The metastable pitting corrosion of 2205 duplex stainless steel under bending deformation

Journal Pre-proof The metastable pitting corrosion of 2205 duplex stainless steel under bending deformation Yan Hou, Jie Zhao, Cong-Qian Cheng, Lei Zhang, Jian Li, Bao-Jun Liu, Tie-Shan Cao PII:

S0925-8388(20)30785-4

DOI:

https://doi.org/10.1016/j.jallcom.2020.154422

Reference:

JALCOM 154422

To appear in:

Journal of Alloys and Compounds

Received Date: 19 October 2019 Revised Date:

17 February 2020

Accepted Date: 17 February 2020

Please cite this article as: Y. Hou, J. Zhao, C.-Q. Cheng, L. Zhang, J. Li, B.-J. Liu, T.-S. Cao, The metastable pitting corrosion of 2205 duplex stainless steel under bending deformation, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154422. 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.

Credit Author Statement： Yan Hou: Conceptualization, Validation, Formal analysis, Investigation, Resources, Data Curation and Writing the original Draft. Jie Zhao: Supervision Cong-Qian Cheng: Conceptualization, Data Curation and Review & Editing Lei Zhang, Jian Li and Bao-Jun Liu: Resources and Visualization. Tie-Shan Cao: Project administration

The metastable pitting corrosion of 2205 duplex stainless steel under bending deformation Yan Hou, Jie Zhao, Cong-Qian Cheng*, Lei Zhang, Jian Li, Bao-Jun Liu, Tie-Shan Cao (School of Materials Science and Engineering, Dalian University of Technology, Dalian city, Liaoning province, 116024, China) (Yan Hou, [email protected]; Jie Zhao, [email protected]; Cong-Qian Cheng

[email protected];

Lei

Zhang,

[email protected];

Jian

Li,

[email protected]; Bao-Jun Liu, [email protected]；Tie-Shan Cao [email protected]) *Corresponding author. School of Materials Science and Engineering, Dalian University of Technology, Dalian, Liaoning province, 116024, China E-mail address: [email protected] Abstract: The effects of tensile and compressive deformation introduced by the bending deformation on metastable pitting corrosion of 2205 duplex stainless steel were studied by microstructure characterization and electrochemical tests. The results demonstrated that both tensile and compressive deformation resulted in severer plastic strains in ferrite than that in austenite, caused the inclusions debonding from matrix and the surrounding micro-cracks. The decrease in critical pitting temperature and pitting potential provides the degradation evidence of the deformed steel. Compared with the as-received samples, bending deformation promoted the initiation frequency and growth rate of metastable pits. It also revealed that higher metastable pitting initiation rate and pit growth under compressive deformation than tensile deformation. The preferential initiation sites and transients’ behavior of metastable pitting were discussed herein. Key words: duplex stainless steel; tensile and compressive; bending deformation; pitting corrosion; metastable pitting;

1

1. Introduction Duplex stainless steel (DSS) has been increasingly implemented as a structural material in a wide range of industries such as nuclear power plant, petrochemical and seawater desalination industries, due to its good combination of mechanical properties and excellent corrosion resistance [1, 2]. For manufacturing DSS components, microstructure deformation with strain is practically induced by cold working, such as rolling, shaping, drawing and bending. Deformation might degrade the pitting corrosion resistance, and sometimes even initiate the stress corrosion cracks at the pits [3, 4]. Thus, it is critically needed to study the pitting corrosion of DSS under deformation condition. Numerous researchers have studied the pitting corrosion of DSS under cold deformation [4-11]. Vignal et al. [5] suggested that the tensile deformation accelerated the pit occurrence. Yang et al. [6] and Renton. et al. [7] revealed that the pitting potential of DSS was decreased noticeably by tensile strain. Lv et al. [8] reported that the doping concentration of the passive film on 2205 DSS increased with increasing tensile deformation. Gennari et al. [9] found that the deformation degraded the corrosion resistance by lowering the critical pitting temperature (CPT). Recently, Örnek et al. [4] reported that the tensile strain introduced by U-bending deformation showed the highest degree of strain and promoted atmospheric-induced stress corrosion cracking (AISCC), compared with the rolling and tensile deformation. Compared to the tensile stain in the outer side of U-bending specimen, the role of compressive deformation in the inner side has not been focused up to now. Previous studies have investigated the effect of compressive deformation on the corrosion behavior of austenitic stainless steel [12-15]. Contradictory results have been reported regarding the effect of compressive deformation on the pitting corrosion. Some researchers stated that the presence of compressive stress in the surface of stainless steel can facilitate the growth of the passive film and maintain the protective properties [12-14]. However, it was also reported that compressive residual stress could induce passive film rupture and had a detrimental effect on the pitting resistance [15]. It is therefore interesting to systematically compare the influence of 2

tensile and compressive strain under U-bending deformation on pitting corrosion of DSS. Metastable pitting as a precursor stage to stable pitting has been extensively investigated [16, 17]. Analysis of the current transients generated during metastable pitting has been proven to be a valid approach to study the initiation, growth, and repassivation of metastable pit. Klapper et al. [18] found the appearance of three types of current transients, such as a slow increase followed by a sharp decay of current (type

), a quick increase followed by a slow decay of current (type

symmetrical shape of current (type

) and a

), and correlated these metastable pitting events

in various environmental conditions. Some researchers [19, 20] pointed out that the type

current transients were associated with pit nucleation events on the inclusions.

Breimesser et al. [21] demonstrated the type

current transients occurred due to

macroscopic cracking event. Feng et al. [22] reported the plastically deformed samples exhibited a small current peak in addition to the main peak. Although previous research mainly concentrated on the metastable pitting behavior of austenite stainless steel, little is understood about the influence of deformation on metastable pitting of DSS. In the current study, the U-bending samples of 2205 DSS were employed to systematically compare the role of tensile and compressive deformation on the metastable pitting during the electrochemical corrosion. The conventional pitting resistance was firstly investigated by CPT measurement and potentiodynamic polarization. The effect of bending deformation on metastable pitting was then extensively explored by current transient analysis during potentiostatic tests. Furthermore, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) was used to understand the morphologies and the preferential initiation sites of metastable pits. 2. Experimental methods 2.1 Materials and specimen preparation The samples with dimensions of 75 mm x 25 mm x 2 mm were cut from a sheet 3

of 2205 DSS with a chemical composition (wt. %): 0.02 C, 22.56 Cr, 5.43 Ni, 3.07 Mo, 1.03 Mn, 0.54 Si, 0.17 N, 0.017 P, 0.008 S and Fe balance. The average composition of ferrite and austenite phases in 2205 DSS was measured by electron probe micro analysis (EPMA-1600). The concentrations of the main alloying elements in ferrite and austenite phases are listed in Table 1. Chromium and molybdenum are enriched in ferrite phase, while nickel and nitrogen are concentrated in austenite phase. The empirical pitting resistance equivalent number (PREN) value (PREN = %Cr + 3.3 (%Mo) + 16 (%N)) was used to evaluate the pitting-resistance capability [23]. As shown in Table 1, the PREN value of austenite phase was lower than that of ferrite phase for the as-received sample. Table 1 Average composition of ferrite and austenite phases in 2205 DSS phases

Cr%

Mo%

N%

Ni%

PREN

ferrite

24.71

3.29

0.045

3.92

36.3

austenite

21.33

2.06

0.34

5.95

33.6

In the current experiment, the flat sheets were bent into a U shape. The rolling direction (RD) of the microstructure is parallel to the length of the specimen, as shown in Fig. 1. Two types of specimens were prepared: specimens under tensile deformation in the outermost side (Fig. 1b) and specimens under compressive deformation in the innermost side of the bend (Fig. 1c). By adjusting R, different levels of plastic deformation were achieved. The tensile and compressive strains at the exposed area can be approximately calculated as [24]: ε = /2R

(1)

where t is the specimen thickness and R is the radius of curvature at the area of interest. The tensile strains of 8 % and 14 % were successively achieved by adjusting the radius of 10 mm and 5 mm, respectively. Accordingly, the compressive strains were 10 % and 20 % at the radius of 10 mm and 5 mm. The U-bending samples were ground sequentially by using SiC paper with 180-2000 grit. Then, they were rinsed with deionized water and alcohol, and dried. Specimens were coated with silica gel, leaving a specific deformed area of 1 cm2 as

4

the exposure area for electrochemical corrosion measurements. A copper wire was connected to each specimen before coating. 2.2. Electrochemical measurements Electrochemical measurements were performed in CS350 workstation by using a conventional three-electrode cell system. A saturated calomel electrode (SCE) and platinum sheet were selected as the reference electrode and counter electrode, respectively. The aggressive chloride anion is essential to the breakdown of passive film and initiation of pits. In addition to the general NaCl solution, MgCl2 electrolyte is another typical electrolyte that has been used for pitting research [25, 26]. In our previous work [27], MgCl2 electrolyte was used for corrosion resistance evaluation under droplet and solution conditions. In the current work, 0.3 M MgCl2 solution with pH value of approximate 5.6 was therefore employed where the chloride concentration is close to that in the general 3.5 % NaCl solution. Prior to electrochemical measurements, samples were allowed to stabilize at the open-circuit potential (OCP) in 0.3 M MgCl2. To evaluate on the CPT, potentiostatic polarization was performed at 0.7 VSCE in solution, where solution temperature was increased at a rate of 0.6 oC/min until the current density exceeded 100 µA/cm2. The temperature associated to such current density was defined as the criterion for CPT assessment [28, 29]. The cyclic potentiodynamic polarization was conducted from the cathodic potentials at a scan rate of 0.5 mV s−1, till the potential corresponding to the anodic current density of 1 mA·cm−2, and then reversed back to the initial potential. To investigate on the metastable pitting, potentiostatic polarization was carried out at 65 oC by applying an anodic potential of 0.15 VSCE with a recording frequency of 20 Hz. To reduce the overlap events, an exposed surface area of 0.1 cm2 was successfully prepared. All the polarization tests were repeated at 6 times for each sample to ensure reproducibility and consistency of the data. 2.3. Microstructure and surface morphology observation Microstructure (ZEISS-SUPARR

of 55)

the and

U-bending EDX.

samples

Electron

was

backscatter

examined diffraction

by

SEM

(EBSD)

measurements were used for material characterization by extracting grain boundary 5

information and local misorientation (LMO). An FEI Quanta 650 SEM interfaced with an EBSD detector from Oxford Instruments with AZtec V2.2 software was used for data acquisition. The acceleration voltage was set at 20 kV under a 70◦ tilt angle and the acquisition step was 0.8 µm. Data post processing was conducted with HKL Channel 5 software. The High-angle grain boundaries (HAGB, > 15° of misorientation, represented by thick black lines), the low-angle grain boundaries (LAGB, between >1° and <15°of misorientation, represented by thin black lines) and Σ3 twin boundaries (represented by red lines) were allowed to distinguish the types of grain boundaries. LMO maps were generated by using a 3 × 3 binning and a 5◦ threshold for the sub-grain angle threshold. This method gives the average LMO for a misorientation below the pre-determined sub-grain angle threshold and can be used to locate regions with higher concentrations of misorientation in microstructure. 3. Results 3.1 Microstructure development under tensile and compressive deformation Fig. 2 shows the SEM microstructure of 2205 DSS after U-bending deformation. The austenite (γ) grains are arrayed in island-like structure and embedded in the ferritic (α) matrix, as shown in Fig. 2a. From Fig. 2b and c, both austenite and ferrite grains have been elongated along the bending direction. Slip lines can be distinctively identified in the austenitic phase, suggesting the high dislocation density induced by both the tensile and compressive strain, as pointed out in the magnification of Fig. 2d. No secondary phases were observed, which is accord to the results by Örnek et al. [30], who have found no intermetallic phases after U-bending deformation. Fig. 3 shows the phase maps and grain boundary maps of 2205 DSS by EBSD analysis. In phase maps, the red and blue areas represented the ferrite (α) and austenite (γ), respectively. After U-bending deformation, local regions with a higher fraction of sub-grain boundaries were observed in both ferrite and austenite grains. For the as-received samples in Fig. 3a, the LAGB fraction of austenite and ferrite was 4.5 % and 16.3 %, respectively. The fraction of LAGB was significantly increased with increasing deformation. The LAGB fraction was increased up to 53.2 % and 68.6 % in austenite and ferrite phase under 20 % compressive strain, respectively. 6

Such high value of 68.6 % LAGB fraction suggests the strain accumulated in the ferrite phase. Meanwhile, the fraction of Σ3 CSL boundaries has been decreased by U-bending deformation. Fig. 4 shows the LMO analysis of the as-received, tensile and compressive deformed samples by U-bending. The LMO degree increased from 2° for the as-received sample up to 4° on the tensile side, and to 5° on the compressive side, respectively. Such increase in LMO degree indicates the presence of large plastic strain after U-bending deformation. For sample with 14 % tensile strain, the LMO presents a broader distribution for ferrite than that for austenite, as shown in Fig. 4b, indicating intensified strain in ferrite phase. For the sample with 20 % compressive strain, strain was also intensified in ferrite phase, as shown in Fig. 4c, due to high density of dislocations and other defects [31]. Fig. 5 shows typical SEM micrographs of inclusions in the 2205 DSS. Most inclusions are round particles in the as-received samples and the samples within 8 % tensile deformation as shown in Fig. 5a. Such inclusions were possibly mixed Al-enriched oxides from the EDX analysis in Fig. 5b. Under the severe tensile plastic deformation, the oxide inclusions seem to be debonded from the matrix, resulted in micro-cracks and crevice between the oxides and matrix, as shown in Fig. 5c. Under the compressive strain of 20 %, as pointed out in Fig. 5d, the deformed inclusions still adhere in the matrix, and no micro-crack was found. Some oval-type hole can be found in the inclusions. Since the inclusions were composed of inner Al2O3 core and the surrounding Mg, Si and Ca bearing oxide [32, 33], it supposes that such hole is possibly attributed to the fracture and detachment of the oxides due to their different composition. 3.2 Critical Pitting Temperature and cyclic polarization results Fig. 6 illustrates the typical current density for CPT evaluation after polarization in 0.3 mol/L MgCl2 solution. Compared to the as-received sample, the current density curves shift toward low temperature region after U-bending deformation. Consequently, the calculated CPT value was decreased from 62 oC to 54.5 oC by the 14 % tensile strain, while the CPT value was decreased down to 53.5 oC as the 7

compressive strain was increased up to 20 %. Fig. 7 shows the cyclic polarization curves of samples in 0.3 mol/L MgCl2 solution at 65 oC and the typical pit morphology after the polarization. For the as-received sample, passivation behavior was apparent during the anodic polarization up to the Epit of 0.51 VSCE, as shown in Fig. 7a. After U-bending deformation, the Epit was decreased significantly, and the area of the hysteresis loop was diminished, indicating the degradation in pitting resistance. The pit morphology after the polarization was changed by deformation. Typical round-type pits with lacy cover were observed on the as-received sample, as shown in Fig. 7b. On the tensile side of the U-bending sample, however, the shape of pits changed from round-type to an irregularly stretched-type, where preferential dissolution along the direction of tensile strain was evident, as shown in Fig. 7c. The pit with irregular shape was enlarged under the compressive strain of 20 %, where the preferential dissolution was perpendicular to the bending direction as shown in Fig. 7d. 3.3 Metastable pitting measurement Fig. 8 shows the current transient curves of samples with different deformation that have been potentiostatic polarized at +0.15 VSCE in 0.3 M MgCl2 at 65 oC for 2 h. The current transients reflect the metastable pitting processes, i.e. initiation, early growth, and repassivation [34, 35]. Both the amount and amplitude of the current transients were enhanced by the tensile and compressive deformation. For example, the number of peak current transients above 1×10-5 A·cm-2 of the as-received samples was 1, and the number of current transients of the samples with 14 % tensile was 11 and that of 20 % compressive was 27. To illustrate metastable pitting behavior due to U-bending deformation, current transients were further analyzed by comparing the current spike shapes as follows. Fig. 9 shows two types of current transients for those metastable pits. Fig. 9a illustrates the type

transients that occurred on all sample surfaces. Such transient

peaks are composed of slow risen in current with a plateau of constant current and followed by a fast decay. Type

transient is characterized by an inverse shape of a

sudden rise and slow decay in current, as shown in Fig. 9b and Fig. 9c. Fig. 10 8

illustrates the number and probability of those current transients after U-bending deformation. Only type I transients were found for the as-received samples, as shown in Fig. 10a. After U-bending deformation, both the amounts of types I and II transients were increased. The fraction of type

transients was increased up to 47 %

and 42 % by the 14 % tensile and 20 % compressive strain, respectively. Each current transient has three key parameters, i.e. pit growth time (tg), pit repassivation time (tr) and pit peak current (Ipeak), as defined in Fig. 9a. Characteristic current transients were integrated to calculate the individual charge values (q), where a baseline correction was applied before integration. The key parameters of metastable events that labelled in Fig. 8 are listed in Table 2. For the type

transients,

the tg value is usually higher than tr, and transient durations range from a few to several tens of seconds. However, the time for each event of type 2 s, and the Ipeak is far below 6 µA. Type dissolution charge than type

transient is within

transients in Table 2 give a higher

, considering the increased number of the two types of

transients, suggesting that the steel suffered far more damage during the metastable pitting events on both sides after bending. Table 2 The tg, tr, Ipeak and q of the individual metastable pits formed under different deformation labelled in Fig. 8 a-e. Peak / transient type

tg / s

tr / s

Ipeak / µA

q / µC

Fig. 8a- 1; Type

0.25

0.1

0.31

0.052

Fig. 8a- 2; Type

12.05

0.8

3.47

11.19

Fig. 8a- 3; Type

1.55

0.25

0.16

0.12

Fig. 8a- 4; Type

2.15

0.15

0.66

0.34

Fig. 8b- 1; Type

0.2

0.4

0.51

0.19

Fig. 8b- 2; Type

10.45

0.6

5.1

18.4

Fig. 8b- 3; Type

0.15

0.3

0.25

0.05

Fig. 8b- 4; Type

0.15

0.3

2.56

0.69

Fig. 8c-1; Type

0.1

0.2

2.09

0.36

Fig. 8c- 2; Type

0.1

1.25

1.27

0.31

9

Fig. 8c- 3; Type

6.3

0.3

12.61

26.85

Fig. 8d- 1; Type

4.6

0.35

5.15

9.14

Fig. 8d- 2; Type

0.2

0.9

0.50

0.15

Fig. 8d- 3; Type

3.55

0.3

1.11

1.03

Fig. 8d- 4; Type

0.15

0.35

0.18

0.043

Fig. 8e- 1; Type

63.7

18.8

49.0

1410

Fig. 8e- 2; Type

0.15

0.8

5.13

2.41

Fig. 11 represents the average frequency of pit initiation λ (cm−2·s−1) for samples under different deformation, where the number of current transients was counted over 1000 s intervals. The large error bars indicated that metastable pitting was a highly random process. In general, the values of λ increased with increasing in tensile and compressive strain, suggesting the enhanced susceptibility on the metastable pitting initiation by the U-bending deformation. The charge of a metastable pit (q) is referred to the integration of current transients from the rapid rise to recovery during the process of birth to death of pit, which is proportional to the dissolution quantity of metal inside a pit [36]. Therefore, the difference between the cumulative charges (Qpit) that passed through the metastable pits, can be used as an indicator of pitting severity, as the following Eq. (2) [37]:



  = ∑

[ |  −  |] 

(2)

where t is the measurement time of each record (7200 s), λ is the nucleation rate of metastable pit. Therefore, the product of λ and t is the total number of metastable pitting events.  and  are the initial time and the terminal time of the n-th current transient, respectively. In(t) is the current as a function of time during the n-th current transient. Ib is the baseline of the current transients. Fig. 12 shows the cumulative charge that passed (in µC) through metastable pits for each sample under different deformation. The value of Qpit increased with increasing tensile and compressive deformation, as can be seen in Fig. 12, where the samples under compressive strain of 20 % showed the highest cumulative charge (3012 µC). This indicates that the 10

samples under tensile and compressive deformation tend to suffer more damage in metastable pitting events, compared to the as-received one. Fig. 13 shows the morphologies of metastable pits after the polarization tests. An electrolytical etch was conducted in 20 wt% KOH solution for 60 seconds to distinguish the austenite phase from the ferrite phase before SEM observation. As shown in Fig. 13a, most tiny metastable pits located in the austenite phase without U-bending deformation. After being bent to tensile strain of 14 %, the preferential initiates of metastable pits were mostly found in ferrite phase near the phase boundaries, as shown in Fig. 13b. Such preferential initiates in the ferrite phase were also observed in the compressive deformation, as shown in Fig. 13c. Fig. 14 depicts the morphologies of metastable pits that initiated at oxide inclusions after the potentiostatic polarization. The metastable pits are characterized as regular hemispherical shape, where there existed remnant inclusions at the bottom of pit according to the morphology and the EDS spectrum, as shown in Fig. 14a and b. For samples under 14 % tensile strain in Fig. 14c, micro-cracks were observed around the metastable pits. The similar results were also found in compression conditions, as shown in Fig. 14d. It can be concluded that tensile and compressive deformation changed the morphology of the metastable pits that initiated at non-metallic inclusions. 4. Discussion The results in this study show that both tensile and compressive deformation increased the initiation frequency, peak current and the cumulative damage of the metastable pits. As a precursor state to stable pitting, metastable pitting directly affects the formation of stable pitting. From a statistical point of view, the initiation probability of metastable pits and its transition probability to stable pits determine the formation probability of stable pits [38, 39]. The effects of tensile and compressive deformation on metastable pitting corrosion are thus discussed from the view point of metastable pitting initiation and the transition probability to stable pitting as follows. 4.1 Effects of U-bending deformation on initiation of metastable pit The dissolution ability around inclusions and the property of passive film are the 11

key factors for the initiation of metastable pitting on DSS. It was reported that the preferential dissolution of metal at the interface between the inclusion and matrix usually introduces the metastable pits due to their intrinsic differences in electrochemical properties [33]. The shape of type

transient that initiated at

inclusions for the as-received sample is consistent with the previous reports [20, 40, 41]. After U-bending deformation, micro-cracks and crevices at the interface between the oxides and matrix were formed (Fig. 5). Given that the deformation and the formed micro-cracks can accelerate the dissolution of matrix around the inclusions, more available sites can be activated at inclusion sites for metastable pitting initiation. As a result, the metastable pitting frequency of 2205 DSS was increased by U-bending deformation (Fig. 10). Furthermore, the presence of micro-cracks and crevices is beneficial to the accumulation of chloride ions and acidification during the dissolution of metastable pits [42], which means a low repassivation ability under such crevice corrosion condition. Suter et al.[43] and Matsch et al. [44] reported the transient peak during the SCC of stainless steel, where similar slow decay of current as type II transient has been observed. Therefore, it is believed that the deformation and crevice during the dissolution of metastable pits (Fig. 14) are responsible for the formation of the type

transient (Fig. 9).

Besides the effect of micro-cracks around the inclusions on the initiation frequency, the passive film difference due to the deformation non-uniformity between the austenite phase and ferrite phase is also critical. The results suggested that the preferential sites for the pitting initiation changed from austenite phase (Fig. 13a) to the ferrite phase near the phase boundaries by U-bending deformation (Fig. 13b and c). In practice, the austenite phase with lower PREN value, was less stable and more susceptible to initiate pitting corrosion than the ferrite phase in Cl- containing solution for the as-received sample. The preferential attack of the austenite phase has been observed in other studies [29, 45, 46]. After U-bending deformation, the distribution of plastic strain due to the presence of dislocations and other defects was higher for ferrite than austenite as shown in Fig. 4. The plastic strain or residual stress could induce passive film rupture and had a detrimental effect [47-49]. Considering the 12

larger strain concentration in the ferrite phase than that in the austenite phase, the preferential initiation of metastable pits at ferrite likely occurs when the passive film on the ferrite breaks down first at the oxide inclusions after U-bending deformation. 4.2 Effects of U-bending deformation on growth and stability of metastable pit Metastable pitting with high nucleation frequency and relatively large size will increase the transition probability to stable pitting [47-49]. The present work has demonstrated that both tensile and compressive deformation increased the dissolution rate of metastable pits (Fig. 8), and the cumulative damage on the U-bending samples was estimated to be severer than the as-received sample (Fig. 12). Moreover, the effect of compressive deformation is more pronounced than the tensile deformation, which is possible due to the larger dissolution of metal cations or Cl- ions at the higher strain level. Many studies have reported that the dissolution rate of metal increases significantly with increasing of plastic strain [50, 51]. The susceptibility of formation of stable pits can be evaluated by the pit stability product. The stability products were calculated by Eq. (3) [17]:  !"!# $%&'( =

)*+,./0 1

×%

(3)

where Ipeak is metastable pit peak current (A) and r (m) is the radius of the pit. In this work, by assuming pits were hemispherical, the metastable pit radius was calculated using Farady equation as follows [52]: 456

% = 3./78 9

(4)

where Q(C) is the total charge during the metastable pit growth, Z is molar mass, n is the average valence number of cations, F is the Faraday constant and ρ (g/cm3) is the density of the alloy. For the 2205 DSS, the value of Z, n, and ρ equal to 56 g/mol, 2.23 and 7.8 g/cm3, respectively. The maximum stability products for different samples were calculated from the largest metastable pits and are given in Table 3. It can be seen that both tensile and compressive deformation significantly increased the stability product of metastable pits. Consequently, it is reasonable to evidence the degradation of CPT and Epit under U-bending deformation (Fig. 6 and Fig. 7), although previous researchers have reported that cold rolling had little effect on the 13

pitting resistance of 2205 DSS [53]. The present study also reveals that the compressive deformation plays more important role on the metastable pitting corrosion than the tensile deformation due to its large strain. The possible explanation of the inconsistent results can be connected to the different deformation mode, because U-bending deformation may induce a much higher level of residual elastic stress and strain Table 3 The pit stability product for different samples calculated from the largest metastable pits by using Eq. (3). samples

8%

14 %

10 %

20 %

tensile

tensile

compressive

compressive

1.27

2.72

1.62

3.39

As-received

stability product / mA·cm-1

1.06

5. Conclusions The effects of tensile and compressive deformation that induced by U-bending on the metastable pitting corrosion of 2205 DSS in magnesium chloride solution were investigated by microstructure observations and electrochemical measurements. The main conclusions can be drawn from the present work: (1) The decrease in CPT and Epit provides the degradation evidence in pitting resistance by tensile and compressive deformation. The U-bending deformation promoted the initiation frequency and growth rate of metastable pits. It also revealed that higher metastable pitting initiation rate and pit growth under compressive deformation than tensile deformation. (2) The type

transients were characterized by a sudden rise and slow decay of

current, found under U-bending deformed conditions. (3) The pit initiation sites transferred from austenite phase to the ferrite phase near the phase boundaries after U-bending deformation.

14

Acknowledgements This work is supported by National Natural Science Foundation of China (NSFC No. 51571051), National Natural Science Foundation of China (NNSF No. U1610256).

15

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Figure Captions Fig. 1 (a) Schematic diagram of the sheet material; Schematic illustration of the U-bending specimens under (b) tensile and (c) compressive deformation. Fig. 2 The SEM microstructure of samples under different deformation, (a) as-received, (b) 14 % tensile, (c) 20 % compressive strain, and (d) the high magnification on the selected location in (c). Fig. 3 EBSD results (phase maps and grain boundary maps) of 2205 DSS under different deformation, with HAGB (thick black lines) and LAGB (thin black lines), and Σ3 twin boundaries (red lines): (a) as-received, (b) 14 % tensile, (c) 20 % compressive strain. Fig. 4 EBSD local misorientation maps of the (a) as-received, (b) 14 % tensile strain, (c) 20 % compressive strain microstructure in Fig. 3. Black lines are phase boundaries. Fig. 5 SEM image of samples under different deformation: (a) 8 % tensile strain, (b) EDS analysis at local positions in (a), (c) 14 % tensile strain and (d) 20 % compressive strain. Fig. 6 Current density versus temperature obtained from different deformation conditions and as-received specimens in 0.3 mol/L MgCl2 solution. Fig. 7 Cyclic potentiodynamic polarization curves and the typical pit morphology of specimens with different deformation and as-received one in 0.3 mol/L MgCl2 solution at 65 oC. Fig. 8 Current transients of 2205 DSS under different deformation in 0.3 M MgCl2 at an applied potential of +0.15 VSCE: (a) as-received, (b) 8 % tensile strain, (c) 14 % tensile strain, (d) 10 % compressive strain, (e) 20 % compressive strain. Fig. 9 Characteristic current transient types encountered during potentiostatic polarization under deformation: (a) type I; (b, c) type II. Fig. 10 The number and probability of the two types of current transients for the as-received and bend samples. Fig. 11 Metastable pit frequency λ with different deformation during the potentiostatic 23

polarization of 2205 DSS at 0.15 VSCE in 0.3 M MgCl2 solution. Fig. 12 The cumulative charge passed (in µC) through metastable pits for each sample under the tensile and compressive deformation. Fig. 13. SEM micrographs of pitted surfaces of 2205 DSS under different deformation after the polarization tests followed by electrolytic etching in KOH solution: (a) as-received, (b) 14 % tensile strain, and (c) 20 % compressive strain. Fig. 14 SEM image of metastable pitting morphologies after the potentiostatic polarization tests. (a) as-received, (b) EDS of sites in a, (c) 14 % tensile strain, and (d) 20 % compressive strain.

24

1、 Both tensile and compressive deformation degraded pitting resistance of 2205 DSS. 2、 The initiation and growth of metastable pits were enhanced by bending deformation. 3、 Sudden rise and slow decay of transients were observed in the deformed conditions. 4、 The pit initiation sites was transferred from the austenite to the ferrite phase.

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: