Effects of Al/Ti Additions on the Corrosion Behavior of Laser-Cladded Hastelloy C276 Coatings

Abstract

This study investigates the effects of aluminum (Al) and titanium (Ti) additions on the porosity, microstructure, and corrosion performance of Hastelloy C276-based coatings fabricated via laser cladding on nodular cast iron substrates. Nickel-based alloy powders blended with varying Ti (1–10 wt.%) and Al (0.5–2.5 wt.%) contents were deposited under optimized laser parameters. Microstructural characterization revealed that Ti addition refined the grain structure and promoted the formation of TiC phases, while Al addition dispersed eutectic networks into isolated island-like structures. Both elements effectively suppressed porosity by reducing gas entrapment during solidification. However, excessive Ti (10 wt.%) induced brittle fracture due to TiC agglomeration, and Al addition caused interfacial cracks owing to Al₂O₃ formation. Electrochemical tests in a 3.5 wt.% NaCl solution indicated that Al/Ti additions enhanced initial passivation but reduced corrosion resistance due to weakened oxide film stability. XPS analysis revealed that Al-enriched coatings formed Al₂O₃ and Al(OH)₃, whereas Ti-modified coatings developed TiO₂ and TiC, both influencing the passivation behavior. These findings provide critical insights into tailoring laser-clad coatings for marine applications by balancing porosity suppression and corrosion resistance.

1. Introduction

In industrial applications, particularly within marine engineering and chemical processing environments, equipment frequently encounters severe corrosion challenges. Ductile cast iron has been widely adopted due to its excellent wide-ranging properties. However, its inferior corrosion resistance limits its utilization in harsh environments [1,2]. To enhance the corrosion resistance of ductile cast iron, surface coating technologies have emerged as critical solutions, with laser cladding receiving considerable attention. As an advanced surface modification technique, laser cladding is primarily employed for surface strengthening and component repair. By fabricating high-performance working layers on low-cost steel substrates to replace bulk precious alloys, this technology conserves rare metallic materials while improving comprehensive properties, such as wear resistance, corrosion resistance, high-temperature stability, and oxidation resistance [3,4,5]. Consequently, laser cladding represents an effective strategy for enhancing the surface performance of cast iron.

Nickel-based superalloys, owing to their exceptional mechanical properties and corrosion resistance (under both ambient and elevated temperatures), have been extensively utilized in nuclear energy, aerospace, and chemical industries, with continuous optimization driven by rapid advancements in these fields [6,7,8]. Liu [9] fabricated functionally graded materials on cast iron substrates using the Inconel 625 nickel-based superalloy as an interlayer and SS420 martensitic stainless steel as a working layer via laser deposition technology, revealing that the clad layer achieved a maximum surface hardness of 650 HV0.3. Chen et al. [10] incorporated TiC particles into nickel-based alloys and optimized laser-cladding parameters, resulting in a coating microhardness approximately 2.5 times that of the substrate. Wang et al. [11] deposited Ni-WC composite coatings on Q345R high-strength low-alloy steel, achieving a surface hardness roughly fourfold higher than the substrate. Nevertheless, inherent limitations in powder preparation and laser-cladding processes often lead to defects, such as pores and cracks in the molten pool, which significantly degrade coating performance and applicability [12,13,14,15]. The current research on pore control in laser cladding predominantly focuses on process parameter optimization [16]. Zeng et al. [17] observed numerous circular pores of varying sizes in Ni60 coatings clad on Q235A substrates, demonstrating that coating porosity conforms to a Weibull distribution rather than normal or log-normal distributions. Studies propose effective porosity mitigation strategies: Carter et al. [18] reported that porosity in selective laser-melted nickel-based alloys decreases markedly when the laser energy density exceeds a critical threshold; and Peng et al. [19] established an energy consumption model for 316L stainless-steel selective laser melting to identify optimized energy ranges for pore elimination. However, the efficacy of process parameter optimization remains constrained by laser system characteristics. Zhang et al. [20] introduced ultrasonic vibration during laser cladding, observing reduced porosity and enhanced coating densification with increasing ultrasonic frequencies.

Hastelloy C276, a Ni-Mo-Cr series nickel-based superalloy, is widely employed in extreme corrosive environments, such as wet chlorine, chloride solutions, and high-temperature contaminated media, due to its superior strength, toughness, and corrosion resistance [21,22]. Significant research has been conducted on its microstructural evolution, mechanical properties, and corrosion behavior. Deng et al. [23] fabricated Hastelloy C276 coatings via wire arc additive manufacturing (WAAM), identifying coarse vertically oriented columnar grains with P-phase precipitates along grain boundaries enriched in Mo and W. Subsequent laser treatment refined grain structures, reduced P-phase size and quantity, and improved mechanical properties. Banamali et al. [24] compared its corrosion–wear behavior with EN 31 stainless steel in sulfuric acid environments, demonstrating significantly lower corrosion rates than other industrial nickel-based alloys. Ma [25] evaluated the electrochemical corrosion characteristics of laser-welded joints in neutral, acidic, and alkaline solutions, revealing reduced corrosion susceptibility in weld zones compared to the base metal under neutral/acidic conditions, while the base metal exhibited superior resistance in alkaline environments. Despite extensive studies on the corrosion properties of nickel-based alloys, the corrosion behavior of laser-cladded C276 alloy coatings in marine environments remains unexplored. With the rapid development of marine technology and the burgeoning of coastal engineering, ocean mining, and subsea operations, the aggressive marine environment poses significant corrosion threats to ships and offshore installations. Therefore, investigating the corrosion behavior of laser-cladded C276 nickel-based alloy coatings in marine environments holds critical scientific and practical importance.

This study prepared nickel-based alloy coatings with varying Al/Ti contents on nodular cast iron substrates through laser cladding using Hastelloy C276 powder blended with Al and Ti powders. The effects of Al/Ti additions on the porosity of clad layers were systematically investigated. The coating microstructure and phase composition were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). Furthermore, the corrosion performance of Al/Ti-modified coatings was evaluated through electrochemical tests and X-ray photoelectron spectroscopy (XPS) analysis.

2. Experimental Procedures

2.1. Materials’ Preparation

The powder mixtures used in this work included commercial Hastelloy C276 powder (supplied by Höganäs, Sweden) and pure aluminum and titanium powder (both 99.9wt.% purity, supplied by Beijing AMC Powders, Beijing, China). All of them were spherically atomized powders with diameters between 53 and 150 µm. Titanium powder and aluminum powder were mixed with nickel powder. The mass ratio of titanium powder to Hastelloy C276 powder was 1%, 2.5%, 5%, 7.5%, and 10%, and aluminum powder to Hastelloy C276 powder was 0.5%, 1%, and 2.5%. All the powders were mixed mechanically by using a planetary ball-milling machine (YXQM-2L, supplied by MITR Instruments, Hu Nan, China) for 3 h. For the mixed powder of aluminum and Hastelloy C276, the rotating rate was 100 rpm because of the light weight (density is 2.702 g/cm³) and low melting point (660 °C) of aluminum powder. The rotating rate was 200 rpm for the mixed powder of titanium and Hastelloy C276. Agate balls and stainless-steel balls with diameters ranging from 4 to 10 mm were used to mix aluminum powder with Hastelloy C276 and titanium powder with Hastelloy C276. The chemical composition of the Hastelloy C276 powder used in this study is shown in

Table 1. Chemical compositions of the Hastelloy C276 powder and cast iron QT500-7.

	C	P	Mo	W	Fe	Mn	Cr	Si	O	V	Ni
Hastelloy C276	0.14	0.01	16.0	4.6	3.1	1.3	15.7	0.7	0.04	0.6	Bal.
	C	P	Si	S	Mn	Fe
QT500-7	3.5–3.9	≤0.08	2.5–3.0	≤0.03	0.3–0.8	Bal.

.

2.2. Experimental Methods

The laser-cladding process was finished on an IPG YLS-4000 laser system (supplied by IPG Photonics Corporation, Oxford, MS, USA), which was equipped with a fiber-delivered 4.0 kW solid-state laser and a coaxial laser deposition head with a focal length of 300 mm. QT500-7 cast iron was employed as the substrate, and its chemical composition is listed in Table 1. In addition, argon was used as the powder carrier and shielding gas. The laser-cladding process parameters were determined in a range to study the effect of process parameters on porosity. The process parameters are summarized in

Table 2. Parameters used for laser cladding.

Laser Power (W)	Scanning Speed (mm/min)	Power Flow Rate (r/min)	Step-over Width (mm)	Shielding Gas Flow (L/min)	Laser Spot Size (mm)	Carrier Gas Flow (L/min)
1200–2500	200–350	1.6–2.2	1.85	40	4.0	5

. In order to satisfy the dimensional requirement of the examination, the coatings were manufactured in three layers. The orthogonal deposition direction was used to minimize the internal stress.

Electrochemical measurements were carried out on an electrochemical work station (CHI760, China) at 25 °C with the samples (10 mm × 10 mm) soaked in 3.5 wt.% NaCl solution. Every test was repeated at least three times to obtain reliable result. The potentiodnamic polarization curves were measured at a scanning speed of 5 mV/s, commencing from −1.5 V to 1.5 V. An electrochemical impedance spectroscopy (EIS) test was carried out at the open circuit potential. The disturbance voltage amplitude was 10 mA and frequency was 100 kHz~10 mHz, the results of which are constructed and fitted by ZsimpWin 3.30 software and Zview 3.1 software. The coating sample was used as the working electrode, the Ag/AgCl/saturated KCl electrode was the reference electrode, and a platinum electrode was used as the counter electrode.

2.3. Material Characterization

The microstructures of cladding layers were characterized using an optical microscope (OM; Olympus, BX51M, Japan) and a field-emission scanning electron microscope (FE-SEM; FEI, XL30S-FEG, State of Oregon, USA) at an acceleration voltage of 20 kV. The chemical compositions of the phases were detected by energy dispersive spectroscopy (EDS; Oxford, X-Max^N, USK). Moreover, X-ray diffraction (XRD) was carried out by a Pert-Pro MPD diffractometer. The scanning range of 2θ was from 10° to 100° with scanning rate 6°/min. X-ray photoelectron spectra (XPS, AXI Sultra DLD, Kratos, Japan) were taken to analysis the chemical state of elements on the coating surface. The laser-cladding samples were cut along the vertical laser scanning direction to obtain the cross-section of the cladding layer, and then the metallographic samples were prepared by inlaying, grinding, and mirror polishing before XRD, SEM, and OM examinations. Moreover, the specimens for OM and SEM were corroded by a chloroazotic acid solution (HNO₃:HCl = 1:3) for 60 s.

3. Results

3.1. Porosity

Porosity is a common defect in the cladding process of ductile cast iron. Pores can destroy the continuity of the coating and cause local stress concentration and initiate cracks. Figure 1 shows the effect of laser-cladding process parameters on the dilution ratio and porosity of the coating without the addition of the Al/Ti element. Figure 1a shows the curve of the dilution ratio and porosity when the laser power varies from 1200 to 2500 W, scanning speed is 350 mm/min, and power flow rate is 1.8 r/min. It is illustrated that both the dilution ratio and porosity increase with the increase in the laser power. Although the change in porosity fluctuates, the overall trend is to grow with the increase in laser power. When the laser power is 1600 W, the powder flow rate is 1.8 r/min, and the scanning rate changes from 200 to 350 mm/min, the curve of the dilution ratio and porosity of the cladding layer with the change in scanning rate are shown in Figure 1b, which indicates that with the increase in the scanning speed, both the dilution rate and porosity increase gradually, but when the scanning rate is higher than 300 mm/min, the porosity decreases. As seen in Figure 1c, both the dilution rate and porosity show a downward trend with the addition of the power flow rate from 1.6 to 2.2 r/min, when the laser power is 1600 W and the scanning speed is 350 mm/min.

Figure 2 illustrates the cross-sectional macro-morphologies of the Hastelloy C276 alloy coating. Some pores could be observed on the coating. The pores in the coating were mainly distributed in the interface between the coating and ductile iron substrate [26]. The size of the pores in the C276 coating was relatively large without the addition of Ti/Al. The maximum value was 220 μm. Because of the large distance between the bubble position and surface of the layer, the gas could not escape and formed a bubble at the interface between the substrate and the coating [27,28]. Moreover, a small amount of metal could be evaporated with laser irradiation. When the overflow rate of the gas was slower than the solidification rate of the molten pool, it was retained within the coating to form pores [29]. In addition, Figure 2b shows that the shape of the pore is regular and spherical, and the inner wall of the pore is smooth. Figure 2c displays the microstructure inside the pore, which shows that the microstructure is flower-like, and the dendrites radiate from the center to the outside. The pores could be induced by the trapped gas in the cladding during the cooling of the molten powders. This gas may be Ar gas used to protect the pool and convey the cladding powder. The energy dispersive spectroscopy (EDS) results of the inner wall of the pore in Figure 2d are shown in

Table 3. EDS of pore (wt.%).

Position	C	Fe	Ni	Mo	Cr	O	W	Si	V	Mn
Sp1	27.6	23.9	16.7	11	9.2	6.9	1.7	1.2	0.5	0.6
Sp2	25.8	26.2	16.5	11.2	10.7	6.1	1.6	1.3		0.5
Sp3	25.8	26.1	17.4	11	10	6.3	2.2	1.1
Sp4	23	32.7	15.7	6.9	14.7	5.7		1.3

, the content of C and O elements is relatively high. The reaction between C element and metal oxide at high temperature will produce CO pores. It also can be the CO₂ or CO gas produced by the reaction of graphite nodules and oxygen. With the solidification of the molten pool, the gas does not have enough time to float, and then forms pores at the interface between coating and substrate. Due to the pressure in the pores, the dendrites grow outward, evenly, and the front edge becomes smooth. On the other hand, the pores form in a regular, spherical shape.

The cross-sections of different Ti contents are showed in Figure 3. It can be seen from Figure 3 that, compared with the coating without Ti, the pore size of the coating with Ti is smaller, and the porosity within the coating gradually decreases with the increase in Ti content (from 1%Ti to 5%Ti); when the Ti content was 7.5 wt.%, pores were not found on the cross-section of the coating, as shown in Figure 4. Notably, a significant number of longitudinal cracks were observed in the 10%Ti-containing coating, extending through to the coating surface and resulting in coating fracture. This can be attributed to how Ti reacts easily with oxygen to form oxides, which reduces the possibility of the graphite making contact with oxygen. The cross-sectional morphologies of the coating with Al are showed in Figure 5. It can be seen that there are no pores on the coating cross-section. The reason is that the affinity between Al and oxygen is stronger than that of Ti, which makes more oxygen react with Al, thus reducing the chance of a reaction with graphite and avoiding the formation of pores. However, the addition of Al resulted in the formation of transverse cracks at the coating/substrate interface, as well as longitudinal cracks propagating perpendicular to the substrate surface along the coating-thickness direction.

3.2. Microstructure

The microstructures of the Hastelloy C276 coating and titanium-modified coating are shown in Figure 6. The energy dispersive spectroscopy (EDS) results corresponding to the marked position in Figure 6 are listed in

Table 4. Spectra of EDS test points in Figure 6 and Figure 7 (wt.%).

	C	Ti	Cr	Fe	Ni	W	Mo	Si	Al
Sp1	18.98		19.16	16.04	40.32	0.77	3.13
Sp2	45.53		5.23	5.79	16.09	1.36	16.8	4.22
Sp3	20.55	3.79	20.46	10.61	42.97	1.62
Sp4	5.73	76.09	16.72	0.77		0.69
Sp5	21.50	2.69	4.73	0.92		1.16
Sp6	29.84	8.28	17.79	8.98	20.84	1.80
Sp7	22.60		16.15		48.03	1.09	8.53	2.58	0.96
Sp8	31.49		8.00	0.51	7.15	3.17	29.24		1.23
Sp9	24.10			19.14	7.46	0.03	1.32		0.58

. As shown in Figure 6a, at the resolution obtained by SEM, the microstructure of the Hastelloy C276 revealed a cellular dentrite and net-like eutectic structure. In order to identify the element distribution in dendrites, an EDS analysis of the C276 coating was performed at high magnification. As shown in Figure 7, the dendrite region is mainly an Ni matrix and Fe. However, Mo and C tend to segregate into the interdendritic area. The aggregation behavior of the Mo element in the γ-Ni matrix is considered to be due to the lower diffusivity of heavier Mo compared with Ni. In the process of the laser-deposited Hastelloy C276, primary dendrites are formed at the initial stage of solidification, and then secondary and tertiary dendrites are developed. According to the partition coefficient K = C_S/C_L (C_S and C_L are the Mo solution concentrations in the solid and liquid phases, respectively) in the process of laser cladding, it is believed to be lower than 1 for the Mo element in the solid and liquid. As a result, the interdendritic regions and the boundaries of solidified grains are rich in Mo.

With the increasing titanium content, the microstructure changes and the original, fine, lamellar net-like eutectic structure coarsens gradually. The isolated irregular particles begin to appear in the structure with a titanium content of 0.5%. Furthermore, the irregular particles begin to grow and gradually transform into regular and sharp cubodial particles, and the lamellar net-like eutectic structure aggregates into large structure when the titanium content added is 7.5%, as shown in Figure 6b–e. It can be seen that there are some black regions, which can be identifies as TiC phases by EDS. During solidification, TiC reacts with the γ-nickel matrix to form a reaction zone around TiC particles, resulting in the formation of eutectic carbides. Carbide constitutional liquefaction and segregation lead to grain boundary penetration, which are responsible for liquefaction cracking behavior [30]. In addition, with the increase in Ti content, the eutectic structure decreases gradually. During solidification, more and more liquid films are formed around TiC particles and gradually turned into flakes, which results in an increase in the trend of hot crack formation. When the aluminum is added to the Hastelloy C276 coating, the original-network eutectic structure becomes dispersed and isolated islands. As the aluminum content increases, the eutectic structure gradually becomes continuous, as seen in Figure 8a–c.

The XRD patterns of the Hastelloy C276 nickel-based alloy and its modified laser-clad coating are shown in Figure 9. The phases mainly consisted of γ-Ni and Cr₃C₂ and Mo₆Ni₆C in all cobalt coatings. Compared with the standard card, it can be seen that the diffraction angle of the Hastelloy C276 cladding layer slightly shifts to a small angle direction, indicating that the lattice constant of nickel becomes larger, which is caused by Cr, W, and Fe entering into Ni species to form a solid solution. Combined with Figure 6a and Figure 7, the dendrite is composed of γ-Ni, which is surrounded by a eutectic network that mainly consists of the Mo₆Ni₆C phase. Moreover, the Ni₃Ti phase was identified in the titanium-modified cobalt coatings, and the Ni₃Al phase was found in the aluminum-modified cobalt coatings. Compared with the Hastelloy C276 coating, the addition of titanium (1%Ti, 2.5%Ti, 5%Ti, and 7.5%Ti) and aluminum (0.5%Al, 1%Al, and 2.5%Al) weakened the peak intensity of γ-Ni. Furthermore, when the titanium content reaches 7.5%, the peaks of TiC appear at 2θ~36°and ~61°, which partially overlaps with the peak of Ni₃Ti, so it is difficult to distinguish whether the two compounds coexist or just one of them is present.

In order to distinguish the distribution of each phase, EBSD maps were created to characterize the features of the phases in the Hastelloy C276, 1%Ti, 5%Ti, 7.5%Ti, and 0.5%Al, 2.5%Al coatings, as shown in Figure 10. The titanium-modified coating consists of γ-Ni matrix, TiC, Cr₃C₂, Ni₃Ti, and NiTi. It can be seen that TiC and NiTi coexist. With the increase in the titanium content, the distribution of TiC gradually increases. Combined with Figure 5, it shows that the irregular particles and cuboidial particles are mainly composed of TiC and NiTi. Furthermore, the NiTi is distributed in the peripheral region of TiC. The aluminum-modified coating consists of the γ-Ni matrix and MC. Because of the low content of aluminum, it is difficult to identify the phase containing aluminum.

3.3. Corrosion Resistance

3.3.1. Polarization Curve

Figure 11 shows the potentiodynamic polarization curves for different coatings. It can be seen that C276 and its modified coating have three stages in the anode region, namely, the active zone, passive zone, and transpassivation zone in 3.5 wt.% NaCl solution. When the corrosion potential (E_corr) of the three coatings exceeds a certain value, the corrosion current (I_corr) hardly changes with the positive shift of potential, indicating that the coatings have been passivated. It can be seen from Table 1 that the Cr content of the C276 coating is more than 15%. Because of the low value of the Cr oxidation constant Kp, it is easy to generate protective dense-film α-Cr₂O₃ on the coating surface, so that the protective coating will not be corroded [31]. When the corrosion potential reaches the breakdown potential, the passive film on the coating surface will be destroyed, and pitting corrosion will occur at the position where the performance of the passive film is relatively weak.

For the C276 coating, the coating surface is in a stable passivation state when the corrosion potential exceeds the self-corrosion potential of −1.06 V/SCE, and then the passivation current hardly increases, which indicates that Cr and Mo elements with oxide form a dense and stable film that hinders or reduces the contact between reactive ions and metals to protect the coating from corrosion. When the corrosion potential exceeded the breaking potential of 0.76 V/SCE, the production of green substances was found on the electrolytic cell solution and alloy samples, as shown in Figure 12, indicating that the corrosion products may contain oxides of hydroxides of Ni and Cr. More importantly, the corrosion degree of three different coating surfaces is also different.

For the coating containing Al, the self-corrosion potential is higher than that of C276 coating. Moreover, with the increase in Al content in the coating, its breaking potential and self-corrosion potential decrease and the passivation range narrows. When the Al content reaches 2.5%, the passivation range is 41% lower than that of C276, indicating that the addition of the Al element can reduce the corrosion tendency of the coating, but once the coating containing Al is corroded, its corrosion rate will be greater than that of the C276 coating, indicating poor stability. The electrochemical corrosion trend of the coating containing Ti in the 3.5%NaCl solution is similar to that of the coating containing Al, that is, compared with the C276 coating, the coating containing Ti has a greater self-corrosion potential and slightly lower breaking potential, and the passivation range is similar to the C276 coating. With the increase in Ti content, its self-corrosion potential and breaking potential gradually decrease, as seen in

Table 5. Electrochemical parameters calculated from polarization curves.

Samples	C276	1%Ti	2.5%Ti	5%Ti	7.5%Ti	0.5%Al	1%Al	2.5%Al
Icorr(A/cm2)	1.16 × 10−4	3.77 × 10−5	5.57 × 10−5	8.24 × 10−5	8.65 × 10−5	1.83 × 10−5	7.02 × 10−5	7.53 × 10−5
Ecorr/V	−1.062	−0.95	−0.975	−1.03	−1.01	−0.965	−0.978	−0.998
Corrosion rate (mm/A)	0.384	0.125	0.185	0.273	0.286	0.061	0.233	0.25
Width of the passivation zone/V	1.57	1.53	1.45	1.49	1.54	1.57	1.54	0.95

. But when the Ti content increase to 7.5%, the passivation range increases, which is slightly lower than that of C276. The possible reason is that when the Ti content increases to 7.5%, the corrosion products deposited on the coating surface increase, which blocks the contact between the coating surface and the corrosion medium and prevents corrosion to a certain extent. It can also be seen from Figure 12 that the surface color of the coating containing Ti or Al after corrosion is deeper.

3.3.2. EIS Measurement

Electrochemical impedance spectroscopy (EIS) measurement is an effective method to study the electrochemical corrosion of metal materials. Nyquist and Bode diagrams of the coatings were assessed according to the open circuit potential, as seen in Figure 13. It can be seen from the Nyquist diagram that the difference in the capacitance arc radiuses reflects the difference in the impedance values. The variation in the impedance modulus and phase angle with time can be seen in the Bode diagram, and the time constant of the research system can be determined from the phase angle diagram. A time constant can be seen in the Bode diagrams in Figure 13c,d. In order to deeply analyze the possible relevant interface characteristics of the studied system, the equivalent circuit of R(CR) shown in Figure 14 was used to fit the impedance results by Zview 3.1 software. Rs represents solution resistance. Rct is the charge transfer resistance. CPE is a constant phase element related to the electric double-layer capacitance. It can be seen in Figure 13a,b that with the increase in the Al and Ti content, the radius of the capacitive arc and the charge transfer resistance (Rct) gradually decrease, as seen in

Table 6. Fitting data of EIS.

Samples	C276	1%Ti	2.5%Ti	5%Ti	7.5%Ti	0.5%Al	1%Al	2.5%Al
W	1.541	1.694	1.161	0.662	1.637	2.367	1.7	2.421
Rct (Ω·cm2)	4.696 × 105	3.74 × 105	3.183 × 105	2.7 × 105	4.614 × 105	5.599 × 105	3.108 × 105	1.272 × 105
CPE-T/μF	2.36 × 10−5	2.822 × 10−5	2.668 × 10−5	2.976 × 10−5	2.886 × 10−5	3.016 × 10−5	3.188 × 10−5	3.687 × 10−5
CPE-P/μF	0.909	0.905	0.905	0.908	0.916	0.885	0.913	0.863

, indicating that the compactness of the passivation film on the surface of the coating decreases and the corrosion reaction is easier, but the Rct of 7.5% Ti increases, which is also consistent with the potential dynamic polarization results.

3.3.3. XPS Measurement

In order to further understand the passivation mechanism of the three alloys in the 3.5%NaCl solution, components on the surfaces of non-corroded and corroded specimens are characterized. Figure 15 and Figure 16 show the results of the XPS survey spectrum scanning and peak fitting of C276, 2.5%Al, and 5%Ti coatings. The primary peaks are generated by the photoelectron process, some of which are composed of double peaks, which is the result of spin orbit splitting, which occurs in all orbits except the s orbit. The spectral lines of the measured samples were calibrated by the C1s (binding energy: BE = 284.8 eV) standard pollution peak. It can be seen from the spectrum that the non-corroded surface, i.e., in air, and the corroded surface mainly contain Ni, O, Cr, and Mo elements. In order to further explore the corrosion products that formed on the coating surface in the 3.5 wt.% NaCl solution and determine their chemical composition and structure, and compare them with the surface composition of the non-corroded product, it is necessary to determine the structural composition of the corrosion products by analyzing the high-resolution narrow spectrum of different elements in the corrosion products formed on the coating surface.

The Ni2p high-resolution narrow spectra of the three coating surfaces are fitted by peak, as shown in Figure 16a,b. Because the Ni2p peak has an obvious split orbit, it is split into Ni2p_1/2 and Ni2p_3/2. In the air, the binding energy values are around 855.6 eV and 872.8 eV, which correspond to two chemical states of Ni⁶⁺ and Ni²⁺, indicating that the oxides of the Ni element on the coating mainly exist in the two forms of Ni₂O₃ and NiO before corrosion. It also can be seen from the spectrum that peaks at about 852.4 eV and 869.4 eV belong to Ni° [32,33]. However, after electrochemical corrosion, only two weak peaks are observed on the coating surface of 2.5%Al, and their corresponding binding energy values are near 856.4 eV and 873.9 eV, which correspond to the two chemical states of Ni²⁺, indicating that the Ni element in the corrosion products on the coating surface mainly exists in the form of Ni(OH)₂ and NiO. In addition, there are few Ni2p peaks on the coating surface of C276 and 5%Ti.

The high-resolution spectrum of the Cr2p peak on the coating surface was fitted. On the non-corroded coating surface, two peaks corresponding to Cr⁶⁺ 2p_1/2 and Cr⁶⁺2p_3/2 are obtained near the binding energy values of 586.3 eV and 576.4 eV, respectively, seen in Figure 16c, showing that the Cr element on the coating surface mainly exists in Cr₂O₃ in the air. On the corroded surface, four peaks are detected. As shown in Figure 16d, the binding energy of the peaks is around 588.7 eV, 587.1 eV, 579.4 eV, and 577.8 eV, and the four peaks are Cr⁶⁺ 2p_1/2, Cr³⁺ 2p_3/2, Cr⁶⁺ 2p_1/2, and Cr³⁺ 2p_3/2, respectively, which come from Cr(OH)₃, Cr₂O₃, and CrO₃.

For the Mo3d spectrum, there are four peaks at around 227.5–230.7 eV, 232 eV, and 235 eV corresponding to Mo°, Mo⁴⁺ 3d_5/3, and Mo⁶⁺3d_3/2 in the non-corroded coating surface, as seen in Figure 16e, which correspond to the oxide of Mo that includes MoO₂ and MoO_3, of which the former can prevent the matter from diffusing to the inner layer through the oxide film, and the latter contributes to stabilizing Cr₂O₃ [34,35]. Figure 16f shows that two peaks with binding energy values of 232 eV and 235 eV are detected on the corrosion coating surface, displaying that the chemical state of Mo has barely changed before and after corrosion.

Figure 16g shows the high-resolution spectrum of Al2p. The binding energy is near 74.5 eV, indicating that the Al element on the coating surface mainly exists in the chemical state of Al⁺³. Al and O mainly exist as Al₂O₃ on the non-corroded surface, while Al(OH)₃ and Al₂O₃ exist on the corroded surface. The Ti2p peak is detected on the non-corroded coating surface of 5%Ti. The binding energy values are 458.2 eV and 464.2 eV, corresponding to Ti⁴⁺2p_1/2 and Ti⁴⁺2p_3/2, which combined with O to form TiO₂. However, the peak of Ti2p was not detected on the corroded surface, as shown in Figure 16h.

4. Discussion

The addition of Al/Ti elements to the C276 alloy during laser cladding significantly influences pore formation and crack initiation mechanisms. Both Al and Ti preferentially react with oxygen to form stable oxides (Al₂O₃ and TiO₂), effectively reducing oxygen availability at the graphite substrate interface. This oxygen-depletion mechanism decreases the probability of CO/CO₂ gas generation through graphite oxidation, thereby considerably reducing pore density with increasing Al/Ti content. However, Ti addition induces distinct microstructural changes. SEM and EBSD analyses reveal that Ti promotes the formation of angular TiC phases with sharp edges in the cladding layer. As the Ti content exceeds 10 wt.%, these TiC phases undergo aggregation and coarsening, creating localized stress concentrations that ultimately lead to macroscopic cracking.

Conversely, Al addition demonstrates different failure mechanisms. Microstructural characterization shows no coarse eutectic formation, but interfacial cracking occurs at the cladding–substrate boundary with crack propagation perpendicular to the interface. This phenomenon originates from two factors: (1) the strong oxygen affinity of Al facilitates Al₂O₃ formation at the interface, creating brittle intermetallic compounds that weaken the metallurgical bonding strength; and (2) these Al₂O₃ particles act as preferential crack initiation sites, with subsequent thermal, stress-driven propagation into the cladding layer. This comparative analysis demonstrates that while both elements effectively mitigate porosity through oxygen scavenging, their distinct phase formation characteristics necessitate careful compositional optimization to balance pore reduction and crack resistance in laser-clad C276 alloy systems.

The corrosion mechanism of Hastelloy C276 and its modified coatings in the 3.5 wt.% NaCl solution was revealed by electrochemical polarization, impedance spectroscopy, and XPS. The surfaces of the coating prepared by laser-cladding technology were oxidized in air to form dense oxide films mainly composed of Cr₂O₃, NiO, Ni₂O₃, MoO₂, and MoO₃ according to the analysis of XPS. A small amount of Al₂O₃ or TiO₂ was also formed on the surface of Al-containing and Ti-containing coatings, respectively. The dense passive film has excellent anti-corrosion properties, which is also reflected in the electrochemical polarization curves with a wide passivation range. There are a small amount of corrosion products on the corroded coating surface, which are mainly composed of binding water in the corrosion product film, which combines with Ni²⁺ and Cr³⁺ on the nickel base surface to form Ni(OH)₂ and Cr(OH)₃, which constitute the outer layer of the corrosion product film on the coating surface. It can be seen from the polarization curve, EIS, and corrosion surface that the corrosion degree of the coating surface with Al or Ti additions is larger. In addition, in the process of potentiodynamic polarization, when the corrosion potential exceeds the breaking potential, the current in the anode region will increase sharply, which easily facilitates pitting corrosion in the weak areas in the passivation film formed in the passivation region. For the coating containing Al or Ti, the corrosion products cannot be accurately detected by XPS, which may be due to the peeling of Al₂O₃ or TiO₂ on the coating surface during the corrosion process. The peeling area becomes the weak area of film.

In the nickel-based alloy containing Al, when the Al content is less than 6%, the oxide film formed on the alloy surface is mainly NiO, and the inner layer is mainly a mixture of Cr₂O₃ and Al₂O₃. Because the oxide of Mo has the characteristics of low melting point and high diffusion coefficient, it reduces the diffusion rate of the Al element and is not conducive to the rapid supplementation of the surface oxide film. Moreover, the growth of the Al2O3 film is uneven, and a cavity is formed at the interface between the coating and the oxide film, resulting in the peeling of the oxide film. For the coating containing Ti, the diffusion rate of Ti^{4 +} ions to the oxide film is faster than that of Cr³⁺ ions, and gradually forms massive TiO₂ particles. With the extension of the oxidation time, the Ti⁴⁺ ions gradually increase, which affects the density and bonding strength of the oxide film and is harmful to the bonding force of the oxide film. When the content of Ti is 7.5%, it can be seen from the SEM results that most of the Ti elements form TiC with C, reducing the content of TiO₂, so that the degree of anti-corrosion of 7.5%Ti is equivalent to C276.

5. Conclusions

This study optimized laser-cladded Hastelloy C276 coatings on ductile cast iron substrates through the incorporation of aluminum (Al) and titanium (Ti) elements, aiming to mitigate porosity and enhance corrosion resistance. The effects of Al/Ti addition on coating porosity, microstructural evolution, and corrosion behavior were systematically investigated. The principal findings are summarized as follows:

(1)

Ti additions (1–7.5 wt.%) significantly reduced the porosity in Hastelloy C276 coatings, with 7.5 wt.% Ti creating defect-free layers by suppressing CO₂/CO gas formation via preferential oxidation. However, excessive Ti (10 wt.%) induced longitudinal cracking due to thermal stress and brittle phase formation. Al additions (0.5–2.5 wt.%) eliminated porosity but caused interfacial cracking, attributed to mismatched thermal expansion coefficients.

(2)

Ti additions promoted TiC and NiTi precipitation, coarsening eutectic structures and increasing crack susceptibility at higher Ti contents. Al additions fragmented the networked eutectic structure into isolated phases, altering the stress distribution. Both elements modified the γ-Ni matrix’s elemental segregation behavior, particularly Mo and Cr redistributions.

(3)

Al/Ti-modified coatings exhibited higher initial corrosion potentials but narrower passivation ranges compared to unmodified C276. Al-enriched coatings formed less-stable Al₂O₃/Al(OH)₃ films prone to localized corrosion, while Ti-modified layers developed TiO₂/TiC phases that marginally improved passivation at 7.5 wt.% Ti. Crack formation in high-Ti/Al coatings further degraded the corrosion resistance by exposing fresh surfaces to electrolytes.

(4)

XPS analysis confirmed that oxide films (Cr₂O₃, NiO, and MoO₂) dominated passivation in unmodified coatings, while Al/Ti additions introduced Al₂O₃ and TiO₂, respectively. The inferior stability of these oxides, coupled with crack-induced defects, accelerated corrosion in aggressive environments.

By optimizing the Al/Ti content and laser processing parameters, nickel-based coatings with a low defect density and superior corrosion resistance can be fabricated, thus providing a viable solution for the surface strengthening of cast iron components in harsh environments, such as marine engineering and chemical processing. Future work should focus on elucidating elemental interactions and evaluating long-term service performance under operational conditions.