Influence of sensitization on microstructure and passive property of AISI 2205 duplex stainless steel

Corrosion Science 104 (2016) 144–151

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Influence of sensitization on microstructure and passive property of AISI 2205 duplex stainless steel Lv Jinlong a,b,∗ , Liang Tongxiang a,b , Dong Limin a,b , Wang Chen a,b a Beijing Key Laboratory of Fine Ceramics, Institute of Nuclear and New Energy Technology, Tsinghua University, Zhongguancun Street, Haidian District, Beijing 100084, China b State Key Lab of New Ceramic and Fine Processing, Tsinghua University, Beijing 100084, China

a r t i c l e

i n f o

Article history: Received 30 May 2015 Received in revised form 7 December 2015 Accepted 9 December 2015 Available online 10 December 2015 Keywords: A. Stainless steel B. EIS B. SEM C. Passive films C. Pitting corrosion

a b s t r a c t The results from double loop electrochemical potentiokinetic reactivation technique, potentiostatic critical pitting temperature technique and electrochemical impedance spectroscopy showed that the resistance of the 2205 duplex stainless steel to pitting corrosion decreased with the increasing of sensitization time due to a decreasing in passivation ability. The 2205 duplex stainless steel showed the increase in pitting corrosion in the ferritic phase and in intergranular corrosion between the austenitic and ferritic phases with the increasing of sensitization time. The precipitated phases changed due to the increasing of the sensitization time, which affected degree of sensitization of the 2205 duplex stainless steel. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction The duplex stainless steels containing austenitic and ferritic phases are widely used for applications such as petroleum, gas refineries and marine environments due to their advantageous corrosion resistance and mechanical properties [1,2]. However, the duplex stainless steels are prone to form some secondary phases such as M23 C6 , ␴ and ␹ phases due to high chromium and molybdenum contents and their high diffusion rate in ferrite between 400 and 1000 ◦ C [3]. The energy spectrum analysis showed the depletion of Cr and Mo at the phase boundaries in 2205 duplex stainless steel after heat treatments [4]. Therefore, the precipitated phases induced some chromium-depleted and molybdenum-depleted zones, leading to a decrease in corrosion resistance of the duplex stainless steel [5,6]. The study showed that the degree of sensitization (DOS) of the 2205 duplex stainless steel increased with increasing sensitization time and the critical pitting temperature (CPT) value decreased at the same time [7]. However, the duplex stainless steels showed complex phase transformation and precipitation behavior due to high content of alloying elements and two phases [8]. For example, a recovery of the pitting and intergranular corrosion resistance of the 2101 duplex stainless steel at

∗ Corresponding author. Fax: +86 10 69771464. E-mail address: [email protected] (L. Jinlong). http://dx.doi.org/10.1016/j.corsci.2015.12.005 0010-938X/© 2015 Elsevier Ltd. All rights reserved.

850 ◦ C was found. This was attributed to the redistribution of the chromium in the depleted zones [9]. The DOS of the stainless steels can be evaluated by eddy current measurements [10], thermoelectric power (TEP) measurements [11], atomic force microscopy [12,13] and double loop electrochemical potentiokinetic reactivation (DLEPR) technique [14]. The effect of sensitization time at 675 ◦ C on the microstructures and the corrosion resistance of the 2205 duplex stainless steel was investigated by X-ray diffraction (XRD), Transmission electron microscopethe (TEM), the DLEPR and electrochemical impedance spectroscopy (EIS) analysis. 2. Experimental methods The 2205 duplex stainless steel has the basic chemical composition (in weight percent) of 0.05 C, 1.05 Mn, 0.022 P, 0.0008 S, 0.75 Si, 21.85 Cr, 5.36 Ni, 3.18 Mo, 0.18 Cu, 0.13 V, 0.054 W, 0.015 Ti, 0.155 N and Fe balance. The as-received samples were annealed at 1100 ◦ C for 1 h and water-quenched. Some samples were isothermally sensitized at 675 ◦ C for different times and then quenched in water. The volume fraction of the ferritic phase in sensitized samples was measured by means of the magneto-inductive device, Feritscope model FMP30 (FISCHER). Cu K␣ (0.154056 nm) radiation at 40 kV and 40 mA with 4◦ /min was used for X-ray diffraction (Rigaku Ultima IV) analysis. The scanning electron microscopic (SEM) observations have been performed in a JSM-5800 microscope. The precipitates in

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Fig. 1. (a) SEM image of solution annealed 2205 duplex stainless steel, the energy dispersive spectrometer of (b) the austenitic phase and (c) the ferritic phase, respectively.

sensitized samples were characterized using JEOL JEM−2011. The samples for TEM examination were firstly ground to a thickness of about 0.06 mm with a diameter of 3 mm and then were electropolished via a twin-jet machine with a 5% (in volume) perchloric acid and 95% alcohol solution at −20 ◦ C. A conventional three-electrode electrochemical cell was used. A platinum counter electrode and a saturated calomel reference electrode (SCE) were connected to a CHI660E electrochemical workstation (Chenhua instrument Co. Shanghai, China) controlled by a computer and software. The DLEPR was conducted in 0.5 M H2 SO4 + 0.01 M KSCN solution [15]. The DLEPR experiments were started after nearly steady state open circuit potential (OCP) had been reached. The potential swept in the anodic direction at 1 mV s−1 until the potential of 0.3 VSCE was reached and then the scan was reversed until the OCP. The role of KSCN was used to help to break the passive film during the reactivation cycle of the test. The EIS test was carried out at OCP after sample was immersed in 0.1 M NaCl solution for 2 h. The EIS measurements were carried out at a frequency range from 100 kHz to 10 mHz and with a 5 mV amplitude of the AC signal. The Mott-Schottky measurement was carried out on at a fixed frequency of 1000 Hz using an excitation voltage of 5 mV. The CPT values were measured in 0.1 M NaCl solution by potentiostatic polarisation method. The samples were polarized at anodic potential +750 mV with respect to OCP and temperature was increased by a rate of 0.6 ◦ C/min [16]. The CPT value also was used to evaluate sensitized 2205 duplex stainless steel. Two or three samples with identical condition were tested to meet the reliability of the test. 3. Results and discussion The SEM image of solid solution 2205 duplex stainless steel is shown in Fig. 1a. The microstructure of the solution annealed sample shows an bulgy ferritic phase and concave austenitic phase. The average grain size of austenitic phase and ferritic phase is 30 ␮m and 25 ␮m, respectively. The twins are not observed in austenitic phase after annealing process. The results of energy dispersive spectrometer show that manganese and nickel enrich austenitic phase in Fig. 1b, while the chromium and molybdenum enrich ferritic phase in Fig. 1c. This is because Cr and Mo are ferrite-stabilizing elements, while Ni and Mn are austenite-stabilizing elements. Fig. 2 shows the microhardness values of ferritic and austenitic phases in sensitized 2205 duplex stainless steels. The increasing

Fig. 2. Effect of sensitization time at 675 ◦ C on the vickers microhardness of the austenitic and the ferritic phases in the 2205 duplex stainless steels.

of the microhardness value in ferritic phase is more significant than that in austenitic phase. This indicates that the sensitization has a greater effect on ferritic phase than austenitic phase. It also was found that the change of microstructure in ferritic phase was more significant than that in austenitic phase after low temperature aging [17,18]. Because diffusion coefficients of chromium and molybdenum are higher in the ferritic phase than in the austenitic phase [19], which leads to a more serious desensitization of the former. In Fig. 3a, the ferritic and austenitic phases are detected in duplex stainless steels by XRD due to their different crystal structures. The strongest (110) reflection indicates a high proportion of the ferritic phase in sample with short sensitization time. The intensity of (110) reflection of the ferritic phase decreases with the increasing of sensitization time, while corresponding intensity of (111) reflection of austenitic phase increases. In Fig. 3b, the volume fraction of the austenitic phase increases with the increasing of the sensitization time, while volume fraction of the ferritic phase decreases at the same time. The volume fraction of the ferritic phase in solution annealed 2205 duplex stainless steel increased from 42% at 1050 ◦ C to 69% at 1250 ◦ C [20]. The different transformation kinetics could be attributed to composition change during sensitization process, considering the little variation of the grain size. If the matrix is depleted in Cr and Mo, then nickel enriches in matrix. The nickel is austenite stabilizing element in stainless steel [21]. Therefore, more austenitic phases are formed after sensitization in the present study. Lee et al. [22] also found that secondary austen-

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ite grew. This was mainly induced by chromium depletion arising from the formation of the carbide. The significant activation and reactivation peaks are observed on the base of the DLEPR measurements in Fig. 4a. It is evident that the intensities of the activation and reactivation current peaks increase with the increasing of the sensitization time. It is noteworthy that three activation current peaks are observed for the sensitized 2205 duplex stainless steels. This phenomenon is rarely observed in previous reports. This suggests that the forming mechanism of the passive film on sensitized 2205 duplex stainless steel was more complicated than that on unsensitized one. In the present study, the DOS can be determined from the current peak values by using the following equation: DOS =

Ir Ia1 + Ia2 + Ia3

(1)

where Ia and Ir are the maximal values of activation and reactivation currents. Fig. 4b displays that the magnitude of the DOS increases with the increasing of the sensitization time. There is not healing phenomenon. The DOS values can be affected by the volume fraction of precipitated carbides in grain boundaries and volume fraction of two phases during sensitization processing. Zhang et al. [23] found that the resistance of UNS S82441 duplex stainless steel to pitting corrosion decreased with the increasing of aging time at 700 ◦ C. This agrees with our results. Fig. 4c shows the potentiodynamic polarization curves of sensitized 2205 duplex stainless steels in 0.5 M H2 SO4 + 0.01 M KSCN solution. It is worthwhile to note that no significant changes occur in the shape of the polarization curves of these sensitized samples, however, anodic current density increases with the increasing of aging time. The increase in the anodic current density with aging time can also be attributed to the microstructural evolution taking place during thermal aging. Fig. 5a represents a typical current density vs. temperature curves obtained from potentiostatic CPT assessment for sensitized 2205 duplex stainless steels in 0.1 M NaCl solution. The current fluctuation with the temperature of NaCl solution is not obvious for short time sensitized samples. This is because Cr-rich carbides don’t completely precipitate in short sensitization time. This indicates that the samples are protected by the passive film. However, with the increasing of sensitization time, the current density increases abruptly due to the occurrence of metastable pits below the CPT. This is related to passive film breakdown occurring in a metastable pit [24–26]. The CPT decreases from 60.3 ◦ C for sample sensitized at 675 ◦ C for 3 h to 50.8 ◦ C for sample sensitized at 675 ◦ C for 15 h in Fig. 5b. The CPT of 2205 duplex stainless steel almost decreases linearly with the increasing of sensitization time and DOS in Fig. 5b and c. Fig. 6a shows the variation of OCP with immersion time in 0.1 M NaCl solution for 2205 duplex stainless steels with different sensi-

Table 1 Electrochemical impedance parameters obtained from the fitting of EIS results. Sensitization time (h)

Rs ( cm2 )

R1 ( cm2 )

Q1 -1

3 6 9 12 15

31.4 32.1 31.5 32.3 31.5

-2

n

Y0 ( cm S )

n

3.25 × 10-5 3.65 × 10-5 3.95 × 10-5 4.21 × 10-5 4.63 × 10-5

0.90 0.89 0.88 0.87 0.85

4.45 × 105 3.95 × 105 3.44 × 105 3.01 × 105 2.12 × 105

tization times. It can be seen that the OCP shifts to more positive potential and gradually stabilizes. The more positive value of the OCP could be attributed to thickening of the passive film spontaneously formed on the surface or more compactness passive film on 2205 duplex stainless steels. In addition, the OCP decreases slightly with the increasing of the sensitization time for 2205 duplex stainless steels in 0.1 M NaCl solution. In Fig. 6b, the real impedance is plotted vs. the imaginary impedance at each frequency for 2205 duplex stainless steels with different sensitization times in 0.1 M NaCl solution. It is seen that Nyquist diagram exhibits a depressed semicircle with a capacitive arc. The diameter of the capacitive semicircle in the Nyquist plot decreases with the increasing of the sensitization time at OCP. A time constant is observed in Fig. 6c. The equivalent circuit in Fig. 6d is proposed for fitting EIS data to quantify the electrochemical parameters. In this equivalent circuit, Rs represents solution resistance; Q1 is double charge layer capacitance; R1 is the charge-transfer resistance [27]. The electrochemical impedance parameters obtained from the fitting of EIS diagrams are shown in Table 1. If the constant-phase element (CPE) impedance response can be associated with an effective capacitance, the dielectric constant or the film thickness may be obtained by the following equation: Ceff =

εε0 ı

(2)

whereı is the film thickness,ε is the dielectric constant, and ε0 is the permittivity of vacuum with a value of ε0 = 8.8542 × 10−14 F/cm. The values of the film thickness can be obtained from power-law model [28], The CPE behavior of oxides on steel was caused by normal rather than surface distributions, which was supported by local electrochemical impedance spectroscopy results [29]. An expression for the effective capacitance is obtained by the following equation Ceff, PL = gQ (␦ εε0 )1−n

(3)

where ␦ is resistivity, ε (≈12) is the relative dielectric constant of the passive film, g is a function of n which can be expressed by the following equation g = 1 + 2.88(1 − n)2.375

(4)

Fig. 3. (a) The X-ray diffraction patterns of the 2205 duplex stainless steels after being sensitized at 675 ◦ C for different times, (b) Volume fraction of austenitic and the ferritic phases.

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Fig. 4. (a) Current changes vs. activation and reactivation potential obtained on the basis of DLEPR measurements for 2205 duplex stainless steels with different sensitization times in 0.5 M H2 SO4 + 0.01 M KSCN solution. (b) The variation of the DOS with sensitization time, (c) potentiodynamic polarization curves in 0.5 M H2 SO4 + 0.01 M KSCN solution for 2205 duplex stainless steels with different sensitization times.

Fig. 5. (a) The results of the evaluation of the CPT of 2205 duplex stainless steels by potentiostatic measurement at 750 mVSCE in 0.1 M NaCl solution and temperature increasing rate of 0.6 ◦ C/min, the variation of the CPT with (b) sensitization time and (c) the DOS.

In addition, the parameter ␦ is assigned a value of ␦ = 500  cm in the power-law model. The value is consistent with the observation of semiconducting properties for the oxide [30].

The solution resistance changes little for sensitized samples, indicating no obvious change of the solution during the form of the passive film. In addition, the charge transfer resistance of 2205 duplex stainless steel decreases from 4.45 × 105  cm2 to

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Fig. 6. (a) The OCP change with time in 0.1 M NaCl solution, (b) Nyquist plots and (c) Bode diagrams of the passive films formed on 2205 duplex stainless steels with different sensitization times in 0.1 M NaCl solution, (d) The electrochemical equivalent circuit for EIS fitting. The variation of the (e) the charge-transfer resistance and (f) the thickness of the passive film with the DOS.

2.12 × 105  cm2 with the sensitization time increasing from 3 h to 15 h. It is clear that the corrosion resistance of passive film decreases with increasing of the DOS for 2205 duplex stainless steel

in 0.1 M NaCl solution in Fig. 6e. The thickness of the passive film also decreases with the sensitization time in Fig. 6f. These results

Fig. 7. (a) The Mott-Schottky plots of the passive films formed on 2205 duplex stainless steels with different sensitization times in 0.1 M NaCl solution, (b) the acceptor concentration and donor concentration in sensitized 2205 duplex stainless steels.

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Fig. 8. Pit morphology of the 2205 duplex stainless steels sensitized at 675 ◦ C for (a) 3 h, (b) 6 h, (c) 9 h, (d) 12 h and (e) 15 h after being electrolytically etched in a 15% oxalic acid solution at 15 V for 60 s.

indicate that the composition of the passive film changes due to sensitization. One of the factors used for ranking the resistance of the stainless steels to pitting corrosion is passive films on the surface of the stainless steels [31,32]. The reduced resistance to localized corrosion of alloy 900 could be related to defects in the formed passive film [33]. The space charge capacitance of the n-type and p-type semiconductor is given by Eqs. (5) and (6) , respectively, according to Mott-Schottky theory. −2 −2 C −2 = CH + CSC =

2 kT (E − Efb − ) εS ε0 qND e

(5)

−2 −2 C −2 = CH + CSC =

−2 kT (E − Efb − ) εS ε0 qNA e

(6)

where ε0 is the vacuum permittivity, εs is the dielectric constant of the specimen, e is the electron charge, k is the Boltzmann constant (1.38 × 10−23 J K−1 ), ND and NA is the donor and acceptor density, respectively. T is the absolute temperature and Efb is the flatband

potential. The donor or acceptor density can be determined from slope in the Mott-Schottky plots [34]. The Mott-Schottky plots recorded for the passive films formed on sensitized 2205 duplex stainless steels in 0.1 M NaCl solution are presented in Fig. 7a. The Mott-Schottky analysis reveals that the passive film formed on sensitized sample exhibits n-type and p-type semiconducting characteristics irrespective of sensitization time. Two slopes in Mott-Schottky plots indicate that the passive films are formed as dual layers which contain iron oxides in outer layer and chromium oxides in inner layer for 2205 duplex stainless steel [35]. However, the passive film formed on Fe–20Cr–xNi (x = 0, 10, 20 wt.%) alloys in deaerated pH 8.5 borate buffer solution exhibited an amorphous structure which was confirmed by TEM and Cs-corrected STEM–EELS analyses [36]. The PDM (point defect model) suggested that the cation vacancies generated at the passive film/solution interface, while cation interstitials and oxygen vacancies generated at metal/passive film interface. The growth and breakdown of the passive film was carried out in terms of for-

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Fig. 9. TEM micrographs and electron diffraction patterns of the 2205 duplex stainless steels sensitized at 675 ◦ C for (a) 6 h and (b) 15 h, respectively.

mation, annihilation and transport of point defects on the atomic scale in the passive film [37]. In Fig. 7b, the donor and the acceptor concentrations in passive films on sensitized 2205 duplex stainless steels are obtained. It is clear that the acceptor concentration is always more than donor concentration. Moreover, with the increasing of sensitization time, the donor and acceptor concentrations increase. This indicates that more chromium-depleted and molybdenum-depleted zones reduce corrosion resistance of the passive films formed on the surface of sensitized 2205 duplex stainless steel. The pitting process was described as being random, sporadic and stochastic in nature, moreover, the prediction of the time and location of the events was extremely difficult [38–40]. The 2205 duplex stainless steels show an increase in pitting corrosion in ferritic phase and an increase in intergranular corrosion between austenitic phase and ferritic phase with the increasing of sensitization time in Fig. 8a–e. Moreover, severe corrosion also occurs in twin boundary in austenitic phase in Fig. 8b and c. These chromium-depleted zones are susceptible to pitting corrosion. The chromium-depleted zones in duplex stainless steels take place mainly in ferritic phase. Because the diffusion rate of chromium in ferritic phase is faster than that in austenitic phase. The sensitization process results in a primary reduction in corrosion resistance in ferritic phase [41,42]. Above results obtained by SEM observation indicate that pitting corrosion between austenitic and ferritic phases becomes serious with the increasing of the DOS. No precipitates are detected by X-ray diffraction, however, TEM observation and the SAD pattern analysis indicate that small precipitated phases occur within the grains for sensitized 2205 duplex stainless steels. Fig. 9a shows that some small precipitates with 100–150 nm in length and 10–20 nm in width are observed. According to the indexing results of the selected area diffraction pattern in Fig. 9a, these precipitates have been identified as M23 C6 carbides. Fig. 9b shows that a large precipitate with 150 nm in size is observed in sensitized 2205 duplex stainless steel. Based on the indexing result of the selected area diffraction pattern in Fig. 9b, this precipitate has been identified as ␴ phase. Due to its high Cr and Mo contents in ␴ phase, the chromium-depleted and molybdenumdepleted zones are formed around the precipitate phase, which results in a decrease in corrosion resistance of the sensitized 2205 duplex stainless steel. The above results suggest that the compositions and structures of precipitated phases change with the increasing of the sensitization time. It was found that the healing occurred due to re-diffusion of Cr and Mo from the ␴ phase to the ␥2 phase in UNS S31803 duplex stainless steel [43]. This could result in

more austenitic phases with the increasing of sensitization time in Fig. 3b. However, the sensitization process exceeded healing process during sensitization process at 675 ◦ C for 2205 duplex stainless steel in the present investigation. Therefore, the DOS increased with the increasing of sensitization time at this temperature. 4. Conclusion The effect of the sensitization of the 2205 duplex stainless steel on the microstructures and corrosion resistance was investigated by XRD, SEM, TEM and electrochemical experiments. The results summarized as below: 1. The volume fraction of the austenitic phase in the 2205 duplex stainless steel increased with the increasing of the sensitization time, while corresponding volume fraction of the ferritic phase decreased at the same time. 2. The DOS of the 2205 duplex stainless steel increased with the increasing of the sensitization time at 675 ◦ C. 3. The CPT of the 2205 duplex stainless steel almost decreased linearly with sensitization time at 675 ◦ C. 4. The precipitate in the 2205 duplex stainless steel changed from initial small M23 C6 carbides to subsequent large ␴ phase with the sensitization process at 675 ◦ C. 5. The sensitization process exceeded the healing process at 675 ◦ C for the 2205 duplex stainless steel. Acknowledgment This work was financially supported by National Natural Science Foundation of China (Grant No. 91326203). References [1] B. Deng, Y. Jiang, J. Gong, C. Zhong, J. Gao, J. Li, Electrochim. Acta 53 (2008) 5220–5225. [2] I.N. Bastos, S.S.M. Tavares, F. Dalarda, R.P. Nogueira, Scr. Mater. 57 (2007) 913–916. [3] K. Ravindranath, S.N. Malhotra, Corros. Sci. 37 (1995) 121–132. [4] N. Sathirachinda, R. Pettersson, J.S. Pan, Corros. Sci. 51 (2009) 1850–1860. [5] C.J. Park, V.S. Rao, H.S. Kwon, Corrosion 61 (2005) 76–83. [6] N. Lopez, M. Cid, M. Puiggali, Corros. Sci. 41 (1999) 1615–1631. [7] N. Ebrahimi, M. Momeni, M.H. Moayed, A. Davoodi, Corros. Sci. 53 (2011) 637–644. [8] M. Pohl, O. Storz, T. Glogowski, Mater. Charact. 58 (2007) 65–71. [9] F. Zanotto, V. Grassi, M. Merlin, A. Balbo, F. Zucchi, Corros. Sci. 94 (2015) 38–47.

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