# Water Vapor Influence on a Nickel-based Alloy NY600 Oxidation -Residual Stress Determination

Henri Buscail, Christophe Issartel, Sébastien Perrier

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Henri Buscail, Christophe Issartel, Sébastien Perrier. Water Vapor Influence on a Nickel-based Alloy NY600 Oxidation -Residual Stress Determination. High Temperature Corrosion and Protection of Materials (HTCPM 2024), Jun 2024, Ile des EMBIEZ, France. hal-05146675

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# Water Vapor Influence on a Nickel-based Alloy NY600 Oxidation – Residual Stress Determination

Henri BUSCAIL<sup>a</sup>, Christophe ISSARTEL<sup>a</sup>, Sébastien PERRIER<sup>a</sup>

<sup>a</sup>Université Clermont Auvergne, Clermont Auvergne INP, CNRS, Institut Pascal, 3 Rue Lashermes, CS 10219, 43009 Le Puy en Velay, France

henri.buscail@uca.fr; christophe.issartel@uca.fr; sebastien.perrier@uca.fr

**Abstract.** The aim of this work is to show the influence of water vapor on the oxide scale formed in wet environment on a nickel-based alloy (NY600) between 900 and 1100 °C. Residual stress determinations were also performed after oxidation to characterise the influence of water vapor on the scale mechanical properties. Between 900 and 1100 °C, 10 vol.% H₂O in air, has little effect on the NY600 superalloy oxidation under isothermal conditions. The adherent oxide scale is mainly composed of Cr₂O₃. Results show that water molecules seem to increase the kinetic rate during the nickel-based alloy oxidation. X-ray diffraction was used to determine the residual stress level in the chromia scale. Results show that on this substrate water vapor as no influence on the residual stress level measured on the adherent chromia scale after oxidation. The alloy studied is found to be an oxidation resistant material at high temperature in wet environments (10 vol.% H₂O) between 900 and 1100 °C.

**Keywords:** Oxidation, Nickel-base alloy NY600, Water Vapor, Residual Stress, Cr₂O₃

## Introduction

Nickel-base superalloys such as Inconel 600, are often used as technologically important materials. It should be noticed that, the titanium content in the NY600 superalloy studied here is higher than in usual Inconel 600 alloys. These nickel-based superalloys are widely used in various sectors of industry such as electric power generation equipment, fuel cells, gas turbines, nuclear reactors or high temperature chemical vessels which often contain water vapor in the gaseous environment. The mechanical properties of this superalloy, such as temperature strength, high hardness, low thermal diffusivity is relatively good. Some previous works reported on the oxidation behavior of alloy 600 [1, 2] but not so much work has been performed yet concerning the oxidation properties of Nickel-based superalloys. Then, the aim of the present work is to study the oxidation NY600 superalloy in the high temperature range 900-1100 °C under very wet conditions (absolute 10 vol.% H₂O). A recent work has been performed on the influence of humidity on high temperature oxidation of Inconel 600 alloy giving results on the residual stress in the oxide scale between 600 and 900 °C [3]. It has been proposed that the residual stress in the oxide layer is in compression. Both humidity and temperature affect the residual stress level. The humidity affects especially the oxide growth stress at high temperature. In order to complete the knowledge in the field of high temperature oxidation of the NY600 alloy, the aim of the present work is to determine the humidity effect on the residual stress levels in oxide layers obtained from oxidation of NY600 ranging from 900 to 1100 °C. Related to the stress determination, attention will be paid on the chromia scale adherence and the role played by titanium and aluminium as alloying elements.

## Experimental

High temperature oxidation was performed during 96 h in flowing (8 l/h) air containing 10 vol.% H₂O. This gaseous environment corresponds to potential industrial oxidizing conditions. An adapted SETARAM TG92-1600 thermobalance was used to perform thermogravimetric measurements in flowing dry and wet air. The oxide scale characterisation was performed by X-ray diffraction (XRD). XRD patterns were obtained by use of a PANalytical X'pert MPD diffractometer (copper radiation, λ = 0.15406 nm). Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) permit to analyse the oxide scale composition and observe the scale morphology. The alloy composition is given in table 1. 1 mm thick rectangular specimens of around 4 cm² surface were polished through a 1200-grit SiC paper, then degreased with ethanol and finally dried. The studied NY600 superalloy, provided by Aperam Alloys Imphy, contains more titanium than the usual Inconel 600. Stress levels in Cr₂O₃ (ICDD 38-1479) were determined by X-ray diffraction. Determinations were made by using ψ goniometer
technique, with Cr-Kα₁ = 0.229 nm radiation, based on the (86.42 ° 2Θ) chromia peak. The ψ angle was varied in steps of 2 ° between -38 ° and +38 °. The mean stress σφ in the plane of the oxide is obtained from the slope (1 + v) σφ/E of the linear relationship between ε and sin²ψ, where ε is the strain, given by Δd/d, d being the interplanar spacing [4, 5]. The oxide was assumed to be isotropic with no stress gradients parallel to the surface components. The Cr₂O₃ Young's modulus, E, was taken as 282 GPa at room temperature and Poisson's ratio v as 0.30 [6]. The technique used is not able to determine the eventual stress gradient inside the oxide scale. Only a mean value is obtained from the whole surface and thickness analysed (21 µm maximum depth with the present analysis conditions).

<table><caption>**Table 1.** NY600 Ni-based alloy composition (weight %).</caption><thead><tr><th>NY600</th><th>Ni</th><th>Cr</th><th>Fe</th><th>Si</th><th>Ti</th><th>Mn</th><th>Al</th><th>Co</th></tr></thead><tbody><tr><th>weight %</th><td>Bal.</td><td>15.8</td><td>8.5</td><td>0.32</td><td>0.27</td><td>0.87</td><td>0.22</td><td>0.04</td></tr></tbody></table>

Kinetics

The mass gain curves per unit area are given in figure 1a. All isothermal thermogravimetric analyses were carried out during 96 h at 900, 1000 and 1100 °C in ambient air and wet air (10 vol.% H₂O). A parabolic regime was observed throughout the 96 h oxidation tests. At 900 °C, the oxidation rates are similar under dry and wet air. At 1000 °C and 1100 °C, the oxidation rates are slightly higher in wet air than in dry air. No breakaway corrosion is observed within the 96 h isothermal oxidation tests. Calculated parabolic rate constants are given in table 2. The oxidation rate is increasing with the temperature.

<table><caption>**Table 2:** Parabolic rate constants for oxidation of NY600 superalloy in dry and wet air (10 vol.% H₂O).</caption><thead><tr><th>Temperature</th><th>Dry air<br>kp (g<sup>2</sup> cm<sup>-4</sup> s<sup>-1</sup>)</th><th>Wet air<br>kp (g<sup>2</sup> cm<sup>-4</sup> s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>900 °C</td><td>0.67 (± 0.05) 10<sup>-12</sup></td><td>0.72 (± 0.05) 10<sup>-12</sup></td></tr><tr><td>1000 °C</td><td>0.57 (± 0.05) 10<sup>-11</sup></td><td>1.27 (± 0.05) 10<sup>-11</sup></td></tr><tr><td>1100 °C</td><td>2.27 (± 0.05) 10<sup>-11</sup></td><td>2.96 (± 0.05) 10<sup>-11</sup></td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>**Fig. 1a** Specific mass gain versus time curves obtained after 96 h isothermal oxidation of NY600 superalloy in dry air and wet air (10 vol.% H₂O) at 900, 1000 and 1100 °C.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 1b** Arrhenius plot obtained from the parabolic rate constants relative to the NY600 specimens' oxidation between 900 and 1100 °C, in dry air, and in wet air (10 vol% H₂O).</figcaption></figure>

This permits the calculation of the parabolic rate constants (kp) at each temperature. The Arrhenius plots of the kp values gives an apparent activation energy (Ea) calculated from the slope of the curve shown on Fig. 1b. Between 800 and 1100 °C the apparent activation energy corresponds to Ea = 236 ± 20 kJ mol⁻¹ in dry air. In wet air (13 vol% H₂O), the calculated apparent activation energy is Ea = 272 ± 20 kJ mol⁻¹ in the same temperature range. These values are
close to those proposed in the literature for the oxidation of chromia-forming alloys considering diffusion-controlled mechanism of a chromia scale. According to other authors Ea values are ranging between 250 to 290 kJ mol⁻¹ for chromia scales [7, 8].

## Morphology and oxide scale composition

The oxide scales cross sections are shown in figure 2. The oxide scales are adherent and mainly composed of chromia. A thin Mn<sub>1.5</sub>Cr<sub>1.5</sub>O<sub>4</sub> scale is found at the external interface mixed with some TiO<sub>2</sub> oxide grains. As also observed on other aluminium containing alloys (Inconel 601) [9] alumina is formed as internal oxide pegs along the alloy grain boundaries. This internal aluminium oxidation seems to be deeper in wet conditions.

<figure><img src="image_4.png" /><figcaption>**Fig. 2** Cross sections obtained after 96 h dry and wet air (10 vol.% H<sub>2</sub>O) oxidation of NY600 between 900 and 1100 °C.</figcaption></figure>

Figure 3 shows that, between 900 and 1100 °C, no scale spallation is observed on the surface neither after dry oxidation nor wet air oxidation. The grain size is also very similar for dry and wet conditions.
<figure><img src="image_5.png" /></figure>

June 9-14 2024 in Les Embiez Island (France)

<figure><img src="image_6.png" /><figcaption>Fig. 3 Surface scale morphology after 96 h dry and wet air (10 vol.% H₂O) oxidation of NY600 between 900 and 1100 °C.</figcaption></figure>

Oxide scale structural analysis

XRD patterns obtained on the oxide scales after dry and wet conditions are shown respectively in figure 4 and in figure 5. The oxide scale is mainly composed of chromia Cr₂O₃. With an increased temperature, manganese chromite seems to be more present on the surface and the alloy is less detected due to the thickening of the oxide scale. Alumina is not observed on the diffraction patterns because this oxide is only formed as internal oxide particles inside the alloy. TiO₂ is well detected by XRD due to its presence at the external interface. Water vapor does not change drastically the scale composition compared with dry air conditions.
<figure><img src="image_7.png" /><figcaption>Fig. 4 XRD patterns obtained after 96 h dry air oxidation of NY600 between 900 and 1100 °C.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5 XRD patterns obtained after 96 h wet air (10 vol.% H₂O) oxidation of NY600 between 900 and 1100 °C.</figcaption></figure>

Stress determination.

After 96 h oxidation at 1000 °C, in wet air (10 vol.% H₂O), figure 6 shows an example of the linear relationship between d and sin²ψ.

<figure><img src="image_9.png" /><figcaption>Fig. 6 Example of stress determination by XRD on an NY600 specimen oxidized 96 h at 1000 °C in wet air (10 vol.% H₂O). It shows the linear relationship between d and sin²ψ. The strain ε used for the stress determination is calculated by Δd/d.</figcaption></figure>

Similar curves were obtained for each 96 h oxidation conditions at 900, 1000 and 1100 °C, in dry or wet air environment. The stress determination was performed 3 times for each oxidation condition. Stress values, given in table 3, are the mean values obtained from the 3 stress determinations.
<table><caption>**Table 3.** Stress determination in Cr₂O₃ on the NY600 oxidized 96 h at 900, 1000 and 1100 °C in dry or wet air environment.</caption><thead><tr><th>Stress / MPa</th><th>Dry air</th><th>Wet air</th></tr></thead><tbody><tr><td>900 °C</td><td>-800 ± 100</td><td>-900 ± 100</td></tr><tr><td>1000 °C</td><td>-1800 ± 100</td><td>-1800 ± 100</td></tr><tr><td>1100 °C</td><td>-2300 ± 100</td><td>-2200 ± 100</td></tr></tbody></table>

The residual compressive stress level increases with the temperature due the higher scale thickness. Results also show that on this substrate water vapor as no important influence on the compressive stress level in the chromia scale after oxidation for a given temperature. This is in accordance with the good adherence of the scale observed on this NY600 alloy under the oxidizing conditions tested in the present work in which it is observed that, no stress relaxation occurred due to scale spallation during cooling to room temperature (see figure 3). In the high temperature range studied here (900-1100 °C) it is then showed that the NY600 superalloy oxidation behavior is different than the one observed in the 600-900 °C range on another Inconel 600 alloy on which both temperature and humidity affects the residual stress level [3]. Residual stress is a result of two contributions: thermal stress and growth stress. Calculation of the thermal stress can be performed as follows. Assuming that, metal and oxide are considered as isotropic, there is no creep during cooling, temperature is homogeneous in the system, growth stress is negligible during cooling; thermal stress can be calculated with Eq. (1) proposed by Liu [4, 11] using data found in references [4, 12]. NY600 Young's modulus E<sub>m</sub> = 210 GPa; Chromia Young's Modulus E<sub>o</sub> = 282 GPa; ΔT = Temperature variation (900, 1000 or 1100 K); t<sub>m</sub> = alloy thickness (1000 µm); t<sub>o</sub> = chromia thickness (5 to 13 µm); alloy thermal expansion coefficient α<sub>m</sub> = 17.6 10<sup>-6</sup> K<sup>-1</sup>; chromia thermal expansion coefficients α<sub>o</sub> = 7.3 10<sup>-6</sup> K<sup>-1</sup>. The oxide scale thickness remains very low compared to the alloy thickness. Then, the denominator remains close to 1 in Eq. 1. Calculations gives σ<sub>th 900</sub> = -2600 MPa σ<sub>th 1000</sub> = -2900 MPa σ<sub>th 1100</sub> = -3200 MPa at 900, 1000 and 1100 °C in dry and wet conditions. Then, calculations show that thermal stress can largely contributes to the residual stress measured and increase with increasing temperature. At this stage, it is not possible to calculate the growth stress generated at high temperature. Only *in situ* stress measurements could complete this work to really determine the growth stress level at high oxidation temperatures.

$$ \sigma_{th} = \frac{E_0 \Delta T(\alpha_0 - \alpha_m)}{1 + 2 \frac{E_0 t_0}{E_m t_m}} \quad (1) $$

## Discussion.

Results show that 10 vol.% water vapor does not induce a modification in the scale composition after oxidation. The homogeneous void distribution inside the chromia scale is in accordance with what is generally proposed in the literature for chromia scales formed under wet conditions on Ni-based alloys [13, 14]. It was indicated by Norby that protons from water vapor might dissolve in oxide scales [15]. As shown on the kinetic curves (Figure 1) it may enhance the oxygen transport in the form of hydroxide ions and increase the kinetic rates. Henry et al. [16] proposed that in wet conditions pores are incorporated in the growing chromia scale whereas under dry conditions porosity was located at the scale/alloy interface. This is attributed to an increased cation vacancies concentration and thus higher outward chromium diffusion which corresponds to the increased values of kp in wet air (see table 2). The effect of the iron content should also be discussed. On iron-based alloys, it has been demonstrated that water vapor drastically enhances the iron oxidation leading to breakaway corrosion [17]. In the present work, the good corrosion behaviour is also certainly related to the low iron content of the alloy studied as it was observed on other nickel-based substrates (such as alloy SY625) [18]. Then, no breakaway corrosion is observed due to iron oxidation in wet environment on the NY600 alloy studied here within the temperature range and temperature duration.

In the 900-1100 °C temperature range investigated, structural analyses (figures 4 and 5) show that the oxide scale is composed of Cr₂O₃ and titanium oxides (TiO₂) crystallites are located at the external interface. In such a scale, a doping effect of the chromia scale by titanium can be envisaged. Some authors have explained that the presence of titanium oxide at the external scale interface indicate that titanium diffuses through chromium oxide layers [19, 20]. A previous work using the *in situ* XRD analysis performed on Fe17Ti alloys has shown that Cr₂TiO₅ forms at the very beginning of the tests, but this oxide is not stable after 2 hours oxidation and transforms into TiO₂ and Cr₂O₃ [21]. In another work, Antoni et al. showed a beneficial effect of titanium on the oxide scale adherence under thermal cycling conditions
[22]. These authors suggested that the presence of titanium as alloying element, can lower the chromium content to 12-14 % to obtain good cyclic oxidation behaviour. It has also been proposed that the parabolic rate constants can be influenced by a doping effect of chromia by titanium [23, 24]. Titanium induces the creation of additional chromium cationic vacancies in the chromia scale when Ti⁴⁺ ions replace Cr³⁺ ions in the oxide lattice [25]. The chromia layer growth is governed by predominant bulk cationic diffusion and a mixed diffusion at grain boundaries [26]. Then, the cationic vacancy increase in the Cr₂O₃ layer will favor the protective chromia scale formation without hindering the scale adherence as proposed by other authors [21].

Results also show that manganese diffuses to the scale external interface in all cases. This phenomenon was reported by many authors [23, 27, 28]. It was indicated that Mn₁.₅Cr₁.₅O₄ could be considered as non-protective because it presents porous and non-adherent properties [27, 29]. Other authors pointed out that Mn₁.₅Cr₁.₅O₄ located at the external interface, could reduce the access of O₂ and H₂O to Cr₂O₃ and its subsequent conversion into a CrO₃ or CrO₂(OH)₂ volatile oxide above 900 °C [30-32]. In the case of chromia-forming materials, the volatilisation of Cr species is one of the key phenomena related to the detrimental effect of water vapor. According to Opila [33], evaporation rates may be ranked in the following order: Al(OH)₃<Si(OH)₄<CrO₂(OH)₂. Indeed, the products of the reaction of water vapor with chromia and silica have significant volatility at temperatures above 800 °C, whereas comparable volatility levels of products from reactions with alumina require temperatures higher than 1400 °C. At each temperature two scales are observed on the surface. The external subscale (Mn₁.₅Cr₁.₅O₄; TiO₂) remains adherent to the internal chromia subscale (Cr₂O₃) during cooling. In this study, manganese chromite, present at the external interface, provides a beneficial effect during the isothermal exposure by hindering the chromia scale volatilisation under dry and wet conditions. Then chromium depletion under the chromia scale does not occurs so much and this phenomenon prevents breakaway oxidation due to iron oxidation. Concerning scale adherence improvement too, the good oxide scale adherence observed on the NY600 superalloy is also attributed to the pegging effect observed on figure 2 with chromia pegs in front of internal alumina formation along the alloy grain boundaries. Internal oxidation of aluminium is a well-known phenomenon [34]. On such nickel-base alloys a continuous alumina scale is not formed at the initial stage of oxidation because the Al content is too low. According to the literature, a continuous Al₂O₃ scale may form with time under the chromia scale on a 4 wt.% containing alloy and depending on the alloy microstructure. In this study, the initial amount of aluminium in the alloy (0.22 wt.%) is not high enough to lead to a continuous alumina scale at the internal interface but it leads to internal oxidation along the alloys grain boundaries. It appears that this internal oxidation promotes the pegging effect of the chromia scale and the absence of voids at the internal interface, which also improves the good chromia scale adherence.

Conclusion

Between 900 and 1100 °C, 10 vol.% H₂O in air has little effect on the NY600 superalloy oxidation under isothermal conditions. The adherent oxide scale is mainly composed of Cr₂O₃. Some TiO₂ crystallites are observed at the external interface and Al₂O₃ is present as internal oxide pegs. X-ray diffraction was used to determine the residual stress level in the chromia scale. Results show that on this substrate water vapor as no influence on the residual stress level on the adherent chromia scale after oxidation. At 900 °C σ = -800 ± 100 MPa (dry) and σ = -900 ± 100 MPa (wet). At 1000 °C σ = -1800 ± 100 MPa (dry) and σ = -1800 ± 100 MPa (wet). At 1100 °C σ = -2300 ± 100 MPa (dry) and σ = -2200 ± 100 MPa (wet). The nickel base alloy studied here is believed to be an oxidation resistant material at high temperature in wet environments up to 1100 °C and 10 vol.% H₂O due to its low iron content.

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