Electrochemical corrosion behavior of 2205 duplex stainless steel in hot concentrated seawater under vacuum conditions

Electrochemical corrosion behavior of 2205 duplex stainless steel in hot concentrated seawater under vacuum conditions

Journal Pre-proof Electrochemical corrosion behavior of 2205 duplex stainless steel in hot concentrated seawater under vacuum conditions Yong Yang, Ho...

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Journal Pre-proof Electrochemical corrosion behavior of 2205 duplex stainless steel in hot concentrated seawater under vacuum conditions Yong Yang, Hongtao Zeng, Sensen Xin, Xueling Hou, Moucheng Li

PII:

S0010-938X(19)31165-5

DOI:

https://doi.org/10.1016/j.corsci.2019.108383

Reference:

CS 108383

To appear in:

Corrosion Science

Received Date:

5 June 2019

Revised Date:

2 December 2019

Accepted Date:

6 December 2019

Please cite this article as: Yang Y, Zeng H, Xin S, Hou X, Li M, Electrochemical corrosion behavior of 2205 duplex stainless steel in hot concentrated seawater under vacuum conditions, Corrosion Science (2019), doi: https://doi.org/10.1016/j.corsci.2019.108383

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Electrochemical corrosion behavior of 2205 duplex stainless steel in hot concentrated seawater under vacuum conditions

Yong Yang, Hongtao Zeng, Sensen Xin, Xueling Hou, Moucheng Li *

Institute of Materials, School of Materials Science and Engineering, Shanghai University, 149

* Corresponding author. E-mail address: [email protected]



The polarization resistance, passive film composition and oxygen reduction reaction

The pitting potential decreases linearly with the vacuum pressure until the appearance of

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violent boiling in the solution. 

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markedly change with the vacuum degree. 

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Research Highlights

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Yanchang Road, Shanghai 200072, China

The pressure reduction and solution boiling may enhance both the occurrence and

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Abstract

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repassivation tendencies of stable pitting corrosion.

The corrosion behavior of 2205 duplex stainless steel (DSS) was investigated in the hot

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concentrated seawater under vacuum pressures by using electrochemical measurement techniques. The vacuum pressure results in the decrease of dissolved oxygen concentration and even boiling of solution. 2205 DSS passivates spontaneously under various vacuum pressures. The vacuum pressures (no lower than 28.4 kPa) greatly influence the polarization resistance, passive film composition and oxygen reduction reaction. The pressure reduction and the violent boiling of solution distinctly decrease the pitting potential, but increase the repassivation potential. They may enhance both the occurrence and repassivation tendencies of stable pitting corrosion. 1

Keywords: Duplex stainless steel; vacuum pressure; pitting corrosion; passivation; hot concentrated seawater

1. Introduction The low temperature multi-effect distillation (LT-MED) has attracted great attentions to supply fresh water in many areas on the earth [1-4]. As an efficient thermal desalination process,

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LT-MED operates at temperatures lower than 75 ºC in consideration of the scaling and corrosion problems. In order to improve the thermal evaporation efficiency and low-energy consumption,

the evaporator chambers of LT-MED work under low pressures such as approximately 0.3 bar (i.e.,

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a vacuum degree of about 0.7) in the first effect [3]. The sprayed seawater forms flowing liquid

films on the shells and tubes [1], where the seawater will boil slightly in accompany with

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collapsing bubbles and evaporation. However, the seawater will boil violently and produce numerous bubbles once the actual working pressure is lower than the rated operating pressure.

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There is a synergistic effect of bubble implosion and corrosion [5-9], which can make materials undergo more aggressive environment. In addition, the concentration of dissolved oxygen in seawater will decrease under vacuum conditions and influence the passive performance of

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stainless steels [10,11]. Thus, the corrosion process of metallic components in the evaporation chamber environment will change with the boiling bubbles and dissolved oxygen content.

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Austenitic and duplex stainless steels such as types 316L and 2205 have been used widely for some major components (e.g., evaporator shell) of LT-MED plants [12-15]. Localized corrosion,

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especially pitting corrosion, will have great influence on their long-term safe application in the practical production. Their pitting corrosion resistance is associated with the formation and local failure of passive films on the stainless steel surfaces. Arjmand et al. [16] found that the values of transpassive potential and corrosion potential for 316L stainless steel in NaCl solution shift positively with the increase of dissolved oxygen concentration. Shen et al. [17] found that there is no limiting current plateau for oxygen reduction reaction under the bulk electrolyte boiling condition. Cáceres et al. [18] observed that there is a critical dissolved oxygen content in NaCl 2

solution for the corrosion rate of steel to reach a maximum value. Qiao et al. [19] obtained that the corrosion potential of high nitrogen stainless steel in oxygen-saturated H2SO4 solution is obviously higher than that in the nitrogen-saturated solution. Feng et al. [20] found that the higher dissolved oxygen content accelerates both anodic and cathodic processes, and produces the larger donor density and lower flat-band potential for the passive film on 316L stainless steel. The micro-jet impact of bubble collapse may accelerate the corrosion of stainless steels [21-25]. However, the corrosion behaviors of metallic materials remain unknown in the vacuum pressure environments as appeared in the LT-MED plants.

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Electrochemical measurement techniques are often used to evaluate pitting corrosion properties of stainless steels. In this work, the electrochemical corrosion behavior of type 2205

duplex stainless steel (DSS) was investigated in the simulated vacuum seawater environments for

the evaporator of LT-MED using cyclic anodic polarization and electrochemical impedance

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spectroscopy (EIS). It will provide a mechanistic understanding of the vacuum pressure effect on

2. Experimental

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2.1. Electrode and test solution

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the electrochemical corrosion of stainless steels in the hot brine environments.

The working electrodes were fabricated from a sheet of commercial type 2205 DSS plate (4

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mm in thickness) with a chemical composition (wt.%): C 0.014,Si 0.39,Mn 1.43,S 0.001,P 0.03,Cr 22.52,Ni 5.74,Mo 3.11,N 0.168, Cu 0.20 and Fe balance. The phase area ratio of ferrite/austenite is about 54.8/45.2. The specimens were cut into 11 × 11 mm, cleaned with

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acetone and polished with the 400# to 800# SiC waterproof abrasive papers. Then, a passivation treatment was conducted in a solution of 400 g L-1 nitric acid + 25 g L-1 potassium dichromate at

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55 ºC for 30 min to prevent the occurrence of crevice corrosion. Each square specimen was welded to a copper wire and embedded in epoxy resin with an exposed steel surface of 1 cm2. Before each experiment, the steel surface was polished with the 600# to 1000# SiC waterproof abrasive papers and rinsed by distilled water and ethanol. All the electrochemical experiments were carried out in the concentrated seawater solutions prepared according to ASTM D1141-2013 standard with a concentration ratio of 2 (i.e., the maximum operation value for the LT-MED process [3]) as shown in Table 1. The pH value was 3

adjusted to about 8.2 by using 0.5 M NaOH solution. All the tests were performed at 72 ºC under different negative pressures. An effective condensation reflux system was adopted to maintain the stability of the electrolyte concentration. As the test solutions were exposed under different pressures, their pH values changed very slightly within a short experiment time of about 4 h due to the slow absorption and desorption reaction processes of CO2 [26] and water evaporation loss. The composition change of test solution during the experiments will have insignificant influence on the measurements. The vacuum degree was controlled by a diaphragm vacuum pump at 0, 0.5, 0.6, 0.7 and 0.72,

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i.e., the pressure about 101.3, 50.7, 40.5, 30.4 and 28.4 kPa, respectively. The boiling point of seawater at different pressures can be calculated from the Antoine equation [27]: (290 K < T < 500 K)

(1)

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where P is the external pressure and T is absolute temperature. The calculated boiling point values are about 69 and 67.8 °C for the solution under 30.4 and 28.4 kPa pressures, respectively. This

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indicates that the boiling of test solution must take place in the cell at 72 °C under these pressures

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and become very violent with the pressure reduction.

2.2. Electrochemical measurements

The specimen was used as the working electrode, a saturated calomel electrode (SCE) as the

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reference electrode and a platinum plate as the counter electrode. All electrochemical tests were conducted through the measurement system comprised an M273A Potentiostat, an M5210 lock-in

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amplifier and the Powersuit software. The electrochemical tests were repeated at least five times with different specimens according to the International Standard ISO 15158-2014.

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After the specimen was immersed in the concentrated seawater under the given pressure, the

corrosion potential (Ecorr) was recorded for 2 h, and subsequently the EIS measurement was performed at the Ecorr with an AC disturbance signal of 10 mV (rms) in the frequency from 99 kHz to 10 mHz. The impedance spectra were analyzed using the ZSimpWin software. The cyclic anodic polarization curve was measured finally with a potential scan rate of 20 mV min-1 until the anodic current density reached 1 mA cm-2. After this point, the potential scan was reversed in order to evaluate the repassivation tendency. From the cyclic polarization curve, the pitting 4

potential (Ep) is determined at the rapid increase point of current density, i.e., the breakdown of passive film and occurrence of stable pitting corrosion [28]. The repassivation potential (Erp) is taken at the crossing point between the reverse scan and the forward scan [29,30]. However, in some cases, there is no intersection between them though their current densities are very close to each other. In these cases, the turning point potential at which a passivation takes place in the reverse scan is regarded as the Erp. The maximum current density (imax) is read from the reverse scan curve. The cathodic polarization curve was determined through the parallel test from the corrosion potential to -0.8 VSCE after 2 h of immersion. In addition, the anodic passivation of the

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specimen was characterized through a potentiostatic polarization of 30 min and the subsequent EIS measurement at 0.1 VSCE in the test solution under the given pressure.

2.3. Dissolved oxygen measurements

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The dissolved oxygen (DO) concentration (CDO) in seawater will decrease with the reduction

of pressure according to the Henry's law [31,32]. The CDO values for the 2 times-concentrated

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seawater at different vacuum pressures were measured by the JSPS-605F Dissolved Oxygen Analyzer with polarographic method, using platinum electrode as the cathode electrode,

electrolyte.

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2.4. Surface analysis

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silver-silver chloride electrode as the anode electrode and potassium chloride solution as the

After the cyclic anodic polarization measurements, the specimen surfaces were observed with

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scanning microscopy (SEM, HITACHI SU-1500). The chemical compositions of the passive films formed on the surface at 0.1 VSCE under various vacuum conditions for 30 min were analyzed by

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X-ray photoelectron spectroscopy (XPS), which were performed using ESCALAB 250Xi with Al Kα radiation source and a hemispherical electron analyzer operating at a pass energy of 30 eV. The C 1s peak from extraneous carbon at 284.8eV was used as a reference to correct for charging shifts. The XPS curves were fitted utilizing the Avantage software. The pit depth was measured by the super depth of field three-dimensional microscope (VHX-100). Prior to the depth measurement, the ultrasonic cleaning in distilled water was carried out on each specimen to remove the remnant lacy metal covers and corrosion products [33]. 5

3. Results 3.1. Dissolved oxygen concentration Fig. 1 shows the CDO values in the 2 times-concentrated seawater at 72 ºC under different vacuum pressures. CDO decreases linearly with reducing the pressure (P) in the degassing process. The relationship between the CDO and pressure can be expressed as: CDO = 0.0346P – 0.578, which follows the Henry's law. It can be calculated that the oxygen content is about 0.46 mg L-1 in the LT-MED evaporator under the operating pressure at 30 kPa [3,34].

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3.2. Electrochemical corrosion characteristics 3.2.1 Evolution of corrosion potential

Fig. 2 gives the Ecorr curves of 2205 DSS in the concentrated seawater solutions under

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different vacuum degree conditions. During 2 h of immersion, the Ecorr values of the specimens

slowly increase and reach the steady states under non-boiling solution conditions with the pressure

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no less than 40.5 kPa. As the pressure reduces to 30.4 kPa, the solution will turn into boiling condition. The Ecorr values shift negatively to reach the relatively steady states. With reducing the

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pressure from 101.3 to 28.4 kPa, the stable Ecorr value at 2 h decreases from about -177 to -566 mVSCE. In particular, there is only a slight decrement of Ecorr value within 100 mV before the occurrence of boiling. However, the Ecorr value drops about 145 mV with the transition from static

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state at 40.5 kPa to slight boiling state at 30.4 kPa. A further reduction of pressure about 2 kPa from 30.4 to 28.4 kPa will result in a violent boiling state and the ensuing big decrement of Ecorr

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value about 151 mV. This is similar to the evolution behavior of Ecorr value under deaeration [35,36] and cavitation conditions [25,37].

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3.2.2 EIS spectra

Fig. 3 shows the EIS plots measured on 2205 DSS at the Ecorr and 0.1 VSCE in the

concentrated seawater solutions under different pressures. A similar impedance feature for all specimens is seen from the Nyquist curves, i.e., one incomplete capacitive semicircle over the whole frequency range. Under the Ecorr conditions (i.e., the free corrosion states) in Fig. 3(a,b), the semicircle size changes slightly as the pressure is no less than 40.5 kPa, but decreases greatly with the pressure reduction from 40.5 kPa to 30.4 and 28.4 kPa. The time constants from the passive 6

film and charge transfer process overlap together as a horizontal platform in the frequency range from about 100 to 0.1 Hz with almost invariable phase angle θ values. A more detailed comparison indicates that the θ value of horizontal platform decreases slightly from about 81 to 75° with reducing the pressure. Under the 0.1 VSCE conditions (i.e., the anodic polarization states in the passive regions) in Fig. 3(c,d), the semicircle shrinks gradually with the pressure reduction. The θ curves almost show no difference between the different pressures, with the horizontal platform value of about 84º.

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3.2.3 Cyclic anodic polarization curves Fig. 4 shows the cyclic anodic polarization curves of 2205 DSS in the concentrated seawater

at 72 ºC under different vacuum degree conditions. The specimens can passivate spontaneously and have insignificant difference in the passive current density with changing the pressure. Stable

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pitting corrosion takes place with increasing the electrode potential from Ecorr to Ep under each

pressure condition. The electrochemical parameters obtained from anodic polarization curves are

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listed in Table 2. The Ep shifts negatively about 122 mV with reducing the pressure from 101.3 to 28.4 kPa. The standard deviation of Ep is less than about 24 mV in each pressure case. This

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indicates that the pitting tendency of 2205 DSS increases with the reduction of pressure to some extent.

The Erp value tends to shift positively about 88 mV from 101.3 to 28.4 kPa. At the same time,

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the imax value noticeably decreases by 61.8 mA cm-2. These indicate that the repassivation tendency of 2205 DSS enhances with reducing the vacuum pressure, especially under the violent

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boiling condition.

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3.2.4 Cathodic polarization curves Fig. 5 gives the cathodic polarization curves of the corrosion processes on 2205 DSS

measured under different pressures. The cathodic reaction process is dominated by the oxygen reduction reaction (ORR). Under each pressure state, the ORR is controlled by the charge transfer process in a wide cathodic over-potential range before the appearance of diffusion process. The limiting diffusion current density (iL) of ORR decreases from about 19.1 to 8.4 μA cm-2 with reducing the vacuum pressure from 101.3 to 40.5 kPa. As the pressure is lower than 30.4 kPa, the 7

limiting diffusion process of dissolved oxygen will gradually disappear due to the occurrence of boiling. No limiting current plateau was observed for ORR as the electrolyte was heated to the boiling state, as reported in the literature [17,38]. In addition, the cathodic polarization curve shifts to more negative potentials and lower current densities with the pressure reduction. The solution boiling may produce vigorous convection mass transfer effect [39] between the bulk electrolyte and electrode surface and remove greatly the dissolved oxygen (Fig. 1). These are mainly responsible for the very low current densities of activation polarization and the disappearance of concentration polarization of ORR on the specimen surfaces.

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3.2.5 SEM Morphologies of pits

Fig. 6 shows the typical SEM morphologies of pits on the specimen surfaces after the cyclic

anodic polarization test under different pressure conditions. The pits formed under different

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pressures have the similar features. Some undermined metallic covers exist over the pit mouths.

These covers have the porous lacelike appearance and some local collapses and cracks, as

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observed in the literature [33,40-42]. In addition, the pit number and depth tend to decrease gradually with the reduction of vacuum pressure. Table 2 gives the average depth of deepest pits

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(dmax) formed on five parallel specimens after the cyclic anodic polarization in Fig. 4. The dmax value changes from about 297 to 139 µm with the vacuum pressure.

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3.3 XPS analysis of passive films

The ex situ XPS analysis was performed to provide the chemical composition information on

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the passive films formed by the potentiostatic polarization at 0.1 VSCE in the concentrated seawater under different pressures (i.e., 101.3, 40.5 and 28.4 kPa). Fig.7 shows the deconvoluted XPS

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spectra of Cr 2p3/2, Fe 2p3/2, Mo 3d, O 1s and Cl 2p3/2 for analyzing the passive film composition. Table 3 gives the binding energies of main species [43-54] in the passive films for the deconvolution analyses. The response peaks of Fe 2p3/2 were fitted with three components at 707.0 ± 0.5, 711.0 ± 0.4

and 711.9 ± 0.4 eV for the metallic state Fe, Fe2O3 and FeOOH, respectively. Under air condition, both Fe2O3 and FeOOH are the dominant Fe3+-species in the passive films because their peaks have the similar intensity features. However, the relative peak intensity of Fe2O3 becomes weaker 8

than that of FeOOH as the pressure changes from 101.3 to 28.4 kPa. This means that FeOOH gradually becomes the dominant Fe3+-species with the pressure reduction. The Cr 2p3/2 response peaks were fitted with three components at 574.0 ± 0.1, 576.0 ± 0.3 and 577.2 ± 0.3 eV for the metallic Cr, Cr2O3 and Cr(OH)3, respectively. Under the air pressure 101.3 kPa, both Cr2O3 and Cr(OH)3 peaks have approximately equal intensities, while the intensity of Cr2O3 becomes significantly lower than that of Cr(OH)3 as the pressure reduces to 40.5 to 28.4 kPa. These indicate that the dominant Cr3+-species change distinctly from both Cr2O3 and Cr(OH)3 to Cr(OH)3 with the pressure reduction.

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It is noted that the Fe 2p3/2 response peaks get very weak under the pressure 28.4 kPa. The Fe/Cr ratio of the passive film decreases from about 2.0 for the non-boiling pressures 101.3 and

40.5 kPa to about 0.7 for the boiling pressure 28.4 kPa. It is clear that the Cr-enrichment in the passive film becomes very noticeable under the violent boiling state. However, this brings no

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improvement in the passive film stability as demonstrated by above Ep and EIS results. According to the literature [43,54,55], it can be inferred that the strong Cr-enrichment here mainly results

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from the enhanced preferential dissolution of Fe species by the boiling bubbles. The O 1s response peaks confirm the appearance of species O2-, OH- and H2O at 530.0 ± 0.3,

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531.4 ± 0.4 and 533.0 ± 0.3 eV, respectively. The relative peak intensity of O2- decreases markedly with the pressure from 101.3 to 28.4 kPa. This indicates that the dominant O species in the passive

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film change clearly from both OH- and O2- to only OH- with the pressure reduction. The Mo 3d response peaks attributed to the Mo 3d5/2 and Mo 3d3/2 orbitals [43,54] can be fitted for the metallic Mo at 227.8 ± 0.4 and 231.0 ± 0.1 eV, Mo4+-species at 233.4 ± 0.4 eV and

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Mo6+-species at 232.1 ± 0.1 and 235.4 ± 0.4 eV, respectively. The relative peak intensities of Mo 3d are much lower than those of Fe 2p3/2 and Cr 2p3/2 under each pressure. This means that only

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small amount of Mo species formed in the passive films. Furthermore, the Cl 2p3/2 response peak appears at 199.2 ± 0.1 eV, which can be assigned to

the Cl- species in the passive films. The intensity of Cl- peak is very weak under the non-boiling conditions, but becomes relatively strong under the violent boiling condition. This illustrates that the boiling may promote the incorporation of Cl- ions into the passive film.

4. Discussion 9

4.1. Influence of vacuum degree on the corrosion resistance of 2205 DSS Thin passive films must spontaneously form on the specimen surfaces during the exposures to air and test solution. An equivalent circuit is proposed in Fig. 8 to fit the EIS spectra in Fig. 3, where Rs is the electrolyte resistance, Rf and Cf are the resistance and capacitance of the passive film, Rt and Cdl are the charge transfer resistance and double layer capacitance, respectively. In the fitting procedure, the constant phase element (CPE) was used to replace both Cf and Cdl because of the non-ideal capacitive response of the interface 2205DSS/solution [56-58]. The impedance of

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CPE is given as (2)

where Y0 is the admittance magnitude of CPE, and α is the exponential term [57].

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Table 4 gives the fitted values from the EIS spectra measured under Ecorr and 0.1 VSCE

conditions. Due to the poorly defined semicircles, it is impossible to obtain the accurate

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capacitance values. The polarization resistance Rp (i.e., corrosion resistance) is the sum of Rt and Rf [57,59]. Rs remains almost constant (about 2 Ω cm2) with changing the vacuum degree due to

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the strong conductivity of the hot test solution. For both measuring conditions, Rp has very high values of the order of 105 Ω cm2 under all vacuum pressures, but the pressure reduction results in the gradual decrease of Rp and the continuous increase of Y0-f and Y0-dl. In addition, due to the

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anodic growth of passive films [36,60], the Rp value of each vacuum pressure is much larger under 0.1 VSCE condition than under Ecorr condition, whereas the Y0-f and Y0-dl values are just the opposite.

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These demonstrate that the reduction of vacuum pressure must degrade the passive film and corrosion resistance of 2205 DSS in the hot concentrated seawater.

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The stable pitting corrosion will take place after the electrode potential reaches the critical potential Ep. Fig. 9 shows that Ep presents a good linear relationship with the pressure (i.e., CDO) no less than 30.4 kPa. Similarly, it is reported that the deaeration with nitrogen may result in higher metastable pit occurrence rates [61] and lower Ep values [36,62] for the stainless steels in chloride solutions. At 28.4 kPa in the presence of violent boiling, Ep negatively deviates from the fitted line. It’s apparent that the vacuum pressure and boiling solution have promotive effects on the occurrence of stable pitting corrosion. 10

The corrosion resistance of stainless steel mainly originates from the protectiveness of passive films on the steel surface in the hot concentrated seawater. According to above XPS analysis and the literature [38,63-65], the passive films are formed mainly through the following primary and secondary electrochemical reactions of the iron and chromium oxidation processes: Fe + 2OH- → Fe(OH)2 + 2e-

(3)

2Fe(OH)2 + 2OH- → Fe2O3 + 3H2O + 2e-

(4)

Cr + 3OH- → Cr(OH)3 + 3e-

(5)

Cr(OH)3 + Cr + 3OH- → Cr2O3 + 3H2O + 3e-

(6)

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The dissolution and deposition of Fe2+-species occur at the passive film/solution interface through the ion transfer reactions [66,67]: (Fe2+)ox + 2H+  (Fe2+)aq + H2O

(7)

(Fe2+)aq + 2OH- → FeOOH + H+ + e-

(8)

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where the subscripts “ox” and “aq” represent the ferrous ions in the oxide/hydroxide and the aqueous solution, respectively. At the same time, the cathodic reaction of corrosion process is:

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O2 + 2H2O + 4e- → 4OH-

(9)

The electrochemical equilibrium potential Eec for reaction (9) at the passive film/solution interface

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where

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is given by:

(10)

is the standard electrode potential, T is the bulk solution temperature, F is Faraday’s

constant, aOH- is the activity of OH- ions and the

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is the partial pressure of oxygen.

These reactions result in forming the passive films with a bilayer structure (i.e., the inner

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Fe-Cr oxide layer and the outer hydroxide-oxyhydroxide layer) under the free corrosion and external polarization states [64,66,68]. The passive film grows through the transport of ionic species and/or vacancies in the oxide film. It should be noted that the initial specimen surfaces must form very thin films in air before the immersion test [68,69]. The air-formed films have influence on the corrosion processes and the XPS analyses of passive films. Fortunately, the impedance results in Fig. 3 and the literature [36,56,68] indicate that the effect of air-form films become very weak with the growth of passive films in the test solutions, especially under anodic 11

polarization conditions in the passive region. Obviously,the environmental pressure plays an important role in the corrosion process of the specimens. The reduction of vacuum pressure will result in the lower

and CDO values, as

measured in Fig. 1. According to equation (10), Eec will shift negatively under the lower conditions. This is a part reason for the decrease of stable Ecorr value with the pressure in Fig. 2. According to reaction (9) and Fig. 5, the cathodic reaction will slow down and then produce less OH- ions on the specimen surface under the lower CDO conditions. It is inferred that the changes of

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Ecorr and cathodic reaction rate are unfavorable to the formation of protective passive film through the reactions (3-6) with reducing the pressure. In particular, due to the insufficiency of oxygen in

the solution, the lower CDO has greater inhibitive effect on the reactions (4,6) than (5,8), but facilitates the reaction (7). As a result, the oxide contents of Fe2O3 and Cr2O3 in the passive films

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decrease with the reduction of pressure. The hydroxide Cr(OH)3 together with oxyhydroxide

FeOOH correspondingly may become the dominant ingredients in the passive films, as observed

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by XPS analysis in Fig. 7. Moreover, the lower CDO is, the more Cl- ions will adsorb on the passive film through the competition with dissolved O2 or OH- ions, thereby helping to the

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reactions (7,8) for the FeOOH deposition [11,28,66]. The passive films must become porous and less protective with the increase of hydroxide content [66,70]. In addition, the anodic polarization of 0.1 VSCE must accelerate the reactions (3-6, 8), leading to the fast growth of passive films on

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the specimen surfaces as mentioned before. It is clear that the pressure reduction will deteriorate the protectiveness of passive films formed on the specimens under both Ecorr and 0.1 VSCE

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conditions. This is mainly responsible for the lower Ecorr, Ep and Rp and larger Y0-f and Y0-dl under the lower pressures. Moreover, the growth of corrosion pits on stainless steel in chloride solution

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takes place in the metastable and stable stages. There are some current spikes under different pressures in Fig. 4, indicating the generation of metastable pits. According to the pit stability product ‘di’ (where d is pit depth and i is current density) proposed by Galvele [71,72], the metastable pit may become stable as it survives to the critical di value of about 3 mA cm-1 for stainless steel in chloride solution. Fig. 4 illustrates that the metastable pits repassivated at the i values less than about 9 μA cm-2 under each vacuum pressure. It is seen from the reactions (3-6,9) that the higher CDO must facilitate the repassivation of metastable pits. So, the propagation of 12

metastable pits will become easier to meet the critical di value (i.e., the transition to stable pits) with reducing the CDO value. This also is the partial reason for the decrease of Ep with the pressure reduction in Fig. 9. Furthermore, the violent boiling of test solution at 28.4 kPa causes the noticeable drop in the Ecorr and Ep values as shown in Figs. 2 and 9. Besides the above actions of low CDO, these may be related to the dynamic effects of boiling bubbles. The continuous collapse of boiling bubbles may produce many micro-jets adjacent to the specimen surface and then cause the mechanical peeling and impingement actions on the specimen surface. These will result in forming less protective

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passive films with more defects [22]. At the same time, the defects facilitate the incorporation of Cl- ions into the passive films under violent boiling conditions as detected by the XPS analysis in

Fig. 7. These indicate that the passive films become more susceptible to pitting attack in the

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presence of violent boiling at 28.4 kPa.

4.2. Repassivation properties of 2205 DSS under different vacuum pressures

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The vacuum pressure plays an important role in the repassivation behavior of 2205 DSS. With reducing the vacuum pressure from 101.3 to 28.4 kPa in Figs. 4 and 10, the anodic hysteresis

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loop and the active potential range ∆E (∆E = Ep - Erp) for the pits tend to shrink distinctly, whereas Erp increases clearly from -99  60.6 to -11  24.4 mVSCE. In general, the higher Erp and the smaller hysteresis loop and ∆E mean the better repassivation ability of the pits [70,73].

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Additionally, after the potential scan is reversed at 1 mA cm-2 in cyclic anodic polarization process, the current density continues to increase for a certain time and attains the maximum value imax due

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to the anodic growth of newly formed pits. As shown in the literature [74], imax has a positive correlation with dmax and an inverse correlation with Erp, which can be used to measure the

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repassivation ability and propagation extent of the pits. It is seen from Table 2 that, with reducing the pressure from 101.3 to 28.4 kPa, the imax value gradually changes from 73.8  21.6 to 6.2  3.9 mA cm-2. Simultaneously, due to the shrinkage of anodic hysteresis loop, the dmax value decreases from 297  24 to 139  20 μm. These indicate that the pit propagation was suppressed during the reverse scan by reducing the pressure. It is apparent that the repassivation of 2205 DSS becomes easier as the pitting corrosion takes place under the lower negative pressures. This is mainly related to the following features. Under 13

the lower vacuum pressures, the lower CDO values (Fig. 1) are adverse to the pit repassivation. On the contrary, with reducing the pressure, Fig. 4 shows that the stable pits were polarized from the lower initial anodic potentials (i.e., Ep), leading to the smaller i values (e.g., imax) for the pit propagation during the reverse scan [72]. The smaller i and ∆E values must result in the smaller d and di values under the lower pressures. Moreover, the d values increase gradually with the continuous reverse scanning in each pressure case. It can be assumed that the specimens reached the highest imax values will have the largest pits (i.e., dmax) and experience the lowest Erp values. According to the literature [71,72], the stable pits will repassivate as the critical di value is

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reached at the potential Erp in the reverse scan. Apparently, the smaller di values mean that the crititcal value (about 3 mA cm-1) for pit repassivation is reached more easily with the pressure

reduction. On the other hand, the hydrolysis of metal ions and the migration of Cl- ions are assumed to take place in the pits during the pitting corrosion process. In the shallower pits, the

-p

fewer dissolved metal cations will cause the less local acidification and accumulation of Cl- ions [28,33,71,72,75]. These imply that the occluded solutions formed inside the stable pits under the

re

lower pressures may be less aggressive and favorable to the repassivation. It is clearly seen that reducing the vacuum pressure will abate the anodic dissolution and occlusion states in the pits but

lP

help the repassivation of pitting corrosion on the specimen surfaces. In addition, under the very low pressures, the violent boiling of solution will enhance the

na

mass transport process of metallic cations from the pits. This must impede the formation of strong occlusion states with aggressive ions in the pits. It is clear that the violent boiling is favorable to the pit repassivation, similar to the cavitation and impingement actions [9,74].

ur

Though further work is necessary to elucidate the electrochemical corrosion characteristics more unambiguously, the above analyses reveal that the generation of stable pits on 2205 DSS in

Jo

the hot concentrated seawater may occur at lower potentials with the reduction of vacuum pressure and the presence of boiling bubbles. However, the narrower active potential range ∆E and the boiling actions will restrain the pitting propagation. Clearly, the reduction of vacuum pressure and the boiling of solution may facilitate both the occurrence and repassivation of stable pitting corrosion on 2205 DSS to some extent in the working environments of the LT-MED plants.

4. Conclusions 14

In the 2 times-concentrated seawater at 72 ºC, the dissolved oxygen concentration decreases linearly with the pressure in the degassing process. The solution will enter a boiling state under the vacuum pressure lower than about 30.4 kPa. 2205 DSS can passivate spontaneously in the hot concentrated seawater under various vacuum pressures, but the pressure reduction noticeably decelerates the cathodic reaction process and simultaneously degrades the passive film and polarization resistance. Due to the decrement of dissolved oxygen content, the dominant ingredients may change from the oxides Fe2O3 and Cr2O3, hydroxide Cr(OH)3 and oxyhydroxide FeOOH to the last two species in the passive films. The

ro of

pitting potential decreases linearly with the pressure before the violent boiling takes place in the solution, whereas the repassivation potential increases distinctly. The reduction of vacuum

pressure may enhance both the occurrence and repassivation tendencies of stable pitting corrosion on 2205 DSS to a certain degree in the simulated environments for the LT-MED plants.

-p

Under the very low pressures, the violent boiling of solution results in the noticeable drop of both corrosion potential and pitting potential of 2205 DSS. This is mainly attributed to the

re

formation of less protective passive film and the incorporation of Cl- ions under the bubble collapse condition. At the same time, the violent boiling markedly accelerates the mass transport

lP

processes of dissolved oxygen in the proximity of electrode surface and the metallic cations produced by anodic dissolution in the pits. These cause the disappearance of oxygen-limiting

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diffusion in the cathodic polarization process and favor the pit repassivation.

Declaration of interests

Jo

ur

No conflict of interest exists in this manuscript.

Acknowledgements Financial support provided by National Natural Science Foundation of China (Grant Nos.

51571139 and U1960103) is greatly appreciated.

15

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resistant alloys after processing for heating-element sheathing, Electrochim. Acta 49 (2004) 3005. [74] A. Neville, T. Hodgkiess, An assessment of the corrosion behaviour of high-grade alloys in

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[75] S.P. Mattin, G.T. Burstein, Detailed resolution of microscopic depassivation events on

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21

Figure Captions: Fig. 1. Variations of dissolved oxygen concentration with the vacuum pressure in the 2

-p

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times-concentrated seawater at 72 ºC.

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ur

na

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72 ºC under different vacuum pressures.

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Fig. 2. Time dependence of corrosion potentials of the specimens in the concentrated seawater at

22

Fig. 3. Impedance plots for the specimens in the concentrated seawater at 72 ºC under different vacuum pressures: (a) Nyquist and (b) Bode at the corrosion potential; (c) Nyquist and (d) Bode at

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ur

na

lP

re

-p

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0.1 VSCE.

23

ro of -p re lP na

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Fig. 4. Cyclic anodic polarization curves for the specimens in the concentrated seawater at 72 ºC

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under different vacuum pressures.

24

Fig. 5. Cathodic polarization curves of the specimens in the concentrated seawater at 72 ºC under

-p

ro of

different vacuum pressures.

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Fig. 6. SEM morphologies of the specimen surfaces after cyclic anodic polarization in the concentrated seawater at 72 ºC under different vacuum pressures: (a) 50.7 kPa, (b) 30.4 kPa and (c)

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ur

na

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28.4 kPa.

25

ro of -p re lP na ur

Fig. 7. The detailed XPS spectra of (a) Fe 2p3/2, (b) Cr 2p3/2, (c) O 1s, (d) Mo 3d and (e) Cl 2p3/2

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for the passive films formed on the specimen surfaces after the potentiostatic polarization in the concentrated seawater under different pressures.

26

27

ro of

-p

re

lP

na

ur

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ro of -p re lP

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ur

vacuum degrees.

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Fig. 8. Equivalent circuit for the electrode system 2205 DSS/concentrated seawater under different

28

Fig. 9. Variation of Ep for the specimens in the concentrated seawater at 72 ºC with vacuum

-p

ro of

pressure. The error bars represent the standard deviations calculated from five parallel specimens.

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Fig. 10. Variation of Erp and E for the specimens in the concentrated seawater at 72 ºC with vacuum pressure. The error bars represent the standard deviations calculated from five parallel

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ur

na

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specimens.

29

Table1 Composition of the concentrated artificial seawater used in the tests (g L-1) NaCl

MgCl2

Na2SO4

CaCl2

KCl

NaHCO3

KBr

49.06

10.40

8.18

2.32

1.39

0.402

0.202

Table 2 The average values and standard deviations of the electrochemical parameters for 2205 DSS in the concentrated seawater solutions under different pressures Ecorr (mVSCE)

Ep (mVSCE)

Erp (mVSCE)

imax (mA·cm-2)

dmax (µm)

101.3

-175 ± 8.2

503 ± 6.1

-99 ± 60.6

73.8 ± 21.6

297 ± 24

50.7

-223 ± 13.8

456 ± 23.9

-45 ± 31.3

31.9 ± 20.6

261 ± 27

40.5

-260 ± 4.0

427 ± 10.3

-42 ± 15.9

28.8 ± 14.5

233 ± 26

30.4

-413 ± 35.9

416 ± 16.7

-16 ± 19.5

9.5 ± 1.3

162 ± 19

28.4

-544 ± 31.1

381 ± 18.7

-11 ± 24.4

6.2 ± 3.9

139 ± 20

ro of

P (kPa)

Table 3 Binding energies and their deviations for fitting the XPS spectra of main species in the passive film on 2205 DSS Peak

Species / Binding energy (eV)

Fe

2p3/2

Fe0 / 707.0±0.5; Fe2O3 / 711.0±0.4; FeOOH / 711.9±0.4

Cr

2p3/2

Cr0 / 574.0±0.1; Cr2O3 / 576.0±0.3; Cr(OH)3 / 577.2±0.3

3d5/2

Mo0 / 227.8±0.4; Mo6+ / 232.1±0.1

3d3/2

Mo0 / 231.0±0.1; Mo4+ / 233.4±0.4; Mo6+ / 235.4±0.4

O

1s

O2- / 530.0±0.3; OH- / 531.4±0.4; H2O / 533.0±0.3

Cl

2p3/2

Cl- / 199.2±0.1

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Mo

-p

Element

Table 4 Fitted results of EIS spectra under different vacuum pressures Rs (Ω cm2)

Y0-f (sα Ω cm-2)

na

P (kPa)

αf

Y0-dl (sα Ω cm-2)

αdl

Rp (Ω cm2)

Measured under Ecorr condition 2.59

5.39×10-5

0.91

2.79×10-5

0.90

3.12×105

50.7

1.72

6.27×10-5

0.90

3.18×10-5

0.97

2.88×105

40.5

2.16

6.90×10-5

0.88

4.42×10-5

0.89

2.73×105

30.4

2.44

1.10×10-4

0.85

6.26×10-5

0.81

2.26×105

28.4

2.44

1.33×10-4

0.83

2.97×10-4

0.85

2.02×105

ur

101.3

Jo

Measured under 0.1 VSCE condition 101.3

2.45

1.96×10-5

0.95

3.50×10-6

0.86

9.04×105

50.7

2.38

2.34×10-5

0.94

1.17×10-5

0.88

7.83×105

40.5

1.87

2.91×10-5

0.94

2.10×10-5

0.83

6.76×105

30.4

2.30

3.80×10-5

0.93

3.82×10-5

0.89

5.22×105

28.4

2.20

4.93×10-5

0.94

7.82×10-5

0.82

4.03×105

30