Study of the Passivation Film on S32750 Super-Duplex Stainless Steel Exposed in a Simulated Marine Atmosphere

Abstract

The corrosion behavior and passivation mechanism of S32750 super-duplex stainless steel exposed in a simulated marine atmosphere were studied using electrochemical methods, XPS and SEM. Passivation and local corrosion occurred on the metal surface when S32750 SDSS was exposed in the simulated marine atmospheric environment. The passivation film is composed of two chromium-enriched layers. The outer layer is a very thin film at the metal/atmosphere interface of the specimen surface with higher chromium content, whereas the chromium in the inner layer seems a little depleted. The outer and inner layers had similar Fe components, and Fe³⁺ oxide/hydroxide was the primary oxide in the film. The outer layer contains CrO₃, whereas the inner layer has Cr³⁺ as its primary oxide. Pitting occurred when exposure time exceeded 24 d, and the Cr content of the specimen decreased. Therefore, S32750 SDSS exposed for 24 d exhibited the best corrosion resistance.

1. Introduction

Duplex stainless steel (DSS) contains high alloy compositions of chromium, nickel and molybdenum and a mixed microstructure that comprises approximately equal amounts of ferrite (α) and austenite (γ) phases [1,2]. These properties make DSS a popular study topic. The elements in the steel form a passive film on the surface when exposed to oxygen [3,4]. The passive film protects the steel from aggressive chemical species, particularly pitting corrosion [5,6]. However, the pitting corrosion of stainless steel only occurs during exposure to marine atmosphere [7,8,9], because chloride ions and deposition particles destroy the passive film, and the structure of passive film considerably affects the corrosion resistance of various microstructures [10]. Investigating the electrochemical behavior and passive film properties on stainless steel surfaces in marine atmosphere is essential in understanding the pitting corrosion process and formation mechanism of the passive film.

The passivity and corrosion of ferritic and austenitic stainless steels are highly interesting and widely studied [11,12,13,14,15]. Raman measurement confirms the presence of the austenite and ferrite phases on the passivation layer created on the surface of 2205 duplex stainless steel [13]. In chloride solution, pitting potential increased and passive current density decreased with the addition of HCO₃⁻and SO₄²⁻ [14]. The passive film structure of UNS S32750 was a single layer, the microstructure of austenite contents and morphologies had no influence on the passive film structure [15]. The XPS analyser has become popular for studying passive films. The Cr oxide in the films is mainly responsible for maintaining passivity [16]. The influence of chloride ions on the passivation behavior of 304L and 316L stainless steel was investigated using XPS; the results showed that Cr₂O₃ and CrO₃ formed in the passive film [17]. Passivation and film chemistry of 2507 super-duplex stainless steel in modified artificial seawater (ASW) were investigated by Cui. [18]. The effects of short-time heat treatment at different temperatures on the microstructure evolution and pitting corrosion behavior of UNS S32750 super-duplex stainless steel welds were investigated [19]. The corrosion behavior of 316L stainless steel in concentrated artificial seawater at 72 °C was studied using electrochemical measurement techniques [20]. The corrosion state changed from spontaneous passivation to pitting after approximately 1150 h of immersion. The passive film acted as a capacitive protection layer before the initiation of pitting corrosion. Klapper [21] used electrochemical noise measurements to investigate the stability and susceptibility of passive layers to pitting corrosion of 304L stainless steel after pickling and passivation at different environmental conditions. The protectiveness of the stainless steel surfaces after pickling was strongly dependent on the relative humidity of the environment in which the surface was subsequently passivated. The corrosion resistances and passivation of austenitic 316 L and duplex 2205 stainless steel were compared [22]. Different passivation processes were observed for different types of steel.

However, few published studies have focused on the passivation mechanism of DSS. Passive film properties vary in terms of alloy composition, environment, film thickness, structure and stoichiometry [23,24,25,26]. Li et al. [27] indicated that crevice corrosion occurred on DSS UNS S32101 in neutral 0.1 mol/L NaCl solutions at room temperature. The current fluxes caused by the passive/active transition of passive films on the crevice wall were the main reason for the immediate crevice corrosion on UNS S32101. Qiao et al. [28] used CSAFM measurements and XPS analysis to characterize the local electrical properties of passive films on DSS formed in air and at various potentials. The passive film acted as a p-type or n-type semiconductor, depending on film formation potential, because of the different chemical compositions of the passive film at different conditions. Vignal et al. [29] used XPS and AES to study the passivity of DSS after long-term ageing in air. The passive film was homogeneous and exhibited capacitive behavior, as described in the CPE. A small thickening was detected after long-term ageing. O²⁻/OH⁻ was significantly higher in austenite than in ferrite. Austenite acted as a blocking electrode, and two capacitive loops were observed in ferrite (low value of O²⁻/OH). High-frequency loops were related to oxygen reduction, whereas low-frequency loops were connected with the passive film. Cheng [30] studied the properties of the passive film formed on the 2205 stainless steel in acetic acid at high temperature, which contained chloride ions. The XPS results indicated that Cr comprised approximately 50% of the metal cations in the near-surface region of the passive film when the potential was in the passivation region; Cr was the main metal constituent in the near-surface region. Fe and Ni had no obvious influence on the formation, dissolution or puncture of the passive film.

Different phases influenced the passive film properties, and the passive film structure on DSS was unclear. A fundamental understanding of the passivity of metals and alloys is important to prevent corrosion and explain why the subject is extensively studied. In order to study the structure and formation process of passivation film on the surface of super-duplex stainless steel S32750, the electrochemical behavior of S32750 super DSS (SDSS) exposed after 8, 16, 24, 32 and 40 d in a cyclic corrosion salt spray chamber was investigated. The formation mechanism of the passive film was also discussed.

2. Experimental

2.1. Materials

A special stainless steel company in China supplied the S32750SDSS.

Table 1. Chemical composition of S32750 SDSS (wt%).

Element (wt%)	C	Si	Mn	P	S	Cr	Ni	Mo	Cu	N	Fe
Sample	0.019	0.44	0.71	0.029	0.001	25.21	6.2	3.33	0.10	0.2512	Bal.
ASTM A240 [31]	≤0.03	≤0.80	≤1.2	≤0.035	≤0.02	24–26	6–8	3–5	≤0.5	0.24–0.32	Bal.

presents the chemical composition by Inductively Coupled Plasma Emission Spectrometer (ICP). Each specimen was ground with 2000 grit SiC paper, and its surface was polished, then ultrasonically cleaned in ethanol and deionized water. The specimens were etched by 30 g KOH + 30 g K₂Fe(CN)₆ + 100 mL H₂O. Figure 1 shows the microstructure of S32750. The percentage of Ferrite phase/Austenite phase in the S32750 microstructure is about 50%/50%.

2.2. Exposure Conditions

The dimensions of the S32750 SDSS samples were 10 mm × 10 mm × 3 mm (Figure 2). A neutral salt spray (NSS) test was conducted on a simulated marine atmosphere in a Q-Fog CCT-1100 cyclic corrosion salt spray chamber according to ASTM B117-09. Three replicates of S32750 SDSS were exposed to the simulated marine atmosphere for 8, 16, 24, 32 and 40 d.

The specimens were specially treated to prevent crevice corrosion. Firstly, the specimen surface was subjected to passivation treatment at 60 °C for 20 min in 35 wt% nitric acid. Then, the passivated specimens were embedded in cold-curing epoxy resin with an exposed surface of 1 cm². Prior to the exposure test, all test specimens were polished down to 2000 grit, cleaned with anhydrous alcohol and deionized water, and dried in a compressed hot air flow.

2.3. Electrochemical Measurements

To obtain the passive potential, passive current density, pitting potential and Randle equivalent circuits of the S32750 SDSS, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were performed. The electrochemical experiments were measured at room temperature in 3.5 wt% NaCl using an Autolab PGSTAT302 electrochemical workstation. A three-electrode cell was employed; the sample acted as a working electrode, saturated calomel electrode (SCE) as the reference electrode and platinum sheet as the counter electrode. The working electrodes were embedded in an epoxy resin to provide insulation, leaving a 1.0 cm² surface in contact with the electrolyte.

The scanning rate of the potentiodynamic polarization (PP) curves was 1 mV/s; the potential range was from −0.1 to +1.8 VSCE (vs. OCP). The scanning stopped immediately when the current densities reached 10 mA/cm².

Electrochemical impedance spectroscopy (EIS) measurement was performed at the open-circuit potential and room temperature, with perturbation amplitude of 10 mV and frequency ranging from 100 kHz to 10 mHz. A delay time of 30 min was set before the EIS measurement was initiated to stabilize the passive film.

2.4. Analysis and Characterization Methods

After exposure testing, the corroded specimens were cleaned with deionized water and dried. The surface morphologies of the exposed samples were observed using FEI QUANTA 250 FEG field emission Scanning Electron Microscope (SEM, FEI, Hillsboro, OR, USA), and the secondary electrons (SE) mode has been performed. The passivation film was analysed using X-ray Photoelectron Spectroscopy (XPS, Axis Ultra DLD, Kratos, Manchester, UK). The photoelectron emission was excited by the monochromatic Al (mono) Kα source operated at 150 W, with initial photo energy of 1486.6 eV. Depth profiling was performed using an Ar-ion gun at a beam energy of 4 kV and spot size of 4 mm. The reference sputtering rate was 0.1 nm/s, which was calculated relative to SiO₂ as the oxygen signal intensity (decreased to half).

3. Results and Discussion

3.1. Electrochemical Measurements after NSS Test

3.1.1. Potentiodynamic Polarization Curves

Figure 3 shows the steady-state polarisation curves of the S32750 SDSS specimens in 3.5 wt% NaCl. The specimens were exposed in a simulated marine atmosphere for 0, 8, 16, 24, 32 and 40 d. The curves exhibited passivation zones and pseudo passivation. The corrosion potentials of the exposed specimens initially increased, peaked at 24 days, and then decreased. A trough passivation current density value was observed. The specimen exposed for 24 d exhibited minimum passivation current density.

Table 2. Tafel results of potentiodynamic polarization.

Exposure Time	Ecorr (V)	Icorr (μA/cm2)	ba (V/dec)
0 d	−0.25	5.49 × 107	0.27
8 d	−0.16	1.10 × 107	0.11
16 d	−0.10	1.23 × 108	0.18
24 d	−0.09	7.69 × 109	0.18
32 d	−0.15	5.49 × 108	0.16
40 d	−0.25	3.11 × 107	0.22

shows the corrosion current densities and self-corrosion potential (E_corr) obtained using the anodic Tafel extrapolation method. The corrosion current density initially decreased, and then increased when exposure time was prolonged. The corrosion current densities decreased sharply until exposure of 16 d, and then stabilized from 16 d to 24 d. The corrosion current densities increased gradually when exposed for 32 and 40 d. The self-corrosion potential of the specimens exhibited a positive value and then shifted to a negative value when exposure time increased. The E_corr of the specimen exposed for 24 d was the highest.

The regularity of corrosion current densities and self-corrosion potential according to kinetics and thermodynamics indicated that the corrosion resistance of S32750 in the simulated marine atmosphere was initially enhanced and then weakened. The corrosion resistance was optimal when the specimens were exposed for 24 d in the simulated marine atmosphere.

3.1.2. Electrochemical Impedance Spectroscopy

Figure 4 shows the typical Nyquist and Bode determined for S32750 SDSS exposed in the simulated marine atmosphere at different exposure times (8, 16, 24, 32 and 40 d). A specimen that was not exposed to the simulated marine atmosphere was designed for comparison.

Figure 4a shows that the diameters of the capacitive loops initially increased, peaked at 24 d, and then decreased with prolonged exposure time. The capacitive loop of the unexposed specimen had the smallest diameter. The impedance value increased as the exposure time was prolonged until 24 d. After 24 d of exposure, the impedance value decreased because of corrosion. Figure 4b shows two relaxation time constants represented by Bode plots. The second time constant represented the formation of a corrosion product and oxide layer on a metal surface.

EIS spectra typical for passivated specimens after exposure to the simulated marine atmosphere were shown as Nyquist plots and Bode plots. A similar circuit with two relaxation time constants was proposed in this study to describe the impedance response of passivation and corrosion in the simulated marine atmosphere, and a good fitting quality was obtained (Figure 5). In the equivalent circuit, Q was a constant phase element (CPE) instead of a capacitance because measured capacitance is often not ideal. The impedance representation of CPE is given by [32,33]

$$Z\left(CPE\right)=\frac{1}{Y_0{\left(jw\right)}^n}$$

(1)

where Y₀ is a fit parameter. In ideal cases when the exponential factor n = 1, CPE acts as a capacitor with Y₀ equal to capacitance C. In practice, n deviates from 1 so that fitting results are given as C and n. CPE behavior increased because microscopic material properties exhibited a distribution. The n-value provided some information on the nature of the surface or passive film. For example, the n-factor was connected to surface roughness [32,33,34].

The specimens were pre-passivated when exposed in the simulated marine atmosphere; the passivated film already formed on the surface. EIS tests were then conducted. Thus, a series connection was noted (Figure 5b). Q_pass is the CPE determined by the double layer and passive film; Q_pit is the double layer related to the pit; and R_s and R_pit are the solution resistance between the specimen and the platinum electrode and the value of the pit charge transfer resistance, respectively. The passivation resistance R_pass is strongly dependent on the passive film and is a measure of the corrosion resistance of the material in the environment [34].

Table 3. EIS fitting data of S32750 SDSS after exposure at different times in the simulated marine atmosphere.

Exposure Time	Rs/ Ω·cm2	Qpass		Rpass/ Ω·cm2	Qpit-pass		Rpit-Pass/Ω·cm2
		Ypass/ Ω−1s−ncm2	n2		Ypit-Pass/ Ω−1s−ncm2	n1
8 d	0.60	3.64 × 10−5	0.89	1.80 × 106	-	-	-
16 d	11.60	4.65 × 10−5	0.92	2.02 × 106	2.87 × 10−4	0.76	26.86
24 d	2.88	6.45 × 10−5	0.93	2.23 × 106	1.57 × 10−4	0.78	58.23
32 d	2.15	6.81 × 10−5	0.91	8.09 × 105	3.20 × 10−4	0.79	36.97
40d	4.17	1.33 × 10−4	0.83	3.93 × 105	5.65 × 10−4	0.72	60.65

shows the EIS fitting data of S32750 SDSS after exposure at different times in the simulated marine atmosphere. The specimen exposed for 24 d had the maximum R_pass value, and thus, the best corrosion resistance. Before the 24 d exposure, the corrosion resistance was enhanced as exposure time was prolonged and the R_pass value increased. As exposure time was prolonged further, the passive film was destroyed by the aggressive ions in the simulated marine atmosphere. Moreover, pitting occurred and the R_pass value decreased.

3.2. XPS Analysis

3.2.1. Surface Analysis of Oxide Films

XPS measurements were conducted to provide detailed information on the structure of surface passive films, which were preferred on the sputter depth profile. Fe, Cr, Ni, Mo and O peaks were identified from the XPS survey spectra of the exposed specimens. The analysis of the passive film in each sample revealed that Cr, Fe and O were the main components of the films. A small difference was observed on the compounds of the outer and inner films. Specimens exposed for 24 d in the simulated marine atmosphere, which exhibited the best corrosion resistance, were observed to investigate the distribution of the main alloy elements and compounds in the passive film (Figure 6). For the outer layer, the Fe 2p_3/2 signal in Figure 6a exhibited three components: metallic state [Fe(met)], bivalent (Fe²⁺) and trivalent (Fe³⁺) species. The relative peak heights of Fe₂O₃ and FeOOH showed that Fe³⁺ was the primary iron-oxidized species in the outer passive film. For the Cr 2p_3/2 spectra in Figure 6b, four constituent peaks that represented Cr(met), Cr(OH)₃, Cr₂O₃ and CrO₃ were observed. For the inner layer, Fe 2p_3/2 also had three similar components (Figure 6c). However, the Cr 2p_3/2 spectra showed little Cr⁶⁺ (Figure 6d). Cr₂O₃/Cr(OH)₃ was the primary oxide.

3.2.2. XPS Depth Profiles

The peaks of the XPS spectra were individually divided by curve-fitting to obtain information on the relative proportion of the different ionic states. The quantitative analysis of XPS depth profiles based on peak areas revealed the element content of passivation films from surface to substrate (Figure 7).

Figure 7a–e shows the depth profiles obtained through XPS analysis combined with argon-ion sputtering sequences of SDSS sample prior to exposure treatments at different times. The concentration of Cr was greater than that of Fe in all passive films during the initial stage of the sputtering period (Cr/Fe>1). Moreover, chromium was enriched in all oxide films, whereas molybdenum exhibited a constant concentration value throughout the oxide film thickness; a slight nickel enrichment occurred near the oxide/metal interface for other iron alloys [35].

Combining the XPS quantitative results with curve-fitting findings, the chromium oxide content (Cr/(Cr+Fe) ratio) in the passive films was estimated as a function of sputtering time (Figure 8). The Cr/(Cr+Fe) ratio of the concentration of Cr and Fe increased gradually when exposure time was prolonged. The Cr/(Cr+Fe) ratio reached a maximum when the specimens were exposed for 24 d. Almost all films exhibited a decrease in Cr/(Cr+Fe) value with depth. The Cr/(Cr+Fe) value reached a maximum outside the surface and then decreased and stabilized for a short time. The oxide film was composed of two chromium-enriched layers. The outer layer had a very thin film at the metal/atmosphere interface of the specimen surface and exhibited higher chromium content than the inner layer, which appeared depleted. The passivated films of the specimens with various exposure times had similar thickness but different Cr content. Thus, the thickness of the passivation film only slightly changed, but became dense when exposure time was prolonged. Cr content decreased because of pitting when the exposure time was more than 24 d.

The deconvolution results from the XPS analysis indicated that the passive film mainly consisted of Cr and Fe oxides/hydroxides. The outer and inner layers had similar Fe components composed of Fe₂O₃/FeOOH and FeO. However, the outer and inner layers had different Cr components. CrO₃, Cr₂O₃/Cr(OH)₃ was the primary oxidized species in the outer layer, whereas Cr₂O₃, Cr(OH)₃ was the primary oxide in the inner layer of the passive film from the XPS peaks.

The specimens exposed for 24 d in the simulated marine atmosphere, which exhibited the best corrosion resistance, were observed to investigate the distribution of Cr elements and compounds in the passive film. The ionic state of the element and the relative proportion of the different ionic states were estimated by the peaks of the XPS spectra. Figure 9 shows the distribution of Cr elements and compounds in the passive films of the specimens exposed to the simulated marine atmosphere for 24 d. The outer layer of the passive film contained Cr³⁺ and a small amount of Cr⁶⁺, whereas the inner layer contained Cr³⁺.

3.3. Surface Morphologies

Figure 10 shows the surface morphologies of the specimens exposed in the simulated marine atmosphere for various times. Before the 8 d exposure, almost no obvious pitting on the surface was observed (Figure 10a). Until the 24 d exposure, pittings were observed on the surface of the specimen (Figure 10c). When the exposure time was more than 24 d, the density and size of the pittings increased (Figure 10d,e).

The passivation film formed on the surface of the Cr-Fe alloy determined its corrosion resistance. The higher the ratio of Cr/(Cr+Fe) in the passivation film, the better the corrosion resistance. After the salt spray test, a passivation film was formed on the surface of the sample, Cr was enriched in the outer layer of the passivation film, the Cr/(Cr+Fe) ratio was higher than 0.5, and the Cr content was higher than the Fe content. The passivation film became thicker, when the salt spray time reached 24 days, the Cr/(Cr+Fe) ratio reached the maximum. This is the reason why the electrochemical results showed the best corrosion resistance of the sample with a salt spray time of 24 days. After 24 days of salt spray time, the thickness of the passivation film on the surface of the sample did not change much. Combined with the SEM morphology of the samples after the salt spray test, the surface corrosion of the samples with the salt spray time of 32 days and 40 days was developed, the passivation film was broken, and pitting corrosion occurred. Therefore, the sample with the salt spray time of 24 days has the best corrosion resistance.

Chromium oxides play an important role in passive films [36]. The passive film on the surface of S32750 SDSS had a duplex structure, a chromium-enriched outer layer and an inner layer with depleted chromium. Cr₂O₃/Cr(OH)₃ was enriched at the interface of the oxide film/atmosphere, and a small amount of CrO₃ was observed. The concentration of Cr₂O₃ reduced in the inner layer of the passive film. The Fe amount was relatively small because of the Cr enrichment in the passive film, and Fe mainly existed in the form of Fe³⁺ in both inner and outer passive films. According to XPS results and reference [11], the formation of passive film mainly involves the following electrochemical reactions on the stainless steel surface:

3Fe + 8OH⁻→Fe₃O₄ + 4H₂O + 8e⁻

(2)

Fe₃O₄ + H₂O + OH⁻→3FeOOH + e⁻

(3)

2Fe₃O₄ + 2OH⁻→3Fe₂O₃ + H₂O + 2e⁻

(4)

Fe + 2OH⁻→Fe(OH)₂ + 2e⁻

(5)

Fe(OH)₂→FeO+ H₂O

(6)

FeO+ OH⁻→FeOOH+ e⁻

(7)

2Fe(OH)₂ + 2OH⁻→Fe₂O₃ + 3H₂O + 2e⁻

(8)

Cr + 3OH⁻→Cr(OH)₃ + 3e⁻

(9)

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

(10)

The outer Cr-rich layer, a kind of compact and thin oxide film, prevented the spread of oxygen to the inner layer and protected the matrix from eroding because of corrosive ions such as Cl⁻ in the simulated marine atmosphere. Figure 11 shows a sketch of the passivation film. According to the electrochemical experiment and XPS analysis results, two processes were observed during exposure in the simulated marine atmosphere: passivation and pitting. The thickness of the passivation film slightly changed, but became dense when exposure time was prolonged. Pitting occurred when exposure time exceeded 24 d, and the Cr content of the specimen decreased. Therefore, S32750 SDSS exposed for 24 d exhibited the best corrosion resistance.

4. Conclusions

(1)

Two processes (passivation and local corrosion) occurred on the metal surface when S32750 SDSS was exposed in the simulated marine atmospheric environment. The specimen exposed for 24 d exhibited the best corrosion resistance. The thickness of the passivation film slightly changed, but became dense when the exposure time was prolonged. Pittings occurred when the exposure time was more than 24 d.

(2)

The passivation film of S32750 SDSS was composed of two chromium-enriched layers. The outer layer had a very thin film at the metal/atmosphere interface of the specimen surface and had higher chromium content than the inner layer, which appeared depleted.

(3)

The outer and inner layers had similar Fe components, and Fe³⁺ oxide/hydroxide was the primary oxide in the film. However, the layers had different Cr components; the outer layer contained CrO₃, whereas the inner layer had Cr₂O₃/Cr(OH)₃ as the primary oxide.