Effect of Solution Treatment on Microstructure and Corrosion Resistance Performance of HIPed Net-Shaped Duplex Stainless Steel SAF2507

Abstract

This study systematically investigates the effect of solution treatment on the microstructure and corrosion resistance of duplex stainless steel SAF2507 fabricated by direct hot isostatic pressing (HIP). The HIP specimens were solution treated at 1080 °C for 1 h, followed by comprehensive characterization using SEM, EDS, EBSD, XRD, XPS, and electrochemical testing in 3.5 wt% NaCl solution. Results indicate that solution treatment effectively dissolved intermetallic precipitates, promoted a more uniform distribution of ferrite and austenite phases, and reduced microstructural heterogeneity. Electrochemical impedance spectroscopy and potentiodynamic polarization tests showed that the treated samples exhibited a wider passive region and higher charge transfer resistance, indicating enhanced passivation behavior. XPS analysis further revealed an increased proportion of Cr₂O₃ and O²⁻ and decreased ${\mathrm{Fe}}_{\mathrm{hy}}^{3+}$ and H₂O content in the passive film, suggesting improved compactness and chemical stability. Surface morphology analysis confirmed a significant reduction in pitting corrosion after treatment. These findings demonstrate that solution treatment is an effective post-processing method to enhance the corrosion resistance of HIP-fabricated SAF2507 duplex stainless steel.

1. Introduction

Duplex stainless steels (DSSs), especially super duplex grades such as SAF2507, are widely employed in marine, petrochemical, and offshore industries due to their excellent combination of high mechanical strength and superior corrosion resistance, particularly in chloride-containing environments [1,2,3]. SAF2507, characterized by a balanced dual-phase microstructure consisting of approximately equal volumes of austenite and ferrite, offers remarkable resistance to localized corrosion, including pitting and crevice corrosion, as well as stress corrosion cracking [4,5,6].

In recent years, hot isostatic pressing (HIP) has received considerable attention as an advanced manufacturing technique for producing duplex stainless steel (DSS) components. HIP consolidates metal powders or preforms under high temperature and pressure, resulting in fully dense materials with improved mechanical properties and reduced porosity [7,8,9]. However, due to the high alloy content of SAF2507, microstructural changes can occur during the HIP process, including the formation of carbides, nitrides, secondary austenite (γ’), and intermetallic phases such as sigma (σ), chi (χ), and R phases. These secondary phases significantly affect the microstructure, mechanical properties, and especially the corrosion resistance of SAF2507 [10,11]. For instance, David et al. [12] demonstrated that increasing σ phase content leads to a decline in corrosion resistance, while Ramirez et al. [13] reported that the presence of secondary austenite is associated with inferior corrosion performance. Therefore, the elimination or mitigation of these detrimental phases is essential for accurately assessing and improving the corrosion behavior of DSS.

Post-HIP heat treatment, particularly solution treatment, plays a key role in refining the microstructure by dissolving harmful secondary phases, promoting a more uniform distribution of alloying elements, and restoring the optimal ferrite-austenite phase balance [14,15,16]. While conventional DSSs have been widely studied, there is limited research on how solution treatment affects the corrosion behavior of HIP-SAF2507.

This study investigates the effects of solution treatment on the microstructural evolution and corrosion resistance of HIP-SAF2507 duplex stainless steel. Microstructural characterization was carried out using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), and corrosion behavior was evaluated through potentiodynamic polarization testing in chloride-containing environments. The results provide valuable insights for optimizing post-HIP thermal treatment strategies to enhance the corrosion resistance and service reliability of DSS components in aggressive environments.

2. Materials and Methods

2.1. Powder Preparation and Hot Isostatic Pressing

Gas-atomized SAF2507 super duplex stainless steel powder (Guanda New Material Technology Co., Ltd., Anshan, China) was encapsulated in a stainless steel capsule, evacuated to below 10 Pa, and sealed. The chemical composition of the stainless steel is shown in

Table 1. Chemical composition of SAF2507 (wt%).

Alloy	Fe	Cr	Ni	Mo	Mn	C	N	Si	P	S
SAF2507	61.225	25.86	7.509	3.710	0.94	0.016	0.23	0.487	0.022	0.001

. The HIP process was carried out at 1200 °C under 150 MPa for 2 h, followed by furnace cooling. The consolidated billets were sectioned into blocks (10 × 10 × 20 mm³) using wire electrical discharge machining for subsequent heat treatment and testing.

2.2. Solution Treatment

The HIP samples were subjected to solution treatment at 1080 °C for 1 h in a KSL-1700X muffle furnace (Hefei Kejing Materials Technology Co., Ltd., Hefei, China), followed by immediate water quenching to room temperature (23 ± 2 °C). The heat treatment parameters were selected based on the parameters reported in the following reference [16,17]. This heat treatment was aimed at dissolving potential deleterious secondary phases and restoring the optimal ferrite-austenite balance. Samples before and after solution treatment are referred to as HIP and HT, respectively.

2.3. Phase and Microstructural Characterization

X-ray diffraction (XRD) analysis was performed using an Ultima IV X-ray diffractometer (Shimadzu Corporation, Kyoto, Japan) with Cu Kα radiation (λ = 1.5406 Å), operated at 40 kV and 40 mA. Scans were conducted in the 2θ range of 30°~60° with a step size of 0.02°. Phase identification and semi-quantitative analysis were carried out using Jade 6 software.

Scanning electron microscopy was conducted on polished cross-sections using a Model JSM-6510La (JEOL Ltd., Tokyo, Japan) at 20 kV. Surface preparation involved grinding with SiC papers up to 1200 grit, followed by polishing with 0.05 µm colloidal silica suspension. Elemental distributions and point analyses were obtained using an EDS system. Electron backscatter diffraction (EBSD) was performed on the same SEM platform equipped with an Oxford Symmetry EBSD detector (Oxford Instruments, Abingdon, UK). Analyses were carried out at 20 kV acceleration voltage with a step size of 0.3 µm and a working distance of ~15 mm. EBSD data were processed and analyzed using AztecCrystal2.1 software to determine grain orientation, phase distribution, and misorientation characteristics.

2.4. Electrochemical Testing

Electrochemical experiments were conducted at room temperature using a CHI 600E workstation (CH Instruments, Austin, TX, USA) in a conventional three-electrode cell. The sample served as the working electrode with an exposed area of 1 cm²; a platinum sheet was used as the counter electrode, and an Ag/AgCl electrode (saturated KCl) was used as the reference. Prior to testing, samples were ground to 1200 grit, ultrasonically cleaned in ethanol, rinsed with deionized water, and dried under nitrogen. Detailed information on the working electrodes is shown in

Table 2. Detailed information on electrochemical samples.

Sample	Process	Number of Samples	Number of Tests
HIP	Hot isostatic pressing	1	3
HT	Hot isostatic pressing + solution treatment	1	3

.

Open-circuit potential (OCP) was monitored for 1 h to ensure stability. Electrochemical impedance spectroscopy (EIS) was performed at OCP in the frequency range from 10⁵ to 10⁻² Hz using a sinusoidal AC amplitude of 10 mV. The resulting impedance spectra were fitted using the Zview2 software. Potentiodynamic polarization was then conducted from −0.8 to +1.3 V at a scan rate of 1 mV/s. All measurements were carried out in a static 3.5 wt% NaCl solution prepared from analytical-grade reagents and deionized water.

2.5. Surface Characterization After Corrosion

Post-electrochemical corroded surfaces were examined by SEM to analyze localized corrosion morphologies, including pit initiation and propagation features. Three-dimensional surface profiling was carried out using an OLYMPUS OLS5000 laser confocal microscope (Olympus Corporation, Tokyo, Japan) to evaluate pit geometry, including depth and volume. Data from multiple fields of view were averaged to ensure reproducibility.

2.6. X-Ray Photoelectron Spectroscopy (XPS)

To investigate the chemical composition of the passive film formed on the surface of steel, samples before and after solid solution treatment were immersed in a 3.5 wt% NaCl solution for 7 days to ensure the formation of a stable passive film on the surface. XPS analysis was performed using the Thermo Scientific ESCALAB 250Xi system (Thermo Fisher Scientific, Waltham, MA, USA), equipped with a monochromatic Al Kα X-ray source at an energy of 1486.6 eV. Survey and high-resolution spectra were collected for Fe 2p, Cr 2p and O 1s regions. Charge correction was performed using the C1s peak at 284.8 eV as a reference. Depth profiling was achieved via Ar⁺ sputtering at 2 kV for successive 10 s intervals. Spectral deconvolution and quantitative analysis were performed using Advantage (6.9) software.

3. Results and Discussion

Figure 1 shows the XRD patterns of SAF2507 duplex stainless steel before and after solution treatment. The HIP sample exhibits a two-phase structure consisting predominantly of ferrite (α) and austenite (γ), along with weak diffraction peaks attributed to secondary intermetallic phases (σ phases). After solution treatment at 1080 °C for 1 h, these secondary phase peaks disappear, indicating effective dissolution during thermal exposure. This confirms that the applied heat treatment successfully homogenizes the microstructure by eliminating detrimental precipitates, in agreement with previous findings on super duplex stainless steels subjected to solution annealing [6]. Furthermore, a noticeable rightward shift in the main diffraction peaks is observed in the HT sample compared to the HIP condition. This shift suggests a change in the lattice parameter, commonly associated with a variation in phase composition. The increased intensity of α peaks relative to γ reflections indicates a phase balance shift toward higher ferrite content, while austenite peaks exhibit decreased intensity, suggesting partial dissolution or transformation of the γ phase during heat treatment. Such ferrite enrichment may result from chromium and molybdenum diffusion during solution treatment, stabilizing the ferritic phase at elevated temperature [18,19].

Figure 2 presents the backscattered electron (BSE) images and corresponding EDS elemental mapping of SAF2507 alloy in the HIP (Figure 2a) and HT (Figure 2b) conditions. Significant microstructural changes are observed after solution treatment. In the HIP condition (Figure 2a), the microstructure is characterized by a dual-phase morphology with γ islands distributed along the boundaries of polygonal α grains. Notably, secondary precipitates are visible at the α/γ interfaces and triple junctions. These are commonly identified in duplex stainless steels as intermetallic phases (σ phase), consistent with the XRD results shown in Figure 1. The distribution of alloying elements in the EDS maps supports this interpretation: Cr and Mo are enriched in σ phase regions. Additionally, the Ni map reveals higher intensity in the austenite phase, which is known to stabilize γ, while the ferrite grains exhibit elevated Cr and Mo levels.

Table 3. Chemical composition (wt%) corresponding to each point in Figure 2.

Piont	Fe	Cr	Mo	Ni	Mn	Si	N
1	56.68	29.51	7.65	4.85	0.67	0.40	0.24
2	62.34	24.35	3.22	8.66	0.55	0.35	0.53
3	60.41	27.94	5.38	5.23	0.70	0.33	0.00
4	61.31	25.01	2.64	8.56	1.71	0.36	0.42
5	59.61	28.40	4.67	5.44	1.28	0.36	0.24

presents the detailed EDS point scan data.

After solution treatment at 1080 °C for 1 h (Figure 2b), the microstructure becomes significantly more homogeneous. The precipitates previously observed at grain boundaries and triple junctions disappear, indicating their complete dissolution during solution annealing. The grain boundaries appear clean and free of secondary phases, suggesting improved microstructural stability. Overall, the solution treatment not only eliminates deleterious secondary phases but also promotes a more stable and chemically uniform duplex microstructure.

Electron backscatter diffraction was used to further evaluate the phase distribution, grain orientation, and texture characteristics of SAF2507 before and after solution treatment. Figure 3 displays the inverse pole figure (IPF) maps, phase maps, and grain size distributions of the HIP and HT samples. As shown in Figure 3a–c, the HIP sample presents a typical dual-phase microstructure composed of ferrite (α, blue) and austenite (γ/γ₂, red), with a significant number of fine precipitates identified as sigma phase (σ, green), primarily located at the α/γ interfaces and triple junctions. The presence of this intermetallic phase is consistent with both XRD (Figure 1) and SEM-EDS (Figure 2) results. The grain structure is relatively fine, with an average grain size of around 10 µm and a high frequency of small grains, as shown by the grain size distribution histogram (Figure 3c). This is attributed to the thermal and pressure conditions of HIP, which promote rapid nucleation and suppress grain growth. After solution treatment (Figure 3 d–f), the sigma phase is almost completely eliminated, and the microstructure becomes cleaner and more homogeneous. The volume fraction of ferrite increases, while austenite content decreases, suggesting a partial transformation of γ into α during thermal exposure. This is quantitatively confirmed by the phase fraction analysis in Figure 3g: the ferrite fraction increases from 26.93% to 46.60%, while austenite drops from 63.61% to 53.40%, and sigma phase is reduced from 9.46% to nearly undetectable levels. These changes are also consistent with the XRD peak intensity shifts discussed previously. In terms of crystallographic orientation, both conditions exhibit a relatively random texture, as indicated by the IPF triangle inset. No strong preferential orientation is observed, suggesting the absence of significant deformation-induced texture.

Figure 4a presents the polarization curves of SAF2507 samples before and after solution treatment in 3.5 wt.% NaCl solution at room temperature. The corresponding electrochemical parameters are summarized in

Table 4. Electrochemical parameters in the 3.5 wt% NaCl solution from potentiodynamic polarization curves.

Sample	Ecorr/V	Icorr/A/cm2	Epit/V
HIP	−0.288 ± 0.023	(4.504 ± 0.114) × 10−7	0.571 ± 0.013
HT	−0.272 ± 0.031	(4.029 ± 0.121) × 10−7	1.008 ± 0.015

. The results reveal that neither the HIP nor the HT samples exhibit a distinct active-passive transition, suggesting that both are capable of spontaneously forming a passive film. Evidence of metastable pitting further indicates the breakdown and subsequent repair of the passive layer. Compared with the HIP condition, the solution-treated sample exhibits a broader passive region, a slightly lower corrosion current density (I_corr), and a higher pitting potential, indicating an improvement in corrosion resistance after solution treatment.

Figure 4b,c show the Nyquist and Bode plots for both samples under OCP. The order of the capacitive arc radius in the Nyquist plot is as follows: HT>HIP. This indicates that after solution treatment, the charge transfer resistance of SAF2507 increases, leading to improved corrosion resistance, which is consistent with the results from the polarization curves. According to the corresponding Bode plot, the impedance modulus decreases with increasing frequency in the low-frequency range. When the frequency range is 10-10² Hz, the phase angle reaches its maximum (~79°), indicating that surface inhomogeneity leads to non-ideal capacitive behavior [20]. Furthermore, the impedance modulus tends toward a wide plateau within the frequency range of 10–10⁵ Hz. Feng et al. [21] suggested that the wide plateau in the impedance modulus is due to the pseudo-capacitive nature of the compact Cr-rich oxide layer. Therefore, a constant phase element (CPE) is used to replace the ideal pure capacitor. The protective performance of the passive film can be quantitatively analyzed by fitting the EIS data. Figure 4d presents the equivalent circuit model (EEC) used for fitting the EIS data, where R_s represents the solution resistance, and R_ct represents the charge transfer resistance at the solution/film interface. CPE is a constant phase element, representing the double-layer capacitance. The equivalent circuit, composed of R_s, R_ct, and CPEs, was selected to characterize the electrochemical response of the passive film and its non-ideal capacitive behavior [22]. The fitting parameters are summarized in

Table 5. Parameters of the equivalent circuit diagram of the electrochemical impedance spectrum of the SAF2507 sample.

Sample	Rs/Ω·cm2	CPE/Ω−1 cm−2 sn	n	Rct/Ω·cm2	χ2
HIP	6.815 ± 0.513	(5.56 ± 0.24) × 10−5	0.923 ± 0.01	92767 ± 4000	(1.05 ± 0.24) × 10−3
HT	6.042 ± 0.341	(6.37 ± 0.31) × 10−5	0.884 ± 0.02	234530 ± 6000	(1.12 ± 0.24) × 10−3

. During the EIS testing, due to the near-capacitive nature of the passive film, the electrochemical system essentially reflects the behavior of the solution resistance in the high-frequency region [23]. Therefore, the impedance modulus at 10⁵ Hz for both samples is approximately 6 Ω·cm², with the corresponding phase angle being close to zero. Furthermore, R_ct is a key parameter for passivating alloys, and its value corresponds to the radius of the semicircle in the Nyquist plot, providing a good evaluation of the compactness and stability of the passive film formed on the alloy surface [24,25]. The results show that the HT samples exhibited the highest R_ct values, indicating that the corrosion rate of the solution-treated samples in the electrolyte was lower. This is attributed to the elimination of the σ-phase, which improved the integrity of the passivation film; the resulting difficulty in charge transfer between the passive film and the steel enhanced the protective capability of the passive film. These findings are consistent with the results reported in the literature [26]. CPE is used to quantitatively characterize the near-capacitive response of the alloy. The impedance of the CPE can be calculated using the following formula [27]:

Z_CPE = Q⁻¹(jω)⁻ⁿ

(1)

where Q is the constant associated with the capacitance of the CPE; j is the imaginary unit; ω is the angular frequency; and n is the CPE exponent related to surface inhomogeneity. The chi-square (χ²) values remain on the order of 10⁻³, indicating a high precision of the EIS fit. From Table 5, it can be seen that the n values for the passive films of the two samples are 0.92 and 0.88, respectively. This deviation from the ideal capacitor value (n = 1) is attributed to surface roughness or other surface phenomena. In summary, both the polarization experiments and electrochemical impedance spectroscopy results indicate that the solution-treated sample exhibits superior corrosion resistance compared to the HIP sample.

Figure 5a,b show the corrosion morphologies of SAF2507 samples after polarization testing, in both the HIP and HT conditions, as observed by 3D laser confocal microscopy. The surface topography of the HIP sample (Figure 5a) reveals significant localized corrosion, with distinct pit formation and deep, sharp pits. These features suggest a relatively poor stability of the passive film under corrosion conditions. The height profile of the HIP sample (Figure 5b) shows large fluctuations, reflecting the pronounced depth and wide distribution of the pits. In contrast, the surface of the HT sample (Figure 5a₁) exhibits much less pronounced pitting corrosion. The overall surface appears smoother, with fewer and smaller pits, indicating improved passivation behavior after solution treatment. The corresponding height profile of the HT sample (Figure 5b₁) shows significantly reduced surface roughness (RSa, RSq) compared to the HIP sample. The smaller fluctuations in the height profile suggest that the passive film on the solution-treated sample is more stable, providing better protection against localized corrosion. This result is consistent with the electrochemical data (Figure 4), where the HT sample shows a lower corrosion current density and a more extensive passive region in the polarization curve. This indicates that solution treatment enhances the formation and stability of the passive film, significantly improving the corrosion resistance of SAF2507 in chloride-containing environments.

Figure 6 presents the corrosion morphologies of SAF2507 duplex stainless steel specimens before and after solution treatment following polarization testing. In chloride-containing environments, the corrosion process mainly involves anodic metal dissolution (Fe → Fe²⁺ + 2e⁻) and cathodic oxygen reduction (O₂ + 2H₂O + 4e⁻ → 4OH⁻). As shown, the HIP specimen exhibits numerous corrosion pits propagating along grain boundaries and phase interfaces. Corrosion primarily occurs in regions containing the σ phase, characterized by selective attack along σ-phase boundaries. According to the literature [28,29], during σ-phase precipitation, significant amounts of Cr and Mo are incorporated into the σ phase, resulting in local depletion of these essential alloying elements in adjacent areas, thereby creating corrosion-prone zones and reducing passive film stability. Therefore, passive film breakdown and localized charge transfer processes are considered the dominant rate-determining factors for corrosion propagation. In contrast, the surface of the solution-treated specimen appears smoother and shows significantly fewer corrosion features, indicating the formation of a more stable and compact passive film that effectively suppresses further corrosion. The dominant corrosion mode in the solution-treated sample is pitting. Line-scan EDS analysis in Figure 6d further reveals that the initiation of pitting corrosion is closely associated with the presence of Mo- and Mn-rich sulfide inclusions. These inclusions are susceptible to preferential dissolution in chloride-containing environments, leading to localized passive film breakdown and subsequent pit initiation [30,31].

Figure 7 presents the composition of the passive film on SAF2507 samples in two conditions after immersion in 3.5 wt% NaCl solution for 7 days. As shown in Figure 7a,d, the Cr 2p_3/2 XPS spectra were deconvoluted into three peaks corresponding to Cr⁰ (573 ± 0.2 eV), Cr₂O₃ (576 ± 0.2 eV), and Cr(OH)₃ (577 ± 0.2 eV). These results are consistent with previous reports in the literature [32]. It is well known that Cr₂O₃ is a key component of the passive film [33]. The Fe 2p_3/2 spectra were fitted with four peaks, corresponding to Fe⁰, ${\mathrm{Fe}}_{\mathrm{ox}}^{3+}$, ${\mathrm{Fe}}_{\mathrm{hy}}^{3+}$, and FeOOH, located at 706 ± 0.2 eV, 707 ± 0.2 eV, 709 ± 0.2 eV, and 711.5 ± 0.2 eV, respectively (Figure 7b,e) [22]. Figure 7c,f display the deconvoluted O 1s spectra of the two samples. Three peaks were identified at binding energies of 529.7 ± 0.2 eV, 531.4 ± 0.2 eV, and 528.9 ± 0.2 eV, corresponding to O²⁻, OH⁻, and H₂O, respectively [34]. The presence of O²⁻ and OH⁻ indicates the formation of metal oxides and hydroxides on the alloy surface [35,36].

To further investigate the elemental composition of the passive films, the atomic percentages of the metallic elements in the passive layers of the two HEAs were calculated using the following equation [37]:

$$C_x={\displaystyle \frac{I_x/S_x}{\sum (I_i/S_i)}}$$

(2)

where $C_x$ is the atomic percentage of element in the passive films, $I_x$ represents the peak intensity corresponding to each element, and $S_x$ denotes the sensitivity factor obtained from the XPS instrument. Figure 8a illustrates the chemical states and corresponding cationic fractions of Fe, Cr, and O elements within the passive film of SAF2507 samples before and after solution treatment. The results indicate that it significantly influences stability and corrosion resistance [38,39]. Prior to solution treatment, Fe primarily exists in the form of Fe₂O₃ (47.41%) and ${\mathrm{Fe}}_{\mathrm{hy}}^{3+}$ (20.28%), with a small amount of FeOOH (7.49%). Cr is predominantly present as Cr(OH)₃ (47.05%), while Cr₂O₃ accounts for only 35.26%. Additionally, the O element consists mainly of unstable species such as OH⁻ (34.40%) and H₂O (36.18%). In contrast, after solution treatment, the FeOOH content markedly increases to 25.23%, while the ${\mathrm{Fe}}_{\mathrm{hy}}^{3+}$ fraction decreases to 28.73%. The Cr₂O₃ content rises to 50.37%, accompanied by a reduction in Cr(OH)₃ to 41.10%. Moreover, the proportion of O²⁻ increases from 29.41% to 34.86%, suggesting that solution treatment promotes the enrichment of more stable oxide species in the passive film, thereby enhancing its density and protective performance [39,40]. To further quantify the stability of the passive film, Figure 8b presents a comparative analysis of the Cr_ox/Cr_hy, O_ox/O_hy, and (Cr_ox+hy)/(Fe_ox+hy) ratios. The results show that the Cr_ox/Cr_hy and O_ox/O_hy ratios increase after solution treatment, with a particularly significant rise in the total (Cr_ox+hy)/(Fe_ox+hy) ratio. This indicates greater enrichment of chromium oxides and a relative reduction of iron oxides in the passive film, contributing to the formation of a denser and more stable passive layer. Consequently, the corrosion resistance of the material in chloride-containing environments is significantly improved [41,42].

4. Conclusions

This study demonstrates that solution treatment significantly improves the microstructural uniformity and corrosion resistance of SAF2507 duplex stainless steel fabricated by direct hot isostatic pressing. The main conclusions are as follows:

(1)

Solution treatment at 1080 °C for 1 h effectively dissolves intermetallic phases and promotes a homogeneous distribution of ferrite and austenite.

(2)

The solution-treated alloy exhibits a slightly lower corrosion current density, together with a wider passive region, higher pitting potential, and improved charge transfer resistance in 3.5 wt.% NaCl solution, indicating enhanced resistance to localized corrosion and improved passive film stability.

(3)

EIS analysis shows that the treated sample has higher charge transfer resistance, suggesting the formation of a denser and more protective passive film.

(4)

XPS results reveal an increased proportion of Cr₂O₃ and O²⁻ and a decrease in Fe₂O₃ and H₂O in the passive layer, confirming improved film stability.

(5)

Surface and corrosion morphology analyses confirm that solution treatment suppresses selective corrosion and pitting, enhancing the uniformity and protectiveness of the passive film.