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wet cycle and immersion environments
Construction and Building Materials 235 (2020) 117440
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Comparative study on corrosion behaviour of rusted X100 steel in dry/ wet cycle and immersion environments Ke Gong a,b, Ming Wu a,b,⇑, Guangxin Liu a,b a b
Key Laboratory of Oil & Gas Storage and Transportation, College of Petroleum Engineering, Liaoning Shihua University, Fushun, Liaoning 113001, China College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong 266555, China
h i g h l i g h t s Simulated marine environment through a self-made dry/wet cycling device.
The simultaneous effects of Cl , O2 and rust layer were studied. Compare corrosion products in dry/wet cycling and full immersion. The three stages of the surface in the dry/wet cycle environment were discussed.
a r t i c l e
i n f o
Article history: Received 14 April 2019 Received in revised form 8 August 2019 Accepted 1 November 2019
Keywords: Dry/wet cycle Immersion Rust layer High strength steel
a b s t r a c t A dry/wet cycle corrosion test device was constructed to study the corrosion behaviour of X100 steel under rust layer in dry/wet cycling and immersion conditions. Results showed that the exposure of X100 steel in dry/wet cycle condition with high concentration of Cl, fast diffusion of O2, and wetness of the rust layers led to an accelerated corrosion process. The initial corrosion rate in the dry/wet cycle environment was higher than that in the immersion environment. As the corrosion time increased, the a-FeOOH content in the rust layer increased, resulting in a decrease in the rate of electrochemical corrosion of metals. In the immersion environment, the corrosion mechanism involved was a typical oxygenabsorbing corrosion mechanism. Under dry/wet cycle conditions, the simultaneous effects of Cl, O2, and rust layer were studied. The three stages of the wet surface in dry/wet cycle environment were described, and the corrosion mechanism was clarified. Ó 2019 Published by Elsevier Ltd.
1. Introduction With the continuous exploration and mining of offshore oil and gas resources, carbon steel is widely used in marine environments for oil and gas transportation [1–3]. Due to the conditions in marine environment, pipelines are highly susceptible to corrosion, which has become a key factor in operational safety. Marine oil and gas leakage accidents caused by pipeline corrosion account for 36% of the total such accidents [4–6].For safety and economic reasons, high-grade pipeline steel is an optimal choice for the efficient storage and transport of oil and gas. Currently, the majority of pipeline steels used in oil and gas pipelines in the marine environment are X65, X70, or X80 pipeline steels [7–9]. X100 pipeline steel ⇑ Corresponding author at: Key Laboratory of Oil & Gas Storage and Transportation, College of Petroleum Engineering, Liaoning Shihua University, Fushun, Liaoning 113001, China. E-mail addresses: [email protected] (K. Gong), [email protected] (M. Wu). https://doi.org/10.1016/j.conbuildmat.2019.117440 0950-0618/Ó 2019 Published by Elsevier Ltd.
is characterised by several economic advantages including high strength, high pressure, and low cost [10–12]. Therefore, the application prospect of X100 high-strength pipeline steel in marine oil and gas transportation is very broad. Although X100 steel has many advantages, it is limited by a high risk of hydrogen embrittlement during use. It is well known that hydrogen embrittlement is one of the most dangerous failure modes and can result in pipeline accidents because of its unpredictability and destructiveness [13– 16]. Therefore, it is important to study the corrosion behaviour of X100 steel in dry/wet cycle environment. In the marine splash zone, carbon steel is subjected to alternate wet and dry states over long periods of time, and the high salt content, seawater scouring, and a sufficient oxygen supply result in severe corrosion of pipeline steel [17,18]. In particular, high aeration contents in dry/wet cycle environment lead to accelerated corrosion of metals [3,19,20]. Moreover, the metal corrosion process occurring in this environment differs significantly from the metal corrosion process in a full immersion environment [3,21–24]. In the dry/wet cycle environment, due to the periodic changes in
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humidity and temperature environment, film thickness decreases and increases alternately, thereby rendering the metal corrosion process more complex [25]. First, Cl is key factor during the corrosion process in the dry/wet cycle environment [22,26]. Following the evaporation and thinning of the liquid film during the drying process, the concentration of Cl increases as the salt content in the liquid layer increases; the conductivity of the liquid layer also increases, thereby promoting the anode reaction [27]. Second, the diffusion of O2 has a considerable influence on the electrochemical corrosion of steel. In the dry/wet cycle environment, due to the faster diffusion of O2, the depolarization speed of the cathode is accelerated, and the local anode corrosion rate depends on the cathode reaction resulting in an accelerated metal corrosion rate [28]. Third, the rust layer formed on the steel surface significantly affects the corrosion behaviour of the steel and the amount of water associated with the surface properties [29]. As the thickness of the rust layer increases, the moisture present in this layer requires more time to evaporate, and the metal surface remains wet for a longer period of time, thereby providing favourable conditions for accelerated metal corrosion [30,31]. However, the rust layer can alter the diffusion trajectory of O2 and prevent Cl from penetrating into the steel matrix, thereby slowing the metal corrosion [32,33]. To date, only the individual effects of Cl [34–36], O2 [37,38], and rust layer [30,39,40] have been studied. However, the simultaneous effects of Cl, O2, and rust have not been investigated. Additionally, as some marine structures such as risers and platform legs are both in a fully immersed environment and in the ocean splash zone, it is necessary to perform a comparative study of the corrosion behaviour of X100 high-strength carbon steel under dry/wet cycle and immersion conditions, and clarify the corrosion mechanism under different environments. To the best of our knowledge, no reports regarding the corrosion mechanism of X100 high-strength pipeline steel in the dry/ wet cycle conditions exist in literature. This study aims to clarify the corrosion mechanism of X100 steel and perform a comparative analysis of corrosion under dry/wet cycling and immersion conditions to provide a theoretical basis for the application of X100 steel in a marine environment.
Fig. 1. Microstructure of X100 steel.
trodes were welded with a Cu-wire for electrical connection, and the samples were embedded in cold curing epoxy resin and mounted in a polyethylene (PE) tube holder with a 1 cm2 working area. Two immersion specimens were required under each condition, one for observing the corrosion product and corrosion morphology, and the other for observing the morphology of the rust layer. All experimental samples were abraded using various grades of silicon carbide papers ranging from 80 to 2000 grit, following by degreasing with acetone, dehydration in absolute ethanol, and storage in a drying cabinet prior to use.
2.2. Dry/wet cyclic tests Based on the marine environment of the South China Sea, the seawater temperature, atmospheric temperature, and relative humidity were controlled using an in-house-developed dry/wet cyclic test device to simulate the marine environment of the South China Sea. The experimental device employed is shown in Fig. 3. A water bath heating device is used to control the simulated seawater temperature, heating tube is used to control the temperature of the test chamber, and salt spray generator controls the relative humidity of the test chamber. The above devices are connected to a computer system and controlled by adjusting the input parameters to obtain a stable experimental environment. Seawater simulation solution consisted of a 3.5 wt% solution of NaCl in deionised water (1 L), and the pH of this solution was adjusted to ~7.5 using 4 wt% glacial acetic acid or 10 wt% sodium hydroxide. According to the environmental parameters of the South China Sea, the tropical ocean conditions were simulated [41,42], which included high salinity and humidity (77%), average surface water temperature of 27 °C, and average temperature of 30 °C. Two groups were created for this experiment. Group A was subjected to the dry/wet cycle environment, while group B was subjected to the immersion environment. Group A samples were immersed in the simulation solution at 25 ± 1 °C for 12 min (the wet period) followed by sample drying at 30 ± 1 °C and 75% humidity for 48 min (the dry period). For group B, the samples were fully immersed in the simulation solution at 25 ± 1 °C for 24, 72, 120, 168, or 240 h, and the solution was replaced every 48 h.
2. Materials and methods 2.1. Test materials The test materials were cut from a thick plate of API 5L X100 high-strength pipeline steel and the chemical composition (wt.%) and mechanical properties are shown in Table 1. The microstructure of the X100 pipeline high-strength steel is presented in Fig. 1, where the acicular ferrite and bainite structures are apparent. The microstructure was revealed by gradual abrasion with a series of silicon carbide papers (150, 360, 600, 800, 1500, and 2000 grit), and subsequent polishing with a 5-lm diamond suspension. The sample was then immersed in an etchant (2 ml of 70% nitric acid and 98 ml of anhydrous, denatured ethyl alcohol) for 15 s prior to treatment with deionised water, dehydration in absolute ethanol, and drying under air. Fig. 2 shows the dimensions of the specimens, i.e., the rectangular electrochemical sample (1 1 cm2) and immersion sample (2 25 50 cm3). In the case of the samples used for electrochemical testing, the working elec-
Table 1 Chemical composition of the X100 steel sample (mass fraction, %). C
Si
Mn
S
P
Al
Nb
Ni
Cu
Ti
Fe
0.05
0.25
1.83
0.002
0.08
0.02
0.10
0.23
0.21
0.01
Bal
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Fig. 2. Schematic of the sample.
Fig. 3. Dry/wet cycle corrosion test device. (1. Integrated ultrasonic salt spray generator; 2. Limit switch; 3. PLC control system; 4. Touch control screen; 5. Temperature and humidity transmitter; 6. Motor; 7. Salt spray exit; 8. Dehumidification fan; 9. Connecting rod; 10. Sample holder; 11. Humidity Sensor; 12. Temperature Sensor; 13. Infrared heating tube; 14. Soaking pool; 15. Water bath; 16. Water bath heater; 17. Slide.)
2.3. Electrochemical measurements An electrochemical workstation (PARSTAT4000, Princeton) equipped with a traditional three-electrode cell was used to conduct the electrochemical measurements. The three electrodes included a specimen as the working electrode (WE), Pt plate as the counter electrode (CE), and saturated calomel electrode (SCE) as the reference electrode (RE). All potentials are reported in the
SCE scale. The test solution is described above. All electrochemical experiments were carried out without stirring or deaeration of the test medium. Initially, the open circuit potential was monitored for 30 min to obtain a stable open circuit potential, after which electrochemical impedance spectroscopy (EIS) was carried out at the open circuit potential using AC signals of 10 mV amplitude (peak-to-peak sinusoidal wave) between 100 kHz and 10 mHz, with 10 points per
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decade. ZSimpWin software was used to analyse the EIS data with a suitable equivalent circuit (EC). Finally, the potentiodynamic polarisation curves were recorded at a scan rate of 0.33 mV/s in the potential range of 0.2 V (vs. OCP) to +0.2 V. All experiments were performed in triplicate under the same experimental conditions to ensure that the results were reproducible and reliable. 2.4. Morphological observations and composition analysis After the removal of corrosion product from the immersion sample, ultrasonic cleaning was carried out for 10 min using distilled water. Thereafter, the sample was dried under warm air, and the surface morphology was observed by scanning electron microscopy (SEM; SU8010, Hitachi). The compositions of the corrosion products were analysed by X-ray diffraction (XRD; D8 Advance, Bruker) using Cu as the target with an electric current of 200 mA and a voltage of 40 kV at a scan rate of 3°/min. The cross-section of the rust layer was characterised by SEM, and the elements present were determined by energy-dispersive X-ray spectroscopy (EDS; SU8010, HITACHI). 3. Results 3.1. Electrochemical properties 3.1.1. EIS Fig. 4 shows the Nyquist diagrams and Bode plots for the X100 steel specimens under dry/wet cycling and immersion conditions.
As indicated, the measured impedance plots differ significantly depending on the environmental conditions and testing time. As shown in Fig. 4(a) and (c), the Nyquist plot shows a capacitive arc, which indicates the common frequency dispersion effect of the X100 steel electrode. Moreover, the size of the capacitive arc decreases gradually with corrosion time. A detailed analysis of the dependence of the corrosion rates under the dry/wet cycling and immersion conditions can be obtained by fitting the measured EIS data with appropriate electrochemical equivalent circuits. The EIS data were fitted with the electrochemical equivalent circuits to determine the dependence of the corrosion rate under dry/wet cycling and immersion conditions, as shown in Fig. 5, where Rs represents the solution resistance, and Qf and Rf correspond to the capacitance and resistance of the corrosion product film, respectively. Qdl and Rct represent the double-charge layer capacitance and charge-transfer resistance, respectively. The constant phase element (CPE) was used for EIS data fitting because the corrosion reaction reduced the uniformity of the sample surface. This can be calculated according to Eq. (1): a
Z Q ¼ Y 0 ðjxÞ :
ð1Þ
where x is the angular frequency (rad/s), and Y0 and a are CPE parameters indicating the deviation of the specimen from the ideal capacitive behaviour [43]. The fitting results are plotted as solid curves in Fig. 4, where the measured EIS data fit well with the circuits, with a fitting error of <10%. The kinetic evolution of the electrochemical processes involved in the experiment can be quantified from the polarisation resis-
Fig. 4. Nyquist and Bode diagrams of the impedance measurements for X100 pipeline steel under (a, b) dry/wet cycling and (c, d) immersion conditions. The black solid lines represent fitted impedance curves with appropriate equivalent circuits.
K. Gong et al. / Construction and Building Materials 235 (2020) 117440
Fig. 5. Electrochemical equivalent circuit used to fit the measured EIS data.
tance Rp, (Rf + Rct), which can reflect the speed of the electrochemical corrosion; the higher is the Rp value, slower is the electrochemical reaction rate [44]. The EIS fitting results of Rp as a function of time under dry/wet cycling and immersion conditions are shown in Fig. 6. In general, the corrosion rate of X100 steel tends to increase with time under both sets of conditions. In addition, in the immersion environment, the value of Rp decreases slowly at the beginning of the experiment, prior to a subsequent rapid decrease, thereby indicating that the corrosion rate is slower at the beginning of the experiment, and is accelerated after 120 h. In the dry/wet cycle environment, the Rp value decreases rapidly in the initial stages of the experiment, and the decreases slowly after 120 h, indicating that the corrosion rate increases rapidly with time prior to 120 h. Furthermore, the initial corrosion rate of X100 steel under dry/wet cycle conditions is higher than that in the immersion environment. However, after 240 h, the corrosion rate of the sample in the immersion environment is higher than that under dry/wet cycle conditions. 3.1.2. Potentiodynamic polarisation The potentiodynamic polarisation curves of X100 steel under (a) dry/wet cycling and (b) immersion conditions are shown in Fig. 7. These curves reveal that the corrosion process of X100 steel under both sets of conditions is under activation (charge transfer) control, with the polarisation curves exhibiting a significant rightward shift upon increasing the experimental time. The corrosion current density (Icorr) and corrosion potential (Ecorr) in the dry/ wet cycle conditions were obtained using the Tafel fitting method, and the fitted results are shown in Fig. 8(a). These results reveal that the corrosion potential shows a gradual decline with an increase in the experimental time, with Icorr values of 22.27, 65.54, 125.23, 147.34, and 165.14 lA/cm2 obtained for experimen-
Fig. 6. Change in Rp as a function of time for X100 steel under dry/wet cycling and immersion conditions.
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tal times of 24, 72, 120, 168, and 240 h, respectively. The change in Icorr is faster from 0 to 120 h, and slower between 120 and 240 h, thereby indicating that the electrochemical corrosion process is faster. Furthermore, the corrosion current density (Icorr) and corrosion potential (Ecorr) values for the immersion conditions were obtained using the Tafel fitting method, and the fitted results are shown in Fig. 8(b). In this case, the Ecorr shows a gradual decline with an increase in the experimental time, with Icorr values of 15.21, 36.77, 89.80, 135.14, and 175.55 lA/cm2 obtained for experimental times of 24, 72, 120, 168, and 240 h, respectively. These fitted results indicate that Icorr increases with the corrosion time, showing an almost linear trend.
3.2. Surface morphologies Fig. 9 shows the morphology of the rust layer during the corrosion of X100 steel under dry/wet cycling and immersion conditions, as well as the surface morphology after the removal of the corrosion products. As shown in Fig. 9(a) and (c), after immersion for 120 h in different environments, the rust layer is relatively uniform and there is no obvious crack on the surface. Furthermore, the rust layer is relatively thin and the surface of the substrate can be partially observed through the rust layer. After 240 h (Fig. 9(b) and (d)), the rust layer exhibits a spore morphology, becoming loose with apparent internal cracks. In general, the macroscopic morphology of the rust layer does not differ significantly, being brownish-yellow in colour and having a low surface density. Moreover, the observation of the surface morphology of the X100 steel after the removal of corrosion products shows that the surface morphology of the metal in the two environments differs significantly. In the dry/wet cycle environment (Fig. 9(a) and (b)), corrosion pits >100 lm in diameter appear on the X100 steel surface due to the non-uniform anodic dissolution behaviour. In the immersion environment (Fig. 9(c) and (d)), the X100 steel exhibits a greater number of shallow corrosion pits at the beginning of the corrosion process, in addition to a local contiguous phenomenon, general corrosion characteristics, and an increased degree of corrosion as the corrosion time increases.
3.3. Cross-sectional morphologies The cross-sectional morphologies of the rust layers under dry/ wet cycling and immersion conditions are shown in Fig. 10. By comparing Fig. 10(a), (c) and (b), (d), it is clear that the rust layer interface morphologies of the corrosion samples do not exhibit any obvious stratification under dry/wet cycling and immersion conditions. Moreover, the rust layer formed in the dry/wet cycle environment is thicker than that formed under immersion conditions. In the dry/wet cycle environment (Fig. 10(a) and (b)), the maximum thickness of the rust layer is 93 lm after 120 h, and the degree of adhesion between the rust layer and substrate is high; voids and cracks are observed in the rust layer and the structural continuity is poor. With an increase in the corrosion time, the maximum thickness of the rust layer is 141 lm after 240 h, and the rust layer shows better compactness and structural continuity. Furthermore, Fig. 10(c) and (d) shows the rust layer formed on the surface of the X100 steel body under immersion conditions. In this case, the thicknesses of the maximum rust layers are 53 and 64 lm after 120 and 240 h, respectively. Transverse cracks of the vertical matrix are visible in addition to longitudinal cracks parallel to the matrix. The presence of these cracks provides a channel for the diffusion of oxygen and chloride ions, causing them to easily penetrate the rust layer and increasing the corrosion of the X100 matrix.
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Fig. 7. Potentiodynamic polarisation curves of X100 pipeline steel under (a) dry/wet cycling and (b) immersion conditions.
Fig. 8. Fitted results for Ecorr and Icorr in the simulated solution under (a) dry/wet cycling and (b) immersion conditions.
3.4. Composition of the corrosion products 3.4.1. XRD Fig. 11 shows the XRD spectra of the corroded X100 steel surfaces obtained following dry/wet cycling and immersion with varying corrosion time. As shown in Fig. 11, the main components of the rust layer formed under the dry/wet cycle environment are Fe3O4, a-FeOOH, b-FeOOH, and c-FeOOH, while the components of the rust layer formed under immersion conditions are mainly Fe3O4, a-FeOOH, and c- FeOOH [45]. Research shows that the presence of b-FeOOH is attributed to the chloride ions present in the liquid film layer on the carbon steel surface during dry/wet cycling. Thus, b-FeOOH can alter the structure of the rust layer to provide channels for the diffusion of corrosive elements such as oxygen and chlorine, thereby allowing the facile penetration of corrosion products and destruction of the corrosion resistance of the carbon steel rust layer, resulting in an acceleration of the corrosion process [46–48]. As stable oxides, Fe3O4 and a-FeOOH are deposited in the rust layer, and upon thickening of the rust layer, these species can prevent chloride ion penetration. 3.4.2. EDS Fig. 12 shows the elemental distribution in the rust layer after 120 h testing under dry/wet cycling and immersion conditions. Through the analysis of the EDS surface distribution of the rust
layer of X100 steel after 120 h, it is determined that the rust layer contains large amounts of Fe and O, which represents the main iron oxide corrosion product. In addition, Cl penetrates the rust layer, indicating that the rust layer formed in the experimental environment does not prevent Cl diffusion into the matrix, and in the case of the dry/wet cycling environment, it can be clearly seen that Cl is deposited in the rust layer. During the thickening of the rust layer under laboratory conditions, the researchers have observed that at low Cl concentrations, the oxyhydroxide formed is lepidocrocite, while high Cl concentrations lead to the formation of akageneite [49]. This finding is consistent with the results obtained in the experimental environment in this study. 4. Discussion 4.1. Comparative analysis of corrosion mechanisms Owing to these periodic variations in the corrosion environment in dry/wet cycle conditions, the corrosion mechanism is distinctly different from the corrosion mechanism of metals in immersion conditions. The corrosion of X100 steel under dry/wet cycle conditions can be regarded as an electrochemical corrosion of metal under a thin liquid layer, i.e., ferrous ions are formed by the dissolution of an anode reaction on the steel surface and are oxidised to
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Fig. 9. Surface corrosion morphology of X100 steel in the corrosion of X100 steel in both environments ((a, b) dry/wet cycling and (c, d) immersion conditions).
form a rust layer. Therefore, the electrochemical corrosion process of a metal under dry/wet cycle conditions includes the reaction occurring at each stage during the formation of an electrolyte layer on the metal. The following can represent the electrochemical reactions in the X100 steel in the initial corrosion stage [19,50,51]: Anodic reaction in the immersion environment:
Fe ! Fe2þ þ 2e
ð2Þ
Cathodic reaction:
O2 þ 2H2 O þ 4e ! 4OH Total reaction:
ð3Þ
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Fig. 10. Cross-sectional morphologies of the rust layers under dry/wet cycling and immersion conditions ((a, b) dry/wet cycling, and (c, d) immersion conditions).
Fig. 11. XRD spectra of the X100 steel following (a) dry/wet cycling and (b) immersion with different corrosion time.
Fe2þ þ 2OH ! FeðOHÞ2
ð4Þ
The corrosion product is Fe(OH)2, which is then oxidised to Fe (OH)3, which is unstable. Subsequently, Fe(OH)3 transforms into goethite (a-FeOOH) and lepidocrocite (c-FeOOH) [52].
FeðOHÞ3 ! FeOOH þ H2 O
ð5Þ
The above reaction can explain the metal corrosion mechanism in a fully immersed environment, but it is not sufficient to explain the complex corrosion mechanism of metals under dry/wet cycle conditions.
Strattmann et al. [53] proposed a three-stage rust formation mechanism during an electrochemical study of phase transitions in already formed rust layers for dry/wet cycle environment:wetting of the dry surface, wet surface, and drying-out of the surface. This clarifies the corrosion process of the metals in dry/wet cycle conditions. Based on this investigation, to clearly explain the corrosion process of X100 steel under dry/wet cycle conditions, this study further divides the wet surface into three stages. In the first stage (Fig. 13(b)), no rust layer is present on the metal surface. A thin liquid layer is formed on the metal surface, and as this simulated solution evaporates, the thin liquid layer becomes more thin-
K. Gong et al. / Construction and Building Materials 235 (2020) 117440
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Fig. 12. Elemental distribution in the rust layer after 120 h testing ((a) dry/wet cycling, and (b) immersion conditions).
ner, thereby increasing the concentration of Cl. This in turn increases the conductivity of the film and promotes the corrosion of the microbattery. The rate of O2 diffusion increases under the thin liquid layer, which in turn affects the rate of cathodic reaction of the metal in the thin liquid layer [38]. In the second stage (Fig. 13(c)), the corrosion progresses, and a rust layer appears on the metal surface. Once a thin rust layer is formed on the steel surface, the effect of the composition and thickness variation of the rust layer on the electrochemical corrosion process of the steel surface must be considered. As shown in Figs. 9 and 10, the early rust layer is thin and does not prevent
O2, Cl ions, or any other corrosive substances from reaching the substrate. Notably, some of the Cl enters the rust layer, while the remainder can pass through the rust layer to reach the surface of the substrate and promote corrosion in a catalytic manner [49]. The deposition of Cl in the rust layer was detected via EDS analysis (Fig. 12), which was linked with the formation of b-FeOOH. The corresponding electrochemical reactions can be expressed as follows [40,54–56]:
Fe2þ ! Fe3þ þ e
ð6Þ
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Fig. 13. Schematic of the corrosion process of X100 steel under dry/wet cycling conditions.
Fe3þ þ Cl þ OH ! FeOCl þ H
ð7Þ
FeOCl þ OH ! b FeOOH þ Cl
ð8Þ
Moreover, in this stage, owing to the thickness of the rust layer, the rate of moisture absorption is high. Therefore, corrosion of the metal surface is prolonged, and the rust layer acts as a strong oxidant during the wetting process, thereby promoting the reduction from Fe3+ to Fe2+. and accelerating the electrochemical corrosion of metal [57,58]. The corrosion at this stage is promoted by the O2 diffusion rate, Cl concentration, and thickness of the rust layer. In the third stage (Fig. 13(d)), the rust layer on the metal surface becomes thicker as shown in Figs. 9 and 10. The content of aFeOOH in the rust layer increases (Fig. 12(a)). It is reported that out of all these oxides, a-FeOOH is the most stable phase [59]. Once the content of a-FeOOH increases in the rust layer, the corrosion rate of the steel is significantly reduced. At this stage, the diffusion of O2 and Cl is hindered to a certain extent. It is not conducive to the electrochemical reaction of the metal surface. In the immersion environment, the corrosion mechanism is a typical oxygen-absorbing corrosion mechanism. The rust layer does not have protective properties in the 240-h immersion environment. The results obtained from the electrochemical experiments also support this conclusion, as shown in Fig. 8(b); under immersion conditions, the initial corrosion current density of X100 steel increases gradually with time, and a linear relationship is observed, indicating that the electrochemical corrosion process is not hindered by the rust layer. Thus, the corrosion rate of X100 steel is controlled by the diffused oxygen as indicated below [60]. In this environment, the corrosion behaviour of the metal can be divided into two main stages. In the first stage, the X100 steel is immersed in the simulated solution and no rust layer is present. The microscopic unevenness of this surface leads to microscopic inhomogeneity of the electrode potential distribution at the interface between the metal and simulated solution, which results in the electrochemical corrosion of the metal [61]. In the second stage, a rust layer is formed on the metal surface. As shown in
Fig. 10(b), the electrochemical corrosion behaviour of the metal is not inhibited because the rust layer is thin, and the porous structure provides a channel for dissolved oxygen to reach the metal substrate. 4.2. Comparison of corrosion product As shown in Fig. 10, with identical experimental durations, the thickness of the rust layer formed under the dry/wet cycling conditions is significantly higher than that formed under immersion conditions. This is attributed to the fact that under cycling, the surface of the rust layer undergoes a drying–wetting–drying process; the rust layer acts as a strong oxidant during the wetting process, thereby promoting the reduction from Fe3+ to Fe2+. During the drying process, oxidation by air results in the re-oxidation of Fe2+ to Fe3+, and the above process is repeated to promote the thickening of the corrosion product layer. The composition of the rust layer formed in dry/wet cyclic environment is different from that formed in the immersion environment. According to the XRD analysis results shown in Fig. 11, the composition of the rust layer in dry/wet cyclic environment includes a-FeOOH, c-FeOOH, b-FeOOH, and Fe3O4. c-FeOOH is one of the main corrosion products. As the exposure time increases, the transition from c-FeOOH to a-FeOOH occurs. The composition of the rust layer in the immersion environment includes a-FeOOH, c-FeOOH, and Fe3O4 (Fig. 11(b)). Additionally, XRD characterization results show that the relative content of Fe3O4 in the surface corrosion products of X100 steel samples is high. Owing to the instability of c-FeOOH and b-FeOOH, these can be reduced with Fe2+. The transformation of c-FeOOH and bFeOOH into Fe3O4 occurs according to the following reactions [62,63]:
2c FeOOH þ Fe2þ ! Fe3 O4 þ 2Hþ
ð9Þ
2b FeOOH þ Fe2þ ! Fe3 O4 þ 2Hþ
ð10Þ
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The peak intensity of the Fe3O4 phase increases with an increase in corrosion time. The difference between dry/wet cycling and immersion is that no b-FeOOH production is observed during immersion. Nishimura et al. [46] found that the dry process progressed with the formation of akageneite at a high corrosion rate in dry/wet cyclic conditions. b-FeOOH can alter the structure of the rust layer, thereby accelerating the corrosion process [46– 48]. This indicates that the pre-corrosion rate is faster owing to the action of b-FeOOH under dry/wet cycling conditions. The data obtained from the electrochemical experiments also verify this conclusion. As shown in the Fig. 8(a), the decrease in corrosion current density at this stage of the electrochemical experiment is the fastest, indicating that the metal corrosion rate is the highest in this stage. As the corrosion time increases, the content of aFeOOH in the rust layer increases, resulting in an enhanced compactness of corrosion products and hindered diffusion of corrosive ions. At this time, the rust layer can hinder metal corrosion to a certain extent and decrease the corrosion rate. As shown in the Fig. 8(a), the corrosion current density of the metal in this stage decreases slowly, indicating that the corrosion rate of the metal changes slightly at this time. 5. Conclusion The corrosion behaviours of X100 steel samples in dry/wet cyclic and immersion environments were observed and were found to differ significantly. Under an identical corrosion time, the thickness of the formed rust layer under dry/wet cycling conditions was greater than that formed in the immersion environment, thereby suggesting that rapid formation of the rust layer was favourable under dry/wet cycling. In addition, under dry/wet cycling conditions, the main corrosion products were c-FeOOH, b-FeOOH, aFeOOH, and Fe3O4, while in the immersion environment, the main corrosion products were c-FeOOH, a-FeOOH, and Fe3O4. Furthermore, due to the presence of a thin liquid layer in the dry/wet cycle environment, the corrosion behaviour of the metal could be attributed to a combination of the thin liquid layer and rust layer, resulting in complex corrosion behaviour. The corrosion behaviour of X100 steel in the wet surface stage could be roughly divided into the following three stages: formation of a thin liquid layer, stage controlled by O2 diffusion rate, Cl concentration, and rust layer thickness, and hindering of corrosion by rust layer. Moreover, the corrosion behaviour of X100 steel in the fully immersed environment was found to be relatively simple, and the corrosion mechanism was a typical oxygen-absorbing corrosion mechanism. The rust layer had no protective effect on the substrate, and the corrosion process was controlled only by the diffusion of O2. These results are important as they can lead to an improved understanding of the corrosion behaviour of X100 steel under two different simulated marine environments, and can ultimately advance the development of materials that are more resistant to corrosion to improve the safety of pipeline transportation. Funding This work was supported by the National Natural Science Foundation of China (Grant numbers 51604150 and 51574147) and Doctor Starup Foundation of Liaoning Province (Grant number 201601324). Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Comparative study on corrosion behaviour of rusted X100 steel in dry/ wet cycle and immersion environments Ke Gong a,b, Ming Wu a,b,⇑, Guangxin Liu a,b a b
Key Laboratory of Oil & Gas Storage and Transportation, College of Petroleum Engineering, Liaoning Shihua University, Fushun, Liaoning 113001, China College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao, Shandong 266555, China
h i g h l i g h t s Simulated marine environment through a self-made dry/wet cycling device.
The simultaneous effects of Cl , O2 and rust layer were studied. Compare corrosion products in dry/wet cycling and full immersion. The three stages of the surface in the dry/wet cycle environment were discussed.
a r t i c l e
i n f o
Article history: Received 14 April 2019 Received in revised form 8 August 2019 Accepted 1 November 2019
Keywords: Dry/wet cycle Immersion Rust layer High strength steel
a b s t r a c t A dry/wet cycle corrosion test device was constructed to study the corrosion behaviour of X100 steel under rust layer in dry/wet cycling and immersion conditions. Results showed that the exposure of X100 steel in dry/wet cycle condition with high concentration of Cl, fast diffusion of O2, and wetness of the rust layers led to an accelerated corrosion process. The initial corrosion rate in the dry/wet cycle environment was higher than that in the immersion environment. As the corrosion time increased, the a-FeOOH content in the rust layer increased, resulting in a decrease in the rate of electrochemical corrosion of metals. In the immersion environment, the corrosion mechanism involved was a typical oxygenabsorbing corrosion mechanism. Under dry/wet cycle conditions, the simultaneous effects of Cl, O2, and rust layer were studied. The three stages of the wet surface in dry/wet cycle environment were described, and the corrosion mechanism was clarified. Ó 2019 Published by Elsevier Ltd.
1. Introduction With the continuous exploration and mining of offshore oil and gas resources, carbon steel is widely used in marine environments for oil and gas transportation [1–3]. Due to the conditions in marine environment, pipelines are highly susceptible to corrosion, which has become a key factor in operational safety. Marine oil and gas leakage accidents caused by pipeline corrosion account for 36% of the total such accidents [4–6].For safety and economic reasons, high-grade pipeline steel is an optimal choice for the efficient storage and transport of oil and gas. Currently, the majority of pipeline steels used in oil and gas pipelines in the marine environment are X65, X70, or X80 pipeline steels [7–9]. X100 pipeline steel ⇑ Corresponding author at: Key Laboratory of Oil & Gas Storage and Transportation, College of Petroleum Engineering, Liaoning Shihua University, Fushun, Liaoning 113001, China. E-mail addresses: [email protected] (K. Gong), [email protected] (M. Wu). https://doi.org/10.1016/j.conbuildmat.2019.117440 0950-0618/Ó 2019 Published by Elsevier Ltd.
is characterised by several economic advantages including high strength, high pressure, and low cost [10–12]. Therefore, the application prospect of X100 high-strength pipeline steel in marine oil and gas transportation is very broad. Although X100 steel has many advantages, it is limited by a high risk of hydrogen embrittlement during use. It is well known that hydrogen embrittlement is one of the most dangerous failure modes and can result in pipeline accidents because of its unpredictability and destructiveness [13– 16]. Therefore, it is important to study the corrosion behaviour of X100 steel in dry/wet cycle environment. In the marine splash zone, carbon steel is subjected to alternate wet and dry states over long periods of time, and the high salt content, seawater scouring, and a sufficient oxygen supply result in severe corrosion of pipeline steel [17,18]. In particular, high aeration contents in dry/wet cycle environment lead to accelerated corrosion of metals [3,19,20]. Moreover, the metal corrosion process occurring in this environment differs significantly from the metal corrosion process in a full immersion environment [3,21–24]. In the dry/wet cycle environment, due to the periodic changes in
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humidity and temperature environment, film thickness decreases and increases alternately, thereby rendering the metal corrosion process more complex [25]. First, Cl is key factor during the corrosion process in the dry/wet cycle environment [22,26]. Following the evaporation and thinning of the liquid film during the drying process, the concentration of Cl increases as the salt content in the liquid layer increases; the conductivity of the liquid layer also increases, thereby promoting the anode reaction [27]. Second, the diffusion of O2 has a considerable influence on the electrochemical corrosion of steel. In the dry/wet cycle environment, due to the faster diffusion of O2, the depolarization speed of the cathode is accelerated, and the local anode corrosion rate depends on the cathode reaction resulting in an accelerated metal corrosion rate [28]. Third, the rust layer formed on the steel surface significantly affects the corrosion behaviour of the steel and the amount of water associated with the surface properties [29]. As the thickness of the rust layer increases, the moisture present in this layer requires more time to evaporate, and the metal surface remains wet for a longer period of time, thereby providing favourable conditions for accelerated metal corrosion [30,31]. However, the rust layer can alter the diffusion trajectory of O2 and prevent Cl from penetrating into the steel matrix, thereby slowing the metal corrosion [32,33]. To date, only the individual effects of Cl [34–36], O2 [37,38], and rust layer [30,39,40] have been studied. However, the simultaneous effects of Cl, O2, and rust have not been investigated. Additionally, as some marine structures such as risers and platform legs are both in a fully immersed environment and in the ocean splash zone, it is necessary to perform a comparative study of the corrosion behaviour of X100 high-strength carbon steel under dry/wet cycle and immersion conditions, and clarify the corrosion mechanism under different environments. To the best of our knowledge, no reports regarding the corrosion mechanism of X100 high-strength pipeline steel in the dry/ wet cycle conditions exist in literature. This study aims to clarify the corrosion mechanism of X100 steel and perform a comparative analysis of corrosion under dry/wet cycling and immersion conditions to provide a theoretical basis for the application of X100 steel in a marine environment.
Fig. 1. Microstructure of X100 steel.
trodes were welded with a Cu-wire for electrical connection, and the samples were embedded in cold curing epoxy resin and mounted in a polyethylene (PE) tube holder with a 1 cm2 working area. Two immersion specimens were required under each condition, one for observing the corrosion product and corrosion morphology, and the other for observing the morphology of the rust layer. All experimental samples were abraded using various grades of silicon carbide papers ranging from 80 to 2000 grit, following by degreasing with acetone, dehydration in absolute ethanol, and storage in a drying cabinet prior to use.
2.2. Dry/wet cyclic tests Based on the marine environment of the South China Sea, the seawater temperature, atmospheric temperature, and relative humidity were controlled using an in-house-developed dry/wet cyclic test device to simulate the marine environment of the South China Sea. The experimental device employed is shown in Fig. 3. A water bath heating device is used to control the simulated seawater temperature, heating tube is used to control the temperature of the test chamber, and salt spray generator controls the relative humidity of the test chamber. The above devices are connected to a computer system and controlled by adjusting the input parameters to obtain a stable experimental environment. Seawater simulation solution consisted of a 3.5 wt% solution of NaCl in deionised water (1 L), and the pH of this solution was adjusted to ~7.5 using 4 wt% glacial acetic acid or 10 wt% sodium hydroxide. According to the environmental parameters of the South China Sea, the tropical ocean conditions were simulated [41,42], which included high salinity and humidity (77%), average surface water temperature of 27 °C, and average temperature of 30 °C. Two groups were created for this experiment. Group A was subjected to the dry/wet cycle environment, while group B was subjected to the immersion environment. Group A samples were immersed in the simulation solution at 25 ± 1 °C for 12 min (the wet period) followed by sample drying at 30 ± 1 °C and 75% humidity for 48 min (the dry period). For group B, the samples were fully immersed in the simulation solution at 25 ± 1 °C for 24, 72, 120, 168, or 240 h, and the solution was replaced every 48 h.
2. Materials and methods 2.1. Test materials The test materials were cut from a thick plate of API 5L X100 high-strength pipeline steel and the chemical composition (wt.%) and mechanical properties are shown in Table 1. The microstructure of the X100 pipeline high-strength steel is presented in Fig. 1, where the acicular ferrite and bainite structures are apparent. The microstructure was revealed by gradual abrasion with a series of silicon carbide papers (150, 360, 600, 800, 1500, and 2000 grit), and subsequent polishing with a 5-lm diamond suspension. The sample was then immersed in an etchant (2 ml of 70% nitric acid and 98 ml of anhydrous, denatured ethyl alcohol) for 15 s prior to treatment with deionised water, dehydration in absolute ethanol, and drying under air. Fig. 2 shows the dimensions of the specimens, i.e., the rectangular electrochemical sample (1 1 cm2) and immersion sample (2 25 50 cm3). In the case of the samples used for electrochemical testing, the working elec-
Table 1 Chemical composition of the X100 steel sample (mass fraction, %). C
Si
Mn
S
P
Al
Nb
Ni
Cu
Ti
Fe
0.05
0.25
1.83
0.002
0.08
0.02
0.10
0.23
0.21
0.01
Bal
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3
Fig. 2. Schematic of the sample.
Fig. 3. Dry/wet cycle corrosion test device. (1. Integrated ultrasonic salt spray generator; 2. Limit switch; 3. PLC control system; 4. Touch control screen; 5. Temperature and humidity transmitter; 6. Motor; 7. Salt spray exit; 8. Dehumidification fan; 9. Connecting rod; 10. Sample holder; 11. Humidity Sensor; 12. Temperature Sensor; 13. Infrared heating tube; 14. Soaking pool; 15. Water bath; 16. Water bath heater; 17. Slide.)
2.3. Electrochemical measurements An electrochemical workstation (PARSTAT4000, Princeton) equipped with a traditional three-electrode cell was used to conduct the electrochemical measurements. The three electrodes included a specimen as the working electrode (WE), Pt plate as the counter electrode (CE), and saturated calomel electrode (SCE) as the reference electrode (RE). All potentials are reported in the
SCE scale. The test solution is described above. All electrochemical experiments were carried out without stirring or deaeration of the test medium. Initially, the open circuit potential was monitored for 30 min to obtain a stable open circuit potential, after which electrochemical impedance spectroscopy (EIS) was carried out at the open circuit potential using AC signals of 10 mV amplitude (peak-to-peak sinusoidal wave) between 100 kHz and 10 mHz, with 10 points per
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decade. ZSimpWin software was used to analyse the EIS data with a suitable equivalent circuit (EC). Finally, the potentiodynamic polarisation curves were recorded at a scan rate of 0.33 mV/s in the potential range of 0.2 V (vs. OCP) to +0.2 V. All experiments were performed in triplicate under the same experimental conditions to ensure that the results were reproducible and reliable. 2.4. Morphological observations and composition analysis After the removal of corrosion product from the immersion sample, ultrasonic cleaning was carried out for 10 min using distilled water. Thereafter, the sample was dried under warm air, and the surface morphology was observed by scanning electron microscopy (SEM; SU8010, Hitachi). The compositions of the corrosion products were analysed by X-ray diffraction (XRD; D8 Advance, Bruker) using Cu as the target with an electric current of 200 mA and a voltage of 40 kV at a scan rate of 3°/min. The cross-section of the rust layer was characterised by SEM, and the elements present were determined by energy-dispersive X-ray spectroscopy (EDS; SU8010, HITACHI). 3. Results 3.1. Electrochemical properties 3.1.1. EIS Fig. 4 shows the Nyquist diagrams and Bode plots for the X100 steel specimens under dry/wet cycling and immersion conditions.
As indicated, the measured impedance plots differ significantly depending on the environmental conditions and testing time. As shown in Fig. 4(a) and (c), the Nyquist plot shows a capacitive arc, which indicates the common frequency dispersion effect of the X100 steel electrode. Moreover, the size of the capacitive arc decreases gradually with corrosion time. A detailed analysis of the dependence of the corrosion rates under the dry/wet cycling and immersion conditions can be obtained by fitting the measured EIS data with appropriate electrochemical equivalent circuits. The EIS data were fitted with the electrochemical equivalent circuits to determine the dependence of the corrosion rate under dry/wet cycling and immersion conditions, as shown in Fig. 5, where Rs represents the solution resistance, and Qf and Rf correspond to the capacitance and resistance of the corrosion product film, respectively. Qdl and Rct represent the double-charge layer capacitance and charge-transfer resistance, respectively. The constant phase element (CPE) was used for EIS data fitting because the corrosion reaction reduced the uniformity of the sample surface. This can be calculated according to Eq. (1): a
Z Q ¼ Y 0 ðjxÞ :
ð1Þ
where x is the angular frequency (rad/s), and Y0 and a are CPE parameters indicating the deviation of the specimen from the ideal capacitive behaviour [43]. The fitting results are plotted as solid curves in Fig. 4, where the measured EIS data fit well with the circuits, with a fitting error of <10%. The kinetic evolution of the electrochemical processes involved in the experiment can be quantified from the polarisation resis-
Fig. 4. Nyquist and Bode diagrams of the impedance measurements for X100 pipeline steel under (a, b) dry/wet cycling and (c, d) immersion conditions. The black solid lines represent fitted impedance curves with appropriate equivalent circuits.
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Fig. 5. Electrochemical equivalent circuit used to fit the measured EIS data.
tance Rp, (Rf + Rct), which can reflect the speed of the electrochemical corrosion; the higher is the Rp value, slower is the electrochemical reaction rate [44]. The EIS fitting results of Rp as a function of time under dry/wet cycling and immersion conditions are shown in Fig. 6. In general, the corrosion rate of X100 steel tends to increase with time under both sets of conditions. In addition, in the immersion environment, the value of Rp decreases slowly at the beginning of the experiment, prior to a subsequent rapid decrease, thereby indicating that the corrosion rate is slower at the beginning of the experiment, and is accelerated after 120 h. In the dry/wet cycle environment, the Rp value decreases rapidly in the initial stages of the experiment, and the decreases slowly after 120 h, indicating that the corrosion rate increases rapidly with time prior to 120 h. Furthermore, the initial corrosion rate of X100 steel under dry/wet cycle conditions is higher than that in the immersion environment. However, after 240 h, the corrosion rate of the sample in the immersion environment is higher than that under dry/wet cycle conditions. 3.1.2. Potentiodynamic polarisation The potentiodynamic polarisation curves of X100 steel under (a) dry/wet cycling and (b) immersion conditions are shown in Fig. 7. These curves reveal that the corrosion process of X100 steel under both sets of conditions is under activation (charge transfer) control, with the polarisation curves exhibiting a significant rightward shift upon increasing the experimental time. The corrosion current density (Icorr) and corrosion potential (Ecorr) in the dry/ wet cycle conditions were obtained using the Tafel fitting method, and the fitted results are shown in Fig. 8(a). These results reveal that the corrosion potential shows a gradual decline with an increase in the experimental time, with Icorr values of 22.27, 65.54, 125.23, 147.34, and 165.14 lA/cm2 obtained for experimen-
Fig. 6. Change in Rp as a function of time for X100 steel under dry/wet cycling and immersion conditions.
5
tal times of 24, 72, 120, 168, and 240 h, respectively. The change in Icorr is faster from 0 to 120 h, and slower between 120 and 240 h, thereby indicating that the electrochemical corrosion process is faster. Furthermore, the corrosion current density (Icorr) and corrosion potential (Ecorr) values for the immersion conditions were obtained using the Tafel fitting method, and the fitted results are shown in Fig. 8(b). In this case, the Ecorr shows a gradual decline with an increase in the experimental time, with Icorr values of 15.21, 36.77, 89.80, 135.14, and 175.55 lA/cm2 obtained for experimental times of 24, 72, 120, 168, and 240 h, respectively. These fitted results indicate that Icorr increases with the corrosion time, showing an almost linear trend.
3.2. Surface morphologies Fig. 9 shows the morphology of the rust layer during the corrosion of X100 steel under dry/wet cycling and immersion conditions, as well as the surface morphology after the removal of the corrosion products. As shown in Fig. 9(a) and (c), after immersion for 120 h in different environments, the rust layer is relatively uniform and there is no obvious crack on the surface. Furthermore, the rust layer is relatively thin and the surface of the substrate can be partially observed through the rust layer. After 240 h (Fig. 9(b) and (d)), the rust layer exhibits a spore morphology, becoming loose with apparent internal cracks. In general, the macroscopic morphology of the rust layer does not differ significantly, being brownish-yellow in colour and having a low surface density. Moreover, the observation of the surface morphology of the X100 steel after the removal of corrosion products shows that the surface morphology of the metal in the two environments differs significantly. In the dry/wet cycle environment (Fig. 9(a) and (b)), corrosion pits >100 lm in diameter appear on the X100 steel surface due to the non-uniform anodic dissolution behaviour. In the immersion environment (Fig. 9(c) and (d)), the X100 steel exhibits a greater number of shallow corrosion pits at the beginning of the corrosion process, in addition to a local contiguous phenomenon, general corrosion characteristics, and an increased degree of corrosion as the corrosion time increases.
3.3. Cross-sectional morphologies The cross-sectional morphologies of the rust layers under dry/ wet cycling and immersion conditions are shown in Fig. 10. By comparing Fig. 10(a), (c) and (b), (d), it is clear that the rust layer interface morphologies of the corrosion samples do not exhibit any obvious stratification under dry/wet cycling and immersion conditions. Moreover, the rust layer formed in the dry/wet cycle environment is thicker than that formed under immersion conditions. In the dry/wet cycle environment (Fig. 10(a) and (b)), the maximum thickness of the rust layer is 93 lm after 120 h, and the degree of adhesion between the rust layer and substrate is high; voids and cracks are observed in the rust layer and the structural continuity is poor. With an increase in the corrosion time, the maximum thickness of the rust layer is 141 lm after 240 h, and the rust layer shows better compactness and structural continuity. Furthermore, Fig. 10(c) and (d) shows the rust layer formed on the surface of the X100 steel body under immersion conditions. In this case, the thicknesses of the maximum rust layers are 53 and 64 lm after 120 and 240 h, respectively. Transverse cracks of the vertical matrix are visible in addition to longitudinal cracks parallel to the matrix. The presence of these cracks provides a channel for the diffusion of oxygen and chloride ions, causing them to easily penetrate the rust layer and increasing the corrosion of the X100 matrix.
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Fig. 7. Potentiodynamic polarisation curves of X100 pipeline steel under (a) dry/wet cycling and (b) immersion conditions.
Fig. 8. Fitted results for Ecorr and Icorr in the simulated solution under (a) dry/wet cycling and (b) immersion conditions.
3.4. Composition of the corrosion products 3.4.1. XRD Fig. 11 shows the XRD spectra of the corroded X100 steel surfaces obtained following dry/wet cycling and immersion with varying corrosion time. As shown in Fig. 11, the main components of the rust layer formed under the dry/wet cycle environment are Fe3O4, a-FeOOH, b-FeOOH, and c-FeOOH, while the components of the rust layer formed under immersion conditions are mainly Fe3O4, a-FeOOH, and c- FeOOH [45]. Research shows that the presence of b-FeOOH is attributed to the chloride ions present in the liquid film layer on the carbon steel surface during dry/wet cycling. Thus, b-FeOOH can alter the structure of the rust layer to provide channels for the diffusion of corrosive elements such as oxygen and chlorine, thereby allowing the facile penetration of corrosion products and destruction of the corrosion resistance of the carbon steel rust layer, resulting in an acceleration of the corrosion process [46–48]. As stable oxides, Fe3O4 and a-FeOOH are deposited in the rust layer, and upon thickening of the rust layer, these species can prevent chloride ion penetration. 3.4.2. EDS Fig. 12 shows the elemental distribution in the rust layer after 120 h testing under dry/wet cycling and immersion conditions. Through the analysis of the EDS surface distribution of the rust
layer of X100 steel after 120 h, it is determined that the rust layer contains large amounts of Fe and O, which represents the main iron oxide corrosion product. In addition, Cl penetrates the rust layer, indicating that the rust layer formed in the experimental environment does not prevent Cl diffusion into the matrix, and in the case of the dry/wet cycling environment, it can be clearly seen that Cl is deposited in the rust layer. During the thickening of the rust layer under laboratory conditions, the researchers have observed that at low Cl concentrations, the oxyhydroxide formed is lepidocrocite, while high Cl concentrations lead to the formation of akageneite [49]. This finding is consistent with the results obtained in the experimental environment in this study. 4. Discussion 4.1. Comparative analysis of corrosion mechanisms Owing to these periodic variations in the corrosion environment in dry/wet cycle conditions, the corrosion mechanism is distinctly different from the corrosion mechanism of metals in immersion conditions. The corrosion of X100 steel under dry/wet cycle conditions can be regarded as an electrochemical corrosion of metal under a thin liquid layer, i.e., ferrous ions are formed by the dissolution of an anode reaction on the steel surface and are oxidised to
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Fig. 9. Surface corrosion morphology of X100 steel in the corrosion of X100 steel in both environments ((a, b) dry/wet cycling and (c, d) immersion conditions).
form a rust layer. Therefore, the electrochemical corrosion process of a metal under dry/wet cycle conditions includes the reaction occurring at each stage during the formation of an electrolyte layer on the metal. The following can represent the electrochemical reactions in the X100 steel in the initial corrosion stage [19,50,51]: Anodic reaction in the immersion environment:
Fe ! Fe2þ þ 2e
ð2Þ
Cathodic reaction:
O2 þ 2H2 O þ 4e ! 4OH Total reaction:
ð3Þ
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Fig. 10. Cross-sectional morphologies of the rust layers under dry/wet cycling and immersion conditions ((a, b) dry/wet cycling, and (c, d) immersion conditions).
Fig. 11. XRD spectra of the X100 steel following (a) dry/wet cycling and (b) immersion with different corrosion time.
Fe2þ þ 2OH ! FeðOHÞ2
ð4Þ
The corrosion product is Fe(OH)2, which is then oxidised to Fe (OH)3, which is unstable. Subsequently, Fe(OH)3 transforms into goethite (a-FeOOH) and lepidocrocite (c-FeOOH) [52].
FeðOHÞ3 ! FeOOH þ H2 O
ð5Þ
The above reaction can explain the metal corrosion mechanism in a fully immersed environment, but it is not sufficient to explain the complex corrosion mechanism of metals under dry/wet cycle conditions.
Strattmann et al. [53] proposed a three-stage rust formation mechanism during an electrochemical study of phase transitions in already formed rust layers for dry/wet cycle environment:wetting of the dry surface, wet surface, and drying-out of the surface. This clarifies the corrosion process of the metals in dry/wet cycle conditions. Based on this investigation, to clearly explain the corrosion process of X100 steel under dry/wet cycle conditions, this study further divides the wet surface into three stages. In the first stage (Fig. 13(b)), no rust layer is present on the metal surface. A thin liquid layer is formed on the metal surface, and as this simulated solution evaporates, the thin liquid layer becomes more thin-
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Fig. 12. Elemental distribution in the rust layer after 120 h testing ((a) dry/wet cycling, and (b) immersion conditions).
ner, thereby increasing the concentration of Cl. This in turn increases the conductivity of the film and promotes the corrosion of the microbattery. The rate of O2 diffusion increases under the thin liquid layer, which in turn affects the rate of cathodic reaction of the metal in the thin liquid layer [38]. In the second stage (Fig. 13(c)), the corrosion progresses, and a rust layer appears on the metal surface. Once a thin rust layer is formed on the steel surface, the effect of the composition and thickness variation of the rust layer on the electrochemical corrosion process of the steel surface must be considered. As shown in Figs. 9 and 10, the early rust layer is thin and does not prevent
O2, Cl ions, or any other corrosive substances from reaching the substrate. Notably, some of the Cl enters the rust layer, while the remainder can pass through the rust layer to reach the surface of the substrate and promote corrosion in a catalytic manner [49]. The deposition of Cl in the rust layer was detected via EDS analysis (Fig. 12), which was linked with the formation of b-FeOOH. The corresponding electrochemical reactions can be expressed as follows [40,54–56]:
Fe2þ ! Fe3þ þ e
ð6Þ
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Fig. 13. Schematic of the corrosion process of X100 steel under dry/wet cycling conditions.
Fe3þ þ Cl þ OH ! FeOCl þ H
ð7Þ
FeOCl þ OH ! b FeOOH þ Cl
ð8Þ
Moreover, in this stage, owing to the thickness of the rust layer, the rate of moisture absorption is high. Therefore, corrosion of the metal surface is prolonged, and the rust layer acts as a strong oxidant during the wetting process, thereby promoting the reduction from Fe3+ to Fe2+. and accelerating the electrochemical corrosion of metal [57,58]. The corrosion at this stage is promoted by the O2 diffusion rate, Cl concentration, and thickness of the rust layer. In the third stage (Fig. 13(d)), the rust layer on the metal surface becomes thicker as shown in Figs. 9 and 10. The content of aFeOOH in the rust layer increases (Fig. 12(a)). It is reported that out of all these oxides, a-FeOOH is the most stable phase [59]. Once the content of a-FeOOH increases in the rust layer, the corrosion rate of the steel is significantly reduced. At this stage, the diffusion of O2 and Cl is hindered to a certain extent. It is not conducive to the electrochemical reaction of the metal surface. In the immersion environment, the corrosion mechanism is a typical oxygen-absorbing corrosion mechanism. The rust layer does not have protective properties in the 240-h immersion environment. The results obtained from the electrochemical experiments also support this conclusion, as shown in Fig. 8(b); under immersion conditions, the initial corrosion current density of X100 steel increases gradually with time, and a linear relationship is observed, indicating that the electrochemical corrosion process is not hindered by the rust layer. Thus, the corrosion rate of X100 steel is controlled by the diffused oxygen as indicated below [60]. In this environment, the corrosion behaviour of the metal can be divided into two main stages. In the first stage, the X100 steel is immersed in the simulated solution and no rust layer is present. The microscopic unevenness of this surface leads to microscopic inhomogeneity of the electrode potential distribution at the interface between the metal and simulated solution, which results in the electrochemical corrosion of the metal [61]. In the second stage, a rust layer is formed on the metal surface. As shown in
Fig. 10(b), the electrochemical corrosion behaviour of the metal is not inhibited because the rust layer is thin, and the porous structure provides a channel for dissolved oxygen to reach the metal substrate. 4.2. Comparison of corrosion product As shown in Fig. 10, with identical experimental durations, the thickness of the rust layer formed under the dry/wet cycling conditions is significantly higher than that formed under immersion conditions. This is attributed to the fact that under cycling, the surface of the rust layer undergoes a drying–wetting–drying process; the rust layer acts as a strong oxidant during the wetting process, thereby promoting the reduction from Fe3+ to Fe2+. During the drying process, oxidation by air results in the re-oxidation of Fe2+ to Fe3+, and the above process is repeated to promote the thickening of the corrosion product layer. The composition of the rust layer formed in dry/wet cyclic environment is different from that formed in the immersion environment. According to the XRD analysis results shown in Fig. 11, the composition of the rust layer in dry/wet cyclic environment includes a-FeOOH, c-FeOOH, b-FeOOH, and Fe3O4. c-FeOOH is one of the main corrosion products. As the exposure time increases, the transition from c-FeOOH to a-FeOOH occurs. The composition of the rust layer in the immersion environment includes a-FeOOH, c-FeOOH, and Fe3O4 (Fig. 11(b)). Additionally, XRD characterization results show that the relative content of Fe3O4 in the surface corrosion products of X100 steel samples is high. Owing to the instability of c-FeOOH and b-FeOOH, these can be reduced with Fe2+. The transformation of c-FeOOH and bFeOOH into Fe3O4 occurs according to the following reactions [62,63]:
2c FeOOH þ Fe2þ ! Fe3 O4 þ 2Hþ
ð9Þ
2b FeOOH þ Fe2þ ! Fe3 O4 þ 2Hþ
ð10Þ
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The peak intensity of the Fe3O4 phase increases with an increase in corrosion time. The difference between dry/wet cycling and immersion is that no b-FeOOH production is observed during immersion. Nishimura et al. [46] found that the dry process progressed with the formation of akageneite at a high corrosion rate in dry/wet cyclic conditions. b-FeOOH can alter the structure of the rust layer, thereby accelerating the corrosion process [46– 48]. This indicates that the pre-corrosion rate is faster owing to the action of b-FeOOH under dry/wet cycling conditions. The data obtained from the electrochemical experiments also verify this conclusion. As shown in the Fig. 8(a), the decrease in corrosion current density at this stage of the electrochemical experiment is the fastest, indicating that the metal corrosion rate is the highest in this stage. As the corrosion time increases, the content of aFeOOH in the rust layer increases, resulting in an enhanced compactness of corrosion products and hindered diffusion of corrosive ions. At this time, the rust layer can hinder metal corrosion to a certain extent and decrease the corrosion rate. As shown in the Fig. 8(a), the corrosion current density of the metal in this stage decreases slowly, indicating that the corrosion rate of the metal changes slightly at this time. 5. Conclusion The corrosion behaviours of X100 steel samples in dry/wet cyclic and immersion environments were observed and were found to differ significantly. Under an identical corrosion time, the thickness of the formed rust layer under dry/wet cycling conditions was greater than that formed in the immersion environment, thereby suggesting that rapid formation of the rust layer was favourable under dry/wet cycling. In addition, under dry/wet cycling conditions, the main corrosion products were c-FeOOH, b-FeOOH, aFeOOH, and Fe3O4, while in the immersion environment, the main corrosion products were c-FeOOH, a-FeOOH, and Fe3O4. Furthermore, due to the presence of a thin liquid layer in the dry/wet cycle environment, the corrosion behaviour of the metal could be attributed to a combination of the thin liquid layer and rust layer, resulting in complex corrosion behaviour. The corrosion behaviour of X100 steel in the wet surface stage could be roughly divided into the following three stages: formation of a thin liquid layer, stage controlled by O2 diffusion rate, Cl concentration, and rust layer thickness, and hindering of corrosion by rust layer. Moreover, the corrosion behaviour of X100 steel in the fully immersed environment was found to be relatively simple, and the corrosion mechanism was a typical oxygen-absorbing corrosion mechanism. The rust layer had no protective effect on the substrate, and the corrosion process was controlled only by the diffusion of O2. These results are important as they can lead to an improved understanding of the corrosion behaviour of X100 steel under two different simulated marine environments, and can ultimately advance the development of materials that are more resistant to corrosion to improve the safety of pipeline transportation. Funding This work was supported by the National Natural Science Foundation of China (Grant numbers 51604150 and 51574147) and Doctor Starup Foundation of Liaoning Province (Grant number 201601324). Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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