Passivation behavior and surface chemistry of 2507 super duplex stainless steel in artificial seawater: Influence of dissolved oxygen and pH

Corrosion Science 150 (2019) 218–234

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Passivation behavior and surface chemistry of 2507 super duplex stainless steel in artificial seawater: Influence of dissolved oxygen and pH ⁎

T

⁎

Zhongyu Cuia, , Shuangshuai Chena, Yunpeng Doua, Sike Hana, Liwei Wangb, Cheng Mana, , ⁎ Xin Wanga, Shougang Chena, , Y. Frank Chenga,c, Xiaogang Lid a

School of Materials Science and Engineering, Ocean University of China, Qingdao, 266100, China College of Electromechanical Engineering, Qingdao University, Qingdao, 266071, China c Department of Mechanical & Manufacturing Engineering, University of Calgary, Calgary, Alberta, T2N 1N4, Canada d Corrosion and Protection Center, University of Science and Technology Beijing, Beijing, 100083, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: A: Stainless steel B: EIS B: XPS C: Passive films

Passivation and film chemistry of 2507 super duplex stainless steel in modified artificial seawater (ASW) are investigated. Removal of dissolved oxygen decreases the electric field strength and point defect diffusivity of the passive film. Passive current density in acidified ASW is higher than that in aerated ASW due to increased potential drop across film/solution interface, donor density and point defect diffusivity. The content of oxidized Cr, Fe(II) and hydroxides increases and film thickness reduces with acidification. In acidified ASW, passive film become denser from pre-passivation to passivation region and pores form accompanied with the increase of oxidized Fe after transpassivation.

1. Introduction Marine corrosion has always been a vital threat to the safe operation of ocean facilities [1]. Therefore, corrosion-resistant alloys are needed to provide long-term stability for the marine engineering equipment. Compared with the easily deteriorated conventional 304 or 316 austenitic steels in marine environments [2], super duplex stainless steels (SDSS) have been used increasingly due to their superior mechanical strength and corrosion resistance [3,4], such as the in the desalination systems and modern oil and gas offshore platforms [5]. Understanding of the corrosion behavior of SDSS in seawater is important for their use in ocean equipment. Corrosion resistance of SDSS in seawater is affected by many factors [6]. In some specific cases, when the steel is covered by marine organisms or crevice forms around it, a local confined environment with high aggressiveness will be generated and thus destroys the SDSS [7–9]. On one hand, oxygen is progressively consumed and cannot be renewed by diffusion or convection [7,8]. On the other hand, local acidity of the electrolyte is increased by the accumulation of H+ due to hydrolysis of iron ions [9–11]. Both the removal of dissolved oxygen and solution pH decrease have non-negligible impact on corrosion susceptibility of steels, which can dominate the electrochemical and passivation behavior of SDSS.

⁎

According to Frankel and Scully [12], both pit stability and passive film properties play important roles in the pitting corrosion process, in which pit stability consideration is the controlling factor under aggressive conditions, while passive film properties are critical in less extreme environments and/or for less susceptible alloys. In artificial seawater (ASW), the passivation properties are more crucial for the performance of SDSS. Some investigations have been devoted to interpret the effect of dissolved oxygen and solution pH on passivation of stainless steels [13–20]. Raja and Jones [13] suggested that dissolved oxygen provided necessary potential rather than chemical species for passivation of 304 stainless steel in H2SO4. Qiao et al. [14] found that dissolved oxygen increased passive current density of a high-nitrogen bearing SS in an acidic solution. As solutions are acidified, chemical composition of the passive film formed on stainless steels has changed significantly, which can influence the corrosion behavior [16,14–20]. In the alkaline buffer solution, a chromium enrichment which improved the film stability of 2205 duplex stainless steel as pH increased was observed by Luo et al. [16], while opposite conclusion was drew by Freire et al. [17] for AISI 316 SS. In acidic solutions, the chromium-rich oxide films are formed due to slower dissolution of chromium oxides when compared with iron oxides [19] and the enhanced preferential dissolution of Fe further facilitates the accumulation of Cr with a content higher than 50% [21]. Liu et al. [18] illustrated that the primary

Corresponding authors. E-mail addresses: [email protected] (Z. Cui), [email protected] (C. Man), [email protected] (S. Chen).

https://doi.org/10.1016/j.corsci.2019.02.002 Received 4 August 2017; Received in revised form 30 December 2018; Accepted 4 February 2019 Available online 11 February 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.

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constituents of the passive film formed on 254SMO steel in the weak (pH 5) and strong acidic solutions (pH 0.8) are iron oxides and Cr(III)containing compounds, respectively. Maurice et al. [22] confirmed a film with a mixed Cr(III) and Fe(III) oxide inner layer enriched with Cr2O3 and a Cr-hydroxide outer layer for Fe-22Cr single crystal in 0.5 M H2SO4. The passivation properties of stainless steels is also associated with the semiconductive characteristics of the film, including the carrier density and diffusion coefficient [23,24]. The carrier densities are confirmed to increase with the increment of pH value in the neutral/ alkaline range [16] and increase with decreasing pH in the acidic range for austenitic stainless steels [20,25], implying the degraded film protection. Now that the chemical composition and microstructure of SDSS are different from austenitic SS, the protectiveness of the film and its determinant roles are also different [26], especially when the environment parameters change as well. Towards a more comprehensive understanding of the passivation behavior of 2507 SDSS in modified ASW, it is indispensable to analyze the electrochemical behavior together with the electric properties and film composition of the passive film. In the present work, the passivation behavior and surface chemistry of SAF 2507 SDSS in the modified ASW are investigated. The influence of dissolved oxygen and solution pH on the passive film growth/dissolution kinetics, electronic characters (carrier density and diffusion coefficient) and passive film properties (morphology and composition) are evaluated. The effect of passive state (pre-passive, passive, and transpassive) on the film properties is also discussed.

adhesion force more than 80 MPa to avoid crevice corrosion before being embedded in a Polyvinyl chloride (PVC) tube using epoxy resin for electrochemical tests [27]. Electrochemical measurements were performed with a Zennium E electrochemical workstation in a conventional three-electrode cell, using a platinum sheet as counter electrode and a saturated calomel electrode (SCE) as reference electrode. Prior to tests, the working electrodes were initially potentiostatically polarized at -1.0 VSCE for 5 min to remove air-formed oxides. The N2 gas flow was maintained through the whole test in the two anaerobic solutions during the test. Potentiodynamic polarization measurements were performed at a scan rate of 0.5 mV/s in a potential range from -0.8 VSCE to 1.3 VSCE. Potentiostatic passivation tests were performed to obtain the steady state current density (iss) and clarify the kinetics of film growth and dissolution. The imposed potentials were selected according to the potentiodynamic curves and applied for 1 h. The iss was obtained from the average value at the end of each current-time transient curve. Electrochemical impedance spectroscopy (EIS) measurements started after the specimens were initially polarized at a certain anodic potential for 1 h, and were conducted in the frequency range from 100 kHz to 10 mHz with 7 points per decade using a 10 mV amplitude signal. The experimental data were analyzed by the commercial software ZsimpWin. The capacitance measurements were performed by sweeping the potential from the determined anodic value to the cathodic direction at a fixed frequency using a 10 mV ac signal and a step rate of 25 mV/s. Different frequencies ranging from 10 to 3000 Hz were used to compensate the frequency dispersion and choose the proper test frequency. The sweeping rate employed here is sufficiently fast to satisfy the assumption of “frozen-in defect structure” for the Mott-Schottky theory. All the electrochemical measurements were carried out at room temperature (23 ± 2 °C), and repeated at least three times to maintain the reproducibility.

2. Experimental 2.1. Materials and solutions The test material was a commercial standard 2507 (UNS S32750) SDSS with a thickness of 3 mm, supplied by Nippon Steel & Sumikin Stainless Steel Corporation. The alloy was provided in the form of hotrolled and solution-annealed at 1100 °C for 20 min, followed by water quenching. The nominal chemical composition (wt.%) was: C 0.03, Cr 25.15, Ni 6.74, Mo 3.43, N 0.27, Si 0.8, Mn 1.2, S 0.02, P 0.035, and Fe balance. The specimens were machined into square sheet with a dimension of 10 × 10 × 3 mm, and then wet ground sequentially to 2000 grit SiC paper, degreased with alcohol, cleaned in water, and then dried in cold air. The composition of the solutions simulating three different forms of seawater conditions are listed in Table 1. Solution A was the conventional artificial seawater prepared according to ASTM D1141. Solution B was the artificial seawater without oxygen that obtained by purging with N2 for 4 h. The residual oxygen concentration was about 0.72 mg/ L. Solution C was the simulation electrolyte of acidified seawater without oxygen, which was adjusted by CH3COOH to maintain a pH of 4.0. The three solutions are written as aerated ASW, deaerated ASW, and acidified ASW in the following statements. All the chemicals used in the experiments were analytic grade reagents. The test solution was freshly prepared with deionized water with a resistivity of 18.2 MΩ cm.

2.3. Surface composition and morphology analysis The chemical composition of passive film on specimens after potentiostatic polarization at different potentials for 1 h was examined by X-ray photoelectron spectroscopy (XPS) using ESCALAB 250Xi (Thermo Fisher Scientific, USA). The surface was sputtered by Ar+ ion bombardment every 10 s with an ion beam of 1 kV to obtain depth profiles. All XPS peaks were corrected to the standard carbon C 1 s binding energy (285 eV). The XPS data were analyzed by the XPSPEAK 4.1 software. The nanoscale lateral structures of the passive film formed under different passive states were observed by an atomic force microscope (AFM, Brucker Multimode VIII) in ScanAsyst-air mode. The specimens were polished and polarized at the designed potential for 1 h in acidified ASW. 3. Results 3.1. Potentiodynamic and potentiostatic polarization

2.2. Electrochemical test Fig. 1 shows the potentiodynamic polarization curves and the current density-time plots at 0.2 VSCE for 2507 SDSS in the three solutions.

Specimens were encased in epoxy powder coating at 220 °C with Table 1 The three test solutions used in this work (Temperature: 23 ± 2 °C). Solution

Solution A Solution B Solution C

Base chemical composition (g/L) NaCl

MgCl2

Na2SO4

CaCl2

KCl

NaHCO3

KBr

24.53 24.53 24.53

5.20 5.20 5.20

4.09 4.09 4.09

1.16 1.16 1.16

0.695 0.695 0.695

0.201 0.201 0.201

0.101 0.101 0.101

219

Dissolved oxygen (mg/L)

pH

4.85 ± 0.068 0.72 ± 0.006 0.49 ± 0.006

8.2 8.2 4.0

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Fig. 1. Potentiodynamic polarization curves (a), current-time transients in linear (b) and double logarithmic coordinates (c) at 0.2 VSCE, and the quasi steady state current density (d) of 2507 SDSS in the three solutions.

value, which is associated with the balance of film formation and dissolution [14]. In the log-log coordinate (Fig. 1c), the decay kinetics is nearly identical for the current measured under different conditions, confirming that the passivation mechanism of 2507 SDSS in the aerated, deaerated, and acidified ASW is consistent. The slopes in the linear region are all higher than 1, indicating the formation of a compact and highly protective passive film [32]. The dependence of quasi steady-state current density (iss) at the film formation potential within the passive region is given in Fig. 1d. Burstein and Daymond [33] reported that the steady state was difficult to be achieved based on the fact that passive current density continued to decline after 15 h. In the present work, the passive current density still decreases after polarization for 1 h (Fig. 1c). However, the purpose of this work is to investigate the passivation variation induced by solution chemistry, and thus the quasi steady current density after 1 h is utilized to interpret the differences as has been used in literature [34–36]. Macdonald [23] suggested that the transmission of cation interstitials and oxygen vacancies yielded an n-type barrier layer and a passive current density which was independent of applied voltage. The steady state current densities in the aerated, deaerated, and acidified ASW are 0.29, 0.28, and 0.41 μA cm−2, respectively. Accordingly, the presence of oxygen slightly affects the passivation of 2507 SDSS while the acidification has a detrimental effect.

In aerated ASW, the cathodic reaction is dominated by oxygen reduction, which is under mixed activation-diffusion control. Removal of oxygen does not alter the shape of the cathodic curve, indicating that corrosion is also driven by reduction of small amount of oxygen in solution and the reduction of H2O contributes relatively little to the total cathodic process [28]. The elimination of oxygen decreases the partial pressure of oxygen and reduces the oxygen reduction current, thus resulting in the left shift of cathodic curve and the negative shift of corrosion potential. In the acidified ASW, reduction of H+ dominates the cathodic process [29]. The Tafel slope is ascertained as -143.9 mV decade−1, implying that the discharge step controls the hydrogen evolution reaction [30]. Solution deaeration has few effects on the anodic polarization curve, attributed to the relatively lower oxygen concentration as compared with the saturated oxygen in literature [14,15], while acidification leads to the appearance of an anodic peak at about -0.46 VSCE, indicating an active surface in this environment as compared with the spontaneous passivation in alkaline solutions. In addition, transpassive dissolution peaks are also detected at more positive potential because of the dissolution of Cr(III) in passive film to CrO42- and Cr2O72-, depending on the solution pH. The typical i-t curves in linear and double logarithmic coordinates for 2507 SDSS potentiostatically polarized at the film formation potential (Ef) of 0.2 VSCE for 1 h are shown in Fig. 1b and c, respectively. The current density at each environment decreases sharply with time during the initial stage of polarization due to the rapid nucleation and growth of the passive film [31]. Then it maintains at a relatively stable 220

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Fig. 2. Nyquist (a, c, e) and Bode diagrams (d, d, f) of 2507 SDSS in aerated (a, b), deaerated (c, d), and acidified ASW (e, f) as well as the corresponding schematic film structure (g–i) and equivalent circuits (j, k) under prepassive (g, j), passive (h, j), and transpassive states (i, k).

passivated potential region, and then decreases up to the potential where transpassivity occurs. Similar variation of impedance with applied potential was observed by Feng et al. [37] and Mohammadi et al. [38].

3.2. Electrochemical impedance spectroscopy analysis In order to further investigate the passive film property formed on 2507 SDSS, EIS measurements were conducted at applied potentials after film formation for 1 h. All the EIS data have been validated using Kramers-Kronig transforms and satisfy the constraints of Linear System Theory. The EIS results are depicted in the form of Bode plots and Nyquist plots and shown in Fig. 2a-f, including the fitting results as solid lines. All the spectra are characterized as unfinished semi-arcs in the Nyquist diagrams, indicating similar passive mechanism. In the Bode diagrams, the impedance moduli display a linear slope close to -1 in the middle and low frequency range, and the phase angles evolve between 60° and 85°, meaning that the film is mainly capacitive. The film formation potential has a considerable effect on the EIS of 2507 SDSS in the three solutions. The impedance of the system increases gradually by raising the potential from negative value to the well-

3.2.1. Selection of equivalent circuit model based on the interface structure Equivalent electrical circuits are employed to analyze the EIS data of 2507 SDSS at different anodic potentials. Some widely used circuits are tried to fit the EIS results with comprehensive consideration of electrochemical mechanism, film structure, and fitting error. Within the pre-passive and passive region, the oxide layer or at least the inner layer remains contact because of the fast formation rate of the latter [39]. In this case, no charge transfer process occurs at the alloy/solution interface and the model including charge transfer process is abandoned. The oxide morphologies are schematically illustrated in Fig. 2g and h which indicate that the system is controlled by the process occurs at 221

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Table 2 Typical fitted electrochemical parameters for EIS of 2507 SDSS in the three solutions at different anodic potentials. Environment

Applied potential (VSCE)

Rs (Ω·cm2)

Q1 (× 10−5 Ω-1·cm-2·sn)

n

Solution A

−0.2 0.2 0.55 −0.2 0.2 0.55 −0.4 −0.2 0.2 0.8

6.36 6.19 7.52 5.96 5.87 6.73 7.47 6.71 6.37 8.35

5.48 2.26 3.77 5.49 2.44 3.40 7.59 10.6 6.97 12.6

0.89 0.93 0.89 0.89 0.93 0.90 0.87 0.87 0.87 0.72

Solution B

Solution C

± ± ± ± ± ± ± ± ± ±

0.15 0.07 0.08 0.39 0.11 0.22 0.62 1.21 0.28 1.54

± ± ± ± ± ± ± ± ± ±

0.014 0.004 0.002 0.019 0.004 0.011 0.010 0.013 0.002 0.015

R1 (kΩ·cm2)

Q2 (× 10−5 Ω-1·cm-2·sn)

n

R2 (kΩ·cm2)

261 ± 33.8 1029 ± 59.9 152 ± 28.4 483 ± 95.1 886 ± 25.6 160 ± 64.6 93.5 ± 13.6 412 ± 57.9 553 ± 13.4 20.5 ± 5.2

– – 1.37 ± 0.61 – – 0.94 ± 0.25 – – – 9.43 ± 0.47

– – 0.80 ± 0.11 – – 0.76 ± 0.08 – – – 0.79 ± 0.13

– – 364 ± 29.6 – – 402 ± 74.7 – – – 157 ± 35.9

3.2.2. EIS fitting and calculation of passive film parameters EIS data are fitted with the equivalent circuits (Fig. 2j and k) and the typical results are listed in Table 2. Fig. 3 shows the parameters derived from the fitting data which reveal the passivation properties of 2507 SDSS in different solutions. Fig. 3a marks the measurement extent of EIS parameters in the polarization curves. The polarization resistance in Fig. 3b illustrates that removal of oxygen has no apparent effect on the film resistance but acidification degrades the film protective ability. In the aerated and deaerated ASW, the increase and decrease trend of the film resistance suggests the competitive process of film formation and dissolution. At low anodic potentials, growth and thickening of the oxide film result in the film increase of resistance with positive shift of Ef. At high anodic potentials, dissolution of the film conceals the thickness increase and leads to the decline of the film resistance. In acidified ASW, a relatively stable Rp region, corresponding to the wellpassivated range in the polarization curve which has a low anodic

film/solution interface. Therefore, the simple Randles circuit (Fig. 2j), which represents an intact layer at the electrode/electrolyte interface [40,41], is used. The symbol Q is used to signify the non-ideal capacitance (CPE, constant phase element), which is caused by a inhomogeneous current flow [42]. When severe transpassive dissolution occurs, the surface film deteriorates due to the transformation from Cr (III) to Cr(VI). Pores are generated within the film, resulting in the contact of bare metal with electrolyte (Fig. 2i). The direct evidence of the film morphology will be presented and discussed in Section 4.4. In this circumstance, the equivalent circuit displayed in Fig. 2k is used to fit the EIS data [43]. It should be noted that the Randles circuit R(QR) is also used when the transpassive dissolution just starts and the film is not destroyed significantly such as 0.4 VSCE in aerated and deaerated ASW.

Fig. 3. Passive states of 2507 SDSS (a) as well as the calculated polarization resistance (b), reciprocal of effective capacitance (c), and film thickness (d) at different anodic potentials in the three solutions. (e) shows the electric field strength and film formation ratio of the film formed in different solutions. 222

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where B is a constant. As shown in Fig. 3e, the film formation ratio lies between 2 and 3 nm/V. The acidification of ASW has few effects on the film formation ratio while removal of oxygen decreases it. 3.3. Mott-Schottky analysis Electrochemical capacitance of the passive film/electrolyte interface is recorded as a function of the applied potential to characterize the semiconductive property and the density of charge carriers of the passive film. According to the Mott-Schottky theory, the space charge capacitance for a semiconductor electrode under the depletion condition can be given by Eqs. (5) and (6):

Fig. 4. Capacitance vs. frequency plot obtained at 0.2 VSCE for 2507 SDSS in the three solutions.

n

(1)

(2)

where α is the polarizability of the film/solution interface (α = 0.7 [50,51]), ε is the dielectric constant (15.6) and ε0 is the vacuum permittivity (8.8542 × 10−14 F/cm). As shown in Fig. 3e, the E0 values are in the range of 2–3 MV/cm, which coincides with the description of the PDM [23]. The elimination of dissolved oxygen increases the filed strength of the passive film while acidification decreases this parameter. Taking the barrier film as a planar capacitor, the film thickness (d) can be calculated using the following relation:

d=

εε0 A Ceff

(3)

where A is the surface area of the film. According to Wallinder et al. [52], the effective area is two times of the geometric area if a roughness factor of 2 is assumed. The film thickness as well as its dependence on the film formation potential is shown in Fig. 3d. The film thickness is in the range of 0.5–2.5 nm, which is consistent with literature. Removal of dissolved oxygen slightly decreases the film thickness and decrease of pH further reduces this parameter. The linear relationship between d and Ef can deduce the film formation ratio (r) according to the following expression [36]:

d = rEf + B=

1−α Ef + B E0

1 2 ⎛ kT ⎞ = Ef − EFB − for p-type C2 εε0 eNA ⎝ e ⎠

(6)

3.3.1. Selection of measurement frequency to compensate the frequency dispersion Semiconducting behavior of the passive film on duplex stainless steels have been reported in several works [26,53,54] with seldom consideration of the possible frequency dispersion, and the results are restricted to the frequency of 1 kHz [55]. In the present work, the frequency dependence of the measured capacitance is examined and the typical curves for the film formed at 0.2 VSCE in the three solutions are shown in Fig. 4. It is seen that the capacitance exhibits relatively fewer changes in the high-frequency range (f > 100 Hz) as compared with the obvious variation within the low frequency region. Therefore, a proper frequency within the high-frequency range should be carefully selected to eliminate the capacitance dependence on the frequency. Fig. 5a and b display the Mott-Schottky (M–S) plots of 2507 SDSS measured at different frequencies in aerated and acidified ASW, respectively. All of the plots are positively sloped, exhibiting the characteristic of n-type semiconductors. But the magnitude and slope of the curves change with measurement frequency, displaying the frequency dispersion phenomenon. According to Dutoit et al. [56] and Paola [55], these curves belong to “(A + B)-type”, with their slopes and intersections with the potential axis are frequency dependent. This type is common for the passive film formed on stainless steels, attributed to the non-uniform distribution of donors, the contribution of surface states to the capacitance response, and the presence of deep donors [55,57]. Several methods have been proposed to compensate the effect of frequency dispersion in literature [58–62]. Firstly, the method proposed by Harrington et al. [59–61] is employed and the calculation procedures are as follows: (i) single point impedance data at each potential in the M–S test along with the five values of frequency (Fig. 5a-b) are put together to build a pseudo-Bode diagram. (ii) The impedance data at each potential is fitted with the equivalent circuit R(QR). Fig. 5c and d present the typical Bode plots from M–S impedance data at selected potentials along with the fitting results. (iii) The Ceff values are calculated using Brug’s formula based on the fitting parameters Rs, Y0, and n. (iv) The inverse square of Ceff is plotted against the applied potential and compared with the measured M–S plots as shown in Fig. 5 e and f. In the aerated ASW, the frequency-compensated data by Harrington’s method and Brug’s formula is very close to the measured data at 1000 Hz (Fig. 5e). In the acidified ASW, the computed results lie between the data measured at 300 Hz and 1000 Hz, while the slope of the corrected curve is similar with that obtained at 1000 Hz (Fig. 5f). Secondly, the approach proposed by Kakaei et al. [62], which allows the arbitrary selection of measurement frequency, is also examined. In

where Y0 is the magnitude of CPE (Q1 in Table 2), Rs is solution resistance, and n is dispersion coefficient. Fig. 3c shows the dependence of C−1 on the film formation potential. The reciprocal of capacitance changes linearly with the potential within the passive range. According to Bojinov [49], the slope of this linear section is directly related to the electrical filed strength E0:

dC −1 1−α = dE ε0 εE0

(5)

where ND and NA are the donor and acceptor densities respectively, e is the electron charge (1.6 × 10−19 C), EFB is the flat-band potential, k is the Boltzman constant (1.38 × 10-23 J/K), T is the absolute temperature (K), ε, ε0 and Ef have the same meaning as before.

current density, is detected besides the rise and fall in the pre-passive and transpassive region. The effective capacitance (Ceff) of the passive film is extracted from the CPE element to interpret the film properties. Three commonly used approaches to obtain the Ceff of a system from the CPE values as summarized by Orazem et al. [44] are checked and the expression proposed by Brug et al. [45] is utilized because it gives reasonable results in the current work. The capacitance and film thickness values calculated by Hsu-Mansfeld equation [46] and Hirschorn equation [47] are deviated from the traditional capacitance of thin passive film and the well-accepted film thickness (1–3 nm) [48]. According to Brug et al. [45], the surface time-constant distribution is assumed and the Ceff can be obtained by the following equation:

Ceff = Y01 n Rs(1 − n)

1 2 ⎛ kT ⎞ = Ef − EFB − for n-type C2 εε0 eND ⎝ e ⎠

(4) 223

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Fig. 5. M–S plots of the film formed on 2507 SDSS (0.2 VSCE) in aerated (a) and acidified ASW (b) at different frequencies. (c) and (d) show the pseudo-Bode plots derived from the M–S data measured in aerated and acidified ASW at different frequencies, respectively. (e) and (f) compare the experimental results with the compensated results using Harrington’s and Kakaei’s methods in aerated and acidified ASW, respectively.

this method, the parameters n and Y0 are firstly calculated at each potential according to the following formula:

n=

−Zimg ⎞ 2 arctan ⎛ π Z ⎝ re − Rs ⎠ ⎜

Y0 = −

nπ 2 Zimg ωn

sin

(8)

where Zre and Zimg are from the M–S test at a given angular frequency ω, and Rs from the EIS test. With these parameters, Ceff is computed with the Brug’s expression and also compared with the measured data in Fig. 5e and f. It should be noted that the frequency-compensated data

⎟

(7)

224

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Fig. 6. Mott-Schottky plots obtained in aerated (a), deaerated (b), and acidified ASW (c) as well as the results acquired at 0.55 VSCE in the three solutions (d). Variation of donor densities and the fitting parameters are shown in (e) and (f), respectively.

current work.

at 3000 Hz is not included because it shows a significant deviation with others. In the aerated ASW, the corrected results from different frequencies using Kakaei’s method lie between the data measured at 1000 Hz and 3000 Hz (Fig. 5e). In acidified electrolytes, most of the compensated data also coincide with the data measured at 1000 Hz (Fig. 5f). In addition, using the method suggested by Rodríguez et al. [58], the calculated measurement frequency is also around 1000 Hz. After considering the three compensating approaches, therefore, the frequency of 1000 Hz is selected to conduct the M–S curves in the

3.3.2. Semiconducting properties of the passive film formed in the three electrolytes Fig. 6a-d shows the typical Mott-Schottky plots of 2507 SDSS measured at different film formation potentials in the three solutions. The fitting results in the linear regions are also included in the figures. The positive slope represents the behavior of an n-type semiconductor, whose predominant donor species are oxygen vacancies and/or cation 225

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interstitials. The variation of the slope with the applied potential in this region is attributed to the modification of the passive film structure and composition. The M–S curves obtained at 0.55 VSCE, which located within the transpassive region (Fig. 1a), are also observed and shown in Fig. 6d. Two linear regions are detected, attributed to the presence of Cr6+ ions in the passive film [36]. The donor densities of the passive film are calculated and shown in Fig. 6e. Within the passive region, the donor concentration decreases with increasing film formation potential since anodic polarization has an annealing effect which eliminates some defects in the passive film [36,63]. Firstly, when comparing the carrier densities in aerated and deaerated ASW, oxygen slightly increases the donor concentration of the passive film. Feng et al. [15] suggested that oxygen vacancies were the dominant defects in the passive film of 316 L stainless steel and their number increased when blowing oxygen into solution. Zhang et al. [64] proposed that oxygen would absorb into surface oxygen vacancies at the barrier layer/outer layer interface:

VO¨ + O2 → OO +O

(9)

and thus blocked the annihilation of oxygen vacancies:

VO¨ + H2 O→ OO +2H+

(10)

Noting that the oxygen vacancies will be generated at a constant rate and accumulate in the barrier layer, leading higher donor densities in film formed in the aerated ASW as compared with that in the deaerated solution. Secondly, the film formed in acidic ASW presents higher donor densities, revealing a thin and highly conductive passive film. Amri et al. [65] indicated that H+ protons would adsorb at the film/electrolyte interface and generate anionic oxygen vacancies increasingly by reacting with oxygen. This is also consistent with the PDM as shown in Eq. (10) that the decrease of pH shifts this reaction to the left hand side, creating more oxygen vacancies which increase the charge carrier density. The dependence of ND on Ef can be theoretically described as:

ND = ω1 exp(−bEff ) + ω2

(11)

where ω1, ω2, and b are constants. The exponential fitting curves to the experimental results are also shown in Fig. 6e and the parameters are listed in Fig. 6f. These parameters are related to defect diffusion within the passive film. 3.4. XPS analysis To investigate the effect of solution chemistry on the passive film composition by XPS, the specimens are polarized at 0.2 VSCE in aerated ASW and acidified ASW. In addition, two additional film formation potentials (-0.4 and 0.8 VSCE) are selected to characterize the variation of the passive film under different passive states in the acidified ASW. The composition of the film formed in deaerated ASW is not considered since the transfer between the laboratory and XPS instrument may encounter oxygen contamination although the specimen is packed in vacuum.

Fig. 7. Detailed XPS spectra of Fe 2p3/2 (a), Cr 2p3/2 (b), and O 1 s (c) of the passive film formed on 2507 SDSS at different anodic potentials.

3.4.1. Surface chemistry of the passive film Fig. 7 presents the detailed spectra of Fe 2p3/2, Cr 2p3/2, and O 1 s after deconvolution along with the percentage of different constituents. The corresponding binding energy and full width at half maximum (FWHM) of each compound are listed in Table 3. In the calculation of Fe (II) and Fe(III) percentage, Fe3O4 is regarded as FeO·Fe2O3. At Ef of 0.2 VSCE, Fe(III) compounds are the primary iron oxidized species in the two solutions (Fig. 7a1 and a3) and Fe2+/Fe3+ increases with acidification due to dissolution of Fe(III) (Fig. 7a3). As for the Cr compounds formed at 0.2 VSCE, Cr(OH)3 is the dominant species and the ratio between Cr2O3 and Cr(OH)3 decreases with decreasing pH. In addition, Fig. 7c1 and c3 show that the oxide/hydroxide ratio decreases from 1.83 to 0.86 as the solution is acidified. This agrees well with the conclusion

that hydroxides are the primary components of the film in aggressive environments while oxides occupy the main position in less corrosive media [18]. The applied potential also has considerable effects on the film composition of 2507 SDSS in acidified ASW. Strehblow et al. [66,67] divided the passivity of Fe-Cr alloys to pre-passive, passive, and transpassive ranges, which correspond to the Ef of -0.4 VSCE, 0.2 VSCE, and 0.8 VSCE in the present work, respectively. Variation of Fe is dominated by the decrease of Fe(II) and increase of Fe(III), which results in the decrease of Fe2+/Fe3+ ratio with increasing potential (Fig. 7a2-a4). Spectra of Cr in Fig. 7b2-b4 show variations including the decrease of 226

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because of the selective dissolution of Fe in the aggressive environments. Moreover, the outer layer of the film formed in the acidic solution has a higher Fe and Cr content and a lower O content than that formed in alkaline solution (Fig. 8e), implying a thinner film in the acidified ASW. The passive state also affects the element composition of the oxide film. As compared with the pre-passive state (Fig. 8b), Cr enriched in the sub-surface layer more intensively under the passive state (Fig. 8c) as indicated by the higher Cr content after the sputtering time exceeds 20 s (Fig. 8e). Under the transpassive state, content of Fe is always higher than that of Cr, suggesting the enhanced transpassive dissolution of Cr at this potential (Fig. 8d). In addition, the oxygen content increases with the positive shift of Ef (Fig. 8e), giving an indirect evidence for the thickening of the film. Further analysis of Fe and Cr spectra is performed and provided in Fig. 9 to determine the depth distribution of the species with different oxidation states. The Fe° signals strengthen and Fe oxide signals weaken with increasing sputtering time. The same tendency occurs on the metallic Cr and oxidized Cr peaks. Because of oxygen pollution, the O element cannot be eliminated even after sputtering for 110 s. The disappearance time of Fe2O3 peak in ASW (Fig. 9a) and acidified ASW (Fig. 9c) is 40 s and 30 s, respectively, suggesting that the Fe(III) compounds mainly exist in the outer layer of the film and solution acidification reduces its thickness. Variation of Cr spectra is similar with that of Fe but more sputtering time is needed for the disappearance of Cr(OH)3 (Fig. 9b and f). As the sputtering time prolongs, the Cr(OH)3 percent gradually decreases and Cr2O3 presents in the inner layer of the passive film. The influence of Ef on the film composition in acidified ASW can be observed from the changing tendency of Cr spectra (Fig. 9d, f, and h). The disappearance time of Cr(OH)3 extends as Ef is shifted to the noble direction, indicating the thickening of the oxide film. After normalization treatment of each constituent to the percent composition of that particular element, the amount of different species can be calculated and the results are plotted in Fig. 10. The inner layer and outer layer are difficult to be distinguished since the contamination of oxygen conceals boundary characters. According to these results, the

Table 3 Binding energy and full width at half maximum of XPS peaks used for deconvolution. Constituents

Binding Energy (eV)

FWHM (eV)

Fe° Fe3O4 FeO Fe2O3 FeOOH Fe(OH)3 Cr° Cr2O3 Cr(OH)3 O2− OH− H2O

707.0 708.0 709.2 710.6 711.5 713.0 574.2 576.1 577.4 530.6 532.0 532.9

1.1 1.1 1.9 2.4 2.2 3.3 1.5 2.0 1.8 1.5 2.0 1.1

± ± ± ± ± ± ± ± ± ± ± ±

0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1

± ± ± ± ± ± ± ± ± ± ± ±

0.1 0.1 0.5 0.8 0.2 0.5 0.2 0.2 0.3 0.2 0.3 0.3

metallic Cr with positive shift of applied potential and the emergence of CrO3 at 0.8 VSCE (Fig. 7b4), attributed to the thickening and the transpassive dissolution of the passive film, respectively [37,68,69]. The percentage of Cr hydroxides decreases with increasing applied anodic potential, which is in accordance with the increasing ratio of O2−/OH- as shown in Fig. 7c2-c4. Haupt et al. [66] suggested that the increasing anodic potential produced films with a smaller degree of hydration. Kong et al. [70] reported that lowering film formation potential would increase the water content in the passive film of pure Cr and thus increased the carrier densities. 3.4.2. Depth profiles of the passive film Fig. 8 shows the atomic percent depth profiles of Fe, Cr, Ni, Mo, and O in the passive films. The content of Fe remains higher than that of Cr in aerated ASW (Fig. 8a), while the Cr content exceeds Fe in the outmost layer of the film formed in acidified ASW (Fig. 8c), indicating that the oxide film is transformed from a Fe-rich outer layer to a Cr-rich outer layer as the solution pH decreases from 8.0 to 4.0. This is in accordance with findings of Liu et al. [18] and Zhang et al. [71] who found that the acid treated stainless steel presented a Cr-rich outer layer

Fig. 8. XPS depth profiles of the elements in the passive film on 2507 SDSS formed in aerated ASW at 0.2 VSCE (a) and in acidified ASW at -0.4 VSCE (b), 0.2 VSCE (c), 0.8 VSCE (d) as well as the comparison in element distribution (e). 227

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Fig. 9. The detailed XPS spectra of Fe 2p3/2 (a, c, e, g) and Cr 2p3/2 (b, d, f, h) of the passive film on 2507 SDSS formed in aerated ASW at 0.2 VSCE (a, b) and in acidified ASW at -0.4 VSCE (c, d), 0.2 VSCE (e, f), 0.8 VSCE (g, h).

dissolved oxygen and acidification of the ASW especially the latter, are the critical factors for corrosion in this environment. The variation of some passivation parameters and characteristics are listed in Table 4.

ratios between oxidized Fe and oxidized Cr at different sputtering time are illustrated in Fig. 10e. In aerated ASW, the content of oxidized Fe is 35.2% at the outmost layer, yielding a Feox+hy/Crox+hy ratio of 1.69. When the ASW is deoxygenated and acidified, the amount of oxidized Cr3+ in the outmost layer increases from 20.9% to 44.1%, yielding a Feox+hy/Crox+hy ratio of 0.55. This is caused by the high stability of Cr oxides and high dissolution rate of Fe species in aggressive environments [71]. After sputtering for 10 and 20 s, the Feox+hy/Crox+hy ratio decreases in aerated ASW and increases in the acidified electrolyte. The reduction of oxidized Fe and oxidized Cr indicates the enrichment of Fe and Cr compounds in the superficial layer. The variation trends of Feox+hy/Crox+hy ratio are similar in the pre-passive and passive ranges, which show a Cr enrichment in the outmost layer and a reversion in the subsurface layer. At the transpassive potential, opposite phenomenon is detected in the initial sputtering. After 20 s, oxidized Fe becomes dominant in the passive film.

4.1. Effect of solution chemistry on the passive film growth and dissolution kinetics According to Burstein and Marshall [72], the passive film grows initially by oxidation of all ingredients in the steel with the film thickening rate being controlled by ion migration under high electronic field. Then film dissolution occurs as the electric field across the growing film relaxes to a particular value, resulting in the leaching of Fe component to the solution. In the present work, the removal of dissolved oxygen has no detectable effect on the film dissolution, while the solution acidification increases the quasi iss (Fig. 1d), which is indicative of the promotion of film dissolution [35]. As reported in literature [23], the applied potential difference across the metal/film/ solution interface, ΔE, is the sum of potential drop across the metal/film interface (φm/f), the film/solution interface (φf/s), and the film (E0×d). Among them, the potential drop at film/solution interface φf/s is the determinative factor in controlling the dissolution rate of the passive film [63,73,74]. According to Moffat et al. [75], the film dissolution

4. Discussion Based on the above results, the 2507 SDSS/modified ASW couple belongs to the less extreme environment and less susceptible alloy. So the passive film properties, which are influenced by removal of 228

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Fig. 10. Distribution of different components within the passive film formed on 2507 SDSS in aerated ASW at 0.2 VSCE and in acidified ASW at -0.4 VSCE (b), 0.2 VSCE (c), and 0.8 VSCE. (e) shows the Feox+hy/Crox+hy ratio of the passive film after sputtering for different time.

through the oxide layer considering the almost constant value of E0 as compared with that in aerated ASW (Fig. 11c), which thus increases φf/s at the film/solution interface. This elevated φf/s provides more driving force for film dissolution and accelerates the film dissolution process of the steel in acidified ASW. In addition, the film formation ratio is not affected by the solution pH as indicated by the unchanged film formation ratio in Fig. 3e.

rate, id, is influenced by the Helmholtz layer potential difference according to the following equation:

i d = id0 exp ⎜⎛ ⎝

αmFφf / s RT

⎞⎟ ⎠

(12)

where id° is the exchange current density, m is the number of electrons. Fig. 11 shows the schematic diagram of the potential distribution across the metal/film/solution system in the three solutions. As shown in Fig. 3, after removal of dissolved oxygen in the ASW, E0 is slightly increased and the film thickness is slightly decreased. This yields a little variation of potential drop inside the passive film, and thus the φf/s shows no detectable change (Fig. 11b). However, in the acidified ASW, the film thickness is significantly reduced as revealed by the EIS and XPS results. Thinning of the film leads to a smaller potential drop

4.2. Effect of solution chemistry on the point defect diffusivity inside the passive film Diffusion coefficient (DO) of point defects (oxygen vacancies and/or cation interstitials in the present work) is a key parameter in describing the transport of point defects and the film properties [76]. Two widely229

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ASW. But for comparison with the literature results, both the high-field model (HFM) and low-field model (PDM) are utilized to obtain the DO values. Fig. 12 shows the calculated DO as well as the reported DO values for contrastive materials in literature. The DO values acquired by the two methods give the same variation trend while the results computed by the HFM are about tow orders lower than that by the PDM. The removal of dissolved oxygen decreases the diffusion coefficient while further acidification significantly increases it. The decrease in DO is supposed to provide better resistance to localized breakdown, particularly in terms of the pit initiation rate [86]. Now that the point defect diffusivity of passive film formed on duplex stainless steels is rarely reported, the results of carbon steel and stainless steels are integrated to the figure. DO of the film formed on 2507 SDSS is lower than that of austenite stainless steels in neutral and acidic solutions, implying a superior corrosion resistance of super duplex stainless steel. The diffusivity obtained by HFM for stainless steels is rare and the comparison is difficult. The passive current density can also reflect the density and the diffusion rate of point defects through the passive film [63]. According to the PDM [35,79], the quasi iss of an n-type semi-conductive film can be expressed as:

Table 4 Summary of solution chemistry effects on the passivation characteristics of 2507 SDSS in modified ASW. Parameters related to the passivation behavior

Corrosion potential, Ecorr Steady-state passive current density, iss Potential drop across film/solution interface, φf/s Electric field strength inside the passive film, E0 Film growth ratio, r Semi-conductive type of passive film Donor density, ND Diffusivity of point defect, DO Film thickness, d Fe2+/Fe3+ Crox/Crhy O2−/OHFeox+hy/Crox+hy

Variation trends Removal of oxygen

Acidification

↓ ↓ →

↓ ↑ ↑

↑

↓

↓ → ↓ ↓ ↓ – – – –

↑ → ↑ ↑ ↓ ↑ ↓ ↓ ↓

↑ means increase; ↓ means decrease; → means remain unchanged.

i ss = 4FKDO ND accepted approaches are used in calculating the point defect diffusivity. The magnitude of the field strength inside the passive film is considered when selecting the two methods. Under low-field condition, that is the parameters meet the following limits: zFaE0/RT ≪ 1, th(zFaE0/RT) = zFaE0/RT, and sh(zFaE0/RT) = zFaE0/RT (a is the half-jump distance). In this case, Macdonald [77] proposed that DO could be obtained based on the Nernst-Plank transport equation:

DO =

JO RT 2Fω2 E0

Accordingly, the increase in point defect diffusivity and carrier density promotes the passive current density, which is consistent with the results in Fig. 1d. In addition, this also indicates the increase in pitting risk since the higher carrier density reveals the higher affinity of chloride ions [23] and pit nucleation ability [87]. 4.3. Effect of removal of oxygen and acidification on the passive film composition

(13)

The composition difference between the film formed in aerated ASW and acidified ASW without oxygen is mainly attributed to the pH decrease. In the alkaline ASW (Solution A), Cr shows a small dissolution rate and dissolution of Fe is not fast enough to results in the Cr enrichment in the outmost layer. Therefore, the surface film is dominated by oxidized Fe. In acidic environment (solution C), dissolution of Fe(III) is much faster than that of Cr(III), resulting in Cr accumulation within the surface passive layer due to the preponderant Fe(III)-losses [88] (Figs. 8 and 10). Meanwhile, the dissolution of Fe(III) also contributes to the increase of Fe(II)/Fe(III) ratio (Fig. 7a). Assuming that Fe(II) is the main cation interstitial, the increase of Fe(II) proportion means the increase in donor density (Fig. 6) [25]. Furthermore, because of the strong Coulomb interaction between a proton and on oxygen ion, hydroxide can be formed according to the following reaction:

where JO is the steady-state flux of donors and can be correlated with iss as J0 = -iss/2e. Under high-field condition, the parameters meet the following limits: zFaE0/RT ≫ 1, th(zFaE0/RT) = 1, and sh(zFaE0/RT) = exp (zFaE0/RT)/2. In this case, DO can be expressed by the following equation:

DO =

2aJO ω2 exp(zFaE0/ RT )

(15)

(14)

The PDM, which adopted the low-field model to calculate DO, has been widely used in literature [15,34,76–81]. However, the field strength in the passive film is usually higher than 1 MV/cm, and Eq. (13) is not appropriate. Bojinov et al. [49] suggested that the high-filed model in Eq. (14) was more proper and has also been used in some published works [82–85]. In the present work (E0 = 2˜3 MV/cm), the high-field model is applicable for the passive film on 2507 SDSS in

H+ + O2 − = OH−

(16)

Fig. 11. Schematic diagrams of the potential distribution across the metal/film/solution interface for 2507 SDSS in aerated (a), deaerated (b), and acidified ASW (c) at 0.2 VSCE. 230

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Fig. 12. Point defect diffusivity of 2507 SDSS calcualted by the High Field Model (HFM) and Point Defect Model (PDM), as well as its comparison with literature results.

This supports the increase of hydroxide in the surface film as obtained by XPS. The film thickness is also reduced by the acidification of the ASW as proved by the EIS and XPS results. In XPS depth profiles, the passive film thickness can be simply estimated using the half-height of oxygen. Taking the rate of Ar ion sputtering (0.1 nm/s) into account, the equivalent thickness of the film formed in ASW and acidified ASW are about 2.47 and 1.14 nm, respectively (Fig. 8e), confirming thinning of the film after solution acidification although this method is not very accurate because of destruction by sputtering. This agrees well with the EIS results, in which the film thickness is calculated to be 2.38 and 1.26 nm for the film formed in ASW and acidified ASW at 0.2 VSCE, respectively, by Ceff which is determined through Brug’s equation. This is in accordance with Mohammadi et al. [38] who reported that Brug’s equation showed better consistency in estimating film thickness.

(Fig. 13b). As the anodic potential is shifted to 0.2 VSCE, the energy bands bend upwards and EF still locates inside the forbidden gap (Fig. 13g). In this case, the three-dimensional growth of oxide particles and their coalescence dominate the film growth process, giving rise to the increase in film thickness (Fig. 8e) [90]. The polished scratch becomes obscure but the surface roughness increases to 3.19 nm, indicating that particle growth rather than the coalescence plays primary role during this period [91]. Meanwhile, the oxide content within the passive film increases regardless of the little change in Feox+hy/Crox+hy ratio, attributed mainly to the increasing Cr2O3 proportion (Fig.13b). At 0.8 VSCE, the band bending becomes more obvious so that the Fermi-level is lower than the valence band edge (EV) at the outmost surface layer of the film, corresponding to the degenerate state of electron energy levels at the film surface [92]. In this case, removal of Dorbital electrons by tunneling induces holes accumulating in the outmost layer, giving rise to the higher valent species in the film and an increase in the Helmholtz potential [75]. This results in the increase of dissolution current and occurrence of transpassive dissolution. The AFM film morphology confirms the preferential dissolution and perforation of the film at some local sites at this potential (Fig. 13e). This also verifies the rationality of the equivalent circuit used in Fig. 2 for fitting EIS data. The pores exhibit a depth exceeding 60 nm, which is comparable with that observed by Man et al. [43]. Marcus et al. [93] suggested that preferential formation of Cr species at some sites lowered Cr enrichment of the passive film in the vicinity of these sites and resulted in the transpassive dissolution. The dissolution of Cr(III) to Cr (IV) as detected by XPS (Fig. 9h) contributes to the significant increase of Feox+hy/Crox+hy ratio (Fig. 13b). Furthermore, Fe(III) content increases to a high level due to its stability at high potential and the Fe (III) compounds become the main constituents that maintain the passivity and protectiveness of the film within this range and the following secondary passive region.

4.4. Characters of the passive film formed in acidified ASW under different passive states Fig. 13 illustrates the composition, surface morphology, and the corresponding energy band structure of the passive film formed under different states. The applied potential -0.4 VSCE can be considered as the point where passivation starts. This potential is negative than the flatband potential (Fig. 6c), yielding a p-type semiconductor and a downward bending of the energy band (Fig. 13f) [89]. Even so, the Fermilevel (EF) at the semiconductor surface is lower than the conduction band edge, and passive film begins to grow. Under this circumstance, the film is very thin and the ridged structure induced by polishing can be observed (Fig. 13c). Some oxide islands partially covering the substrate are formed at local sites, yielding a surface roughness of 0.2 nm. Hydroxides, which mainly composed of FeOOH, Fe(OH)3, and Cr(OH)3, dominate the surface film with the Cr-hydroxides maintaining the passivity despite the higher oxidized Fe content in the passive film 231

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Fig. 13. Film composition (b), film nano-morphology (c–e), and energy band structure (f–h) of the passive film formed on 2507 SDSS in acidified ASW at -0.4 VSCE (c, f), 0.2 VSCE (d, g), and 0.8 VSCE (e, h). The passive states can be refered in the polariztaion curve (a).

5. Conclusion

diffusivity. (2) Acidification of ASW increases the passive current density by increasing the potential drop across the film/solution interface (φf/s), the point defect diffusivity (DO), and the donor density (ND). (3) Film composition is characterized by XPS and film thickness is calculated by both XPS depth profiles and EIS (Brug’s equation is used). Acidification of ASW increases the content of oxidized Cr, Fe (II) and hydroxides inside the passive film, and reduces the film thickness. (4) In acidified ASW, a thin passive film with Cr hydroxides as the primary constituent forms in the pre-passive region, while a more dense film with higher oxide content is generated in passive region.

Electrochemical passivation and surface chemistry of 2507 SDSS in the aerated, deaerated, and acidified artificial seawater have been investigated in this work. The main conclusions can be drawn as follows: (1) To determine the influence of solution chemistry on passive film properties, the potential distribution across the metal/film/solution interface is characterized and the point defect diffusivity is calculated using both High Field Model and the Point Defect Model. Removal of dissolved oxygen decreases the electric field strength (E0) inside the passive film and slightly decreases the point defect 232

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Further increase the potential to transpassive region, the content of oxidized Fe (especially Fe3+ components) and Cr2O3 increases and some pores form on the film due to transpassive dissolution.

[27] [28]

Acknowledgements [29]

The authors wish to acknowledgement the financial support of National Natural Science Foundation of China (Nos. 51701102and51601182), Fundamental Research Funds for the Central Universities (No. 201762008), and the National Environmental Corrosion Platform.

[30]

[31]

References [32] [1] X. Li, D. Zhang, Z. Liu, Z. Li, C. Du, C. Dong, Materials science: share corrosion data, Nature 527 (2015) 441–442. [2] A. Ul-Hamid, H. Saricimen, A. Quddus, A.I. Mohammed, L.M. Al-Hems, Corrosion study of SS304 and SS316 alloys in atmospheric, underground and seawater splash zone in the Arabian Gulf, Corros. Eng. Sci.Technol. 52 (2017) 134–140. [3] I. Alvarez-Armas, Duplex stainless steels: brief history and some recent alloys, Recent Patents Mech. Eng. 1 (2008) 51–57. [4] Y. Zhao, E. Zhou, Y. Liu, S. Liao, Z. Li, D. Xu, T. Zhang, T. Gu, Comparison of different electrochemical techniques for continuous monitoring of the microbiologically influenced corrosion of 2205 duplex stainless steel by marine Pseudomonas aeruginosa biofilm, Corros. Sci. 126 (2017) 142–151. [5] J. Olsson, M. Snis, Duplex—a new generation of stainless steels for desalination plants, Desalination 205 (2007) 104–113. [6] M. da Cunha Belo, B. Rondot, C. Compere, M. Montemor, A. Simoes, M. Ferreira, Chemical composition and semiconducting behaviour of stainless steel passive films in contact with artificial seawater, Corros. Sci. 40 (1998) 481–494. [7] S. Marcelin, N. Pébère, S. Régnier, Electrochemical investigations on crevice corrosion of a martensitic stainless steel in a thin-layer cell, J. Electroanal. Chem. Lausanne (Lausanne) 737 (2015) 198–205. [8] S. Marcelin, N. Pébère, S. Régnier, Electrochemical characterisation of a martensitic stainless steel in a neutral chloride solution, Electrochim. Acta 87 (2013) 32–40. [9] B.J. Little, J.S. Lee, R.I. Ray, The influence of marine biofilms on corrosion: a concise review, Electrochim. Acta 54 (2008) 2–7. [10] Z. Cui, Z. Liu, X. Wang, Q. Li, C. Du, X. Li, W. Zhang, Crack growth behaviour and crack tip chemistry of X70 pipeline steel in near-neutral pH environment, Corros. Eng. Sci. Technol. 51 (2016) 352–357. [11] H. Tian, X. Wang, Z. Cui, Q. Lu, L. Wang, L. Lei, Y. Li, D. Zhang, Electrochemical corrosion, hydrogen permeation and stress corrosion cracking behavior of E690 steel in thiosulfate-containing artificial seawater, Corros. Sci. 144 (2018) 145–162. [12] G.S. Frankel, T. Li, J.R. Scully, Perspective—localized corrosion: passive film breakdown vs Pit growth stability, J. Electrochem. Soc. 164 (2017) C180–C181. [13] K. Raja, D. Jones, Effects of dissolved oxygen on passive behavior of stainless alloys, Corros. Sci. 48 (2006) 1623–1638. [14] Y. Qiao, Y. Zheng, P. Okafor, W. Ke, Electrochemical behaviour of high nitrogen bearing stainless steel in acidic chloride solution: effects of oxygen, acid concentration and surface roughness, Electrochim. Acta 54 (2009) 2298–2304. [15] Z. Feng, X. Cheng, C. Dong, L. Xu, X. Li, Effects of dissolved oxygen on electrochemical and semiconductor properties of 316L stainless steel, J. Nucl. Mater. Phys. Mater. Sci. Radiat. Appl. 407 (2010) 171–177. [16] H. Luo, C. Dong, X. Li, K. Xiao, The electrochemical behaviour of 2205 duplex stainless steel in alkaline solutions with different pH in the presence of chloride, Electrochim. Acta 64 (2012) 211–220. [17] L. Freire, M. Catarino, M. Godinho, M. Ferreira, M. Ferreira, A. Simões, M. Montemor, Electrochemical and analytical investigation of passive films formed on stainless steels in alkaline media, Cement Concrete Comp. 34 (2012) 1075–1081. [18] C. Liu, J. Wu, Influence of pH on the passivation behavior of 254SMO stainless steel in 3.5% NaCl solution, Corros. Sci. 49 (2007) 2198–2209. [19] M. Kumagai, S.-T. Myung, Y. Katada, H. Yashiro, Stability of type 310S stainless steel bipolar plates tested at various current densities in proton exchange membrane fuel cells, Electrochim. Acta 211 (2016) 754–760. [20] H. Luo, H. Su, C. Dong, K. Xiao, X. Li, Influence of pH on the passivation behaviour of 904L stainless steel bipolar plates for proton exchange membrane fuel cells, J. Alloys. Compd. 686 (2016) 216–226. [21] C.-O. Olsson, D. Landolt, Passive films on stainless steels—chemistry, structure and growth, Electrochim. Acta 48 (2003) 1093–1104. [22] V. Maurice, W. Yang, P. Marcus, XPS and STM study of passive films formed on Fe22Cr (110) single-crystal surfaces, J. Electrochem. Soc. 143 (1996) 1182–1200. [23] D.D. Macdonald, The history of the point defect model for the passive state: a brief review of film growth aspects, Electrochim. Acta 56 (2011) 1761–1772. [24] Z. Cui, L. Wang, H. Ni, W. Hao, C. Man, S. Chen, X. Wang, Z. Liu, X. Li, Influence of temperature on the electrochemical and passivation behavior of 2507 super duplex stainless steel in simulated desulfurized flue gas condensates, Corros. Sci. 118 (2017) 31–48. [25] M. Carmezim, A. Simoes, M. Montemor, M.D.C. Belo, Capacitance behaviour of passive films on ferritic and austenitic stainless steel, Corros. Sci. 47 (2005) 581–591. [26] X. Cheng, Y. Wang, C. Dong, X. Li, The beneficial galvanic effect of the constituent

[33]

[34]

[35]

[36]

[37]

[38] [39]

[40] [41]

[42] [43]

[44]

[45]

[46] [47]

[48] [49] [50]

[51] [52]

[53] [54] [55] [56]

[57]

233

phases in 2205 duplex stainless steel on the passive films formed in a 3.5% NaCl solution, Corros. Sci. 134 (2018) 122–130. Y. Yang, T. Zhang, Y. Shao, G. Meng, F. Wang, In situ study of dew point corrosion by electrochemical measurement, Corros. Sci. 71 (2013) 62–71. H.J. Flitt, D.P. Schweinsberg, A guide to polarisation curve interpretation: deconstruction of experimental curves typical of the Fe/H2O/H+/O2 corrosion system, Corros. Sci. 47 (2005) 2125–2156. A. Davydov, K.V. Rybalka, L.A. Beketaeva, G.R. Engelhardt, P. Jayaweera, D.D. Macdonald, The kinetics of hydrogen evolution and oxygen reduction on Alloy 22, Corros. Sci. 47 (2005) 195–215. Z. Cui, L. Wang, M. Zhong, F. Ge, H. Gao, C. Man, C. Liu, X. Wang, Electrochemical behavior and surface characteristics of pure titanium during corrosion in simulated desulfurized flue gas condensates, J. Electrochem. Soc. 165 (2018) C542–C561. C. Escrivà-Cerdán, E. Blasco-Tamarit, D. García-García, J. García-Antón, R. Akid, J. Walton, Effect of temperature on passive film formation of UNS N08031 Cr–Ni alloy in phosphoric acid contaminated with different aggressive anions, Electrochim. Acta 111 (2013) 552–561. D.A. Jones, N. Greene, Electrochemical measurement of low corrosion rates, Corrosion 22 (1966) 198–205. G.T. Burstein, B.T. Daymond, The remarkable passivity of austenitic stainless steel in sulphuric acid solution and the effect of repetitive temperature cycling, Corros. Sci. 51 (2009) 2249–2252. A. Fattah-Alhosseini, M. Golozar, A. Saatchi, K. Raeissi, Effect of solution concentration on semiconducting properties of passive films formed on austenitic stainless steels, Corros. Sci. 52 (2010) 205–209. Z. Cui, S. Chen, L. Wang, C. Man, Z. Liu, J. Wu, X. Wang, S. Chen, X. Li, Passivation behavior and surface chemistry of 2507 super duplex stainless steel in acidified artificial seawater containing thiosulfate, J. Electrochem. Soc. 164 (2017) C856–C868. R. Fernández-Domene, E. Blasco-Tamarit, D. García-García, J. García-Antón, Passive and transpassive behaviour of Alloy 31 in a heavy brine LiBr solution, Electrochim. Acta 95 (2013) 1–11. Z. Feng, X. Cheng, C. Dong, L. Xu, X. Li, Passivity of 316L stainless steel in borate buffer solution studied by Mott–Schottky analysis, atomic absorption spectrometry and X-ray photoelectron spectroscopy, Corros. Sci. 52 (2010) 3646–3653. F. Mohammadi, T. Nickchi, M. Attar, A. Alfantazi, EIS study of potentiostatically formed passive film on 304 stainless steel, Electrochim. Acta 56 (2011) 8727–8733. Z. Lu, D.D. Macdonald, Transient growth and thinning of the barrier oxide layer on iron measured by real-time spectroscopic ellipsometry, Electrochim. Acta 53 (2008) 7696–7702. J.E.G. González, a.F.J.H. Santana, J.C. Mirza-rosca, Effect of bacterial biofilm on 316 SS corrosion in natural seawater by eis, Corros. Sci. 40 (1998) 2141–2154. C. Boissy, C. Alemany-Dumont, B. Normand, EIS evaluation of steady-state characteristic of 316L stainless steel passive film grown in acidic solution, Electrochem. Commun. 26 (2013) 10–12. J.-B. Jorcin, M.E. Orazem, N. Pébère, B. Tribollet, CPE analysis by local electrochemical impedance spectroscopy, Electrochim. Acta 51 (2006) 1473–1479. C. Man, C. Dong, Z. Cui, K. Xiao, Q. Yu, X. Li, A comparative study of primary and secondary passive films formed on AM355 stainless steel in 0.1M NaOH, Appl. Surf. Sci. 427 (2018) 763–773. M.E. Orazem, I. Frateur, B. Tribollet, V. Vivier, S. Marcelin, N. Pébère, A.L. Bunge, E.A. White, D.P. Riemer, M. Musiani, Dielectric properties of materials showing Constant-Phase-Element (CPE) impedance response, J. Electrochem. Soc. 160 (2013) C215–C225. G.J. Brug, A.L.G. van den Eeden, M. Sluyters-Rehbach, J.H. Sluyters, The analysis of electrode impedances complicated by the presence of a constant phase element, J. Electroanal. Chem. 176 (1984) 275–295. C.H. Hsu, F. Mansfeld, Technical Note: concerning the conversion of the constant phase element parameter Y0 into a capacitance, Corrosion 57 (2001) 747–748. B. Hirschorn, M.E. Orazem, B. Tribollet, V. Vivier, I. Frateur, M. Musiani, ConstantPhase-Element behavior caused by resistivity distributions in films: I. Theory, J. Electrochem. Soc. 157 (2010) C452–C457. M.A. Rodríguez, R.M. Carranza, R.B. Rebak, Passivation and depassivation of alloy 22 in acidic chloride solutions, J. Electrochem. Soc. 157 (2010) C1–C8. M. Bojinov, The ability of a surface charge approach to describe barrier film growth on tungsten in acidic solutions, Electrochim. Acta 42 (1997) 3489–3498. C.B. Breslin, D.D. Macdonald, J. Sikora, E. Sikora, Influence of uv light on the passive behaviour of SS316—effect of prior illumination, Electrochim. Acta 42 (1997) 127–136. D.D. Macdonald, A. Sun, An electrochemical impedance spectroscopic study of the passive state on Alloy-22, Electrochim. Acta 51 (2006) 1767–1779. D. Wallinder, J. Pan, C. Leygraf, A. Delblanc-Bauer, EIS and XPS study of surface modification of 316LVM stainless steel after passivation, Corros. Sci. 41 (1998) 275–289. H. Luo, C.F. Dong, K. Xiao, X.G. Li, Characterization of passive film on 2205 duplex stainless steel in sodium thiosulphate solution, Appl. Surf. Sci. 258 (2011) 631–639. J. Yao, D.D. Macdonald, C. Dong, Passive film on 2205 duplex stainless steel studied by photo-electrochemistry and ARXPS methods, Corros. Sci. 146 (2019) 221–232. A.D. Paola, Semiconducting properties of passive films on stainless steels, Electrochim. Acta 34 (1989) 203–210. E.C. Dutoit, R.L. van Meirhaeghe, F. Cardon, W.P. Gomes, Investigation on the frequency-dependence of the impedance of the nearly ideally polarizable semiconductor electrodes CdSe, CdS and TiO2, Ber, Bunsenges Phys. Chem. 79 (1975) 1206–1213. V.M.-W. Huang, V. Vivier, M.E. Orazem, N. Pébère, B. Tribollet, The apparent constant-phase-element behavior of a disk electrode with faradaic reactions: a

Corrosion Science 150 (2019) 218–234

Z. Cui, et al.

global and local impedance analysis, J. Electrochem. Soc. 154 (2007) C99–C107. [58] M.A. Rodríguez, R.M. Carranza, Properties of the passive film on alloy 22 in chloride solutions obtained by electrochemical impedance, J. Electrochem. Soc. 158 (2011) C221–C230. [59] S.P. Harrington, T.M. Devine, Analysis of electrodes displaying frequency dispersion in Mott-Schottky tests, J. Electrochem. Soc. 155 (2008) C381–C386. [60] S.P. Harrington, T.M. Devine, Relation between the semiconducting properties of a passive film and reduction reaction rates, J. Electrochem. Soc. 156 (2009) C154–C159. [61] S.P. Harrington, F. Wang, T.M. Devine, The structure and electronic properties of passive and prepassive films of iron in borate buffer, Electrochim. Acta 55 (2010) 4092–4102. [62] M.N. Kakaei, J. Neshati, H. Hoseiny, T. Poursaberi, Compensating the effects of frequency dispersion in Mott–Schottky analyses of passive films, Corrosion 73 (2017) 970–978. [63] X.-Z. Wang, H. Luo, J.-L. Luo, Effects of hydrogen and stress on the electrochemical and passivation behaviour of 304 stainless steel in simulated PEMFC environment, Electrochim. Acta 293 (2019) 60–77. [64] Y. Zhang, M. Urquidi-Macdonald, G.R. Engelhardt, D.D. Macdonald, Development of localized corrosion damage on low pressure turbine disks and blades: I. Passivity, Electrochim. Acta 69 (2012) 1–11. [65] J. Amri, T. Souier, B. Malki, B. Baroux, Effect of the final annealing of cold rolled stainless steels sheets on the electronic properties and pit nucleation resistance of passive films, Corros. Sci. 50 (2008) 431–435. [66] S. Haupt, H.-H. Strehblow, A combined surface analytical and electrochemical study of the formation of passive layers on FeCr alloys in 0.5 M H2SO4, Corros. Sci. 37 (1995) 43–54. [67] P. Keller, H.-H. Strehblow, XPS investigations of electrochemically formed passive layers on Fe/Cr-alloys in 0.5 M H2SO4, Corros. Sci. 46 (2004) 1939–1952. [68] L. Ries, M.D.C. Belo, M. Ferreira, I. Muller, Chemical composition and electronic structure of passive films formed on Alloy 600 in acidic solution, Corros. Sci. 50 (2008) 676–686. [69] H. Tian, X. Cheng, Y. Wang, C. Dong, X. Li, Effect of Mo on interaction between α/γ phases of duplex stainless steel, Electrochim. Acta 267 (2018) 255–268. [70] D.-S. Kong, S.-H. Chen, C. Wang, W. Yang, A study of the passive films on chromium by capacitance measurement, Corros. Sci. 45 (2003) 747–758. [71] J. Liu, T. Zhang, G. Meng, Y. Shao, F. Wang, Effect of pitting nucleation on critical pitting temperature of 316L stainless steel by nitric acid passivation, Corros. Sci. 91 (2015) 232–244. [72] G.T. Burstein, P.I. Marshall, The coupled kinetics of film growth and dissolution of stainless steel repassivating in acid solutions, Corros. Sci. 24 (1984) 449–462. [73] G. Tranchida, M. Clesi, F. Di Franco, F. Di Quarto, M. Santamaria, Electronic properties and corrosion resistance of passive films on austenitic and duplex stainless steels, Electrochim. Acta 273 (2018) 412–423. [74] L. Guan, Y. Li, G. Wang, Y. Zhang, L.-C. Zhang, pH dependent passivation behavior of niobium in acid fluoride-containing solutions, Electrochim. Acta 285 (2018) 172–184. [75] T.P. Moffat, R.M. Latanision, An electrochemical and X-Ray photoelectron

[76]

[77] [78] [79]

[80] [81]

[82] [83]

[84] [85]

[86]

[87]

[88] [89] [90]

[91]

[92] [93]

234

spectroscopy study of the passive state of chromium, J. Electrochem. Soc. 139 (1992) 1869–1879. G. Meng, Y. Li, Y. Shao, T. Zhang, Y. Wang, F. Wang, Effect of Cl− on the properties of the passive films formed on 316L stainless steel in acidic solution, J. Mater. Sci. Technol. 30 (2014) 253–258. E. Sikora, J. Sikora, D.D. Macdonald, A new method for estimating the diffusivities of vacancies in passive films, Electrochim. Acta 41 (1996) 783–789. Y.F. Cheng, C. Yang, J.L. Luo, Determination of the diffusivity of point defects in passive films on carbon steel, Thin Solid Films 416 (2002) 169–173. S.J. Ahn, H.S. Kwon, Effects of solution temperature on electronic properties of passive film formed on Fe in pH 8.5 borate buffer solution, Electrochim. Acta 49 (2004) 3347–3353. H.X. Guo, B.T. Lu, J.L. Luo, Study on passivation and erosion-enhanced corrosion resistance by Mott-Schottky analysis, Electrochim. Acta 52 (2006) 1108–1116. A. Fattah-alhosseini, F. Soltani, F. Shirsalimi, B. Ezadi, N. Attarzadeh, The semiconducting properties of passive films formed on AISI 316 L and AISI 321 stainless steels: a test of the point defect model (PDM), Corros. Sci. 53 (2011) 3186–3192. S. Ahn, H. Kwon, Diffusivity of point defects in the passive film on Fe, J. Electroanal. Chem. 579 (2005) 311–319. D.G. Li, J.D. Wang, D.R. Chen, Influence of potentiostatic aging, temperature and pH on the diffusivity of a point defect in the passive film on Nb in an HCl solution, Electrochim. Acta 60 (2012) 134–146. G. Vázquez, I. González, Diffusivity of anion vacancies in WO3 passive films, Electrochim. Acta 52 (2007) 6771–6777. M. Bojinov, G. Fabricius, T. Laitinen, K. Mäkelä, T. Saario, G. Sundholm, Influence of molybdenum on the conduction mechanism in passive films on iron–chromium alloys in sulphuric acid solution, Electrochim. Acta 46 (2001) 1339–1358. B. Ter-Ovanessian, C. Alemany-Dumont, B. Normand, Electronic and transport properties of passive films grown on different Ni-Cr binary alloys in relation to the pitting susceptibility, Electrochim. Acta 133 (2014) 373–381. X. Feng, X. Lu, Y. Zuo, N. Zhuang, D. Chen, The effect of deformation on metastable pitting of 304 stainless steel in chloride contaminated concrete pore solution, Corros. Sci. 103 (2016) 223–229. H.-H. Strehblow, Passivity of metals studied by surface analytical methods, a review, Electrochim. Acta 212 (2016) 630–648. T.D. Burleigh, Anodic photocurrents and corrosion currents on passive and activepassive metals, Corrosion 45 (1989) 464–472. T. Massoud, V. Maurice, L.H. Klein, A. Seyeux, P. Marcus, Nanostructure and local properties of oxide layers grown on stainless steel in simulated pressurized water reactor environment, Corros. Sci. 84 (2014) 198–203. C. Man, C. Dong, Z. Cui, K. Xiao, Q. Yu, X. Li, Characterization of the outer layer nanostructure in the electrochemical response of stainless steel in aqueous sodium hydroxide, Anal. Lett. 51 (2018) 1384–1399. N. Sato, Toward a more fundamental understanding of corrosion processes, Corrosion 45 (1989) 354–368. T. Massoud, V. Maurice, L.H. Klein, P. Marcus, Nanoscale morphology and atomic structure of passive films on stainless steel, J. Electrochem. Soc. 160 (2013) C232–C238.