Characterization of NiCrAlY Layers Deposited on 310H Alloy Using the EB-PVD Method After Oxidation in Water at High Temperature and Pressure

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As mentioned in the title, this paper is dedicated to investigating an advanced procedure of the EB-PVD deposition method to increase the performance of 310H alloys used in aggressive pressure and temperature conditions. The proposed coating is NiCrAlY and the potential application is for cleaner nuclear energy, which is the material being used for generation IV SCW reactors.

Abstract

In this paper, the oxidation behavior of the 310H alloy coated with NiCrAlY using the EB-PVD method is studied after exposure to water at a high temperature and pressure (550 °C and 25 MPa) for different periods (720 h, 1440 h, and 2160 h). The Electron Beam Physical Vapor Deposition (EB-PVD) method was used to obtain the NiCrAlY coating. After testing, the coating performance was carried out by gravimetric analysis, grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), and the linear polarization method. GIXRD analysis highlighted the presence of chromium oxide (Cr₂O₃) and the Corundum phase (Al₂O₃) on the surface of the oxidized NiCrAlY-coated 310H samples. On the surface of the 310H alloy, the existence of the NiCrAlY coating and of the oxide film generated during oxidation are evident according to the EIS spectra, which show two capacitive semicircles in the Nyquist diagram. Furthermore, an increase in diameter semicircles with the oxidation time increasing was observed in the Nyquist diagram. Very low corrosion rates of 4.8 × 10⁻⁵ mm × year⁻¹, which were observed for oxidization for 2160 h NiCrAlY-coated samples, indicated that the oxide films are more protective and provide better corrosion resistance, which is also evidenced by the EIS analysis. Considering the obtained results, a significant relationship between the electrochemical technique, scanning electron microscopy, and gravimetric analysis was established.

1. Introduction

Today, considering the global energy crisis as well as climate change, alternatives are being sought worldwide to decrease the dependence on fossil fuels; thus, a particular interest can be observed in nuclear energy [1]. In this context, Gen IV nuclear reactors present a significant global challenge as a sustainable and economical mode of energy production [2]. Compared to current nuclear reactor systems, the new Gen IV reactors operate in more severe conditions and require new materials for the new performance in an aggressive environment. One of the six innovative nuclear fission reactor concepts is the Supercritical Water-Cooled Reactor (SCWR), which uses water above its critical thermodynamic point (374 °C, 22.1 MPa). In such an environment, the structural materials used in conventional nuclear reactors can no longer meet the operating requirements, and new selections were proposed as the material substrates and their coatings [3]. The suggested substrates come from various alloy classes, including Ni-based alloys (IN718, IN625), austenitic stainless steels (304, 304L, 316, 316L, 310), and ferrite–martensitic steels (HT-9, T91, T92, and HCM12A). Following heat treatment and exposure to high temperatures, the nonmagnetic austenitic stainless steel varieties, known as the chromium–nickel 300-series and the chromium–nickel–manganese 200-series steels, demonstrate exceptional mechanical and corrosion resistance [4].

Due to their mechanical and corrosion resistance properties, as well as their performance against radiation, austenitic stainless steels [5] have attracted special interest for applications in higher temperatures and pressures. With a crystalline structure of the FCC type, the 310H type is a 300-series chromium–nickel austenitic stainless steel material that was developed for utilization at temperatures ranging from 800 °C to 900° C. It combines exceptional high-temperature qualities with resistance to creep, weldability, and strong ductility.

To improve corrosion resistance, existing alloys can be modified by applying thin protective ceramic or metallic layers on the surface using different surface modification techniques. Currently, there are several techniques for thin layers on metal surface deposition, which are as follows: physical vapor deposition (PVD) [6,7] and chemical vapor deposition (CVD). An early technology was EB-PVD, which is still in use. The coatings obtained via such procedures have high density, remarkable oxidation resistance, and a strong bonding force [8,9,10].

For the EB-PVD technology, alloy targets are mostly used, and the coating composition is affected by the target content. When a high-energy ion beam bombards the target, after melting, sublimation, and evaporation, the deposition of the target takes place. Since each element’s saturated vapor pressure is different, the coating composition on the surface sample will change.

When placed in a thin, homogeneous layer, vapor-deposited coatings are dense. These types of coatings result in a decrease in the quantity of gas or moisture that can pass through the film. Consequently, these coatings are perfect for corrosion reduction in a nuclear environment. This applies to Generation IV reactors, such as SCW-type or liquid metal-cooled reactors, as well as existing LWR reactors (BWR, PWR) and CANDU reactors [11,12].

In this paper, 310H stainless steel has been selected for testing because the tests carried out so far, as well as the data from the literature, proved that this type of steel has a high oxidation resistance in water at high temperatures and pressures, while remaining susceptible to intergranular corrosion [13,14,15] and sigma phase precipitation when exposed to aggressive conditions like 550 °C and 25 MPa for a longer period of time [16]. Therefore, applying protective ceramic or metallic layers on the surface of this alloy has proven to be one of the ways to improve corrosion resistance.

Ceramic materials such as nitrides, carbides, silicones, transition metal oxides, and metallic materials—such as MCrAlY (M = Ni, NiCo, CoNi, or Fe)—show good resistance in corrosive environments, and are thus recommended for applications in water at supercritical temperatures. NiCrAlY-based coatings, widely used as independent layers or bond layers in TBCs (thermal barrier coatings), are selected in this paper based on their good adhesion, high modulus, high strength, and good oxidation resistance [17,18,19]. The development of a corrosion-resistant layer (Cr₂O₃, and Al₂O₃) on the material’s surface, which shields the deposited layer and the base material (substrate), is what gives the material its high oxidation resistance [20]. Additionally, dense layers rich in α-Al₂O₃-forming serve as a protective barrier, thus effectively preventing the diffusion-controlled oxidation process and rapid deterioration of the coated superalloys [21].

In recent years, only a few authors have studied the oxidation behavior of coated austenitic alloys under extreme conditions [22], even though knowledge of the oxidation of austenitic steel 316 L at high temperatures [23] is more present in the literature [24,25,26]. An example of this is the corrosion behavior of CrN_x-coated 310H SS before and after exposure to water under supercritical pressure and temperature (550 °C and 25 MPa) for up to 2160 h [22,27,28].

The present paper aims to study the corrosion behavior of the NiCrAlY-coated 310H alloy in thermally degassed demineralized water (2 ppm O₂) at a temperature of 550 °C and a pressure of 25 MPa for up to 2160 h. After oxidation, all tested samples were characterized by the following various methods: gravimetric analysis, grazing incidence X-ray diffraction (GIXRD), or scanning electron microscopy (SEM). The corrosion susceptibility has been assessed using the following two electrochemical techniques: linear potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The motivation for choosing such an investigation is based on the fact that the selected 310H alloy coating was not sufficiently studied at high temperatures despite its remarkable behavior. It is also crucial to mention the novel character of this article, which has a detailed electrochemical investigation of NiCrAlY film on 310 alloys studied in aggressive temperature and pressure conditions. The electrochemical aspect has not been studied until now. The original character of this article is not only the fabrication, but also the characterization for a longer time. All such aspects are important for us to gain more knowledge about nuclear material candidates for generation IV reactors.

2. Materials and Methods

2.1. Coating Material

Using the Electron Beam Physical Vapor Deposition (EB-PVD) technique, NiCrAlY layers with a thickness of approximately 1.5 µm were applied to the surface of 310H stainless steel samples that were bought from Outokumpu Stainless AB Company (Degerfors, Sweden) and cut to dimensions of 25 mm × 15 mm × 2 mm. After being hot rolled and heated to 1100 degrees Celsius, the plate was quenched with water.

Table 1. The chemical composition of 310 H SS.

Alloying Elements, [wt. %]
C	Si	Mn	P	S	Cr	Ni	N	Fe
0.063	0.71	1.61	0.016	0.001	24.13	19.03	0.04	54.34

lists this material’s chemical makeup [29] in weight percentage (wt. %).

The samples were provided with a hole 3 mm in diameter at one end for mounting on the autoclave holder. The samples were mechanically sanded with abrasive paper of various granulations (#120, #240, #400, and #600 µm) before deposition, after which they were exposed to the acetone ultrasound for 30 min. The samples were ultrasonically cleaned, dried, and then weighed with an analytical balance.

The present coating selection is based on the investigation from a previous paper [30], which established such coatings as viable candidates for protective coatings for high-temperature and high-pressure work, both of which were the basis for this coating’s selection.

Table 2. Coating chemical composition.

Powder Type	Particle Size, μm	Element, wt. %
		Ni	Cr	Al	Y
Amperit@413.006 NiCrAlY atomized gas	125/45	67	22	10	0.9

[31] shows the chemical composition of NiCrAlY.

2.2. Coating Method

This work examines the deposition of NiCrAlY coatings, which have been produced by the Electron Beam Physical Vapor Deposition (EB-PVD) technique. The installation used for the deposition of NiCrAlY is presented in Figure 1a, and the evaporation process is schematically represented in Figure 1b [32,33]. The coating method used a TORR 5X300EB-45 kW electron flux deposition system (Thermionics NW, Auburn, WA, USA). The coating composition is changed compared to the target content by considering the different vapor pressures of each element after the electron beam bombarding and evaporation process. The deposition parameters used for the deposition of these layers by the EB-PVD method are presented in

Table 3. Deposition parameters (EB-PVD method).

Parameter	Value
Starting vacuum	5.3 × 10−3 Pa
Working vacuum	2.7 × 10−3 Pa
Deposit speed	2 A/s
E-Beam beam power	1–2 KW
Distance between the crucible and the substrate	1.2 m
Substrate heater	set to the maximum value of 800 °C
Total deposition time	4 h

.

2.3. Testing Method

All samples were tested in static thermally degassed demineralized water (2 ppm O₂) at a temperature of 550 °C and a pressure of 25 MPa, for up to 2880 h of exposure, and with the autoclave stopping every 720 h. The pH of the testing solution was about 6.1 ÷ 6.3 and the water conductivity at 25 °C was 0.2 ÷ 0.25 μS cm⁻¹. At regular intervals, all samples were taken out of the autoclave to weigh the specimens following drying and rinsing. The morphological and functional characterization of the samples, as well as the electrochemical tests, were carried out. The autoclave solution was changed by adding a new one following each inspection. A balance with an accuracy of 1 × 10⁻⁵ g was used to calculate weight increases brought on by oxidation. Determination of the mass variation required knowledge of the starting weight and the weight obtained after oxidation.

The oxidation rates were calculated as follows, according to Equation (1) [34]:

$$v=m_d\times {\displaystyle \frac{0.0365}{\rho }}$$

(1)

where m_d is the mass variation per unit area S and exposure time t of the sample, expressed in mg × dm⁻² × day⁻¹, and ρ is the density. The unit of measurement for the oxidation rate is mm × year⁻¹. According to the XRD results, a mixture of Al₂O₃ and Cr₂O₃ is formed on the coated 310H stainless steel surface when exposed to water at high temperatures and pressures [35].

2.3.1. Structural and Morphological Characterization

GIXRD and SEM examination were used to examine the morphological and structural characteristics of the surfaces of the NiCrAlY-coated 310H stainless steel samples both before and after they were exposed to supercritical water.

GIXRD analysis was conducted using an X’Pert PRO MPD Diffractometer (PANalytical B. V., Almelo, The Netherlands), wherein the structure and the chemical composition of samples before and after oxidation were investigated. The GIXRD patterns were made in a θ–2θ geometry using CuKα radiation (λ = 1.5406 Å) at a grazing angle of 5° in the 10–95° range. The X’Pert Data Collector software (v.2.2) was used to establish parameters for experimental study. The acquired X-ray scans were manually analyzed by comparing them to the reference patterns from the International Center for Diffraction Data (ICDD) (PDF-4+) database. The morphologies of all coated sample surfaces before and after oxidation were investigated by SEM using a TESCAN VEGA II LMU microscope (Tescan Orsay Holding, Brno-Kohoutovice, Czech Republic).

2.3.2. Electrochemical Tests

Two electrochemical methods, linear potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), have been used to evaluate the corrosion susceptibility.

An electrochemical system, PARSTAT 2273 potentiostat/galvanostat (Princeton Applied Research, AMETEK, Oak Ridge, TN, USA), was used for all electrochemical measurements. Electrochemical studies were conducted using a traditional three-electrode electrochemical cell, which consists of an auxiliary electrode (graphite rod), a saturated calomel reference electrode (SCE), and a working electrode (the NiCrAlY-coated 310H SS sample). The characteristics of the oxide layer were unaffected by the electrochemical tests, which were conducted at room temperature (22 ± 2 °C) in a chemically inert solution with a pH of 7.7–7.8 (0.05 M boric acid with 0.001 M borax solution). Nyquist and Bode diagrams were drawn using the impedance spectra, acquired at open-circuit potential (OCP) with an ac amplitude of 10 mV in the frequency range from 100 kHz to 100 mHz, following OCP stabilization [36]. To perform a quantitative analysis, the experimental EIS data were simulated using analogous electrical circuits and Zview 2.90c software (Scribner Associates Inc., Southern Pines, NC, USA).

To observe the corrosion behavior of the samples, the potentiodynamic polarization plots were recorded in a potential range of −0.250 V vs. OCP up to +1.0 V. The scan rate was 0.5 mV/s. Based on the polarization plots, using the Tafel slope method and the polarization resistance (Rp) method, the main electrochemical parameters that were specific to the corrosion process were determined, as follows: corrosion potential (Ecorr), corrosion rate (vcorr), corrosion current density (icorr), and polarization resistance (Rp). Based on these parameters, the oxide protection efficiency (Pi) and porosity coefficient (P) were calculated as well using Equations (2) and (3) [37,38].

$$P_i\left(\%\right)=\left[1-\left({\displaystyle \frac{i_{corr}}{i_{corr}^0}}\right)\right]\times 100$$

(2)

where

i_corr is the corrosion current density of the deposited layers;

${\mathrm{i}}_{\mathrm{corr}}^0$ is the corrosion current density of the substrate.

$$P\left(\%\right)=\left({\displaystyle \frac{R_{ps}}{R_p}}\right)\times {10}^{-\left({\displaystyle \frac{\mathsf{\Delta }{E}_{corr}}{{\beta }_a}}\right)}$$

(3)

where

P is the total porosity of the deposited layer;

R_ps is the polarization resistance of the uncoated substrate;

R_p is the polarization resistance of the coated sample;

β_a is the anodic Tafel slope;

∆E_corr is the difference between the corrosion potentials of the coating and the substrate.

3. Results and Discussion

3.1. Oxidation Kinetics

To carry out the gravimetric analysis, every 720 h of oxidation in water at a temperature of 550 °C and a pressure of 25 Mpa, three stainless steel samples coated with NiCrAlY were taken out of the autoclave, after which they were washed, dried, and weighed with an accuracy of 1 × 10⁻⁵ g, using the KERN analytical balance. Based on the mass variation, oxidation kinetics were also determined for all coated samples after testing them in water at high temperatures, as can be seen in Figure 2. The oxidation kinetics are represented by the mass variation as a function of the oxidation time.

Gravimetric analysis reveals that all tested samples gained weight over time due to oxidation.

The mass values determined by weighing in the case of the tested samples fell between 5.36065 mg × dm⁻² and 7.14577 mg × dm⁻². The oxidation kinetics (Figure 2) were also determined for all tested samples using the obtained mass gains values.

The oxidation process is described as follows by the kinetic Equation (4):

$${\displaystyle \frac{\mathrm{\Delta }m}{s}}=k{t}^n$$

(4)

where

Δm is the mass gain, [mg];

s is the sample surface, [dm²];

k is the rate constant of the oxidation reaction;

t is the exposure time, [h];

n is the time constant.

By fitting Equation (4) through non-linear regression, the oxidation constant, k, and the time constant, n, were obtained and included in

Table 4. Kinetic parameters determined for NiCrAlY-coated 310H stainless steel samples after oxidation in water at supercritical temperature.

Kinetics Equation	k	n	R2
y = 1.3029 × t0.2154	1.3029	0.2154	0.9991

. The value of the correlation coefficient (R² = 0.9991) indicates that the experimental data of the oxidation of 310H samples coated with NiCrAlY by the EB-PVD method fit very well.

In our previous studies [39], the substrate was tested under the same conditions and for the same periods of time. The oxidation process was characterized by parabolic kinetics because n = 0.564. This fact indicates that oxidation is controlled by the diffusion of metal ions and oxygen through a generally protective oxide.

Average values of oxide thicknesses (µm) and oxidation rates (µm × year⁻¹) calculated from mass measurements for 310H stainless steel samples coated with NiCrAlY and oxidized for 720 h, 1440 h, 2160 h, and 2880 h in water at 550 °C and 25 Mpa are graphically shown in Figure 3.

Figure 3 shows that as the oxidation period increased, the oxide thicknesses increased, and the corrosion rates decreased. The lowest value of the corrosion rate of 0.27 μm × year⁻¹, obtained for a coated sample and oxidized for the longest period of time, shows a better corrosion resistance.

It should be mentioned that compared to the data obtained by electron microscopy, where layer thicknesses are determined only on the areas visualized under the microscope (the samples may have a thicker or thinner layer in other areas), global layer thicknesses were calculated by gravimetric analysis.

3.2. Morphological and Structural Characterization of the Oxides Formed Following Oxidation

3.2.1. Grazing Incidence X-Ray Diffraction (GIXRD)

Figure 4 shows the X-ray diffraction spectra obtained for the 310H substrates coated with a layer of NiCrAlY before and after exposure for 720 h, 1440 h, and 2160 h in water at 550 °C and 25 Mpa. The spectra were obtained at the incidence angle of 5⁰.

The diffractograms presented in Figure 4 show the changes induced in the composition of the NiCrAlY layer following the sample oxidation in the environment simulating the conditions in an SCW reactor. Thus, in the figure above, on the surface of the coated and unoxidized samples, the NiCrAlY phase was identified as a deposited layer, while on the surface of all oxidized samples, chromium oxide (Cr₂O₃) and the Corundum phase (Al₂O₃) were identified. The diffraction spectrum of Cr₂O₃ (PDF card no. 00-38-1479) shows some diffraction peaks, at 24.5°, 33.6°, 36.2°, 41.9°, 54.3°, 63.5°, and 65.1° 2 thetas, corresponding to (012), (104), (110), (113), (116), (214), and (300) planes, respectively. The presence of the Eskolite phase of the Cr₂O₃ structure, where the peaks are attributed to the rhombohedral structure, is confirmed by Tsegay et al. [38]. The presence of the Corundum phase (PDF card no. 00-46-1212) is highlighted by the appearance of a high peak at 43.3°, corresponding to (113), which increases with increasing oxidation time.

3.2.2. Scanning Electron Microscopy (SEM)

The morphologies of coated samples before and after oxidation for various periods of time in water at a supercritical temperature are displayed by SEM examination.

Figure 5a–d shows the SEM images at magnifications of 10 kx and 50 kx obtained for NiCrAlY-coated samples before and after 720 h, 1440 h, and 2160 h of oxidation in water at 550 °C and 25 Mpa.

To measure the thickness of the deposited layer, SEM in cross-section analyses were also carried out (Figure 6a–d).

From the SEM micrographs shown in Figure 6a, we can see that the deposition of an initial layer of NiCrAlY is not very continuous, and its cross-section thickness, measured by SEM, was approximately 1.48 μm. The deposition layer is granular, without visible defects (pores, cracks).

With an increasing oxidation time, a slight increase in layer thickness is observed from 1.48 µm to 2.02 µm (the case of samples coated and oxidized 2160 h), which means that a thin, compact, and fairly uniform oxide layer has been deposited on the sample surfaces (see Figure 5b–d and Figure 6b–d). A similar behavior, with comparable values of the oxide layer, was also obtained by other authors who studied the behavior at 1100 °C of some NiCrAlY coatings obtained by different methods, such as cathode arc evaporation [40], magnetron sputtering [40,41], hybrid arc/sputtering deposition [40], arc ion plating [42,43], or spraying with high-velocity oxygen fuel [44]. In principle, based on the literature data, the deposited oxide layer is Cr₂O₃, which, being extremely thin, did not allow for measurement with the electronic microscope. Therefore, the layer thickness values determined by SEM in the cross-section represent the thickness of the NiCrAlY layer deposited by EB-PVD and the thicknesses of the oxide formed following oxidation.

Even if the test conditions are not like those used in this study, it can be assumed that the mechanism of the oxidation process is comparable to that proposed in other works from the literature [40,42,45] and that it is composed of a rapid growth stage and stable oxidation stage. Fast oxidation is the first stage, in which the chromium and aluminum elements from the coating are oxidized rapidly under the influence of high partial pressure of oxygen, forming Al₂O₃ and Cr₂O₃, as demonstrated by XRD tests.

$$4\left(Al, Cr\right)+3O_2=2{\left(Al, Cr\right)}_2{O}_3$$

(5)

The development of the oxide layer significantly reduced the oxygen partial pressure at the coated surface. The literature demonstrates that Gibbs free energy predicts that the elements inside of the coating will oxidize in the following order: Y, Al, Cr, and Ni. As a result, the major reaction that occurs at the interface between the oxide layer and the coating can be identified as the following reaction:

$$4Al+3O_2=2A{l}_2{O}_3$$

(6)

Consequently, the oxide layer that developed on the surface of the coating at the end of the first oxidation step could be divided into the following two independent layers: an Al₂O₃ underlying layer and a mixed oxide layer on the surface. Following the formation of the protective oxide layer on the surface, the initial coating reached a stable oxidation state, during which the underlying Al₂O₃ layer primarily caused the thickness to increase as the oxide layer gradually grew. The material may have anti-corrosive qualities due to the aluminum oxide layer. Figure 7 shows the oxidation process of NiCrAlY coatings.

3.3. Corrosion Susceptibility Assessment Tests

3.3.1. Electrochemical Impedance Spectroscopy (EIS)

To evaluate the protective nature of the NiCrAlY layers deposited by the EB-PVD method, the electrochemical impedance spectroscopy (EIS) method was applied, which is a method that does not accelerate reactions at a metal/solution interface [46,47,48,49].

Figure 8a,b shows the Nyquist (a) and Bode diagrams (b) obtained for the 310H stainless steel samples coated with a layer of NiCrAlY and oxidized for different periods of time in water at 550 °C and 25 MPa. A qualitative characterization of the oxides formed on the sample surfaces after oxidation in an aqueous medium at high temperatures can be made using these diagrams.

Following the Nyquist diagrams shown in Figure 8a, it can be observed that for both the uncoated sample and the sample coated with NiCrAlY, a single capacitive semicircle was obtained. In the case of NiCrAlY-coated samples oxidized for different periods of time, two capacitive semicircles were obtained. These can be associated with two created interfaces. The semicircle recorded at medium and low frequencies corresponds to the NiCrAlY-deposited layer and the semicircle recorded at high frequencies correspond to the oxide layer formed on the sample surface following the oxidation tests.

Furthermore, these diagrams show an increase in diameter semicircles with the oxidation time increase. This determines a higher polarization resistance value; thus, there is a smaller corrosion rate for coated samples that were oxidized for 2160 h, indicating a better corrosion resistance of these samples compared to the other samples.

The Bode diagrams (Figure 8b) make the existence of two maximum values of phase angles evident for each analyzed sample, one at medium and low frequencies and another at high frequencies, corresponding to the two semicircles on the Nyquist diagram (Figure 8a).

Corrosion processes at the substrate–coating interface are represented by impedances recorded at medium and low frequencies, while corrosion processes at the oxide–solution interface are represented by impedances measured at high frequencies [50,51,52,53,54].

To obtain the quantitative data, all experimental EIS data were fitted with the ZView 2.90 software. The equivalent electrical circuit (EEC) is represented in Figure 9. The equivalent electrical circuit model is similar to the one used by our group for the study of the uncoated substrate [39], or those that were coated with CrxN and oxidized under similar conditions [22]. The values of the elements of the equivalent circuits determined for all tested samples are presented in

Table 5. Equivalent circuit element values for unoxidized and oxidized coated samples for different periods in water at 550 °C and 25 MPa.

Element Circuit	Uncoated Sample		Coated Sample
		Unoxidized	Oxidized Sample
			720 h	1440 h	2160 h
Rs, Ω·cm2	331.4	325.6	259.8	300.3	242.7
Rox, Ω·cm2	-	-	1.9 × 104	5.8 × 105	6.2 × 105
CPEox—T, F·cm−2	-	-	1.5 × 10−4	1.95 × 10−4	5.7 × 10−5
CPEox—P	-	-	0.68	0.53	0.54
Rcoat, Ω·cm2	-	8291	11455	2.54 × 106	3.1 × 106
CPEcoat—T, F·cm−2	-	2.53 × 10−4	3.4 × 10−4	8.6 × 10−5	6.4 × 10−5
CPEcoat—P	-	0.786	0.97	0.95	0.97
Rct, Ω·cm2	8810	160.9	522.1	529.6	619
CPEdl—T, F·cm−2	2.61 × 10−4	5.91 × 10−7	2.4 × 10−5	3.6 × 10−5	3.7 × 10−5
CPEdl—P	0.856	0.52	0.68	0.65	0.79
Chi-squared (χ2)	3.1 × 10−3	1.6 × 10−3	4.8 × 10−4	2.5 × 10−4	6.3 × 10−4

.

Table 5 shows that in the case of all tested samples, for the capacity of the oxide layer, the capacity of the deposited layer, a capacitive element, could not be used; thus, a distributive element consisting of the constant phase element CPE-T and the CPE-P parameter was used instead.

The coating resistance (R_coat) shows a progressive increase with the following oxidation times: 11,455 Ω × cm² at 720 h, 2.54 × 10⁶ Ω × cm² at 1440 h, and 3.1 × 10⁶ Ω × cm² at 2160 h. This trend indicates a strengthening of the oxide layer’s protective properties as the exposure time increases. The substantial increase in R_coat between 720 and 1440 h, followed by a more moderate increase at 2160 h, suggests a gradual densification and stabilization of the oxide layer on the NiCrAlY coating. This sequential increase in resistance, mirrored by the rising Rox values over time, reflects the coating’s enhanced ability to impede corrosion with prolonged exposure. Additionally, the high CPE_ox-p values, which approach 1, reflect a strong capacitive behavior that is indicative of a more uniform and protective oxide layer after extended oxidation. Table 5 shows that, in the case of all tested samples, a value of Chi-square (χ²) ≈ 10⁻³ was found, and the fitting errors were quite small.

An approach based on the Kramers–Kronig transforms was used to determine the polarization resistance values (Rp). The imaginary impedance component summed over the measured frequency domain is used in this method to estimate the polarization resistance [55].

Table 6. R_p and i₀ values obtained for unoxidized and oxidized NiCrAlY-coated 310H samples for different periods in water at 550 °C and 25 MP.

Sample Type	Polarization Resistance (From the Extrapolation of Nyquist Diagrams), (MΩ × cm2)	i0 (Calculated from Polarization Resistance). (nA × cm−2)
310H_uncoated	0.0342	370
310H/NiCrAlY_0 h	0.039	320
310H/NiCrAlY_720 h	0.297	42.6
310H/NiCrAlY_1440 h	0.389	32.6
310H/NiCrAlY_2160 h	0.514	24.6

presents R_p and i₀ values obtained for uncoated and coated samples with NiCrAlY, unoxidized and oxidized for 720 h, 1440 h, and 2160 h in water at 550 °C and 25 MPa.

Comparing the R_p values estimated from the Nyquist diagrams (Figure 8a) and using a method derived from the Kramers–Kronig transforms (Table 6), it is observed that for the samples coated and oxidized in a supercritical environment for a longer period of time (2160 h), the highest polarization resistance value (0.514 MΩ × cm²) and the lowest corrosion current density value (24.6 nA × cm⁻²) were obtained. According to the Buttler–Volmer equation (Equation (7)), polarization resistance is inversely proportional to the corrosion current density; therefore, a higher polarization resistance value indicates a lower corrosion rate value. The conclusion, which is congruent with the findings of the gravimetric study, would be that samples of oxidized steel that had been exposed to a supercritical environment over a longer period of time had lower corrosion rates than samples that were unoxidized or oxidized for a shorter period of time.

$$R_T={\displaystyle \frac{\mathrm{\Delta }\eta }{\mathrm{\Delta }i}}={\displaystyle \frac{RT}{zF}}{\displaystyle \frac{1}{i_0}}$$

(7)

where:

i₀: corrosion current density [A × cm⁻²];

R: gas constant [8.314 J × mol⁻¹ × K⁻¹];

F: Faraday’s constant [9.65 × 10⁴ C × mol⁻¹];

T: absolute temperature, [K];

z: the valence of the ion;

R: polarization resistance, [Ω].

3.3.2. Potentiodynamic Linear Polarization

To study the corrosion behavior of the 310H stainless steel samples coated with a layer of NiCrAlY using the EB-PVD method after exposure for different periods in the specific environment of an SCW reactor, the potentiodynamic polarization plots were recorded starting from a potential of −0.250 V vs. OCP up to +1.0 V. A scan rate of 0.5 mV/s was used. The obtained potentiodynamic polarization plots are shown in Figure 10.

Following the shape of the polarization plots shown in Figure 10, it can be seen that in the case of the samples coated with a layer of NiCrAlY and oxidized for 2160 h, more electropositive values of the corrosion potential and lower corrosion current density values were recorded, which indicate a better corrosion resistance of these samples compared to unoxidized ones or those oxidized under the same conditions for a shorter period of time. These observations indicate that the oxide films are more protective and provide better corrosion resistance, which is also evidenced by the EIS analysis.

The electrochemical parameters determined from potentiodynamic polarization plots using the Tafel slopes extrapolation and polarization resistance (Rp) methods, as well as the protection efficiency (P_i) and porosity (P) values, are presented in

Table 7. Electrochemical parameter values calculated from Tafel slopes and Rp methods, Pi and P values determined for NiCrAlY-coated 310H stainless steel samples unoxidized and oxidized for different periods in water at 550 °C and 25MPa.

Sample Type	Electrochemical Parameters					Pi, [%]	P, [%]
	Tafel Slopes Method			Rp Method
	Ecorr, [mV]	icorr, [nA × cm−2]	Vcorr, [mm × year−1]	Rp, [KΩ × cm2]	icorr, [nA × cm−2]
uncoated	−256	1030	0.011	26.07	1190	-	-
0 h	−219	200	0.002	82.61	227	80.58	0.135
720 h	−164	8.43	8.9 × 10−5	2.5 × 103	8.14	99.18	1.25 × 10−3
1440 h	−161	7.17	7.5 × 10−5	3.6 × 103	7.25	99.30	8.01 × 10−4
2160 h	−123	4.62	4.8 × 10−5	7.3 × 103	4.29	99.55	1.67 × 10−4

.

As this table shows, similar values of the electrochemical parameters were obtained by the Tafel and R_p methods. Thus, it is observed that the sample coated with a layer of NiCrAlY and oxidized for 2160 h has the best corrosion protection, which is indicated by the lowest corrosion current density value (4.62 nA × cm⁻²) and the corrosion rate (4.8 × 10⁻⁵ mm × year⁻¹), which possess a greater electropositive corrosion potential (−123 mV) and the highest polarization resistance value (7.3 × 10³ KΩ × cm²). These values are consistent with the observations made on the polarization plots. The improvement of the corrosion rate by almost two orders of magnitude can be explained by the appearance in the first stage of a mixture of chromium oxide and aluminum oxide with an underlying Al₂O₃ (in the fast stage of the mechanism of the oxidation process) [40,42,43]. The slower decrease in the corrosion rate for the samples oxidized for 1440 h or 2160 h, compared to samples oxidized for 720 h, can be explained by the predominantly slow growth of the Al₂O₃ layer, according to the second stage of the proposed oxidation mechanism.

Furthermore, in the case of this sample, the highest protective efficiency value (99.55%) and the lowest porosity value (1.67 × 10⁻⁴%) indicate the formation of more protective films on the surface of these samples.

To better illustrate the dependence of the corrosion speed calculated from the potentiodynamic polarization curves on the coating thickness, the graph in Figure 11 was made. It can be noted that a rapid decrease in the value of the corrosion speed for the samples coated with a layer of NiCrAlY and oxidized under conditions of high temperature and pressure are comparable to the non-oxidized sample. At the same time, it can be observed that, for the samples tested under supercritical conditions, the increase in the thickness of the coating leads to a decrease in the value of the corrosion speed, indicating good corrosion behavior of the oxidized samples.

Porosity, a measure of the density of the oxide defect, and its corrosion behavior, could be related. Several studies in the literature [56,57,58] have found that samples with lower porosity, lower corrosion current densities and rates, higher polarization potentials, and higher polarization resistivities exhibit the best oxide protective efficiencies (and can prevent corrosion the best). These findings are supported by the results.

4. Conclusions

In the present study, the morphological, structural, and electrochemical characterization of samples coated with a layer of NiCrAlY deposited using the EB-PVD method, before and after exposure in water at 550 °C and 25MPa for up to 2160 h, was investigated.

All of the studied samples gained weight according to the gravimetric analysis, and the oxidation of the 310H alloy covered with NiCrAlY in water at 550 °C follows a logarithmic law; with an oxidation time increase, the oxide thicknesses increased, and the corrosion rates decreased. The lowest value of the corrosion rate of 0.27 μm × year⁻¹, obtained for the coated sample and oxidized for the longest period, has better corrosion resistance.

The GIXRD analyses revealed the presence of NiCrAlY as the deposition layer in the coated and unoxidized samples. In the coated and oxidized samples, chromium oxide (Cr₂O₃) and the Corundum phase (Al₂O₃) were identified. Additionally, an increase in the intensity of peaks characteristic of Cr₂O₃ and Al₂O₃ was observed with longer oxidation times.

SEM analysis highlighted the deposition of an initial layer of NiCrAlY on the surface of stainless steel, which exhibited discontinuities, and whose thickness measured by SEM in a cross-section was 1.48 μm. The coating layer is granular, without visible defects (pores, cracks). After oxidation, a thin, compact, and uniform oxide layer was formed on sample surfaces. A mechanism for the growth of the oxide layer consisting of two stages was proposed, as follows: one fast and the other slower, after which a mixed oxide layer on the surface and an underlying Al₂O₃ is formed.

The EIS analysis with Bode and Nyquist diagrams and equivalent circuits showed that the 310H steel samples coated with NiCrAlY using the EB-PVD method and oxidized for the longest period of time have a better corrosion resistance, which is a conclusion evidenced by the potentiodynamic method as well. The highest protective efficiency value (99.55%) and the lowest porosity value (1.67 × 10⁻⁴%) observed over a longer time in the case of oxidized samples suggested the development of greater protective coatings on their surface.

This is further supported by the lower values of corrosion current densities, corrosion rates, higher corrosion potentials, and higher polarization resistances reported in oxidized samples for a longer period of time.

Based on the obtained and detailed results, including the electrochemical aspects, a more complete behavior in the aggressive media of an EB-PVD coating on 310H alloys was discussed, and a good correlation between gravimetric analysis, scanning electron microscopy, and the electrochemical method was found.