1. Investigation of deep water corrosion of carbon and low alloy steel
Compared to shallow water, factors such as hydrostatic pressure, temperature, salinity, dissolved oxygen concentration and pH value in the deep sea environment change with the depth of sea water, and the effect of these factors on the corrosion behavior of carbon steel and low carbon steel alloy steel The mechanism is complex , and it will inevitably lead to significant differences in its corrosion behavior in deep and shallow water environments. Venkatesan et al., National Institute of Ocean Technology, India, studied the corrosion behavior of carbon steel at depths of 500, 1200, 3500 and 5100 m in the Indian Ocean using real offshore installation methods. affecting the uniform corrosion process. The corrosion rate of medium carbon steel in the deep sea decreases with a decrease in the concentration of dissolved oxygen. The Civil Engineering Laboratory of the Naval Construction Battalion Center in Port Huene, USA, conducted large-scale environmental testing of seawater in the Pacific Ocean from 1962 to 1970. The results showed that the corrosion rate of carbon steel and low alloy steel in 1828 m depth of the sea was in the surface layer. About 33% in sea water, the corrosion rate at a depth of 762 m is also lower than at a depth of 1828 m, and the average corrosion rate of steel exposed for one year is linearly related to oxygen concentration. The results of real sea tests reflect the key role of dissolved oxygen concentration in the process of deep sea corrosion of carbon and low alloy steel, but some scientists have also pointed out that the role of high hydrostatic pressure in deep sea conditions cannot be ignored. For example, researchers from the 725th Institute of the China Shipbuilding Industry Corporation and Sun Haijing from the Metal Research Institute of the Chinese Academy of Sciences studied the effect of deep sea hydrostatic pressure on the corrosion behavior of low alloy steel through indoor simulation experiments. and found that high hydrostatic pressure does not greatly affect the cathodic process, but can increase Cl- activity and accelerate the rate of anode dissolution. Yang et al. also studied the behavior of Ni-Cr-Mo-V steel under deep sea corrosion conditions and concluded that high hydrostatic pressure can reduce its corrosion resistance. The morphology of the corrosion surface tends to be uniform.
2. Study of deep sea corrosion of aluminum alloy
Aluminum alloys can be passivated in the marine environment, and the amount of corrosion mass loss is small, but the corrosion resistance of aluminum alloys in various seawater environments can still be preliminarily estimated by the amount of corrosionno loss of mass. Venkatesan studied the corrosion of 1060 aluminum alloy exposed to various depths in the Indian Ocean for 168 days and found that the corrosion rate gradually increased with depth (500-5100 m). In addition, the corrosion rates of 2000 series aluminum alloys in seawater at different depths in the Pacific and Indian Oceans also show similar patterns. Based on the available data alone, it is not possible to explain that the corrosion rate of aluminum alloys in deep water increases linearly with depth, and there are many abnormal situations, so other data such as pitting and crevice corrosion should be taken into account. combined with deep sea corrosion assessment of aluminum alloys.
The main forms of marine corrosion of aluminum alloys are pitting and crevice corrosion, and stress corrosion problems still exist in the application of high-strength aluminum alloys. Deep sea exposure results in India have shown that Al-Mg alloys have a lower corrosion rate than pure Al or Al-Cu alloys at different depths. Al-1100 caused pitting in deep water, and pitting was most severe at 5100 m. The aluminum-magnesium alloy showed uniform corrosion and a small amount of rare pitting. The exposed surface exhibits the characteristics of mud cracking. In particular, 5000 and 6000 series aluminum alloys have good corrosion resistance in shallow water, but their sensitivity to pitting and crevice corrosion increases in deep sea. Beccaria et al. believe that the reason for the aggravation of localized corrosion is that an increase in pressure causes a change in the ionic radius and degree of hydrolysis of metal ions, which changes the activity of metal ions and the composition of metal complexes, resulting in higher reaction constants for aluminum compounds. Boyd et al. and Reinhart respectively investigated the corrosion behavior of aluminum-magnesium alloys in seawater at the surface of the Pacific Ocean and in the deep sea, and found that the pitting corrosion rate of 5000-series aluminum-magnesium alloys increased in deep water conditions, and the pitting corrosion rate was the highest in a seawater environment at a depth of 700m, which was the highest in a surface seawater environment. It is three times higher than in sea water, and decreases by half at a depth of 1700 m. It is believed that the main factor influencing pitting corrosion of aluminum-magnesium alloys of the 5000 series is the oxygen content. Studies of stress corrosion of various series of aluminum alloys in deep water conditions show that under stress conditions of 50% and 75% yield strength after 402 days exposure at a depth of 760 m, with the exception of the 7000 series, other series of aluminum alloys are not susceptible to stress corrosion, a series of aluminumalloys 7000, 7075, 7079 and 7178 have stress corrosion cracking.
3. Investigation of deep-water corrosion of copper alloy
Copper alloys are still subject to uniform corrosion in deep sea conditions, and the corrosion rate calculated based on mass loss can be reliably applied to the design of the structure, but this is not suitable for copper-based alloys, which are susceptible to decomposition corrosion. The results of real sea tests of the plates showed that in sea water at different depths, all brass containing from 10% to 42% zinc exhibits decomposition corrosion. Studies have shown that copper alloys corrode more slowly in deep waters than in surface sea waters, but this trend is not obvious. With the exception of copper and silicon bronze, the corrosion rate of other copper alloys increases with increasing oxygen concentration . Savant et al. studied the corrosion behavior of copper, brass and copper-nickel alloys in the shallow waters of the Arabian Sea and the Bay of Bengal, at a depth of 1000-2900 m for one year, and found that the corrosion rate of brass is not related to depth, but other materials at a depth of 2900 m is lower than the corrosion rate at a depth of 1000 m and shallow water in the marine environment. At the same time, it is also indicated that the copper alloy corrosion rate is controlled by the content of dissolved oxygen. The research team of Li Xiaogang from Beijing University of Science and Technology conducted real seawater impact experiments at depths of 500m and 1200m in the South China Sea using the China Shipbuilding Industry Corporation's 725th Research Institute Environmental Testing Platform and studied H62. brass, aluminum bronze QAl9-2, corrosive behavior of tin bronze QSn6.5-0.1 under deep water exposure for 3 years. The results show that with increasing water depth, the corrosion rate of brass H62 decreases linearly, the corrosion rate of aluminum bronze QAl9-2 and tin bronze QSn6.5-0.1 first decreases and then increases with increasing water depth, and the minimum value of the corrosion rate occurs at depth from 800 to 1200 m, and the corrosion rate is in the following order: brass H62>tin bronze QSn6.5-0.1>aluminum bronze QAl9-2. It can be seen that the research results in related fields at home and abroad show good agreement. The law of deep-sea corrosion of copper alloys is relatively simple, and the existing data show that any copper alloy is not susceptible to stress corrosion, all of which are deep-sea environments. Selection and application copper alloys provides a database and guide.
4. Deep Sea Corrosion Study of Stainless Steel
For stainless steel, as the sea water depth increases, the corrosion rate usually tends to decrease, and the values do not differ much. Deep sea testing of stainless steel specimens in the Indian Oceanshow that stainless steel can still form a dense passivation film at depths of 500, 1200, 3500 and 5100 m and the corrosion rate is close to zero after 168 days of exposure. Reinhart studied the effect of seawater depths of 1000, 1500 and 2000 m on the corrosion of AISI 300 and 400 series stainless steels and obtained similar results. Stainless steel is often subject to localized corrosion in seawater, as well as pitting, crevice and even tunneling corrosion in deep water. After holding stainless steel 301 for 1064 days at a depth of 1615 m in the Pacific Ocean, tunneling corrosion has gone through almost the entire sample, and AISI304 did not undergo tunneling corrosion when held at a depth of 5300 m for the same time. It can be seen that the probability of crevice corrosion of stainless steel from different materials in deep water conditions is not the same.
In deep sea environments, stainless steel components are subjected to high static pressure, increased susceptibility to stress corrosion, and mechanical properties deteriorate, jeopardizing the operational safety of deep sea structures/equipment. Studies have shown that the tensile strength, yield strength and elongation of AISI405 steel and AISI316 welded and sensitized stainless steel were exposed for almost 400 days at a depth of 1830m and 762m respectively. m; The mechanical properties of AISI201 and AISI300 series stainless steel have not been adversely affected by various deep sea conditions. This also indicates that the sensitivity of stainless steel to deep sea localized corrosion is closely related to the material. Overall, AISI300 series stainless steel outperforms AISI400 series stainless steel and precipitation hardened stainless steel in terms of frequency and severity of several types of corrosion.
5. Investigation of deep sea corrosion of titanium alloy
Titanium alloys have excellent corrosion and pitting resistance in sea water, and are virtually unaffected by corrosion in deep water environments. The US Civil Engineering Laboratory studied the susceptibility of titanium alloys to deep sea stress corrosion. The results show that, in addition to the weldable alloy 13V-11Cr-3Al, when using any other non-weldable and weldable alloy, a value equal to 75% of the yield strength and exposure to sea water in the surface layer 13a 80 days, 402 days at a depth of 762 m, and 1751 days at a depth of 1828 m, there was no failure due to stress corrosion cracking.
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