Research Progress on corrosion characteristics of CO2 marine storage system pipeline
With global climate change becoming the main threat to human sustainable development, reducing CO2 emissions to mitigate climate change has become a hot topic in the international community. Carbon capture and storage (CCS) technology is of great significance in reducing CO2 emissions, which can effectively alleviate the climate change caused by CO2 emissions . In CCS technology, the carbon capture, utilization and storage (CCUs) technology is added, which can not only store CO2 permanently, but also use CO2 to improve crude oil recovery, produce economic benefits and reduce the cost of CO2 storage. Among the existing schemes for long-term storage of CO2, CO2 injection into marine or terrestrial geological structures is considered to be a very promising storage method .
CO2 Marine Storage refers to a storage method that stores the captured CO2 under the deep sea or seabed sediment and isolates it from the atmosphere. Many scholars have studied the technology of CO2 storage in the ocean. CO2 storage in the ocean is considered to accelerate the process of CO2 absorption in the ocean, and it is also an effective and feasible storage scheme. Compared with geological storage, using the ocean as a sealed storage place for CO2 not only has a larger storage capacity, but also has a safer and more stable storage effect . Domestic research on CCS technology started late, and there is still a big gap compared with foreign countries, and most of the research is focused on land CO2 storage technology, and the research on CO2 marine storage technology is relatively less. The large sedimentary basins in the coastal areas of China have good CO2 storage conditions and huge storage capacity. The application of CO2 marine storage technology can store a large amount of CO2.
CO2 transportation is the key link of CCS technology. It has been proved that pipeline transportation is the most efficient way to transport a large amount of CO2 in medium or long distance. However, some impurities, such as H2O, NO2, SO2, O2, etc., are unavoidable in the CO2 captured from the flue gas of coal-fired power station. Once free water exists in the pipeline, CO2 and other impurities will dissolve in the water phase, change its chemical composition and increase its corrosiveness , resulting in the corrosion of metal pipes. At present, many researches on the corrosion of CO2 transportation pipeline are focused on the high temperature supercritical phase CO2 corrosion in the process of land transportation and geological storage, while the research on the corrosion caused by the low temperature liquid phase CO2 transportation in the process of marine storage is relatively lacking. Moreover, in the process of CO2 marine storage, the environment of transportation pipeline is more complex. The complex external marine environment and impurities in CO2 will affect the corrosion of pipeline.
CO2 Marine Storage Technology
In fact, CO2 ocean storage is to inject CO2 into the deep sea or deep-sea strata to form hydrate or heavier plume to sink into the bottom of the ocean, so as to achieve the purpose of stable CO2 storage. The reason why the ocean can be used as a storage place for a large amount of CO2 is determined by the special environment of the ocean and the characteristics of CO2 in the marine environment.
Characteristics of CO2 in the marine environment
Basic properties of CO2
In the atmospheric environment, pure CO2 is a heavier and stable gas than air. In the marine environment, due to the gradual decrease of temperature and pressure with the increase of depth, CO2 can only exist as a gas in the initial shallow area hundreds of meters deep, and as a liquid under this shallow area. Therefore, CO2 is usually injected in liquid form in marine storage. CO2 is soluble in water, and its solubility increases with the increase of pressure and decreases with the increase of temperature and salinity of water. In the environment rich in water, under the condition of low temperature and high pressure, CO2 will form solid hydrate with higher density than water . Understanding these basic properties of CO2 can provide a reference for choosing appropriate methods and sites for CO2 treatment and storage.
Due to the higher compressibility of liquid CO2 compared with water, its density will increase with the increase of depth. When the injection depth is fixed, the density of liquid CO2 becomes larger than that of water under high pressure and low temperature. This kind of low temperature and high pressure condition is not common in land environment, but it is common in deep sea environment. When the injection depth is 3000 m, the liquid CO2 with higher density than water will sink, and form a CO2 Lake in the low-lying area of the sea bottom and exist stably .
If CO2 is injected into seabed sediments with a depth of more than 3000 m, at this time, because the density of CO2 is higher than that of pore fluid in seabed sediments, the pore fluid with a smaller density will float to fill the lower CO2 storage cap layer and ensure the gravity stability of the lower layer . The gravity stability makes CO2 naturally gather at the bottom of the sediment. Even if there are cracks in the sediment cover, CO2 will not leak, and even the geological and mechanical disturbances such as earthquakes will not cause CO2 to escape. It is worth noting that when pure CO2 is injected into the formation below the marine sediment, there is a critical buoyancy depth of injected liquid CO2 due to the existence of geothermal gradient. Above the critical buoyancy depth, the density of liquid CO2 is higher than that of the environmental pore fluid. When below the critical buoyancy depth, the density of liquid CO2 becomes lower than that of the environmental pore fluid because the formation temperature increases with the depth. The range between the bottom of the ocean and the critical buoyancy depth is called the negative buoyancy zone (NbZ) . If injected under NbZ, CO2 will rise under the action of buoyancy until it reaches the bottom of NbZ, that is, the area where the density of liquid CO2 becomes higher than that of surrounding void fluid. In this area, the negative buoyancy acting on CO2 will prevent it from rising further.
Formation of CO2 hydrate
When liquid CO2 is in the environment of high pressure and low temperature, the formation of CO2 hydrate is rapid and stable. CO2 hydrate (5.75 H2O-CO2) is a non stoichiometric crystalline compound, which is formed under high pressure and low temperature by trapping CO2 molecules in hydrogen bonded H2O cages. These compounds exist as three-phase metastable equilibrium between CO2 (L), CO2 (AQ) and hydrate .
In the marine environment, injected CO2 can be converted into hydrate form in the sea water below a certain depth. In the sediment, because the temperature will increase with the increase of the depth of the sediment due to the existence of geothermal gradient, and the pressure that hydrate can stably exist will also increase. Therefore, there is an area that can stably generate CO2 hydrate in the sediment layer, which is called hydrate forming area (HFZ) . On the one hand, the formation of hydrate in sediment will lead to the decrease of effective porosity and permeability of submarine sediment fluid, and may even block the formation. On the other hand, the formation of CO2 hydrate can also significantly limit the flow of CO2 stored in the deeper area , which is conducive to storage.
CO2 storage methods in the marine environment
Koide et al.  proposed for the first time to store CO2 in the seabed aquifer, and determined three types of marine CO2 storage methods based on the ocean depth: shallow layer (300 m), deep layer (300-3700 m) and ultra deep layer (> 3700 m). After that, many scholars have carried out extensive research on marine CO2 storage technology [12, 13, 14, 15].
In terms of current research, the storage methods of marine CO2 can be divided into the following types: shallow aquifer storage, marine water storage, marine solid storage and deep-sea sediment storage. Considering the storage cost and effect, CO2 deep-sea sediment storage is a hot research topic, which is closely related to the gravity stability of CO2 in the marine environment and the formation of hydrate.
The key point of deep-sea sediment storage is to inject CO2 under the 3000 m deep-sea sedimentary layer, so that the CO2 injected into the sediment will migrate in the sediment and form a CO2 hydrate cover, so as to achieve the purpose of CO2 storage . This storage method requires CO2 (usually liquid) to be injected below the corresponding NbZ. Below the NbZ, the density of CO2 becomes smaller than that of the surrounding fluid, and will rise under the action of slight buoyancy (Fig. 1a). When CO2 rises into HFZ area of sediment, CO2 will form stable and dense hydrate. A large amount of CO2 hydrate in the pores of marine sediment will form a cover layer to hinder the upward migration of CO2 and force CO2 to migrate laterally (Fig. 1b). On a large scale, liquid CO2 in the lower layer of hydrate will diffuse to form a “pool”, which is covered with an impermeable CO2 hydrate . House et al.  carried out a simulation study on the migration rule of CO2 in the seabed sediment with an injection depth of 3700 m, and the results show that this method can store CO2 permanently. After about 106 years, the liquid CO2 and CO2 hydrate in the formation will completely dissolve and form CO2 solution. The solution will permeate through the porous matrix, diffuse and mix in the sediment gap, forming a zero buoyancy solution. The CO2 solution penetrating into the porous matrix even reacts with the sediment to form carbonate minerals which are permanently sealed in the sediment (Fig. 1c). It is worth noting that the cap formed by CO2 hydrate and sediment in the deep sea area is still within the HFZ range. Therefore, even if there are holes or cracks in the cap, any liquid or gaseous CO2 that can escape through the CO2 hydrate cap will generate CO2 hydrate again once it contacts the water rich sediments. This self sealing of CO2 hydrate cap in the sediment Capacity is of great significance for effective CO2 storage .
Fig.1 Principle of CO2 sequestration in deep-sea sediments: (a) small amounts of hydrate form when the top of the plume enters the HFZ, (b) the self-forming hydrate cap will have expanded laterally and trapped substantial quantities of CO2 (l), (c) eventually the CO2 (l) and CO2 hydrates will have dissolved and formed a CO2 (aq) solution. Once the solution is neutrally buoyant, further solute migration will only occur by diffusion
The advantage of deep-sea sediment CO2 storage is that a stable CO2 hydrate cover can be formed in a large area of the seabed with a depth of more than 3000 m, so as to realize the stable storage of a large amount of CO2, with little impact on marine ecology and long storage period . Although the deep-sea sediment CO2 storage has a good storage effect, the disadvantage of this technology is that the cost of deep-sea drilling is very high. With the continuous research of CO2 storage combined with combustible ice mining technology, the cost of marine CO2 storage is expected to be greatly reduced .
The corrosion problem of CO2 ocean sealing system
The storage and transportation of CO2 is an important part of CCS technology. Pipeline transportation has become the most important way of CO2 transportation with the advantages of large capacity, long distance and good economy. In the past few years, the problem of CO2 corrosion to steel in pipeline transportation has been widely concerned.
CO2 ocean storage system includes four basic processes of CO2 capture, pressurized storage, transportation and ocean injection. In this series of processes, large-scale CO2 storage and transportation are involved. Due to the lack of large-scale application of CO2 marine storage technology, the current research on CO2 transportation pipeline corrosion in CCS technology is mainly focused on land pipelines, while the research on pipeline corrosion in CO2 marine storage system is relatively less.
Transport characteristics of CO2 Marine Storage
Characteristics of marine environment
Due to the relatively stable land conditions, the land transportation pipeline is basically in a static external environment, while the marine environment has many unstable factors, and the marine transportation pipeline is in a dynamic and complex environment. In the marine environment, with the increase of the ocean depth, the temperature and pressure of the external seawater will change accordingly. In the 3000 m deep marine area, the temperature of the sea surface and the storage site can be even different by 25 ℃. Therefore, in the relatively short transportation process of injecting CO2 from the Sea into the storage site, the pipeline will cross a large temperature range, with different depths at the same time Tao will be in a different temperature environment. In addition, due to the influence of current and wave near the ocean surface, the fast flowing sea water will produce violent scour on the pipeline on the ocean surface, while the deep sea sea water will flow slowly, and the scour effect is not obvious; moreover, due to the different water temperature at different ocean depths, the dissolved oxygen, pH value, and types of marine microorganisms will also vary with the change of ocean depth However, these dynamic characteristics in the marine environment will have an important impact on the corrosion of transportation pipelines.
Injection mode of CO2 Marine Storage
The captured CO2 will be transported to the storage site. Geological storage only involves the land transportation process, but if the inland CO2 gas source is stored in the sea, it will involve two processes: land transportation and sea transportation. At present, there are two ways to inject CO2 into the ocean: as shown in Figure 2a, the captured CO2 is stored in a tank near the coast under pressure, and then injected directly into the designated marine storage site through the submarine pipeline; as shown in Figure 2B, the high-pressure CO2 is transported to the offshore operation platform through the transport ship, and injected into the designated storage depth through the pipeline on the offshore operation platform.
Fig.2 CO2 transportation by subsea pipeline (a) or oceanic tankers (b) for deep ocean sequestration
The phase state of CO2 transport in ocean storage is different from that in land transport. During land transportation, CO2 is generally in supercritical phase, and the injected CO2 is also in supercritical state; while the marine storage site is generally located in high-pressure and low-temperature environment, which is generally transported and injected in the way of liquid CO2. Compared with supercritical CO2, the density and viscosity of liquid CO2 are larger, the diffusion coefficient is smaller, and the solubility of impurity gas in liquid and supercritical CO2 systems are also different, which will inevitably lead to different corrosion characteristics.
The internal corrosion of the pipeline is closely related to the nature of the transport medium. In the CO2 marine storage system, the transport medium is liquid CO2. In general, pure CO2 will not cause corrosion to carbon steel pipes. However, in CCS technology, due to the existence of combustion impurities in the captured CO2 source, it is difficult to thoroughly purify it, which will make the CO2 in transportation contain water, O2, SO2, H2S and NO2 and other impurities. The existence of impurity gas will increase the corrosiveness of CO2 system, and then cause pipeline failure. It is not only difficult to remove all the impurities in the captured CO2, but also the cost of CO2 capture and storage will be greatly increased. It can be said that the internal corrosion of the CO2 transport pipeline is mainly caused by the impurities contained in the CO2 system.
CO2 corrosion mechanism
Water is very important for electrochemical reaction. It provides necessary solution phase for electrochemical reaction. Once the water molecules precipitate on the surface of the pipe, CO2 will dissolve in the water quickly to form carbonic acid (H2CO3), which will reduce the pH value of the liquid phase and lead to metal corrosion. The total reaction formula of CO2 corrosion can be written as follows:
The electrochemical corrosion process of CO2 is usually accompanied by the formation of FeCO3 products (or other iron compounds such as Fe2O3). According to the experimental conditions for their formation, they can be protective or unprotected. External factors (such as temperature, pressure, pH value, reaction time, impurity gas, solution chemistry, flow rate and material, etc.) can affect the corrosion rate by affecting the protection of the corrosion product film on the substrate.
Local corrosion is the most common type of CO2 corrosion. Although the quality of local loss is smaller than that of uniform corrosion, it is a more serious way of corrosion. Local corrosion can cause serious perforation, fracture and damage. Field data show that almost all pipeline accidents are caused by local corrosion.
Difference between liquid and supercritical CO2 corrosion
Many scholars have studied the influencing factors of supercritical CO2 corrosion, including CO2 partial pressure, temperature and impurities [18,19], but the research on liquid CO2 corrosion is relatively rare. The experimental results of Choi et al.  show that the corrosion rate of carbon steel in liquid (8 MPa, 25 ℃) CO2 environment is much lower than that in supercritical (12 MPa, 80 ℃) CO2 environment in water saturated CO2 system containing 0.02% (volume fraction) H2S. The experiment of Su I et al.  compared the difference of corrosion rate of X65 steel in gas, liquid and supercritical phase CO2 containing H2S impurities, and found that X65 steel had the lowest corrosion rate in gas phase CO2, and the results are shown in Figure 3. Fareras et al.  found that in the CO2 system containing 0.065% (volume fraction) H2O and 0.1% (volume fraction) SO2, the corrosion rate in the liquid environment was higher than that in the supercritical state.
Fig.3 Corrosion rates of X65 steel exposed to gas, liquid and supercritical CO2 containing H2S impurity: (a) p=8 MP; (b) T=35 ℃
At present, there is no systematic study on the corrosion characteristics of liquid CO2. Only some scholars have carried out a few experiments on the corrosion of liquid CO2 in the process of studying supercritical CO2 corrosion. Mahlobo et al.  believe that the different corrosion rate of metals in liquid CO2 and supercritical CO2 is due to the change of CO2 phase state which affects the solubility of impurity gas in CO2 system. The research on the corrosion mechanism of liquid CO2 is still limited, especially how the impurity gas affects the corrosion process in the liquid CO2 system is not clear, and further experimental research is needed.
Effect of water content on internal corrosion
In the liquid CO2 system, water content plays a key role in the corrosion behavior of pipeline steel, because the existence of water provides solution conditions for the electrochemical reaction on the surface of metal pipeline, and further determines the degree of corrosion.
When the water content in CO2 system is lower than the solubility of water in CO2, the water will completely dissolve in CO2, and the corrosion effect is lighter. Once the water content is higher than its solubility, the water will easily precipitate in the inner wall of the pipeline. Obviously, a lot of CO2 and impurity gas (SO2, NO2, H2S, O2, etc.) will be dissolved in the water separated from the inner wall of the pipeline, forming a low pH solution phase, causing metal corrosion. In CO2 system, the corrosion rate increases with the increase of water content, so it is necessary to strictly control the water content of CO2 in pipeline transportation.
The solubility curve of water in CO2 system at different pressures and temperatures is shown in Figure 4 . It can be seen that the solubility of water in liquid CO2 system decreases with the decrease of temperature at a certain pressure. In the special marine environment, with the increase of the injection depth, the external ocean temperature will significantly reduce, which may lead to the supersaturation of water in the liquid CO2 system during the injection process and precipitation, which will aggravate the corrosion of the pipeline.
Fig.4 Solubility of water in CO2 phase
Effect of impurity gas on internal corrosion
In CCS system, impurity gas is the main cause of internal corrosion. With the development of CCUs technology, the influence mechanism of impurity gas on Corrosion in supercritical CO2 system has become a research hotspot. Sun et al.  found that when a small amount of O2, SO2, H2S impurity gas was introduced, the corrosion rate of water saturated supercritical CO2 system would significantly increase; through XRD and EDS analysis of the corrosion product film, it was found that when there was impurity gas in supercritical CO2 system, the content of FeCO3 in the corrosion product film was significantly reduced, and the impurity gas introduced became the main factor controlling the corrosion rate. Choi et al.  found that the corrosion rate of carbon steel and 1Cr was very low (< 0.01 mm / a) in the water saturated liquid CO2 environment (12 MPa, 25 ℃), and the corrosion rate increased sharply after adding 0.02% (volume fraction) H2S (carbon steel 0.07 mm / A, 1gr 0.13 mm / a). Fareras et al.  found that in the liquid CO2 system with water content of 0.065% (volume fraction) (8 MPa, 25 ℃), when the content of SO2 is lower than 0.05%, X65 carbon steel almost does not corrode; but when the content of SO2 is 0.1%, the corrosion is relatively obvious (about 0.1 mm / a). It can be seen that whether it is liquid or supercritical CO2, trace impurity gas will have an important impact on corrosion. Therefore, the concentration of impurity gas in CO2 system must be strictly controlled. However, at present, there is no uniform standard for the content of impurity gas in CCS technology.
In CCS system, there are many kinds of impurity gases in the captured CO2 at the same time. When free water exists, these impurity gases will dissolve in the water phase to produce complex reaction, and even produce more corrosive substances. For example, when O2 and SO2 exist at the same time, SO2 will be oxidized to form H2SO4. Sun et al.  have studied the situation that there are many kinds of impurity gases in water saturated supercritical CO2 at the same time. The results show that when two or more kinds of impurity gases exist, the corrosion situation is more serious. The reason for this phenomenon is that there is a reaction between the impurity gases, which produces a more corrosive substance, or the product destroys the continuity of the corrosion product film and reduces its protection Low.
In CO2 system, impurity gas mainly affects the corrosion process by accelerating the precipitation of water and changing the morphology of corrosion product film . On the one hand, the impurities in CO2 can change the solubility of water in CO2 system, and intensify the corrosion by promoting the precipitation of dissolved water and changing the chemical characteristics of water. The experiments of Ahmad and gersen  show that the presence of trace O2, N2 and CH4 impurities can reduce the solubility of water in the CO2 system of gas phase or liquid phase at a pressure of 12 MPa. In the presence of a small amount of SO2, NO2, H2S and other impurity gases, the impurity gas is easy to combine with water to form a strong acid solution phase, so that the water in the CO2 system will precipitate when it is far lower than its solubility, and with the increase of the concentration of impurity gas, the faster the precipitation rate of water . On the other hand, the existence of impurity gas will make the composition of the corrosion product film complex, obviously change the morphology and structure of the corrosion product film, and reduce its protection to the metal matrix. For example, when there is O2 impurity, the Fe2O3 produced by corrosion will destroy the integrity of the FeCO3 product film, make the corrosion product film loose and porous, and lead to more opportunities for corrosive ions to contact with metal matrix ; when there is SO2 impurity, the feso3 produced will cause many cracks on the corrosion product film, and a large number of voids on the product film provide a channel for corrosive ions to contact with metal, and then add Fast corrosion process: NO2 impurities will not only make the corrosion product film loose, but also have a clear tendency to induce pitting corrosion. Therefore, NO2 impurities should be avoided in CO2 pipeline transportation [28,29].
In addition, the presence of impurities will also affect the electrochemical process of corrosion reaction. Xiang et al.  studied the coupling corrosion mechanism of carbonic acid and sulphurous acid by potentiodynamic scanning method. It was found that 0.01% (mass fraction) sulphurous acid can obviously promote the dissolution of anode in the saturated solution of CO2 at 25 ℃, which may introduce the cathode reaction of direct electricity generation by sulphurous acid or direct electron generation by hydrogen sulfite. The reaction mechanism of the cathode process is shown in Figure 5. Under the same pH value (2.0 or 3.0), the corrosion rate of the sample with sulphurous acid is higher than that without sulphurous acid.
Fig.5 Comparison of experimental potentiodynamic sweeps at different pH conditions with 0.01% (mass fraction) sulfurous acid (a) and an illustration of hypothetical polarization sweeps at different pH conditions with sulfurous acid (b)
Other influencing factors of internal corrosion
For CO2 transportation pipeline, in addition to impurity gas and water content, temperature and flow rate will also have a great impact on corrosion.
Temperature will accelerate almost all the processes involved in corrosion (mass transfer rate, chemical reaction rate, etc.), so it may be inferred that the corrosion rate of CO2 will also accelerate with the increase of temperature. In fact, this assumption is true without the coating of corrosion products. In high-pressure CO2 system, on the one hand, temperature will affect the solubility of water in CO2 system and impurities in water film. With the increase of temperature, the solubility of water in CO2 system will increase and it is difficult to separate out, and the solubility of acid impurity gas in water film will decrease; on the other hand, with the increase of temperature, the corrosion product film will be more compact, showing a stronger protection for metal matrix. The temperature in the CO2 system will affect the crystallization process and morphology of the corrosion product film. The corrosion rate increases first and then decreases with the increase of temperature, and there is a temperature value corresponding to the maximum corrosion rate . In the ocean storage system, because the sea water temperature will change with the depth, it is necessary to cross a relatively large temperature range to transport CO2 from the sea to the storage site. On the one hand, the decrease of the external ocean environment temperature will accelerate the precipitation of water in the CO2 system and intensify the corrosion, on the other hand, the decrease of the temperature will reduce the rate of the corrosion reaction. How will the temperature in the ocean environment affect the CO2 pipe The corrosion rate of the channel needs further study.
The influence mechanism of flow rate on CO2 corrosion is complex. The increase of flow rate will make the diffusion of CO32 -, H + in the fluid faster, increase the mass transfer speed of corrosive medium reaching the metal surface, cause the enhancement of cathodic depolarization and the rapid departure of Fe2 + from the metal surface caused by corrosion, resulting in the increase of corrosion rate .
There are two main aspects of flow-induced local corrosion in CO2 pipeline: on the one hand, flow will lead to the peeling off of amorphous FeCO3 product film in the early stage of corrosion, which will reduce its protection, thus causing serious local corrosion; on the other hand, flow will damage the integrity of FeCO3 product film, and accelerate the material exchange between metal matrix and corrosive medium .
Although the flow does not change the composition of the corrosion product film, it has a great influence on the formation process of the product film and the shape of the product film. Moreover, under the flow condition, local corrosion is the main type of Corrosion . Wei et al.  found that the integrity of the corrosion product film was seriously damaged under the flow condition when studying the impact of flow on Corrosion in supercritical CO2 system. With the increase of flow rate, the peeling area and pitting number of the corrosion product film increased. Although under the flow condition, the thickness of the corrosion product film was much larger than that under the static condition, the corrosion product film became loose and porous, It’s not very protective.
External marine environment corrosion of CO2 marine storage system
The marine environment is a very complex corrosion system. On the one hand, the seawater is a very corrosive electrolyte solution, on the other hand, the marine atmospheric environment and marine microorganisms will also affect the corrosion behavior and mechanism of metal materials.
Sea water is a natural electrolyte, which contains a lot of salt substances. At the same time, there are dissolved O2, N2, CO2 and other gases in the sea water. The composition is very complex. In the sea water, it not only has the role of micro corrosion battery, but also the role of macro corrosion battery. If there is a gap on the metal pipe surface, the sea water medium will enter the gap on the pipe surface to have an electrochemical reaction and accelerate the corrosion. In addition, in the surface of the sea, the flowing sea water will have a strong erosion on the metal surface, which will cause the falling off of the corrosion product film and local corrosion. Once the sea water is mixed with solid particles, it will also have erosion on the metal pipeline, which will accelerate the failure of the pipeline.
Because the relative humidity of the ocean atmosphere is large, even higher than its critical value, the metal surface exposed to the ocean atmosphere is very easy to form corrosive water film, which will cause the corrosion of the ocean atmosphere. Generally, the corrosion rate will accelerate with the increase of the thickness of the water film . A large amount of sea salt and oxygen in the atmosphere will dissolve in the water film on the metal surface, which will enhance the corrosiveness of the water film and further intensify the corrosion of offshore equipment.
At the same time, there are many kinds of microorganisms in the ocean, which are easy to adhere to the surface of engineering materials and form a microbial membrane. The formation, development and extinction of microbial film will affect the electrochemical state and corrosion process of metal . The metabolism of microorganisms will exchange substances with the outside world, and discharge ammonia, CO2, H2S and other gases, so that the internal pH value, dissolved oxygen content, types of organic and inorganic substances and other factors of the biofilm are completely different from the marine environment, forming an internal corrosion environment of the biofilm. The activity of microorganisms in the biofilm controls the rate and type of electrochemical reaction .
Corrosion protection of CO2 marine storage system
The marine external environment is highly corrosive, so it is necessary to strengthen the metal surface anti-corrosion treatment. The surface rust removal and protective coating will have a good anti-corrosion effect. The anti-corrosion alloy can also be used, or the multi-layer protective coating after surface heat treatment can be formed before the metal steel forming to improve the anti-corrosion performance of the pipeline. Referring to the anti-corrosion measures of the land gathering and transmission pipeline, the cathodic protection method can also be used to protect the marine pipeline . In addition, special protection can be provided for the pipeline, such as adding outer protective casing in some places with active seawater and microorganism activities.
For CO2 transportation pipeline, the use of stainless steel or low Cr steel can significantly improve the corrosion resistance of the pipeline . Adding appropriate inhibitors can also effectively reduce the corrosion rate, but for this high-pressure liquid-phase CO2 environment, whether the traditional inhibitors can play a role needs further study, it is necessary to screen or develop new inhibitors suitable for CCS system [39,40]. At present, the research on the corrosion characteristics of liquid-phase CO2 is still very limited. For the internal corrosion problem of the marine storage system, further research on the corrosion mechanism in the liquid-phase CO2 environment is needed, including the research on the corrosion characteristics of near critical area CO2. Under the premise of economy, the concentration and water content of the impurity gas should be reasonably controlled to reduce the corrosion rate in the pipe.
Conclusion and Prospect
In CCS technology, CO2 ocean storage technology can effectively store a large amount of CO2 for a long time, which has a broad development prospect. The deep-sea sediment storage method has become a research hotspot because of its large storage capacity and long storage period. Compared with land-based storage, the advantage of CO2 marine storage is that the storage effect is more secure and stable, but the disadvantage is that the technology is not mature enough and the corresponding research is lacking.
At present, many researches on the corrosion of CO2 transportation pipeline in CCS technology are focused on the supercritical phase CO2 corrosion under the condition of land geological storage, while the research on the low temperature liquid phase CO2 corrosion in marine storage is still limited. Impurity gas (including moisture) is the main cause of CO2 transport pipeline corrosion. In the low temperature liquid phase CO2 system, the mechanism of the influence of the presence of impurity gas on the corrosion is still unclear, which needs further experimental research. At the same time, the environment of marine CO2 transportation pipeline is more complex, and many researches are not perfect, such as the changes of ocean temperature, marine sediments, marine microorganisms and possible CO2 hydrate will affect the pipeline corrosion. With the development of CO2 marine storage technology, the cost of CO2 marine storage will be greatly reduced, so it is necessary to study the mechanism of low temperature liquid-phase CO2 corrosion.
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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