Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Abstract:

Carbon steels as common structural material have been widely used for basic facilities with the development of the city. In these service environments, carbon steel would inevitably encounter the atmospheric corrosion. Especially, the corrosion of carbon steels exposed to coastal-industrial atmosphere is very outstanding. However, the initial corrosion mechanism of carbon steel subjected to coastal-industrial environment still need to be clarified, which would be vital for predicating the subsequent corrosion process. In addition, although many scholars studied the synergism of SO2 and Cl, which obviously accelerates the corrosion of steel and reduces its service life, there is few research about the effect of the synergism of SO2 and Cl (in different proportion) on the early corrosion behavior of the carbon steel. Therefore, it is of great importance to investigate the initial corrosion mechanism of carbon steel and the effect of the synergism of SO2 and Cl (in different proportion) in the coastal-industrial atmosphere. In present work, the initial corrosion behavior of Q235 carbon steel exposed to a simulated coastal-industrial atmosphere has been studied by weight loss, XRD, SEM and electrochemical measurements. Also, the effect of the synergism of SO2 and Cl (in different proportion) on the early corrosion behavior of Q235 car bon steel has been investigated. The results indicate that the initial corrosion behavior of carbon steel exposed to a simulated coastal-industrial atmosphere presented a transition from corrosion acceleration to deceleration, and the kinetics of accelerated corrosion process followed the empirical equation D=Atn. A double-layered corrosion product was formed on the surface of carbon steel after 24 h: the loose outer layer and relative dense inner layer; the synergistic effect between SO2 and Cl accelerated the corrosion of carbon steel. However, the change in the ratio of SO2 and Cl had no significant effect on the corrosion loss of carbon steel, and had not changed the composition of corrosion products formed on carbon steel surface. SO2 caused the corrosion morphology of carbon steel to tend to uniform corrosion.

Key wordscarbon steel;atmospheric corrosion;synergism

GUO Mingxiao1,2, PAN Chen1, WANG Zhenyao1, HAN Wei1
  • 1 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
  • 2 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China

As the most commonly used structural material, carbon steel is widely used in transportation, resource environment, energy power, construction and agriculture [1]. In most cases, carbon steel is inevitably directly in contact with the atmosphere in the service environment, and atmospheric corrosion occurs [2]. Therefore, research on atmospheric corrosion of carbon steel has been receiving much attention. Wang Zhenduo et al [3] conducted an experimental study on the corrosion of carbon steel under atmospheric exposure, and summarized the climatic conditions and atmospheric corrosivity of the natural environment in China. The results show that the climate of different regions has different climate and the corrosion rate of carbon steel exists. Significant differences. Some studies [4~7] have shown that the corrosion of carbon steel exposed to the marine industrial atmosphere is particularly prominent. At present, many southeast coastal cities in China are affected by the rapid development of industry, and SO2 pollution is becoming more and more serious. It has developed into a marine industrial atmosphere containing both Cl- and SO2. Therefore, it is necessary to carry out research on the corrosion behavior of carbon steel in the marine industry atmosphere.
At present, research on the corrosion of steel in the atmosphere mainly focuses on long-term corrosion behavior research. Kucera et al [8] conducted an 8 a exposure test in Sweden and the Czech Republic. The results show that the corrosion rate of carbon steel in a corrosive environment containing SO2 pollutants and chlorides can be described by a linear equation, linear coefficient and SO2 deposition. Related. Liang Caifeng et al [9] selected seven experimental points in China to carry out a 16-a exposure corrosion test on 17 steels. The results show that the corrosion development of steel in the long-term atmospheric environment follows the power function law. However, studies on the initial corrosion behavior of carbon steel in the marine industry’s atmospheric environment have rarely been reported. Since the corrosion products formed by carbon steel in the initial stage of atmospheric corrosion exhibit different compositions and structures due to differences in atmospheric environment, which significantly affect the subsequent corrosion behavior of carbon steel, the corrosion behavior of carbon steel in the initial stage is explored to understand the corrosion mechanism. It is important.
In addition, SO2 and Cl- are recognized as the most important corrosive components affecting atmospheric corrosion in steel. Cai Jianping [10] studied the effects of chloride and sulfur pollutants on atmospheric corrosion of carbon steel. The results show that there is a synergistic effect in the initial stage of corrosion, but this synergistic effect is weakened with the progress of corrosion. Qu Qing et al [11] explored the effect of NaCl and low concentration of SO2 on the atmospheric corrosion behavior of A3 steel. The results show that due to the synergy between NaCl and low concentration of SO2, the corrosion loss of steel is much greater than that caused by 2 alone. The corrosion is weightless. It can be seen that in the atmospheric environment of marine industry, the synergistic effect of SO2 and Cl- can significantly accelerate the corrosion damage of steel and reduce its service life [12~14]. However, the mechanism of the effect of the ratio of SO2 to Cl- on the synergistic effect of carbon steel initial corrosion behavior is still unclear and needs further detailed study.
In summary, this work carried out the laboratory simulation acceleration method, using traditional weightless analysis, scanning electron microscopy, X-ray diffraction (XRD) analysis and electrochemical testing methods to carry out the initial corrosion behavior mechanism of carbon steel in the simulated marine industrial atmosphere. Detailed study and further study the effects of different ratios of SO2 and Cl- on the initial corrosion behavior of carbon steel.
1 Experimental method
1.1 Experimental materials
The experimental material is Q235 steel, and its chemical composition (mass fraction, %) is: C 0.18, Si 0.25, Mn 0.5, S 0.018, P 0.016, Fe balance. The sample size for weight loss analysis and rust layer analysis is 50 mm × 25 mm × 2.5 mm. All the samples were degreased by acetone mechanical cleaning before the experiment, then dehydrated with alcohol and placed in a desiccator for use. After 24 h, the weight loss analysis samples were weighed and recorded with an analytical balance with an accuracy of 0.001 g. The sample size for electrochemical analysis was 10 mm × 10 mm × 2.5 mm.
1.2 Laboratory accelerated simulation experiment
Accelerated corrosion experiments were carried out using a C4-180Pro high and low temperature damp heat test chamber. Because HSO3- is more in line with the conversion behavior and characteristics of SO2 under the liquid film in the corrosion process, this experiment uses NaHSO3 and NaCl to simulate SO2 and Cl- in the atmosphere, and builds a laboratory simulation accelerated experimental method. The NaHSO3 and NaCl solutions were used as the corrosive medium. The corrosive medium was evenly spread on the surface of the sample by a micro-syringe, then placed in an oven for drying, and finally placed in a test chamber for accelerated corrosion experiments. The experiment was carried out according to the “wet/dry” alternating method, and it was cycled every 3 h, in which it was wet (relative humidity 90%, temperature 30 °C) for 1 h, dry (relative humidity 60%, temperature 30 °C) for 2 h, every 24 h. Deposit the salt once. In order to deepen the understanding of the synergistic effect of SO2 and Cl-, and at the same time, it is beneficial to realize the control of the corrosion behavior of carbon steel under different corrosive environments. This work also utilizes the indoor dry-wet alternate cycle accelerated corrosion test method, the amount of S and Cl substances. Under the premise of constant, the effects of different ratios of SO2 and Cl- on the initial corrosion behavior of carbon steel were investigated by adjusting the ratio of NaHSO3 and NaCl in the corrosive medium, and the synergistic mechanism of SO2 and Cl- was further revealed. The following two sets of accelerated corrosion experiments were carried out according to the above-mentioned dry-wet alternating experimental method:
(1) Accelerate experiment 1 to simulate the initial corrosion behavior of carbon steel in the atmospheric environment of marine industry. 0.15 mol/L NaCl+0.05 mol/L NaHSO3 (the amount of salt deposited on the surface of the sample was 10 μL/cm2, and the mixed salt ratio was 3:1) was deposited as a corrosive medium on the surface of the sample for accelerated dry-wet cycle alternating corrosion accelerated experiment. . The experimental periods were 24, 48, 72, 96 and 120 h, respectively. Three parallel samples were taken for each weight loss analysis, one for morphology analysis, and four electrochemical parallel samples.
(2) Accelerated experiment 2, the effect of different ratios of SO2 and Cl- synergy on the initial corrosion mechanism of carbon steel. Single NaCl (0.2 mol/L NaCl), single NaHSO3 (0.2 mol/L NaHSO3) and different ratios of NaCl and NaHSO3 mixed salt (0.15 mol/L NaCl+0.05 mol/L NaHSO3, 0.1 mol/L NaCl+0.1 mol) /L NaHSO3, 0.05 mol/L NaCl+0.15 mol/L NaHSO3), ie, the mixing ratio is 3:1, 1:1, 1:3, respectively, deposited as corrosion medium on the surface of the sample for alternating wet and dry cycle corrosion test The experimental period is 120 h.
1.3 Analysis of corrosion weight loss
The sample was derusted using 500 mL HCl (38% concentrated hydrochloric acid) + 500 mL distilled water + 20 g hexamethylenetetramine solution. After the rust removal, the sample was washed with distilled water and alcohol, finally blown dry with a hair dryer, placed in a desiccator, and weighed after 24 h (accurate to 0.001 g). The weight loss data for each cycle takes the average of the weight loss data of three parallel samples.
1.4 analysis of rust layer composition
A 50 mm × 25 mm × 2.5 mm sample taken for each rust layer analysis in each cycle was cut into small pieces of 10 mm × 10 mm × 2.5 mm with a saw, and then the composition of the rust layer was analyzed. The Rigaku-D/max-2500PC XRD was used for phase analysis. The Cu target was used to qualitatively analyze the corrosion products at a scanning rate of 2 °/min at 50 kV and 250 mA.
1.5 rust layer morphology analysis
The surface morphology and cross-section of the corrosion samples were observed and analyzed by ESEM XL30 FEG scanning electron microscopy (SEM). The rust layer cross-section analysis sample was encapsulated with epoxy resin at room temperature. After curing, it was sanded from No. 600 to No. 2000 with sandpaper, then polished with W2.5 abrasive paste. After polishing, it was washed with alcohol and blown under a hair dryer. dry. Finally, the sample was placed under SEM for observation.
1.6 Electrochemical analysis
Electrochemical measurements were performed using a PARSTAT 2273 electrochemical workstation using a standard three-electrode system with a saturated calomel electrode (SCE) as the reference electrode, a Pt electrode as the auxiliary electrode, and a corrosion sample as the working electrode. The potentials measured in the experimental results are all potentials relative to SCE. The electrolyte was a 0.1 mol/L Na2SO4 solution prepared from analytically pure reagent and distilled water, and the experimental temperature was room temperature. The scanning rate of the potentiodynamic polarization curve was 0.33 mV/s. Since the self-corrosion potential of the working electrode is unstable at the beginning of the measurement, it is necessary to stabilize until the open circuit potential does not exceed 1 mV within 1 min before performing the above electrochemical test.
2 Experimental results and analysis
2.1 Initial corrosion mechanism of Q235 carbon steel in simulated atmospheric environment of marine industry
2.1.1 Corrosion Dynamics The weight loss measurement method is the most commonly used method for atmospheric corrosion evaluation and is usually expressed by the weight loss per unit surface area or the average corrosion depth. The corrosion depth of steel can be obtained by the following formula [15]:
20180804095035 48869 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere
Where D is the corrosion depth, μm; Wt is the corrosion weight loss, g; ρ is the density of Q235 carbon steel, 7.86 g/cm3; S is the total area of the sample, cm2.
According to the corrosion depth of carbon steel, the average corrosion rate Va can be calculated by the following formula [15]:
20180804095100 59126 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere
Where t is the corrosion time, h; the subscript m represents the sampling period (when m = 1, it represents corrosion for 24 h, and so on, when m = 2, 3, 4, 5, the corrosion time is 48, 72, 96, 120 h). Figure 1 shows the trend of the average corrosion rate of carbon steel in the atmospheric environment of simulated marine industry with corrosion time. It can be seen that the average corrosion rate of carbon steel increases gradually throughout the corrosion cycle, reaching a maximum at 96 h of corrosion, and then the corrosion rate drops significantly. That is to say, the initial corrosion process of Q235 carbon steel in this environment is divided into two stages, namely the corrosion acceleration process and the corrosion deceleration process. This phenomenon should be closely related to the evolution process of carbon steel surface corrosion products.
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Fig.1 Average corrosion rates of Q235 carbon steel exposed to a simulated coastal-industrial atmosphere as a function of exposure time

The traditional view is that the law of long-term atmospheric corrosion dynamics of steel follows the law of power function, as shown in the following equation [15]:
20180804095213 78187 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere
Where A and n are constants whose values are related to the material and the corrosive environment. In order to verify whether the power function law is applicable to the development of corrosion kinetics of carbon steel in the atmospheric environment of the marine industry, especially the dynamic development law of the initial corrosion acceleration process, the power function relationship is used to simulate the marine industry of Q235 carbon steel. The weight loss data of the initial corrosion acceleration stage (first 96 h) in the atmospheric environment was fitted, and the fitting results are shown in Fig. 2. The correlation coefficient R2 (0.9998) is close to 1, indicating that the initial corrosion behavior of carbon steel in the simulated marine industrial atmosphere still follows the law of power function development. In this experiment, n=1.3998, which is greater than 1, indicating that the rust layer formed by carbon steel in the initial stage of corrosion has no protection to the substrate. With the extension of the corrosion cycle (the first 96 h), the corrosion rate of carbon steel increases gradually.
20180804095325 20279 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Fig.2 Thickness reductions of Q235 carbon steel exposed to a simulated coastal-industrial atmosphere as a function of exposure time

2.1.2 Composition of rust layer Figure 3 shows the XRD spectrum of corrosion products formed on the surface of Q235 carbon steel in different atmospheric corrosion environments under simulated atmospheric conditions. It can be seen that the corrosion products mainly have α-FeOOH and γ-FeOOH after 24 h of corrosion, and a small amount of FeSO32.5H2O is also detected. For the initial stage of carbon steel corrosion, the corrosion products are mainly γ-FeOOH, and the presence of SO2 promotes the preferential generation of α-FeOOH [16]. When corroded to 48 h, β-FeOOH began to appear in the corrosion products, and the formation of β-FeOOH was related to the presence and concentration of Cl-. At this time, the Cl- concentration on the surface of the sample reaches a critical concentration, which provides the necessary conditions for the formation of β-FeOOH. β-FeOOH has a strong reducing activity, which promotes the corrosion process [17]. After that, as the corrosion time prolonged, the composition and relative content of the corrosion products no longer changed significantly. In addition, the presence of the corrosion product FeSO32.5H2O can always be detected throughout the corrosion process. The ferrous sulfate salt increases the conductivity of the thin liquid film and promotes the electrochemical corrosion reaction on the surface of the sample. At the same time, according to the acid regeneration cycle mechanism, FeSO3 will be formed into oxygen by FeSO4, and then FeSO4 is hydrolyzed and oxidized to form iron oxyhydroxide. And free sulfuric acid [18,19], thereby accelerating atmospheric corrosion of carbon steel.

20180804095428 71672 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Fig.3 XRD patterns for the scraped rust formed on Q235 carbon steel surface after different exposure time

2.1.3 Analysis of rust layer morphology Macroscopically, as the exposure time is prolonged, the color of the corrosion product gradually deepens, and the orange-yellow color changes to reddish brown and tan, and the corrosion products are evenly distributed on the surface of the sample. Figure 4 is a diagram showing the surface topography of rust layer in different corrosion cycles of Q235 carbon steel in simulated marine industrial atmosphere. At the first 24 h of corrosion, spherical corrosion products formed and the corrosion product layer was very thin, which caused the corrosive medium to easily contact the substrate and cause corrosion [20] (Fig. 4a). When the corrosion time was extended to 48 h, the surface of the sample was corroded. The product gradually increases, but the corrosion product contains a large number of pores, which can not effectively prevent the intrusion of corrosive media (Fig. 4b); and then, as the corrosion time prolongs, the surface corrosion product compactness gradually increases (Fig. 4c~e).

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Fig.4 Surface morphologies of Q235 carbon steel exposed to a simulated coastal- industrial atmosphere for 24 h (a), 48 h (b), 72 h (c), 96 h (d) and 120 h (e)

Figure 5 is a cross-sectional view of the cross-section of the rust layer of Q235 carbon steel under different atmospheric corrosion conditions in a simulated marine industrial atmosphere. At the first 24 h of corrosion, the rust layer is thinner and looser. There are many transverse and longitudinal cracks, which have no inhibitory effect on O2 and aggressive media, and the corrosion products have little protection to the steel matrix. When corroded to 72 h, the corrosion products showed obvious delamination, that is, the loose rust layer with a large number of holes and the cracked but relatively dense inner rust layer [21, 22], but the inner rust layer and the outer rust layer The boundaries between the two are not obvious. With the prolongation of corrosion time, the thickness of the inner and outer rust layers gradually increases and the compactness gradually increases. When corroded to 120 h, the inner layer corrosion products are dense and crack-free, which can effectively prevent the penetration of O2 and corrosive media. The corrosion rate of steel begins to decrease.

20180804101920 19691 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Fig.5 Cross-sectional morphologies of Q235 carbon steel exposed to a simulated coastal-industrial atmosphere for 24 h (a), 48 h (b), 72 h (c),96 h (d) and 120 h (e)

2.1.4 Electrochemical analysis Figures 6 and 7 show the potentiodynamic polarization curves of the rust and blank samples of carbon steel in different corrosion cycles in 0.1 mol/L Na2SO4 solution and the corrosion current density obtained by fitting. It can be seen from the figure that the cathode process of the blank sample is controlled by the diffusion diffusion of dissolved oxygen, and the cathode current density of the rust sample is significantly larger than that of the blank sample, which is mainly related to the reduction of the cathode corrosion product of the rust sample. After 24~72 h corrosion, the cathode current density of the rust sample increased gradually, indicating that the reduction reaction of the corrosion product gradually became the main cathode process. After 96 h of corrosion, the cathode current density did not change much, indicating that the cathodic reduction process gradually became stable. Stabilization of the cathode process with rust samples may be related to the stabilization of the relative content of γ-FeOOH in the corrosion products [23].
For the anode process, the anode reaction of the rust sample is in addition to the dissolution reaction of the carbon steel matrix, and the oxidation reaction of the corrosion product FeSO3 is performed, and the anode current density of the rust sample is larger than that of the blank sample. However, it is worth noting that as the corrosion time prolongs, the anode current density of the rust sample decreases gradually, indicating that the anode reaction process is gradually suppressed. The corrosion current density obtained by curve fitting is shown in Fig. 7. Corrosion current density of rust samples increased significantly after 24~72 h of corrosion. After 72 h of corrosion, the corrosion current density of rust samples increased, and the change became stable. This result is consistent with the corrosion rate results obtained from the weight loss curve.
20180804102255 47936 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Fig.6 Potentiodynamic polarization curves of Q235 carbon steel with different exposure time (E—potential, i—current density)

2.1.5 Analysis of initial corrosion evolution process In the simulated atmospheric environment of marine industry, Q235 carbon steel undergoes corrosion process of matrix anode dissolution and dissolved oxygen cathode reduction under a thin liquid film, thereby forming a hydrolyzate layer of Fe(OH)2, followed by During the drying process, Fe(OH)2 is oxidized by air to form a large amount of divalent and trivalent oxides and hydroxides of Fe, and some oxides are converted into γ-FeOOH and α-FeOOH. The presence of SO2 promotes α-FeOOH. Priority generation. When corroded to 48 h, the concentration of Cl- reached a critical concentration, which provided the necessary conditions for the formation of β-FeOOH. At this time, β-FeOOH began to appear in the corrosion products. HSO3- in the simulated environment makes the thin liquid film on the sample surface acidic, resulting in a uniform chemical dissolution reaction [24], and FeSO32.5H2O is gradually formed on the surface of Q235 carbon steel. In the initial stage of corrosion, the rust layer is thin, and the outer rust layer is loose, and the inner rust layer is cracked (Fig. 5). Such corrosion products can not effectively block the corrosion of the corrosive medium, but can prolong the time that the corrosive medium stays on the surface of the carbon steel. That is, prolonging the surface wetting time and promoting the rapid development of corrosion, the corrosion rate is gradually increased. Until corrosion for 120 h, the thickness of the inner and outer rust layers increases, the compactness increases, and the cracks decrease. The corrosion products can effectively block the intrusion of O2 and corrosive media, providing some protection for the matrix, which shows that the corrosion rate begins to decrease.

20180804102637 20655 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Fig.7 Corrosion current densities (icorr) of corroded Q235 carbon steel as a function of exposure time

2.2 Effect of different ratios of SO2 and Cl- on initial corrosion mechanism
2.2.1 Analysis of corrosion weight loss Figure 8 shows the corrosion depth of Q235 carbon steel after corrosion for 120 h in different corrosive media. It can be seen that the corrosion weight loss of carbon steel in SO2 and Cl-mixed corrosive medium environment is significantly greater than that of single corrosive medium, indicating that there is a synergistic effect between SO2 and Cl- in the corrosion process, which accelerates the corrosion of carbon steel [25~28]. The corrosion effect of corrosive medium on Q235 carbon steel is: NaHSO3+NaCl>NaHSO3>NaCl. Among them, when the ratio of NaCl:NaHSO3 is 1:1, the corrosion loss of carbon steel reaches a maximum value.
20180804103025 98895 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Fig.8 Thickness losses of Q235 carbon steel exposed to different corrosive mediums for 120 h

Cl- is a natural pollutant that is found in the ocean atmosphere. Due to its small volume, Cl- can pass through the rust layer to reach the surface of the substrate to promote the corrosion reaction, and is not consumed during the reaction, acting as a catalyst [29, 30]. At the same time, the presence of Cl- increases the conductivity of the thin liquid film and promotes the role of corrosion of the microbattery. The presence of Cl- also causes partial dissolution of Fe(OH)2 adsorbed by the Fe matrix:
20180804103154 84629 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere
The above effects may cause localized dissolution of the relatively dense corrosion products to cause loose outer rust layers and fine cracks (Fig. 5). O2 and corrosive media are transmitted to the surface of the substrate through cracks to promote corrosion.
SO2 is recognized as the most corrosive gas in the atmosphere. In this experiment, HSO3- reacts with the matrix to form FeSO3, while FeSO3 is gradually oxidized to FeSO4, and then FeSO4 is hydrolyzed and oxidized to form oxyhydroxide and free sulfuric acid:
20180804103227 89742 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere
H2SO4 accelerates the corrosion of Fe:
20180804103456 69183 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere
The newly formed FeSO4 is hydrolyzed to the free acid, so that it can reciprocate, so that one molecule of SO2 can generate many molecules of rust [31~33], and the free acid generated during the cycle dissolves the unstable corrosion products in the rust layer. Provides a channel for the intrusion of O2 and corrosive media to further accelerate corrosion. The synergistic effect of the local dissolution of the corrosion products and the regeneration cycle of the acid makes the corrosion weight loss of the Q235 carbon steel in the mixed corrosive medium significantly higher than that in the single corrosive medium. However, this synergistic effect does not continue to increase with increasing SO2 concentration in the corrosive medium, but the corrosion weight loss of carbon steel reaches a maximum when the ratio of Cl- and SO2 is 1:1. This is because as the concentration of SO2 increases, the amount of H2SO4 generated during the regeneration cycle of the acid increases, and the rust layer is more easily destroyed, so that the corrosion accelerates. However, when the SO2 concentration continues to rise to a certain critical value, SO2 significantly promotes the formation of the corrosion product α-FeOOH. α-FeOOH is a stable corrosion product with good continuity and compactness, which can provide good protection to the substrate, which shows that the corrosion weight loss of carbon steel decreases. However, the effect of the ratio of SO2 and Cl- in the corrosive medium on the corrosion weight loss of carbon steel is not significant, indicating that the effect of the ratio of SO2 and Cl- on the synergy is not obvious.
2.2.2 Corrosion product composition analysis Figure 9 shows the XRD spectrum of Q235 carbon steel after corrosion for 120 h in a single NaCl, NaHSO3 corrosive medium and different ratios of NaCl and NaHSO3 corrosion media. It can be seen that the corrosion products formed in different proportions of mixed medium mainly include α-FeOOH, β-FeOOH, γ-FeOOH and FeSO32.5H2O, indicating that the ratio of SO2 to Cl- in the corrosive medium does not change the corrosion products formed on the surface of carbon steel. Ingredients, but as the proportion of NaCl decreases, the relative content of β-FeOOH in the corrosion products decreases significantly, while the relative content of α-FeOOH gradually increases. The simultaneous presence of β-FeOOH and FeSO32.5H2O promotes the corrosion process, which shows that the corrosion weight loss of carbon steel in mixed corrosive media is significantly higher than that of single corrosive media. Q235 carbon steel in a single NaCl corrosion environment, the corrosion products contain a large amount of β-FeOOH and a small amount of γ-FeOOH, because the presence of high concentration of Cl- promotes the formation of β-FeOOH; and in the case of corrosion in a single NaHSO3 environment, The corrosion product contains a large amount of α-FeOOH and a small amount of γ-FeOOH, and it can be seen that α-FeOOH is preferentially formed in an environment containing SO2 [16].
20180804103544 78683 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Fig.9 XRD patterns for the scraped rust formed on Q235 carbon steel exposed to different corrosive mediums for 120 h(a) the mixed corrosive mediums(b) the single corrosive mediums

2.2.3 Analysis of Corrosion Product Morphology Figure 10 shows the cross-sectional shape of carbon steel after corrosion for 120 h in different corrosive media. It can be seen from the figure that when Q235 carbon steel is corroded in the environment of single NaCl, the rust layer is thin, but there is a large local corrosion pit, which is caused by NaCl induced pitting; when Q235 carbon steel exists in single NaHSO3 When corroded in the environment, the rust layer is also very thin, but there is no significant local corrosion compared to the single NaCl environment.

20180804103714 66408 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Fig.10 Cross-sectional morphologies of Q235 carbon steel exposed to different corrosive mediums for 120 h
           (a) NaCl (b) NaCl∶NaHSO3=3∶1 (c) NaCl∶NaHSO3=1∶1 (d) NaCl∶NaHSO3=1∶3 (e) NaHSO3
When Q235 carbon steel is corroded in the environment where both NaCl and NaHSO3 coexist, the thickness of corrosion products formed is significantly higher than that of single corrosive medium, indicating that the corrosion rate of Q235 carbon steel in NaCl and NaHSO3 mixed corrosion medium is faster, the result and weightlessness. The results are consistent. It is worth noting that the corrosion product layer of Q235 carbon steel in a 3:1 ratio of NaCl and NaHSO3 corrosive medium is a two-layer structure, that is, a loose, porous outer layer structure and a relatively dense inner layer structure [ 21, 22], and this corrosion product stratification is not clearly observed in other mixed proportions of the corrosive environment. Due to the presence of Cl-, the corrosion products formed on the surface of the carbon steel are locally dissolved. The higher the Cl- concentration, the more severe the local dissolution of the corrosion products, resulting in a loose outer layer structure of the corrosion products, thereby inferring the surface of the carbon steel. The stratification of corrosion products is related to the Cl- concentration in the corrosive medium.
Figure 11 shows the surface topography of Q235 carbon steel after corrosion for 120 h in different corrosive media. It can be seen from the figure that when the carbon steel is corroded in a single NaCl, the surface of the sample after removing the corrosion product exhibits atypical local corrosion characteristics, and the corrosion pit is large and small, which is caused by NaCl (Fig. 11a); When carbon steel is corroded in the mixed medium of NaCl and NaHSO3, it can be seen that the corrosion pit on the surface of the sample is small and much, and the corrosion pit of the surface becomes more as the proportion of NaHSO3 in the mixed corrosion medium increases. Small and dense (Fig. 11b~d); when carbon steel is corroded in a single NaHSO3, the surface of the sample exhibits its own uniform corrosion characteristics (Fig. 11e). The above analysis shows that when carbon steel is corroded in the environment where SO2 and Cl- coexist, SO2 promotes the corrosion pattern of carbon steel to tend to uniform corrosion.
20180804104105 20005 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Fig.11 Morphologies of Q235 carbon steel exposed to different corrosive mediums after removing corrosion products for 120 h(a) NaCl (b) NaCl∶NaHSO3=3∶1 (c) NaCl∶NaHSO3=1∶1 (d) NaCl∶NaHSO3=1∶3 (e) NaHSO3

2.2.4 Electrochemical analysis Figure 12 shows the potentiodynamic polarization curves of Q235 carbon steel after corrosion for 120 h in different corrosive media. It can be seen from the figure that the cathode current density of Q235 carbon steel when corroded in a single NaHSO3 is significantly smaller than the cathode current density corroded in a single NaCl. The reason for the analysis is that the corrosion product is mainly β-FeOOH when corroded in a single NaCl, and the α-FeOOH is mainly corroded in NaHSO3. The reduction of α-FeOOH is less than that of β-FeOOH, so the corrosion of the corrosion product β-FeOOH is more active when corroded in a single NaCl, which is characterized by a higher cathode current density. Although the carbon steel is corroded in a single NaHSO3, the cathode process is somewhat inhibited, but the anode process is significantly promoted. This is because the corrosion product of carbon steel in a single NaHSO3 environment contains FeSO3, that is, the anode reaction process In addition to the carbon steel matrix dissolution reaction, there is also the oxidation reaction of the corrosion product FeSO3, so the anode current density is significantly higher than the anode current density of the carbon steel corroded in a single NaCl. Corrosion current density of rust samples after corrosion of Q235 carbon steel in different corrosive media for 120 h was obtained according to the results of dynamic potential polarization curves (Fig. 13). It can be seen that the corrosion current density of Q235 carbon steel in different NaCl and NaHSO3 mixed medium environments is comparable, but significantly greater than its corrosion current density in single NaCl and single NaHSO3; the corrosion current density of carbon steel in a single NaHSO3 environment is greater than that in single Corrosion current density in NaCl. This result is consistent with the corrosion weight loss results.

20180804104246 67862 - Study on initial corrosion behavior of carbon steel in simulated marine industrial atmosphere

Fig.12 Potentiodynamic polarization curves of Q235 carbon steel exposed to different corrosive mediums for 120 h

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Fig.13 icorr of Q235 carbon steel exposed to different corrosive mediums for 120 h

3 Conclusion
(1) The initial corrosion rate of Q235 carbon steel in the simulated marine industrial atmosphere first increased and then decreased. The dynamics of corrosion acceleration process still followed the power function formula D=Atn.
(2) The presence of corrosion products β-FeOOH and FeSO3 promotes an increase in the corrosion rate of carbon steel. After 24 h of corrosion, the corrosion product has a loose outer layer and a relatively dense inner layer structure. The leaching of corrosion products may be related to the Cl- concentration in the corrosive medium.
(3) At the initial stage of corrosion, the synergistic effect of SO2 and Cl- accelerates the corrosion of carbon steel, but the change of the ratio of SO2 and Cl- in the corrosive medium has no obvious effect on the corrosion weight loss and does not change the corrosion product composition formed on the surface of carbon steel. SO2 promotes the corrosion pattern of carbon steel to tend to uniform corrosion.
The authors have declared that no competing interests exist.

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