Hot corrosion behaviour of constant and pulsed current welded Hastelloy X in Na2SO4, V2O5, and NaCl salt mixture at 900 °C

The high-temperature corrosion behavior of constant current gas tungsten arc (GTA) and pulsed current gas tungsten arc (PCGTA) welded Hastelloy X with different filler wires (C263 and ERNiCr-3) are studied for 50 cycles at 900 °C. Molten salt I (MS I) (75% Na2SO4 + 25% V2O5) and molten salt II (MS II) (75% Na2SO4 + 20% V2O5 + 5% NaCl) were coated on the welded specimens. MS II coated substrate shows the highest weight gain than MS I with a parabolic constant for GTA ERNiCr-3 as 21.440 × 10–6 mg2/(cm4.s). Whereas PCGTA C263 welded sample with MS I, revealed parabolic constant (lowest) of 0.008 × 10–6 mg2/ (cm4.s). Based on the results, an increasing pattern of hot corrosion resistance of substrates is arranged as GTA ERNiCr-3 < GTA C263 < PCGTA C263 < PCGTA ERNiCr-3. PCGTA shows more refined grains, higher grain boundary volume, better corrosion resistance, and more protective phases like Cr2O3, NiO, NiCr2O4, CoCr2O4, Al2O3, NiFe2O4, NbO than GTA weldment. But phases such as Fe2O3, MoO3, and Cr2S3 (non-protective phases) decrease corrosion resistance due to acid fluxing of alloying elements that promote the oxide scale exfoliation, spallation, chipping, and cracking. This study observed that PCGTA with C263 filler in MS I and MS II environment provides good corrosion resistance at high temperatures.


Introduction
The aerospace industry's tremendous development creates a demand for operating materials because of a continuous increase in combustion chamber temperature for improving performance. It needs the material to withstand a very high temperature without degrading its strength and corrosion resistance [1][2][3][4]. Hastelloy X is the best candidate material for this kind of application with Nickel as a base element with other major elements like Mo, Fe, Cr. Solid strengthening mechanism is used to strengthen the material, and it withstands properties at high temperatures up to 1200°C [5][6][7][8]. It is used in afterburner, gas-cooled reactor, thrust reverser [9], and mainly in combustion chamber parts of aerospace components. Sometimes, the poor-quality fuel used during the combustion process causes impurities like V, S, Cl & Na [10]. These impurity elements form MS compounds like NaCl, V 2 O 5 , and Na 2 SO 4 , coated on the substrate. These salts reduce the life of the substrate during hightemperature conditions.
The low melting eutectic salts are formed from Na 2 SO 4 , V 2 O 5 & NaCl blends at above 650°C. These eutectic salts increase the reduction and oxidation reaction when operated at an elevated temperature. There are two forms of hot corrosion: Type II (600°C to 800°C) and Type I (800°C to 950°C). The combustion chamber parts made of Hastelloy X material are operated at 900°C [10,11] temperature. In the combustion chamber and aerospace components, the Type I hot corrosion exhibits a vital [12], and hence it is considered for the present work.
Hot corrosion performance of gas turbine components made of Inconel 738LC coated with high activity Craluminite [13] is investigated in different salts environment like, 25% V 2 O 5 +75% Na 2 SO 4 , Na 2 SO 4 , 5% NaCl+20% V 2 O 5 +75% Na 2 SO 4 for 150 h and 300 h. The authors concluded that molten salts with NaCl possess a higher corrosion rate than the other two salts, and it exhibits cauliflower-like, plate-like, and broken plate-like structures. Fu guangyan et al [14] studied hot corrosion behaviour of pure Ni, Ni+10% Cr, Ni+10% Cr+5% Al alloy in NaCl+Na 2 SO 4 molten salt mixture at 900°C. The presence of Cr and Al in the Nickel-based alloy reduces the corrosion rate than pure Ni. Also, the presence of NaCl causes loose and porous particles.
Inconel 718 and Hastelloy coated by Alumina-Titania are investigated for the high-temperature behavior [15] for 50 h at 1000°C. The authors concluded that the FeO scale formation on the thermally coated substrate started spalling due to repeated thermal cycles for a prolonged period. The Hastelloy has also shown less weight gain, low porosity, micro-cracks, and better corrosion resistance with respect to the Inconel 718. The authors recommended the Hastelloy materials for high-temperature applications over Inconel 718.
Surface modification of Inconel 718 alloy [16] was investigated in molten salts (100% NaCl, 10% V 2 O 5 +15% NaCl+75% Na 2 SO 4 ) environment at 700°C after nickel aluminide coating by ultrasonic shot peening and pack aluminizing method. The authors concluded that the nickel aluminide-coated samples showed better corrosion resistance, while ultrasonic shot peened samples revealed the worst corrosion resistance with respect to uncoated samples. The change in the surface activity of elements in Inconel 718 during the ultrasonic shot peening process affects the corrosion resistance.
Superni 601, 600, and 75 are coated with Ni-20Cr at 900°C in the air environment [17] and studied for its hot corrosion behavior. A parabolic corrosion rate was observed in all the samples after 50 h by thermogravimetric analysis. NiO, NiCr 2 O 4 , Cr 2 O 3 phases are found, and spallation-free coating is achieved. Choi et al [16] investigated the effects of a bond coat (NiCrAlY) and topcoat (ZrO 2 -8% Y 2 O 3 ) on Hastelloy X by a plasma spray technique. Authors revealed that the substrate bond strength and fatigue life increase with decreased coating residual stress and thickness.
Hari et al [18] studied hot corrosion propensity at 700°C of alloy 617 used in the ultra-supercritical power plant. The authors coated the material with a molten salt mixture of Na 2 SO 4 , K 2 SO 4 , Fe 2 O 3 , SiO 2 , Al 2 O 3 and tested the coated material in a gas mixture environment and air environment. The sulfidation reaction occurs in both internal and external layers of the substrate in a gas mixture environment, and no such reactions occur in the air environment. Karimi et al [19] studied the top and bond coat remelting on Hastelloy X using GTA welding. The authors concluded that the three-time remelting using GTA welding revealed the best corrosion resistance compared to one-time remelting.
Hastelloy X's carburation kinetics in the CH 4 environment [20] was investigated at 1000°C and 800°C. The development of (Cr, Mo) 23 C 6 , (Cr, Mo) 3 C 2 , (Cr, Mo) 7 C 3 , carbide precipitates were observed on both outer and inner side parts. The internal carburization kinetics follows the parabolic rate law. Min Liu et al [21] studied Inconel 600, Hastelloy C276, and Hastelloy X in the molten fluoride environment at 750°C. The results indicated that the Hastelloy C276 provided better corrosion resistance than Hastelloy X and Inconel 600 due to more Mo and low Cr content.
In a nitrogen gas environment, Yan Wang et al [22] studied Ni-Cr-Al-Fe alloy for its oxidation mechanism in the 850°C to 1000°C temperature range. The oxidized substrate reveals the presence of NiO, Al 2 O 3 , NiCr 2 O 4 , and Cr 2 O 3 . During the initial stages, the gamma prime phase favors the formation of these oxides. Sreenivasulu et al [22,23] investigated the corrosion behavior of Nimonic 80A coated with metallic powder through air plasma and HVOF technique in 40%+60% of Na 2 SO 4 +V 2 O 5 MS environment and air environment for 50 h at 900°C. The authors concluded that spinel oxides and Cr 2 O 3 enhance corrosion resistance, whereas the MoO 3 decreases the corrosion resistance at the prescribed condition.
From the above literature, it has been clear that only minimal literature is reported on the oxidation behavior of welded Hastelloy X at a higher temperature. The substrate's salt mixture (NaCl, V 2 O 5 , and Na 2 SO 4 ) reduces the corrosion resistance. In the present work, the corrosion behavior of Hastelloy X welded with GTA and

Welding on Hastelloy X
The PCGTA and GTA welding was done on the substrate material with two filler wires, C263 and ERNiCr-3. The process conditions utilized for welding are given in table 2. The welding was conducted by adopting the following conditions: application of argon shielding gas at 15 l min −1 , filler wire diameter of 1.2 mm, tungsten electrode, standoff distance of 2 mm, 60 A of base current (I bc ), 120 A of peak current (I pc ), 120 A of continuous current (I cc ), pulse frequency of 6 Hz and welding efficiency of 70% [8]. The heat input utilized during welding is displayed in table 2, and the full penetration is obtained in four passes. The welded substrate was tested by visual observation and radiography testing, which show no cracks and porosity in the weldment of both GTA and PCGTA. The samples for hot corrosion testing are then extracted from the weldment with a dimension of 25 mm × 10 mm × 7 mm (figure 1).

Hot corrosion experiment
The samples for hot corrosion were preheated for 2 h at 200°C to eradicate the impurities, moisture from the substrate surface, and the uniform application of molten salts all over the surface. The MS I (25% V 2 O 5 +75% Na 2 SO 4 ) was mixed with acetone to make a slurry, and it was uniformly applied on all six surfaces. For moisture and impurities removal of coated samples, it was heated at 200°C in the tubular furnace for 2 h. The above steps are followed until a uniform coating is obtained on the substrate surface between 3 mg mm −2 to 5 mg mm −2 . Similar measures have been observed for coating MS II (5% NaCl+20% V 2 O 5 +75% Na 2 SO 4 ) on the substrate surface. The specimens are introduced into the furnace in an alumina boat for 50 cycles at 900°C. Each cycle has 5 h of heating in the furnace and then air cooling for 40 min to attain room temperature. The specimens were then weighed using a weighing machine to identify the weight gain or loss due to oxidation of the surface after each cycle. Then the samples were kept back into the furnace for the next cycle, followed until the 50th cycle.

Metallography analysis
The residual salt on the substrate was removed by cleaning with distilled water. Scanning Electron Microscope (SEM) with Energy dispersive spectroscopy (EDS) attachment (Make: ZEISS EVO 18) was used to identify the chemical compositions and surface morphology [28][29][30][31]. The x-ray Diffraction (XRD) analysis (BRUKER D8 Advance) provided the substrate phases due to oxidation and reduction reaction.

Visual examination
The surface morphology of the welded substrate after each cycle was captured with an optical macroscope.  cycle, MS I coated substrate revealed a light greenish color, whereas MS II showed a light yellowish color. The substrate active elements enhance the rate of corrosion rapidly during the initial few cycles, and then it increases gradually until the 50th cycle [22]. In the GTA weldment of C263 and ERNiCr-3 fillers coated with MS I, during the 2nd cycle, the substrate color changes to light grey from light greenish and then to a dark grey color. Spalling and chipping start from the 6th cycle, continuing until the final cycle. A similar color change is found for PCGTA weldments of C263 and ERNiCr-3 filler, but very little spalling and chipping of the substrate are observed. The GTA weldment made with C263 and ERNiCr-3 fillers of the MS II coated substrate indicates the change of color from a light yellow to a light grey after the 2nd cycle after the 5th cycle gets changed to dark grey color, and it remains the same till 50th cycle. The PCGTA weldment with MS II displays similar color changes, but a small amount of chipping and spalling occurs.

Corrosion kinetics
The high-temperature corrosion kinetics (number of cycles vs. weight gain per unit area) with MS I and MS II coating on GTA and PCGTA weldment is illustrated in figures 4 and 5. The thermogravimetric analysis revealed that the MS I have shown a lesser weight gain per unit area than MS II for both the weldments. The MS I coated substrate offers better corrosion resistance when compared to MS II for the given condition. The absence of     NaCl salt in the mixture of MS I is the reason for better corrosion resistance [13]. The drastic increase in weight gain square (WGS) curve and weight gain curve is observed until the 3rd cycle due to the rapid oxidation of Ni and Cr on the substrate surface and gradually increases. In MS I, GTA ERNiCr-3 showed the highest weight gain, followed by GTA C263, PCGTA C263, and PCGTA ERNiCr-3. The formation of high accelerated oxides Fe, Mo, S, V, Na in GTA causes higher weight gain for both fillers. Whereas in the case of PCGTA, the formation of more Ni, Cr, and Al oxides showed lesser weight gain and better corrosion resistance for C263 and ERNiCr-3 fillers. Similarly, for MS II, the highest weight gain is observed for GTA with ERNiCr-3 filler followed by GTA with C263 filler, PCGTA with C263 filler, and PCGTA with ERNiCr-3 filler. Figures 6 and 7 show the WGS and number of cycles graphs of the coated samples welded by PCGTA and GTA using C263 and ERNiCr-3 fillers. The parabolic constant is determined to identify the corrosion rate from the following equation (1) hereas K p -parabolic constant in mg 2 (cm 4 .s) −1 , S A -the surface area of the substrate in cm 2 , Δw-pre and post weight difference of the substrate (mg), t-time taken (s). Table 3 shows the parabolic constant value for the coated samples (MS I and MS II). There comes a gradual increase in WGS after the 3rd cycle due to continuous oxidation of Mo, Fe, S, Cr, V, and Na. During hot corrosion testing, the sudden reduction in the WGS of some substrates is observed in figures 6 and 7 because Cr and Na elements evaporated at 900°C. In MS I, GTA with ERNiCr-3 filler exhibits maximum WGS per unit area trailed by GTA with C263 filler, PCGTA with C263 filler, and PCGTA with ERNiCr-3 filler. For MS II, the highest WGS is obtained in GTA with ERNiCr-3 filler followed by GTA with C263 filler, PCGTA with C263 filler, and PCGTA with ERNiCr-3 filler.
The resistance to corrosion of GTA and PCGTA weldments coated with MS I at 900°C for both the filler materials is given by,

GTA ERNiCr
GTA C PCGTA C PCGTA ERNiCr MS I 3 263 The corrosion resistance for GTA and PCGTA substrate with C263 and ERNiCr-3 filler wires tested at 900°C for 250 h and coated using MS II is given below,

XRD analysis
The corrosive and protective phases on the weldment coated with MS I and MS II and base metal are evaluated through the XRD test. The phases present on the weldments and base metal after the 50th cycle are displayed in figures 12 and 13.  [22,33]. In PCGTA MS II substrate, the higher crystalline grain refinement provides better corrosion than GTA for both the filler materials (C263, ERNiCr-3). MS II has a higher rate of corrosion than MS I. This is due to the presence of NaCl in MS II accelerates corrosion, whereas NaCl is not included in the mixture of MS I [34]. In PCGTA C263, the more protective phases of Cr 2 6 and 7).
The crystalline grain size for PCGTA and GTA weldments for both C263 and ERNiCr-3 filler wires is evaluated through Scherer's formula using the Gaussian method XRD test. The PCGTA shows the crystalline grain size of 26 nm and 22 nm, whereas GTA reveals the crystalline grain size of 30 nm and 24 nm for C263 and Figure 11. EDS area mapping of MS II (5% NaCl+25% V 2 O 5 +70% Na 2 SO 4 ) and base metal.
ERNiCr-3 filler wires, respectively. The refined grains in PCGTA show a 7.64% and 13.52% reduction in grain size compared to GTA for ERNiCr-3 and C263 filler wires. Scherer's formula for evaluating the grain size is given by (2) [24].

Chemical reactions
The V 2 O 5 and Na 2 SO 4 salt mixture reacts and forms the NaVO 3 oxides during the initial stages of hot corrosion testing [35]. This NaVO 3 oxide is the reason for the rapid oxidation of the initial few cycles and drastically increases the substrate's weight gain. The reaction of salts is given in the below equation ( The NiO and Cr 2 O 3 are formed as the product of a reaction between Ni and Cr with O during hot corrosion testing, and this acts as a diffusion barrier to oxidation through other elements [36]. The gradual improvement in WGS rate and weight gain follows the parabolic law [35,37]. The more amount of Cr in all MS I and MS II substrates is confirmed by EDS analysis [16]. The chemical reactions of Ni, Cr, O during initial cycles are given in the below equations (4) and (5) [33].
The presence of Na 2 SO 4 salt also reacts with Ni and forms the NiO, NiS, and Na 2 O. This reaction is given in the below equation (6) [13,38].

Ni
The reaction of Na 2 SO 4 and Cr 2 O 3 forms the CrVO 4 oxides, and this oxide is not protecting the substrate from oxidation. The formation of oxides creates the Cr 2 O 3 dissolution in most places and causes Cr depletion, further improving the corrosion rate. Due to this, most of the region of substrate pores are started forming, which improves the weight gain of the substrate, and reaction is given in equation (7)  spinel oxides, providing the protective layer to the substrate for further corrosion. The anions and cations diffusion co-efficient for the spinel oxides is much lower than other oxides and forms more protection for all the substrates at high temperatures. The higher corrosion resistance in PCGTA weldments is due to the refined grain structure and spinel oxides' presence. The equations (8) and (9) for the formation of spinel oxides are given below [13,17]. ( ) + = The development of Cr 2 S 3 is found at the molten salt coating and weldment interface region. This improves the corrosion rate of the substrate by destroying the alloying elements. It also forms Cr 2 O 3 by reacting with oxygen, and the equation (10) is given by [14,39].

Conclusions
In the present study, the high-temperature corrosion behavior of GTA and PCGTA welded Hastelloy X with C263 and ERNiCr-3 in MS I and MS II environments are studied, and its results are as follows: 1. The MS II coated substrate at 900°C for 50 cycles shows the highest weight gain for all the PCGTA and GTA weldment joined using C263 and ERNiCr-3 filler wires to MS I coated substrates.
2. The change of color in each cycle depicts the occurrence of hot corrosion in both GTA and PCGTA weldments coated with MS I and MS II. The parabolic constant for GTA ERNiCr-3 coated with MS II showed the highest value of 21.440×10 -6 mg 2 /(cm 4 .s). The MS I coated PCGTA C263 weldment, revealed the parabolic constant of 0.008×10 -6 mg 2 /(cm 4 .s) (lowest).
3. The resistance against hot corrosion of MS I and MS II coated substrates in the increasing pattern is GTA ERNiCr-3<GTA C263<PCGTA ERNiCr-3<PCGTA C263.
4. In MS I and MS II, the crystalline grain refinement observed for C263 and ERNiCr-3 fillers causes more grain boundaries and grains in the substrate, which develops more protective phases and better corrosion resistance in PCGTA than GTA.  3 , and Cr 2 S 3 , which act as the non-protective phase, decrease corrosion resistance. These phases cause acid fluxing of alloying elements, which promote the spallation, chipping, exfoliation of oxide scale, and cracking.
6. Based on the chosen welding conditions, PCGTA C263 weldment with MS I and MS II coating shows excellent corrosion resistance. Hence, it is recommended for welding aerospace components like thrust reverser and combustion chamber, which are operated at higher temperatures when compared to ERNiCr weldments.