- Email: [email protected]
On the propagation of a single pit in stainless steel
Journal Pre-proofs Full Length Article On the Propagation of a Single Pit in Stainless Steel Jinyang Zhu, Zhangshuai Fan, Lining Xu PII: DOI: Reference:
S0169-4332(20)30380-9 https://doi.org/10.1016/j.apsusc.2020.145624 APSUSC 145624
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
28 November 2019 8 January 2020 1 February 2020
Please cite this article as: J. Zhu, Z. Fan, L. Xu, On the Propagation of a Single Pit in Stainless Steel, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145624
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
On the Propagation of a Single Pit in Stainless Steel Jinyang Zhu a,1, Zhangshuai Fan b,1, Lining Xu b,* a National
Center for Materials Service Safety, University of Science and Technology Beijing, Beijing 100083, China
b Corrosion
and Protection Center, Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
1These
authors contributed equally to this study and share the first authorship
*Correspondence
author, Email: [email protected], Tel: +86 10 62334410
Abstract Single pits were produced on 10Cr and 12Cr stainless steels through an in-house experimental setup. The pitting propagation was investigated by confocal laser scanning microscope (CLSM), scanning electron microscope (SEM), electron diffraction spectroscopy (EDS), and electrochemical tests for simulating potentials inside and outside the pit in chloride solution. The results showed that the pit depth of 10Cr and 12Cr steel increased rapidly in the earlier stage, whereas the rate of increase declined significantly in later periods. The pit mouth diameter increases almost linearly with time. The pit cover and the metal salt film in the pit played important roles in pitting propagation. The cover served as a primary barrier to diffusion in the early stage of corrosion, and the pit bottom was under diffusion control. After a longer immersion time, the cover might collapse and disappear. A metal salt film formed at the pit bottom, and the pit bottom propagation was under diffusion control. Keywords: Pitting corrosion; Stainless steel; Single pit; Metal salt film; Pit cover
1
Introduction Pitting frequently occurs on passivated metal surfaces that are exposed to an aggressive
environment [1-3]. The most commonly aggressive ion is chloride ion, which is easily found in many natural and industrial environments. Once pit formation initiates on the metal surface, aggressive chemistry quite different from the bulk solution develops inside the pit. It 1
is characterised by a higher concentration of aggressive anions and lower pH, both of which are necessary to maintain stable pit growth [4-10]. The remarkably reduced pH values are mainly attributed to Cr ions [11]. The electrolyte inside the pit has high viscosity because of the mass of chloride and metal ions [12-14]. Frankel [15] reported that metastable pits would repassivate when the pit cover ruptured to form openings. Many researchers have also observed a layer covering mature pits with a number of holes distributed around the pit mouth [5-8,16-19]. Covered pits are dangerous in practice because they are stable against the loss of their internal environment by diffusion or convection, especially if there is precipitated material over the pit mouth [20]. Whether such layers are the residue of the passive film [6,21-23], metal [24-26], or both has long been subject to debate, and this depends upon the size of the pit being considered [18,19]. When the pit reaches a critical size, the pit cover is destroyed (or at least becomes ineffectual as a diffusion barrier), resulting in the formation of an open hemispherical cavity [27,28]. Once a pit has achieved stability, the pit depth can provide its own diffusion barrier to sustain the pit anolyte, and the cover is no longer necessary [18]. At the pit bottom, when the concentration of metal cations in the pit reaches solid solubility, the metal salt deposits on the surface of the pit bottom [18,29]. Vetter and Strehblow [30] reported the formation of a nonporous salt film. Isaacs [31] used an artificial pit electrode to study the pitting corrosion of stainless steel in chloride solutions and demonstrated that a resistive layer existed at the electrode interface. He also reported that this layer was approximately 5 μm thick and had a resistivity of approximately 108 Ω·cm. The main component was FeCl2•4H2O and a small amount of CrCl3•6H2O [18,32-34]. The salt film had a duplex structure, which included an inner compact anhydrous layer with high field conduction and an outer porous hydrated layer with low field conduction [35-37]. In the past few decades, many researchers have studied the propagation of single pits. These studies have investigated 1-D single pits where pit propagation was simulated by dissolving a small wire embedded in an insulator [38-41], 2-D single pits grown in stainless steel foils [13,18,19,42-45], and 3-D single pits obtained by injecting sodium chloride 2
solution [46-49]. Almost all single pits were initiated and propagated by applying potential, rather than being formed under practical environments. In the present study, an improved method for inducing a single pit without applying potential on steel is presented. Two stainless steels (10Cr and 12Cr) were chosen as the experimental materials. It is worthwhile to note that no Mo was added into the steels used in this work. By using these steels and the method for inducing a single pit without applying potential, the pitting propagation of single pits on 10Cr and 12Cr stainless steels in sodium chloride solution was investigated, and the potentials of the pit bottom and passive metal surrounding the pit were also determined. Special attention was paid to the pit cover and salt film in investigating the pitting propagation mechanism for stainless steel in chloride solution. 2
Experimental details
2.1. Experimental set up for initiation and propagation of a single pit The components of 10Cr and 12Cr steels used in this study are displayed in Table 1. Quenching was performed through heating the steel samples at 900 °C for 40 min, followed by water cooling. Tempering was performed at 660 °C for 50 min and followed by air cooling. The specimens used in this experiment were sealed into epoxy, and the size of the exposed surface was 10 mm × 10 mm. As depicted in Fig. 1, an improved experimental installation was set up on the basis of previous methods [46-49]. It mainly consisted of a glass microcapillary, a peristaltic pump, two beakers, and a water bath. Beaker A containing 0.05 M NaCl solution was immersed in a water bath at a temperature of 80 ℃. One end of the peristaltic pump hose was extended into Beaker B containing 5 M NaCl solution, and the other end was connected to a capillary tube with an inner diameter of 100 μm and an outer diameter of 1000 μm. A plexiglass plate was fixed on top of Beaker A to position the capillary and specimen. To induce a single pit instead of several pits, the distance from the microcapillary to the surface of the specimen could not exceed 300 μm. The peristaltic pump pumped 5 M NaCl solution through the capillary, so that the concentration of Cl− in a small portion of the specimen surface immediately below the ending of capillary could reach 5 M. 3
A single pit was initiated and propagated in this area of high Cl− concentration. It is worthwhile to note that this method was not a new method, but an improved method for inducing a single pit. Almost all single pits were initiated and propagated by applying potential in previous studies. The applied potential might change the electric double layer and some other electrochemical characteristics. Increasing the temperature only controlled the kinetics and thermodynamics, purely accelerating the pitting propagation. Therefore, in order to reduce the effect of the applied potential on the pitting process, no potential was applied on the steel in this work. Instead, the temperature and the Cl− concentration were increased for inducing a single pit. In this experiment, by instilling 5 M NaCl solution, a single pit was induced and allowed to initiate and propagate for 1 day on the steel surface in 0.05 M NaCl solution at 80 ℃; then, the specimens with single pits were immersed in 0.6 M NaCl solution at 30 ℃ for a long period to research the pit propagation on 10Cr steel. Pitting corrosion on 12Cr steel was conducted for comparison. Notably, the flow rate of the solution in the capillary tube was controlled at 0.5 mL per h through the peristaltic pump. Thus, the concentration of the bulk solution was kept low (i.e., no higher than 0.06 M after 1 day). Stolica [50] reported 0.1 M as the minimum concentration of Cl− necessary for pit initiation on Fe-20Cr when anodic potential was applied. During the experiment, it was observed that only a single pit initiated and propagated on the surface of the specimen; thus, the Cl− concentration of 0.06 M did not affect other parts of the specimen surface. Pit morphology after pickling was investigated using a LEXT OLS-4100 Confocal Laser Scanning Microscope (CLSM) with a 405-nm laser diode and double confocal optical system. It is possible to perform high quality imaging using OLS-4100 for a maximum resolution of 10 nm. OLS-4100 can measure pit depth and pit mouth diameter precisely to construct the 3D pit model. The pickling process was performed according to the ASTM G1-03 standard [51]. The cross-sectional morphologies of the pit were observed using the quadrant back scattering mode of a scanning electron microscope (SEM). The composition of the salt film 4
and corrosion products were investigated using electron diffraction spectroscopy (EDS). The cross-sections of the pits were grinded with silicon carbide sandpaper, which did not always pass through the pit centres. Therefore, sometimes, the pit mouth was not displayed in cross-sectional morphology. 2.2. Electrochemical tests for simulating potentials inside and outside the pit The active metal inside the pit acts as a small anode, and the passive metal surrounding the pit mouth acts as a big cathode; galvanic coupling between the cathode and anode occurs, and this process accelerates the development of the pit. Because it is difficult to directly measure the potentials inside and outside the pit, a simple method was proposed to measure the potentials of a small anode and a large cathode in chloride solution to simulate the potential inside and outside the pit. Although this method could not completely simulate the real galvanic effect on steel surface, it could be used as an approximate equivalent to help us to obtain the potential information inside and outside the pit. As depicted in Fig. 2, the anode and cathode were fabricated from the same material, such that a small anode (surface area 1 mm2) was close to but separated from a large cathode (surface area 1000 mm2). Electrical wires were welded to the anode and cathode, and the whole electrode was mounted in epoxy resin. A switch was installed to control the anode–cathode connection externally. During the test, the anode and cathode were immersed in the solution, so that they were connected by it. Two steel surfaces, serving as the cathode and anode, were polished using a 200-, 400-, and 600-grit sand paper in sequence. The specimens were kept in air for 24 h to ensure the formation of a passive film. After that, the anode was potentiostatically polarised at 0.8 V/SCE for 40 min, so that the surface of the anode would be corroded and filled with corrosion products. The anode surface, which was covered by corrosion products, was lower than that of the epoxy and cathode. Therefore, a 1-D artificial pit formed at the position of the anode. The mixed potential was monitored while the anode and cathode were connected, and the open circuit potential (OCP) of the disconnected anode and cathode was also measured. The test solution in the simulated experiment was 0.6 M NaCl solution, and the test 5
temperature was 30 ℃. 3
Results and discussion
3.1 Formation of a single pit on stainless steel Fig. 3(a) illustrates the macroscopic morphologies of a single pit, which was formed by instilling 5 M NaCl solution for 1 day only (without immersion in 0.6 M NaCl solution). The estimated concentration of Cl− in bulk solution after instilling 5 M NaCl solution for 1 day was 0.06 M. Therefore, except for the area near the glass microcapillary, the sample surface was immersed in a solution with a low Cl− concentration. Thus, a single pit was created on stainless steel, as illustrated in Fig. 3(a). Fig. 3(b) depicts the macroscopic morphology of a single pit, which was formed by instilling 5 M Cl− solution for 1 day and by subsequently immersing the steel in 0.6 M NaCl solution. After the additional day of immersion in 0.6 M NaCl solution, no other new pits were initiated, and the formation of an obvious product ring around the initial pit was observed. These results confirmed the feasibility of the experimental method for forming a single pit on stainless steel surface in the present study. 3.2 CLSM results Fig. 4 displays the CLSM images of single pits on 10Cr steel after cleaning. These pits were formed by instilling 5 M NaCl solution at 80℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for various periods. Overall, the pits on 10Cr steel were open and hemispherical. With prolonged immersion, the pits grew larger and deeper. Moreover, no other pits were observed around the pits after instilling 5 M NaCl solution for 1 day. Even after the longer period of immersion in 0.6 M NaCl solution (7~35 days), no new pit was found on the steel surface, as shown in Figs. 4(b-f). Fig. 5 presents the morphological parameters of single pits formed at various immersion times at 30 °C in 0.6 M NaCl solution. The pit depth was measured at the deepest position inside the pit. At least three samples were measured for each period, which confirmed that the data was more reliable. From Fig. 5, the increase in pit depth generally conformed to a parabolic curve. The pit depth can be expressed as a parabolic function of time: D = −0.4t2 + 6
26.9t + 208.4, where D is the pit depth, and t is the immersion time for pit development in 0.6 M NaCl solution. The pit depth increased rapidly in the first 14 days, whereas the rate of increase declined significantly in later periods. This trend of pit depth increase was probably due to the corrosion in the pit being under diffusion control [5-8,18,52-54]. The shape of the pit mouth was often irregular. As depicted in Fig. 6, the diameter of the pit mouth was obtained by calculating the area of the pit mouth and then calculating the corresponding diameter in a perfect circle. The diameter of the pit mouth increased almost linearly with time and demonstrated a higher rate of increase than pit depth. These results indicated that corrosion at the pit mouth was probably under activation control. Fig. 7 provides the CLSM images of single pits formed on 12Cr steel immersed in 0.6 M NaCl solution for 28 days and 35 days after pickling. From Fig. 7, it can be observed that the side walls of the 12Cr pit were always vertical, which meant that the true morphology of the pit could not be reflected using the CLSM method because the pit on the 12Cr steel was pear shaped. The true morphology of the pit on 12Cr steel will be displayed in the cross-section images below. From the CLSM images in Figs. 4 and 7, the depth and mouth diameter of the pits on 10Cr and 12Cr after immersion for 28 days and 35 days were obtained and compared, as depicted in Fig. 8. The depth of the pit on 12Cr steel was slightly smaller than that of the pit on 10Cr (Fig. 8(a)). However, as illustrated in Fig. 8(b), the diameter of the pit mouth on 10Cr steel was nearly three times larger than that of the pit on 12Cr steel after both 28 and 35 days. In general, the pitting resistance of these 10Cr and 12Cr steels was relatively poor, of which the pitting depth have reached more than 500 μm after immersion for 38 days. 3.3 Metal salt inside pit After instilling 5 M Cl solution on the surface of 10Cr steel for 1 day and immersing the steel in 0.6 M NaCl solution for 35 days, a single pit was formed. The cross-sectional morphology and EDS map scanning results of this pit are displayed in Fig. 9. Due to peeling caused by the corrosion product in the pit during the preparation of the cross-section sample, a 7
part of corrosion products at the pit bottom was absent. At the pit bottom, when the concentration of metal cations in the pit reaches saturation, the metal salt will be deposited. The main components of the metal salt film are FeCl2•4H2O and a small amount of CrCl3•6H2O [18,32-34]. Therefore, regarding composition, the metal salt film is characterised by a large amount of Cl, whereas the proportion of Fe is higher than that of Cr. The metal salt film can be traced by the distribution of Cl in the elemental maps. As depicted in Fig. 9(b), a Cl-enriched layer was observed at the pit bottom and sidewall. Cl was enriched both in the salt film at the pit bottom and in the corrosion product that filled the pit. The thickness of the metal salt film that formed at the pit bottom was approximately 14.5 µm. To further explore the differences in the composition of different regions of the pit, a detailed EDS analysis was performed at five typical positions, as demonstrated in Fig. 9(a). Points (A, B) and C were located on the salt film covering the pit sidewall and the pit bottom, respectively. Points D and E were located on corrosion products inside the pit and corrosion products stacked around the pit mouth, respectively. As shown in Fig. 10, the concentrations of Cl at points A and B reached more than 60%, which were much higher than that at point C (~25%). The distribution of Cl in the salt film was not uniform. However, the concentration of O increased as metal cation hydrolysed inside the pit. The concentration of O reached approximately 30% and 75% at points D and E, respectively, which meant that the metal cations away from pit bottom and sidewall hydrolysed more seriously [40]. Fig. 11 displays the cross-section morphology and elemental maps of a single pit on 12Cr steel surface. This pit was formed by instilling 5 M Cl solution for 1 day and by immersing the steel in 0.6 M NaCl solution for 35 days. The pit mouth is absent in Fig. 11. This was caused by the pear shape of pits on 12Cr steel, as displayed in Fig. 12. The pit on the 10Cr steel surface often had a hemispherical shape (Fig. 9(a)), whereas the pit on 12Cr steel often demonstrated a pear shape. The difference in pit morphologies between 12Cr and 10Cr may be related to the different Cr contents in substrates. The formation mechanism of the pear-shaped pit on 12Cr steel requires further exploration. Similar to 10Cr steel, Cl was 8
enriched in the pit bottom and sidewall of the 12Cr steel, as seen in Fig. 11(b), indicating the formation of a metal salt film inside the pit. The direct observation of the metal salt film has seldom been reported in the literature. Fig. 13 displays the back scattered electron image and elemental maps of the cross-section of a single pit on 12Cr steel. This pit was formed by instilling 5 M Cl solution for 1 day and by immersing the steel in 0.6 M NaCl solution for 49 days. The single pits displayed in Figs. 11 and 13 are not from the same specimen. Compared with the pit after 35-day immersion (Fig. 11), the total amount of Cl inside the pit after 49-day immersion (Fig. 13) significantly increased. The pit was filled with corrosion products containing Cl. In addition, the Cl content at the pit bottom in region A (Fig. 13(b)) was approximately 42%, which was obviously higher than that for corrosion products in region B (~ 25%) in the pit. After immersion for 49 days, a considerable quantity of Cl shifted from the solution outside the pit to inside the pit; furthermore, metal chloride was deposited at the pit bottom, and a salt film also formed there. 3.4 Observations of the cover on pit Some researchers have suggested that the 2D pit contained a perforated lacy metal cover, which hindered the diffusion of ions in the pit and which collapsed with a prolonged period of applied potential on the specimens [13,18,19,42-45]. However, the cross-section morphology of the cover on the pit was difficult to obtain for observation. The direct observation of the cover has rarely been reported. In the present study, a single pit was produced, and the cover on the pit was observed directly from the cross-section. Fig. 14 displays the cross-section morphologies and elemental maps of a single pit on 10Cr steel formed by instilling 5 M NaCl solution for 1 day and immersing the steel in 0.6 M NaCl solution for 7 days. A Fe-depletive and Cr-rich cover was found on the pit mouth, as indicated by the arrow in Fig. 14(a). This might be caused by the selective dissolution on the steel substrate. Fe dissolved preferentially and entered into the solution, whereas Cr-rich film grew in situ. However, the cover could not be maintained at all times and tended to collapse when corrosion proceeded; thus, the residual cover was extremely small, as depicted in Fig. 14. On the left side, the sidewall of the pit was 9
filled with corrosion products. The collapsed cover might be contained in these corrosion products. The Cr content of the cover on the right was higher than that of the corrosion product filling the left sidewall. Fig. 15 also displays the cross-section morphologies and elemental maps of a single pit on 10Cr steel formed by instilling 5 M NaCl solution for 1 day and immersing the steel in 0.6 M NaCl solution for 7 days. Compared with the pit in Fig. 14, the cover appears longer but thinner. The cover was inclined towards the bottom of the pit and suggested a certain degree of collapse. This observation represented direct evidence that the cover on the pit could collapse during the corrosion process. Compared with the pits in Figs. 9, 14, and 15, the cover existed on the pit after immersion for 7 days; however, it collapsed and disappeared after immersion for 35 days. This observation further proved that the cover tended to collapse with corrosion. The pit cover served as a primary barrier to diffusion of Fe2+ and Cr3+. These ions hydrolysed considerably inside the pit, and the anolyte became acidic with the generation of H+. In addition, some metal lamellae on the pit sidewall were observed (Fig. 9), which may also hinder the diffusion of ions. Therefore, the pH of the anolyte within an active pit can become extremely low for stainless steel—which promotes the corrosion of substrates in the pit. In addition, Cl− from bulk solution migrated into the pit to maintain electrical neutrality. The content of Cl in a local area of pit often surpassed 70% after immersion for 35 days (see Fig. 10). In summary, the cover of the pit, corrosion products, and metal salt film in the pit consisted of the diffusion barrier of ions. A local aggressive environment with lower pH and higher Cl− concentration was produced in the pit. This development explained the spontaneous propagation of the single pit. However, Cl enrichment was not observed in the pit, which had developed after immersion in 0.6 M NaCl solution for 7 days. It seems that the metal salt film only existed in the pits that had developed over a longer period. 3.5 Galvanic effect between the inside and outside pit From the electrochemical measurements of the specimen depicted in Fig. 2, the potentials of isolated Specimens A and B were obtained, as depicted in Fig. 16. For both 10Cr 10
and 12Cr steel, the potentials of Specimen A were more negative than those of Specimen B. The coupled potential was between and closer to the potential of isolated Specimen B due to the much larger surface of Specimen B. As mentioned in Section 2.2, Specimen A simulated the potential inside the pit, and Specimen B simulated the potential outside the pit. Therefore, from the results in Fig. 16, it can be concluded that the potential inside the pit on 10Cr steel (−0.65 V/SCE) was more negative than that inside the pit on 12Cr steel (−0.43 V/SCE). This result indicated that the activity of the metal inside the pit on 10Cr steel was higher than that of the metal inside the pit on 12Cr steel. The potential difference between the isolated Specimens A and B of 10Cr steel was 0.16 V, whereas that was only 0.04 V for 12Cr steel. This result indicated that the acceleration effect caused by the potential difference between the pit and the surrounding metal for 10Cr steel tended to be more obvious than for 12Cr steel. This explains the difference in pit propagation between 10Cr and 12Cr steel, as depicted in Fig. 8. When Specimens A and B were coupled, a relatively low coupled potential (−0.55 V/SCE) was observed for 10Cr steel. At such low potentials cathodic reduction of hydrogen ions would be expected to constrain a very low pH in a pit. By contrast, the coupled potential for 12Cr steel was more positive. Therefore, the pH in pit on 10Cr steel should be lower than that on 12Cr steel. This also explains the difference in pit propagation between 10Cr and 12Cr steel, as depicted in Fig. 8. Above all, the pitting propagation of stainless steel in chloride solutions was mainly controlled by two factors: self-catalytic effect in the pit and galvanic effect between the inside and outside of the pit. For the self-catalytic effect, the cover on the pit and the metal salt film played a crucial role. In the early stage (~7 days), the cover served as a primary barrier to diffusion and maintained the pit’s internal electrolytes at a sufficiently aggressive concentration [11,15,25,33]. The cover on the pit produced a closed environment, which made substance exchange difficult between the pit and external solution. Fe2+ and Cr3+ hydrolysed severely inside the pit, and the anolyte became acidic with the generation of H+. Furthermore, Cl− with a small diameter from the bulk solution would migrate into the pit to maintain electrical neutrality. In the presence of FeCl2 and CrCl3, H+ activity increased 11
considerably. Therefore, severe acidification occurred in the solution inside the pit. Such an aggressive environment makes pit propagation spontaneous and rapid. After a longer immersion time (~35 days), the cover may collapse and disappear. The metal salt film formed at the pit bottom and sidewall. The local concentration gradient around the metal salt film promoted the corrosion of substrate under the metal salt film. The pit bottom propagation was under diffusion control. For the galvanic effect, the electrochemical tests simulating the internal and external potentials of a single pit confirmed a potential difference between the inside and outside of the pit, which could promote the propagation of pits. What is noticeable is the low coupled potential, especially for 10Cr steel. At such low potentials (below −0.5 V/SCE) cathodic reduction of hydrogen ions would be expected to constrain a very low pH in a pit, which could also promote the propagation of pits. However, it is not clear which of the aforementioned two effects was stronger or dominant in the different pitting propagation stages for stainless steel. This issue requires further in-depth research. 4
Conclusions In the present study, a single pit was produced on 10Cr and 12Cr stainless steel using an
in-house experimental setup. The evolution of both the pit depth and pit diameter was investigated. The pit cover and metal salt film in the pit were observed, and the pit propagation mechanism was discussed. From this study, the following conclusions can be drawn: An improved method to create single pits on the steel surface based on previous work is presented in the present paper, and the test results confirm the feasibility of this method. The pit depth increased rapidly in the earlier stage, whereas the rate of increase declined significantly in later periods. This indicates that the pit bottom might be under diffusion control. The pit mouth diameter increases almost linearly with time, which indicates a much higher rate of increase compared with pit depth. The pitting propagation of stainless steel is mainly controlled by the self-catalytic effect in the pit and the galvanic effect between the inside and outside of the pit. 12
The pit cover and the metal salt film in the pit play important roles in pitting propagation. The cover serves as a primary barrier to diffusion in the early stage of corrosion. After a longer immersion time, the cover might collapse and disappear. A metal salt film forms at the pit bottom.
Acknowledgments This work was supported by the National Nature Science Foundation of China under grant No. 51871025 and Fundamental Research Funds for the Central Universities under grant No. 06500118.
Data Availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
References [1]
Z.H. Jin, H.H. Ge, W.W. Lin, Corrosion behavior of 316L stainless steel and anti-corrosion materials in a high acidified chloride solution, Appl. Surf. Sci. 322 (2014) 47–56.
[2]
H. Feng, Z.H. Jiang, H.B. Li, P.C. Lu, S.C. Zhang, B.B. Zhang, T. Zhang, D.K. Xu, Z.G. Chen, Influence of nitrogen on corrosion behavior of high nitrogen martensitic stainless steels manufactured by pressurized metallurgy, Corros. Sci. 144 (2018) 288– 300.
[3]
Z.B. Wang, H.X. Hu, Y.G. Zheng, Synergistic effects of fluoride and chloride on general corrosion behavior of AISI 316 stainless steel and pure titanium in H2SO4 solutions, Corros. Sci. 130 (2018) 203–217.
[4]
G.T. Burstein, C. Liu, R.M. Souto, S.P. Vines, Origins of pitting corrosion, Corros. Eng. Sci. Technol. 39 (2004) 25–30.
[5]
P.C. Pistorius, G.T. Burstein, Growth of corrosion pits on stainless steel in chloride solution containing dilute sulphate, Corros. Sci. 33 (1992) 1885–1897.
[6]
G.T. Burstein, S.P. Mattin, Nucleation of corrosion pits on stainless steel, Philos. Mag. Lett. 66 (1992) 127–131.
13
[7]
P.C. Pistorius, G.T. Burstein, Aspects of the effects of electrolyte composition on the occurrence of metastable pitting on stainless steel, Corros. Sci. 36 (1994) 525–538.
[8]
G.T. Burstein, S.P. Vines, Repetitive nucleation of corrosion pits on stainless steel and the effects of surface roughness, J. Electrochem. Soc. 148 (2001) B504–B516.
[9]
K. Sasaki, G.T. Burstein, Erosion–corrosion of stainless steel under impingement by a fluid jet, Corros. Sci. 49 (2007) 92–102.
[10] S.P. Mattin, G.T. Burstein, Detailed resolution of microscopic depassivation events on stainless steel in chloride solution leading to pitting, Philos. Mag. Lett. 76 (1997) 341– 347. [11] T. Suzuki, M. Yamabe, Y Kitamura, Composition of anolyte within pit anode of austenitic stainless steels in chloride solution, Corrosion 29 (1973) 18–22. [12] W. Tian, N. Du, S. Li, S. Chen, Q, Wu, Metastable pitting corrosion of 304 stainless steel in 3.5% NaCl solution, Corros. Sci. 85 (2014) 372–379. [13] N.J. Laycock, S.P. White, Computer simulation of single pit propagation in stainless steel under potentiostatic control, J. Electrochem. Soc. 148 (2001) B264–B275. [14] N.J. Laycock, R.C. Newman, Localised dissolution kinetics, salt films and pitting potentials, Corros. Sci. 39 (1997) 1771–1790. [15] G.S. Frankel, L. Stockert, F. Hunkeler, H. Boehni, Metastable pitting of stainless steel, Corrosion 43 (1987) 429–436. [16] N.J. Laycock, S.P. White, J.S. Noh, P.T. Wilson, R.C. Newman, Perforated covers for propagating pits, J. Electrochem. Soc. 145 (1998) 1101–1108. [17] M.H. Moayed, R.C. Newman, Evolution of current transients and morphology of metastable and stable pitting on stainless steel near the critical pitting temperature, Corros. Sci. 48 (2006) 1004–1018. [18] P. Ernst, R.C. Newman, Pit growth studies in stainless steel foils. I. Introduction and pit growth kinetics, Corros. Sci. 44 (2002) 927–941. [19] P. Ernst, R.C. Newman, Pit growth studies in stainless steel foils. II. Effect of temperature, chloride concentration and sulphate addition, Corros. Sci. 44 (2002) 943– 954. [20] P. Ernst, N.J. Laycock, M.H. Moayed, R.C. Newman, The mechanism of lacy cover formation in pitting, Corros. Sci. 39 (1997) 1133–1136. [21] J. Yahalom, L.K. Ives, J. Kruger, On the nature of films over corrosion pits in stainless steel, J. Electrochem. Soc. 130 (1973) 384–386. [22] H.S. Isaacs, G. Kissel, Surface preparation and pit propagation in stainless steels, J. Electrochem. Soc. 119 (1972) 1628–1632. [23] G.S. Frankel, L. Stockert, F. Hunkeler, H. Boehni, Metastable pitting of stainless steel, Corrosion 43 (1987) 429–436. 14
[24] N. Sato, T. Nakagawa, K. Kudo, M. Sakashita, Chloride pitting dissolution of rotating stainless steel electrode in acid solution, in: R.W. Staehle, B.F. Brown, J. Kruger, A. Agrawal (Eds.), U.R. Evans Conference on Localized Corrosion, NACE, 1971, p. 447. [25] W. Schwenk, Theory of stainless steel pitting, Corrosion 20 (1964) 129t–137t. [26] J. Mankowski, Z. Szklarska-Smialowska, Studies on accumulation of chloride ions in pits growing during anodic polarization, Corros. Sci. 15 (1975) 493–501. [27] G.T. Gaudet, W.T. Mo, T.A. Hatton, J.W. Tester, J. Tilly, H.S. Isaacs, R.C. Newman, Mass transfer and electrochemical kinetic interactions in localized pitting corrosion, AIChE J. 32 (1986) 949–958. [28] R.C. Newman, Local chemistry considerations in the tunnelling corrosion of aluminium, Corros. Sci. 37 (1995) 527–533. [29] P. Ernst, R.C. Newman, Explanation of the effect of high chloride concentration on the critical pitting temperature of stainless steel, Corros. Sci. 49 (2007) 3705–3715. [30] K.J. Vetter, H.H. Strehblow, Formation and shape of corrosion pits in localized corrosion on iron theoretical results pertaining to localized corrosion. Berichte Der Bunsen-Gesellschaft Fur Physikalische Chemie. 74 (1970) 1024–1035. [31] H.S. Isaacs, The behavior of resistive layers in the localized corrosion of stainless steel, J. Electrochem. Soc. 120 (1973) 1456–1462. [32] H.S. Isaacs, J.H. Cho, M.L. Rivers, S.R. Sutton, In-situ X-ray microprobe study of salt layers during anodic-dissolution of stainless-steel in chloride solution, J. Electrochem. Soc. 142 (1995) 1111–1118. [33] N. Sridhar, D.S. Dunn, Effect of applied potential on changes in solution chemistry inside crevices on type 304l stainless steel and alloy 825, Corrosion 50 (1994) 857–872. [34] T. Rayment, A.J. Davenport, A.J. Dent, J.P. Tinnes, R.J.K. Wiltshire, C. Martin, G. Clark, P. Quinn, J.F.W. Mosselmans, Characterisation of salt films on dissolving metal surfaces in artificial corrosion pits via in situ synchrotron X-ray diffraction, Electrochem. Commun. 10 (2008) 855–858. [35] C. Clerc, D. Landolt, AC impedance study of anodic films on nickel in LiCl, Electrochim. Acta. 33 (1988) 859–871. [36] R. D. Grimm, D. Landolt, Salt films formed during mass transport controlled dissolution of iron-chromium alloys in concentrated chloride media. Corros. Sci. 36 (1994) 1847–1868. [37] R.D. Grimm, A.C. West, D. Landolt, AC impedance study of anodically formed salt films on iron in chloride solution, J. Electrochem. Soc. 139(1992) 1622–1629. [38] E.D. Parsons, H.H. Cudd, H.L. Lochte, Synthetic corrosion pits and the analysis of their contents, J. Phys. Chem. 45 (1941) 1339–1345. 15
[39] J.W. Tester, H.S. Isaacs, Diffusional effects in simulated localized corrosion, J. Electrochem. Soc. 122 (1975) 1438–1445. [40] W. Tian, S. Li, N. Du, S. Chen, Q. Wu, Effects of applied potential on stable pitting of 304 stainless steel, Corros. Sci. 93 (2015) 242–255. [41] N. Greene, M. Fontana, A critical analysis of pitting corrosion, Corrosion 15 (1959) 41–47. [42] N.J. Laycock, J.S. Noh, S.P. White, D.P. Krouse, Computer simulation of pitting potential measurements, Corros. Sci. 47 (2005) 3140–3177. [43] S.
Scheiner,
C.
Hellmich,
Stable
pitting
corrosion
of
stainless
steel
as
diffusion-controlled dissolution process with a sharp moving electrode boundary, Corros. Sci. 49 (2007) 319–346. [44] G.S. Frankel, The growth of 2-d pits in thin film aluminum, Corros. Sci. 30 (1990) 1203–1218. [45] S. Ghahari, D. Krouse, N. Laycock, T. Rayment, C. Padovani, T. Suter, R. Mokso, F. Marone, M. Stampanoni, M. Monir, Pitting corrosion of stainless steel: measuring and modelling pit propagation in support of damage prediction for radioactive waste containers, Corros. Eng. Sci. Techn. 46 (2011) 205–211. [46] N. Aouina, F. Balbaud-Célérier, F. Huet, S. Joiret, H. Perrot, F. Rouillard, V. Vivier, A flow microdevice for studying the initiation and propagation of a single pit, Corros. Sci. 62 (2012) 1–4. [47] N. Aouina, F. Balbaud-Célérier, F. Huet, S. Joiret, H. Perrot, F. Rouillard, V. Vivier, Initiation and growth of a single pit on 316L stainless steel: Influence of SO42− and ClO4− anions, Electrochim. Acta. 104 (2013) 274–281. [48] S. Heurtault, R. Robin, F. Rouillard, V. Vivier, Initiation and propagation of a single pit on stainless steel using a local probe technique, Faraday. Discuss. 180 (2015) 267–282. [49] S. Heurtault, R. Robin, F. Rouillard, V. Vivier, On the propagation of open and covered pit in 316l stainless steel, Electrochim. Acta. 203 (2016) 316–325. [50] N. Stolica, Pitting corrosion on FeCr and FeCrNi alloys, Corros. Sci. 9 (1969) 455–470. [51] ASTM G1-03, “Standard practice for preparing, cleaning, and evaluating corrosion test specimens” (West Conshohocken, PA: ASTM, 2011). [52] J.W. Tester, H.S. Isaacs, Diffusional effects in simulated localized corrosion, J. Electrochem. Soc. 122 (1975) 1438–1445. [53] L. Peguet, A. Gaugain, C. Dussart, B. Malki, B. Baroux, Statistical study of the critical pitting temperature of 22-05 duplex stainless steel, Corros. Sci. 60 (2012) 280–283. [54] T.S. Li, J.R. Scully, G.S. Frankel, Localized Corrosion: Passive Film Breakdown vs. Pit Growth Stability: Part III. A Unifying Set of Principal Parameters and Criteria for Pit Stabilization and Salt Film Formation, J. Electrochem. Soc. 165 (2018) C762–C770. 16
17
Figure Captions Figure 1. Schematic diagram of the experimental setup for generating a single pit on the steel surface. A glass microcapillary and a peristaltic pump were used to locally and slowly supply chloride ions on the steel surface. Figure 2. Schematic diagram of the specimen showing a small anode and a large cathode used in electrochemical tests for simulating the potentials inside and outside the pit: (a) top view; (b) side view. Figure 3. Photos of specimens with a single pit: (a) formed by instilling 5 M NaCl solution at 80 ℃ for 1 day, (b) formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 1 day. Figure 4. CLSM images of a single pit on 10Cr steel after cleaning. Pits were formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for various periods: (a) 1 day; (b) 7 days; (c) 14 days; (d) 21 days; (e) 28 days; (f) 35 days. Figure 5. Pit depth of a single pit on 10Cr steel as a function of immersion time in a 0.6 M NaCl solution at 30 ℃. Depth data were obtained from the deepest position inside the pit. All tests were repeated at least three times. Figure 6. Pit mouth diameter of a single pit on 10Cr steel as a function of immersion time in 0.6 M NaCl solution at 30 ℃. Diameter data were obtained by calculating the area of the pit mouth and then calculating the corresponding diameter in a perfect circle. All tests were repeated at least three times. Figure 7. CLSM images of a single pit on 12Cr steel after cleaning. Pits were formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for various periods: (a) 28 days, (b) 35 days. Figure 8. A comparison of pit depth (a) and pit mouth diameter (b) between 10Cr and 12Cr steel with various immersion times in a 0.6 M NaCl solution at 30 ℃. The initial depth and diameter data were obtained from the pit formed by instilling 5 M NaCl solution at 80 ℃ for 1 day. Figure 9. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 10Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 35 days. Figure 10. Elemental concentration (O, Cl, Cr, and Fe) of different positions displayed in Fig. 9(a). 18
Figure 11. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 35 days. Figure 12. Schematic diagram showing that the cross-section morphology of a single pit on 12Cr steel in Fig. 11 did not pass through the pit mouth. Figure 13. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 49 days. Figure 14. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 10Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 7 days. Figure 15. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 7 days. Figure 16. Potentials of isolated Specimens A and B in Fig. 2 and coupled potentials for 10Cr and 12Cr steel in 0.6 M NaCl solution at 30 ℃. The results were obtained using the specimen shown in Fig. 2 for simulating the potentials inside and outside the pit.
19
Figure 1. Schematic diagram of the experimental setup for generating a single pit on the steel surface. A glass microcapillary and a peristaltic pump were used to locally and slowly supply chloride ions on the steel surface.
20
Figure 2. Schematic diagram of the specimen showing a small anode and a large cathode used in electrochemical tests for simulating the potentials inside and outside the pit: (a) top view; (b) side view.
21
(a)
(b) Corrosion product
A single pit
A single pit
5mm
5mm
Figure 3. Photos of specimens with a single pit: (a) formed by instilling 5 M NaCl solution at 80 ℃ for 1 day, (b) formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 1 day.
22
500μm
500μm
500μm
500μm
500μm
500μm
Figure 4. CLSM images of a single pit on 10Cr steel after cleaning. Pits were formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for various periods: (a) 1 day; (b) 7 days; (c) 14 days; (d) 21 days; (e)
28 days; (f) 35 days.
23
900
Measured Fitted
800
Pit depth, μm
700 600 500 400 300 200 100 0
5
10
15
20
25
30
35
40
Time, d
Figure 5. Pit depth of a single pit on 10Cr steel as a function of immersion time in a 0.6 M NaCl solution at 30 ℃. Depth data were obtained from the deepest position inside the pit. All tests were repeated at least three times.
24
2500
Measured Fitted
Pit mouth diameter, um
2000
1500
1000
500
0 0
5
10
15
20
25
30
35
40
Time, d
Figure 6. Pit mouth diameter of a single pit on 10Cr steel as a function of immersion time in 0.6 M NaCl solution at 30 ℃. Diameter data were obtained by calculating the area of the pit mouth and then calculating the corresponding diameter in a perfect circle. All tests were repeated at least three times.
25
500μm
500μm
Figure 7. CLSM images of a single pit on 12Cr steel after cleaning. Pits were formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for various periods: (a) 28 days, (b) 35 days.
26
900
10Cr 12Cr
800
716.482
Pit depth, μm
700 600
573.427
500
552.795
617.036
400 300 200 100
initial pit depth for 10Cr
175.797 46.926
initial pit depth for 12Cr
0 1
25
0 30
35
Time, d
2500
10Cr 12Cr
Pit mouth diameter, μm
2000
1500
1000 initial pit mouth diameter for 10Cr
500 initial pit mouth diameter for 12Cr
0 0 1
25
30
35
Time, d
Figure 8. A comparison of pit depth (a) and pit mouth diameter (b) between 10Cr and 12Cr steel with various immersion times in a 0.6 M NaCl solution at 30 ℃. The initial depth and diameter data were obtained from the pit formed by instilling 5 M NaCl solution at 80 ℃ for 1 day.
27
Figure 9. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 10Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 35 days.
28
O Cl Cr Fe
80
Concentration, at %
70 60 50 40 30 20 10 0 A
B
C
D
E
Position in Fig. 8a
Figure 10. Elemental concentration (O, Cl, Cr, and Fe) of different positions displayed in Fig. 9(a).
29
Figure 11. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 35 days.
30
Figure 12. Schematic diagram showing that the cross-section morphology of a single pit on 12Cr steel in Fig. 11 did not pass through the pit mouth.
31
B A
Figure 13. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 49 days.
32
Figure 14. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 10Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 7 days.
33
Figure 15. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 7 days.
34
-0.35
Potential, V vs. SCE
-0.40 -0.45 -0.50 -0.55 12Cr-isolated A 12Cr-isolated B 12Cr-coupled
-0.60
10Cr-isolated A 10Cr-isolated B 10Cr-coupled
-0.65
0
1
2
3
4
Time, h
Figure 16. Potentials of isolated Specimens A and B in Fig. 2 and coupled potentials for 10Cr and 12Cr steel in 0.6 M NaCl solution at 30 ℃. The results were obtained using the specimen shown in Fig. 2 for simulating the potentials inside and outside the pit.
35
Table Captions
Table 1. Chemical composition of the materials used in this study (wt%) Materials
Cr
C
Si
Mn
P
S
Fe
10Cr
10.00
0.07
0.20
0.55
0.003
0.003
Bal.
12Cr
12.10
0.19
0.30
0.23
0.015
0.003
Bal.
36
Credit Author Statement Jinyang Zhu: Methodology, Writing - Original Draft, Funding acquisition Zhangshuai Fan: Data Curation, Investigation Lining Xu: Conceptualization, Validation, Writing- Reviewing and Editing
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which
may be considered as potential competing interests:
Highlights
An improved method to create single pits without applying potential is
presented.
Detailed investigation on the pitting propagation of stainless steel.
Observation of the pit cover and the metal salt film in the pit. Self-catalytic effect in the pit and galvanic effect between inside and outside
of the pit. 37
S0169-4332(20)30380-9 https://doi.org/10.1016/j.apsusc.2020.145624 APSUSC 145624
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
28 November 2019 8 January 2020 1 February 2020
Please cite this article as: J. Zhu, Z. Fan, L. Xu, On the Propagation of a Single Pit in Stainless Steel, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145624
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
On the Propagation of a Single Pit in Stainless Steel Jinyang Zhu a,1, Zhangshuai Fan b,1, Lining Xu b,* a National
Center for Materials Service Safety, University of Science and Technology Beijing, Beijing 100083, China
b Corrosion
and Protection Center, Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
1These
authors contributed equally to this study and share the first authorship
*Correspondence
author, Email: [email protected], Tel: +86 10 62334410
Abstract Single pits were produced on 10Cr and 12Cr stainless steels through an in-house experimental setup. The pitting propagation was investigated by confocal laser scanning microscope (CLSM), scanning electron microscope (SEM), electron diffraction spectroscopy (EDS), and electrochemical tests for simulating potentials inside and outside the pit in chloride solution. The results showed that the pit depth of 10Cr and 12Cr steel increased rapidly in the earlier stage, whereas the rate of increase declined significantly in later periods. The pit mouth diameter increases almost linearly with time. The pit cover and the metal salt film in the pit played important roles in pitting propagation. The cover served as a primary barrier to diffusion in the early stage of corrosion, and the pit bottom was under diffusion control. After a longer immersion time, the cover might collapse and disappear. A metal salt film formed at the pit bottom, and the pit bottom propagation was under diffusion control. Keywords: Pitting corrosion; Stainless steel; Single pit; Metal salt film; Pit cover
1
Introduction Pitting frequently occurs on passivated metal surfaces that are exposed to an aggressive
environment [1-3]. The most commonly aggressive ion is chloride ion, which is easily found in many natural and industrial environments. Once pit formation initiates on the metal surface, aggressive chemistry quite different from the bulk solution develops inside the pit. It 1
is characterised by a higher concentration of aggressive anions and lower pH, both of which are necessary to maintain stable pit growth [4-10]. The remarkably reduced pH values are mainly attributed to Cr ions [11]. The electrolyte inside the pit has high viscosity because of the mass of chloride and metal ions [12-14]. Frankel [15] reported that metastable pits would repassivate when the pit cover ruptured to form openings. Many researchers have also observed a layer covering mature pits with a number of holes distributed around the pit mouth [5-8,16-19]. Covered pits are dangerous in practice because they are stable against the loss of their internal environment by diffusion or convection, especially if there is precipitated material over the pit mouth [20]. Whether such layers are the residue of the passive film [6,21-23], metal [24-26], or both has long been subject to debate, and this depends upon the size of the pit being considered [18,19]. When the pit reaches a critical size, the pit cover is destroyed (or at least becomes ineffectual as a diffusion barrier), resulting in the formation of an open hemispherical cavity [27,28]. Once a pit has achieved stability, the pit depth can provide its own diffusion barrier to sustain the pit anolyte, and the cover is no longer necessary [18]. At the pit bottom, when the concentration of metal cations in the pit reaches solid solubility, the metal salt deposits on the surface of the pit bottom [18,29]. Vetter and Strehblow [30] reported the formation of a nonporous salt film. Isaacs [31] used an artificial pit electrode to study the pitting corrosion of stainless steel in chloride solutions and demonstrated that a resistive layer existed at the electrode interface. He also reported that this layer was approximately 5 μm thick and had a resistivity of approximately 108 Ω·cm. The main component was FeCl2•4H2O and a small amount of CrCl3•6H2O [18,32-34]. The salt film had a duplex structure, which included an inner compact anhydrous layer with high field conduction and an outer porous hydrated layer with low field conduction [35-37]. In the past few decades, many researchers have studied the propagation of single pits. These studies have investigated 1-D single pits where pit propagation was simulated by dissolving a small wire embedded in an insulator [38-41], 2-D single pits grown in stainless steel foils [13,18,19,42-45], and 3-D single pits obtained by injecting sodium chloride 2
solution [46-49]. Almost all single pits were initiated and propagated by applying potential, rather than being formed under practical environments. In the present study, an improved method for inducing a single pit without applying potential on steel is presented. Two stainless steels (10Cr and 12Cr) were chosen as the experimental materials. It is worthwhile to note that no Mo was added into the steels used in this work. By using these steels and the method for inducing a single pit without applying potential, the pitting propagation of single pits on 10Cr and 12Cr stainless steels in sodium chloride solution was investigated, and the potentials of the pit bottom and passive metal surrounding the pit were also determined. Special attention was paid to the pit cover and salt film in investigating the pitting propagation mechanism for stainless steel in chloride solution. 2
Experimental details
2.1. Experimental set up for initiation and propagation of a single pit The components of 10Cr and 12Cr steels used in this study are displayed in Table 1. Quenching was performed through heating the steel samples at 900 °C for 40 min, followed by water cooling. Tempering was performed at 660 °C for 50 min and followed by air cooling. The specimens used in this experiment were sealed into epoxy, and the size of the exposed surface was 10 mm × 10 mm. As depicted in Fig. 1, an improved experimental installation was set up on the basis of previous methods [46-49]. It mainly consisted of a glass microcapillary, a peristaltic pump, two beakers, and a water bath. Beaker A containing 0.05 M NaCl solution was immersed in a water bath at a temperature of 80 ℃. One end of the peristaltic pump hose was extended into Beaker B containing 5 M NaCl solution, and the other end was connected to a capillary tube with an inner diameter of 100 μm and an outer diameter of 1000 μm. A plexiglass plate was fixed on top of Beaker A to position the capillary and specimen. To induce a single pit instead of several pits, the distance from the microcapillary to the surface of the specimen could not exceed 300 μm. The peristaltic pump pumped 5 M NaCl solution through the capillary, so that the concentration of Cl− in a small portion of the specimen surface immediately below the ending of capillary could reach 5 M. 3
A single pit was initiated and propagated in this area of high Cl− concentration. It is worthwhile to note that this method was not a new method, but an improved method for inducing a single pit. Almost all single pits were initiated and propagated by applying potential in previous studies. The applied potential might change the electric double layer and some other electrochemical characteristics. Increasing the temperature only controlled the kinetics and thermodynamics, purely accelerating the pitting propagation. Therefore, in order to reduce the effect of the applied potential on the pitting process, no potential was applied on the steel in this work. Instead, the temperature and the Cl− concentration were increased for inducing a single pit. In this experiment, by instilling 5 M NaCl solution, a single pit was induced and allowed to initiate and propagate for 1 day on the steel surface in 0.05 M NaCl solution at 80 ℃; then, the specimens with single pits were immersed in 0.6 M NaCl solution at 30 ℃ for a long period to research the pit propagation on 10Cr steel. Pitting corrosion on 12Cr steel was conducted for comparison. Notably, the flow rate of the solution in the capillary tube was controlled at 0.5 mL per h through the peristaltic pump. Thus, the concentration of the bulk solution was kept low (i.e., no higher than 0.06 M after 1 day). Stolica [50] reported 0.1 M as the minimum concentration of Cl− necessary for pit initiation on Fe-20Cr when anodic potential was applied. During the experiment, it was observed that only a single pit initiated and propagated on the surface of the specimen; thus, the Cl− concentration of 0.06 M did not affect other parts of the specimen surface. Pit morphology after pickling was investigated using a LEXT OLS-4100 Confocal Laser Scanning Microscope (CLSM) with a 405-nm laser diode and double confocal optical system. It is possible to perform high quality imaging using OLS-4100 for a maximum resolution of 10 nm. OLS-4100 can measure pit depth and pit mouth diameter precisely to construct the 3D pit model. The pickling process was performed according to the ASTM G1-03 standard [51]. The cross-sectional morphologies of the pit were observed using the quadrant back scattering mode of a scanning electron microscope (SEM). The composition of the salt film 4
and corrosion products were investigated using electron diffraction spectroscopy (EDS). The cross-sections of the pits were grinded with silicon carbide sandpaper, which did not always pass through the pit centres. Therefore, sometimes, the pit mouth was not displayed in cross-sectional morphology. 2.2. Electrochemical tests for simulating potentials inside and outside the pit The active metal inside the pit acts as a small anode, and the passive metal surrounding the pit mouth acts as a big cathode; galvanic coupling between the cathode and anode occurs, and this process accelerates the development of the pit. Because it is difficult to directly measure the potentials inside and outside the pit, a simple method was proposed to measure the potentials of a small anode and a large cathode in chloride solution to simulate the potential inside and outside the pit. Although this method could not completely simulate the real galvanic effect on steel surface, it could be used as an approximate equivalent to help us to obtain the potential information inside and outside the pit. As depicted in Fig. 2, the anode and cathode were fabricated from the same material, such that a small anode (surface area 1 mm2) was close to but separated from a large cathode (surface area 1000 mm2). Electrical wires were welded to the anode and cathode, and the whole electrode was mounted in epoxy resin. A switch was installed to control the anode–cathode connection externally. During the test, the anode and cathode were immersed in the solution, so that they were connected by it. Two steel surfaces, serving as the cathode and anode, were polished using a 200-, 400-, and 600-grit sand paper in sequence. The specimens were kept in air for 24 h to ensure the formation of a passive film. After that, the anode was potentiostatically polarised at 0.8 V/SCE for 40 min, so that the surface of the anode would be corroded and filled with corrosion products. The anode surface, which was covered by corrosion products, was lower than that of the epoxy and cathode. Therefore, a 1-D artificial pit formed at the position of the anode. The mixed potential was monitored while the anode and cathode were connected, and the open circuit potential (OCP) of the disconnected anode and cathode was also measured. The test solution in the simulated experiment was 0.6 M NaCl solution, and the test 5
temperature was 30 ℃. 3
Results and discussion
3.1 Formation of a single pit on stainless steel Fig. 3(a) illustrates the macroscopic morphologies of a single pit, which was formed by instilling 5 M NaCl solution for 1 day only (without immersion in 0.6 M NaCl solution). The estimated concentration of Cl− in bulk solution after instilling 5 M NaCl solution for 1 day was 0.06 M. Therefore, except for the area near the glass microcapillary, the sample surface was immersed in a solution with a low Cl− concentration. Thus, a single pit was created on stainless steel, as illustrated in Fig. 3(a). Fig. 3(b) depicts the macroscopic morphology of a single pit, which was formed by instilling 5 M Cl− solution for 1 day and by subsequently immersing the steel in 0.6 M NaCl solution. After the additional day of immersion in 0.6 M NaCl solution, no other new pits were initiated, and the formation of an obvious product ring around the initial pit was observed. These results confirmed the feasibility of the experimental method for forming a single pit on stainless steel surface in the present study. 3.2 CLSM results Fig. 4 displays the CLSM images of single pits on 10Cr steel after cleaning. These pits were formed by instilling 5 M NaCl solution at 80℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for various periods. Overall, the pits on 10Cr steel were open and hemispherical. With prolonged immersion, the pits grew larger and deeper. Moreover, no other pits were observed around the pits after instilling 5 M NaCl solution for 1 day. Even after the longer period of immersion in 0.6 M NaCl solution (7~35 days), no new pit was found on the steel surface, as shown in Figs. 4(b-f). Fig. 5 presents the morphological parameters of single pits formed at various immersion times at 30 °C in 0.6 M NaCl solution. The pit depth was measured at the deepest position inside the pit. At least three samples were measured for each period, which confirmed that the data was more reliable. From Fig. 5, the increase in pit depth generally conformed to a parabolic curve. The pit depth can be expressed as a parabolic function of time: D = −0.4t2 + 6
26.9t + 208.4, where D is the pit depth, and t is the immersion time for pit development in 0.6 M NaCl solution. The pit depth increased rapidly in the first 14 days, whereas the rate of increase declined significantly in later periods. This trend of pit depth increase was probably due to the corrosion in the pit being under diffusion control [5-8,18,52-54]. The shape of the pit mouth was often irregular. As depicted in Fig. 6, the diameter of the pit mouth was obtained by calculating the area of the pit mouth and then calculating the corresponding diameter in a perfect circle. The diameter of the pit mouth increased almost linearly with time and demonstrated a higher rate of increase than pit depth. These results indicated that corrosion at the pit mouth was probably under activation control. Fig. 7 provides the CLSM images of single pits formed on 12Cr steel immersed in 0.6 M NaCl solution for 28 days and 35 days after pickling. From Fig. 7, it can be observed that the side walls of the 12Cr pit were always vertical, which meant that the true morphology of the pit could not be reflected using the CLSM method because the pit on the 12Cr steel was pear shaped. The true morphology of the pit on 12Cr steel will be displayed in the cross-section images below. From the CLSM images in Figs. 4 and 7, the depth and mouth diameter of the pits on 10Cr and 12Cr after immersion for 28 days and 35 days were obtained and compared, as depicted in Fig. 8. The depth of the pit on 12Cr steel was slightly smaller than that of the pit on 10Cr (Fig. 8(a)). However, as illustrated in Fig. 8(b), the diameter of the pit mouth on 10Cr steel was nearly three times larger than that of the pit on 12Cr steel after both 28 and 35 days. In general, the pitting resistance of these 10Cr and 12Cr steels was relatively poor, of which the pitting depth have reached more than 500 μm after immersion for 38 days. 3.3 Metal salt inside pit After instilling 5 M Cl solution on the surface of 10Cr steel for 1 day and immersing the steel in 0.6 M NaCl solution for 35 days, a single pit was formed. The cross-sectional morphology and EDS map scanning results of this pit are displayed in Fig. 9. Due to peeling caused by the corrosion product in the pit during the preparation of the cross-section sample, a 7
part of corrosion products at the pit bottom was absent. At the pit bottom, when the concentration of metal cations in the pit reaches saturation, the metal salt will be deposited. The main components of the metal salt film are FeCl2•4H2O and a small amount of CrCl3•6H2O [18,32-34]. Therefore, regarding composition, the metal salt film is characterised by a large amount of Cl, whereas the proportion of Fe is higher than that of Cr. The metal salt film can be traced by the distribution of Cl in the elemental maps. As depicted in Fig. 9(b), a Cl-enriched layer was observed at the pit bottom and sidewall. Cl was enriched both in the salt film at the pit bottom and in the corrosion product that filled the pit. The thickness of the metal salt film that formed at the pit bottom was approximately 14.5 µm. To further explore the differences in the composition of different regions of the pit, a detailed EDS analysis was performed at five typical positions, as demonstrated in Fig. 9(a). Points (A, B) and C were located on the salt film covering the pit sidewall and the pit bottom, respectively. Points D and E were located on corrosion products inside the pit and corrosion products stacked around the pit mouth, respectively. As shown in Fig. 10, the concentrations of Cl at points A and B reached more than 60%, which were much higher than that at point C (~25%). The distribution of Cl in the salt film was not uniform. However, the concentration of O increased as metal cation hydrolysed inside the pit. The concentration of O reached approximately 30% and 75% at points D and E, respectively, which meant that the metal cations away from pit bottom and sidewall hydrolysed more seriously [40]. Fig. 11 displays the cross-section morphology and elemental maps of a single pit on 12Cr steel surface. This pit was formed by instilling 5 M Cl solution for 1 day and by immersing the steel in 0.6 M NaCl solution for 35 days. The pit mouth is absent in Fig. 11. This was caused by the pear shape of pits on 12Cr steel, as displayed in Fig. 12. The pit on the 10Cr steel surface often had a hemispherical shape (Fig. 9(a)), whereas the pit on 12Cr steel often demonstrated a pear shape. The difference in pit morphologies between 12Cr and 10Cr may be related to the different Cr contents in substrates. The formation mechanism of the pear-shaped pit on 12Cr steel requires further exploration. Similar to 10Cr steel, Cl was 8
enriched in the pit bottom and sidewall of the 12Cr steel, as seen in Fig. 11(b), indicating the formation of a metal salt film inside the pit. The direct observation of the metal salt film has seldom been reported in the literature. Fig. 13 displays the back scattered electron image and elemental maps of the cross-section of a single pit on 12Cr steel. This pit was formed by instilling 5 M Cl solution for 1 day and by immersing the steel in 0.6 M NaCl solution for 49 days. The single pits displayed in Figs. 11 and 13 are not from the same specimen. Compared with the pit after 35-day immersion (Fig. 11), the total amount of Cl inside the pit after 49-day immersion (Fig. 13) significantly increased. The pit was filled with corrosion products containing Cl. In addition, the Cl content at the pit bottom in region A (Fig. 13(b)) was approximately 42%, which was obviously higher than that for corrosion products in region B (~ 25%) in the pit. After immersion for 49 days, a considerable quantity of Cl shifted from the solution outside the pit to inside the pit; furthermore, metal chloride was deposited at the pit bottom, and a salt film also formed there. 3.4 Observations of the cover on pit Some researchers have suggested that the 2D pit contained a perforated lacy metal cover, which hindered the diffusion of ions in the pit and which collapsed with a prolonged period of applied potential on the specimens [13,18,19,42-45]. However, the cross-section morphology of the cover on the pit was difficult to obtain for observation. The direct observation of the cover has rarely been reported. In the present study, a single pit was produced, and the cover on the pit was observed directly from the cross-section. Fig. 14 displays the cross-section morphologies and elemental maps of a single pit on 10Cr steel formed by instilling 5 M NaCl solution for 1 day and immersing the steel in 0.6 M NaCl solution for 7 days. A Fe-depletive and Cr-rich cover was found on the pit mouth, as indicated by the arrow in Fig. 14(a). This might be caused by the selective dissolution on the steel substrate. Fe dissolved preferentially and entered into the solution, whereas Cr-rich film grew in situ. However, the cover could not be maintained at all times and tended to collapse when corrosion proceeded; thus, the residual cover was extremely small, as depicted in Fig. 14. On the left side, the sidewall of the pit was 9
filled with corrosion products. The collapsed cover might be contained in these corrosion products. The Cr content of the cover on the right was higher than that of the corrosion product filling the left sidewall. Fig. 15 also displays the cross-section morphologies and elemental maps of a single pit on 10Cr steel formed by instilling 5 M NaCl solution for 1 day and immersing the steel in 0.6 M NaCl solution for 7 days. Compared with the pit in Fig. 14, the cover appears longer but thinner. The cover was inclined towards the bottom of the pit and suggested a certain degree of collapse. This observation represented direct evidence that the cover on the pit could collapse during the corrosion process. Compared with the pits in Figs. 9, 14, and 15, the cover existed on the pit after immersion for 7 days; however, it collapsed and disappeared after immersion for 35 days. This observation further proved that the cover tended to collapse with corrosion. The pit cover served as a primary barrier to diffusion of Fe2+ and Cr3+. These ions hydrolysed considerably inside the pit, and the anolyte became acidic with the generation of H+. In addition, some metal lamellae on the pit sidewall were observed (Fig. 9), which may also hinder the diffusion of ions. Therefore, the pH of the anolyte within an active pit can become extremely low for stainless steel—which promotes the corrosion of substrates in the pit. In addition, Cl− from bulk solution migrated into the pit to maintain electrical neutrality. The content of Cl in a local area of pit often surpassed 70% after immersion for 35 days (see Fig. 10). In summary, the cover of the pit, corrosion products, and metal salt film in the pit consisted of the diffusion barrier of ions. A local aggressive environment with lower pH and higher Cl− concentration was produced in the pit. This development explained the spontaneous propagation of the single pit. However, Cl enrichment was not observed in the pit, which had developed after immersion in 0.6 M NaCl solution for 7 days. It seems that the metal salt film only existed in the pits that had developed over a longer period. 3.5 Galvanic effect between the inside and outside pit From the electrochemical measurements of the specimen depicted in Fig. 2, the potentials of isolated Specimens A and B were obtained, as depicted in Fig. 16. For both 10Cr 10
and 12Cr steel, the potentials of Specimen A were more negative than those of Specimen B. The coupled potential was between and closer to the potential of isolated Specimen B due to the much larger surface of Specimen B. As mentioned in Section 2.2, Specimen A simulated the potential inside the pit, and Specimen B simulated the potential outside the pit. Therefore, from the results in Fig. 16, it can be concluded that the potential inside the pit on 10Cr steel (−0.65 V/SCE) was more negative than that inside the pit on 12Cr steel (−0.43 V/SCE). This result indicated that the activity of the metal inside the pit on 10Cr steel was higher than that of the metal inside the pit on 12Cr steel. The potential difference between the isolated Specimens A and B of 10Cr steel was 0.16 V, whereas that was only 0.04 V for 12Cr steel. This result indicated that the acceleration effect caused by the potential difference between the pit and the surrounding metal for 10Cr steel tended to be more obvious than for 12Cr steel. This explains the difference in pit propagation between 10Cr and 12Cr steel, as depicted in Fig. 8. When Specimens A and B were coupled, a relatively low coupled potential (−0.55 V/SCE) was observed for 10Cr steel. At such low potentials cathodic reduction of hydrogen ions would be expected to constrain a very low pH in a pit. By contrast, the coupled potential for 12Cr steel was more positive. Therefore, the pH in pit on 10Cr steel should be lower than that on 12Cr steel. This also explains the difference in pit propagation between 10Cr and 12Cr steel, as depicted in Fig. 8. Above all, the pitting propagation of stainless steel in chloride solutions was mainly controlled by two factors: self-catalytic effect in the pit and galvanic effect between the inside and outside of the pit. For the self-catalytic effect, the cover on the pit and the metal salt film played a crucial role. In the early stage (~7 days), the cover served as a primary barrier to diffusion and maintained the pit’s internal electrolytes at a sufficiently aggressive concentration [11,15,25,33]. The cover on the pit produced a closed environment, which made substance exchange difficult between the pit and external solution. Fe2+ and Cr3+ hydrolysed severely inside the pit, and the anolyte became acidic with the generation of H+. Furthermore, Cl− with a small diameter from the bulk solution would migrate into the pit to maintain electrical neutrality. In the presence of FeCl2 and CrCl3, H+ activity increased 11
considerably. Therefore, severe acidification occurred in the solution inside the pit. Such an aggressive environment makes pit propagation spontaneous and rapid. After a longer immersion time (~35 days), the cover may collapse and disappear. The metal salt film formed at the pit bottom and sidewall. The local concentration gradient around the metal salt film promoted the corrosion of substrate under the metal salt film. The pit bottom propagation was under diffusion control. For the galvanic effect, the electrochemical tests simulating the internal and external potentials of a single pit confirmed a potential difference between the inside and outside of the pit, which could promote the propagation of pits. What is noticeable is the low coupled potential, especially for 10Cr steel. At such low potentials (below −0.5 V/SCE) cathodic reduction of hydrogen ions would be expected to constrain a very low pH in a pit, which could also promote the propagation of pits. However, it is not clear which of the aforementioned two effects was stronger or dominant in the different pitting propagation stages for stainless steel. This issue requires further in-depth research. 4
Conclusions In the present study, a single pit was produced on 10Cr and 12Cr stainless steel using an
in-house experimental setup. The evolution of both the pit depth and pit diameter was investigated. The pit cover and metal salt film in the pit were observed, and the pit propagation mechanism was discussed. From this study, the following conclusions can be drawn: An improved method to create single pits on the steel surface based on previous work is presented in the present paper, and the test results confirm the feasibility of this method. The pit depth increased rapidly in the earlier stage, whereas the rate of increase declined significantly in later periods. This indicates that the pit bottom might be under diffusion control. The pit mouth diameter increases almost linearly with time, which indicates a much higher rate of increase compared with pit depth. The pitting propagation of stainless steel is mainly controlled by the self-catalytic effect in the pit and the galvanic effect between the inside and outside of the pit. 12
The pit cover and the metal salt film in the pit play important roles in pitting propagation. The cover serves as a primary barrier to diffusion in the early stage of corrosion. After a longer immersion time, the cover might collapse and disappear. A metal salt film forms at the pit bottom.
Acknowledgments This work was supported by the National Nature Science Foundation of China under grant No. 51871025 and Fundamental Research Funds for the Central Universities under grant No. 06500118.
Data Availability The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
References [1]
Z.H. Jin, H.H. Ge, W.W. Lin, Corrosion behavior of 316L stainless steel and anti-corrosion materials in a high acidified chloride solution, Appl. Surf. Sci. 322 (2014) 47–56.
[2]
H. Feng, Z.H. Jiang, H.B. Li, P.C. Lu, S.C. Zhang, B.B. Zhang, T. Zhang, D.K. Xu, Z.G. Chen, Influence of nitrogen on corrosion behavior of high nitrogen martensitic stainless steels manufactured by pressurized metallurgy, Corros. Sci. 144 (2018) 288– 300.
[3]
Z.B. Wang, H.X. Hu, Y.G. Zheng, Synergistic effects of fluoride and chloride on general corrosion behavior of AISI 316 stainless steel and pure titanium in H2SO4 solutions, Corros. Sci. 130 (2018) 203–217.
[4]
G.T. Burstein, C. Liu, R.M. Souto, S.P. Vines, Origins of pitting corrosion, Corros. Eng. Sci. Technol. 39 (2004) 25–30.
[5]
P.C. Pistorius, G.T. Burstein, Growth of corrosion pits on stainless steel in chloride solution containing dilute sulphate, Corros. Sci. 33 (1992) 1885–1897.
[6]
G.T. Burstein, S.P. Mattin, Nucleation of corrosion pits on stainless steel, Philos. Mag. Lett. 66 (1992) 127–131.
13
[7]
P.C. Pistorius, G.T. Burstein, Aspects of the effects of electrolyte composition on the occurrence of metastable pitting on stainless steel, Corros. Sci. 36 (1994) 525–538.
[8]
G.T. Burstein, S.P. Vines, Repetitive nucleation of corrosion pits on stainless steel and the effects of surface roughness, J. Electrochem. Soc. 148 (2001) B504–B516.
[9]
K. Sasaki, G.T. Burstein, Erosion–corrosion of stainless steel under impingement by a fluid jet, Corros. Sci. 49 (2007) 92–102.
[10] S.P. Mattin, G.T. Burstein, Detailed resolution of microscopic depassivation events on stainless steel in chloride solution leading to pitting, Philos. Mag. Lett. 76 (1997) 341– 347. [11] T. Suzuki, M. Yamabe, Y Kitamura, Composition of anolyte within pit anode of austenitic stainless steels in chloride solution, Corrosion 29 (1973) 18–22. [12] W. Tian, N. Du, S. Li, S. Chen, Q, Wu, Metastable pitting corrosion of 304 stainless steel in 3.5% NaCl solution, Corros. Sci. 85 (2014) 372–379. [13] N.J. Laycock, S.P. White, Computer simulation of single pit propagation in stainless steel under potentiostatic control, J. Electrochem. Soc. 148 (2001) B264–B275. [14] N.J. Laycock, R.C. Newman, Localised dissolution kinetics, salt films and pitting potentials, Corros. Sci. 39 (1997) 1771–1790. [15] G.S. Frankel, L. Stockert, F. Hunkeler, H. Boehni, Metastable pitting of stainless steel, Corrosion 43 (1987) 429–436. [16] N.J. Laycock, S.P. White, J.S. Noh, P.T. Wilson, R.C. Newman, Perforated covers for propagating pits, J. Electrochem. Soc. 145 (1998) 1101–1108. [17] M.H. Moayed, R.C. Newman, Evolution of current transients and morphology of metastable and stable pitting on stainless steel near the critical pitting temperature, Corros. Sci. 48 (2006) 1004–1018. [18] P. Ernst, R.C. Newman, Pit growth studies in stainless steel foils. I. Introduction and pit growth kinetics, Corros. Sci. 44 (2002) 927–941. [19] P. Ernst, R.C. Newman, Pit growth studies in stainless steel foils. II. Effect of temperature, chloride concentration and sulphate addition, Corros. Sci. 44 (2002) 943– 954. [20] P. Ernst, N.J. Laycock, M.H. Moayed, R.C. Newman, The mechanism of lacy cover formation in pitting, Corros. Sci. 39 (1997) 1133–1136. [21] J. Yahalom, L.K. Ives, J. Kruger, On the nature of films over corrosion pits in stainless steel, J. Electrochem. Soc. 130 (1973) 384–386. [22] H.S. Isaacs, G. Kissel, Surface preparation and pit propagation in stainless steels, J. Electrochem. Soc. 119 (1972) 1628–1632. [23] G.S. Frankel, L. Stockert, F. Hunkeler, H. Boehni, Metastable pitting of stainless steel, Corrosion 43 (1987) 429–436. 14
[24] N. Sato, T. Nakagawa, K. Kudo, M. Sakashita, Chloride pitting dissolution of rotating stainless steel electrode in acid solution, in: R.W. Staehle, B.F. Brown, J. Kruger, A. Agrawal (Eds.), U.R. Evans Conference on Localized Corrosion, NACE, 1971, p. 447. [25] W. Schwenk, Theory of stainless steel pitting, Corrosion 20 (1964) 129t–137t. [26] J. Mankowski, Z. Szklarska-Smialowska, Studies on accumulation of chloride ions in pits growing during anodic polarization, Corros. Sci. 15 (1975) 493–501. [27] G.T. Gaudet, W.T. Mo, T.A. Hatton, J.W. Tester, J. Tilly, H.S. Isaacs, R.C. Newman, Mass transfer and electrochemical kinetic interactions in localized pitting corrosion, AIChE J. 32 (1986) 949–958. [28] R.C. Newman, Local chemistry considerations in the tunnelling corrosion of aluminium, Corros. Sci. 37 (1995) 527–533. [29] P. Ernst, R.C. Newman, Explanation of the effect of high chloride concentration on the critical pitting temperature of stainless steel, Corros. Sci. 49 (2007) 3705–3715. [30] K.J. Vetter, H.H. Strehblow, Formation and shape of corrosion pits in localized corrosion on iron theoretical results pertaining to localized corrosion. Berichte Der Bunsen-Gesellschaft Fur Physikalische Chemie. 74 (1970) 1024–1035. [31] H.S. Isaacs, The behavior of resistive layers in the localized corrosion of stainless steel, J. Electrochem. Soc. 120 (1973) 1456–1462. [32] H.S. Isaacs, J.H. Cho, M.L. Rivers, S.R. Sutton, In-situ X-ray microprobe study of salt layers during anodic-dissolution of stainless-steel in chloride solution, J. Electrochem. Soc. 142 (1995) 1111–1118. [33] N. Sridhar, D.S. Dunn, Effect of applied potential on changes in solution chemistry inside crevices on type 304l stainless steel and alloy 825, Corrosion 50 (1994) 857–872. [34] T. Rayment, A.J. Davenport, A.J. Dent, J.P. Tinnes, R.J.K. Wiltshire, C. Martin, G. Clark, P. Quinn, J.F.W. Mosselmans, Characterisation of salt films on dissolving metal surfaces in artificial corrosion pits via in situ synchrotron X-ray diffraction, Electrochem. Commun. 10 (2008) 855–858. [35] C. Clerc, D. Landolt, AC impedance study of anodic films on nickel in LiCl, Electrochim. Acta. 33 (1988) 859–871. [36] R. D. Grimm, D. Landolt, Salt films formed during mass transport controlled dissolution of iron-chromium alloys in concentrated chloride media. Corros. Sci. 36 (1994) 1847–1868. [37] R.D. Grimm, A.C. West, D. Landolt, AC impedance study of anodically formed salt films on iron in chloride solution, J. Electrochem. Soc. 139(1992) 1622–1629. [38] E.D. Parsons, H.H. Cudd, H.L. Lochte, Synthetic corrosion pits and the analysis of their contents, J. Phys. Chem. 45 (1941) 1339–1345. 15
[39] J.W. Tester, H.S. Isaacs, Diffusional effects in simulated localized corrosion, J. Electrochem. Soc. 122 (1975) 1438–1445. [40] W. Tian, S. Li, N. Du, S. Chen, Q. Wu, Effects of applied potential on stable pitting of 304 stainless steel, Corros. Sci. 93 (2015) 242–255. [41] N. Greene, M. Fontana, A critical analysis of pitting corrosion, Corrosion 15 (1959) 41–47. [42] N.J. Laycock, J.S. Noh, S.P. White, D.P. Krouse, Computer simulation of pitting potential measurements, Corros. Sci. 47 (2005) 3140–3177. [43] S.
Scheiner,
C.
Hellmich,
Stable
pitting
corrosion
of
stainless
steel
as
diffusion-controlled dissolution process with a sharp moving electrode boundary, Corros. Sci. 49 (2007) 319–346. [44] G.S. Frankel, The growth of 2-d pits in thin film aluminum, Corros. Sci. 30 (1990) 1203–1218. [45] S. Ghahari, D. Krouse, N. Laycock, T. Rayment, C. Padovani, T. Suter, R. Mokso, F. Marone, M. Stampanoni, M. Monir, Pitting corrosion of stainless steel: measuring and modelling pit propagation in support of damage prediction for radioactive waste containers, Corros. Eng. Sci. Techn. 46 (2011) 205–211. [46] N. Aouina, F. Balbaud-Célérier, F. Huet, S. Joiret, H. Perrot, F. Rouillard, V. Vivier, A flow microdevice for studying the initiation and propagation of a single pit, Corros. Sci. 62 (2012) 1–4. [47] N. Aouina, F. Balbaud-Célérier, F. Huet, S. Joiret, H. Perrot, F. Rouillard, V. Vivier, Initiation and growth of a single pit on 316L stainless steel: Influence of SO42− and ClO4− anions, Electrochim. Acta. 104 (2013) 274–281. [48] S. Heurtault, R. Robin, F. Rouillard, V. Vivier, Initiation and propagation of a single pit on stainless steel using a local probe technique, Faraday. Discuss. 180 (2015) 267–282. [49] S. Heurtault, R. Robin, F. Rouillard, V. Vivier, On the propagation of open and covered pit in 316l stainless steel, Electrochim. Acta. 203 (2016) 316–325. [50] N. Stolica, Pitting corrosion on FeCr and FeCrNi alloys, Corros. Sci. 9 (1969) 455–470. [51] ASTM G1-03, “Standard practice for preparing, cleaning, and evaluating corrosion test specimens” (West Conshohocken, PA: ASTM, 2011). [52] J.W. Tester, H.S. Isaacs, Diffusional effects in simulated localized corrosion, J. Electrochem. Soc. 122 (1975) 1438–1445. [53] L. Peguet, A. Gaugain, C. Dussart, B. Malki, B. Baroux, Statistical study of the critical pitting temperature of 22-05 duplex stainless steel, Corros. Sci. 60 (2012) 280–283. [54] T.S. Li, J.R. Scully, G.S. Frankel, Localized Corrosion: Passive Film Breakdown vs. Pit Growth Stability: Part III. A Unifying Set of Principal Parameters and Criteria for Pit Stabilization and Salt Film Formation, J. Electrochem. Soc. 165 (2018) C762–C770. 16
17
Figure Captions Figure 1. Schematic diagram of the experimental setup for generating a single pit on the steel surface. A glass microcapillary and a peristaltic pump were used to locally and slowly supply chloride ions on the steel surface. Figure 2. Schematic diagram of the specimen showing a small anode and a large cathode used in electrochemical tests for simulating the potentials inside and outside the pit: (a) top view; (b) side view. Figure 3. Photos of specimens with a single pit: (a) formed by instilling 5 M NaCl solution at 80 ℃ for 1 day, (b) formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 1 day. Figure 4. CLSM images of a single pit on 10Cr steel after cleaning. Pits were formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for various periods: (a) 1 day; (b) 7 days; (c) 14 days; (d) 21 days; (e) 28 days; (f) 35 days. Figure 5. Pit depth of a single pit on 10Cr steel as a function of immersion time in a 0.6 M NaCl solution at 30 ℃. Depth data were obtained from the deepest position inside the pit. All tests were repeated at least three times. Figure 6. Pit mouth diameter of a single pit on 10Cr steel as a function of immersion time in 0.6 M NaCl solution at 30 ℃. Diameter data were obtained by calculating the area of the pit mouth and then calculating the corresponding diameter in a perfect circle. All tests were repeated at least three times. Figure 7. CLSM images of a single pit on 12Cr steel after cleaning. Pits were formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for various periods: (a) 28 days, (b) 35 days. Figure 8. A comparison of pit depth (a) and pit mouth diameter (b) between 10Cr and 12Cr steel with various immersion times in a 0.6 M NaCl solution at 30 ℃. The initial depth and diameter data were obtained from the pit formed by instilling 5 M NaCl solution at 80 ℃ for 1 day. Figure 9. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 10Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 35 days. Figure 10. Elemental concentration (O, Cl, Cr, and Fe) of different positions displayed in Fig. 9(a). 18
Figure 11. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 35 days. Figure 12. Schematic diagram showing that the cross-section morphology of a single pit on 12Cr steel in Fig. 11 did not pass through the pit mouth. Figure 13. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 49 days. Figure 14. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 10Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 7 days. Figure 15. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 7 days. Figure 16. Potentials of isolated Specimens A and B in Fig. 2 and coupled potentials for 10Cr and 12Cr steel in 0.6 M NaCl solution at 30 ℃. The results were obtained using the specimen shown in Fig. 2 for simulating the potentials inside and outside the pit.
19
Figure 1. Schematic diagram of the experimental setup for generating a single pit on the steel surface. A glass microcapillary and a peristaltic pump were used to locally and slowly supply chloride ions on the steel surface.
20
Figure 2. Schematic diagram of the specimen showing a small anode and a large cathode used in electrochemical tests for simulating the potentials inside and outside the pit: (a) top view; (b) side view.
21
(a)
(b) Corrosion product
A single pit
A single pit
5mm
5mm
Figure 3. Photos of specimens with a single pit: (a) formed by instilling 5 M NaCl solution at 80 ℃ for 1 day, (b) formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 1 day.
22
500μm
500μm
500μm
500μm
500μm
500μm
Figure 4. CLSM images of a single pit on 10Cr steel after cleaning. Pits were formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for various periods: (a) 1 day; (b) 7 days; (c) 14 days; (d) 21 days; (e)
28 days; (f) 35 days.
23
900
Measured Fitted
800
Pit depth, μm
700 600 500 400 300 200 100 0
5
10
15
20
25
30
35
40
Time, d
Figure 5. Pit depth of a single pit on 10Cr steel as a function of immersion time in a 0.6 M NaCl solution at 30 ℃. Depth data were obtained from the deepest position inside the pit. All tests were repeated at least three times.
24
2500
Measured Fitted
Pit mouth diameter, um
2000
1500
1000
500
0 0
5
10
15
20
25
30
35
40
Time, d
Figure 6. Pit mouth diameter of a single pit on 10Cr steel as a function of immersion time in 0.6 M NaCl solution at 30 ℃. Diameter data were obtained by calculating the area of the pit mouth and then calculating the corresponding diameter in a perfect circle. All tests were repeated at least three times.
25
500μm
500μm
Figure 7. CLSM images of a single pit on 12Cr steel after cleaning. Pits were formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for various periods: (a) 28 days, (b) 35 days.
26
900
10Cr 12Cr
800
716.482
Pit depth, μm
700 600
573.427
500
552.795
617.036
400 300 200 100
initial pit depth for 10Cr
175.797 46.926
initial pit depth for 12Cr
0 1
25
0 30
35
Time, d
2500
10Cr 12Cr
Pit mouth diameter, μm
2000
1500
1000 initial pit mouth diameter for 10Cr
500 initial pit mouth diameter for 12Cr
0 0 1
25
30
35
Time, d
Figure 8. A comparison of pit depth (a) and pit mouth diameter (b) between 10Cr and 12Cr steel with various immersion times in a 0.6 M NaCl solution at 30 ℃. The initial depth and diameter data were obtained from the pit formed by instilling 5 M NaCl solution at 80 ℃ for 1 day.
27
Figure 9. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 10Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 35 days.
28
O Cl Cr Fe
80
Concentration, at %
70 60 50 40 30 20 10 0 A
B
C
D
E
Position in Fig. 8a
Figure 10. Elemental concentration (O, Cl, Cr, and Fe) of different positions displayed in Fig. 9(a).
29
Figure 11. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 35 days.
30
Figure 12. Schematic diagram showing that the cross-section morphology of a single pit on 12Cr steel in Fig. 11 did not pass through the pit mouth.
31
B A
Figure 13. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 49 days.
32
Figure 14. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 10Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 7 days.
33
Figure 15. Cross-section morphology (a) and EDS map scanning analysis (b, c, and d) of a single pit on 12Cr steel. The pit was formed by instilling 5 M NaCl solution at 80 ℃ for 1 day and then immersing the steel in 0.6 M NaCl solution at 30 ℃ for 7 days.
34
-0.35
Potential, V vs. SCE
-0.40 -0.45 -0.50 -0.55 12Cr-isolated A 12Cr-isolated B 12Cr-coupled
-0.60
10Cr-isolated A 10Cr-isolated B 10Cr-coupled
-0.65
0
1
2
3
4
Time, h
Figure 16. Potentials of isolated Specimens A and B in Fig. 2 and coupled potentials for 10Cr and 12Cr steel in 0.6 M NaCl solution at 30 ℃. The results were obtained using the specimen shown in Fig. 2 for simulating the potentials inside and outside the pit.
35
Table Captions
Table 1. Chemical composition of the materials used in this study (wt%) Materials
Cr
C
Si
Mn
P
S
Fe
10Cr
10.00
0.07
0.20
0.55
0.003
0.003
Bal.
12Cr
12.10
0.19
0.30
0.23
0.015
0.003
Bal.
36
Credit Author Statement Jinyang Zhu: Methodology, Writing - Original Draft, Funding acquisition Zhangshuai Fan: Data Curation, Investigation Lining Xu: Conceptualization, Validation, Writing- Reviewing and Editing
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which
may be considered as potential competing interests:
Highlights
An improved method to create single pits without applying potential is
presented.
Detailed investigation on the pitting propagation of stainless steel.
Observation of the pit cover and the metal salt film in the pit. Self-catalytic effect in the pit and galvanic effect between inside and outside
of the pit. 37