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Effect of recrystallization degree on properties of passive film of super ferritic stainless steel S44660

  • Bin Wang , Yugui Li ORCID logo EMAIL logo , Huaying Li , Guanghui Zhao , Yaohui Song and Hui Xu
Published/Copyright: April 26, 2024
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Corrosion Reviews
From the journal Corrosion Reviews Volume 42 Issue 4

Abstract

The effect of the recrystallization degree on the properties of passive films formed in 0.1 M HNO3 solution for super ferritic stainless steel S44660 was examined in this study. The initial specimens, in their cold-rolled state, showed a high dislocation density, as observed through electron backscatter diffraction (EBSD) experiments. Analysis of potentiodynamic polarization (PDP) curves and electrochemical impedance spectroscopy (EIS) measurements suggested that with the increase of recrystallization degree, the corrosion current density reduced and the corrosion potential increased. As revealed by Mott–Schottky analysis, the passive film showed a dual structure of n-type and p-type semiconductors, with the carrier density of the passive film decreasing as the recrystallization degree increased. X-ray photoelectron spectroscopy (XPS) provided insights into the film composition, indicating that the Fe2O3 and Cr2O3 content, which improved the stability of the passive film, increased with the degree of recrystallization. In summary, the increase in recrystallization degree reduced the number of defects in the microstructure, thereby creating favorable conditions for the formation of highly protective passive films. The passive film formed after complete recrystallization exhibited enhanced corrosion resistance.

Keywords: super ferritic stainless steel; recrystallization degree; passive film

1 Introduction

Ferritic stainless steels (FSS) have been extensively employed in nuclear power cooling systems, and for seawater heat exchange pipes and offshore construction materials over the past few years (Borri et al. 2019; Gao et al. 2013; Ma et al. 2016). These steels have become promising alternatives to austenitic and duplex stainless steels due to their high thermal conductivity, low linear expansion, excellent corrosion resistance, and low cost. As a “fourth generation” ferritic stainless steel (Azevedo and Padilha 2019), the SFSS series products were originally designed to have a high Cr content and a certain amount of Mo, while the C and N contents were further reduced and stabilizing elements (e.g., Nb and Ti) were added to improve corrosion resistance and processability. Compared with conventional FSS stainless steel, the addition of a small amount of Ni can enhance the impact toughness of ferritic stainless steel (Azuma et al. 2004; Chowwanonthapunya 2019). In general, the high corrosion resistance of SFSS was largely achieved through high alloying, hyper purification, and stabilization.

Super ferritic stainless steel S44660 was originally designed for marine applications (Dowling et al. 1999); it has been one of the most cost-effective alloys for high-chloride corrosive environments and marine corrosive environments. Moreover, S44660 features excellent pitting resistance (PREN value ≥ 36), excellent resistance to stress corrosion in chloride ion environments, high yield strength ratio, high surface hardness, and a coefficient of thermal expansion close to that of carbon steel. It outperforms austenitic and duplex stainless steels in terms of thermal conductivity. Most of S44660 has been employed in seawater and high-chloride environment in thin-walled pipe or pipe plate section.

Stainless steel exhibits excellent corrosion resistance due to the formation of a continuous, stable and renewable oxide passive film on its surface, thus protecting the metal substrate from the corrosive environment (Hakiki 2011; Olsson and Landolt 2003). The structure, stability, and compactness of the passive film are significantly correlated with several factors, such as alloy composition, environmental media, surface condition, and the semiconductor properties of the film (Tranchida et al. 2018). Cold rolling annealing process is important in the production of stainless steel cold plate, a series of evolution of stainless steel microstructure will occur in the process, besides, the impact exerted by mechanics and corrosion resistance is important. During cold working of stainless steel, the strength and hardness are significantly increased due to grain fragmentation, internal lattice distortion, large internal stresses and work hardening, and high density of microstructure defects (dislocation and grain boundary density) (Vafaeian et al. 2016). Moreover, residual strain can remain in the substrate microstructure after cold rolling, which changes significantly the semiconductor properties of the passive film (Maric et al. 2018; Tao et al. 2021), and affects its corrosion resistance (Fattah-alhosseini and Vafaeian 2015). According to Lv et al. (2016), the stability of the passive film is significantly decreased due to the presence of microstructural defects caused by cold working. However, some studies have also reported enhanced localized corrosion resistance due to faster chromium diffusion resulting from cold-working microstructural defects, which accelerates the formation of passive films (Monrrabal et al. 2019; Phadnis et al. 2003). Therefore, the effect of microstructural evolution of stainless steel brought about by cold working on the corrosion resistance of passive films shows different and even controversial characteristics. After recrystallization annealing treatment, the lattice distortion of cold rolled plate will be restored, the density of various organizational defects to reduce the internal stress disappears, the strength and hardness of the material decreases, the plastic toughness improves, and its performance is restored to the level before the deformation (Birol 2008; Rios 2003). At the completion of recrystallization, the cold deformed grains with high dislocation density are replaced by new distortion-free grains with low dislocation density. It has been suggested that recrystallization is conducive to the formation of dense and stable passive films (de Oliveira et al. 2020; Yamamoto et al. 2010). However, these reports lacked a systematic study and necessary mechanistic explanations on the effect of microstructure evolution on the passive film properties of stainless steel.

In recent years, most of the studies on the effect of cold working on the passive film properties of ferritic stainless steels have been focused on low to medium chromium ferritic stainless steels. Fattah-alhosseini and Vafaeian (2015) obtained two grain sizes of AISI 430 ferritic stainless steel cold plate by controlling the annealing temperature after cold rolling, and found that grain refinement provided better conditions for the formation of passive films with higher protective properties due to the growth of thicker and less defective films. Hadi Irani et al. came to a similar conclusion that for Fe-18.5 % Cr ferritic stainless steel, refining the grain structure improves the corrosion and passivation behavior of ferritic stainless steel in 0.1 M HNO3 solution (Irani and Shaban Ghazani 2020). The evolution of passive film after cold working of austenitic stainless steel has different characteristics compared with ferritic stainless steel. By investigating the effect of cold deformation on the microstructural evolution and corrosion behavior of 316L stainless steel, Ma et al. (2022) found that when the deformation was less than 50 %, the corrosion resistance increased with the increase of deformation, but when the deformation increased to 70 %, the stability of the passive film was reduced due to the formation of martensite. Controlling the cold working process of 304 stainless steel, S.K. Pradhan et al. (2021) found that grain refinement intensified the formation of defects in the passive film, while “special” grain boundaries and triple junctions inhibited it. The formation of oxygen vacancies at the metal/film interface is accelerated by microstructural defects caused by strain generation, resulting in reduced passive film’s protective properties.

Currently, most of the studies focus on the effect of cold deformation on the electrochemical behavior and corrosion resistance of low to medium Cr ferritic stainless steels and austenitic stainless steels (Luo et al. 2017a; Pradhan et al. 2021). The evolution of stainless steel passive film properties and corrosion resistance during cold rolling and annealing of high Cr and Mo super ferritic stainless steels has rarely been reported. The corrosion resistance of SFSS S44660 used as a stainless steel material in marine engineering has attracted much attention, therefore, it is necessary to study the law of the evolution of passive film with the microstructure.

Corrosion reaction and passive film performance of stainless steel are affected by alloy composition, microstructure, and environment. The effect of recrystallization on the corrosion resistance of super ferritic stainless steel S44660 was investigated in this study. The effect of microstructure defect density on the barrier properties of passive films was examined through potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). In addition, the semiconductor behavior of the passive film was characterized through Mott–Schottky analysis. The chemical composition and structure of the passive film were examined under X-ray photoelectron spectroscopy (XPS).

2 Materials and methods

2.1 Materials preparation

In this study, super ferritic steel S44660 hot plate in the thickness of 4 mm was selected, of which the chemical composition is presented in Table 1. Being cold rolled to 0.8 mm, three-state specimens were selected for analysis (including cold rolling, annealing at 1000 °C for 1.5 min after cold rolling and water cooling, and annealing at 1060 °C for 1.5 min after cold rolling and water cooling). The specimens were cut to the size of 1 cm × 1 cm, and the surfaces of rolled specimen were tested.

Table 1:

Composition of S44660 super FSS grade (wt-%).

Specimen C Si Mn S Cr Mo Ni N Ti Nb Fe
S44660 0.015 0.340 0.280 0.050 27.8 3.440 1.910 0.020 0.097 0.212 Bal.

2.2 Microstructure characterization

The orientation contrast image of the three-state specimens was captured by using scanning electron microscopy backscattering electron technique to study the evolution of microstructure during recrystallization of super ferritic steel S44660. Moreover, the microstructure of the specimens was characterized through electron backscatter diffraction (EBSD) analysis based on the information kernel average misorientation (KAM), geometrically necessary dislocations (GND), grain boundary area fraction, and dislocation density. The EBSD specimens were pre-polished with 0.5 μm diamond abrasive, and prepared for EBSD analysis through electrolytic polishing in a mixture of 20 mL HClO4 + 80 mL ethanol for 12 s at 32 V DC. High-resolution EBSD measurements were performed by using a Thermo Fisher Apreo 2S FE-SEM and an Oxford/Symmetry S2 detector. All EBSD measurements were performed based on a working distance of 13 mm and a step size of 0.3 μm. Post-processing was performed by using Azteca software based on the EBSD measurements.

2.3 Electrochemical measurements

Electrochemical measurements (potentiodynamic polarization, electrochemical impedance spectroscopy and Mott–Schottky analysis) were used to study the electrochemical properties of the passive films formed on the specimens in the cold rolling state, 1000 °C for 1.5 min, and 1060 °C for 1.5 min. Thermocouple wires were spot welded to the bottom of the specimens to ensure a good connection between the working electrode and the electrochemical cell, and embedded in polyester resin so as to avoid cracks and the exposure of area of 1 cm2. All specimens were ground with 1000 and 2000 size silicon carbide paper and polished with 0.1 μm diamond powder before electrochemical testing. Subsequently, the polished specimens were ultrasonically cleaned in ethanol. Electrochemical measurements were performed by using a conventional three-electrode electrochemical cell. The device includes a specimen as the working electrode, a platinum plate as the counter electrode, and an Ag/AgCl (saturated KCl) reference electrode. All the potential values in the article are relative to the Ag/AgCl (saturated KCl) reference electrode. Besides, 0.1 M HNO3 solution at 25 ± 1 °C served as the test solution. The working electrode was cathodically polarized at −1.0 VAg/AgCl for 5 min before the electrochemical measurements to remove the passive film formed in air. Then, the working electrode was immersed in 0.1 M HNO3 solution for 1800 s under open-circuit potential conditions to prepare a steady-state passive film.

The scan rate of the potentiodynamic curve was 0.33 mV/s, and the test potential range was −0.3∼1.1 VAg/AgCl (relative to Ecorr). The corrosion current density (icorr) and self-corrosion potential (Ecorr) were determined, respectively based on the Tafel linear extrapolation method.

EIS tests were performed under open-circuit potential and AC potential with an amplitude of 10 mV as well as the frequency ranging typically from 100 kHz to 10 mHz. EIS measurements were performed after OCP stabilization, and the measured data were analyzed by using ZSimpWin software.

The semiconductor behavior of passive films formed on the surface of three-state specimens was analyzed using the Mott–Schottky method. The tests were performed based on a 10 mV AC signal and a 25 mV step potential at a frequency of 1 kHz. In the cathodic direction, the potential range was selected from 1.2 VAg/AgCl to −0.6 VAg/AgCl. It is noteworthy that the specimens reached a stable corrosion potential before the Mott–Schottky measurements.

All the respective experiments were repeated three times to examine reproducibility under ambient temperature.

2.4 XPS characterization

X-ray photoelectron spectroscopy (XPS) measurements were performed to examine the chemical composition and structure of the oxide layer in the passive film. The specimens were polished with 0.1 μm diamond powder, and then cleaned before measurements. In addition, the specimens were immersed in a 0.1 M HNO3 solution for 1800 s to achieve the same surface conditions. After that, the specimens were immediately transferred to an XPS vacuum chamber for measurements. XPS tests were performed by using AXIS ULTRADLDXPS spectrometer with monochromated Al-Ka (hν = 1486.6 eV) as X-ray source. The XPS depth profiles were obtained by Ar+ ion bombardment with an ion beam of 2 Kv at the sputtering rate of approximately 2.5 nm/min. Casa software is employed to combine Shirley background subtraction and Gaussian Lorentz function for better spectral fitting. The binding energy of the C 1s peak on the specimen surface was observed at 284.8 eV, corrected, and identified as a reference peak.

3 Results

3.1 Microstructure observations

Figure 1 presents the orientation contrast images obtained by scanning electron microscopy after the specimens were electrolytically polished in three states, showing the microstructure of the rolled surface of the specimens at different recrystallization degrees. Compared with secondary electron images taken using the conventional erosion method, after electrolytic polishing the backscattered orientation contrast images can indicate the morphology of the deformed microstructure more clearly. As depicted in Figure 1a, the deformed ferrite grains exhibited considerable shear bands after cold rolling. As shown in Figure 1b, partial recrystallization occurred after annealing treatment at 1000 °C for 1.5 min. The grains were all equiaxed crystals with an average size of about 40 μm after being heated at 1060 °C for 1.5 min, as shown in Figure 1c.

Figure 1: BSE images of cold-rolled and after two temperature heat treated S44660 super FSS: (a) cold-rolled, (b) 1000 °C-1.5 min, (c) 1060 °C-1.5 min.
Figure 1:

BSE images of cold-rolled and after two temperature heat treated S44660 super FSS: (a) cold-rolled, (b) 1000 °C-1.5 min, (c) 1060 °C-1.5 min.

High-resolution electron backscatter diffraction (EBSD) measurements were performed to observe the microstructure at different levels of recrystallization to confirm the defect density (grain boundary area fraction and dislocation density). Figure 2 shows BC maps, IQ maps, kernel average misorientation (KAM) maps and geometrically necessary dislocation (GND) maps for the cold rolling condition, 1000 °C-1.5 min and 1060 °C-1.5 min specimens. The similar results of Figure 1 can be identified from the BC maps, and the degree of microstructure recrystallization increased with the increase of the annealing temperature. Red lines in the IQ maps represent low-angle grain boundaries between 2° and 5°, green lines represent angular grain boundaries between 5° and 15°, and black lines represent high-angle grain boundaries above 15°. As illustrated in the above figure, after cold rolling (Figure 2a2), there were considerable low-angle grain boundaries (2°–5°) in deformed grains, accounting for 63.7 %; after annealing at 1000 °C for 1.5 min (Figure 2b2), the microstructure comprised both recrystallized grains and reverted grains, and the number of low-angle grain boundaries was decreased, accounting for nearly 43.3 %; after annealing at 1060 °C for 1.5 min (Figure 2c2), the grain boundaries of the recrystallized grains were all high-angle grain boundaries. KAM value is usually used to estimate the residual strain and plastic deformation inside the material. As depicted in the KAM maps, after cold rolling (Figure 2a3) high local orientation difference regions existed in most of the deformed grains, indicating the higher and unevenly distributed local strains (Subedi et al. 2015); after annealing at 1000 °C for 1.5 min (Figure 2b3), the small-sized recrystallization grains were identified to be defect-free grains without strain, and the presence of local orientation difference was identified only in the deformed grains. After annealing at 1060 °C for 1.5 min (Figure 2c3), the recrystallization was completed, the deformed grains were transformed into equiaxed grains without strain, and the residual strain disappeared. Figure 2(a4, b4, c4) shows the distribution of geometrically necessary dislocations (GND), and it is evident that the cold-rolled specimen has the highest dislocation density, with an average of 7.5 × 1014 m−2, the partially recrystallized specimen’s dislocation density is 3.4 × 1014 m−2, and that of the fully recrystallized one is 0.4 × 1014 m−2.

Figure 2: BC (band contrast) maps, IQ (image quality) maps, kernel average misorientation (KAM) maps and geometrically necessary dislocation (GND) maps of cold-rolled and after two temperature heat treated S44660 super FSS: (a1), (a2), (a3), (a4) cold-rolled; (b1), (b2), (b3), (b4) 1000 °C-1.5 min; (c1), (c2), (c3) (c4) 1060 °C-1.5 min.
Figure 2:

BC (band contrast) maps, IQ (image quality) maps, kernel average misorientation (KAM) maps and geometrically necessary dislocation (GND) maps of cold-rolled and after two temperature heat treated S44660 super FSS: (a1), (a2), (a3), (a4) cold-rolled; (b1), (b2), (b3), (b4) 1000 °C-1.5 min; (c1), (c2), (c3) (c4) 1060 °C-1.5 min.

In general, with the increase of annealing temperature, on one hand, the recrystallization degree of the specimens was increased, the grains transformed into equiaxed grains, the proportion of high-angle grain boundaries was increased, and the microstructure became homogeneous; on the other hand, the average value of KAM and dislocation density were decreased significantly, resulting in the decrease of local strain and microstructure defects.

3.2 Electrochemical characterization

Figure 3 shows the potentiodynamic polarization curves of S44660 stainless steel in the cold rolling state, after annealing at 1000 °C-1.5 min and 1060 °C-1.5 min in 0.1 M HNO3 solution. It can be seen from the above figure that all three curves show similar trends, and exhibit typical passive behavior. It can be identified that the Epit values of the specimens in the three states are similar, indicating that the recrystallization degree has no significantly influence on the pitting potential of S44660 stainless steel. The corrosion potential Ecorr, corrosion current density icorr and passive current density ip were calculated by using the Tafel extrapolation method (Table 2). The passive current density can approximately represent the dissolution rate of the passive film in the passive process (Fu et al. 2009; Schultze and Lohrengel 2000), which can be selected as a characterization of the stability of the passive film. The corrosion potential Ecorr(V) of the cold-rolling state specimen reached the minimum, whereas its corrosion current density and passive current density were maximum, suggesting that in the cold-rolling state, the electron and/or ionic conductivity of the passive film was highest, and the corrosion rate was maximum. After annealing at 1000 °C for 1.5 min, the Ecorr(V) was slightly increased, and the corrosion current density and passive current density were decreased with partial recrystallization of the grains. After recrystallization (1060 °C-1.5 min), the Ecorr(V) of the specimen showed a significantly positive shift, and the corrosion current density and passive current density were reduced. In conclusion, under the same acidic conditions, the stability and corrosion resistance of the passive film formed by fully recrystallized grains improved.

Figure 3: Potentiodynamic polarization curves of cold-rolled and after two temperature heat treated S44660 super FSS in 0.1 M HNO3 solution at 25 °C.
Figure 3:

Potentiodynamic polarization curves of cold-rolled and after two temperature heat treated S44660 super FSS in 0.1 M HNO3 solution at 25 °C.

Table 2:

Parameters of potentiodynamic polarization curves.

Specimen E corr (VAg/AgCl) i corr (mA/cm2) i p (mA/cm2)
Cold-rolled −0.150 5.492 × 10−4 1.479 × 10−3
1000 °C-1.5 min −0.084 5.133 × 10−4 1.288 × 10−3
1060 °C-1.5 min 0.201 2.821 × 10−4 6.670 × 10−4

EIS tests were conducted at open circuit potential. Figure 4 shows the Nyquist and equivalent circuit image obtained by testing the specimens in the cold rolling state, after annealing at 1000 °C-1.5 min and 1060 °C-1.5 min in 0.1 M HNO3 solution. The Nyquist plot reflects the correlation between the real part of impedance and the imaginary part. As depicted in Figure 4, the three curves were all single capacitive arc within the whole test frequency range, suggesting that the corrosion rate was primarily controlled by the charge transfer process generated by the electrochemical reaction on the electrode surface. The radius of the capacitive arc of the annealed specimen prepared at 1060 °C for 1.5 min was significantly larger than that in the other two states, and the radius of the capacitive arc can directly indicate the corrosion resistance of the passive films, that is, the larger the radius, the better the corrosion resistance (Xu et al. 2020). An equivalent circuit was fitted by using ZSimpWin software to quantify the electrochemical parameters and analyze the results of the impedance spectrum. In the equivalent circuit, Rs represents the solution resistance; QCPE (constant phase angle element) denotes the double charge layer capacitance; and Rct expresses the charge transfer resistance. The electrochemical impedance parameters obtained by fitting are listed in Table 3 where Y0 denotes a constant indicating the CPE (Ω−1 cm−2 Sn), n expresses the exponent of the CPE, and ω represents the angular frequency (rad s−1). It can be seen that Rct becomes larger as the degree of recrystallization increases, indicating that electrons and/or ions are subjected to greater resistance in the passive film and when migrating across the interface (Fernández-Domene et al. 2014), making the passive film more resistant to penetration of the corrosive medium. Besides, the corrosion reactions at the interface between the passive film and the corrosive medium are hindered. The EIS test findings show that the passive film generated on the surface of the completely recrystallized specimen has the highest stability, protection, and corrosion resistance, which agrees with the polarization curve test results.

Figure 4: Nyquist and equivalent electrical circuits of cold-rolled and after two temperature heat treated specimens at open-circuit potential in 0.1 M HNO3 solution.
Figure 4:

Nyquist and equivalent electrical circuits of cold-rolled and after two temperature heat treated specimens at open-circuit potential in 0.1 M HNO3 solution.

Table 3:

Equivalent circuit parameters of impedance spectrum.

Specimen R s Q CPE R ct
Ω cm2 Y 0 n Ω cm−2
(Ω−1 cm−2 Sn)
Cold-rolled 13.90 7.105 × 10−5 0.89 3.870 × 104
1000 °C-1.5 min 16.44 5.790 × 10−5 0.91 4.368 × 104
1060 °C-1.5 min 20.89 4.425 × 10−5 0.88 2.594 × 105

3.3 Mott–Schottky analysis

The semiconductor properties of the passive films were characterized using Mott–Schottky analysis. In general, nanoscale passive films have considerable point defects, which behave as extrinsic semiconductors. In accordance with the point defect model (PDM), the point defects in passive films are cation vacancies (VMχ′), oxygen vacancies (VO..), and cation gap (Miχ+). Based on the PDM model, the electron acceptor in the p-type semiconductor is the cation vacancy, whereas the oxygen vacancy and the cation gap serve as electron donors for the n-type semiconductor (Zhang et al. 2012). The semiconductor type of the passive film and the defect density in the oxide film was determined through Mott–Schottky (M−S) analysis (Feng et al. 2010). Based on the Mott–Schottky theory, the contribution of the Helmholtz layer to the capacitance of the passive layer is negligible (Fattah-alhosseini et al. 2010a; Luo et al. 2011). Accordingly, the capacitance of the passive film can be considered as the capacitance of the space charge layer. n-type and p-type semiconductors are expressed by Eqs. (1) and (2).

(1)1C2=2εε0eND(E−EFB−kTe) for n‐type semiconductor
(2)1C2=−2εε0eNA(E−EFB−kTe) for p‐type semiconductor

where C and E denote the capacitance and applied potential of the space charge layer, respectively; ND and NA represent the donor and acceptor densities (cm−3) of n- and p-type semiconductors, respectively; e expresses the electronic charge (1.602 × 10−19 C); ε represents the dielectric constant of the passive film (usually taken as 15.6 for stainless steel (Fattah-alhosseini et al. 2010b)); ε0 denotes the vacuum dielectric constant; k is the Boltzmann constant (1.38 × 10−23 J/K); T refers to the absolute temperature; and EFB is the flat band potential. The donor and acceptor densities of the semiconductor material can be determined by the slope of the C−2 versus E curve.

Figure 5 illustrates the Mott–Schottky curves of the passive films formed on the specimens in the cold-rolling state, after annealing at 1000 °C for 1.5 min and at 1060 °C for 1.5 min in 0.1 M HNO3 solution. As depicted in Figure 5, the correlations between capacitance and applied potential of the three-state specimens follow the same trend, and all three curves consist of three different regions. According to their n-type and p-type semiconductor behavior, the passive films of all three-state specimens exhibit a dual structure. The above phenomena are consistent with the results obtained for other stainless steels in acidic solutions (Fattah-alhosseini et al. 2011). The linear portions of all the three curves in Region I and Region III have negative slopes, and exhibit p-type semiconductor behavior, while the linear portions in Region II show positive slopes, and exhibit n-type semiconductor behavior (Wijesinghe and Blackwood 2007). The slope of the linear part of the M−S curve was inversely proportional to the carrier density; the higher the slope, the lower the carrier density. In addition, a flat plateau exists between Region I and Region II, which is often referred to as the flat band potential (EFB), and the donor and acceptor densities are in equilibrium (Pradhan et al. 2021). Numerous studies on stainless steel passive films have suggested that Cr oxide in the passive film serves as a p-type semiconductor, while Fe oxide/hydroxide layer serves as an n-type semiconductor (Bautista et al. 2009; Hakiki et al. 1995, 2000; Trisnanto et al. 2022).

Figure 5: Mott–Schottky plots of cold-rolled and after two temperature heat treated specimens in 0.1 M HNO3 solution.
Figure 5:

Mott–Schottky plots of cold-rolled and after two temperature heat treated specimens in 0.1 M HNO3 solution.

The slope of the linear part of the M−S curve was inversely proportional to the carrier density; and the higher the slope, the lower the carrier density. The values of ND and NA can be determined based on the slope of C−2 versus E. In accordance with Eq. (3), the charge carrier density N is expressed as follows:

(3)N=2meεε0

where m denotes the slope of the Mott–Schottky curve in the relevant linear region; e expresses the electronic charge; ε represents the relative dielectric constant of the semiconductor, which was assumed as 15.6 for passive films formed on stainless steel; and ε0 represents the vacuum dielectric constant, reaching 8.85 × 10−12 F m−1.

The order of magnitude of the donor and acceptor densities for the three-state specimens ranges from nearly 1021 to 1022 cm−3 (Table 2), which is consistent with that of carrier densities in the literature (Fattah-alhosseini and Vafaeian 2015). As depicted in Table 4, in all the three regions of the M−S curves, the cold-rolling state specimens exhibited the maximum donor/acceptor densities, and the fully recrystallized specimens (1060 °C-1.5 min) achieved the minimum donor/acceptor densities. The PDM model indicates that the carrier density is a vital indicator of the point defects condition in the passive film (Vázquez and González 2007). The larger the carrier density, the more the defects in the passive film, and the higher the conductivity of the passive film (Carmezim et al. 2005; Escrivà-Cerdán et al. 2012), leading to a higher reaction rate and poorer protection of the passive film, the reason is that the increase of the defects concentration in the passive film can facilitate the electrode reaction. In comparison with the cold-rolling and partially recrystallized specimens, the fully recrystallized specimens achieved a lower carrier density and had fewer defects in the passive film formed in the 0.1 M HNO3 solution, which exhibited better corrosion resistance.

Table 4:

Donor and acceptor densities of cold-rolled and after two temperature heat treated S44660 super FSS in 0.1 M HNO3 solution.

Specimen N A N D
Cold-rolled 2.01 × 1021 1.45 × 1021
1000 °C-1.5 min 1.40 × 1021 4.64 × 1020
1060 °C-1.5 min 8.45 × 1020 1.55 × 1020

3.4 XPS results

The corrosion behavior of super ferritic stainless steels in acidic environments is significantly correlated with the composition and structure of their passive films. Thus, the effect of the recrystallization degree on the composition and structure of the passive film were analyzed by using X-ray photoelectron spectroscopy (XPS) observations. The results showed that the passive films of the three-state specimens were composed of Cr, Fe, Mo, and O.

Figure 6 presents the high-resolution XPS spectra of Cr 2p3/2, Fe 2p3/2, and Mo 3d of the passive films formed on S44660 in three states. The Cr spectra are split into four peaks denoting the metallic Cr(met) (574.1 ± 0.1 eV), Cr2O3 (576.1 ± 0.1 eV), and Cr(OH)3 (577.5 ± 0.1 eV), respectively (Chen et al. 2021; Donik and Kocijan 2014). In comparison with Cr(OH)3, Cr2O3 exhibited a lower concentration of point defects (i.e., cation vacancies) and higher stability (Jinlong et al. 2016; Ma et al. 2022). Seen from Table 5, the Crox/Crhy ratio reached the minimum in the passive film of the cold-rolling state specimens; with the increase of the annealing temperature, the recrystallization degree was increased, and the Crox/Crhy values were increased accordingly with the increase of the Cr2O3 content in the passive film. It can be seen that the increase of the degree of recrystallization facilitates the improvement of the corrosion resistance of S44660. For the Fe 2p3/2 spectrum, as shown in Figure 6, the Fe 2p3/2 spectrum was divided into four peaks, representing metallic Fe (706.6 ± 0.1 eV), FeO (708.4 ± 0.1 eV), Fe2O3 (710.5 ± 0.1 eV), and Fe(OH)3 (712.5 eV), respectively (Biesinger et al. 2011; Freire et al. 2012). The doping characteristics of Fe2+ reveal that more oxygen vacancies will be formed in the passive film to charge balance with the increase of the Fe2+ content, leading to the breakdown of the passive film (Li and Luo 2007). The Fe2+/Fe3+ ratio was decreased as the recrystallization degree increased, which indicates a shift from Fe(II) compounds to Fe(III) compounds. With the decrease of the Fe2+/Fe3+ ratio, the Fe3+ content in the passive films is increased, the oxygen vacancies in the passive films are decreased, and the stability of the passive films is increased. A double-peak structure due to the spin–orbit coupling of Mo 3d5/2 and Mo 3d3/2 can be seen. The above figure shows that the Mo 3d spectra in the passive films of all the three states of the specimens comprised three sets of peaks, representing the metal (Mo (met)) (227.6 eV ± 0.1 and 230.8 ± 0.1 eV), the tetravalent (Mo4+) (230.2 ± 0.2 eV and 233.2 ± 0.2 eV) and the hexavalent (Mo6+) (232.5 ± 0.2 eV and 235.3 ± 0.2 eV), respectively (Luo et al. 2017b). Pardo et al. (2008) has suggested that molybdenum reduced the donor and acceptor densities in the passive film, and improved the passive status of stainless steel.

Figure 6: XPS spectra of Cr 2p3/2, Fe 2p3/2, and Mo 3d of the passive film formed on cold-rolled and two temperature heat treated S44660 super FSS in 0.1 M HNO3 solution.
Figure 6:

XPS spectra of Cr 2p3/2, Fe 2p3/2, and Mo 3d of the passive film formed on cold-rolled and two temperature heat treated S44660 super FSS in 0.1 M HNO3 solution.

Table 5:

The ratios of Fe2+/Fe3+, Crox/Crhy, and O2−/OH− of the passive film formed on cold-rolled and two temperature heat treated specimens in 0.1 M HNO3 solution.

Specimen Fe2+/Fe3+ Crox/Crhy O2−/OH−
Cold-rolled 1.15 2.53 0.43
1000 °C-1.5 min 1.01 3.69 0.44
1060 °C-1.5 min 0.81 5.96 0.51

As depicted in Figure 7, the O1s can be fitted to three peaks, including O2− (530.4 ± 0.2 eV), OH− (531.6 ± 0.2 eV) and H2O (533.0 ± 0.2 eV) (Kong et al. 2018). O2− refers to the metal oxides in the passive films, OH− represents the formation of Fe(OH)3 and Cr(OH)3, and the peak at the binding energy of 533 eV indicates bound water (H2O). Research has shown that OH− in passive films contributes less to the improvement of the corrosion resistance of passivated films compared to O2−. As shown in Table 5, the O2−/OH− ratio increases with the increase of annealing temperature, the O2−/OH− ratio reaches the maximum value of 0.51 when recrystallization is completed. Therefore, the oxide content in the passive films increases with the increase of recrystallization, and the passive films becomes denser.

Figure 7: XPS spectra of O1s of the passive films formed on cold-rolled and two temperature heat treated S44660 super FSS.
Figure 7:

XPS spectra of O1s of the passive films formed on cold-rolled and two temperature heat treated S44660 super FSS.

To examine the distribution of alloying elements in the passive film, an X-ray photoelectron spectroscopy (XPS) depth profiling test was conducted on the three states of S44660. The results are shown in Figure 8 which illustrates the curves of the distribution of the major elements (Fe, Cr, Mo, O) with depth. The thickness of the passive film was determined using the 50 % value of the maximum amplitude of the oxygen element, which is a common method used in this field (Mischler et al. 1991). As the sputtering rate of XPS was 2.5 nm/min, the sputtering time could be converted to sputtering depth. Thus, the sputtering depth was 4.67 nm for the cold-rolled specimen, 3.52 nm for the specimen obtained after annealing at 1000 °C for 1.5 min, and 3.02 nm for the specimen obtained after annealing at 1060 °C for 1.5 min. It can be seen from the test results shown in Figure 8 that the major elements’ distribution trends in the passive films are similar. As sputtering time increased, the O content gradually decreased, the Fe content gradually increased, and the Mo content slightly increased. Meanwhile, the Cr content was initially greater than that of Fe, but when the sputtering time exceeded 60 s, the Cr content started to decrease. These results confirmed that the outer layer of the passive film after acid passivation is a Cr-rich oxide, while the inner layer had a higher content of Fe oxide. Zheng and Zheng (2016) also reported the same situation in previous studies.

Figure 8: XPS depth profiles of the passive films formed on cold-rolled and two temperature heat treated S44660 super FSS.
Figure 8:

XPS depth profiles of the passive films formed on cold-rolled and two temperature heat treated S44660 super FSS.

4 Discussion

M−S analysis and EBSD test data have revealed that the passive films formed on cold-rolled and partially recrystallized specimens in 0.1 M HNO3 solution exhibit a higher carrier density than that of fully recrystallized specimens, and the carrier density in the passive film increases with the microstructural defect (dislocation) density. The reason is that dislocations in the interface of substrate and passive film promote electrochemical reaction, resulting in a higher passive current density ip (as shown in Table 2). The accumulation of substrate strain due to various microscopic defects (Figure 2a1) enhances substrate surface activity (Feng et al. 2019; Yamamoto et al. 2010), and accelerates the formation of the passive film, resulting in a thicker film with a higher carrier density. Several studies have demonstrated that the presence of substrate defects has a significant impact on doping concentration of the passive film, as well as the stability and compactness (Feng et al. 2014; Lv et al. 2016). While after completely recrystallization S44660 stainless steel has the lowest substrate dislocation density (Figure 2(c4)) and local strain (Figure 2(c3)), leading to the lowest carrier density in the formed passive film. Although the passive film is the thinnest, its compactness and stability are the best.

For a deeper understanding the impact of microstructural defects on the passive films stability, the Point Defect Model (PDM) was employed to elucidate the formation and breakdown of passive films (Fattah-alhosseini et al. 2011; Macdonald 2006, 2011). The PDM’s diagrammatic representation (as shown in Figure 9) explains key reactions occurred at the interfaces of metal substrate, passive film, and electrolyte. In this model, it is hypothesized that the majority of substrate dislocations consist of point defects, and the dislocations at the substrate/passive film interface are considered as the edges of point defect assemblage. Additionally, most of the defects in the passive film comprise oxygen vacancies (VO..), cationic gaps (Miχ+), and cationic vacancies (VMχ′) (Liu and Macdonald 2001). Defect generation and annihilation occur at the interface between the metal substrate and the passive film (i.e., Interface I), and between the passive film and the electrolyte (i.e., Interface II), as portrayed in (Equations a–g) in Figure 9. Equation (a) describes the depletion of cation vacancies (Vm) at Interface I generated by Interface II (Equation (d)). Equation (b) describes the injection of metal cation interstitial (Miχ+) from the substrate into the passive film at Interface I; while Equation (e) describes the migration of Miχ+ from the passive film into the substrate at Interface II. Equation (c) explains the growth process of the passive film on the stainless steel surface, the metal cation (MM) in the cationic position formed by the metal cation (m) entering the passive film from the substrate, and the oxygen ion (OO) injected into the passive film in the reaction according to Equation (f) combined with an oxide (MOx/2) on the substrate, leading to the passive film growth into the substrate. Equation (f) describes the oxygen vacancies (VO..) annihilation and oxygen ion (OO) injection into the passive film. Equation (g) represents the dissolution of the passive film at Interface II, which leads to the thickness decrease of passive film even to a breakdown. Equations (a), (b), (d), and (f) are responsible for keeping the electrochemical equilibrium of the passive film system, while equations (c) and (g) are responsible for describing the formation and dissolution of the passive film, respectively.

Figure 9: Schematic illustration of defect generation–annihilation reactions that occur in the Interfaces-I and Interface-II according to the PDM. Here m = metal atom, MM = metal cation at cation site in passive film, Vm = vacancy in metal phase, Miχ+ = cation interstitial, MΓ+ = cation in the solution, VM χ’ = cation vacancy, VÖ = oxygen (anion) vacancy, OO = oxygen ion at anion site in passive film, MOx/2 = stoichiometric passive film.
Figure 9:

Schematic illustration of defect generation–annihilation reactions that occur in the Interfaces-I and Interface-II according to the PDM. Here m = metal atom, MM = metal cation at cation site in passive film, Vm = vacancy in metal phase, Miχ+ = cation interstitial, MΓ+ = cation in the solution, VM χ’ = cation vacancy, VÖ = oxygen (anion) vacancy, OO = oxygen ion at anion site in passive film, MOx/2 = stoichiometric passive film.

The experiments conducted in this study involved cold rolled specimens and partially recrystallized specimens with numerous dislocation defects (T) originated from the substrate at Interface I. These dislocation defects can react as follows (Feng et al. 2014):

(6)m+T→mT

where mT denotes the metal atom located near the dislocation at Interface I.

The dislocations at Interface I participated in the oxide formation at the substrate/passive film interface, and accelerated the (a), (b), (c) reactions, leading to the increase of the initial growth rate of the passive film. The acceleration of various reactions limited the time for the atoms/ions to diffuse to the “correct” location, and resulted in more defects. The interaction between dislocation defects and major point defects in the passive film can be explained by using Equations (7)–(9).

Equation (7) explains the effect of dislocations on oxygen vacancies (Feng et al. 2014):

(7)mT→MM+x2VO¨+xe′

where VO¨ denotes the oxygen vacancy, MM refers to the metal cation at the cationic position in the passive film, and T represents the dislocation.

Equation (7) shows that an increase of 1 M point defect will result in an increase of x/2 M oxygen vacancies, implying that the presence of microstructural defects leads to the formation of oxygen vacancies in the passive film, and accelerates reactions at the interface, which is one of the reasons for the high passive current density of cold-rolled and partially recrystallized specimens, and the formation of passive films with a high density of carriers.

Equation (8) explains the impact of dislocations on interstitial cations (Yamamoto et al. 2010):

(8)mT→Miχ++Vm+χe′

where Vm denotes the vacancy in the substrate, and Miχ+ refers to the interstitial cation in the passive film.

The interstitial cations can be formed at Interface I through Equation (b), and then migrate to Interface II to become cations in the electrolyte by Equation (e), which leaves vacancies in the substrate at Interface I. The accumulation of dislocation defects at Interface I accelerates the formation of interstitial cations, which in turn reduces the compactness of the passive film.

Equation (9) explains the effect of dislocation on cation vacancies in passive films (Feng et al. 2014):

(9)Null=VMχ′+x2VO¨

where VMχ′ and VO¨ denote cation vacancies and oxygen vacancies, respectively.

The process described by Equation (9) is autocatalytic, which means that an increase in the number of oxygen vacancies (Equation (7)) may lead to more cationic vacancies in the passive film. The dislocations generated by cold rolling promote the inward diffusion of cation vacancies at Interface I. And the rapid diffusion of cationic vacancies accelerates the dissolution of the passive film.

Based on Equations (7)–(9), it is evident that the existence of a high number of microstructural defects, such as dislocations accelerates the formation of oxygen vacancies, interstitial cations, and cation vacancies in the passive film. Consequently, the passive films formed on the surface of cold-rolled and partially recrystallized specimens have lower compactness, stability, and resistance to electron and/or ion migration, leading to lower Rct values (Table 3) compared to fully recrystallized specimens.

In summary, the dislocations enhance the reactions occurred at the interface between the metal substrate and the passive film, and have a significant impact on the formation and migration of point defects. This can explain the high passive current density and high carrier density in the passive film on the cold-rolled and partially recrystallized specimens, resulting in a thicker but higher defect density passive film. However, the low density of microscopic defects in the substrate of S44660 super ferritic stainless steel after complete recrystallization makes the passive film thinner but denser and more stable, which has a stronger protective effect on the substrate. In this study, the effect of microscopic defect density on passive film properties in high Cr and Mo super ferritic stainless steels has been derived for the first time, and the mechanism of the evolution of microstructure morphology on the corrosion resistance of the material during the annealing treatment has been revealed.

5 Conclusions

In this study, the effect of the recrystallization degree on the passive behavior of S44660 super FSS in 0.1 M HNO3 solution was studied. The conclusions were drawn as follows.

  1. As indicated by the results of microstructure observation, the cold-rolled specimen exhibited a greater degree of strain and the highest density of microstructural defects. At the completion of recrystallization, the residual strain disappeared, and the dislocations density reached the minimum.

  2. The electrochemical results suggested that the cold-rolled specimen exhibited greater corrosion current density icorr and passive current density ip, and the charge transfer resistance Rct was minimum.

  3. As revealed by Mott–Schottky analysis, the carrier density in the passive film and the concentration of microscopic defects were decreased with the increase of recrystallization degree.

  4. The results of XPS analysis showed that with the increase of the recrystallization degree of the microstructure, the Fe2+/Fe3+ ratio decreased, the Crox/Crhy ratio increased and the O2−/OH− ratio increased. The findings of the XPS depth profiling analysis proved that the cold-rolled and partially recrystallized specimens had a thicker passive film.

  5. As the degree of recrystallization increased, the compactness and stability of the passive film got higher, which improved the corrosion resistance and resulted in stronger protection.

Corresponding author: Yugui Li, School of Mechanical Engineering, Taiyuan University of Science and Technology, Taiyuan030024, China, E-mail:

Funding source: Excellent Innovation Project for Graduate Students in Shanxi Province

Award Identifier / Grant number: 2023KY632

Funding source: National Natural Science Foundation of China

Award Identifier / Grant number: 52375364

Funding source: Central Guiding Local Science and Technology Development Fund Project

Award Identifier / Grant number: YDZJSX2021A036

Funding source: Basic Research Program of Shanxi Province

Award Identifier / Grant number: TZLH20230818001

  1. Research ethics: Not applicable.

  2. Author contributions: The authors have accepted responsibility for the entire content of this manuscript and approved its submission. Bin Wang: investigation, methodology, validation, writing – original draft, writing – review & editing. Yugui Li: methodology, investigation, data curation, writing – original draft, writing – review & editing, visualization, project administration. Huaying Li: methodology, investigation, validation, data curation, visualization. Guanghui Zhao: methodology, validation, supervision. Yaohui Song: conceptualization, validation, supervision. Hui Xu: conceptualization, validation, supervision.

  3. Competing interests: The authors state no conflict of interest.

  4. Research funding: This project was supported by National Natural Science Foundation of China (52375364); Central Guiding Local Science and Technology Development Fund Project (YDZJSX2021A036); Basic Research Program of Shanxi Province (TZLH20230818001); Excellent Innovation Project for Graduate Students in Shanxi Province (2023KY632).

  5. Data availability: The raw data can be obtained on request from the corresponding author.

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Received: 2023-06-17
Accepted: 2024-02-22
Published Online: 2024-04-26
Published in Print: 2024-08-27

© 2024 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.1515/corrrev-2023-0069
Keywords for this article
super ferritic stainless steel; recrystallization degree; passive film

Articles in the same Issue

  1. Frontmatter
  2. Reviews
  3. The importance and evolution of corrosion inhibitors in the drilling fluid for the offshore drilling industry
  4. Recent progress on the research of steel corrosion behavior and corrosion control in the context of CO2 geological utilization and storage: a review
  5. Original Articles
  6. Effect of recrystallization degree on properties of passive film of super ferritic stainless steel S44660
  7. Corrosion inhibition properties of sweet crude oil for carbon steel 1018 in NaCl solution
  8. Surface modified molybdenum disulfide nanosheets for corrosion resistance improvement on polyurethane coatings
  9. Comparison of the hydrogen damage of different rolling surfaces of TC4 Ti alloy sheet
  10. Study on the preparation and anticorrosive performance of silica/modified kaolin/epoxy resin composite coating
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