Evaluation of Hydrogen Permeation into High-Strength Steel during Corrosion in Different Marine Corrosion Zones
Evaluation of Hydrogen Permeation into High-Strength Steel during Corrosion in Different Marine...
Xu, Yong;Huang, Yanliang;Cai, Fanfan;Wang, Zhengquan;Lu, Dongzhu;Wang, Xiutong;Yang, Lihui
2022-03-09 00:00:00
applied sciences Article Evaluation of Hydrogen Permeation into High-Strength Steel during Corrosion in Different Marine Corrosion Zones 1 , 2 , 3 , 4 1 , 2 , 4 , 1 , 2 , 3 , 4 1 , 2 , 3 , 4 1 , 2 , 4 Yong Xu , Yanliang Huang * , Fanfan Cai , Zhengquan Wang , Dongzhu Lu , 1 , 2 , 4 1 , 2 , 4 Xiutong Wang and Lihui Yang CAS Key Laboratory of Marine Environmental Corrosion and Bio-Fouling, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China; xuyong182@mails.ucas.ac.cn (Y.X.); caifanfan20@mails.ucas.ac.cn (F.C.); 17854268797@163.com (Z.W.); ldz@qdio.ac.cn (D.L.); wangxiutong@qdio.ac.cn (X.W.); lhyang@qdio.ac.cn (L.Y.) Open Studio for Marine Corrosion and Protection, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, China University of Chinese Academy of Sciences, Beijing 100049, China Center for Ocean Mega-Science, Chinese Academy of Sciences, 7 Nanhai Road, Qingdao 266071, China * Correspondence: hyl@qdio.ac.cn Abstract: Hydrogen permeation into high-strength steel during the corrosion process can deteriorate their mechanical properties, thus seriously threatening the safety of steel structures. However, the hydrogen permeation behavior of steels in corrosive marine environments is not well understood. In this study, the hydrogen permeation behavior and mechanism of AISI 4135 steel in different marine corrosion zones was investigated for the first time using an in situ hydrogen permeation monitoring system via outdoor and indoor tests. The three-month outdoor hydrogen permeation test showed Citation: Xu, Y.; Huang, Y.; Cai, F.; that the diffusible hydrogen content of the steels exposed to the marine atmospheric, splash, tidal and Wang, Z.; Lu, D.; Wang, X.; Yang, L. 3 2 2 2 immersion zone was 3.15 10 , 7.00 10 , 2.06 10 and 3.33 10 wt ppm, respectively. Evaluation of Hydrogen Permeation Meanwhile, results showed that the hydrogen permeation current density was positively correlated into High-Strength Steel during with the corrosion rate of the steel in the marine environments. This research is of great significance Corrosion in Different Marine Corrosion Zones. Appl. Sci. 2022, 12, for guiding the safe application of high-strength steel in the marine environments. 2785. https://doi.org/10.3390/ app12062785 Keywords: high-strength steel; hydrogen permeation; sensor; marine corrosion environments Academic Editors: Giuseppe Lacidogna, Sanichiro Yoshida, Guang-Liang Feng, Jie Xu, 1. Introduction Alessandro Grazzini and Gianfranco Piana With the development of the iron and steel manufacturing industry, high-strength steel has been gradually used in various industries such as civil buildings, energy transportation Received: 29 January 2022 and marine engineering due to its superior mechanical properties. However, high-strength Accepted: 28 February 2022 steel endures severe corrosion and other potential threats during its service in offshore Published: 9 March 2022 engineering facilities, owing to the harsh marine environments. Among these threats, the Publisher’s Note: MDPI stays neutral mechanical degradation of steel caused by hydrogen permeation during the corrosion with regard to jurisdictional claims in process has received widespread attention in the last few decades [1–4]. Wang et al. [4] published maps and institutional affil- found that diffusible hydrogen led to a decrease in the notch tensile strength for the iations. AISI 4135 steels at 1320 MPa in a power law manner. Liu et al. [5] found that E690 steel exhibited high susceptibility to stress corrosion cracking (SCC) under simulated marine thin electrolyte layer and indicated that the SCC process was jointly determined by local anodic dissolution (AD) and hydrogen embrittlement (HE). Li et al. [6] found that the Copyright: © 2022 by the authors. change in fracture stress of steels showed apparent correspondence with the change in Licensee MDPI, Basel, Switzerland. hydrogen content during the evaluation on delayed fracture property of outdoor-exposed This article is an open access article high-strength AISI 4135 steels. To date, it has been recognized that hydrogen plays a critical distributed under the terms and role in the fracture failure of steels, while the sensitivity of steels to hydrogen increases conditions of the Creative Commons sharply with strength [7,8]. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Generally, there are two main sources of hydrogen permeated into steels: a part of 4.0/). hydrogen comes from the steel production and subsequent processing process, such as Appl. Sci. 2022, 12, 2785. https://doi.org/10.3390/app12062785 https://www.mdpi.com/journal/applsci Appl. Sci. 2022, 12, 2785 2 of 20 pickling and plating; while most of the hydrogen absorbed into steel comes from the corrosion processes, especially the hydrolysis of corrosion products [9,10]. It has been found that hydrogen permeation behavior into steel is closely related to the corrosion behavior of steel [11–16]. Our previous studies [11,12] found that the amount of hydrogen permeated into steel is positively correlated with the corrosion weight loss of steel under the simulated wet–dry cycle condition. Meanwhile, the positive relationship between the hydrogen permeation and steel corrosion in the automobile moving environment and the atmospheric corrosion environment has also been reported in previous studies [13,14]. Furthermore, it is well known that the amount of hydrogen permeated into steel increases significantly in the case of salts being loaded on the surface of steels [9,15,16]. Therefore, hydrogen permeation behavior into steel should be more serious and complex in the marine corrosive environments according to above analysis. Traditionally, the marine corrosive environments are divided into five zones including the marine atmospheric zone, the splash zone, the tidal zone, the full immersion zone and the seabed sediment zone [17]. For offshore facilities, the steel structures run through different corrosion zones, and the hydrogen permeation behavior caused by varying degrees of corrosion may also be diverse at different locations. Since the amount of hydrogen permeated into steel plays an essential role in the hydrogen embrittlement or delayed fracture of steel, it is significant to clarify how much hydrogen can be permeated into steel during service in a certain environment [17,18]. Over the past few decades, important progress has been made on understanding the hydrogen permeation behavior of steel in atmospheric environments [14,19–21], but relevant infor- mation on such behavior of steel in more complex and harsh marine environments remains limited. Therefore, systematic and scientific research should be carried out on the hydrogen permeation behavior of steels in marine environments. Among the methods for the determi- nation of hydrogen content in steel, the electrochemical hydrogen permeation technique is widely used in various scenarios due to its excellent reliability and adaptability [22–24]. In this work, the electrochemical hydrogen permeation test was performed in outdoor marine environments using an in situ monitoring system developed by the authors to investigate the hydrogen permeation behavior of AISI 4135 steel in different marine corrosion zones. In addition, combined with a laboratory test, the hydrogen permeation mechanism of steels in marine environments was analyzed. 2. Materials and Methods 2.1. Material and Outdoor Test Site The material used in this work was as-received AISI 4135 high-strength steel produced by Beijing Shougang New Steel Co., Ltd., Beijing, China, and its chemical composition is provided in Table 1. The mechanical properties include a tensile strength of 830 MPa, a yield strength of 780 MPa and an elongation of 10.4% at ambient temperature (20 C). To observe the microstructure of the steel, a steel sample of 10 mm in diameter and 10 mm in height was ground with a series of silicon carbide (SiC) papers (from 400 to 1000 grade) and then polished with a 2.5 m diamond abrasive. Then, the sample was etched in a nital etchant (4% nitric acid and 96% anhydrous alcohol) for 10 s, treated with deionized water, swabbed with alcohol and finally dried in a cold air stream. The microstructure of the AISI 4135 high-strength steel is shown in Figure 1, where pearlite and ferrite can be observed. The specimens are hollow cylindrical steels with open ends, an inner diameter of 34 mm and a wall thickness of 0.5 mm (See Figure S1 for more information on the dimensions of the specimens). Both the inner and outer surfaces of the cylindrical wall were grinded, with a final roughness average (Ra) of 0.4. A palladium (Pd) film of ca. 0.6 m in thickness was electroplated on the inner surface of the hollow cylindrical steel for facilitating hydrogen detection, inhibiting corrosion, etc. [25–27]. The effective hydrogen diffusion coefficient, D , of the specimen was determined through the breakthrough-time method [28], the value eff 7 2 1 of which was 4.13 10 cm s . The outdoor hydrogen permeation test was conducted 0 0 in Xuejia Island Harbor (36 06 N; 120 10 E), Qingdao, China. The tide in the sea area is Appl. Sci. 2022, 12, x FOR PEER REVIEW 3 of 20 Appl. Sci. 2022, 12, 2785 3 of 20 film of ca. 0.6 μm in thickness was electroplated on the inner surface of the hollow cy- lindrical steel for facilitating hydrogen detection, inhibiting corrosion, etc. [25–27]. The of typical semidiurnal one, and the record shows that the annual average tidal range and temperatur effective he ywer drogen d e ~2.7 i m ffusio and ~12 n coeffic C, respectively ient, Deff, of the speci . men was determined through the −7 2 −1 breakthrough-time method [28], the value of which was 4.13 × 10 cm ·s . The outdoor Table 1. Chemical composition of the AISI 4135 steel used in this work (wt.%). hydrogen permeation test was conducted in Xuejia Island Harbor (36°06′ N; 120°10′ E), Qingdao, China. The tide in the sea area is of typical semidiurnal one, and the record C Si Mn P S Ni Cr Mo Fe shows that the annual average tidal range and temperature were ~2.7 m and ~12 °C, re- 0.399 0.293 0.509 0.0146 0.0144 0.0804 0.903 0.204 Bal. spectively. Figure 1. Microstructure of AISI 4135 high-strength steel: OM micrograph (a), SEM micrograph (b); Figure 1. Microstructure of AISI 4135 high-strength steel: OM micrograph (a), SEM micrograph (b); α—Ferrite, P—Pearlite. —Ferrite, P—Pearlite. 2.2. In Situ Hydrogen Permeation Monitoring Test Table 1. Chemical composition of the AISI 4135 steel used in this work (wt.%). 2.2.1. In Situ Hydrogen Permeation Monitoring Set-Up C Si Mn P S Ni Cr Mo Fe In this study, the hydrogen permeated into the hollow cylindrical steel specimen 0.399 0.293 0.509 0.0146 0.0144 0.0804 0.903 0.204 Bal. during exposure to the marine environments was monitored in situ using the hydro- gen permeation monitoring set-up shown in Figure 2. Based on Devanathan–Stachurski technique 2.2. In situ H [29], ydro thegdeveloped en Permeation Monitorin set-up can evaluate g Testhe t hydrogen permeated into steel via measuring the so-called “anodic oxidation current”. To make it clear, the set-up shown in 2.2.1. In situ Hydrogen Permeation Monitoring Set-Up Figure 2 is referred to “hydrogen permeation sensor” and unless otherwise specified, the In this study, the hydrogen permeated into the hollow cylindrical steel specimen sensor mentioned below is a hydrogen permeation sensor. Inside the sensor was a closed during exposure to the marine environments was monitored in situ using the hydrogen three-electrode system: the inner surface of the hollow cylindrical steel specimen was the working electrode, a Pt wire as counter electrode, a Hg/HgO/0.2 mol/L NaOH electrode permeation monitoring set-up shown in Figure 2. Based on Devanathan–Stachurski as reference electrode and the nitrogen deaerated 0.2 mol/L NaOH + 30 vol% dimethyl technique [29], the developed set-up can evaluate the hydrogen permeated into steel via sulfoxide (DMSO) solution as the inner electrolyte. The outer surface of the sensor was measuring the so-called “anodic oxidation current”. To make it clear, the set-up shown in sealed with custom-built poly tetra fluoroethylene (PTFE) shells and epoxy resin except for Figure 2 is referred to “hydrogen permeation sensor” and unless otherwise specified, the the area reserved for exposure, as shown in Figure 2. During hydrogen permeation test, the sensor mentioned below is a hydrogen permeation sensor. Inside the sensor was a closed inner surface with Pd-plated acted as so-called “hydrogen detection side”; while the outer three-electrode system: the inner surface of the hollow cylindrical steel specimen was the surface of the sensor exposed to the marine environments acted as so-called “hydrogen working electrode, a Pt wire as counter electrode, a Hg/HgO/0.2 mol/L NaOH electrode entry side”. The multi-core copper wires connecting the electrodes were led out through a as reference electrode and the nitrogen deaerated 0.2 mol/L NaOH + 30 vol% dimethyl waterproof connector, and the wire joints were sealed with epoxy resin inside the sensor. sulfoxide (DMSO) solution as the inner electrolyte. The outer surface of the sensor was sealed with custom-built poly tetra fluoroethylene (PTFE) shells and epoxy resin except for the area reserved for exposure, as shown in Figure 2. During hydrogen permeation test, the inner surface with Pd-plated acted as so-called “hydrogen detection side”; while the outer surface of the sensor exposed to the marine environments acted as so-called “hydrogen entry side”. The multi-core copper wires connecting the electrodes were led out through a waterproof connector, and the wire joints were sealed with epoxy resin inside the sensor. Appl. Sci. 2022, 12, x FOR PEER REVIEW 4 of 20 Appl. Sci. 2022, 12, 2785 4 of 20 Figure 2. Schematic diagram of the hydrogen permeation sensor. Figure 2. Schematic diagram of the hydrogen permeation sensor. Before the in situ hydrogen permeation monitoring test, the Pd-plated surface of Before the in situ hydrogen permeation monitoring test, the Pd-plated surface of the the sensor was polarized at 0 mv (vs. Hg/HgO) and left for at least 24 h to ensure the sensor was polarized at 0 mv (vs. Hg/HgO) and left for at least 24 h to ensure the polar- polarization current density monitored decreased to below 0.1 Acm . In addition, as −2 ization current density monitored decreased to below 0.1 μA·cm . In addition, as shown shown in Figure 2, the sensor whose outer surface was fully sealed with epoxy resin and in Figure 2, the sensor whose outer surface was fully sealed with epoxy resin and PTFE PTFE shell served as a control sample, for correcting the background current fluctuations shell served as a control sample, for correcting the background current fluctuations caused by temperature variations. This control sample is also referred to as a completely caused by temperature variations. This control sample is also referred to as a completely sealed sensor in later sections. Since no surface of the steel was exposed to corrosive sealed sensor in later sections. Since no surface of the steel was exposed to corrosive me- media because of the covering by epoxy resin and PTFE, it is regarded that there is not dia because of the covering by epoxy resin and PTFE, it is regarded that there is not a a electrochemical reaction (including corrosion reaction) occurring on the surface of the electrochemical reaction (including corrosion reaction) occurring on the surface of the completely sealed sensor. completely sealed sensor. 2.2.2. In Situ Hydrogen Permeation Monitoring Test in the Outdoor Marine Environments 2.2.2. In situ Hydrogen Permeation Monitoring Test in the Outdoor The in situ hydrogen permeation monitoring test was carried out to investigate the Marine Environments hydrogen permeation behavior into steel in outdoor marine environments. During the The in situ hydrogen permeation monitoring test was carried out to investigate the test, sensors were individually suspended on an insulated wire rope and fixed at differ- hydrogen permeation behavior into steel in outdoor marine environments. During the ent positions in the vertical direction of the sea by clamps. The hydrogen permeation test, sensors were individually suspended on an insulated wire rope and fixed at differ- sensor was connected to a multi-channel Potentiostat (made in Japan) via copper wires. ent positions in the vertical direction of the sea by clamps. The hydrogen permeation Meanwhile, a temperature sensor was installed near the hydrogen permeation sensor to sensor was connected to a multi-channel Potentiostat (made in Japan) via copper wires. monitor temperature changes. According to the tidal record of the experimental sea area, Meanwhile, a temperature sensor was installed near the hydrogen permeation sensor to the position distribution of the sensors in the vertical direction of the sea was determined, monitor temperature changes. According to the tidal record of the experimental sea area, including the marine atmospheric zone, the splash zone, the tidal zone and the immersion the position distribution of the sensors in the vertical direction of the sea was determined, zone. Details of the position distribution of the sensors along the vertical direction of the isea ncludi aren shown g the ma in ri Figur ne aetmospheri 3. c zone, the splash zone, the tidal zone and the immer- sion zone. De As a blank tails of the po control, the sition d completely istribution sealed of the sensors sensor wasalon suspended g the vertical direction of alone on another insulated wire rope and placed side by side with the sensor exposed to the marine atmo- the sea are shown in Figure 3. spheric zone. Tide heights were monitored in real time through a pressure sensor placed at the bottom of the immersion zone. Both the tidal data and above temperature data were recorded by a HIOKI MEMORY HILOGGER. In addition, due to the important influence of humidity on the corrosion of steel in the atmospheric zone, the changes in marine at- mospheric humidity were also monitored and recorded by a hygrothermograph. The in situ hydrogen permeation monitoring test started on 6 July 2020, and ended on 12 October 2020, a period encompassing the summer and early autumn of the Northern hemisphere. Appl. Sci. 2022, 12, 2785 5 of 20 Appl. Sci. 2022, 12, x FOR PEER REVIEW 5 of 20 Figure 3. Schematic diagram of the outdoor hydrogen permeation test. Figure 3. Schematic diagram of the outdoor hydrogen permeation test. As a blank control, the completely sealed sensor was suspended alone on another 2.2.3. Measurement of Corrosion Thickness Loss insulated wire rope and placed side by side with the sensor exposed to the marine at- The hydrogen permeation sensor was also used as a corrosion coupon. The parameter mospheric zone. Tide heights were monitored in real time through a pressure sensor of corrosion thickness loss was adopted to quantify the corrosion degree of sensors after placed at the bottom of the immersion zone. Both the tidal data and above temperature outdoor exposure. The hydrogen permeation sensor was cut into two sections from the data were recorded by a HIOKI MEMORY HILOGGER. In addition, due to the important middle with a hacksaw. One section of the sensor was sealed with epoxy resin, and then influence of humidity on the corrosion of steel in the atmospheric zone, the changes in the cross-section of the sensor was grinded with SiC paper up to 1000 grade. Subsequently, marine atmospheric humidity were also monitored and recorded by a hygrothermo- it was polished with a 3 m diamond abrasive and then rinsed with distilled water and graph. The in situ hydrogen permeation monitoring test started on 6 July 2020, and alcohol to obtain a smooth surface for observation. Measurement of the residual thickness of ended on 12 October 2020, a period encompassing the summer and early autumn of the the sensor wall was performed with a Jangnan MR-2000 optical metallographic microscope Northern hemisphere. and accompanying software of ScopeImage 9.0. 2.2.4. Analysis of Corrosion Products 2.2.3. Measurement of Corrosion Thickness Loss The The hydro corrosion genpr permeation oduct layers senso scraped r wafr s om also the use sensors d as a corrosion were grinded coupto on. The p fine powde aram- rs and then analyzed by using X-ray diffraction (XRD) and Raman spectroscopy. XRD patterns eter of corrosion thickness loss was adopted to quantify the corrosion degree of sensors were obtained with an Ultima IV X-ray diffractometer with Cu K radiation ( = 1.5406 Å) after outdoor exposure. The hydrogen permeation sensor was cut into two sections from operating at 40 kV and 40 mA. The 2 ranged from 5 to 80 at a scanning rate of 2 min the middle with a hacksaw. One section of the sensor was sealed with epoxy resin, and and the data were processed with software of MDI Jade 6.5. Raman analysis was performed then the cross-section of the sensor was grinded with SiC paper up to 1000 grade. Sub- on a micro-Raman spectrometer (Renishaw MZ20-FC) equipped with a solid-state diode sequently, it was polished with a 3 μm diamond abrasive and then rinsed with distilled pumped green laser (wavelength = 532 nm), and the Raman shift range was 0~1800 cm . water and alcohol to obtain a smooth surface for observation. Measurement of the re- sidual thickness of the sensor wall was performed with a Jangnan MR-2000 optical 2.3. Indoor Simulated Hydrogen Permeation Test metallographic microscope and accompanying software of ScopeImage 9.0. To investigate the hydrogen permeation behavior and mechanism of the steel under a wet–dry cycle condition, a hydrogen permeation test was carried out in a simulated tidal 2.2.4. Analysis of Corrosion Products cycling condition in the laboratory. The simulated tidal cycles were realized through a The corrosion product layers scraped from the sensors were grinded to fine powders platform and water tank that can be regularly raised and lowered, as shown in Figure 4. The and then analyzed by using X-ray diffraction (XRD) and Raman spectroscopy. XRD pat- indoor tidal cycles simulate semidiurnal tides, which were 6 h of immersion in sea water, terns were obtained with an Ultima IV X-ray diffractometer with Cu Kα radiation (λ = then 6 h of air exposure, and so on. During the indoor hydrogen permeation monitoring 1.5406 Å) operating at 40 kV and 40 mA. The 2θ ranged from 5° to 80° at a scanning rate tests, in addition to the continuous monitoring of hydrogen permeation behavior, open −1 of 2° min and the data were processed with software of MDI Jade 6.5. Raman analysis circuit potential (OCP) of the steel was continuously monitored through a calomel electrode was performed on a micro-Raman spectrometer (Renishaw MZ20-FC) equipped with a which was always immersed in sea water. A cotton thread is connected between the sensor Appl. Sci. 2022, 12, x FOR PEER REVIEW 6 of 20 solid-state diode pumped green laser (wavelength λ = 532 nm), and the Raman shift −1 range was 0~1800 cm . 2.3. Indoor Simulated Hydrogen Permeation Test To investigate the hydrogen permeation behavior and mechanism of the steel under a wet–dry cycle condition, a hydrogen permeation test was carried out in a simulated tidal cycling condition in the laboratory. The simulated tidal cycles were realized through a platform and water tank that can be regularly raised and lowered, as shown in Figure 4. The indoor tidal cycles simulate semidiurnal tides, which were 6 h of immersion in sea water, then 6 h of air exposure, and so on. During the indoor hydrogen permeation monitoring tests, in addition to the continuous monitoring of hydrogen permeation be- Appl. Sci. 2022, 12, 2785 6 of 20 havior, open circuit potential (OCP) of the steel was continuously monitored through a calomel electrode which was always immersed in sea water. A cotton thread is connected between the sensor and the water tank to ensure that the OCP of the sensor can be con- and the water tank to ensure that the OCP of the sensor can be continuously monitored tinuously monitored during tidal cycles. Meanwhile, the potential between the sensor during tidal cycles. Meanwhile, the potential between the sensor and another calomel electr and ode anot fixed her calom on theesensor l electrwas ode continuously fixed on the sen monitor sor ed, was as cont an indicator inuously potential monitoto red, as an indicate the drying or wetting stage. indicator potential to indicate the drying or wetting stage. Figure 4. Photos of the hydrogen permeation test set-up under simulated tidal cycling condition in Figure 4. Photos of the hydrogen permeation test set-up under simulated tidal cycling condition in laboratory (a), enlarged view of electrolytic cell (b). laboratory (a), enlarged view of electrolytic cell (b). 3. Results and Discussion 3. Results and Discussion 3.1. Hydrogen Permeation Monitoring Test in the Outdoor Marine Environments 3.1. Hydrogen Permeation Monitoring Test in the Outdoor Marine Environments 3.1.1. Hydrogen Permeation Behavior of Steels in Different Marine Corrosion Zones 3.1.1. Hydrogen Permeation Behavior of Steels in Different Marine Corrosion Zones Figure 5 shows the macro-morphologies of sensors after being exposed for 3 months Figure 5 shows the macro-morphologies of sensors after being exposed for 3 months in outdoor marine environments. It can be seen that the sensors exposed to different corrosion zones endured varying degrees of corrosion. The sensor exposed to the marine in outdoor marine environments. It can be seen that the sensors exposed to different atmospheric zone was less corroded in comparison with the sensors exposed to other corrosion zones endured varying degrees of corrosion. The sensor exposed to the marine corrosion zones. Meanwhile, the sensor exposed to the tidal zone was obviously affected by atmospheric zone was less corroded in comparison with the sensors exposed to other biofouling, and the senor surface was covered with a large number of barnacles (Figure 5c). corrosion zones. Meanwhile, the sensor exposed to the tidal zone was obviously affected To date, the effect of macro biofouling on steel corrosion remains controversial. On the one by biofouling, and the senor surface was covered with a large number of barnacles (Fig- hand, macro-fouling organisms such as oyster, barnacle and filamentous macroalgae may ure 5c). To date, the effect of macro biofouling on steel corrosion remains controversial. induce crevice corrosion, or accelerate the corrosion process by facilitating the growth of On the one hand, macro-fouling organisms such as oyster, barnacle and filamentous microbial communities, as elaborated in literatures [30–32]; on the other hand, based on outdoor measurement data, it was found that there was a negative relationship between biomass data and corrosion mass loss, suggesting that macro-fouling played a positive role in reducing mass loss [33,34]. In addition, due to the scouring action of sea water and prolonged immersion, it was observed that the rust layer formed on the surface of the sensor exposed to the immersion zone was relatively loose, and the surface was covered with silt-like sediments. Given the positive relationship between hydrogen permeation and corrosion of steel, varying degrees of corrosion for steel may imply differences in the amount of hydrogen permeated into the steel. Appl. Sci. 2022, 12, x FOR PEER REVIEW 7 of 20 macroalgae may induce crevice corrosion, or accelerate the corrosion process by facili- tating the growth of microbial communities, as elaborated in literatures [30–32]; on the other hand, based on outdoor measurement data, it was found that there was a negative relationship between biomass data and corrosion mass loss, suggesting that mac- ro-fouling played a positive role in reducing mass loss [33,34]. In addition, due to the scouring action of sea water and prolonged immersion, it was observed that the rust layer formed on the surface of the sensor exposed to the immersion zone was relatively loose, and the surface was covered with silt-like sediments. Given the positive relation- Appl. Sci. 2022, 12, 2785 7 of 20 ship between hydrogen permeation and corrosion of steel, varying degrees of corrosion for steel may imply differences in the amount of hydrogen permeated into the steel. Figure Figure 5. 5. Macr Mac o-morphologies ro-morphologi ofes the of the sensors sens after orsbeing after bei exposed ng expos for 3 months ed for 3 inmonths the marine in the mari atmo- ne at- spheric mospheric zon zone (a), e the (asplash ), the splas zone h (b zon ), the e ( tidal b), the zone tidal zone ( (c) and thecimmersion ) and the immersion zone ( zone (d). d). Figure 6a shows the polarization current of the completely sealed sensor exposed Figure 6a shows the polarization current of the completely sealed sensor exposed to to the atmospheric zone, and environment temperature from 4 August to 9 August. It the atmospheric zone, and environment temperature from 4 August to 9 August. It is is observed that there is a positive relationship between the background current and en- observed that there is a positive relationship between the background current and envi- vironment temperature. Since there is no electrochemical reaction (including corrosion ronment temperature. Since there is no electrochemical reaction (including corrosion re- reaction) occurring on the surface of the completely sealed sensor, it is considered that the action) occurring on the surface of the completely sealed sensor, it is considered that the variation of the polarization current density in Figure 6a is caused by temperature change rather variation than of the polarizatio hydrogen permeation. n curr Accor ent den dingly sity , for in a Figure sensor 6a is c that isanot used compl by temperat etely sealed, ure change the polarization current contributed by hydrogen permeation (i.e., hydrogen permeation rather than hydrogen permeation. Accordingly, for a sensor that is not completely sealed, current) can be obtained by subtracting the polarization current totally caused by tempera- the polarization current contributed by hydrogen permeation (i.e., hydrogen permeation ture (i.e., background current). In other words, the hydrogen permeation current can be current) can be obtained by subtracting the polarization current totally caused by tem- expressed as Equation (1). perature (i.e., background current). In other words, the hydrogen permeation current can i = i i (1) H b be expressed as Equation (1). where i is the hydrogen permeation current, i and i are the total anodic polarization H a current and background current, respectively. Based on a large amount of polarization current data of the completely sealed sensor (i.e., background current data) and temperature data, the quantitative relationship between the background current and temperature was obtained by fitting, as shown in Figure 6b. Accordingly, changes in the background current can be determined via the variation of environment temperature during tests. Appl. Sci. 2022, 12, x FOR PEER REVIEW 8 of 20 iH = ia − ib (1) where iH is the hydrogen permeation current, ia and ib are the total anodic polarization current and background current, respectively. Based on a large amount of polarization current data of the completely sealed sensor (i.e., background current data) and temper- ature data, the quantitative relationship between the background current and tempera- ture was obtained by fitting, as shown in Figure 6b. Accordingly, changes in the back- Appl. Sci. 2022, 12, 2785 8 of 20 ground current can be determined via the variation of environment temperature during tests. Figure Figure 6. 6. VVari ariations ations of of the the b backgr ackground curr ound current with ent with envi environment ronment temperature ( temperature (a), and athe ), an relationship d the rela- tionship between the background current and environment temperature (b). between the background current and environment temperature (b). Figure 7 shows the hydrogen permeation current of the sensor exposed to the ma- Figure 7 shows the hydrogen permeation current of the sensor exposed to the ma- rine atmospheric zone, temperature and relative humidity, as well as weather records, rine atmospheric zone, temperature and relative humidity, as well as weather records, from 4 August to 9 August. The data show that the relative humidity and temperature of from August 4th to 9th. The data show that the relative humidity and temperature of the the marine atmosphere were relatively high during this period (Figure 7a), which provided marine atmosphere were relatively high during this period (Figure 7a), which provided favorable conditions for the corrosion of steel in the marine atmospheric zone. It is seen favorable conditions for the corrosion of steel in the marine atmospheric zone. It is seen that the hydrogen permeation current increases significantly when the temperature rises. that the hydrogen permeation current increases significantly when the temperature rises. On one hand, the increase in temperature facilitates the corrosion of the steel, thereby pro- On one hand, the increase in temperature facilitates the corrosion of the steel, thereby moting hydrogen permeation into steel [13]; on the other hand, an increased temperature promoting hydrogen permeation into steel [13]; on the other hand, an increased temper- can increase the H diffusion activation energy, leading to more hydrogen permeation into ature can increase the H diffusion activation energy, leading to more hydrogen permea- steel, which may also be a reason for the increased hydrogen permeation current [35,36]. In tion into steel, which may also be a reason for the increased hydrogen permeation current addition, it is found that the hydrogen permeation current is generally low, when it is rainy. [35,36]. In addition, it is found that the hydrogen permeation current is generally low, The apparent decrease in hydrogen permeation current during rainfall can be attributed to when it is rainy. The apparent decrease in hydrogen permeation current during rainfall both the loss of pollutions deposited on the steel surface and the decrease in temperature. can be attributed to both the loss of pollutions deposited on the steel surface and the de- It is well known that pollutions such as Cl and SO have an accelerated effect on the crease in temperature. It is well known that pollutions such as Cl and SO2 have an ac- corrosion of steel, so that the corrosion rate of steel decreases as pollutions are washed off celerated effect on the corrosion of steel, so that the corrosion rate of steel decreases as from steel surface when it rains [37]. Meanwhile, it can be seen in later discussions that pollutions are washed off from steel surface when it rains [37]. Meanwhile, it can be seen the hydrogen permeation current has a positive relationship with the corrosion rate due in later discussions that the hydrogen permeation current has a positive relationship with to the hydrolysis of the corrosion products, hence a lower corrosion rate corresponds to a the corrosion rate due to the hydrolysis of the corrosion products, hence a lower corro- lower hydrogen permeation current. Accordingly, for steels being exposed to the marine sion rate corresponds to a lower hydrogen permeation current. Accordingly, for steels atmospheric zone, pollutions deposition and salts loading which are related to the humidity being exposed to the marine atmospheric zone, pollutions deposition and salts loading and weather conditions also play an important role in hydrogen permeation behavior. which are related to the humidity and weather conditions also play an important role in Figure 8 shows transients of the hydrogen permeation current density of the sensor hydrogen permeation behavior. exposed to the splash zone, and changes in temperature and tide. Taking the position of the pressure sensor in the full immersion zone as the reference zero point, the specific position of the sensor in the splash zone is as shown in Figure 8b. A large hydrogen permeation current density was observed, with the value approximately 10 times that of the current density for the sensor exposed to the marine atmospheric zone (Figures 7a and 8a). It is well known that steel exposed to the splash zone is prone to endure more severe corrosion [38,39]. Therefore, a large hydrogen permeation current of the steel exposed to the splash zone is reasonable, according to the positive relationship between corrosion of steel and hydrogen permeation. For steel exposed to the splash zone, the wet–dry alternation of the steel surface is mainly controlled by splashing waves. It can be observed that the hydrogen permeation current was relatively high when the surface of the steel was in a wetting stage caused by spindrift, suggesting a corrosion accelerated process in the wetting stage. Furthermore, the promoting effect of a high temperature on hydrogen permeation was more obvious in the splash zone, on the basis of a higher hydrogen permeation current, as shown in Figure 8a. Appl. Sci. 2022, 12, x FOR PEER REVIEW 9 of 20 Appl. Sci. 2022, 12, x FOR PEER REVIEW 9 of 20 Appl. Sci. 2022, 12, 2785 9 of 20 Figure 7. Time variations of the hydrogen permeation current density of the sensor exposed to the marine atmospheric zone (a), and temperature/relative humidity changes (b) from 4 August to 9 August during outdoor hydrogen permeation test. Figure 8 shows transients of the hydrogen permeation current density of the sensor exposed to the splash zone, and changes in temperature and tide. Taking the position of the pressure sensor in the full immersion zone as the reference zero point, the specific position of the sensor in the splash zone is as shown in Figure 8b. A large hydrogen permeation current density was observed, with the value approximately 10 times that of the current density for the sensor exposed to the marine atmospheric zone (Figures 7a and 8a). It is well known that steel exposed to the splash zone is prone to endure more severe corrosion [38,39]. Therefore, a large hydrogen permeation current of the steel ex- posed to the splash zone is reasonable, according to the positive relationship between corrosion of steel and hydrogen permeation. For steel exposed to the splash zone, the wet–dry alternation of the steel surface is mainly controlled by splashing waves. It can be observed that the hydrogen permeation current was relatively high when the surface of the steel was in a wetting stage caused by spindrift, suggesting a corrosion accelerated process in the wetting stage. Furthermore, the promoting effect of a high temperature on Figure 7. Time variations of the hydrogen permeation current density of the sensor exposed to the Figure 7. Time variations of the hydrogen permeation current density of the sensor exposed to the hydrogen permeation was more obvious in the splash zone, on the basis of a higher hy- marine atmospheric zone (a), and temperature/relative humidity changes (b) from 4 August to marine atmospheric zone (a), and temperature/relative humidity changes (b) from 4 August to 9 drogen permeation current, as shown in Figure 8a. 9 August during outdoor hydrogen permeation test. August during outdoor hydrogen permeation test. Figure 8 shows transients of the hydrogen permeation current density of the sensor exposed to the splash zone, and changes in temperature and tide. Taking the position of the pressure sensor in the full immersion zone as the reference zero point, the specific position of the sensor in the splash zone is as shown in Figure 8b. A large hydrogen permeation current density was observed, with the value approximately 10 times that of the current density for the sensor exposed to the marine atmospheric zone (Figures 7a and 8a). It is well known that steel exposed to the splash zone is prone to endure more severe corrosion [38,39]. Therefore, a large hydrogen permeation current of the steel ex- posed to the splash zone is reasonable, according to the positive relationship between corrosion of steel and hydrogen permeation. For steel exposed to the splash zone, the wet–dry alternation of the steel surface is mainly controlled by splashing waves. It can be observed that the hydrogen permeation current was relatively high when the surface of the steel was in a wetting stage caused by spindrift, suggesting a corrosion accelerated process in the wetting stage. Furthermore, the promoting effect of a high temperature on hydrogen permeation was more obvious in the splash zone, on the basis of a higher hy- drogen permeation current, as shown in Figure 8a. Figure 8. Time variations of the hydrogen permeation current density of the sensor exposed to the splash zone and the corresponding temperature (a), as well as tidal changes (b) from 4 August to 9 August during outdoor hydrogen permeation test. Figure 9 shows transients of the hydrogen permeation current density of the sensor exposed to the splash zone, and changes in temperature and tide. The specific position of Appl. Sci. 2022, 12, x FOR PEER REVIEW 10 of 20 Figure 8. Time variations of the hydrogen permeation current density of the sensor exposed to the splash zone and the corresponding temperature (a), as well as tidal changes (b) from 4 August to 9 August during outdoor hydrogen permeation test. Figure 9 shows transients of the hydrogen permeation current density of the sensor exposed to the splash zone, and changes in temperature and tide. The specific position of Appl. Sci. 2022, 12, 2785 10 of 20 the sensor exposed to the tidal zone is shown in Figure 9b. The obvious fluctuations of temperature with tides were observed in Figure 9a, owing to the difference in the specific the sensor exposed to the tidal zone is shown in Figure 9b. The obvious fluctuations of heat capacity between sea water and air, as well as the evaporation of water. Meanwhile, temperature with tides were observed in Figure 9a, owing to the difference in the specific a current peak was observed in each tidal cycle. It is believed that the appearance of heat capacity between sea water and air, as well as the evaporation of water. Meanwhile, current peak is related to the temperature effect on the one hand and may be related to a current peak was observed in each tidal cycle. It is believed that the appearance of the difference in the cathodic reactions between drying and wetting process on the other current peak is related to the temperature effect on the one hand and may be related to the difference in the cathodic reactions between drying and wetting process on the other hand hand (details refer to Section 3.2). Furthermore, it was also found that the hydrogen (details refer to Section 3.2). Furthermore, it was also found that the hydrogen permeation permeation current of the sensor exposed to the tidal zone was approximately one quar- current of the sensor exposed to the tidal zone was approximately one quarter that of the ter that of the sensor exposed to the splash zone. This indicates that the corrosion of steel sensor exposed to the splash zone. This indicates that the corrosion of steel in the tidal zone in the tidal zone is relatively light in comparison with that of steel in the splash zone. is relatively light in comparison with that of steel in the splash zone. Figure 9. Time variations of the hydrogen permeation current density of the sensor exposed to the Figure 9. Time variations of the hydrogen permeation current density of the sensor exposed to the tidal zone and the corresponding temperature (a), as well as tidal changes (b) from 4 August to tidal zone and the corresponding temperature (a), as well as tidal changes (b) from 4 August to 9 9 August during outdoor hydrogen permeation test. August during outdoor hydrogen permeation test. Figure 10a shows transients of the hydrogen permeation current density of the sensor Figure 10a shows transients of the hydrogen permeation current density of the exposed to the immersion zone and temperature. It can be seen that the hydrogen perme- ation current of the sensor exposed to the immersion zone was much larger than that of sensor exposed to the immersion zone and temperature. It can be seen that the hydrogen the sensor exposed to the tidal zone. The relatively large hydrogen permeation current in permeation current of the sensor exposed to the immersion zone was much larger than the immersion zone may be caused by the stronger reproduction and metabolic activity that of the sensor exposed to the tidal zone. The relatively large hydrogen permeation of sulfate-reducing bacteria (SRB) on the surface of the steel in the immersion zone. It can current in the immersion zone may be caused by the stronger reproduction and meta- be observed that part of the surface of the hydrogen permeation sensor in the immersion zone was covered by a layer of gray-black sediments, both the sediments and relatively bolic activity of sulfate-reducing bacteria (SRB) on the surface of the steel in the immer- low concentration of dissolved oxygen in the immersion zone providing favorable living sion zone. It can be observed that part of the surface of the hydrogen permeation sensor conditions for SRB. Raman spectroscopy shown in Figure 11b confirmed that the presence in the immersion zone was covered by a layer of gray-black sediments, both the sedi- of ferrous sulfide (FeS) in the corrosion products formed in the immersion zone, with the ments and relatively low 1 concentration of dissolved oxygen in the immersion zone characteristic peak at 284 cm [40]. The formation of FeS is related to the activities of SRB, providing favorable living conditions for SRB. Raman spectroscopy shown in Figure 11b and published study showed that SRB were tend to be present in the inner black stratum of the corrosion products layer [41]. Meanwhile, sulfate green rust [GR(SO )], akaganeite confirmed that the presence of ferrous sulfide (FeS) in the corrosion products formed in ( -FeOOH) and magnetite (Fe O ) were also detected in the corrosion products, with the 3 4 −1 the immersion zone, with the characteristic peak at 284 cm [40]. The formation of FeS is 1 1 1 characteristic peaks at 217 cm , 385 cm and 638 cm , respectively [42,43]. However, no related to the activities of SRB, and published study showed that SRB were tend to be present in the inner black stratum of the corrosion products layer [41]. Meanwhile, sulfate Appl. Sci. 2022, 12, x FOR PEER REVIEW 11 of 20 Appl. Sci. 2022, 12, 2785 11 of 20 2− green rust [GR(SO4 )], akaganeite (β-FeOOH) and magnetite (Fe3O4) were also detected −1 −1 −1 in the corrosion products, with the characteristic peaks at 217 cm , 385 cm and 638 cm , Appl. Sci. 2022, 12, x FOR PEER REVIEW 11 of 20 respectively [42,43]. However, no FeS was found in the corrosion products formed in the FeS was found in the corrosion products formed in the tidal zone, only -FeOOH, Fe O 3 4 tidal zone, only β-FeOOH, Fe3O4 and lepidocrocite (γ-FeOOH) were detected, 1 with the and lepidocrocite (
-FeOOH) were detected, with the characteristic peaks at 312 cm , 1 1 −1 −1 −1 characteristic peaks at 312 cm , 540 cm and 706 cm , respectively (Figure 11a) [43]. 540 cm and 706 cm , respectively (Figure 11a) [43]. 2− green rust [GR(SO4 )], akaganeite (β-FeOOH) and magnetite (Fe3O4) were also detected −1 −1 −1 in the corrosion products, with the characteristic peaks at 217 cm , 385 cm and 638 cm , respectively [42,43]. However, no FeS was found in the corrosion products formed in the tidal zone, only β-FeOOH, Fe3O4 and lepidocrocite (γ-FeOOH) were detected, with the −1 −1 −1 characteristic peaks at 312 cm , 540 cm and 706 cm , respectively (Figure 11a) [43]. Figure 10. Time variations of the hydrogen permeation current density of the sensor exposed to the Figure 10. Time variations of the hydrogen permeation current density of the sensor exposed to the Figure 10. Time variations of the hydrogen permeation current density of the sensor exposed to the immersion zone and the corresponding temperature (a), as well as tidal changes (b), from 4 August immersion zone and the corresponding temperature (a), as well as tidal changes (b), from 4 August immersion zone and the corresponding temperature (a), as well as tidal changes (b), from 4 August to 9 August during outdoor hydrogen permeation test. to 9 August during outdoor hydrogen permeation test. to 9 August during outdoor hydrogen permeation test. Figure 11. Raman spectra of the corrosion products formed on the surface of hydrogen permeation Figure 11. Raman spectra of the corrosion products formed on the surface of hydrogen permeation sensors exposed to the tidal zone (a) and the immersion zone (b). sensors exposed to the tidal zone (a) and the immersion zone (b). 2 2− − It is considered that the sulfide products of SRB metabolic activity (S , HS and H2S) It is considered that the sulfide products of SRB metabolic activity (S , HS and H S) Figure 11. Raman spectra of the corrosion products formed on the surface of hydrogen permeation can act as a “catalyst” in facilitating hydrogen ions reduction and a “poisoning agent” in can act as a “catalyst” in facilitating hydrogen ions reduction and a “poisoning agent” in sensors exposed to the tidal zone (a) and the immersion zone (b). inhibiting hydrogen recombination, respectively, thus promoting hydrogen permeation inhibiting hydrogen recombination, respectively, thus promoting hydrogen permeation into steels [44–46]. Many studies have reported the promoting effect of SRBs on hydrogen into steels [44–46]. Many studies have reported the promoting effect of SRBs on hydrogen 2− − permeation [47,48]. Zhu et al. [49] found that the hydrogen permeation current of steel in It is considered that the sulfide products of SRB metabolic activity (S , HS and H2S) sea mud containing SRB was about three times to that in sterilized sea mud in the hy- can act as a “catalyst” in facilitating hydrogen ions reduction and a “poisoning agent” in drogen permeation investigation of API X56 steel in sea mud. Furthermore, due to a large inhibiting hydrogen recombination, respectively, thus promoting hydrogen permeation into steels [44–46]. Many studies have reported the promoting effect of SRBs on hydrogen permeation [47,48]. Zhu et al. [49] found that the hydrogen permeation current of steel in sea mud containing SRB was about three times to that in sterilized sea mud in the hy- drogen permeation investigation of API X56 steel in sea mud. Furthermore, due to a large Appl. Sci. 2022, 12, 2785 12 of 20 Appl. Sci. 2022, 12, x FOR PEER REVIEW 12 of 20 permeation [47,48]. Zhu et al. [49] found that the hydrogen permeation current of steel in specific heat capacity of sea water, it was observed that changes in temperature of sea sea mud containing SRB was about three times to that in sterilized sea mud in the hydrogen water in the immersion zone were very small and the maximum fluctuation range was permeation investigation of API X56 steel in sea mud. Furthermore, due to a large specific heat capacity of sea water, it was observed that changes in temperature of sea water in about 2 °C. However, similarly, based on the background of a large hydrogen permeation the immersion zone were very small and the maximum fluctuation range was about 2 C. current, the fluctuations of hydrogen permeation current caused by small changes in However, similarly, based on the background of a large hydrogen permeation current, the temperature can be obviously observed from Figure 10a. fluctuations of hydrogen permeation current caused by small changes in temperature can be obviously observed from Figure 10a. 3.1.2. The Relationship between Hydrogen Permeation and Corrosion Loss of Steel 3.1.2. The Relationship between Hydrogen Permeation and Corrosion Loss of Steel The residual thickness of the side wall of the sensor was measured by using an op- tical mi The croscope so residual thickness as to ofobta the side in iwall ts thickn of theess sensor loss da wasta measur , accordi ed by ng to the method i using an optical ntro- microscope so as to obtain its thickness loss data, according to the method introduced in the duced in the published study [50]. The cross-sectional morphologies of the sensors en- published study [50]. The cross-sectional morphologies of the sensors endured corrosion dured corrosion exposure are shown in Figure S2 (refer to supplementary file). The exposure are shown in Figure S2 (refer to supplementary file). The widest and narrowest widest and narrowest sites of corrosion residual thickness in each field of vision were sites of corrosion residual thickness in each field of vision were selected for measurement, selected for measurement, and the procedures were repeated for 72 times at different and the procedures were repeated for 72 times at different sites on the cross-section, as sites on the cross-section, as shown in Figure 12. After multiple measurements, the av- shown in Figure 12. After multiple measurements, the average corrosion thickness loss erage corrosion thickness loss of the sensors in the marine atmospheric, splash, tidal and of the sensors in the marine atmospheric, splash, tidal and immersion zones are 54 m, immersion zones are 54 μm, 245 μm, 127 μm and 230 μm, respectively. This result sug- 245 m, 127 m and 230 m, respectively. This result suggests that sensors in the splash and gestimmersion s that sens zones ors in have the as higher plash corr and immers osion rateiin on z compari ones ha sonvwith e a hi that ghof er c sensors orrosiin on rate in the marine atmospheric and tidal zones. comparison with that of sensors in the marine atmospheric and tidal zones. Figure 12. Residual thickness measurement on the cross-section of the sensors after being exposed Figure 12. Residual thickness measurement on the cross-section of the sensors after being exposed for 3 months in the marine atmospheric zone (a), the splash zone (b), the tidal zone (c), and the for 3 months in the marine atmospheric zone (a), the splash zone (b), the tidal zone (c), and the immersion zone (d), respectively. immersion zone (d), respectively. Hydrogen permeation behavior into the steel during exposure period was continu- Hydrogen permeation behavior into the steel during exposure period was contin- ously monitored using electrochemical hydrogen permeation technique, as displayed in uously monitored using electrochemical hydrogen permeation technique, as displayed in Figures 7–10. Based on the hydrogen permeation data continuously collected for 3 months, Figures 7–10. Based on the hydrogen permeation data continuously collected for 3 months, the average hydrogen permeation current density (jaH) and the sub-surface hy- drogen concentration C0 were calculated according to Equations (2)–(4): QH = |𝑖 (𝑡)| d𝑡 (2) jaH = (3) C0 = (4) ·· Appl. Sci. 2022, 12, 2785 13 of 20 the average hydrogen permeation current density (j ) and the sub-surface hydrogen aH concentration C were calculated according to Equations (2)–(4): Q = ji (t)j dt (2) H H j = (3) aH At Li aH C = (4) nFD e f f where Q represents the quantity of hydrogen permeation charge, A is the total working area of the hydrogen permeation sensor, t is the corresponding test duration, L is the thick- ness of the steel specimen, F represents Faraday’s constant with a value of 96,485 C/mol, n is number of transferred electrons in the hydrogen reduction reaction. Table 2 shows results of the j and C of steel in different marine corrosion zones. It can be found that the aH 0 hydrogen permeation into steel in the splash zone is the most severe, while that of the steel in the atmospheric zone is the least. Meanwhile, the amount of the hydrogen permeated into the steel in the immersion zone is about 1.5 times that the hydrogen permeated into the steel in the tidal zone. Table 2. The parameters for hydrogen permeation of AISI 4135 steel in different marine corrosion zones, including the average hydrogen permeation current density (j ), sub-surface hydrogen aH concentration (C ), and diffusible hydrogen content (H ). 0 C 2 3 Exposure Site j /nAcm C /molcm H /wt ppm aH 0 8 3 Atmospheric zone 19.6 2.46 10 3.15 10 7 2 Splash zone 435.3 5.46 10 7.00 10 7 2 Tidal zone 128.1 1.61 10 2.06 10 7 2 Immersion zone 206.9 2.60 10 3.33 10 Figure 13 shows the relationship between the time-averaged hydrogen permeation current density (i.e., j ) and the time-averaged corrosion thickness loss rate (r) of steels aH exposed to different marine corrosion zones. In Figure 13, it can be found that the average hydrogen permeation current density increases with the increase in corrosion thickness loss of steels. This suggests that hydrogen permeation into steel should be enhanced when the corrosion of steel is accelerated in the marine environment, which applies for all marine corrosion zones. The mechanisms of hydrogen embrittlement were summarized as internal pressure autocatalytic, Vacancy-Agglomeration and Lattice decohesion [51]. The hydrogen source is either internal or external. The low internal hydrogen concentration can be achieved by quality control of production. The hydrogen concentration dependence on corrosion rate indicates that the corrosion protection is an important way for hydrogen entry inhibition into steel. Earlier work showed that petrolatum tape cover was effective in inhibiting hydrogen entry under simulated marine splash conditions [52]. The validity of this technique for the hydrogen entry inhibition in other marine corrosion zones will be confirmed in future works. 3.1.3. Corrosion Products Analysis and Its Effect on Hydrogen Permeation Different corrosive environments between marine corrosion zones may not only lead to differences in the corrosion rates of steel, but also differences in the composition of corrosion products. Figure 14 shows the XRD spectra of the corrosion products formed on the surface of sensors exposed to different marine corrosion zones. It can be seen that the corrosion products were mainly iron oxides, consisting of goethite (-FeOOH), lepidocrocite (
-FeOOH), akaganeite ( -FeOOH) and magnetite (Fe O ). No sign of FeS 3 4 was found in the XRD pattern of the corrosion products formed in the immersion zone, owing to the overlap of some peaks and less content. The XRD pattern shows a high Appl. Sci. 2022, 12, x FOR PEER REVIEW 13 of 20 where QH represents the quantity of hydrogen permeation charge, A is the total working area of the hydrogen permeation sensor, t is the corresponding test duration, L is the thickness of the steel specimen, F represents Faraday’s constant with a value of 96,485 C/mol, n is number of transferred electrons in the hydrogen reduction reaction. Table 2 shows results of the jaH and C0 of steel in different marine corrosion zones. It can be found that the hydrogen permeation into steel in the splash zone is the most severe, while that of the steel in the atmospheric zone is the least. Meanwhile, the amount of the hydrogen permeated into the steel in the immersion zone is about 1.5 times that the hydrogen permeated into the steel in the tidal zone. Table 2. The parameters for hydrogen permeation of AISI 4135 steel in different marine corrosion zones, including the average hydrogen permeation current density (jaH), sub-surface hydrogen concentration (C0), and diffusible hydrogen content (HC). −2 −3 Exposure Site jaH/nA·cm C0/mol·cm HC/wt ppm −8 −3 Atmospheric zone 19.6 2.46 × 10 3.15 × 10 −7 −2 Splash zone 435.3 5.46 × 10 7.00 × 10 −7 −2 Appl. Sci. 2022, 12, 2785 14 of 20 Tidal zone 128.1 1.61 × 10 2.06 × 10 −7 −2 Immersion zone 206.9 2.60 × 10 3.33 × 10 proportion of -FeOOH in the corrosion products formed in the marine atmospheric zone. Figure 13 shows the relationship between the time-averaged hydrogen permeation Previous studies have shown that a corrosion environment with a low content of chloride current density (i.e., jaH) and the time-averaged corrosion thickness loss rate (r) of steels ions can facilitate the formation of -FeOOH in the corrosion products, thus leading to a exposed to different marine corrosion zones. In Figure 13, it can be found that the average denser corrosion products layer. This dense corrosion products layer can largely hinder the hydrogen permeation current density increases with the increase in corrosion thickness penetration of O and Cl to steel surface, thereby suppressing electrochemical corrosion loss of steels. This suggests that hydrogen permeation into steel should be enhanced and decreasing hydrogen permeation ultimately (Figure 7a) [53,54]. On the contrary, an when the corrosion of steel is accelerated in the marine environment, which applies for environment with a high Cl content favors the formation of -FeOOH [55,56] -FeOOH al can l m alter arine the co str rrosion ucture of zones corr.osion The m pre oducts chanism layer s of h such ydr asogen embr facilitatingittlement were the formation of summa- cracks and pores, thereby promoting corrosion and hydrogen permeation into steel via rized as internal pressure autocatalytic, Vacancy-Agglomeration and Lattice decohesion providing extra channels for the penetration of corrosive substances [57]. Meanwhile, [51]. The hydrogen source is either internal or external. The low internal hydrogen con- published studies have shown that the corrosion products layer formed on steel surface centration can be achieved by quality control of production. The hydrogen concentration can suppress the anodic reaction of iron dissolution and simultaneously promote the dependence on corrosion rate indicates that the corrosion protection is an important way hydrogen evolution reaction at the cathode, thus enhancing hydrogen permeation [58–60]. for hydrogen entry inhibition into steel. Earlier work showed that petrolatum tape cover Furthermore, as mentioned above, corrosion product-FeS (as shown in Figure 11b) can was effective in inhibiting hydrogen entry under simulated marine splash conditions simultaneously act as both “catalyst” and “poisoning agent” during hydrogen permeation [52]. The validity of this technique for the hydrogen entry inhibition in other marine process. Accordingly, the category of corrosion products can, in turn, affect the corrosion corrosion and hydrogen zonpermeation es will be con behaviors firmedof in steel futu in re wor a direct ks.or indirect way. Figure 13. Relationship between the time-averaged hydrogen permeation current density (j ) and aH the time-averaged corrosion thickness loss rate (r) of the hydrogen permeation sensors in the entire duration of the outdoor hydrogen permeation test. 3.2. Hydrogen Permeation into Steel under the Simulated Wet–Dry Cycle Condition It is observed from Figure 9 that the hydrogen permeation current does not vary monotonically during both the wetting and drying stages. To further verify the hydrogen permeation behavior of steels in the wet–dry cycle condition and investigate its mechanism, hydrogen permeation test was performed under the simulated wet–dry cycle condition. Figure 15a,b show the variations of the hydrogen permeation current density and open cir- cuit potential of the sensor in laboratory simulated tidal cycling environment, respectively. Figure 15c shows changes in the indicator potential, indicating which state the hydrogen permeation sensor is in (wetting state or drying state). When the indicator potential shows 0 mV, it indicates that the sensor is in the drying state, otherwise the sensor is in the wetting state. It is found that changes in the hydrogen permeation current are nonmonotonic in the drying or wetting stages (Figure 15a). During the drying stage, the hydrogen permeation current firstly decreased and then increased, while during the wetting stage, it showed the Appl. Sci. 2022, 12, x FOR PEER REVIEW 14 of 20 Figure 13. Relationship between the time-averaged hydrogen permeation current density (jaH) and the time-averaged corrosion thickness loss rate (r) of the hydrogen permeation sensors in the entire duration of the outdoor hydrogen permeation test. 3.1.3. Corrosion Products Analysis and Its Effect on Hydrogen Permeation Different corrosive environments between marine corrosion zones may not only lead to differences in the corrosion rates of steel, but also differences in the composition of corrosion products. Figure 14 shows the XRD spectra of the corrosion products formed on the surface of sensors exposed to different marine corrosion zones. It can be seen that the corrosion products were mainly iron oxides, consisting of goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH) and magnetite (Fe3O4). No sign of FeS was found in the XRD pattern of the corrosion products formed in the immersion zone, owing to the overlap of some peaks and less content. The XRD pattern shows a high proportion of α-FeOOH in the corrosion products formed in the marine atmospheric zone. Previous studies have shown that a corrosion environment with a low content of chloride ions can facilitate the formation of α-FeOOH in the corrosion products, thus leading to a denser corrosion products layer. This dense corrosion products layer can largely hinder the penetration of O2 and Cl to steel surface, thereby suppressing elec- trochemical corrosion and decreasing hydrogen permeation ultimately (Figure 7a) [53,54]. On the contrary, an environment with a high Cl content favors the formation of β-FeOOH [55,56] β-FeOOH can alter the structure of corrosion products layer such as facilitating the formation of cracks and pores, thereby promoting corrosion and hydrogen permeation into steel via providing extra channels for the penetration of corrosive sub- stances [57]. Meanwhile, published studies have shown that the corrosion products layer formed on steel surface can suppress the anodic reaction of iron dissolution and simul- taneously promote the hydrogen evolution reaction at the cathode, thus enhancing hy- Appl. Sci. 2022, 12, 2785 15 of 20 drogen permeation [58–60]. Furthermore, as mentioned above, corrosion product-FeS (as shown in Figure 11b) can simultaneously act as both “catalyst” and “poisoning agent” during hydrogen permeation process. Accordingly, the category of corrosion products opposite characterization. In addition, it is obvious that the overall hydrogen permeation can, in turn, affect the corrosion and hydrogen permeation behaviors of steel in a direct or current in the wetting stage is higher than that during the drying stage, which is consistent indirect way. with the outdoor experiment results (Figure 9). Appl. Sci. 2022, 12, x FOR PEER REVIEW 15 of 20 3.2. Hydrogen Permeation into Steel under the Simulated Wet–Dry Cycle Condition It is observed from Figure 9 that the hydrogen permeation current does not vary monotonically during both the wetting and drying stages. To further verify the hydro- gen permeation behavior of steels in the wet–dry cycle condition and investigate its mechanism, hydrogen permeation test was performed under the simulated wet–dry cy- cle condition. Figure 15a,b show the variations of the hydrogen permeation current den- sity and open circuit potential of the sensor in laboratory simulated tidal cycling envi- ronment, respectively. Figure 15c shows changes in the indicator potential, indicating which state the hydrogen permeation sensor is in (wetting state or drying state). When the indicator potential shows 0 mV, it indicates that the sensor is in the drying state, otherwise the sensor is in the wetting state. It is found that changes in the hydrogen permeation current are nonmonotonic in the drying or wetting stages (Figure 15a). Dur- ing the drying stage, the hydrogen permeation current firstly decreased and then in- creased, while during the wetting stage, it showed the opposite characterization. In ad- dition, it is obvious that the overall hydrogen permeation current in the wetting stage is higher than that during the drying stage, which is consistent with the outdoor experi- Figure 14. XRD spectra of the corrosion products formed on the surface of sensors exposed to Figure 14. XRD spectra of the corrosion products formed on the surface of sensors exposed to dif- dif ment ferent resu marine lts (Fig corrosion ure 9). zones. ferent marine corrosion zones. Figure 15. Variations of the hydrogen permeation current density (a) and open circuit potential (b) of Figure 15. Variations of the hydrogen permeation current density (a) and open circuit potential (b) the sensor, and indicator potential (c) under the simulated wet–dry cycle condition. of the sensor, and indicator potential (c) under the simulated wet–dry cycle condition. Under the wet–dry cycle condition, the anodic dissolution of iron during the elec- trochemical corrosion process can be balanced by the hydrogen reduction reaction Equation (5), the dissolved oxygen reduction reaction Equation (6) and the reduction re- actions of corrosion products represented by Equations (7) and (8) [61–63]: + − H + e → Had (5) − − O2 + 2H2O + 4e → 4OH (6) + − γ-FeOOH + H + e → γ-FeOHOH (7) + − 3γ-FeOOH + H + e → Fe3O4 + 2H2O (8) Under the wet–dry cycle condition, the dominant cathodic reactions of steel corro- sion in different states possess some differences. Evans [64] and Stratmann et al. [65,66] proposed that the corrosion process of steel in the simulated atmospheric corrosion en- Appl. Sci. 2022, 12, 2785 16 of 20 Under the wet–dry cycle condition, the anodic dissolution of iron during the electro- chemical corrosion process can be balanced by the hydrogen reduction reaction Equation (5), the dissolved oxygen reduction reaction Equation (6) and the reduction reactions of corro- sion products represented by Equations (7) and (8) [61–63]: H + e ! H (5) ad O + 2H O + 4e ! 4OH (6) 2 2 -FeOOH + H + e !
-FeOHOH (7) 3
-FeOOH + H + e ! Fe O + 2H O (8) 3 4 2 Under the wet–dry cycle condition, the dominant cathodic reactions of steel corrosion in different states possess some differences. Evans [64] and Stratmann et al. [65,66] proposed that the corrosion process of steel in the simulated atmospheric corrosion environment can be divided into three stages: wetting of the dry surface, wet surface and drying-out of the surface. Meanwhile, they found that the cathodic reduction reactions of ferric species within the corrosion products were significantly enhanced when the oxygen reduction reaction was weakened in the wetting stage, whereas the oxygen reduction reaction was dominant in the wet stage. During tidal cycles, the oxygen is prone to diffuse through a thin electrolyte film to steel surface when it is exposed to atmosphere. In this case, the oxygen reduction reaction (Equation (6)) and ferric species reduction reactions (Equations (7) and (8)) are dominant in cathodic reactions. However, this does not mean that hydrogen reduction reaction (Equation (5)) is inactive in this stage, in fact the opposite is true, because the hydrolysis of corrosion products can result in acidification of the electrolyte beneath the rust layer (Equations (9) and (10)). Meanwhile, the acidity of the electrolyte is further enhanced with the evaporation of water during exposure, the pH value of which can be even lower than 3 with the presence of chloride ions [11,67]. 3+ + Fe + Cl + OH ! FeOCl + H (9) + + + FeCl + H O ! FeOH + H + Cl (10) Based on Nernst equation, the equilibrium potential of the hydrogen evolution reaction at 25 C can be expressed as: E = 0.0591pH (11) H /H The equilibrium potential of the hydrogen evolution reaction in natural seawater (pH = 7.5) is about 0.684 V , whereas it is only 0.177 V when the pH of the SCE SCE electrolyte under the rust layer drops to 3 during the drying stage. It is seen from Figure 15c that the value of the OCP of steel is about 0.66 V during the drying stage. Therefore, a SCE lower OCP and pH during the middle and late drying stage can enhance the activity of the hydrogen evolution reaction, thereby resulting in a higher hydrogen permeation current, as shown in Figure 15a. However, the pH of the electrolyte beneath the rust layer is still high in the initial drying stage, which brings difficulties to the hydrogen evolution owing to a higher overpotential, thus leading to the persistent decrease in the hydrogen permeation current. Accordingly, cathodic reactions (Equations (5)–(8)) can actively proceed in the drying stage during the tidal cycles, although the contribution of the hydrogen reduction reaction to corrosion weight loss of steels may be only about 0.1% [11]. In contrast, the pH of the electrolyte beneath the rust layer is basically the same as that of sea water owing to the good ion transport between sea water and the electrolyte beneath the rust layer. Therefore, hydrogen evolution under immersion condition is more difficult to proceed in the case where the OCP of the steel is almost equal during the drying and wetting stages. This reasonably explains the continuous decrease in the hydrogen permeation current in the middle and the last wetting stages. Meanwhile, the supply of oxygen to the surface of the rusty steel is insufficient in the wetting stage. This can be attributed to two aspects. On one hand, the steel surface left in contact with electrolyte by the rust layer is very small, limiting substances exchange. On the other hand, tortuous Appl. Sci. 2022, 12, 2785 17 of 20 pores of nanometric width in the thick rust layer add more difficulty to the diffusion of oxygen. Therefore, the corrosion of the steel is relatively mild in the wetting stage, and the corrosion process is controlled by the diffusion of dissolved oxygen [68]. Therefore, the lower corrosion rate of the steel is also another reason for the lower hydrogen permeation current in the wetting stage. Meanwhile, it is also due to the temporary inhibition of the rust layer on the diffusion of hydrogen ions, which causes the brief rise in the hydrogen permeation current at the initial wetting stage, as shown in Figure 15. 4. Conclusions Hydrogen permeation behavior and the mechanism of AISI 4135 steel in different marine corrosion zones were investigated using an in situ hydrogen permeation monitor- ing system via outdoor and indoor hydrogen permeation tests. The conclusions can be summarized as follows: (1) A good performance of the in situ hydrogen permeation monitoring system was present in in both the outdoor and indoor hydrogen permeation tests. Hydrogen permeation into steel during corrosion in different marine corrosion zones can be evaluated with good accuracy through the collected hydrogen permeation current data during outdoor testing. (2) The 3-month outdoor hydrogen permeation tests showed that the diffusible hydrogen content of steels in the marine atmospheric, splash, tidal and immersion zones were 3 2 2 2 3.15 10 , 7.00 10 , 2.06 10 and 3.33 10 wt ppm, respectively. (3) It was found that the hydrogen permeation current density is positively correlated with the corrosion rate of the steel in the marine environment. A large hydrogen permeation current of the steel exposed to the immersion zone was observed during the outdoor test, which is considered to be related to the formation of FeS in the corrosion products caused by SRB. (4) There are no obvious changes in the OCP of the steel during tidal cycles, and the change in the hydrogen permeation current is mainly controlled by the pH of elec- trolyte and oxygen concentration beneath the rust layer. Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/app12062785/s1, Figure S1: Details of dimensional information for metal specimens; Figure S2: Macro-morphologies on the cross-section of the sensors after exposure in the atmospheric zone (a), the splash zone (b), the tidal zone (c), and the immersion zone (d), respectively. Author Contributions: Data curation, Y.X.; Formal analysis, D.L.; Funding acquisition, Y.H.; Method- ology, L.Y.; Project administration, Y.H.; Software, F.C.; Validation, Z.W.; Writing—original draft, Y.X.; Writing—review & editing, X.W. All authors have read and agreed to the published version of the manuscript. 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