment in the corresponding time period.

[0082] Specifically, when simulating a short-term frozen soil environment, the temperature curve range is between -5°C and 20°C, and the preset initial temperature is 20°C;

[0083] Phase 1: Target temperature -5±1℃, cooling rate 0.5~3℃ / hour, keep warm after reaching the target temperature, a total of 12~16h;

[0084] The second stage: target temperature 20±1℃, heating rate 0.5~5℃ / hour, keep warm after reaching the target temperature, a total of 8~12h;

[0085] The first and second stages total 24 hours;

[0086] With a cycle of 24 hours, the total detection time is 1 to 30 days, for example 1, 2, 3, 5, 10, 15, 20, 25, 30 days.

[0087] Specifically, since the short-term frozen soil environment is relatively mild, the above-mentioned general temperature curve can be used for corrosion simulation unless there is a special need, and the simulation evaluation results still have high reliability and reference value.

[0088] It is worth noting that, regardless of whether it is an alternating low-temperature frozen soil environment or a short-term frozen soil environment simulation, the cooling and heating rates are variable. The specific heating and cooling rates can be changed according to the actual temperature curve of the simulated environment, such as slow first and then fast, fast first and then slow, etc.

[0089] Specifically, different months in the simulated area may belong to different types of frozen soil environments. The actual simulation evaluation can be implemented by combining different types of temperature curve cycles. From an engineering perspective, soil containing solid water and frozen for two years or more should be determined as permafrost, also known as permafrost. Seasonal frozen soil refers to soil or loose rock layers that freeze in winter and melt in spring. The depth of the frozen soil layer is determined by factors such as natural geographical conditions and soil physical properties. This type of frozen soil will freeze and thaw periodically with seasonal changes, that is, it freezes when the temperature drops below freezing in winter and melts when the temperature rises in spring. Short-term frozen soil refers to frozen soil with a freezing time of several hours to half a month. This type of frozen soil is characterized by a short freezing time, which usually does not last for a long time. Compared with seasonal frozen soil and permafrost (permafrost), the freezing and thawing cycle of short-term frozen soil is shorter, and the impact on engineering construction and the natural environment is relatively small.

[0090] For example, if a certain area is seasonally frozen, then a constant temperature frozen soil environment simulation can be used from November to February (winter), and an alternating low-temperature frozen soil environment simulation and / or a short-term frozen soil environment simulation can be used from March to October (spring, summer, and autumn). Different temperature curves are used to perform different numbers of cycles in sequence to simulate the corrosion conditions in different time periods, and finally the annual corrosion rate is calculated or estimated by weighting and other methods.

[0091] Preferably, the metal sample can be in two shapes, and the specific specifications are as follows:

[0092] Plate specimen: Overall dimensions: length l × width b × thickness h;

[0093] Circular specimen: Overall dimensions: diameter φ×thickness h;

[0094] l, b, φ≤100mm, h is 2~5mm, and the surface area of ​​each sample is ≥10cm 2The shape and size of the specimen depend on the original conditions of the test material and the test container used. Specimens with large surface area per unit mass and small ratio of side to total area should be used as much as possible. In general, the area perpendicular to the rolling or forging direction shall not be greater than half of the total area of ​​the specimen, but the specimen should not be too large. Specimens of other shapes and sizes may also be used depending on the purpose of the test. The specimen is suspended in the solution by hanging.

[0095] Specifically, the internal gas atmosphere includes nitrogen and oxygen, Vnitrogen: Voxygen=7~8:3~2, and the internal gas atmosphere pressure is 50~100kPa. Since metal materials are buried or semi-buried underground in actual situations, the influence of atmospheric corrosion is relatively small, and its influence is omitted in the present invention; however, oxygen in the atmosphere has an important influence on metal corrosion in the soil. It not only participates in the metal oxidation reaction, but also acts as an electron acceptor in the corrosion process, accelerating the occurrence and development of corrosion. The corrosion rate of oxygen on metals increases with increasing temperature and concentration, and the presence of water will also accelerate the corrosion rate of metals. In order to restore the air pressure conditions of the frozen soil environment as much as possible (generally speaking, the frozen soil environment is mostly plateaus with low air pressure) and atmospheric components, the present invention can adjust the composition and air pressure of the circulating gas to make the simulated corrosion process as close to the actual situation as possible.

[0096] Specifically, the specific operations of step S4 are:

[0097] S4-1: The metal to be tested is made into metal samples of certain specifications, and the metal samples are weighed and recorded;

[0098] S4-2: The metal sample is completely immersed in the simulated corrosion solution, and the container containing the simulated corrosion solution is placed in a closed simulation chamber and pre-temperatured;

[0099] S4-3: According to a preset temperature curve, a temperature-controllable circulating gas is introduced into the closed simulation chamber to adjust the temperature in the simulation chamber and start timing, so that the temperature in the chamber changes according to the preset temperature curve;

[0100] S4-4: After reaching the preset detection time, the corroded metal sample is taken out and cleaned to remove the surface corrosion part, and the corrosion amount is calculated by the weight difference before and after.

[0101] Specifically, the specific operation of pre-temperature adjustment in step S4-2 is: close the simulation chamber and start temperature adjustment, and proceed to the next step when the simulated corrosion solution and the metal sample reach the preset initial temperature; the pre-temperature adjustment time is 0.5 to 2 hours, the purpose is to avoid a large difference between the target temperature and the initial temperature, which will cause the temperature of the simulated corrosion solution and the metal sample to fail to reach the preset temperature curve.

[0102] Specifically, the specific composition of the simulated corrosion solution is calculated by mass percentage as follows: Na2SO4 0.06-1%, MgCl2 0.05-0.1%, NaCl 3-5%, and the rest is deionized water; the pH value of the simulated corrosion solution is 6-8. The hydrological conditions in permafrost areas are very different from those in plain areas. Taking the salt lake area as an example, it has gradually desalinated due to the warming and humidification of the Qinghai-Tibet Plateau. The Qingshui River downstream has changed from brackish water to salt water due to the discharge of the salt lake; the hydrochemical types of perennial surface water such as the Salt Lake and Qingshui River and their shallow groundwater are all Cl·SO4-Na type, with evaporation and crystallization as the dominant role, while the chemical type of the northern river water is HCO3-Mg·Ca, and its shallow groundwater has similar ion composition to the deep groundwater of the Qingshui River. The water quality is weakly alkaline as a whole, and the pH value fluctuates between 7.8 and 8.5 with different seasons. The cation in the water is mainly Ca 2+ Mg 2+ The main anion is Cl - 、SO4 2- 、HCO3 - Mainly, Cl - Ion concentration 40~190mg / l, SO4 2- Ion concentration 8~25mg / l, HCO3 - The ion concentration is 190-480 mg / l, which varies with the seasons. The water chemical composition is mainly affected by rock weathering and dissolution. The main component and related analysis show that Na + , K + , Cl - Mg 2+ and SO4 2- Mainly from evaporative salt rocks such as rock salt and magnesium sulfate, Ca 2+ 、HCO3 - It mainly comes from silicate rocks such as dolomite and calcium feldspar. - 、SO4 2- 、HCO3 - Plasma will corrode metals, so the corrosion resistance requirements of these ions need to be considered when using steel for highway construction in this area; among them, Cl - 、SO4 2- The contribution to metal corrosion is greater. Theoretical analysis and experimental verification show that the composition of the simulated corrosion solution can basically restore the metal corrosion situation in the frozen soil environment.

[0103] It is worth noting that due to the long corrosion test time, when accelerated experiments are required, the concentration of the reagents needs to be increased accordingly, and the ambient temperature needs to be appropriately increased to speed up the reaction rate. The temperature control curve adjustment cycle needs to be adjusted synchronously according to the acceleration ratio. When the temperature alternation of freezing and thawing affects the corrosion rate, the purpose of accelerating corrosion can be achieved by shortening the heating / cooling time and increasing the number of cycles.

[0104] Furthermore, in order to simulate the effect of physical damage to the surface of metal materials caused by freeze-thaw on corrosion, a portion of sand and soil components can be added to the simulated solution, and the sample is inserted into the sand and soil and is completely covered by the simulated corrosion solution to simulate the additional wear caused by the change in volume of soil freezing under an alternating environment. The corrosion conditions of the metal sample buried in the sand and soil will be closer to the corrosion conditions in the real environment.

[0105] The present invention also provides a device for implementing the simulation evaluation method designed by the design method: the device comprises two parts: a temperature control module (1) and a simulation test module, the temperature control module is used to control the temperature change inside the simulation test module, and the simulation test module is used to implement a full immersion corrosion simulation corrosion test of a metal sample.

[0106] The temperature control module 1 includes a heat exchange medium circulation system composed of a radiator 13, a four-way reversing valve 11, a first heat exchanger 16, a second expansion valve 151, and a first expansion valve 141 connected in sequence, wherein the four-way reversing valve 11 is also connected to a compressor 12; the first expansion valve 141 is provided with a first check valve 142 in parallel; the second expansion valve 151 is provided with a second check valve 152 in parallel;

[0107] The simulation test module includes a closed simulation chamber 2, and a second heat exchanger 21, a circulating fan 22, a sample pool 23 and a sensor 25 are arranged inside the simulation chamber. The second heat exchanger 21, the sample pool 23 and the circulating fan 22 are arranged in sequence to form an annular air path. It is worth noting that the components arranged in sequence here do not have a physical connection relationship, but are only arranged in sequence on the gas circulation path / annular closed road to achieve temperature control of the closed simulation chamber 2. The sensor 25 is arranged in the middle and upper part of the sample pool 23. The simulation chamber is provided with an exhaust hole, and a negative pressure filter 24 is provided at the exhaust hole;

[0108] The first heat exchanger 16 and the second heat exchanger 21 are installed close to each other to achieve contactless heat exchange.

[0109] The temperature control module 1 adopts a reverse Carnot cycle to control the temperature of the simulation test module by changing the direction of heat flow. The first heat exchanger 16 is placed in the environment and can exchange heat with the atmosphere; the first heat exchanger 16 adopts an isolation design to exchange heat with the simulation test module (mainly the second heat exchanger 21).

[0110] When cooling is required, the heat pump transfers heat from the second heat exchanger 21 to the first heat exchanger 16; after the temperature of the second heat exchanger 21 is reduced, it absorbs heat from the closed simulation chamber 2 circulation system, causing its temperature to decrease; after the first heat exchanger 16 obtains heat, its temperature increases, and it releases the heat to the surrounding atmosphere. When heating is required, the heat pump reverses, and heat flows from the first heat exchanger 16 to the second heat exchanger 21. At this time, the temperature of the first heat exchanger 16 decreases, and it absorbs heat from the surrounding atmosphere; after the second heat exchanger 21 obtains heat, its temperature increases, and it releases heat to the simulation chamber circulation system, raising its temperature. The second heat exchanger 21 itself is provided with an electric heater. When the surrounding environment is insufficiently heated, the heater can be used for auxiliary heating to quickly increase the temperature of the simulation chamber;

[0111] The simulation test module adopts an air-cooling design, and the surrounding environment of the sample is simulated by changing the temperature and circulation speed of the circulating air. Since the simulated corrosion solution has a certain volatility, the circulating gas contains a certain amount of corrosive components. The closed simulation chamber 2 adopts a corrosion-resistant design to ensure its service life; the simulation chamber is equipped with temperature, humidity, pressure and other sensors 25 for detecting the current environmental conditions; the gas components of the circulation system are mainly nitrogen and oxygen, and their proportion and pressure are set according to the atmospheric conditions of the simulated environment;

[0112] In order to protect the internal equipment and the surrounding environment, the temperature control module 1 and the simulation test module are isolated and exchange heat through the heat exchange system to prevent the simulated gas from affecting the temperature control module 1.

[0113] Specifically, the sample pool 23 is provided with a matching reagent tank for holding the simulated corrosion solution, and the reagent tank is provided with fins. To improve the heat exchange effect, fins are provided on the outside of the reagent tank along the airflow direction to increase the heat exchange area with the gas, so that the reagent in the reagent tank reaches the predetermined temperature faster.

[0114] Specifically, the operating process / principle of the temperature control module 1 is as follows:

[0115] The various components inside the temperature control module 1 are connected by pipes, and the inside of the pipes is a heat exchange medium. When cooling is required, the heat exchange medium enters from the inlet of the compressor 12, and changes from gas to liquid after compression, and releases heat through the radiator 13. The heat exchange medium in a liquid state evaporates from liquid to gas due to the pressure reduction after passing through the first check valve 142 and the second expansion valve 151, and absorbs heat through the heat exchanger.

[0116] The first heat exchanger 16 of the temperature control module 1 is installed close to the second heat exchanger 21. As the heat is transferred to the first heat exchanger 16, the temperature of the gas passing through the second heat exchanger 21 decreases, thereby achieving the purpose of refrigeration. The refrigerated gas is pressurized by the circulating fan and enters the space where the sample pool 23 is located, where the temperature is reduced to simulate the low temperature environment of frozen soil (see Figure 1 ).

[0117] When the frozen soil thawing process needs to be simulated, the four-way valve is reversed, and the heat exchange medium is compressed from gas to liquid after passing through the compressor 12, and then enters the first heat exchanger 16, releasing heat at the same time. The heat exchange medium that has become liquid passes through the second one-way valve 152 and the first expansion valve 141 and becomes gas due to the pressure reduction. Then, it exchanges heat with the outside air through the radiator 13 and returns to the compressor 12 through the four-way valve to complete the cycle. The heating process of the temperature control system (see Figure 2 ).

[0118] Refrigeration and heating are completely completed through the heat pump system. A sensor 25 is provided in the closed simulation chamber 2 to control the refrigeration and heating amount according to the current temperature so that the environment in the chamber meets the detection requirements.

[0119] An isolation design is adopted between the temperature control module 1 and the simulation test module (closed simulation chamber 2), and the gases between the two are not connected, so as to avoid the corrosive gas generated by the volatilization of the reagent in the sample pool 23 from affecting the equipment; the simulation chamber adopts a closed design and is also provided with a negative pressure filter 24, which can extract and purify the gas in the chamber before discharging it when the sample is placed in and taken out, so as to avoid the direct discharge of corrosive gas and cause environmental pollution.

[0120] Application Examples

[0121] Taking the metal corrosion simulation of the frozen soil environment (December) in the Mohe Basin in Northeast China as an example, the following Figure 1 The device shown carries out corrosion simulation evaluation on four kinds of steel materials to be tested, namely, Q235 steel, Q460 steel, Q690 steel, and Ni-Cr-Mo-V low alloy high strength steel.

[0122] According to the average temperature curve of Mohe in December ( Figure 4 ) It can be seen that the highest temperature is around -20℃ and the lowest temperature is around -35℃, which belongs to the alternating freezing environment in the alternating low-temperature permafrost environment. The average daily sunshine time in Mohe in December is about 7.5 to 8 hours.

[0123] The temperature curve is:

[0124] The temperature curve range is between -35℃ and -20℃, and the preset initial temperature is -20℃;

[0125] Phase 1: Target temperature -35±1℃, cooling rate 0.5~10℃ / hour, keep warm after reaching the target temperature, a total of 16h;

[0126] The second stage: target temperature -20±1℃, heating rate 0.5~5℃ / hour, keep warm after reaching the target temperature, a total of 8h;

[0127] With a cycle of 24 hours, the total detection time is 3 days.

[0128] According to the China Soil Database, the soluble salt content of soil in Mohe area is 0.6-1.0%, the anion composition is mainly chloride ions, with only a small amount of sulfate ions, the equivalent ratio of chloride ions to sulfate ions is greater than 4, and the texture is mostly loamy clay. The soil is mostly slightly alkaline, with a pH of 7.4-8.4, and the exchangeable base is mainly calcium and magnesium ions.

[0129] Prepare a simulated corrosion solution, Na2SO4 0.06%, MgCl2 0.1%, NaCl 3.5%, the rest is deionized water, pH = 8;

[0130] Sample specifications: 50mm×40mm×3mm;

[0131] Sand addition amount: 80g / L (based on the total volume of simulated corrosion solution);

[0132] Circulating gas parameters: Nitrogen: oxygen (volume ratio) = 8:2, gas pressure 95kPa; (the local air pressure in Mohe is about 95kPa)

[0133] The corroded metal sample is taken out and cleaned to remove the surface corrosion part, and the corrosion amount is calculated by the weight difference before and after. The corrosion rate calculation formula is as follows:

[0134]

[0135] X---------Test piece corrosion rate, mm / a

[0136] W1--------Weighing of the test piece before testing, g

[0137] W2--------Weighing of the test piece after the test, g

[0138] 87600----Calculation constant

[0139] A---------Surface area of ​​the test piece, cm 2

[0140] T----------Test time, h

[0141] D---------Test piece material density, g / cm 3

[0142] Table 1 Summary of the simulated corrosion results of each sample in alternating low temperature environment

[0143]

[0144] Since the corrosion rate decreases with decreasing temperature, in the simulation of the December environment in Mohe, although the corrosion resistance of several samples is different, the absolute value difference of the corrosion rate is relatively small. Samples 1 to 3 have similar material prices, similar corrosion rates, and strengths that differ by 2 to 3 times. Therefore, high-strength materials can be selected. After calculating the total cumulative corrosion thickness based on a 20-year service life, sufficient thickness margins need to be reserved during structural design. Sample 4 (Ni-Cr-Mo-V low-alloy high-strength steel) has the best corrosion resistance, but its cost is 3 to 4 times that of other samples. Unless extreme environments or applications have restrictions on the weight of material structures, samples 2 and 3 (Q460 steel, Q690 steel) are sufficient to meet construction needs. Compared with samples buried in the field, the corrosion rate error is ≤5%, which can meet the needs of corrosion resistance evaluation and preliminary screening of metal materials.

[0145] This test only screens materials from the perspective of corrosion resistance, and the specific material selection must also be combined with the low-temperature mechanical properties of the material. The embodiment is aimed at evaluating the corrosion resistance of a certain area in a specific time period. When calculating the cumulative corrosion amount, it is necessary to calculate the annual average corrosion rate after weighted averaging of each time period throughout the year.

[0146] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A design method for evaluating the corrosion resistance of metals in frozen soil environments, characterized in that: The specific steps include: S1: Collect monthly temperature curves, air pressure range, atmospheric composition, hydrology and soil composition information of the area to be simulated; S2: Design and prepare a test device according to the upper and lower limits of the temperature curve, determine the composition of the simulated corrosion solution according to the hydrological and soil composition information, and prepare the simulated corrosion solution; S3: Preset the temperature curve according to the annual and monthly temperature curves, and adjust the gas atmosphere inside the testing device according to the air pressure variation range and the atmospheric composition information; S4: The metal to be tested is made into a metal sample of a certain specification, and a full immersion corrosion simulation experiment of at least one temperature curve cycle is carried out using the test device and the simulated corrosion solution, and the corrosion behavior and corrosion resistance of the corresponding metal in the frozen soil environment are estimated according to the corrosion simulation results; wherein one temperature curve cycle is 24 hours.

2. The design method according to claim 1, characterized in that: The monthly temperature curve maximum temperature T max Not higher than the local freezing point T0 and the difference between the upper and lower limits of the monthly temperature curve T max -T min ≤10℃, it is determined that the simulated area is a constant temperature frozen soil environment in the corresponding time period.

3. The design method according to claim 1, characterized in that: The difference between the upper and lower limits of the monthly temperature curve T max -T min >10℃ and the maximum temperature T max ≤ local freezing point T0, it is determined that the simulated area is in an alternating freezing environment during the corresponding time period; The difference between the upper and lower limits of the monthly temperature curve T max -T min >10℃ and the maximum temperature T max >Local freezing point T0, and T max -T0<T0-T min When , it is determined that the area to be simulated is an alternating and gradual melting environment in the corresponding time period; The alternating freezing environment and the alternating gradual thawing environment are collectively referred to as alternating low-temperature frozen soil environment.

4. The design method according to claim 1, characterized in that: The maximum temperature T of the monthly temperature curve max >Local freezing point T0 and minimum temperature T min < local freezing point T0, and T max -T0≥T0-T min When the simulated area is in a short-term frozen soil environment in the corresponding time period.

5. The design method according to any one of claims 2 to 4, characterized in that: Different months in the area to be simulated may belong to different permafrost environment types. The actual simulation evaluation can be implemented by combining different types of temperature curve cycles.

6. The design method according to claim 3, characterized in that: A temperature curve of alternating low-temperature frozen soil environment used to simulate the environment of the northeastern basin of Mohe in December is: The temperature curve range is between -35°C and -20°C, and the preset initial temperature is -20°C; Phase 1: Target temperature -35±1℃, cooling rate 0.5~10℃ / hour, keep warm after reaching the target temperature, a total of 12~16h; The second stage: target temperature -20±1℃, heating rate 0.5~5℃ / hour, keep warm after reaching the target temperature, a total of 8~12h; With a cycle of 24 hours, the total detection time is 1 to 30 days.

7. The design method according to claim 1, characterized in that: The internal gas atmosphere includes nitrogen and oxygen, Vnitrogen:Voxygen=7-8:3-2, and the internal gas atmosphere pressure is 50-100 kPa.

8. The design method according to claim 1, characterized in that: The specific operations of step S4 are: S4-1: The metal to be tested is made into metal samples of certain specifications, and the metal samples are weighed and recorded; S4-2: The metal sample is completely immersed in the simulated corrosion solution, and the container containing the simulated corrosion solution is placed in a closed simulation chamber and pre-temperatured; S4-3: According to a preset temperature curve, a temperature-controllable circulating gas is introduced into the closed simulation chamber to adjust the temperature in the simulation chamber and start timing, so that the temperature in the chamber changes according to the preset temperature curve; S4-4: After reaching the preset detection time, the corroded metal sample is taken out and cleaned to remove the surface corrosion part, and the corrosion amount is calculated by the weight difference before and after.

9. The design method according to claim 1, characterized in that: The specific composition of the simulated corrosion solution is calculated by mass percentage: Na2SO4 0.06-1%, MgCl2 0.05-0.1%, NaCl 3-5%, and the rest is deionized water; the pH value of the simulated corrosion solution is 6-8.

10. A device for implementing the simulation evaluation method designed by the design method according to any one of claims 1 to 9, characterized in that: The device comprises two parts: a temperature control module (1) and a simulation test module. The temperature control module is used to control the temperature change inside the simulation test module. The simulation test module is used to implement a full immersion corrosion simulation corrosion test on a metal sample.

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