Study on corrosion behavior of typical carbon steel and low alloy steel in deep sea of different sea areas

1 Introduction

With the progress of science and technology and the depletion of earth resources, human beings have been seeking to expand the space for activities in deep space, deep sea, and deep earth. Among them, the deep sea is rich in seabed mineral resources, which will provide a strong support for sustainable development of human beings. At present, the exploration of deep-sea oil and gas, minerals, and biological resources have become the key content of the development of marine economy. In order to better develop these ocean resources, application of deep-sea equipment/facilities, like oil and gas pipelines and platforms, has been extended to the Western Pacific Ocean, Indian Ocean, and other ocean waters. However, due to the particularity of the deep-sea environment, these equipment/facilities are facing severe corrosion threat. The huge economic loss and ecological disaster caused by the deepwater horizon platform accident in the Gulf of Mexico are still fresh in memory (Kimes et al. 2013; Romero et al. 2015). The deep-sea corrosion has become a technical bottleneck in the development of deep-sea resources, the utilization of deep-sea space and the construction of deep-sea engineering systems, which affects the long-term development of the deep-sea economy.

There are significant differences for the physical and chemical properties of deep-sea water in different sea areas, and the corrosion behavior of various materials in the deep sea environment need to be clarified (Cao et al. 2015; Traverso and Canepa 2014). In order to accumulate corrosion data of various materials in the deep sea environment and provide a basis for the design and selection of materials for deep-sea engineering and equipment/facilities, it is necessary to conduct corrosion coupon test research (Cai et al. 2020). However, with the increase of the depth of seawater and the distance from the mainland, it is more difficult to obtain corrosion data and characterize corrosion properties of materials. At present, only a few countries in the world have carried out corrosion field tests of materials in deep sea environment. The United States, the former Soviet Union, Japan, the United Kingdom and other countries began the research on the corrosion of materials in the deep-sea environment in the 1960s. Subsequently, Norway, India and other countries also carried out research in this field (Dexter 1980; Sparks et al. 1983; Venkatesan et al. 2002). However, in recent years, deep-sea test technology has become the forefront of the entire marine science, and it is mostly used in military affairs. As a result, the relevant field test data available are becoming limited. For this reason, we launched a deep-sea corrosion test device for the first time in the South China Sea, the Western Pacific Ocean, and the Indian Ocean, respectively, and studied the environmental adaptability of deep sea engineering materials. In this work, the corrosion behaviors of typical carbon steel and low-alloy steels exposed for one year at different depths of the Western Pacific Ocean and Indian Ocean waters were investigated, and the results were compared with those of the South China Sea.

2 Materials and methods

The test materials are Q235 and three kinds of low alloy steels (they are represented by ①, ②, and ③, respectively, containing 0.6–1.2 wt % Cr). The main composition of test material is shown in Table 1. The coupon size is 200 mm × 100 mm, and the long side of the coupon is perpendicular to the metal rolling direction. Before test, degreasing was done, and the coupon size and weight were accurately measured and recorded. The test sites were chosen as the South China Sea, Western Pacific Ocean, and Indian Ocean. The self-designed deep-sea environmental test device was used, with 3–4 test stands located at different depths of seawater to obtain corrosion data simultaneously. The test depths in the South China Sea were 1200 m, 2000 m, and 3000 m, respectively, while the test depths in the Western Pacific Ocean and Indian Ocean were 500 m, 800 m, 1200 m, and 2000 m, respectively. Photo of the field launching of the device is shown in Figure 1.

Figure 1:

Photo of the field launching of the deep-sea test device.

After exposure for one year, the coupons were retrieved and transported into the lab for further analysis. The surface morphologies of the retrieved coupons before and after rust removal were recorded using digital camera. After removal of the corrosion products, the corrosion rate was calculated and the micro-morphologies of representative regions were revealed by the optical microscope (Hirox 8700). Based on the acquired deep-sea environmental factors data and the corrosion data of the test materials, the gray correlation analysis was applied to explore the environmental impact mechanism on deep-sea corrosion behavior.

3 Corrosion data analysis

Figure 2 shows the macroscopic corrosion morphologies of Q235 exposed for one year at different depths of the South China Sea. As can be seen, a large number of barnacles, oysters, and other ocean organisms were attached to the surface of Q235 in the shallow seawater environment, and the rust layer was thick with an intact structure. In the deep-sea environment, basically no bio-fouling occurred. The entire surface of coupon was covered with reddish brown rust products, and the structure of the outer rust layer was relatively loose, with the phenomenon of falling off. After rust removal, the corrosion of coupons in the shallow seawater was uneven, and several large corrosion pits were formed locally. For the deep-sea corrosion coupons, the morphologies were similar. All the coupons lost its metallic luster, but their surfaces were still relatively flat.

Figure 2:

Macro-morphologies of Q235 exposed for one year at different depths of the South China Sea. (a) Shallow seawater before rust removal, (b) 1200 m before rust removal, (c) 2000 m before rust removal, (d) 3000 m before rust removal, (e) shallow seawater after rust removal, (f) 1200 m after rust removal, (g) 2000 m after rust removal, (h) 3000 m after rust removal.

Figure 3 shows the microscopic corrosion morphologies of Q235 exposed for one year at different depths of the South China Sea. As can be seen, in the shallow seawater environment, corrosion pits formed on the surface of Q235 had different sizes and irregular shapes. While in the deep-sea environment, uneven general corrosion occurred, with numerous small cavities distributed on the coupon surface.

Figure 3:

Micro-morphologies of Q235 exposed for one year at different depths of the South China Sea. (a) Shallow seawater, (b) 1200 m, (c) 2000 m, (d) 3000 m.

Figure 4 shows the corrosion rates of Q235 exposed for one year at different depths of the South China Sea, the Western Pacific Ocean, and the Indian Ocean. As can be seen, the corrosion rates of Q235 in the deep sea environment of the South China Sea were 39–48 μm/a, far lower than that in the shallow sea water, which may be caused by the huge difference in oxygen content between deep sea and shallow seawater. In the Western Pacific Ocean, the corrosion rates of Q235 were 53–115 μm/a within the depth range of 500–2000 m. In the Indian Ocean, the corresponding corrosion rates were 43–77 μm/a, slightly lower than that in the Western Pacific Ocean. On the whole, for the Western Pacific Ocean and Indian Ocean, the corrosion rate of Q235 was highest at the depth of 500 m, and the corrosion rate tended to be stable with slightly fluctuation after the depth exceeded 800 m, which was closely related to the relatively stable and uniform environmental factors in this depth range.

Figure 4:

Corrosion rates of Q235 exposed for one year at different depths of the South China Sea (a), the Western Pacific Ocean (b) and the Indian Ocean (c).

The corrosion rates evolution law of low alloy steels with depth was basically consistent with that of Q235. Taking the Western Pacific Ocean as an example, Figure 5 shows the corrosion rates of several low alloys exposed for one year at different depths of the Western Pacific Ocean. As can be seen, the corrosion rate of all low alloy steels was highest at the depth of 500 m. With the increase of the depth, the corrosion rate had a significant decline process, and then the corrosion rate tended to be relatively stable and its fluctuation was reduced. On the whole, at the same depth, the corrosion rate of all low alloy steels was higher than that of Q235. The addition of alloy elements did not promote their corrosion resistance, which was contrary to some corrosion coupon test results obtained in shallow sea environment. Compared with carbon steel, the low alloy steels had higher content of Cr and Ni elements, especially for Cr, whose addition will form a dense protective film containing Cr₂O₃ on the surface of the coupon substrate during the corrosion process (Wang and Cao 2014). As the dissolved oxygen concentration in the shallow seawater is basically saturated, self-repair can be carried out in time when the protective film is damaged. To some extent, this can inhibit the further development of corrosion, so the low alloy steels has relatively good corrosion resistance in shallow seawater. In the deep-sea environment, the situation changed. On the one hand, the corrosion damage of Cl⁻ to the protective film is enhanced. On the other hand, due to the low content of dissolved oxygen, the protective film cannot be repaired in time after local corrosion damage. In addition, relevant research showed that Cr can also reduce the potential of the substrate in the corrosion pits (Cao et al. 2010; Yang et al. 2009), promoting the developing of the corrosion pits. As a result, the corrosion resistance of these low alloy steel steels in the deep-sea was reduced. Guo et al. (2019) studied the corrosion behavior of 10CrNi₃MoV and E47 steel in the deep-sea environment, and found that the corrosion resistance of 10CrNi₃MoV was inferior to that of E47 steel, which may also be due to the higher content of Cr in 10CrNi₃MoV steel.

Figure 5:

Corrosion rates of the low alloy steel ① (a), ② (b) and ③ (c) exposed for one year at different depths of the Western Pacific Ocean.

4 Analysis of deep-sea environmental factors

The deep-sea corrosion behavior of carbon steel and low alloy steel is determined by the specific seawater environmental factors at different depths. These environmental factors include seawater depth (hydrostatic pressure), temperature, dissolved oxygen, salinity, pH, etc. (Natesan and Palaniswamy 2009; Soares et al. 2009) In order to deeply evaluate the deep-sea corrosion behavior and mechanism, the main marine environmental factors at different depths of the South China Sea were monitored, as shown in Figure 6. Furthermore, these environmental factors data were compared with that of the Western Pacific Ocean and the Indian Ocean.

Figure 6:

Curves of main seawater environmental factors changing with depth in the South China Sea.

5 Analysis of corrosion impact law

In order to determine the impact of these deep-sea environmental factors on the corrosion rate of carbon steel and low alloy steel, gray relational analysis (Ding et al. 2020) was applied. The corrosion rates of Q235 exposed for one year at different depths in the South China Sea, the Western Pacific Ocean, and the Indian Ocean were set as reference data series (as shown in Table 2), and the corresponding seawater environmental factors were set as comparison data series. All the data were dealt with method of dimensionless processing, and then the gray relational degree (Table 3) of environmental factors to the corrosion rate was calculated and sorted according to the following formula. Furthermore, the same method was used to obtain the gray relational degree for low alloy steels. The results were also shown in Table 3.

In the formula, i, respectively, takes the value of 1–5, corresponding to five kinds of environmental factors (comparison data series); k takes the value of 1–12, corresponding to different sea areas and different depth test conditions; Δ_i(k) is the absolute difference of each data point between the comparison data series i and the reference data series at the same test condition; miniminkΔi(k) is the secondary minimum difference among Δ_i(k) between all the comparison data series and the reference data series; maximaxkΔi(k) is the secondary maximum difference among Δ_i(k); parameter ρ is the resolution coefficient (0 < ρ < 1), which generally takes the value of 0.5; γ_0i is the gray relational degree of comparison data series i, and it indicates a good correlation when its value is close to 1.

It can be seen from Table 3 that the main environmental factor affecting the corrosion rate of carbon steel and low alloy steel in deep-sea environment of different sea areas was temperature, followed by dissolved oxygen concentration. Van Hoff once summarized an empirical rule based on a large number of experimental data: the reaction rate will increase approximately 2–4 times when temperature increases 10° (Ding et al. 2018). Taking the South China Sea as an example, the corrosion rate of carbon steel and low alloy steel in the deep sea decreased to less than 1/4 of that in the shallow seawater as the temperature dropped by more than 20 °C (from shallow to 3000 m). However, when the depth dropped from 2000 m to 3000 m, the corrosion rate rose slightly, presumably due to the change of dissolved oxygen concentration. Since dissolved oxygen participated in the cathodic oxygen reduction reaction process of steel, it had a very important influence on its corrosion rate. Especially in the deep-sea environment, the temperature was low with limited variation range, and the dissolved oxygen began to dominate, which was consistent with the relevant results of foreign deep-sea corrosion coupon test.

The Civil Engineering Laboratory of the US Navy once carried out large-scale seawater corrosion coupon tests in the Pacific Ocean between 1962 and 1970. These results showed that the corrosion rate of carbon steel and low-alloy steel at 1828 m of the deep sea was about 33 % of that in surface seawater, while the corrosion rate at 762 m was lower than that at 1828 m. The average corrosion rate of steel exposed for one year at different depth was linear with the change of dissolved oxygen concentration (Schumacher 1979). The corrosion field test carried out by the National Institute of Oceanography of India at 1000–2900 m in the Arabian Sea (Sawant and Wagh 1990) showed that the carbon steel presented an overall corrosion feature, and the corrosion product was about 2 mm thick after exposure for one year. With the increase of seawater depth, the corrosion rate of carbon steel increased slightly. The change of corrosion rate with depth was consistent with the distribution of dissolved oxygen concentration, and the correlation coefficient r reached 0.97. This indicated that the dissolved oxygen concentration was the key factor affecting the deep sea corrosion of carbon steel. They also carried out a comparative study on corrosion results of carbon steel between the deep-sea corrosion test in the Arabian Sea and the Bay of Bengal and the shallow seawater corrosion test in the coastal test station (Sawant et al. 1993). It showed that the surface of carbon steel in the shallow seawater was seriously polluted by marine organisms (the biological content was 1.8–2.3 kg/m²), and the corrosion products were firmly attached, containing more sulfide (caused by sulfate reducing bacteria corrosion). In the contrary, the surface of the deep-sea coupons was free of fouling organisms, and the corrosion products were relatively loose. The corrosion rate of carbon steel in shallow seawater (0.5 m and 6 m below the water surface) was 0.15 and 0.31 mm/a, respectively, while that in deep sea (1000–2900 m) was 0.020–0.058 mm/a, significantly lower than that in shallow seawater. This was related to the higher concentration of dissolved oxygen and temperature in shallow seawater.

In addition, Venkatesan et al. (2002) from the National Institute of Marine Technology of India studied the corrosion resistance of carbon steel at depths of 500, 1200, 3500, and 5100 m in the Indian Ocean through the corrosion coupon test. The results showed that the corrosion rate in the deep sea was significantly lower than that in the shallow seawater. The dissolved oxygen concentration was the main environmental factor affecting the corrosion process. The corrosion rate of carbon steel in the deep sea decreased with the decrease of dissolved oxygen concentration. It indicated that dissolved oxygen concentration played a key role in the deep-sea corrosion process of carbon steel and low alloy steel, which acted as the cathodic depolarizer. Low dissolved oxygen concentration will reduce the rate of oxygen reduction reaction, thus inhibiting the corrosion of carbon steel and low alloy steel. The corrosion product formed on the surface of carbon steel in deep sea was mainly FeOOH, which was loose with weak protection effect. Experiments on maraging steel, deformed steel and isothermal quenched ductile cast iron showed that these metals had similar deep-sea corrosion behavior with carbon steel, and their corrosion rate in deep sea was about 4 times lower than that in the shallow seawater.

In this work, the deep-sea corrosion data obtained in different sea areas also showed similar laws: the corrosion rate was mainly affected by dissolved oxygen. Comparing the changing trend of Q235 corrosion rate in different depths in the South China Sea, the Western Pacific Ocean, and the Indian Ocean, it can be found that with the increase of depth, the corrosion rate had a significant decline at the beginning. As the seawater depth exceeded 800 m, the corrosion rate tended to be relatively stable. Even sometimes, it increased a little with depth. This was consistent with the change trend of dissolved oxygen concentration at different depths. As shown in Figures 4 and 6, the curve of corrosion rate changing with depth was very similar to the curve of dissolved oxygen, which further confirmed this conclusion.

It can also be found from Table 3 that hydrostatic pressure was the weakest environmental impact factor on the corrosion rate of carbon steel and low alloy steel at different depths in the South China Sea, the Western Pacific Ocean, and the Indian Ocean. Schumacher (1979) also believed that the corrosion rate of steel was basically not affected by seawater pressure through the research on the deep-sea corrosion coupon test in the Pacific Ocean, which to some extent confirmed the reliability of the gray correlation analysis. However, through indoor short-term deep-sea simulation corrosion test, we found that the high hydrostatic pressure had a certain promoting effect on the corrosion of carbon steel and low alloy steel. Other relevant research institutes have also obtained similar results. Liu et al. (2011) studied the influence of hydrostatic pressure on the corrosion behavior of two high-strength low-alloy steels containing Cr and Ni in the simulated deep-sea environment. The corrosion medium was natural seawater at room temperature, and the test time was 24 h. The results showed that the corrosion rate of the two steels gradually increased with the increase of hydrostatic pressure.

When the hydrostatic pressure increased from 0.1 MPa to 4.5 MPa, the corrosion rates of the two steels increased from 0.351 and 0.328 mm/a to 0.486 and 0.426 mm/a, respectively. Hydrostatic pressure had influence on the structure of rust layer of low alloy steel, and formed γ-FeOOH and Fe₃O₄ under deep sea had higher electrochemical activity or conductivity, which promoted the corrosion of steel. Some studies also showed that (Sun et al. 2013a,b), hydrostatic pressure had little effect on the cathodic polarization process of steel, but it accelerated the anodic dissolution rate by improving the activity of chloride ions and promoting the adsorption of chloride ions on the surface. However, the simulation test was limited to the study of short-term corrosion behavior. From the results of long-term real sea tests, the effect of high hydrostatic pressure was obviously weaker than that of dissolved oxygen and other environmental factors. With the increase of depth (hydrostatic pressure), the corrosion rate decreased significantly. On the one hand, this was related to the protective effect of the rust layer. On the other hand, hydrostatic pressure had a limited role in promoting the corrosion rate, which was mainly to promote pitting nucleation rather than accelerating its growth. Yang et al.’s research on the corrosion behavior of Ni–Cr–Mo–V low alloy steel under different hydrostatic pressures in sodium chloride solution showed that high hydrostatic pressure accelerated the metastable pitting initiation rate and reduced the pitting growth probability (Yang et al. 2010). As a result, the corrosion surface morphology tended to be uniform, which was consistent with the deep-sea corrosion morphology.

6 Conclusions

1. In deep sea environment, the corrosion morphology of carbon steel and low alloy steel was more uniform than that in shallow seawater. The surface of coupon from deep sea was relatively flat, covered with a large number of small cavities caused by corrosion. High hydrostatic pressure can promote the corroded surface morphology to become uniform, but its effect on accelerating the corrosion rate was very limited.

2. In the deep sea environment of the South China Sea, the Western Pacific Ocean, and the Indian Ocean, the corrosion rates of low alloy steels exposed for one year at different depths were all higher than that of Q235. The alloy element doping may not improve their corrosion resistance due to the particularity of the deep sea environment.

3. With the increase of seawater depth, the evolution law of the corrosion rates for carbon steel and low alloy steel in different sea areas was similar, with an obvious decline of corrosion rate in the beginning. The gray correlation results indicated that the evolution law of the corrosion rates with depth was mainly controlled by temperature and dissolved oxygen.