Effect of Nitrogen on the Corrosion Resistance of 6Mo Super Austenitic Stainless Steel
Abstract
6Mo super austenitic stainless steels (SASS) with nitrogen contents of 0.2 and 0.4
(wt.%) were melted and solution treated at 1100, 1180, and 1250 °C for 30 min
then subsequently tested. The effects of nitrogen on the microstructure and pitting
resistance of the two steels labelled as 0.2N and 0.4N samples were investigated.
At a heat-treatment temperature of 1180 °C, the alloy demonstrates the highest
corrosion resistance, attributed to the combined effects of grain size and
precipitates. The structure of the passivation film changes with increasing nitrogen
content, with the Cr/Fe ratio being significantly higher in the 0.4N sample
compared to the 0.2N sample. Moreover, the increase in nitrogen content results
in thicker Cr and Mo oxide layers and higher levels of NH3 and NH4+, thereby
improving the corrosion resistance of the stainless steel.
1. Introduction
The 6Mo super austenitic stainless steel (SASS) is rich in a large amount of
chromium, molybdenum and nickel. These elements facilitate the spontaneous
formation of a continuous dense passive film on the surface, which significantly
enhances the corrosion resistance and mechanical properties of stainless steel
[1,2,3]. Currently, 6Mo SASS is widely used in aggressive environments such as
the petrochemical industry, flue gas desulfurization, seawater desalination
systems, and pulp mill bleaching systems. The research on the performance of
6Mo SASS has also attracted more and more researchers’ interest. For instance,
Sandra et al. [4] found that 6Mo high-nitrogen grade UNS S31266 showed
excellent corrosion resistance in acidic media containing chloride. Behrens et al.
[5] found that the combination of high nickel content and nitrogen in 6Mo super
austenitic stainless steel resulted in favorable low sigma solvent temperatures, and
high chromium and molybdenum content resulted in resistance to common forms
of corrosion and localized corrosion in chlorine-containing media. Ren et al. [6]
studied the effect of boron addition on the oxide skin of S31254 super austenitic
stainless steel in high-temperature air, and the results showed that the addition of
B could inhibit the diffusion of Mo to the surface and promote the diffusion of Cr to
the surface to form a dense Cr2O3 film. Yang et al. [7] found that the grain
boundary segregation of B and Ce had a significant inhibitory effect on precipitates,
and the addition of B and Ce could greatly improve the corrosion resistance of
S31254 SASS. Dou et al. [8] investigated the passivation performance of S31254
SASS in simulated condensate after flue gas desulfurization at different
temperatures. The results revealed that acidic condensation accelerated the
preferential dissolution and subsequent precipitation of Fe and the oxidation of Mo,
resulting in the formation of a mixed Cr-Fe-Mo oxide/hydroxide surface layer. Chen
et al. [9] studied the effect of Cl− on the corrosion and passivation behavior of the
same steel in blast furnace gas (BFG) electrolyte. The results indicated that the
passivation ability of S31254 SASS was greater under low-chloride ion
concentrations compared to high concentrations. In addition, Liu et al. [10] reported
that the pH value significantly influenced the passivation behavior of S31254 SASS
in NaCl solution. In weakly acidic and strongly acidic solutions, the main
components of the passive film outer layer were iron oxide and Cr(OH)3,
respectively. Li et al. [11] demonstrated that the corrosion resistance of 6Mo SASS
initially improved but later weakened with increasing copper content.
The influence of nitrogen on the properties of stainless steel has drawn significant
attention from researchers. Nitrogen not only enhances the re-passivation ability
of stainless steel but also plays a crucial role in improving the passive film [12,13].
For example, Aamani et al. [14] observed that nitrogen addition leads to grain
refinement and an increased proportion of low-energy boundaries, thereby
enhancing corrosion resistance. Loable et al. [15] reported a synergistic effect
between nitrogen and molybdenum on the pitting potential. Moreover, Dai et al.
[16] demonstrated that nitrogen significantly enhances the corrosion resistance of
316 L stainless steel in a thiosulfate-chlorine solution by promoting the formation
of chromium oxide and iron oxide in the passive film. Gao et al. [17] reported a
higher proportion of stable oxides, such as chromium oxide, in the passivation film
of high-nitrogen stainless steel. The results also revealed that the content of
ammonium ions is higher in high-nitrogen stainless steel, effectively inhibiting
passive film breakdown and pitting. Furthermore, Zhang et al. [18] investigated the
impact of nitrogen on precipitation behavior, intergranular corrosion resistance,
and mechanical properties of S32654 SASS. The results indicated that S32654
with medium nitrogen content exhibits the smallest total amount of precipitates
after aging treatment and the best intergranular corrosion resistance.
Nevertheless, the influence of nitrogen content on the corrosion resistance and
passive film structure of 6Mo SASS remains unclear.
In this study, 6Mo SASS with nitrogen contents of 0.2 and 0.4 (wt.%) was melted
and solid solution treatments at 1100, 1180, and 1250 °C for 30 min were performed. The effects of nitrogen on the microstructure and pitting resistance of
the two steels with different solution treatments were investigated. The impact of
nitrogen on the structure of passive film was studied using X-ray photoelectron
spectroscopy (XPS) analysis.
2. Experimental Procedure
Material Preparation
Super austenitic stainless steels with nitrogen contents of 0.2 and 0.4 wt.% were
prepared for this study. The steels were melted in a 25 kg vacuum induction
furnace and deoxidized using nickel magnesium. The steels were cast into ingots
and subsequently rolled into plates through forging. The chemical composition of
the steel was quantitatively analyzed using the inductance method, and the results
are presented in Table 1.
Table 1. Chemical composition of test steel/wt.%
The hot-rolled plates were then cut into blocks with dimensions of 15 mm × 15 mm × 2 mm using a wire-cutting machine. After solution treatment at temperatures of 1100 °C, 1180 °C, and 1250 °C for 30 min, the samples were rapidly quenched in
water. The samples after solution treatment were denoted as 0.2N-ST and 0.4NST.
Microstructure Characterization
All samples were polished with SiC papers up to 3000 grit and cleaned with ethanol
and distilled water in an ultrasonic bath. Then, the samples were etched with aqua
regia for microstructure observation. The microstructure was observed using a
Leica DM 6 M optical microscopy (OM) (Leica, Solms, Germany), an EVO18
scanning electron microscope (SEM) (JEOL, Tokyo, Japan), and a Rigaku
Smatlab (Malvern Panalytical, Malvern, UK) X-ray diffraction (XRD). The number
and size of grains and precipitates were quantified using Image Pro Plus 6.0
software.
Electrochemical Experiment
The open circuit potential (OCP) of solution-heat treated samples was tested in a
3.5% NaCl solution with a pH of 1 for 60 min to ensure the stability of the open
potential before conducting the potential polarization test. In the potentiodynamic
polarization test, the scanning rate was set to 1.667 mV/s, with a scanning range
between −0.5 V and 1.5 V, and scanning stopped when the current density
reached 10−2 mA. For the potentiostatic polarization test, a potential of 0.4 V was
applied for 10 min, as this potential falls within the passivation range of a 3.5%
NaCl solution with a pH of 1. Electrochemical impedance spectroscopy (EIS) was
performed under a 10 mV sinusoidal AC disturbance with a frequency of 100 kHz
to 0.01 Hz to study the corrosion resistance of passive films. The experimental
data were fitted and analyzed using Zview software (3.0a Scribner Associates, Inc., Southern Pines, NC, USA). Each experiment was repeated at least three times to ensure repeatability. Mott–Schottky curves were generated by setting the frequency to 1000 Hz, the amplitude to 10 mV, and the potential step size to 10
mV within the range of 0 to 1.2 V. Each experiment was repeated at least three times to ensure repeatability.
XPS Analysis
To elucidate the relationship between the passive film composition of 0.2N-ST and
0.4N-ST stainless steel with depth, the passive film composition of samples heattreated at 1180 °C was characterized using XPS. The passive film of the sample
was polarized at 0.4 V in 3.5% NaCl solution with a pH of 1. The sputtering rate was calibrated with silica and was about 3 nm/min. The photoelectron peak binding
energy of other elements was corrected using the position of the hydrocarbon C 1s peak of 284.8 eV. XPS peak 41 software was utilized to fit the spectrum, and a
Shirley-type correction method was applied for background subtraction.
3. Results and Discussion
Microstructure Characterization
Figure 1 and Figure S1 are OM images of 6Mo super austenitic stainless steel
after hot rolling and solution treatment at different temperatures, respectively. It
can be seen from Figure S1 that the grain size of the two alloys without solution
treatment is relatively small. However, there are a lot of precipitated phases in both
samples, which will seriously reduce the corrosion resistance of the hot-rolled steel. Figure 2 shows the SEM image after solution treatment. No precipitates are observed in the samples heat-treated at 1180 °C and 1250 °C. Meanwhile, an undissolved second phase is presented in the samples treated at 1100 °C due to a lower heat treatment temperature. The change in particle size with solution
temperature is illustrated in Figure 3. The results indicate that the average grain size of both samples increases with the increase in solution temperature. Furthermore, under all conditions, the grain size of the 0.4N-ST sample is smaller than that of the 0.2N-ST sample, suggesting that the increase in nitrogen content
has a positive effect on grain refinement.
Figure 1. The OM observation of 0.2N-ST (a–c), 0.4N-ST (d–f) alloy solution at
1100 °C (a,d), 1180 °C (b,e), and 1250 °C (c,f) for 30 min.
Figure 2. The SEM observation of 0.2N-ST (a–c), 0.4N-ST (d–f) alloy solution at
1100 °C (a,d), 1180 °C (b,e), and 1250 °C (c,f) for 30 min.
Figure 3. Average grain size as a function of solution temperature. Figure 4 shows the XRD diffractograms of the two alloys after solution treatment
at 1180 °C. All peaks belong to the austenitic phase of the face-centered cubic
structure, mainly including γ (111), γ (200), and γ (220). It can be seen from Figure
4b that the γ(111) diffraction peak of the 0.4N sample slightly shifts to a lower angle
than that of 0.2N sample. The change in the peak position of the austenite
diffraction peaks is related to the change in the lattice parameters of the austenite
phase, and the increase of nitrogen atoms may cause lattice distortion [19,20].
Figure 4. XRD diffractograms of the two alloys after solution treatments at 1180
°C. (a) XRD diffractograms; (b) high magnification in (a) at 2θ range from 42° to
52°.
3.2. Polarization Curve Measurements
Figure 5a shows the potentiodynamic polarization curve of 6Mo SASS in a 3.5%
NaCl solution with pH = 1. Each curve exhibits a wide passive region with a passive
potential value between −0.3 and 0.9 VSCE. During the process of spontaneous
passivation, active/passive transient peaks are observed in each curve. Table
2 lists the electrochemical parameters derived from the polarization curve,
including corrosion potential (Ecorr), corrosion current density (icorr), and pitting
potential (Ep). It can be seen that the change in nitrogen content and solution
temperature has no effect on Ecorr and Ep, but the corrosion current is different.
The icorr of both samples reaches its minimum under the solution heat-treatment
condition of 1180 °C. However, under the same condition, the icorr of 0.4N-ST
samples is lower than that of 0.2N-ST samples, indicating that the corrosion
resistance of passivated film is better [21,22]. Figure S2 shows the surface
morphology of the 0.2N sample after solution treatment at 1180 °C following
potentiodynamic polarization measurement. It can be seen that pitting pits appear
on the surface of the material after potentiodynamic polarization measurement.
Figure 5. Potentiodynamic (a) and potentiostatic (b) polarization curve of 6Mo
SASS sample in 3.5% NaCl with pH = 1.
Table 2. Electrochemical parameters derived from potentiodynamic polarization
curves in Figure 5a.
The change in current density with passivation time is presented in Figure 5b. In
the initial stage of potentiostatic polarization, the current density of all samples
sharply decreases with time, indicating that the growth rate of the passive film
exceeds the dissolution rate [23]. Subsequently, the current density gradually
decreases and eventually stabilizes. Each curve exhibits a smooth shape,
suggesting no breakdown in the passive film [10], and almost no metastable pitting
process occurs during the entire measurement period. In addition, the current
density first decreases and then increases with the increase in heat treatment
temperature. The current density of both samples reaches its minimum when the
heat-treatment temperature is 1180 °C. Notably, the current density of 0.4N-ST
consistently remains lower than that of 0.2N-ST, indicating that passivation is more
likely to occur at higher nitrogen contents.
Grain size significantly influences the corrosion resistance of stainless steel,
generally improving as grain size decreases. A study reported that an increase in
grain boundary density promotes Cr diffusion to the surface, facilitating the rapid
formation of a uniform passivation film rich in Cr, thus enhancing corrosion
resistance [24]. Although the sample after the solid solution at 1100 °C has a small
grain size, the corrosion resistance is decreased due to incomplete dissolution of
the second phase. Therefore, considering the combined effects of grain size and
precipitates, the sample heat-treated at 1180 °C exhibits the best corrosion
resistance.
Electrochemical Impedance Spectroscopy Measurements
The EIS are presented in Figure 6. It can be seen that compared with the sample
of 0.2N-ST, the impedance mode and phase angle of the sample of 0.4N-ST in the
low-frequency region are reduced, indicating that the corrosion resistance is
improved [25,26]. Figure 6a indicates that the Nyquist diagram of each sample
exhibits an arc feature, with larger arc radii indicating greater charge transfer
resistance and thus better corrosion resistance. The arc radius of both samples is
the largest for solution heat treatment at 1180 °C. At the same condition, the arc
radius of the 0.4N-ST sample is larger than that of the 0.2N-ST sample, indicating
superior corrosion resistance.
Figure 6. EIS of a 6Mo SASS sample in 3.5% NaCl with pH = 1. (a) Nyquist. (b) Byrd.
The EIS results were fitted using the equivalent circuit diagram shown in Figure 6a, and the fitting results are presented in Table 3. Here, Rs represents the
resistance of the electrolyte solution, while R and CPE represent the resistance
and capacitance of the passive film, respectively. The total impedance value of the
circuit is expressed by
Z = Rs. +. 1/(R + (jω)n Q)
where Q is the CPE constant, ω is the angular frequency, j is the imaginary unit,
and n is the CPE adjustment parameter. CPE behaves as an ideal capacitor when
n is equal to 1 [27]. The χ2 sum of each fitting value is small, indicating a small
fitting error; thus, the EIS data after fitting has high reliability. With the increase in
solution temperature, the R-value initially increases and then decreases. The Rvalue increases with the increase in nitrogen content, indicating a decrease in the
charge transfer rate [28] and an improvement in the corrosion resistance of the
steel.
Table 3. Impedance spectrum fitting parameters of 6Mo SASS samples in 3.5%
NaCl with pH = 1.
Mott–Schottky Analysis
Mott–Schottky theory was employed to study the semiconductor properties of the
stainless-steel surface passive film. The relationship between the space charge
capacitance of the passive film and the applied potential is shown as follows:
1/C2.+ 2/ɛɛ0eN (E-Efb – KTe) = (2)
where the positive sign and the negative sign represent n-type semiconductor and
p-type semiconductor, respectively, ε is the dielectric constant, ε0 is the vacuum
dielectric constant (8.854 × 10−14 F cm−2), e is the electron charge (1.6 ×
10−19 C), N is the donor concentration (ND) or acceptor concentration (NA), T is
the absolute temperature, K is the Boltzmann constant (1.38 × 10−23 J K−1), E is
the external electrode potential (VSCE), and Efb is the flat band potential. Figure
7 illustrates the Mott–Schottky diagram of the 6Mo SASS samples under a 3.5%
NaCl solution with pH = 1. All samples exhibit n-type semiconductor characteristics
in the range of 0.1 to 0.75 VSCE, attributed to the outer layer’s richness in Cr-Fe
oxide and hydroxide. At higher potentials, the passive dissolution of the passive
film leads to the accumulation of cation vacancies, resulting in all samples
exhibiting p-type semiconductor characteristics above 0.75 VSCE [29].
Figure 7. Mott–Schottky plots of a 6Mo SASS sample in 3.5% NaCl with pH = 1.
The conductivity of the passive film is a critical parameter dictating the
electrochemical corrosion resistance. The values of ND and NA determine the
conductivity of the passive film. Figure 8 illustrates ND and NA values calculated
from the slope of the Mott–Schottky curve according to Equation (2). The ND and
NA values of the samples with different nitrogen contents reach their minimum
under the solution heat-treatment condition of 1180 °C. However, in comparison
with the sample of 0.2N-ST under the same condition, 0.4N-ST exhibits lower NA and ND values, indicating that the passive film defects formed by the 0.4N-ST sample are fewer than those of the 0.2N-ST sample. This suggests that the
passive film of 0.4N-ST is more effective in preventing charge transfer, thereby reducing the possibility of passive film breakdown and pitting.
Figure 8. Density of ND (a) and NA (b) of 6Mo SASS sample in 3.5% NaCl with pH = 1.
XPS Analysis
Figure 9 illustrates the variation in element contents within the passive film over
sputtering time after solution heat treatment at 1180 °C for the two samples. As
sputtering time increases, the oxygen content consistently decreases, while the
iron content progressively increases until stabilizing after 60 s. This phenomenon
indicates the transformation of passive film to matrix. In the 0.2N-ST sample, the
Fe content is consistently greater than that of Cr, whereas in the 0.4N-ST sample,
Cr content exceeds Fe content in the initial sputtering stages (before 10 s) and is
smaller between 10 s and 20 s. The Ni content shows significant depletion in the
surface layer of the passive film, reaching its maximum at 30 s, indicating that Ni
mainly exists in the inner layer of the passive film. No Ni was detected on the
surface layer of the 0.2N-ST sample, suggesting that the increase in N content
could partially inhibit Ni depletion on the film’s surface.
Figure 9. The change in passive film element concentration with sputtering time in 0.2N-ST (a) and 0.4N-ST (b) samples.
Ni and N exhibit a synergistic effect in enhancing the corrosion resistance of
nitrogenous stainless steel [30]. N significantly contributes to improving the alloy’s
corrosion resistance. However, it also reduces the protective effect of the passive
film at a certain depth by reducing the fraction of chromium ions in the film. When
there is a small amount of Ni present in the alloy, N further enhances the protective
capability of the passive film by increasing the Cr fraction within the film. As a
result, the impact of increased N content on the pitting potential level is more
pronounced. The depletion of Ni in the surface layer may be one reason for the
alloy’s decreased corrosion resistance.
Cr(OH)3-2Cr3++60H- (3)
2Cr202N-+V-o. → Cr203 + 2CrN (4)
Fe-2e-→Fe2+ (5)
4Fe2++2H2O+202→4Fe3++4OH- (6)
2Fe3++Fe→3Fe2+ (7)
[N]+3H++3e-→NH3 (8)
[N]+4H++4e-→NH4+ (9)
CrN+3H2O Cr2O3+2NH3 (10)
Figures S3–S5 show the spectral changes of Cr 2p3/2, Fe 2p3/2, and Ni
2p3/2 passive films with sputtering time, respectively. As sputtering time increases,
the content of Cr(OH)3 decreases, and the spectral intensity of metal Cr increases.
This indicates that Cr(OH)3 predominantly exists in the outer layer of the passive
film. Cr(OH)3 may undergo hydrolysis into Cr3+ (as shown in Equation (3)),
resulting in a depletion of Cr in the passive film and a reduction in corrosion
resistance [31]. Some N atoms existing in the form of Cr2O2N− can neutralize
oxygen vacancies and form Cr2O3 and CrN (as shown in Equation (4)), thereby
reducing donor density and enhancing the protective ability of passive film [32], As
sputtering time increases, the content of FeOOH decreases, indicating that
FeOOH primarily appears in the outer layer of the passive film. According to the
Fe2+ to Fe3+ ratio, the outer layer of the passive film consists mainly of Fe3+ (as
shown in Equations (5) and (6)). As sputtering time further increases, the content
of Fe2+ increases (as shown in Equation (7)). As we all know, Fe3+ is more stable
than Fe2+; Fe3+ and Cr2O3 are the effective components of passive film, and their
content ratio can determine the performance of the passive film [33]. At the same
sputtering depth, the content of Cr2O3 in the 0.4N-ST sample is higher than that in
the 0.2N-ST sample, and the Fe2+/Fe3+ ratio of 0.4N-ST sample is lower than that
of the 0.2N-ST sample. Throughout the entire sputtering process, Ni mainly exists
in the metallic state, indicating that Ni oxides and hydroxides are not the main
components of the passive film. However, the presence of Ni reduces the
conductivity of the passive film, promoting the formation of more resistive oxide
films and thereby improving the film’s protective effect [34].
Figure 10 illustrates the change in Mo 3d spectra of passive films for the two
samples over sputtering time. Due to spin-orbit coupling, Mo 3d5/2 and Mo
3d3/2 exist in Mo elements, resulting in six component peaks in Mo 3d
photoelectron spectra. These include metallic Mo at 227.7 ± 0.1 eV and 230.8 ±
0.1 eV, Mo4+ at 228.6 ± 0.1 eV and 231.7 ± 0.1 eV, and Mo6+ at 232.2 ± 0.1 eV
and 235.3 ± 0.1 eV, respectively [11,35]. Notably, Mo6+ represents the
predominant Mo oxide which effectively prevents the outward dissolution of certain
cations, thereby favoring the passivation of stainless steel. At the initial stage of
sputtering, the Mo6+ content is highest. However, as sputtering time increases, the
Mo6+ content gradually decreases, and the content of Mo4+ gradually increases.
The results show that Mo mainly exists in the form of Mo6+ and Mo4+ in the outer
and inner layers of the passive film, respectively. After sputtering for 30 s, only
metallic Mo and Mo4+ were detected in the 0.2N-ST sample, while a small amount
of Mo6+ persists in the 0.4N-ST sample even after 30 s, and the content of
Mo4+ was larger.
Figure 10. The change in Mo 3d spectra of 0.2N-ST (a) and 0.4N-ST (b)
samples with sputtering time.
Figure 11 illustrates the change in N 1s spectra of passive films for the two
samples with sputtering time. N mainly exists in the form of CrN, NH3, and NH4+,
with corresponding binding energies of 396.8 ± 0.1 eV, 399.2 ± 0.1 eV, and 400.3
± 0.1 eV, respectively [16]. As sputtering time increases, the contents of NH3 and
NH4+ gradually decrease. After sputtering for 30 s and 60 s, only CrN is detected
in the 0.2N-ST and 0.4N-ST samples, respectively, indicating that NH3 and
NH4+ are confined to the outer layer of the passive film. CrN effectively promotes
the formation of Cr2O3 and enhances the protection of passive film [36],
Meanwhile, N improves the re-passivation ability of stainless steel by consuming
H+ in the pit cavity to inhibit acidification (as shown in Equations (8) and (9)),
thereby preventing the development of metastable pits and inhibiting thiosulfate
reduction [37,38]. The increase in N content, as confirmed by the CrN, NH3, and
NH4+ content in Figure 11, enhances the corrosion resistance of the steel. The
possible reactions involved are as follows.
Figure 11. The change in N 1s spectra of 0.2N-ST (a) and 0.4N-ST (b) samples
with sputtering time.
Figure 12 illustrates the composition of the passive films of the two samples after
sputtering for 10 s. The Cr/Fe ratio of the 0.4N-ST sample is significantly higher
than that of the 0.2N-ST sample. This indicates that the repair rate of the passive
film increases relative to the dissolution rate with the increase in nitrogen content.
Furthermore, the increase in nitrogen content promotes the formation of more Crrich oxides, thereby enhancing the stability of the passive film. Previous studies [15] have demonstrated that N and Mo synergistically improve the corrosion resistance of the alloy. Nitrogen effectively promotes the formation of metastable re-passivation, thereby inhibiting pit growth. In addition, nitrogen buffers the drop in pH and stabilizes molybdate, with molybdate also contributing to the formation
of ammonium. Figure 12 indicates that the contents of NH3 and NH4+, as well as
Mo6+ and Mo4+, on the surface layer of the passive film increase with the increase
in nitrogen content.
Figure 12. Surface passive film composition of 0.2N-ST (a) and 0.4N-ST (b)
samples.
Based on the content of each element, the XPS fitting results and the model
diagram of the mass fraction of the passive film with the sputtering depth were
obtained. Figure 13 demonstrates the gradual transition from the surface layer of
the passive film to the metal matrix. The outer layer of the passive film is rich in Cr,
while the inner layer is rich in Fe. The hydroxides of Fe, Cr, and Ni mainly appear
in the outer layer, while the inner layer mainly consists of oxides, with Cr oxide
being the thickest. The Cr oxide of the 0.4N-ST sample is thicker than that of the
0.2N-ST sample. NH3 and NH4+ mainly appear in the outer layer, while N in the
inner layer mainly exists in the form of CrN. NH3 and NH4+, as well as Mo6+ and
Mo4+, in the 0.4N-ST sample are thicker than those in the 0.2N-ST sample.
Figure 13. Mass fraction of passive films in 2N-ST (a) and 0.4N-ST (b) samples
varies with sputtering depth.
4. Conclusions
The 6Mo SASS samples containing 0.2 and 0.4 wt.% nitrogen were prepared and
subjected to solution treatment at temperatures of 1100, 1180, and 1250 °C for 30
min. Then, the influence of nitrogen on the microstructure, corrosion resistance,
and passivation film structure of 6Mo SASS was studied. The findings are
summarized as follows.
1. With increasing solution temperature, the grain size gradually increases, and
alloys treated at 1100 °C exhibit incompletely dissolved second phases. The
addition of nitrogen effectively inhibits the grain growth of 6Mo SASS during
solution heat treatment.
2. At a heat-treatment temperature of 1180 °C, the alloy demonstrates the
highest corrosion resistance, attributed to the combined effects of grain size
and precipitates. In 3.5% NaCl solution with pH = 1, the corrosion resistance
and re-passivation ability of 6Mo SASS were improved when the nitrogen
content was increased from 0.2% to 0.4% wt.%.
3. Mott–Schottky and XPS analyses revealed that when the nitrogen content
increased from 0.2% to 0.4%, the density of the donors and acceptors
decreased and the structure of the passivation film changed, the Cr/Fe ratio
at 0.4N being significantly higher than that at 0.2N. Moreover, the increase
in nitrogen content results in thicker Cr and Mo oxide layers and higher levels
of NH3 and NH4+, thereby improving the corrosion resistance of the
stainless steel.
Supplementary Materials
The following supporting information can be downloaded
at: https://www.mdpi.com/article/10.3390/met14040391/s1, Figure S1. The OM
observation of 0.2 N (a) and 0.4 N (b) hot-rolled alloys. Figure S2. Corrosion
morphology after potentiodynamic polarization test in 3.5% NaCl with pH = 1.
Figure S3. The change of Cr 2p spectra of 0.2N-ST (a) and 0.4N-ST (b) samples
with sputtering time. Figre S4. The change of Fe 2p spectra of 0.2N-ST (a) and
0.4N-ST (b) samples with sputtering time. Figure S5. The change of Ni 2p spectra
of 0.2N-ST (a) and 0.4N-ST (b) samples with sputtering time.
Authors
Haiyu Tian, Jian Wang *, Zhiqiang Liu and Peide Han
College of Materials Science and Engineering, Taiyuan University of Technology,
Taiyuan 030024, China
Authors to whom correspondence should be addressed.
Metals 2024, 14(4), 391; https://doi.org/10.3390/met14040391
Submission received: 23 February 2024 / Revised: 22 March
2024 / Accepted: 25 March 2024 / Published: 26 March 2024
Author Contributions
Conceptualization, H.T., J.W. and P.H.; methodology, H.T., J.W. and P.H.;
software, H.T.; validation, H.T., J.W., Z.L. and P.H.; formal analysis, H.T., J.W. and
Z.L.; investigation, H.T. and Z.L.; resources, J.W. and P.H.; data curation, H.T.;
writing—original draft, H.T.; writing—review and editing, H.T., J.W. and Z.L.;
visualization, H.T. and Z.L.; supervision, J.W. and P.H.; project administration,
J.W.; funding acquisition, J.W. All authors have read and agreed to the published
version of the manuscript.
Funding
The research was financially supported by the National Natural Science
Foundation of China (Grant Nos. 52104338) and China Scholarship Council (Grant
No. 202006935035).
Data Availability Statement
The original contributions presented in the study are included in the article, further
inquiries can be directed to the corresponding author.
Conflicts of Interest
The authors declare no conflict of interest.
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