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Corrosion behavior of wire arc additive manufactured Inconel 718 superalloy
Journal Pre-proof Corrosion behavior of wire arc additive manufactured Inconel 718 superalloy L.N. Zhang, O.A. Ojo PII:
S0925-8388(20)30818-5
DOI:
https://doi.org/10.1016/j.jallcom.2020.154455
Reference:
JALCOM 154455
To appear in:
Journal of Alloys and Compounds
Received Date: 6 December 2019 Revised Date:
19 February 2020
Accepted Date: 21 February 2020
Please cite this article as: L.N. Zhang, O.A. Ojo, Corrosion behavior of wire arc additive manufactured Inconel 718 superalloy, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.154455. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit Author Statement Lina Zhang: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Writing-Original draft. Olanrewaju A. Ojo: Supervision, Reviewing, and Editing.
Corrosion Behavior of Wire Arc Additive Manufactured Inconel 718 Superalloy L.N. Zhang*, O. A. Ojo Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, Canada R3T 5V6
ABSTRACT The corrosion behavior of Inconel 718 (IN718) superalloy fabricated using wire-arc additive manufacturing (WAAM) and post-deposition heat treatment is investigated and compared to that of wrought alloy. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) analyses show that the corrosion resistance of heat-treated WAAM IN718 (WAAM-HT IN718) is inferior to that of heat-treated wrought alloy (wrought-HT). Potentiostatic polarization and X-ray photoelectron spectroscopy (XPS) are used to analyze passivation properties of the materials. The results indicate that in comparison to the wrought-HT material, the passive film formed on the WAAM-HT IN718 contains more NiO, which is porous and less protective than Cr2O3. The formation of more NiO and lesser Cr2O3 in the passive film formed on the WAAMHT IN718 could be related to its {100} <001> (Cube) texture. Moreover, based on thermodynamic calculations, Nb depletion in its gamma matrix, due to selective partitioning of Nb into Laves phase and delta phase particles, can also contribute to formation of the porous passive film.
KEYWORDS Additive Manufacturing; Inconel 718 Superalloy; Corrosion Behavior; Microstructure
*Corresponding author Tel: 1-204-474-7693; E-mail: [email protected]; Present address: Room E2-327 Department of Mechanical Engineering, 75A Chancellors Circle, University of Manitoba, Winnipeg, MB. Canada. R3T5V6
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1. INTRODUCTION Nickel-based superalloys have been extensively applied in various industries due to their excellent combination of mechanical properties and corrosion resistance both at room and elevated temperatures. INCONEL 718 (IN718) superalloy, as one of the most highly utilized nickel-based alloys, is used for hot section components in aerospace industry, nuclear reactor parts in nuclear application, oil drilling shafts in oil and gas exploration, as well as some critical supporting structures and fasteners [1-5]. However, conventional fabrication methods for IN718 superalloy are quite complex in processing routes [6-8]. Moreover, it is significantly challenging to manufacture IN718 components with complicated geometries, and also it is likely impossible to fix worn parts using traditional fabrication techniques [9-10]. Hence, intensive efforts have been invested in developing more efficient and effective manufacturing technologies to address the problems as mentioned above regarding the traditional manufacturing methods.
In the recent decade, there has been a rapidly growing interest in fabricating and repairing IN718 components employing additive manufacturing (AM) technology [8-9, 11-14]. The AM is a layer-by-layer manufacturing process which can produce geometrically complex components with highly precise dimensions, and also shorten processing steps and reduce production costs [15-17]. Wire arc additive manufacturing (WAAM), one of the most commonly used AM techniques, employs an electric arc or plasma as the heat source and a welding wire as the feedstock. Compared to other AM methods, the WAAM is outstanding because of its high deposition rate, unlimited deposition capabilities, relatively low costs as well as environmental friendliness [16, 18]. So far, research work on IN718 alloy fabricated by WAAM (WAAM IN718) is mainly focused on studied of microstructure and mechanical properties [19-21]. Xu et al. systematically investigated the effect of oxide, wire source, and heat treatment on mechanical properties of WAAM IN718 [19]. Their results show that the strength of the heat-treated WAAM IN718 is lower than the wrought alloy, and the main reason is due to the formation of large columnar grain structure in the WAAM IN718.
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Nevertheless, apart from the mechanical performance, corrosion behavior in aggressive circumstances is equally significant in affecting the durability and service life of IN718 components. For instance, turbine vanes being made of IN 718 may suffer from roomtemperature corrosion during downtime, and downhole drilling parts which are fabricated by IN 718 fail in service environments with different corrosive media [4, 9, 22]. Thus, the corrosion resistance of IN718 alloy in ambient conditions is another important fundamental property which is worthwhile to study for practical applications. Presently, limited work is available in the literature on the corrosion performance study of WAAM IN718 alloy. It is known that heat treatment is a necessity for IN718 to optimize microstructure and achieve desirable properties. Based on previous studies, the microstructure of WAAM IN718 is dramatically different from that of the wrought IN718 even after identical heat treatments [19]. However, it is not clear yet how the corrosion behavior will be for the heat-treated WAAM IN 718 alloy with the distinctive microstructure.
The objective of this study is to investigate the corrosion performance of the heat-treated WAAM IN718 (WAAM-HT IN718) under different corrosion environments in comparison to the heat-treated wrought IN718 (wrought-HT IN718). The techniques of scanning electron microscope (SEM) equipped with both electron backscatter diffraction (EBSD), and energy dispersive spectroscopy detectors (EDS) are employed for microstructural analyses. The corrosion performance of the materials in corrosive environments are analyzed by potentiodynamic
polarization,
potentiostatic
polarization,
electrochemical
impedance
spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS).
2. EXPERIMENTAL 2.1. Material preparation IN718 superalloy was used for both substrate material and filler wire in this study. Its nominal chemical composition (wt. %) is Fe-17.60, Cr-18.00, Nb-5.40, Mo-3.00, Ti-0.94, Al-0.46, Mn0.05, Si-0.10, and Ni-balance. The as-deposited IN718 specimens were produced using a
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tungsten inert gas (TIG) welding system attached to a 6-DOF Panasonic VR-004 robot. The filler wire with a diameter of 0.787mm was fed by a wire feeder of the 6-DOF Panasonic VR-004 robot into an arc on the substrate plate. The WAAM process was carried out using the following optimized parameters: arc current of 100 A, arc length of 3.0 mm, wire feed speed of 0.5 m/min, travel speed of 0.13 m/min. A straight-wall structure with a geometric size of 80mm × 10mm × 25mm was deposited in an argon protection environment. Experimental specimens were sectioned from the as-deposited part in an orientation parallel to the build direction with physical dimensions of 10 mm × 5 mm × 10 mm. A two-step heat treatment was applied on the asdeposited WAAM IN718 specimens: solution treatment at 1010 ºC for 1 hour, water quenching to room temperature; aging treatment, i.e. holding at 720 ºC for 8 hours followed by furnace cooling to 620 ºC in 20 min, and then holding at 620 ºC for 8 hours and air cooling to room temperature. In this study, the commercial wrought IN 718 plate with the composition (wt.%) 17.9% Cr, 17.6% Fe, 5.4 % Nb, 0.94% Ti, 0.46% Al, 2.9% Mo, 0.13% Co, 0.05% Si, 0.05% Mn, and balance Ni was used as a reference. The wrought IN 718 alloy was produced using vacuum induction melting (VIM) plus vacuum arc remelting (VAR); then followed by homogenizing at 1163 °C for 16 hours, forging to billet at 1093 °C, and further rolling to plates at 1066 °C [23-24]. The wrought IN718 specimens with dimensions 10 mm × 5 mm × 10 mm were subjected to identical two-step heat treatment as WAAM IN718 were used for comparative study.
2.2. Microstructure characterization The crystallographic orientation was identified by using an Oxford Instrument Nordlys EBSD detector attached to a scanning electron microscope (SEM, Philips XL 30). The specimens were ground by wet silicon carbide paper with decreasing grit size (180, 600, 800 1200), then mechanically polished to 1 µm and finally followed by vibratory polishing with 0.04 µm colloidal silica solution for 8 hours to obtain mirror-like and strain-free surfaces. The EBSD data were analyzed by using HKL Channel 5 software. Microstructure of the materials were examined employing the SEM equipped with an energy dispersive spectroscopy (EDS). The specimens were mechanically polished to 1 µm and then etched using a solution of 15 g chromium trioxide,
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10 ml sulfuric acid, and 150 ml phosphoric acid for 15 to 20 seconds at a voltage of 5 V.
2.3. Electrochemical tests Electrochemical corrosion experiments were carried out with a three-electrode electrochemical cell (volume of 500 ml), using a saturated calomel electrode (SCE) for a reference electrode and carbon rods as counter electrodes. The tests were performed using a Princeton Applied Research Potentiostat with VersaStudio analysis software. Considering adequate study has not yet been performed in corrosive acid environments, the corrosion behavior of materials was studied in 1M nitric acid (1M HNO3), and 1M sulfuric acid (1M H2SO4). Specimens for electrochemical corrosion tests were mounted in bakelite leaving exposed working areas of 0.5 cm2, mechanically polished to 1 µm, then followed by rinsing in acetone and distilled water. The freshly prepared specimens were immediately assembled and transferred into the corrosion cell filled with the electrolytes as mentioned above to avoid air-formed oxides. Before each electrochemical polarization measurement, an open circuit potential measurement was conducted for 1 hour to achieve a relatively stabilized potential.
Potentiodynamic polarization tests were performed with a scan rate of 1 mV/s. The polarization curves were used to characterize corrosion current density (icorr), critical current density (icrit), passivation current density (ipass), corrosion potential (Ecorr), passivation potential (Epp), and passivation potential range (∆Epp). To obtain details of corrosion performance, specifically, the passivation behavior of materials, potentiostatic polarization and electrochemical impedance spectroscopy (EIS) techniques were employed as well. The potentiostatic polarization experiments were conducted at a passivation potential of 1 V for 1 hour in 1 M H2SO4 solution to form passive films on the specimens and followed by the EIS measurements to further characterize the corrosion behavior of the materials. The EIS spectra were recorded at an AC potential amplitude of 10 mV with a frequency range of 10-2-104 Hz. The impedance data were
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analyzed by using ZSimpWin software. All experiments were conducted at room temperature and repeated at least three times to ensure reproducibility.
X-ray photoelectron spectroscopy (XPS) analysis was carried out on the specimens after potentiostatic polarization to characterize the chemical composition of passive films formed on the materials. All XPS spectra were collected using a Kratos Axis Ultra DLD X-ray Photoelectron Spectrometer with an Al
(1486.6 eV) monochromatic radiation source under a
vacuum pressure below 10-8 Torr. For each specimen, a survey scan was recorded firstly and then followed by high-resolution scans of Cr 2p, Ni 2p, Fe 2p, Mo 3d, and Nb 3d regions. The C 1s peak with a binding energy of 285 eV was used as a reference for calibration. All highresolution spectra were fitted using Gaussian-Lorentzian function after Shirley background subtraction with Casa XPS software. Position constraints were set according to Moulder [25]. Peak identifications were carried out by reference to the NIST XPS database.
3. RESULTS and DISSCUSSION 3.1. Microstructure analysis 3.1.1. WAAM-HT IN718 Figure 1 shows an inverse pole figure (IPF) colored orientation map (OIM), as well as the corresponding pole figure of WAAM-HT IN718. The microstructure of the WAAM-HT IN718 specimen mainly consists of columnar grains with long axis almost parallel to the build direction. Most of the grains exhibit a strongly preferred orientation close to (001), indicating that the microstructure is significantly textured (depicted in red in Figure 1a). The maximum intensity of the {100} pole figure is about 28.70 times that of the randomly oriented planes near the center. (Figure 1c). The EBSD results indicate that the predominant texture component of WAAM-HT IN718 specimen is {100} <001> (Cube) texture.
SEM micrographs of WAAM-HT IN718 specimen as given in Figure 2 show strip-like distributions of secondary phases (white precipitates). The irregularly island-shaped precipitates
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are Laves phases [26]. A large amount of needle/plate-like δ phases are also detected and a lot of them associate with the Laves particles which was observed and explained in previous studies [20, 27-29]. Apart from Laves and δ phases, some spherical/square-shaped particles are identified as (Nb, Ti)C carbides.
Fig. 1 EBSD analysis of the WAAM-HT IN718 specimen (a) inverse pole figure colored orientation image map (OIM), (b) index map of IPF and the reference coordinate, (c) pole figure of {100}
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Fig. 2 SEM micrographs showing microstructure of WAAM-HT IN718 specimens (a) low magnification, (b) high magnification
3.1.2. Wrought-HT IN718 Microstructure characteristics of wrought-HT IN718 specimens were also investigated for comparison with those of the WAAM-HT IN718. In the EBSD map shown in Figure 3, the whole region is composed of equiaxed grains, indicating the microstructure contains randomly oriented grains with a variety of crystallographic planes (Figure 3a). Compared to the WAAMHT IN718, the {100} pole figure of the wrought-HT sample is fairly weak without the maximum intensity in the center (Figure 3c), which further confirms the untextured microstructure. Also, no Laves particles are observed for the wrought-HT IN 718 specimen, as presented in Figure 4.
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Fig. 3 EBSD analysis of the wrought-HT IN718 specimen (a) inverse pole figure colored orientation image map (OIM), (b) index map of IPF and the reference coordinate, (c) pole figure of {100}
Fig. 4 SEM micrographs showing microstructure of wrought-HT IN718 specimens (a) low magnification, (b) high magnification
3.2. Corrosion behavior 3.2.1. Potentiodynamic polarization analysis The corrosion behavior of WAAM-HT IN718 was investigated in comparison to wrought-HT alloy using different electrochemical techniques. Figure 5 illustrates the potentiodynamic polarization curves of WAAM-HT and wrought-HT specimens in 1M HNO3 and 1M H2SO4. As can be seen, a typical active-passive-transpassive behavior is observed for both specimens under
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the aforementioned corrosive environments. To evaluate corrosion characteristics of the specimens, electrochemical parameters: corrosion current density (icorr), corrosion potential (Ecorr), critical current density (maximum current density of active-passive transition, icrit), passivation current density (ipass), and passivation potential range (∆Epass) are measured from the polarization plots. Tables I and II summarize the electrochemical characteristics of the specimens obtained from the potentiodynamic polarization tests in HNO3 and H2SO4, respectively. In the 1M HNO3 (Table I), the value of icorr of WAAM-HT IN718 is about 1.4 times higher than that of the wrought-HT IN718. Furthermore, significant differences are observed in the passive region. The values of icrit and ipass of WAAM-HT specimen are over an order of magnitude higher than those of the wrought-HT sample. It is known that excellent corrosion resistance of nickel-based superalloys and stainless steels is attributable to the formation of protectively passive films [3031]. Generally speaking, the higher passive current density is, the more difficult is the formation of passive film, resulting in poorer corrosion resistance in the corresponding corrosive environment [32]. It is clear that compared the wrought-HT specimen, the ipass of the WAAMHT sample is more than ten times higher, which indicates the corrosion resistance of WAAMHT IN718 is significantly lower than that of the wrought-HT IN718 in 1M HNO3. Also, the passivation potential range of ∆Epass usually describes the stability of the passive film. The ∆Epass of WAAM-HT IN718 is smaller than that of the wrought-HT IN718, which suggests less stability of the passive film formed on WAAM-HT IN718 compared to the one that formed on the wrought-HT sample. Similar to the case in 1M HNO3, the WAAM-HT IN718 shows higher values of icorr, icrit, ipass, and narrower ∆Epass in comparison to those of the wrought-HT sample in the 1M H2SO4 (Table II). This indicates that the WAAM-HT specimen exhibits lower corrosion resistance than the wrought-HT specimen in 1M H2SO4 as well. Hence, the results demonstrate that compared to the wrought-HT IN718, the WAAM-HT IN718 exhibits inferior resistance to corrosion in the two different corrosive environments.
To further understand corrosion behavior, another importance electrochemical technique, electrochemical impedance spectroscopy (EIS), was used and discussed next.
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Fig. 5 Potentiodynamic polarization curves of WAAM-HT IN718 specimen in comparison to wrought-HT specimen in different corrosion environments at room temperature (a) 1M HNO3 solution, (b) 1M H2SO4 solution
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Table I. Corrosion Characteristics of WAAM-HT IN718 in Comparison to the Wrought-HT Specimen in 1 M HNO3 at Room Temperature Specimen
icorr (µA/ cm2)
Ecorr (mV)
icrit (µA/ cm2)
ipass (µA/ cm2)
Epp (mV)
Passive range ∆Epass (mV)
WAAM-HT
16.0±0.7
471.5±1.5
523±31
421±57
708±6.3
756±1.3
wrought-HT
11.6±0.8
510±8.5
66±2.9
27.6±4.9
590±6.1
850±5.4
Table II. Corrosion Characteristics of WAAM-HT IN718 in Comparison to the Wrought-HT Specimen in 1M H2SO4 at Room Temperature
WAAM-HT
icorr (µA/ cm2) 3.89±0.1
Ecorr (mV) 371.67±38
icrit (µA/ cm2) 541±11.7
ipass (µA/ cm2) 251±37
Epp (mV) 678±3.2
Passive range ∆Epass (mV) 808±3.5
wrought-HT
2.80±0.4
400.67±56
83.1±3.5
22.7±7.6
555±6.8
891±6.3
Specimen
3.2.2. Electrochemical impedance spectroscopy (EIS) measurements Electrochemical impedance spectroscopy (EIS), as a versatile non-destructive technique, provides a unique approach to directly investigate passive film properties and evaluate the corrosion resistance behavior of materials [33]. EIS measurements were performed on both WAAM-HT and wrought-HT specimens after potentiostatic polarization at 1 V for 1 hour in 1 M H2SO4 at room temperature. The impedance spectra are presented in both Nyquist and Bode forms, as shown in Figure 6. The Nyquist plots exhibit incomplete semicircle arcs (Figure 6a), which are due to charge transfer processes at the electrolyte/electrode interface [34]. Normally, the diameter of the semicircular arc is related to the corrosion resistance of the material, i.e. the larger the diameter is, the better corrosion resistance of the material [35]. As can be seen in Figure 6, the diameter of semicircular arc of WAAM-HT IN718 is markedly less than that of the wrought-HT IN718. This implies that the corrosion resistance of WAAM-HT IN718 is considerably lower than the wrought-HT alloy, which is in good agreement with potentiodynamic polarization results. In the Bode diagram (log modulus of impedance versus log frequency), as shown in Figure 6(b), the impedance modulus value (|Z|) at high frequency (f >1k Hz) represents the solution resistance, while the value of |Z| at low frequency (f <1 Hz) 12
corresponds to the sum resistance which approximates to the polarization resistance of material surface [36]. It is obvious that the polarization resistance of the WAAM-HT IN718 is significantly lower than that of the wrought-HT IN718, which is strongly affected by the properties of surface films. Moreover, two phase-angle peaks are observed for both specimens in Bode-phase plots (phase angle versus log frequency), as shown in Figure 6(c), which suggest two time constants. The phase angle maxima at low frequency is attributable to the formation of protective passive films, while the maxima at high frequency is due to the formation of electrical double layers [33, 37-38]. It can be seen that the wrought-HT IN718 displays a higher phase angle maximum at low frequency compared to the WAAM-HT IN718, which indicates that the passive film formed on wrought-HT specimen exhibits a better protective capability.
Figure 7 depicts the equivalent circuit used for fitting the measured EIS spectra. In this circuit, Rs refers to the solution resistance, Qp represents the passive film capacitance, Rp stands for the passive film resistance, Qdl corresponds to the double-layer capacitance, and Rct is the charge transfer resistance between the matrix and the passive film. The constant phase element (CPE) is used to describe capacitive behavior, and the impedance of CPE is defined by the following Equation (1): = 1⁄
(
)
(-1<
n
<1)
(1) where
represents the magnitude of CPE, j is the imaginary number (j2=-1),
frequency (in rad/s), and
is the CPE exponential factor. The CPE is used to describe several
behavior depending on the value of : Warburg element,
=1, represents a perfect capacitance,
=0, represents a resistance,
Normally, the value of
is the angular
=0.5, represents a
=-1, represents pure inductance [39-40].
ranges from 0 to 1 in experimental conditions.
The fitted results are plotted in solid lines as included in Figure 6. The values of fitting parameters are given in Table III. As can be seen, the small values of chi-squared χ2 (≈10-3) reveal good correlations between experimental and simulated data for both specimens. The Rp reflects the protective property of passive film which is vitally dependent on characteristics of the passive film. Generally, a high value of Rp indicates a strong protective capability of the passive film and superior corrosion resistance of the material [41]. Also, the Qp demonstrates the
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diffusion capability of ionic species within the passive film, and a low value of Qp shows a weak capability of ionic diffusion within passive film [42]. There are significant differences in Rp and Qp for WAAM-HT and wrought-HT specimens, as presented in Table III, i.e. the WAAM-HT sample shows a lower value of Rp, while a higher value of Qp compared to the wrought-HT specimen. This result implies that the passive film formed on the WAAM-HT specimen displays weaker protection capability, corresponding to inferior corrosion resistance. Therefore, the EIS measurements are consistent with potentiodynamic polarization results.
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Fig. 6 Electrochemical impedance spectroscopic (EIS) data of WAAM-HT IN718 specimen in comparison to the wrought-HT IN 718 specimen in 1M H2SO4 after potentiostatic polarization at 1 V for 1 hour at room temperature (a) Nyquist plots, (b) Bode plots - impedance, (c) Bode plots - phase angle
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Fig. 7 Electrical equivalent circuit model applied to simulate EIS spectra of IN718 specimens
Table III. Equivalent Circuit Parameters of EIS Data of WAAM–HT IN718 and Wrought–HT IN718 Specimens in 1M H2SO4 after Potentiostatic Polarization at 1 V for 1 Hour at Room Temperature Specimen
Rs (Ω cm2)
WAAM-HT
0.17
wrought-HT
7.4
ndl
Rct (Ω cm2)
Qp (F s / cm2)
np
Rp (Ω cm2)
χ2
2.24×10-4
0.81
9.80×10
2.19×10-3
0.69
4.03×102
2.6×10-3
7.72×10-5
0.83
1.09×104
6.44×10-4
0.83
2.09×104
3.5×10-3
(F s
Qdl (n-1)
2
/ cm )
(n-1)
3.2.3. X-ray photoelectron spectroscopy (XPS) analysis The results obtained from both potentiodynamic polarization and EIS measurements conclude that the corrosion resistance of the WAAM-HT IN718 is inferior to that of the wrought-HT IN718, which is mainly due to the difference in passivation. Research work performed by Peng et al. demonstrated that the protective properties of passive films depend upon both its chemical composition and structure [43]. To further investigate the nature of passivation, XPS analysis was carried out to examine the chemical composition of the passive films formed on WAAM-HT and wrought-HT specimens. Survey scans were carried out to identify all detectable elements in the passive films formed on specimens after potentiostatic polarization at 1 V for 1 hour in 1 M H2SO4. Figure 8 shows the XPS survey spectra, and C 1s, O 1s, Ni 2p, Cr 2p, Fe 2p, Mo 3d, and
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Nb 3d peaks are detected. High-resolution spectra of primary compounds are deconvoluted after Shirley background subtractions. The Cr 2p spectra of WAAM-HT and wrought-HT samples are shown in Figures 9(a) and (b) respectively, which consist of two peaks at the binding energy (BE) of 576.2 ± 0.3 eV and 586.3 ± 0.2 eV representing Cr2O3, and their associated satellite peaks at 573.9 ±0.2 eV and 583.5 ±0.1 eV represent metallic Cr. Figure 10(a) shows the Ni 2p spectra recorded from the surface of WAAM-HT specimens, and the peaks decompose into metallic Ni at the BE of 852.4 eV and 870.0 eV, and NiO oxide with BE of 855.5 eV and 873.8 eV. The identification of Ni chemical states in the passive film formed on the wrought-HT specimens is presented in Figure 10(b). One signal located at the BE of 855.5 eV is assigned as NiO oxide, and the other two peaks at 852.4 eV and 870.0 eV are Ni metal. The presences of Cr in 3+ and 0 states, and Ni in 2+ and 0 states are in agreement with published studies [41, 44]. The XPS spectra of Fe 2p shown in Figures 11(a) and (b) indicate that Fe peaks of WAAM-HT and wrought-HT specimens decompose into metallic Fe and Fe2O3 oxide with the BE of 706.7 ± 0.2 eV and 711.3 ±0.3 eV, respectively. V. Maurice et al. detected Fe2O3 in the oxide layer formed on Fe-18Cr-13Ni single-crystal alloy as well [45]. The Mo 3d spectra in Figure 12 represent the oxide state of MoO3 at the BE of 232.4 ± 0.3 eV and 235.6 ±0.2 eV, along with metallic Mo at the BE of 227.4 ± 0.3 eV and 231.0 ± 0.1eV, obtained from the WAAM-HT and wrought-HT specimens. Llody and Chen reported the existence of MoO3 in passive films of nickel-based superalloys in different corrosive environments [44, 46]. The XPS spectra of Nb 3d show that two peaks located at the BE of 206.9 ± 0.2 eV and 209.9 ±0.2 eV belong to Nb2O5, one peak with BE 204.7 ±0.1 eV belongs to NbO, and one peak at 202.5 ± 0.3 eV corresponds to the metallic Nb, as shown in Figure 13. Previous study shows that the Nb2O5 and NbO oxides exist in the passive layer formed on nickel-based superalloy Inconel 625 [47]. The fitting results reveal that Cr 2p, Ni 2p, Fe 2p, Mo 3d, and Nb 3d peaks are made up of metallic and oxidized metallic components in the passive films formed on both WAAM-HT and wrought-HT specimens.
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Fig. 8 XPS survey scan peaks of passive films formed on WAAM-HT IN718 and wrought-HT specimen after potentiostatic polarization at 1V for 1hour in 1M H2SO4
Fig. 9 High-resolution XPS spectra of Cr 2p of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718
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Fig. 10 High-resolution XPS spectra of Ni 2p of passive films formed on different specimens (a)WAAM-HT IN718, (b) wrought-HT IN718
Fig. 11 High-resolution XPS spectra of Fe 2p of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718
Fig. 12 High-resolution XPS spectra of Mo 3d of passive films formed on different specimens
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(a) WAAM-HT IN718, (b) wrought-HT IN718
Fig. 13 High-resolution XPS spectra of Nb 3d of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718
Table IV summarizes the composition of oxide films determined from the integrated intensity of XPS spectra of WAAM-HT and wrought-HT samples. The result shows that significant differences are observed in the quantity of Cr2O3 and NiO in the passive films formed on WAAM-HT and wrought-HT specimens. To be specific, the percentage of NiO content (~25.65 at. %) in the passive film formed on the surface of the WAAM-HT sample is over four times higher than that of the wrought-HT specimen (~5.87 at.%). The percentage of Cr2O3 in passive films formed on WAAM-HT IN718 is about 12 at.% lower as compared to the that of the wrought-HT specimen. Moreover, the surface oxide films of WAAM-HT IN718 contain lesser MoO3 compared to the wrought-HT specimen. It has been found that oxide films enriched in Cr2O3 and MoO3 are normally dense, stable and protective, resulting in low passive current densities and superior resistance to corrosion [48-49]. On the contrary, the passive films that contain high NiO exhibit porous structure and low protective capabilities [41]. Thus, the XPS results suggest that WAAM-HT IN718 and wrought-HT718 display different corrosion performance due to difference in the composition of their surface passive films.
Additionally, the porosity of passive film can affect the corrosion performance of the alloy as well. However, the passive film is normally too thin (≈ a few nm) to examine its morphology via conventional optical microscope and SEM. Thus, the further study of structural characteristic of
20
the passive films was performed based on potentiostatic polarization and the results are discussed next.
Table IV. Chemical Composition of Passive Films Formed on WAAM-HT IN718 and Wrought-HT Specimen from XPS Analysis (at. %) Specimen
Cr2O3
NiO
Fe2O3
MoO3
Nb2O5
NbO
WAAM-HT IN718
47.40
25.65
10.43
3.79
9.92
2.81
wrought-HT IN718
59.12
5.87
12.22
12.49
8.95
1.35
3.2.4. Potentiostatic polarization analysis The variation of anodic current density with time was measured at a potential within the passivation region of the materials by potentiostatic polarization technique. The initial decrease of current density is related to passive film growth on the surface of the samples, if the effect of double-layer charge is neglected [50]. The current density decreases with time according to the following Equation (2) i = 10
(
)
(2)
where i is the current density, t is the time, k and b are constants. In a double-logarithmic plot of Equation (2), k is the slope of the plot, and k = -1.0, indicates a high field-controlled surface film formation that produces well compact passive film with highly protective property, while k = 0.5, indicates a diffusion-controlled surface film formation that produces porous film with weakly protective capability [41, 51-52].
As can be seen from the double-logarithmic plots presented in Figure 14, the slope k of the WAAM-HT sample is -0.34 at the initial stage, and changes to -0.67 at the later stage. In the case of the wrought –HT specimen, the initial slope value of k is -0.53 and then approaches the values of -0.96 and -0.85. The variation in the slope values indicates that the passive film of the
21
WAAM-HT specimen is consistently porous, while the oxide film formed on the wrought-HT sample is porous initially but then becomes relatively compact. These results agree with the XPS results, which show that there is more NiO, which is known to be porous [41], formed on WAAM-HT specimen compared to the wrought-HT sample. It has been reported that passive films with porous structure exhibit low protective capability, which degrade corrosion resistance of the materials [41]. Thus, the relatively more porous passive film formed on WAAM-HT IN718 can explain its inferior corrosion resistance compared to the wrought-HT specimen.
Fig. 14 Double-logarithmic plots of current-time for WAAM-TH IN718 and wrought-HT IN718 as obtained from potentiostatic polarization tests (at 1 V for 1hour in 1M H2SO4)
3.3. Possible influence of microstructure on the formation of surface passive films The XPS results and double-log plots show that the passive films formed on WAAM-HT IN718 and wrought-HT IN718 are different in terms of chemical composition and structure, resulting in their different resistance to corrosion. Specifically, the passive film formed on the WAAM-HT specimen with porous structure contains 25.65 at. % NiO and 47.40 at.% Cr2O3 , while there is less porous film formed on the wrought-HT alloy which comprises 5.87 at.% NiO and 59.12 at.% Cr2O3. It is known that microstructure characteristics play important roles in the corrosion
22
behavior of materials. The formation of a passive film that contains more NiO and less Cr2O3 on WAAM-HT specimen compared to the wrought-HT specimen could relate to some microstructural factors. According to the microstructure analysis performed in this work, the WAAM-HT IN718 specimen exhibits distinctly coarse elongated grain structure with a strongly preferred orientation of (001), which is significantly different from the wrought-HT IN718 specimen with randomly oriented equiaxed grains. The difference of formation of passive film on the WAAM-HT specimen could be related to the {100} <001> (Cube) texture of the material. Bonfrisco et al. studied the effects of crystallographic orientation on the early stages of oxidation behavior of nickel (Ni) and chromium (Cr) [53]. Their results show that the oxidation rate of Ni changes with surface orientation in the order of (111) < (011) < (001), while the oxidation rate of Cr increases in the order of (001) < (011) < (111). Therefore, the {100} <001> (Cube) texture may have enhanced the formation of porous NiO at the expense of compact Cr2O3 in the passive film formed on the surface of WAAM-HT specimen. Secondly, another possible factor is related to the micro-segregation of Nb, which occurs during the solidification stage of the WAAM process. The micro-segregation of Nb during solidification of IN 718 superalloy is known to result in the formation of Laves phase micro-constituent and enhances the formation of delta phase particles during heat treatment [10, 28]. Since Laves phase and delta phase particles contain high concentrations of Nb, their formations are normally accompanied by a decrease of Nb concentration in the surrounding gamma matrix. As stated earlier, in the present work, Laves phase particles are observed only in the WAAM-HT IN718 and not in the wrought-HT specimen. Also, the formation of delta phase particles is observed to be more pronounced in the WAAMHT IN718 compared to the wrought-HT specimen. Accordingly, averaged concentration of Nb in the gamma matrix region of the WAAM-HT IN718 is lower than the Nb content in the gamma matrix of wrought-HT specimen, as shown in Figure 15. One major factor that can influence the formation of NiO and Cr2O3 is the values of activity of Ni and Cr in the alloy. Thermodynamic calculations by JMatPro software were performed to evaluate the influence of decrease in Nb concentration on the activity values of Ni and Cr in alloy IN 718 and the results are presented in Figure 16. It shows that the decrease in Nb concentration increases the activity of Ni, but reduces the activity of Cr. Therefore, the reduction in Nb concentration in the gamma matrix due to the formation of Laves phase and delta phase particles may cause an increase in the activity of Ni and a decrease in the activity of Cr for the WAAM-HT IN718 compared to the wrought-HT
23
specimen. Such changes in the activity values may cause an increase in the affinity of Ni and a decrease in the affinity of Cr for oxygen to form oxides, which could explain the observation of more NiO and lesser Cr2O3 in the surface passive film of WAAM-HT IN718 compared to that of the wrought-HT specimen.
Fig. 15 Comparison of Nb concentration in the γ matrixes of WAAM-HT IN718 and wrought-HT IN718 (a) SEM micrograph of WAAM-HT IN718, (b) composition profile acquired using EDS
24
Fig. 16 Effect of Nb content on activity values of Ni and Cr for IN 718 alloy (a) activity of Ni, (b) activity of Cr
4. CONCLUSIONS 1. The microstructure of WAAM-HT IN718 is characterized by columnar grains with a strongly preferential crystal orientation of (001) and Nb-rich microconstituents distributed along the build direction.
2. Potentiodynamic polarization and EIS measurements reveal that the WAAM-HT IN718 exhibits lower corrosion resistance in comparison to the wrought-HT alloy in acidic environments at room temperature.
3. Analyses of XPS data and double-log plots of potentiostatic data show that the passive film formed on WAAM-HT IN718 contains lesser and higher extents of Cr2O3 and NiO respectively, and as such is more porous and exhibits lower corrosion protection compared to the film formed on the wrought-HT alloy.
4. The formation of more NiO and lesser Cr2O3 in the passive film formed on the WAAM-HT IN718 could be related to its {100} <001> (Cube) texture, and Nb depletion in its gamma matrix, due to selective partitioning of Nb into Laves phase and delta phase particles.
25
ACKNOWLEDGEMENT Financial support from NSERC of Canada is gratefully acknowledged
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Figure captions Fig. 1 EBSD analysis of the WAAM-HT IN718 specimen (a) inverse pole figure colored orientation image map (OIM), (b) index map of IPF and the reference coordinate, (c) pole figure of {100}
Fig. 2 SEM micrographs showing microstructure of WAAM-HT IN718 specimens (a) low magnification, (b) high magnification
Fig. 3 EBSD analysis of the wrought-HT IN718 specimen (a) inverse pole figure colored orientation image map (OIM), (b) index map of IPF and the reference coordinate, (c) pole figure of {100} Fig. 4 SEM micrographs showing microstructure of wrought-HT IN718 specimens (a) low magnification, (b) high magnification
Fig. 5 Potentiodynamic polarization curves of WAAM-HT IN718 specimen in comparison to wroughtHT specimen in different corrosion environments at room temperature (a) 1M HNO3 solution, (b) 1M H2SO4 solution Fig. 6 Electrochemical impedance spectroscopic (EIS) data of WAAM-HT IN718 specimen in comparison to the wrought-HT IN 718 specimen in 1M H2SO4 after potentiostatic polarization at 1 V for 1 hour at room temperature (a) Nyquist plots, (b) Bode plots - impedance, (c) Bode plots - phase angle
Fig. 7 Electrical equivalent circuit model applied to simulate EIS spectra of IN718 specimens
Fig. 8 XPS survey scan peaks of passive films formed on WAAM-HT IN718 and wrought-HT specimen after potentiostatic polarization at 1V for 1hour in 1M H2SO4
Fig. 9 High-resolution XPS spectra of Cr 2p of passive films formed on different specimens (a) WAAMHT IN718, (b) wrought-HT IN718 Fig. 10 High-resolution XPS spectra of Ni 2p of passive films formed on different specimens (a) WAAMHT IN718, (b) wrought-HT IN718 Fig. 11 High-resolution XPS spectra of Fe 2p of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718 Fig. 12 High-resolution XPS spectra of Mo 3d of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718 Fig. 13 High-resolution XPS spectra of Nb 3d of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718
Fig. 14 Double-logarithmic plots of current-time for WAAM-TH IN718 and wrought-HT IN718 as obtained from potentiostatic polarization tests (at 1 V for 1 hour in 1M H2SO4) Fig. 15 Comparison of Nb concentration in the γ matrixes of WAAM-HT IN718 and wrought-HT IN718 (a) SEM micrograph of WAAM-HT IN718, (b) composition profile acquired using EDS
Fig. 16 Effect of Nb content on activity values of Ni and Cr for IN 718 alloy (a) activity of Ni, (b) activity of Cr
Table I. Corrosion Characteristics of WAAM-HT IN718 in Comparison to the Wrought-HT Specimen in 1 M HNO3 at Room Temperature Specimen
icorr (µA/ cm2)
Ecorr (mV)
icrit (µA/ cm2)
ipass (µA/ cm2)
Epp (mV)
Passive range ∆Epass (mV)
WAAM-HT
16.0±0.7
471.5±1.5
523±31
421±57
708±6.3
756±1.3
wrought-HT
11.6±0.8
510±8.5
66±2.9
27.6±4.9
590±6.1
850±5.4
Table II. Corrosion Characteristics of WAAM-HT IN718 in Comparison to the Wrought-HT Specimen in 1M H2SO4 at Room Temperature
WAAM-HT
icorr (µA/ cm2) 3.89±0.1
Ecorr (mV) 371.67±38
icrit (µA/ cm2) 541±11.7
ipass (µA/ cm2) 251±37
Epp (mV) 678±3.2
Passive range ∆Epass (mV) 808±3.5
wrought-HT
2.80±0.4
400.67±56
83.1±3.5
22.7±7.6
555±6.8
891±6.3
Specimen
Table III. Equivalent Circuit Parameters of EIS Data of WAAM–HT IN718 and Wrought–HT IN718 Specimens in 1M H2SO4 after Potentiostatic Polarization at 1 V for 1 Hour at Room Temperature Specimen
Rs (Ω cm2)
Qdl (F s(n-1)/ cm2)
ndl
Rct (Ω cm2)
Qp (F s(n-1)/ cm2)
np
Rp (Ω cm2)
χ2
WAAM-HT
0.17
2.24×10-4
0.81
9.80×10
2.19×10-3
0.69
4.03×102
2.6×10-3
wrought-HT
7.4
7.72×10-5
0.83
1.09×104
6.44×10-4
0.83
2.09×104
3.5×10-3
Table IV. Chemical Composition of Passive Films Formed on WAAM-HT IN718 and Wrought-HT Specimen from XPS Analysis (at. %) Specimen
Cr2O3
NiO
Fe2O3
MoO3
Nb2O5
NbO
WAAM-HT IN718
47.40
25.65
10.43
3.79
9.92
2.81
wrought-HT IN718
59.12
5.87
12.22
12.49
8.95
1.35
Highlights Corrosion behavior of WAAM IN718 was studied by various electrochemical methods. The WAAM IN718 exhibits inferior corrosion resistance to wrought alloy. XPS results show that the inferior performance is related to porous passive film. Thermodynamic calculations suggest porous film formed due to Nb micro-segregation.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30818-5
DOI:
https://doi.org/10.1016/j.jallcom.2020.154455
Reference:
JALCOM 154455
To appear in:
Journal of Alloys and Compounds
Received Date: 6 December 2019 Revised Date:
19 February 2020
Accepted Date: 21 February 2020
Please cite this article as: L.N. Zhang, O.A. Ojo, Corrosion behavior of wire arc additive manufactured Inconel 718 superalloy, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.154455. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit Author Statement Lina Zhang: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Writing-Original draft. Olanrewaju A. Ojo: Supervision, Reviewing, and Editing.
Corrosion Behavior of Wire Arc Additive Manufactured Inconel 718 Superalloy L.N. Zhang*, O. A. Ojo Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, Canada R3T 5V6
ABSTRACT The corrosion behavior of Inconel 718 (IN718) superalloy fabricated using wire-arc additive manufacturing (WAAM) and post-deposition heat treatment is investigated and compared to that of wrought alloy. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) analyses show that the corrosion resistance of heat-treated WAAM IN718 (WAAM-HT IN718) is inferior to that of heat-treated wrought alloy (wrought-HT). Potentiostatic polarization and X-ray photoelectron spectroscopy (XPS) are used to analyze passivation properties of the materials. The results indicate that in comparison to the wrought-HT material, the passive film formed on the WAAM-HT IN718 contains more NiO, which is porous and less protective than Cr2O3. The formation of more NiO and lesser Cr2O3 in the passive film formed on the WAAMHT IN718 could be related to its {100} <001> (Cube) texture. Moreover, based on thermodynamic calculations, Nb depletion in its gamma matrix, due to selective partitioning of Nb into Laves phase and delta phase particles, can also contribute to formation of the porous passive film.
KEYWORDS Additive Manufacturing; Inconel 718 Superalloy; Corrosion Behavior; Microstructure
*Corresponding author Tel: 1-204-474-7693; E-mail: [email protected]; Present address: Room E2-327 Department of Mechanical Engineering, 75A Chancellors Circle, University of Manitoba, Winnipeg, MB. Canada. R3T5V6
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1. INTRODUCTION Nickel-based superalloys have been extensively applied in various industries due to their excellent combination of mechanical properties and corrosion resistance both at room and elevated temperatures. INCONEL 718 (IN718) superalloy, as one of the most highly utilized nickel-based alloys, is used for hot section components in aerospace industry, nuclear reactor parts in nuclear application, oil drilling shafts in oil and gas exploration, as well as some critical supporting structures and fasteners [1-5]. However, conventional fabrication methods for IN718 superalloy are quite complex in processing routes [6-8]. Moreover, it is significantly challenging to manufacture IN718 components with complicated geometries, and also it is likely impossible to fix worn parts using traditional fabrication techniques [9-10]. Hence, intensive efforts have been invested in developing more efficient and effective manufacturing technologies to address the problems as mentioned above regarding the traditional manufacturing methods.
In the recent decade, there has been a rapidly growing interest in fabricating and repairing IN718 components employing additive manufacturing (AM) technology [8-9, 11-14]. The AM is a layer-by-layer manufacturing process which can produce geometrically complex components with highly precise dimensions, and also shorten processing steps and reduce production costs [15-17]. Wire arc additive manufacturing (WAAM), one of the most commonly used AM techniques, employs an electric arc or plasma as the heat source and a welding wire as the feedstock. Compared to other AM methods, the WAAM is outstanding because of its high deposition rate, unlimited deposition capabilities, relatively low costs as well as environmental friendliness [16, 18]. So far, research work on IN718 alloy fabricated by WAAM (WAAM IN718) is mainly focused on studied of microstructure and mechanical properties [19-21]. Xu et al. systematically investigated the effect of oxide, wire source, and heat treatment on mechanical properties of WAAM IN718 [19]. Their results show that the strength of the heat-treated WAAM IN718 is lower than the wrought alloy, and the main reason is due to the formation of large columnar grain structure in the WAAM IN718.
2
Nevertheless, apart from the mechanical performance, corrosion behavior in aggressive circumstances is equally significant in affecting the durability and service life of IN718 components. For instance, turbine vanes being made of IN 718 may suffer from roomtemperature corrosion during downtime, and downhole drilling parts which are fabricated by IN 718 fail in service environments with different corrosive media [4, 9, 22]. Thus, the corrosion resistance of IN718 alloy in ambient conditions is another important fundamental property which is worthwhile to study for practical applications. Presently, limited work is available in the literature on the corrosion performance study of WAAM IN718 alloy. It is known that heat treatment is a necessity for IN718 to optimize microstructure and achieve desirable properties. Based on previous studies, the microstructure of WAAM IN718 is dramatically different from that of the wrought IN718 even after identical heat treatments [19]. However, it is not clear yet how the corrosion behavior will be for the heat-treated WAAM IN 718 alloy with the distinctive microstructure.
The objective of this study is to investigate the corrosion performance of the heat-treated WAAM IN718 (WAAM-HT IN718) under different corrosion environments in comparison to the heat-treated wrought IN718 (wrought-HT IN718). The techniques of scanning electron microscope (SEM) equipped with both electron backscatter diffraction (EBSD), and energy dispersive spectroscopy detectors (EDS) are employed for microstructural analyses. The corrosion performance of the materials in corrosive environments are analyzed by potentiodynamic
polarization,
potentiostatic
polarization,
electrochemical
impedance
spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS).
2. EXPERIMENTAL 2.1. Material preparation IN718 superalloy was used for both substrate material and filler wire in this study. Its nominal chemical composition (wt. %) is Fe-17.60, Cr-18.00, Nb-5.40, Mo-3.00, Ti-0.94, Al-0.46, Mn0.05, Si-0.10, and Ni-balance. The as-deposited IN718 specimens were produced using a
3
tungsten inert gas (TIG) welding system attached to a 6-DOF Panasonic VR-004 robot. The filler wire with a diameter of 0.787mm was fed by a wire feeder of the 6-DOF Panasonic VR-004 robot into an arc on the substrate plate. The WAAM process was carried out using the following optimized parameters: arc current of 100 A, arc length of 3.0 mm, wire feed speed of 0.5 m/min, travel speed of 0.13 m/min. A straight-wall structure with a geometric size of 80mm × 10mm × 25mm was deposited in an argon protection environment. Experimental specimens were sectioned from the as-deposited part in an orientation parallel to the build direction with physical dimensions of 10 mm × 5 mm × 10 mm. A two-step heat treatment was applied on the asdeposited WAAM IN718 specimens: solution treatment at 1010 ºC for 1 hour, water quenching to room temperature; aging treatment, i.e. holding at 720 ºC for 8 hours followed by furnace cooling to 620 ºC in 20 min, and then holding at 620 ºC for 8 hours and air cooling to room temperature. In this study, the commercial wrought IN 718 plate with the composition (wt.%) 17.9% Cr, 17.6% Fe, 5.4 % Nb, 0.94% Ti, 0.46% Al, 2.9% Mo, 0.13% Co, 0.05% Si, 0.05% Mn, and balance Ni was used as a reference. The wrought IN 718 alloy was produced using vacuum induction melting (VIM) plus vacuum arc remelting (VAR); then followed by homogenizing at 1163 °C for 16 hours, forging to billet at 1093 °C, and further rolling to plates at 1066 °C [23-24]. The wrought IN718 specimens with dimensions 10 mm × 5 mm × 10 mm were subjected to identical two-step heat treatment as WAAM IN718 were used for comparative study.
2.2. Microstructure characterization The crystallographic orientation was identified by using an Oxford Instrument Nordlys EBSD detector attached to a scanning electron microscope (SEM, Philips XL 30). The specimens were ground by wet silicon carbide paper with decreasing grit size (180, 600, 800 1200), then mechanically polished to 1 µm and finally followed by vibratory polishing with 0.04 µm colloidal silica solution for 8 hours to obtain mirror-like and strain-free surfaces. The EBSD data were analyzed by using HKL Channel 5 software. Microstructure of the materials were examined employing the SEM equipped with an energy dispersive spectroscopy (EDS). The specimens were mechanically polished to 1 µm and then etched using a solution of 15 g chromium trioxide,
4
10 ml sulfuric acid, and 150 ml phosphoric acid for 15 to 20 seconds at a voltage of 5 V.
2.3. Electrochemical tests Electrochemical corrosion experiments were carried out with a three-electrode electrochemical cell (volume of 500 ml), using a saturated calomel electrode (SCE) for a reference electrode and carbon rods as counter electrodes. The tests were performed using a Princeton Applied Research Potentiostat with VersaStudio analysis software. Considering adequate study has not yet been performed in corrosive acid environments, the corrosion behavior of materials was studied in 1M nitric acid (1M HNO3), and 1M sulfuric acid (1M H2SO4). Specimens for electrochemical corrosion tests were mounted in bakelite leaving exposed working areas of 0.5 cm2, mechanically polished to 1 µm, then followed by rinsing in acetone and distilled water. The freshly prepared specimens were immediately assembled and transferred into the corrosion cell filled with the electrolytes as mentioned above to avoid air-formed oxides. Before each electrochemical polarization measurement, an open circuit potential measurement was conducted for 1 hour to achieve a relatively stabilized potential.
Potentiodynamic polarization tests were performed with a scan rate of 1 mV/s. The polarization curves were used to characterize corrosion current density (icorr), critical current density (icrit), passivation current density (ipass), corrosion potential (Ecorr), passivation potential (Epp), and passivation potential range (∆Epp). To obtain details of corrosion performance, specifically, the passivation behavior of materials, potentiostatic polarization and electrochemical impedance spectroscopy (EIS) techniques were employed as well. The potentiostatic polarization experiments were conducted at a passivation potential of 1 V for 1 hour in 1 M H2SO4 solution to form passive films on the specimens and followed by the EIS measurements to further characterize the corrosion behavior of the materials. The EIS spectra were recorded at an AC potential amplitude of 10 mV with a frequency range of 10-2-104 Hz. The impedance data were
5
analyzed by using ZSimpWin software. All experiments were conducted at room temperature and repeated at least three times to ensure reproducibility.
X-ray photoelectron spectroscopy (XPS) analysis was carried out on the specimens after potentiostatic polarization to characterize the chemical composition of passive films formed on the materials. All XPS spectra were collected using a Kratos Axis Ultra DLD X-ray Photoelectron Spectrometer with an Al
(1486.6 eV) monochromatic radiation source under a
vacuum pressure below 10-8 Torr. For each specimen, a survey scan was recorded firstly and then followed by high-resolution scans of Cr 2p, Ni 2p, Fe 2p, Mo 3d, and Nb 3d regions. The C 1s peak with a binding energy of 285 eV was used as a reference for calibration. All highresolution spectra were fitted using Gaussian-Lorentzian function after Shirley background subtraction with Casa XPS software. Position constraints were set according to Moulder [25]. Peak identifications were carried out by reference to the NIST XPS database.
3. RESULTS and DISSCUSSION 3.1. Microstructure analysis 3.1.1. WAAM-HT IN718 Figure 1 shows an inverse pole figure (IPF) colored orientation map (OIM), as well as the corresponding pole figure of WAAM-HT IN718. The microstructure of the WAAM-HT IN718 specimen mainly consists of columnar grains with long axis almost parallel to the build direction. Most of the grains exhibit a strongly preferred orientation close to (001), indicating that the microstructure is significantly textured (depicted in red in Figure 1a). The maximum intensity of the {100} pole figure is about 28.70 times that of the randomly oriented planes near the center. (Figure 1c). The EBSD results indicate that the predominant texture component of WAAM-HT IN718 specimen is {100} <001> (Cube) texture.
SEM micrographs of WAAM-HT IN718 specimen as given in Figure 2 show strip-like distributions of secondary phases (white precipitates). The irregularly island-shaped precipitates
6
are Laves phases [26]. A large amount of needle/plate-like δ phases are also detected and a lot of them associate with the Laves particles which was observed and explained in previous studies [20, 27-29]. Apart from Laves and δ phases, some spherical/square-shaped particles are identified as (Nb, Ti)C carbides.
Fig. 1 EBSD analysis of the WAAM-HT IN718 specimen (a) inverse pole figure colored orientation image map (OIM), (b) index map of IPF and the reference coordinate, (c) pole figure of {100}
7
Fig. 2 SEM micrographs showing microstructure of WAAM-HT IN718 specimens (a) low magnification, (b) high magnification
3.1.2. Wrought-HT IN718 Microstructure characteristics of wrought-HT IN718 specimens were also investigated for comparison with those of the WAAM-HT IN718. In the EBSD map shown in Figure 3, the whole region is composed of equiaxed grains, indicating the microstructure contains randomly oriented grains with a variety of crystallographic planes (Figure 3a). Compared to the WAAMHT IN718, the {100} pole figure of the wrought-HT sample is fairly weak without the maximum intensity in the center (Figure 3c), which further confirms the untextured microstructure. Also, no Laves particles are observed for the wrought-HT IN 718 specimen, as presented in Figure 4.
8
Fig. 3 EBSD analysis of the wrought-HT IN718 specimen (a) inverse pole figure colored orientation image map (OIM), (b) index map of IPF and the reference coordinate, (c) pole figure of {100}
Fig. 4 SEM micrographs showing microstructure of wrought-HT IN718 specimens (a) low magnification, (b) high magnification
3.2. Corrosion behavior 3.2.1. Potentiodynamic polarization analysis The corrosion behavior of WAAM-HT IN718 was investigated in comparison to wrought-HT alloy using different electrochemical techniques. Figure 5 illustrates the potentiodynamic polarization curves of WAAM-HT and wrought-HT specimens in 1M HNO3 and 1M H2SO4. As can be seen, a typical active-passive-transpassive behavior is observed for both specimens under
9
the aforementioned corrosive environments. To evaluate corrosion characteristics of the specimens, electrochemical parameters: corrosion current density (icorr), corrosion potential (Ecorr), critical current density (maximum current density of active-passive transition, icrit), passivation current density (ipass), and passivation potential range (∆Epass) are measured from the polarization plots. Tables I and II summarize the electrochemical characteristics of the specimens obtained from the potentiodynamic polarization tests in HNO3 and H2SO4, respectively. In the 1M HNO3 (Table I), the value of icorr of WAAM-HT IN718 is about 1.4 times higher than that of the wrought-HT IN718. Furthermore, significant differences are observed in the passive region. The values of icrit and ipass of WAAM-HT specimen are over an order of magnitude higher than those of the wrought-HT sample. It is known that excellent corrosion resistance of nickel-based superalloys and stainless steels is attributable to the formation of protectively passive films [3031]. Generally speaking, the higher passive current density is, the more difficult is the formation of passive film, resulting in poorer corrosion resistance in the corresponding corrosive environment [32]. It is clear that compared the wrought-HT specimen, the ipass of the WAAMHT sample is more than ten times higher, which indicates the corrosion resistance of WAAMHT IN718 is significantly lower than that of the wrought-HT IN718 in 1M HNO3. Also, the passivation potential range of ∆Epass usually describes the stability of the passive film. The ∆Epass of WAAM-HT IN718 is smaller than that of the wrought-HT IN718, which suggests less stability of the passive film formed on WAAM-HT IN718 compared to the one that formed on the wrought-HT sample. Similar to the case in 1M HNO3, the WAAM-HT IN718 shows higher values of icorr, icrit, ipass, and narrower ∆Epass in comparison to those of the wrought-HT sample in the 1M H2SO4 (Table II). This indicates that the WAAM-HT specimen exhibits lower corrosion resistance than the wrought-HT specimen in 1M H2SO4 as well. Hence, the results demonstrate that compared to the wrought-HT IN718, the WAAM-HT IN718 exhibits inferior resistance to corrosion in the two different corrosive environments.
To further understand corrosion behavior, another importance electrochemical technique, electrochemical impedance spectroscopy (EIS), was used and discussed next.
10
Fig. 5 Potentiodynamic polarization curves of WAAM-HT IN718 specimen in comparison to wrought-HT specimen in different corrosion environments at room temperature (a) 1M HNO3 solution, (b) 1M H2SO4 solution
11
Table I. Corrosion Characteristics of WAAM-HT IN718 in Comparison to the Wrought-HT Specimen in 1 M HNO3 at Room Temperature Specimen
icorr (µA/ cm2)
Ecorr (mV)
icrit (µA/ cm2)
ipass (µA/ cm2)
Epp (mV)
Passive range ∆Epass (mV)
WAAM-HT
16.0±0.7
471.5±1.5
523±31
421±57
708±6.3
756±1.3
wrought-HT
11.6±0.8
510±8.5
66±2.9
27.6±4.9
590±6.1
850±5.4
Table II. Corrosion Characteristics of WAAM-HT IN718 in Comparison to the Wrought-HT Specimen in 1M H2SO4 at Room Temperature
WAAM-HT
icorr (µA/ cm2) 3.89±0.1
Ecorr (mV) 371.67±38
icrit (µA/ cm2) 541±11.7
ipass (µA/ cm2) 251±37
Epp (mV) 678±3.2
Passive range ∆Epass (mV) 808±3.5
wrought-HT
2.80±0.4
400.67±56
83.1±3.5
22.7±7.6
555±6.8
891±6.3
Specimen
3.2.2. Electrochemical impedance spectroscopy (EIS) measurements Electrochemical impedance spectroscopy (EIS), as a versatile non-destructive technique, provides a unique approach to directly investigate passive film properties and evaluate the corrosion resistance behavior of materials [33]. EIS measurements were performed on both WAAM-HT and wrought-HT specimens after potentiostatic polarization at 1 V for 1 hour in 1 M H2SO4 at room temperature. The impedance spectra are presented in both Nyquist and Bode forms, as shown in Figure 6. The Nyquist plots exhibit incomplete semicircle arcs (Figure 6a), which are due to charge transfer processes at the electrolyte/electrode interface [34]. Normally, the diameter of the semicircular arc is related to the corrosion resistance of the material, i.e. the larger the diameter is, the better corrosion resistance of the material [35]. As can be seen in Figure 6, the diameter of semicircular arc of WAAM-HT IN718 is markedly less than that of the wrought-HT IN718. This implies that the corrosion resistance of WAAM-HT IN718 is considerably lower than the wrought-HT alloy, which is in good agreement with potentiodynamic polarization results. In the Bode diagram (log modulus of impedance versus log frequency), as shown in Figure 6(b), the impedance modulus value (|Z|) at high frequency (f >1k Hz) represents the solution resistance, while the value of |Z| at low frequency (f <1 Hz) 12
corresponds to the sum resistance which approximates to the polarization resistance of material surface [36]. It is obvious that the polarization resistance of the WAAM-HT IN718 is significantly lower than that of the wrought-HT IN718, which is strongly affected by the properties of surface films. Moreover, two phase-angle peaks are observed for both specimens in Bode-phase plots (phase angle versus log frequency), as shown in Figure 6(c), which suggest two time constants. The phase angle maxima at low frequency is attributable to the formation of protective passive films, while the maxima at high frequency is due to the formation of electrical double layers [33, 37-38]. It can be seen that the wrought-HT IN718 displays a higher phase angle maximum at low frequency compared to the WAAM-HT IN718, which indicates that the passive film formed on wrought-HT specimen exhibits a better protective capability.
Figure 7 depicts the equivalent circuit used for fitting the measured EIS spectra. In this circuit, Rs refers to the solution resistance, Qp represents the passive film capacitance, Rp stands for the passive film resistance, Qdl corresponds to the double-layer capacitance, and Rct is the charge transfer resistance between the matrix and the passive film. The constant phase element (CPE) is used to describe capacitive behavior, and the impedance of CPE is defined by the following Equation (1): = 1⁄
(
)
(-1<
n
<1)
(1) where
represents the magnitude of CPE, j is the imaginary number (j2=-1),
frequency (in rad/s), and
is the CPE exponential factor. The CPE is used to describe several
behavior depending on the value of : Warburg element,
=1, represents a perfect capacitance,
=0, represents a resistance,
Normally, the value of
is the angular
=0.5, represents a
=-1, represents pure inductance [39-40].
ranges from 0 to 1 in experimental conditions.
The fitted results are plotted in solid lines as included in Figure 6. The values of fitting parameters are given in Table III. As can be seen, the small values of chi-squared χ2 (≈10-3) reveal good correlations between experimental and simulated data for both specimens. The Rp reflects the protective property of passive film which is vitally dependent on characteristics of the passive film. Generally, a high value of Rp indicates a strong protective capability of the passive film and superior corrosion resistance of the material [41]. Also, the Qp demonstrates the
13
diffusion capability of ionic species within the passive film, and a low value of Qp shows a weak capability of ionic diffusion within passive film [42]. There are significant differences in Rp and Qp for WAAM-HT and wrought-HT specimens, as presented in Table III, i.e. the WAAM-HT sample shows a lower value of Rp, while a higher value of Qp compared to the wrought-HT specimen. This result implies that the passive film formed on the WAAM-HT specimen displays weaker protection capability, corresponding to inferior corrosion resistance. Therefore, the EIS measurements are consistent with potentiodynamic polarization results.
14
Fig. 6 Electrochemical impedance spectroscopic (EIS) data of WAAM-HT IN718 specimen in comparison to the wrought-HT IN 718 specimen in 1M H2SO4 after potentiostatic polarization at 1 V for 1 hour at room temperature (a) Nyquist plots, (b) Bode plots - impedance, (c) Bode plots - phase angle
15
Fig. 7 Electrical equivalent circuit model applied to simulate EIS spectra of IN718 specimens
Table III. Equivalent Circuit Parameters of EIS Data of WAAM–HT IN718 and Wrought–HT IN718 Specimens in 1M H2SO4 after Potentiostatic Polarization at 1 V for 1 Hour at Room Temperature Specimen
Rs (Ω cm2)
WAAM-HT
0.17
wrought-HT
7.4
ndl
Rct (Ω cm2)
Qp (F s / cm2)
np
Rp (Ω cm2)
χ2
2.24×10-4
0.81
9.80×10
2.19×10-3
0.69
4.03×102
2.6×10-3
7.72×10-5
0.83
1.09×104
6.44×10-4
0.83
2.09×104
3.5×10-3
(F s
Qdl (n-1)
2
/ cm )
(n-1)
3.2.3. X-ray photoelectron spectroscopy (XPS) analysis The results obtained from both potentiodynamic polarization and EIS measurements conclude that the corrosion resistance of the WAAM-HT IN718 is inferior to that of the wrought-HT IN718, which is mainly due to the difference in passivation. Research work performed by Peng et al. demonstrated that the protective properties of passive films depend upon both its chemical composition and structure [43]. To further investigate the nature of passivation, XPS analysis was carried out to examine the chemical composition of the passive films formed on WAAM-HT and wrought-HT specimens. Survey scans were carried out to identify all detectable elements in the passive films formed on specimens after potentiostatic polarization at 1 V for 1 hour in 1 M H2SO4. Figure 8 shows the XPS survey spectra, and C 1s, O 1s, Ni 2p, Cr 2p, Fe 2p, Mo 3d, and
16
Nb 3d peaks are detected. High-resolution spectra of primary compounds are deconvoluted after Shirley background subtractions. The Cr 2p spectra of WAAM-HT and wrought-HT samples are shown in Figures 9(a) and (b) respectively, which consist of two peaks at the binding energy (BE) of 576.2 ± 0.3 eV and 586.3 ± 0.2 eV representing Cr2O3, and their associated satellite peaks at 573.9 ±0.2 eV and 583.5 ±0.1 eV represent metallic Cr. Figure 10(a) shows the Ni 2p spectra recorded from the surface of WAAM-HT specimens, and the peaks decompose into metallic Ni at the BE of 852.4 eV and 870.0 eV, and NiO oxide with BE of 855.5 eV and 873.8 eV. The identification of Ni chemical states in the passive film formed on the wrought-HT specimens is presented in Figure 10(b). One signal located at the BE of 855.5 eV is assigned as NiO oxide, and the other two peaks at 852.4 eV and 870.0 eV are Ni metal. The presences of Cr in 3+ and 0 states, and Ni in 2+ and 0 states are in agreement with published studies [41, 44]. The XPS spectra of Fe 2p shown in Figures 11(a) and (b) indicate that Fe peaks of WAAM-HT and wrought-HT specimens decompose into metallic Fe and Fe2O3 oxide with the BE of 706.7 ± 0.2 eV and 711.3 ±0.3 eV, respectively. V. Maurice et al. detected Fe2O3 in the oxide layer formed on Fe-18Cr-13Ni single-crystal alloy as well [45]. The Mo 3d spectra in Figure 12 represent the oxide state of MoO3 at the BE of 232.4 ± 0.3 eV and 235.6 ±0.2 eV, along with metallic Mo at the BE of 227.4 ± 0.3 eV and 231.0 ± 0.1eV, obtained from the WAAM-HT and wrought-HT specimens. Llody and Chen reported the existence of MoO3 in passive films of nickel-based superalloys in different corrosive environments [44, 46]. The XPS spectra of Nb 3d show that two peaks located at the BE of 206.9 ± 0.2 eV and 209.9 ±0.2 eV belong to Nb2O5, one peak with BE 204.7 ±0.1 eV belongs to NbO, and one peak at 202.5 ± 0.3 eV corresponds to the metallic Nb, as shown in Figure 13. Previous study shows that the Nb2O5 and NbO oxides exist in the passive layer formed on nickel-based superalloy Inconel 625 [47]. The fitting results reveal that Cr 2p, Ni 2p, Fe 2p, Mo 3d, and Nb 3d peaks are made up of metallic and oxidized metallic components in the passive films formed on both WAAM-HT and wrought-HT specimens.
17
Fig. 8 XPS survey scan peaks of passive films formed on WAAM-HT IN718 and wrought-HT specimen after potentiostatic polarization at 1V for 1hour in 1M H2SO4
Fig. 9 High-resolution XPS spectra of Cr 2p of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718
18
Fig. 10 High-resolution XPS spectra of Ni 2p of passive films formed on different specimens (a)WAAM-HT IN718, (b) wrought-HT IN718
Fig. 11 High-resolution XPS spectra of Fe 2p of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718
Fig. 12 High-resolution XPS spectra of Mo 3d of passive films formed on different specimens
19
(a) WAAM-HT IN718, (b) wrought-HT IN718
Fig. 13 High-resolution XPS spectra of Nb 3d of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718
Table IV summarizes the composition of oxide films determined from the integrated intensity of XPS spectra of WAAM-HT and wrought-HT samples. The result shows that significant differences are observed in the quantity of Cr2O3 and NiO in the passive films formed on WAAM-HT and wrought-HT specimens. To be specific, the percentage of NiO content (~25.65 at. %) in the passive film formed on the surface of the WAAM-HT sample is over four times higher than that of the wrought-HT specimen (~5.87 at.%). The percentage of Cr2O3 in passive films formed on WAAM-HT IN718 is about 12 at.% lower as compared to the that of the wrought-HT specimen. Moreover, the surface oxide films of WAAM-HT IN718 contain lesser MoO3 compared to the wrought-HT specimen. It has been found that oxide films enriched in Cr2O3 and MoO3 are normally dense, stable and protective, resulting in low passive current densities and superior resistance to corrosion [48-49]. On the contrary, the passive films that contain high NiO exhibit porous structure and low protective capabilities [41]. Thus, the XPS results suggest that WAAM-HT IN718 and wrought-HT718 display different corrosion performance due to difference in the composition of their surface passive films.
Additionally, the porosity of passive film can affect the corrosion performance of the alloy as well. However, the passive film is normally too thin (≈ a few nm) to examine its morphology via conventional optical microscope and SEM. Thus, the further study of structural characteristic of
20
the passive films was performed based on potentiostatic polarization and the results are discussed next.
Table IV. Chemical Composition of Passive Films Formed on WAAM-HT IN718 and Wrought-HT Specimen from XPS Analysis (at. %) Specimen
Cr2O3
NiO
Fe2O3
MoO3
Nb2O5
NbO
WAAM-HT IN718
47.40
25.65
10.43
3.79
9.92
2.81
wrought-HT IN718
59.12
5.87
12.22
12.49
8.95
1.35
3.2.4. Potentiostatic polarization analysis The variation of anodic current density with time was measured at a potential within the passivation region of the materials by potentiostatic polarization technique. The initial decrease of current density is related to passive film growth on the surface of the samples, if the effect of double-layer charge is neglected [50]. The current density decreases with time according to the following Equation (2) i = 10
(
)
(2)
where i is the current density, t is the time, k and b are constants. In a double-logarithmic plot of Equation (2), k is the slope of the plot, and k = -1.0, indicates a high field-controlled surface film formation that produces well compact passive film with highly protective property, while k = 0.5, indicates a diffusion-controlled surface film formation that produces porous film with weakly protective capability [41, 51-52].
As can be seen from the double-logarithmic plots presented in Figure 14, the slope k of the WAAM-HT sample is -0.34 at the initial stage, and changes to -0.67 at the later stage. In the case of the wrought –HT specimen, the initial slope value of k is -0.53 and then approaches the values of -0.96 and -0.85. The variation in the slope values indicates that the passive film of the
21
WAAM-HT specimen is consistently porous, while the oxide film formed on the wrought-HT sample is porous initially but then becomes relatively compact. These results agree with the XPS results, which show that there is more NiO, which is known to be porous [41], formed on WAAM-HT specimen compared to the wrought-HT sample. It has been reported that passive films with porous structure exhibit low protective capability, which degrade corrosion resistance of the materials [41]. Thus, the relatively more porous passive film formed on WAAM-HT IN718 can explain its inferior corrosion resistance compared to the wrought-HT specimen.
Fig. 14 Double-logarithmic plots of current-time for WAAM-TH IN718 and wrought-HT IN718 as obtained from potentiostatic polarization tests (at 1 V for 1hour in 1M H2SO4)
3.3. Possible influence of microstructure on the formation of surface passive films The XPS results and double-log plots show that the passive films formed on WAAM-HT IN718 and wrought-HT IN718 are different in terms of chemical composition and structure, resulting in their different resistance to corrosion. Specifically, the passive film formed on the WAAM-HT specimen with porous structure contains 25.65 at. % NiO and 47.40 at.% Cr2O3 , while there is less porous film formed on the wrought-HT alloy which comprises 5.87 at.% NiO and 59.12 at.% Cr2O3. It is known that microstructure characteristics play important roles in the corrosion
22
behavior of materials. The formation of a passive film that contains more NiO and less Cr2O3 on WAAM-HT specimen compared to the wrought-HT specimen could relate to some microstructural factors. According to the microstructure analysis performed in this work, the WAAM-HT IN718 specimen exhibits distinctly coarse elongated grain structure with a strongly preferred orientation of (001), which is significantly different from the wrought-HT IN718 specimen with randomly oriented equiaxed grains. The difference of formation of passive film on the WAAM-HT specimen could be related to the {100} <001> (Cube) texture of the material. Bonfrisco et al. studied the effects of crystallographic orientation on the early stages of oxidation behavior of nickel (Ni) and chromium (Cr) [53]. Their results show that the oxidation rate of Ni changes with surface orientation in the order of (111) < (011) < (001), while the oxidation rate of Cr increases in the order of (001) < (011) < (111). Therefore, the {100} <001> (Cube) texture may have enhanced the formation of porous NiO at the expense of compact Cr2O3 in the passive film formed on the surface of WAAM-HT specimen. Secondly, another possible factor is related to the micro-segregation of Nb, which occurs during the solidification stage of the WAAM process. The micro-segregation of Nb during solidification of IN 718 superalloy is known to result in the formation of Laves phase micro-constituent and enhances the formation of delta phase particles during heat treatment [10, 28]. Since Laves phase and delta phase particles contain high concentrations of Nb, their formations are normally accompanied by a decrease of Nb concentration in the surrounding gamma matrix. As stated earlier, in the present work, Laves phase particles are observed only in the WAAM-HT IN718 and not in the wrought-HT specimen. Also, the formation of delta phase particles is observed to be more pronounced in the WAAMHT IN718 compared to the wrought-HT specimen. Accordingly, averaged concentration of Nb in the gamma matrix region of the WAAM-HT IN718 is lower than the Nb content in the gamma matrix of wrought-HT specimen, as shown in Figure 15. One major factor that can influence the formation of NiO and Cr2O3 is the values of activity of Ni and Cr in the alloy. Thermodynamic calculations by JMatPro software were performed to evaluate the influence of decrease in Nb concentration on the activity values of Ni and Cr in alloy IN 718 and the results are presented in Figure 16. It shows that the decrease in Nb concentration increases the activity of Ni, but reduces the activity of Cr. Therefore, the reduction in Nb concentration in the gamma matrix due to the formation of Laves phase and delta phase particles may cause an increase in the activity of Ni and a decrease in the activity of Cr for the WAAM-HT IN718 compared to the wrought-HT
23
specimen. Such changes in the activity values may cause an increase in the affinity of Ni and a decrease in the affinity of Cr for oxygen to form oxides, which could explain the observation of more NiO and lesser Cr2O3 in the surface passive film of WAAM-HT IN718 compared to that of the wrought-HT specimen.
Fig. 15 Comparison of Nb concentration in the γ matrixes of WAAM-HT IN718 and wrought-HT IN718 (a) SEM micrograph of WAAM-HT IN718, (b) composition profile acquired using EDS
24
Fig. 16 Effect of Nb content on activity values of Ni and Cr for IN 718 alloy (a) activity of Ni, (b) activity of Cr
4. CONCLUSIONS 1. The microstructure of WAAM-HT IN718 is characterized by columnar grains with a strongly preferential crystal orientation of (001) and Nb-rich microconstituents distributed along the build direction.
2. Potentiodynamic polarization and EIS measurements reveal that the WAAM-HT IN718 exhibits lower corrosion resistance in comparison to the wrought-HT alloy in acidic environments at room temperature.
3. Analyses of XPS data and double-log plots of potentiostatic data show that the passive film formed on WAAM-HT IN718 contains lesser and higher extents of Cr2O3 and NiO respectively, and as such is more porous and exhibits lower corrosion protection compared to the film formed on the wrought-HT alloy.
4. The formation of more NiO and lesser Cr2O3 in the passive film formed on the WAAM-HT IN718 could be related to its {100} <001> (Cube) texture, and Nb depletion in its gamma matrix, due to selective partitioning of Nb into Laves phase and delta phase particles.
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ACKNOWLEDGEMENT Financial support from NSERC of Canada is gratefully acknowledged
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Figure captions Fig. 1 EBSD analysis of the WAAM-HT IN718 specimen (a) inverse pole figure colored orientation image map (OIM), (b) index map of IPF and the reference coordinate, (c) pole figure of {100}
Fig. 2 SEM micrographs showing microstructure of WAAM-HT IN718 specimens (a) low magnification, (b) high magnification
Fig. 3 EBSD analysis of the wrought-HT IN718 specimen (a) inverse pole figure colored orientation image map (OIM), (b) index map of IPF and the reference coordinate, (c) pole figure of {100} Fig. 4 SEM micrographs showing microstructure of wrought-HT IN718 specimens (a) low magnification, (b) high magnification
Fig. 5 Potentiodynamic polarization curves of WAAM-HT IN718 specimen in comparison to wroughtHT specimen in different corrosion environments at room temperature (a) 1M HNO3 solution, (b) 1M H2SO4 solution Fig. 6 Electrochemical impedance spectroscopic (EIS) data of WAAM-HT IN718 specimen in comparison to the wrought-HT IN 718 specimen in 1M H2SO4 after potentiostatic polarization at 1 V for 1 hour at room temperature (a) Nyquist plots, (b) Bode plots - impedance, (c) Bode plots - phase angle
Fig. 7 Electrical equivalent circuit model applied to simulate EIS spectra of IN718 specimens
Fig. 8 XPS survey scan peaks of passive films formed on WAAM-HT IN718 and wrought-HT specimen after potentiostatic polarization at 1V for 1hour in 1M H2SO4
Fig. 9 High-resolution XPS spectra of Cr 2p of passive films formed on different specimens (a) WAAMHT IN718, (b) wrought-HT IN718 Fig. 10 High-resolution XPS spectra of Ni 2p of passive films formed on different specimens (a) WAAMHT IN718, (b) wrought-HT IN718 Fig. 11 High-resolution XPS spectra of Fe 2p of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718 Fig. 12 High-resolution XPS spectra of Mo 3d of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718 Fig. 13 High-resolution XPS spectra of Nb 3d of passive films formed on different specimens (a) WAAM-HT IN718, (b) wrought-HT IN718
Fig. 14 Double-logarithmic plots of current-time for WAAM-TH IN718 and wrought-HT IN718 as obtained from potentiostatic polarization tests (at 1 V for 1 hour in 1M H2SO4) Fig. 15 Comparison of Nb concentration in the γ matrixes of WAAM-HT IN718 and wrought-HT IN718 (a) SEM micrograph of WAAM-HT IN718, (b) composition profile acquired using EDS
Fig. 16 Effect of Nb content on activity values of Ni and Cr for IN 718 alloy (a) activity of Ni, (b) activity of Cr
Table I. Corrosion Characteristics of WAAM-HT IN718 in Comparison to the Wrought-HT Specimen in 1 M HNO3 at Room Temperature Specimen
icorr (µA/ cm2)
Ecorr (mV)
icrit (µA/ cm2)
ipass (µA/ cm2)
Epp (mV)
Passive range ∆Epass (mV)
WAAM-HT
16.0±0.7
471.5±1.5
523±31
421±57
708±6.3
756±1.3
wrought-HT
11.6±0.8
510±8.5
66±2.9
27.6±4.9
590±6.1
850±5.4
Table II. Corrosion Characteristics of WAAM-HT IN718 in Comparison to the Wrought-HT Specimen in 1M H2SO4 at Room Temperature
WAAM-HT
icorr (µA/ cm2) 3.89±0.1
Ecorr (mV) 371.67±38
icrit (µA/ cm2) 541±11.7
ipass (µA/ cm2) 251±37
Epp (mV) 678±3.2
Passive range ∆Epass (mV) 808±3.5
wrought-HT
2.80±0.4
400.67±56
83.1±3.5
22.7±7.6
555±6.8
891±6.3
Specimen
Table III. Equivalent Circuit Parameters of EIS Data of WAAM–HT IN718 and Wrought–HT IN718 Specimens in 1M H2SO4 after Potentiostatic Polarization at 1 V for 1 Hour at Room Temperature Specimen
Rs (Ω cm2)
Qdl (F s(n-1)/ cm2)
ndl
Rct (Ω cm2)
Qp (F s(n-1)/ cm2)
np
Rp (Ω cm2)
χ2
WAAM-HT
0.17
2.24×10-4
0.81
9.80×10
2.19×10-3
0.69
4.03×102
2.6×10-3
wrought-HT
7.4
7.72×10-5
0.83
1.09×104
6.44×10-4
0.83
2.09×104
3.5×10-3
Table IV. Chemical Composition of Passive Films Formed on WAAM-HT IN718 and Wrought-HT Specimen from XPS Analysis (at. %) Specimen
Cr2O3
NiO
Fe2O3
MoO3
Nb2O5
NbO
WAAM-HT IN718
47.40
25.65
10.43
3.79
9.92
2.81
wrought-HT IN718
59.12
5.87
12.22
12.49
8.95
1.35
Highlights Corrosion behavior of WAAM IN718 was studied by various electrochemical methods. The WAAM IN718 exhibits inferior corrosion resistance to wrought alloy. XPS results show that the inferior performance is related to porous passive film. Thermodynamic calculations suggest porous film formed due to Nb micro-segregation.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: