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Article

Effect of Cr:Al Ratio on Corrosion Mechanism of Ni-Cr-Mo-Al Alloys in 3.5 wt.% NaCl Solution: Microstructure and Electrochemical and Passive Characteristics

by
Chenggang Lian
1,2,3,
Wei Xie
1,*,
Huanjie Fang
2,3,*,
Wenqian Wang
2,3,
Jianhao Yu
2,3,
Jicheng Li
2,3 and
Xiaodong He
2,3
1
School of Marine Engineering Equipment, Zhejiang Ocean University, Zhoushan 316022, China
2
State Key Laboratory of Advanced Marine Materials, Ningbo Institute of Materials Technology and Engineering of the Chinese Academy of Sciences, No. 1219 Zhongguan West Road, Ningbo 315201, China
3
Qianwan Institute of CNITECH, Zhongchuang 1st Road, Zhongchuang Park, Qianwan New Area, Ningbo 315336, China
*
Authors to whom correspondence should be addressed.
Materials 2025, 18(10), 2177; https://doi.org/10.3390/ma18102177
Submission received: 3 April 2025 / Revised: 28 April 2025 / Accepted: 29 April 2025 / Published: 8 May 2025
(This article belongs to the Section Corrosion)
Browse Figures
Versions Notes

Abstract

:
In this study, the microstructure and electrochemical and passive characteristics of NiCr25.2−xMoAlx (x = 0, 1.25, 2.5, and 5 mol.%) alloys were investigated. The results show that Ni-Cr-Mo-Al alloys with varying Cr:Al ratios both had a single FCC structure without any second structure precipitates, and decreases in dislocation density and grain size were observed as the Al content in NiCrMoAl alloys increased. It was found from the electrochemical results that the NiCr23.95MoAl1.25 alloys had the maximum radius for a semicircle and the lowest Icorr, indicating an enhanced anti-corrosion performance (Rct: 8.08 ± 0.368 × 105 Ω cm2, Icorr: 1.05 ± 0.003 × 10−7 A/cm2). In this study, it was found that the anti-corrosion performance of the alloys had an approximate connection to the composition and density of passive films. Denser and more stable microstructures in NiCr23.95MoAl1.25 alloys were further proven by potentiostatic polarization tests and Mott–Schottky experiments, showing a lower stable current density and acceptor density (NA: 9.79 ± 0.4 × 10−20 cm−3). In addition, the results of XPS show that the Al1.25 specimen had the highest Cr2O3 in the passive film’s content among the NiCrMoAl alloys. Cr2O3 was the main component, suggesting an enhanced protective influence of passive film. The present study can offer guidance for the application of nickel-based alloys with high anti-corrosion resistance in marine environments.

1. Introduction

With the depletion of terrestrial resources caused by human over-exploitation, the development of marine resources is of particular importance, and it heavily relies on the rapid development of cutting-edge marine machinery. However, the marine environment is extremely harsh and complex. Seawater is widely considered to be a naturally strong electrolyte with high salt content, strong conductivity, and high microorganism content; therefore, a series of measures have been employed to ensure the successful long-term operation of marine equipment [1,2,3]. Among these numerous methods, nickel-based alloy is expected to become an effective solution because of its superior mechanical qualities and resistance to corrosion. In order to meet the requirements of working conditions, several types of nickel-based alloys, such as Monel [4], Inconel [5,6], Hastelloy [7], and Incoloy [8] have been developed and successfully applied in the industries of warships, gas turbines, and petrochemical engineering [9,10,11].
Nickel-based alloys have attracted widespread attention for their outstanding passivation ability [12,13,14,15,16]. When exposed to the marine environment, it is possible for the metal surface to quickly develop a protective oxide film [17,18], which can prevent the substrate from undergoing electrochemical reactions with the corrosive medium. The alloy’s composition affects the passivation film’s chemical makeup but is usually composed of oxides. The ability of nickel-based alloys to be corrosion-resistant can be effectively improved by adding a certain proportion of chromium (Cr), molybdenum (Mo), iron (Fe) and trace amounts of aluminum (Al), titanium (Ti), and other elements. For example, the passive film’s composition with more Cr2O3 can be promoted by introducing chromium (Cr), while the addition of molybdenum (Mo) into nickel-based alloys can improve pitting resistance and resistance to corrosion, particularly in acidic situations, which results from the makeup of oxides containing Mo6+ [19]. Consequently, the versatility of nickel-based alloys benefits from the incorporation of elements like Cr and Mo, enabling their application in complex corrosive conditions. However, when the amount of Mo is excessive, the σ phase is inevitably precipitated, causing decreased resistance to corrosion [20]. The present study focused on the density and stability of a passive film on the alloy surface, which has been widely recognized by researchers as a critical factor [21]. The dense passive film formed on the alloy surface, typically 1–3 nm thick, was identified as a key contributor to the alloy’s outstanding corrosion resistance.
Furthermore, many studies have explored the impact of Al addition on the mechanical qualities and resistance to corrosion of passive films. Pan et al. [22] examined how the addition of Al affects CoNiVAlx medium-entropy alloy’s microstructure and resistance to corrosion, and Al addition was demonstrated to positively influence resistance to corrosion. This investigation further proposed that with Al addition, the passive film’s stability and protective qualities were improved when the oxide content increased and the concentration of defects decreased. The corrosion resistance ability of AlxCrFeNi3-x high-entropy alloys with different Al elements was also investigated, employing electrochemical tests by Zhao et al. [23] Their findings were contrary to previous studies, and a decrease in the passive film’s stability was found as the Al content increased. Inconel 625 coatings with different Al contents were prepared by Feng et al. [24] Phase composition analysis showed that the addition of Al formed a body-centered cubic (BCC) phase, increasing the coatings’ hardness and wear resistance. In addition, an enhanced corrosion resistance was found with the addition of trace amounts of Al (wt.% 2.5%), but decreased as the Al content increased beyond this threshold. It is worth noting that when Al and Cr are added into nickel-based alloys, it is possible to see the development of an oxide layer, and the thermal stability of Al2O3 film was superior to that of Cr2O3 [25]. However, its corrosion resistance at room temperature has not been extensively studied.
To this end, this work systematically examined the influence of Cr:Al ratio on the corrosion mechanism of NiCr25.2-xMoAlx (x = 0, 1.25, 2.5, 5 at%) alloys in 3.5 wt.% NaCl electrolyte solution. Specifically, electrochemical tests like the electrochemical impedance spectroscopy curve (EIS) and potentiodynamic polarization curve methods, were employed to evaluate electrochemical property of NiCrMoAl alloys with different Cr:Al ratio. In addition, constant potential polarization, Mott–Schottky (M-S) curve, and X-ray photoelectron spectroscopy (XPS) method were employed to study the characteristics of passive film. This study can offer valuable guidance for designing alloys with good corrosion resistance in marine environments.

2. Experimental Procedures

2.1. Sample Preparation

Mixtures of high-purity metals (purity > 99.95%) were arc-melted to create alloy ingots with different aluminum (Al) percentages; all the raw materials (Linyiyan Innovative Materials Technology Co., Ltd., Linyi, China) were irregular particles of about 3 mm in size. To avoid contamination from leftover oxygen and nitrogen, the entire procedure was conducted in an argon environment that absorbed titanium. To minimize compositional inhomogeneity caused by the volatilization of high-melting-point elements (Mo and Cr) and low-melting-point elements (Al), Ni particles were placed as protective layers on both the top and bottom of the material stack; these ingots were remelted at least five times while being stirred by a magnetic device, and then suction cast using a water-cooled copper mold into bulk samples measuring 60 × 20 × 6 mm3. After that, the castings were chopped into cuboid samples measuring 10 × 10 × 3 mm3. All the alloy samples were ground successively using SiC sandpaper (400#, 800#, 1200#, 2000#, and 3000#), polished using a polishing compound to a thickness of 1 µm to observe their microstructural characteristics, and then further polished using 0.05 µm silica suspension for electron backscatter diffraction (EBSD) analysis. Other copper wires were connected with the alloys with regard to the specimens prepared for electrochemical tests, and afterwards, the specimens were covered with epoxy resin, exposing just 1 cm2. The chemical compositions of NiCr25.2−xMoAlx (x = 0, 1.25, 2.5, and 5 mol.%) alloys are listed in Table 1. For simplicity, the NiCrMoAl alloys with varying Cr:Al ratios are referred to as Al0, Al1.25, Al2.5, and Al5 in the following manuscript.

2.2. Electrochemical Measurements

All electrochemical tests, including dynamic potential polarization, constant potential current transient, and capacitance measurements, were conducted utilizing the Gamry workstation (American, Reference 620), and a regular three-electrode test setup, consisting of the counter electrode (in platinum, 10 × 10 × 0.2 mm3), an Ag/AgCl reference electrode (KCl saturated), 300 mL of 3.5 wt.% NaCl electrolyte solution, and the sample serving as the working electrode. All potentials were set relative to the Ag/AgCl reference electrode. Before testing, constant potential polarization was performed for 300 s at −0.5 V to reduce the effect of the oxide film.
To ensure that the system had stabilized, an open-circuit potential test was conducted for an hour before the EIS test, which was further confirmed by an allowable fluctuation over 10 min of no greater than ±5 mV. With an AC perturbation amplitude of 10 mV in a frequency range of 102 kHz to 10 mHz, the EIS test was carried out. ZSimpWin (version 3.60) software was then applied to examine the EIS results. Subsequently, at a scan rate of 60 mV/min, a dynamic potential polarization test was conducted, beginning at −0.46 V until the polarization current density reached 2 mA/cm2.
A Mott–Schottky curve was applied to examine the passive film’s semiconductor characteristics, using a 50 mV/s scan rate, Eb − 0.2 V scan range, and a frequency of 1 kHz. Prior to the test, a constant potential polarization lasting for 600 s at −0.5 V was strictly necessary to remove any oxide film that may have formed. To create a stable oxide film in a 3.5 wt.% NaCl electrolyte solution, 18,000 s of polarization at 0.2 V was then carried out. All electrochemical tests were carried out at least 3 times to guarantee the accuracy and repeatability of the results.

2.3. Characterization

Utilizing X-ray diffraction (XRD), the alloy’s phase structure and the 2θ range of 20° to 100° were determined. To grasp the crystallographic data, including dislocation density and the size and orientation of the grains, a Gemini 300 scanning electron microscope system equipped a QUANTAX EBSD probe (Bruker, Billerica, MA, USA) was adopted. The resultant data were further analyzed using AZtecCrystal (version 3.1) software. Moreover, X-ray photoelectron spectroscopy (XPS, Axis Supra, Kratos, Japan) was used to describe the composition of the passive film that resulted from constant potential polarization. The XPS date was calibrated using the general peak with (C 1s, 284.8 eV), and the findings were analyzed using Avantage (version 5.9922) software, with the Shirley model applied for background subtraction.

3. Results and Discussion

3.1. Microstructure Analysis

The XRD spectrum of the cast NiCr25.2−xMoAlx alloy is shown in Figure 1. All characteristic peaks were corresponded to the single FCC structure, with peaks at (111), (200), (220), and (311), indicating that NiCr25.2−xMoAlx was composed of a single FCC phase. This result was consistent with typical NiCrMo alloys [26].
The EBSD data presented in Figure 2 show the inverse pole figure (IPF), dislocation density (GND), and the average grain size for Al0, Al1.25, Al2.5, and Al5 alloys. From the IPF maps in Figure 2(a1–a4), it can be observed that the alloy grains with varying Cr:Al ratio exhibited uniform equiaxed structures. The GND maps in Figure 2(b1–b4) further revealed that the dislocation density decreased with the continuous addition of Al content. This phenomenon can be attributed to the large atomic radius and low elastic modulus of Al, which caused a local stress field that inhibited dislocation movement and generation, thus leading to a reduction in dislocation density [23]. Some studies [7,27] proposed that an increased dislocation density would enhance the passive film’s nucleation sites, thereby promoting its formation. In addition, Figure 2(c1–c4) gives the G-S statistics of the NiCrMoAl alloys, and the results demonstrated that the average grain size reduced as the amount of Al rose, which was consistent with the reports of other studies [28]. However, it is worth pointing out that grain size is negatively correlated with corrosion resistance [21,29]. That is, dislocation density and grain size have the opposing effects on corrosion resistance, and corrosion resistance falls as grain size decreases. Therefore, it is difficult to accurately speculate the anti-corrosion resistance of NiCrMoAl alloys with varying Cr:Al ratio based solely on their grain microstructure.

3.2. Electrochemical Characteristic Analysis

EIS was employed for comprehensive evaluation in order to assess the corrosion resistance of the alloy samples with varying Cr:Al ratios. As shown in Figure 3a, the Nyquist plot exhibited a classic capacitive semicircle, which can be attributed to the formation of protective oxide film on the sample surface after rapid oxidation due to pre-treatment of open-circuit potential. The Nyquist plot displayed a classic capacitive semicircle, as seen in Figure 3a. This is because of the open-circuit voltage test, causing a protective oxide film to form on the sample surface following rapid oxidation. Additionally, the radius of the semicircle followed the order Al1.25 > Al0 > Al2.5 > Al5, indicating an enhanced resistance to electrochemical corrosion when the content of Al reached to 1.25 mol.%. In the Bode plot shown in Figure 3b, a broad peak can be seen in the range of 100–103 Hz, suggesting the oxide film’s capacitive qualities on the sample surface. Figure 4 displays the circuit model that was utilized to fit the data, with Rs standing for the electrolyte solution’s resistance, and CPE is used to denote the constant phase element that reflecting the capacitance of the oxide film on the alloy’s surface [28,29]. This CPE is connected in parallel with Rct, the charge transfer resistance, which indicates the resistance encountered by corrosive ions as they pass through the oxide film to reach the alloy interface. The impedance value of CPE can be calculated using the following formula:
Z C P E = 1 Q ( j ω ) n
where ZCPE is the impedance value, Q is the admittance value, ω is the angular frequency, j is an imaginary number, and n (0 ≤ n ≤ 1) is the fitting parameter. n = 0 represents pure resistance, n = 0.5 represents Warburg impedance, and n = 1 represents an ideal capacitor. The parameters obtained from fitting the test results using ZSimpWin software are listed in Table 2, and a small value of chi-square (χ2) (on the order of 10−4) indicates a good fit. In Table 2, the CPEdl value of the Al1.25 sample was the lowest, while that of the Al5 sample was the highest, indicating that the Al1.25 sample exhibited the best corrosion resistance, whereas the Al5 sample showed the poorest corrosion resistance. Moreover, the Rct value of the Al1.25 sample was the highest, suggesting that its oxide film provided the most effective protection. These results demonstrate that the addition of a small amount of Al significantly enhanced the corrosion resistance of the alloy.
Figure 5a plots the dynamic potential polarization of NiCr25.2−xMoAlx alloys. The polarization curves revealed that all samples exhibited similar characteristics, including visible passivation regions [30]. As the voltage increased in the early dissolution zone, the corrosion current density (Icorr) increased quickly, but this behavior was mitigated in the subsequent transition zone. Because of the passive film’s protective impact, the growth rate of the corrosion current was lowered in the passivation zone, though it worsened in the over-passivation region. The Tafel extrapolation method was used to fit the pertinent values from the polarization curves (Figure 5b) [31], and other values are compiled in Table 3, including the pitting potential (Ep), passive current density (Ip), corrosion current density (Icorr), and self-corrosion potential (Ecorr) for each alloy. Among all samples, the highest Ecorr was found in the Al0 specimen, and as Al content increased, Ecorr gradually decreased, which was likely due to the lower electronegativity of Al. However, it was notable that the Al1.25 specimen unexpectedly showed the lowest Icorr, which was in agreement with the results of EIS. Therefore, it can be concluded that the trace amounts of introduced Al can effectively improve the anti-corrosion performance of NiCr25.2−xMoAlx alloys, but further increases in Al content resulted in a weak corrosion resistance.

3.3. Passivation Characteristic Analysis

To explore the possible causes for different corrosion resistance, passivation behavior of the samples with different Cr:Al ratios was invested. It is well acknowledged that the density of the passive film plays a significant role in determining how resistant an alloy is to corrosion. A stable passivation potential was applied for 5 h of constant potential polarization, with current density measured over time, and the results of potentiostatic polarization are shown in Figure 6, revealing a sharp decrease in current density during the test’s first phase, credited to the passive film’s quick formation [32]. Throughout the polarization process, sudden fluctuations in current density were also observed, which were likely caused by the localized dissolution of the passive film [33]. After 5 h of constant potential polarization, the current density reached a stable state. The steady-state current densities were found to follow the order of 5 nA (Al1.25) < 12 nA (Al0) < 23 nA (Al2.5) < 31 nA (Al5). Notably, the Al1.25 sample exhibited much smaller dispersion and a lower passivation current density, demonstrating the formation of a denser and more stable passive film.
The connection between the imposed voltage (E) and the space charge capacitance (C) was described by the M-S equation, as presented below [34,35].
1 C 2 = ± 2 ε ε 0 e N ( E − E F B − K T e )
where ε represented the vacuum permittivity (8.854 × 10−14 F/cm), while ε0 denoted the relative permittivity (typically 12). E is the applied external potential, and EFB referred to the flat band potential (V). In addition, the electron charge was given as e = 1.602 × 10−19 C, and N corresponded to the carrier defect concentration (cm−3). K and T denoted the Boltzmann constant and temperature (K). Generally, the defect concentration can be inferred from the slope of the M-S curve. A higher defect concentration would result in a smaller absolute value of slope, showing a slower variation in the curve. Furthermore, the slope’s indication could be used to identify the type of defect. The passive film displayed p-type semiconductor characteristics when the slope was negative, where the carrier defects corresponded to cation vacancies, denoted as NA [36].
Figure 7a shows the Mott–Schottky curves for NiCr25.2−xMoAlx alloys, illustrating their passive films’ semiconductor characteristics. Every curve exhibited a negative slope, confirming the passive film’s p-type semiconductor properties, with cation vacancies as the predominant point defects [37,38]. Significant variations in the curves’ slope were noted as a result of the passive film’s non-uniform acceptor density distribution [39]. The acceptor density (NA) of each sample was determined by calculating the linear region’s slope, as shown in Figure 7b. The numerical range of NA was found to be between 1020 and 1021, which aligned with the defect concentration of conventional stainless steels [40,41]. The conclusion that the Al1.25 sample had a lower defect density was demonstrated again by Mott–Schottky experiments. With the further addition of Al, NA significantly increased, and accordingly, the ability to inhibit the passage of electrolytes weakened.
Additionally, the composition of passive films directly influenced its protective performance [42]. Figure 8 presents the comprehensive spectra of Ni 2p, Cr 2p, Mo 3d, and O 1s obtained through fitting analysis. The Ni 2p spectrum was discovered to have four component peaks, corresponding to Ni metal, Niox2+, Nihy2+, and Ni2+ Sat, respectively. Among these, the peak associated with metallic nickel was the most prominent. The main cause of this was that, during the characterization procedure, some nickel oxide was reduced to metallic Ni. Additionally, the higher electronegativity of metallic nickel compared to Cr and Al resulted in most of the oxygen reacting with Cr and Al during the oxidation process, leaving the passive film with a greater concentration of metallic nickel [43,44]. Furthermore, the Cr 2p spectrum could be decomposed into the peaks that represent metal Cr0, Crox3+, and Crhy3+. The results indicate that variations in the types of chromium shown in the passive film were not substantially impacted by the Cr:Al ratio. In Figure 8(c1–c4), Mo0 3d5/2, Mo0 3d3/2, Mo4+ 3d5/2, Mo4+ 3d3/2, Mo6+ 3d5/2, and Mo6+ 3d3/2 are the peaks that were identified in the Mo 3d spectrum. Notably, metallic Mo was the dominant species, suggesting that Mo was less likely to undergo passivation in a neutral solution, which resulted in a relatively low oxide content. Figure 8(d1–d4) presents the O 1s spectrum, which can be categorized into three characteristic peaks for H2O, OH−, and O2−, corresponding to the passive film’s bound water, metal hydroxides, and oxides, respectively. Bound water is a critical component of the passive film, as it can interact with the solution’s metal ions, facilitating the formation of additional oxide layers [45,46]. In addition, the Al 2p spectrum, as shown in Figure 8(e1–e3), can be decomposed into peaks for Al0 2p3/2, Al0 2p1/2, Alox3+ and Alhy3+. Some studies proposed that alumina (Al2O3) can inhibit alloy oxidation under high-temperature conditions; however, the porous nature of alumina raised concerns about its effectiveness in strengthening the alloys’ resistance to corrosion in certain environments [47,48]. The detailed parameters are provided in the Supplementary Material.
By calculating the area of each fitting peak, the percentage of each valence state for each element can be intuitively quantified and compared. For NiCr-based alloys, the presence of Cr2O3 is principally responsible for the passive film’s protective action. Among these NiCrMoAl alloys, the Al1.25 sample has the highest Cr2O3 concentration, as seen in Figure 9b. In addition, the O2− contents in NiCr25.2−xMoAlx samples were 19%, 25%, 19%, and 15%, respectively. When measuring the oxygen content, the oxide content of the passivation film was relatively reliable. The highest content of O2− in the Al1.25 sample was significantly higher, suggesting that the addition of trace amounts of Al encourage the film’s oxide development. A previous study [27] also found that increasing the Al content can improve the corrosion resistance of FeCoCrNiAlx by simultaneously promoting the formation of Cr2O3 in the passive film, making it thicker in the H2SO4 solution.

4. Conclusions

This study systematically investigated the effect of Cr:Al ratio on the corrosion mechanism of NiCr25.2-xMoAlx (x = 0, 1.25, 2.5 and 5 mol.%) alloys in 3.5 wt.% NaCl solution. Specifically, electrochemical tests were employed to evaluate electrochemical behavior of NiCrMoAl alloys with different Cr:Al ratios. In addition, constant potential polarization, M-S curve, and XPS were used to study the characteristics of passivation film. The following conclusions were drawn:
(1)
The Ni-Cr-Mo-Al alloys with varying Cr:Al ratio both exhibited a single face-centered cubic (FCC) phase without any second-phase precipitates. In addition, uniform equiaxed structures were observed in all specimens; however, the results of EBSD showed a decreased dislocation density and grain size as Al content in NiCrMoAl alloys increased. The decrease in grain size was beneficial to the corrosion resistance of the alloy, while a decreasing dislocation density affected the nucleation and growth of passivation film, thereby decreasing anti-corrosion performance.
(2)
The corrosion resistance of the alloy samples with different Cr:Al ratio was compared by electrochemical methods, including EIS and PDP. A maximum radius of a semicircle was found in the Al1.25 specimen on EIS test, whereas the highest Ecorr was found in the Al0 specimen, and as Al content increased, Ecorr gradually decreased. However, the Al1.25 specimen unexpectedly showed the lowest Icorr. The results of EIS and PDP both indicated that the Al1.25 sample exhibited the best electrochemical performance (Rct: 8.08 ± 0.368 × 105 Ω cm2, Icorr: 1.05 ± 0.003 × 10−7 A/cm2).
(3)
The density of passivation film has an essential influence on the corrosion resistance of alloys. In the potentiostatic polarization test, steady-state current densities were observed to follow the order 5 nA (Al1.25) <12 nA (Al0) <23 nA (Al2.5) < 31 nA (Al5), indicating that the passive film with a denser and more stable microstructure was formed in Al1.25 specimen. The similar conclusion was demonstrated again in Mott–Schottky experiments. When the Al content was 1.25 mol.%, the amount of NA was 9.79 × 1020 cm−3, which was lowest among these alloys.
(4)
The composition of a passive film was another important factor affecting its protective performance. It can be found from XPS results that the content of Cr2O3 in the passive film, the main component of the passive film, first increased and then decreased. Among these Ni-Cr-Mo-Al alloys, the Al1.25 specimen had a highest Cr2O3 content, implying an enhanced protective effect of the passive film.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma18102177/s1. Table S1: XPS fitting parameters of components on the Ni 2p, Cr 2p, Mo 3d, O 1s, and Al 2p spectra.

Author Contributions

Methodology, C.L., W.W., J.Y., J.L. and X.H.; Investigation, C.L.; Resources, H.F.; Writing—original draft, C.L.; Visualization, H.F.; Supervision, W.X. and H.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research is supported by National Natural Science Foundation of China (Grant No. 52305169), China Postdoctoral Science Foundation (Grant No. 2022M723249), Zhejiang Provincial Natural Science Foundation of China (Grant No. LQ23E050006), Natural Science Foundation of Ningbo (Grant No. 2023J328) and the 173 Project.

Data Availability Statement

The data presented in this study are available on request from the corresponding author due to privacy.

Conflicts of Interest

We declare that we have no known competing financial interest or personal relationships that could have influenced the work reported in this paper.

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Materials 18 02177 g001
Figure 1. XRD patterns of the Al0, Al1.25, Al2.5, and Al5 alloys.
Figure 1. XRD patterns of the Al0, Al1.25, Al2.5, and Al5 alloys.
Materials 18 02177 g001
Materials 18 02177 g002
Figure 2. EBSD results including (a1–a4) IPF maps, (b1–b4) GND and (c1–c4) grain size of the (a1–c1) Al0, (a2–c2) Al1.25, (a3–c3), Al2.5, and (a4–c4) Al5 alloys.
Figure 2. EBSD results including (a1–a4) IPF maps, (b1–b4) GND and (c1–c4) grain size of the (a1–c1) Al0, (a2–c2) Al1.25, (a3–c3), Al2.5, and (a4–c4) Al5 alloys.
Materials 18 02177 g002
Materials 18 02177 g003
Figure 3. (a) Nyquist plot and (b) Bode plot of the Al0, Al1.25, Al2.5, and Al5 in 3.5 wt.% NaCl electrolyte solution. The fitting curves are shown by solid lines.
Figure 3. (a) Nyquist plot and (b) Bode plot of the Al0, Al1.25, Al2.5, and Al5 in 3.5 wt.% NaCl electrolyte solution. The fitting curves are shown by solid lines.
Materials 18 02177 g003
Materials 18 02177 g004
Figure 4. The equivalent circuit applied in the EIS date fitting.
Figure 4. The equivalent circuit applied in the EIS date fitting.
Materials 18 02177 g004
Materials 18 02177 g005
Figure 5. (a) Dynamic potential polarization curves of Al0, Al1.25, Al2.5, and Al5 specimens in 3.5 wt.% NaCl electrolyte solution and (b) Tafel extrapolation.
Figure 5. (a) Dynamic potential polarization curves of Al0, Al1.25, Al2.5, and Al5 specimens in 3.5 wt.% NaCl electrolyte solution and (b) Tafel extrapolation.
Materials 18 02177 g005
Materials 18 02177 g006
Figure 6. (a) Current–time curves and (b) the results for 18,000 s at an imposed potential of Al0, Al1.25, Al2.5, and Al5 alloys in the 3.5 wt.% NaCl electrolyte solution.
Figure 6. (a) Current–time curves and (b) the results for 18,000 s at an imposed potential of Al0, Al1.25, Al2.5, and Al5 alloys in the 3.5 wt.% NaCl electrolyte solution.
Materials 18 02177 g006
Materials 18 02177 g007
Figure 7. (a) M-S curves of Al0, Al1.25, Al2.5, and Al5 in 3.5 wt.% NaCl electrolyte solution and (b) the calculated values of the NA according to the M-S curves.
Figure 7. (a) M-S curves of Al0, Al1.25, Al2.5, and Al5 in 3.5 wt.% NaCl electrolyte solution and (b) the calculated values of the NA according to the M-S curves.
Materials 18 02177 g007
Materials 18 02177 g008
Figure 8. XPS spectrums of (a1–a4) Ni 2p, (b1–b4) Cr 2p, (c1–c4) Mo 3d, (d1–d4) O 1s, and (e1–e3) Al 2p for the passive film formed on (a1–d1) Al0, (a2–d2) Al1.25, (a3–d3), Al2.5, and (a4–d4) Al5 in 3.5 wt.% NaCl electrolyte solution.
Figure 8. XPS spectrums of (a1–a4) Ni 2p, (b1–b4) Cr 2p, (c1–c4) Mo 3d, (d1–d4) O 1s, and (e1–e3) Al 2p for the passive film formed on (a1–d1) Al0, (a2–d2) Al1.25, (a3–d3), Al2.5, and (a4–d4) Al5 in 3.5 wt.% NaCl electrolyte solution.
Materials 18 02177 g008
Materials 18 02177 g009
Figure 9. The atomic percentages of the component peaks to the total intensity of (a) Ni 2p, (b) Cr 2p, (c) Mo 3d, (d) O 1s, and (e) Al 2p in the passive film of Al0, Al1.25, Al2.5, and Al5 alloys in 3.5 wt.% NaCl electrolyte solution.
Figure 9. The atomic percentages of the component peaks to the total intensity of (a) Ni 2p, (b) Cr 2p, (c) Mo 3d, (d) O 1s, and (e) Al 2p in the passive film of Al0, Al1.25, Al2.5, and Al5 alloys in 3.5 wt.% NaCl electrolyte solution.
Materials 18 02177 g009
Table 1. Chemical compositions of NiCrMoAl alloys with varying Cr:Al ratio.
Table 1. Chemical compositions of NiCrMoAl alloys with varying Cr:Al ratio.
Alloy Elements
NiCrMoAl
Al0Nominal (at.%)71.725.23.1-
Weight (wt.%)72.422.55.1-
Al1.25Nominal (at.%)71.723.953.11.25
Weight (wt.%)72.821.55.10.6
Al2.5Nominal (at.%)71.722.73.12.5
Weight (wt.%)73.120.55.21.2
Al5Nominal (at.%)71.720.23.15
Weight (wt.%)73.918.55.22.4
Table 2. Equivalent circuit model fitting parameters based on the EIS results.
Table 2. Equivalent circuit model fitting parameters based on the EIS results.
AlloyRs (Ω·cm2)CPE × 10−5 (Ω−1 cm−2 sn)nRct × 10−5 (Ω·cm2)
Al06.788 ± 0.042.23 ± 0.020.9417 ± 0.0027.66 ± 0.362
Al1.256.276 ± 0.042.22 ± 0.010.9461 ± 0.0018.08 ± 0.368
Al2.56.807 ± 0.032.53 ± 0.020.9381 ± 0.0026.95 ± 0.356
Al56.614 ± 0.053.52 ± 0.030.8961 ± 0.0026.26 ± 0.522
Table 3. Electrochemical values fitting from the test results of Al0, Al1.25, Al2.5, and Al5 alloys in 3.5 wt.% NaCl electrolyte solution.
Table 3. Electrochemical values fitting from the test results of Al0, Al1.25, Al2.5, and Al5 alloys in 3.5 wt.% NaCl electrolyte solution.
AlloyIcorr × 10−7 (A)Ecorr (V)Ip × 10−6 (A)Eb (V)
Al01.55 ± 0.004−0.23 ± 0.0053.14 ± 0.0020.61 ± 0.02
Al1.251.05 ± 0.003−0.24 ± 0.0033.01 ± 0.0020.61 ± 0.01
Al2.51.58 ± 0.005−0.26 ± 0.0043.50 ± 0.0040.61 ± 0.01
Al52.21 ± 0.002−0.33 ± 0.0026.17 ± 0.0030.46 ± 0.03
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Lian, C.; Xie, W.; Fang, H.; Wang, W.; Yu, J.; Li, J.; He, X. Effect of Cr:Al Ratio on Corrosion Mechanism of Ni-Cr-Mo-Al Alloys in 3.5 wt.% NaCl Solution: Microstructure and Electrochemical and Passive Characteristics. Materials 2025, 18, 2177. https://doi.org/10.3390/ma18102177

AMA Style

Lian C, Xie W, Fang H, Wang W, Yu J, Li J, He X. Effect of Cr:Al Ratio on Corrosion Mechanism of Ni-Cr-Mo-Al Alloys in 3.5 wt.% NaCl Solution: Microstructure and Electrochemical and Passive Characteristics. Materials. 2025; 18(10):2177. https://doi.org/10.3390/ma18102177

Chicago/Turabian Style

Lian, Chenggang, Wei Xie, Huanjie Fang, Wenqian Wang, Jianhao Yu, Jicheng Li, and Xiaodong He. 2025. "Effect of Cr:Al Ratio on Corrosion Mechanism of Ni-Cr-Mo-Al Alloys in 3.5 wt.% NaCl Solution: Microstructure and Electrochemical and Passive Characteristics" Materials 18, no. 10: 2177. https://doi.org/10.3390/ma18102177

APA Style

Lian, C., Xie, W., Fang, H., Wang, W., Yu, J., Li, J., & He, X. (2025). Effect of Cr:Al Ratio on Corrosion Mechanism of Ni-Cr-Mo-Al Alloys in 3.5 wt.% NaCl Solution: Microstructure and Electrochemical and Passive Characteristics. Materials, 18(10), 2177. https://doi.org/10.3390/ma18102177

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