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# Oxidation of additive manufactured 699XA Ni-based alloy under different atmospheres

Fernando Pedraza, Antoine Duval, Ivo Šulák, Benedikt Nowak, Thomas Kepa, Germain Boissonnet, Gilles Bonnet

► To cite this version:

Fernando Pedraza, Antoine Duval, Ivo Šulák, Benedikt Nowak, Thomas Kepa, et al. Oxidation of additive manufactured 699XA Ni-based alloy under different atmospheres. Corrosion Science, 2024, 237, pp.112333. 10.1016/j.corsci.2024.112333. hal-04907098

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https://hal.science/hal-04907098v1

Submitted on 22 Jan 2025

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OXIDATION OF ADDITIVE MANUFACTURED 699XA Ni-BASED ALLOY UNDER DIFFERENT
ATMOSPHERES

Fernando PEDRAZA¹,* Antoine DUVAL¹, Ivo ŠULÁK², Benedikt NOWAK³, Thomas KEPA¹, Germain
BOISSONNET¹, Gilles BONNET.

¹ La Rochelle Université. LaSIE UMR 7356- CNRS. Avenue Michel Crépeau, 17042 La Rochelle cedex 1 France.

² Institute of Physics of Materials. Czech Academy of Sciences. Žižkova 22. 61600 Brno, Czech Republic

³ VDM Metals International GmbH, Kleffstraße 23, 58762 Altena, Germany

Corresponding author: fpedraza@univ-lr.fr

Abstract

The oxidation behaviour of an additively manufactured (AM) NiCrAl alloy (VDM® Powder 699XA) was compared with the hot rolled counterpart in different atmospheres (dry air, Ar-10%H₂ and Ar-10%H₂-10%H₂O) for 48h at 950 and 1100°C. Internal oxidation occurred through the twin boundaries and dislocations in the AM material. Such defects promoted the outward diffusion of Al and the growth of a protective Al₂O₃ scale at reduced P₂O₅ (Ar-10%H₂ and Ar-10%H₂-H₂O), hence restraining the internal oxidation. It is thus demonstrated that this high Cr-containing alloy is a marginal alumina former under controlled conditions like the hot rolled alloy.

Keywords: High temperature oxidation; additive manufacturing; Ni-based alloy; microstructure; kinetics.

Introduction

The nickel-based VDM Alloy 699XA was designed to resist metal dusting conditions [1]. This was thoroughly addressed in a paper from Schlereth et al. at various carbon activities, 620°C and 19 bar [2]. The authors also highlighted that greater resistance against metal dusting could be achieved with around 29.5 wt.% Cr, more than 2 wt.% Al and additions of iron (2.7 wt.% or 4.2 wt.%) in model alloys because of the change of the Al activity. White and collab. applied various surface modifications (mill grinding, shot peening and mild pickling) on the same alloy to evaluate the optimum surface finish ensuring protective behaviour in harsh metal dusting conditions [3]. The authors concluded that fast Al and Cr diffusion induced by the deformation of the mill grinding resulted in improved metal
dusting resistance due to a high dislocation density. However, shot peening introduced Fe
contamination and pickling lowered the Cr content, hence lowering the metal dusting resistance.
Such studies demonstrate that protection against aggressive environments at high temperatures is
highly reliant on establishing dense protective oxide scales. In this view, Mathieu and collab.
investigated the preoxidation in air and Ar-H₂O of four chromia forming Ni-based alloys including
Alloy 602CA [4], which is also a high temperature NiCrAl alloy like 699XA. Despite the relatively low Al
content (slightly above 2 wt.%) and high Cr amount (around 25 wt.%), the alloys investigated by
Mathieu et al. [4] behaved as alumina formers in air, with the growth of inhomogeneous scales at the
surface. In contrast, severe internal oxidation and growth of chromia occurred in the H₂O containing
atmosphere in line with the suppression of protective scale formation and the enhanced internal
oxidation in water vapour, also proposed by Esuman et al. in model Ni-Cr alloys [5]. While the
incorporation of H₂ in Fe-Cr model steels was found to majorly contribute to internal oxidation of Cr
[6], this was not put forward in the Ni-Cr model alloys [5] possibly because the solubility of hydrogen
in nickel is reputed to be lower than in iron [7].

Therefore, this paper investigates the mechanisms of formation of the oxide scales at 950 and 1100°C for short times (48h) on the nickel-based Alloy 699XA at high temperatures in different atmospheres (synthetic air, Ar+10H₂ and Ar+10H₂+10H₂O, vol.%) in view of elucidating the development of protective oxide scales. As opposed to the previous studies [1-3], the alloy investigated here was additively manufactured by laser powder bed fusion at RWTH Aachen (partner of the top-AM European project [8]) for burner head applications. The effect of the gas compositions is highlighted by comparing the behaviour of the additively manufactured material with the state-of-the-art counterpart, in this case, hot rolled plate material, which is solution annealed like the AM material.

**2.- Experimental**

The nickel-based alloy 699XA, whose nominal composition is given in **Table 1**, was hot rolled and solution annealed at VDM Metals (Germany) (**Figure 1a**), while the additively manufactured counterpart was fabricated by laser powder bed fusion at the Digital Additive Production (DAP) department of RWTH Aachen (Germany) in the shape of cylindrical rods and then solution annealed (1200°C for 1 h) and hot isostatically pressed (HIP) at 1.5 10⁸ Pa, 1300°C for 1h at the Lukasiewcz Institute of Technology of Krakow (Poland) with an AIP 10-30H (American Isostatic Presses) to reach a density of 99.30 ± 0.33 % (**Figures 1b and 1c**). The rods were subsequently cut into 2 mm thick sections and polished finely for later characterization. The band contrast map in **Figure 1b** obtained
via electron backscattered diffraction (EBSD, Zeiss Auriga system) shows a density of twins as high as that of grain boundaries. A dislocation density was established using transmission electron microscopy to be ~ 10¹³ m⁻² [9]. **Figure 1c** reveals an equiaxed microstructure in all 3D, with a grain size of 29.3 ± 26.9 µm with no preferential orientation as revealed by EBSD mapping. The building direction is parallel to the Z axis. In contrast, the plate material has a coarser grain size (69.3 ± 12.0 µm) and some twins [10] but the chemical composition (X-ray fluorescence -Bruker Tornado M4, λ = KαRh- and combustion analysis - LECO 928 Series-) is very similar to that of the additively manufactured material (Table 1).

<table><caption>Table 1. Chemical composition (wt.%) of the AM (additive manufactured) and the hot rolled 699XA Ni-based alloy.</caption><thead><tr><th></th><th>Ni</th><th>Cr</th><th>Al</th><th>Fe</th><th>Mn</th><th>Si</th><th>Nb</th></tr></thead><tbody><tr><th>AM</th><td>Bal.</td><td>29.4</td><td>2.3</td><td>0.1</td><td>&lt;0.01</td><td>0.04</td><td>0.1</td></tr><tr><th>Hot rolled</th><td>Bal.</td><td>29.5</td><td>2.3</td><td>0.2</td><td>&lt;0.01</td><td>0.05</td><td>0.1</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>Figure 1 SEM micrographs of 699XA alloy microstructures of the (a) hot rolled 699XA nickel-based alloy and of (b), (c) the additively manufactured and HIPed counterpart. (b) is the band contrast map and c) EBDS maps with inverse pole figure colouring for the X, Y and Z directions.</figcaption></figure>
The coupon samples for the high temperature oxidation tests were ground with SiC P180. The latter were conducted in the thermogravimetric (TGA) devices Setaram TG92 and Setsys models the latter being coupled to a Setaram Wetsys system to generate water vapour. In all cases, the samples were suspended with Pt wires. The chambers were pumped down well below 10⁻³ bar at least twice to remove impurities. The samples were then heated at 20°C/min under a 20 mL/min flow of Ar (99.999% purity, 2 vpm O₂, 5 vpm H₂O) till the reaction temperature (950 and 1100°C) was reached. Then, the chambers were pumped again and let the reactive gas enter at a flow of 16 mL/min. The P₀₂ (in bar) calculated with the FactSage software at the Dechema Institute (Germany) was 2.1 10⁻¹, 2.6 10⁻²⁷ and 5.2 10⁻¹⁸ for synthetic (dry) air, Ar-10%H₂ and Ar-10%H₂-10%H₂O, respectively at 950°C. The onset of the isothermal tests was considered when the internal pressure reached 1 bar. The duration of the reaction was set to be 48h. After that, the samples were cooled down at 50°C/min. Complementary tests at shorter times (20h) were also performed to elucidate the effect of each gas constituent in the oxidation process. Such complementary tests included the reaction of a blank (pure alumina) sample, static and flowing tests with Ar, heating in vacuum and finer surface finish (SiC P2400) for the hot rolled samples.

After the high temperature tests, the samples were characterised by X-ray diffraction (XRD -Bruker AXS D8, λ = KαCu-), Raman spectroscopy (Jobin-Yvon Labram HR Evolution, λ = 532 nm), optical (Leica DMRM) and scanning electron microscopy (SEM, FEI Quanta200F) and energy dispersive spectrometry (EDS, EDAX detector) on the samples' surfaces and the metallographic cross-sections.

The latter were prepared with a Cu electrodeposit on top of the oxide scales to protect them from spallation upon metallographic preparation.

**3.- Results**

**Figure 2a** shows the evolution of the specific mass with time at 950°C (solid lines) and 1100°C (dot lines) under the different gas atmospheres for the additively manufactured 699XA. Irrespective of the gas, the exposure to 1100°C brings about greater mass variations than at 950°C, especially when water vapour is added to the gas mixture. Similarly, the application of the complete law for calculating the parabolic rate constants [11] and from the parabolic plot of **Figure 2b** lead to the kₚ values gathered in **Table 2**. At 950°C, the oxidation rate is greater in air than in the H₂-containing atmospheres, which is nevertheless relatively significant. In contrast, at 1100°C, the fastest reaction occurs in Ar-10%H₂-10%H₂O and the slowest in air. Unexpectedly, the oxidation kinetics in Ar-10%H₂ is faster than in air. Indeed, at low P₀₂ (Ar-10%H₂ with and without H₂O) the kₚ values are about one order of magnitude higher than the ones reported by Ramanarayanan and collab. (kₚ = 3.5 10⁻¹² g²cm⁻³
$^4$S⁻¹ at 950°C and 8 $10^{-11}$ g²cm⁻⁴s⁻¹ at 1100°C) for the exclusive formation of Cr₂O₃ in model Ni30Cr under a low Pₐ₂ of 1.4 $10^{-17}$ bar [12]. At high Pₐ₂ (air), the kₚ value is lower than that found in model Ni30Cr at 1100°C by P. Berthod (kₚ = 2.3 $10^{-11}$ g²cm⁻⁴s⁻¹), who considered minor volatilization of CrO₃ in air [13]. However, the kₚ is one order of magnitude higher than the one obtained ($1.76 \times 10^{-13}$ g²cm⁻⁴s⁻¹) by Kemdehoundja and collab. upon the oxidation of model Ni30Cr at 900°C in air [14]. Therefore, the TGA results indicate that an exclusive Cr₂O₃ layer did not develop. Various characterization methods including SEM/EDS, XRD and Raman spectroscopy were thereafter employed to understand this.

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Figure 2.- (a) Evolution of the specific mass with time at different temperatures in air ($2.1 \times 10^{-1}$ bar), Ar-10%H₂ ($2.6 \times 10^{-27}$ bar) and in Ar-10%H₂-10%H₂O ($5.2 \times 10^{-18}$ bar) for the additively manufactured 699XA. (b) is the parabolic plot of (a).</figcaption></figure>

<table><caption>Table 2.- Parabolic rate constants, kₚ (in g² cm⁻⁴ s⁻¹) calculated from the complete parabolic law [10] for the additively manufactured 699XA Ni-based alloy exposed to different gas atmospheres at 950 and 1100°C for 48h.</caption><thead><tr><th></th><th>Air<br>(2.1 10<sup>-1</sup> bar)</th><th>Ar-10%H<sub>2</sub><br>(2.6 10<sup>-27</sup> bar)</th><th>Ar-10%H<sub>2</sub>-10%H<sub>2</sub>O<br>(5.2 10<sup>-18</sup> bar)</th></tr></thead><tbody><tr><td>950°C</td><td>1.3 10<sup>-12</sup></td><td>6.7 10<sup>-13</sup></td><td>6.5 10<sup>-13</sup></td></tr><tr><td>1100°C</td><td>2.7 10<sup>-12</sup></td><td>7.2 10<sup>-12</sup></td><td>1.5 10<sup>-11</sup></td></tr></tbody></table>

The intensity of the oxide peaks is very low in the X-ray patterns obtained in the symmetric mode compared to that of the face-centred cubic structure of the 699XA alloy substrate (Figure 3). It can be noticed that in all three gas atmospheres, the peaks of the face-centred cubic (fcc) structure of the substrate material are observed, which indicates sufficiently thin oxide scales. However, the substrate peaks shift slightly to the right after oxidation suggesting a decrease in the lattice parameter. Such reduction in lattice parameters may be related to the exit of the alloying elements
from the substrate structure to the surface upon oxidation. This is confirmed by the various peaks of the α-Cr₂O₃ (JCPDS-ICDD File No 38-1479) phase formed particularly in air. In Ar-10%H₂-10%H₂O, the peaks of the α-Cr₂O₃ shift slightly and suggest the formation of Al-rich mixed (Cr,Al)₂O₃ oxides. In contrast, the only tiny peak is exclusively related to α-Al₂O₃ in the samples exposed to Ar-10%H₂ at 1100°C but not at 950°C, possibly because of the insufficient volume of oxide probed under the experimental conditions employed here. It shall be noted though that α-Al₂O₃ is also very minorly detected in the other atmospheres but cannot be observed in the X-ray patterns of Figure 3. Therefore, many different areas of the surface were analysed using Raman spectroscopy after exposure to the different atmospheres and the results are gathered in Table 3.

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>Figure 3.- X-ray patterns of the additive manufactured 699XA at (a) 950 °C and (b) 1100 °C under different gas atmospheres for 48 h. The pattern of the unoxidized (raw) substrate is also included for comparison.</figcaption></figure>

<table><caption>Table 3.- Summary of the main oxide phases obtained by Raman spectroscopy (also from the fluorescence region) in representative areas of the surface after exposure of additively manufactured 699XA to the different gas atmospheres and temperatures.</caption><thead><tr><th></th><th>950°C</th><th>1100°C</th></tr></thead><tbody><tr><th>Air</th><td>Major: Cr₂O₃<br>Minor: Ni(Cr,Fe)₂O₄, α-Al₂O₃</td><td>Major: Cr₂O₃<br>Minor: Ni(Cr,Fe)₂O₄, α-Al₂O₃</td></tr><tr><th>Ar-10%H₂</th><td>Major: α-Al₂O₃<br>Minor: Cr₂O₃ (nodules)</td><td>Major: α-Al₂O₃</td></tr><tr><th>Ar-10%H₂-10%H₂O</th><td>Major: Al₂O₃ (surface)<br>Minor: α-(Al,Cr)₂O₃, NiCr₂O₄ (nodules)</td><td>Major: α-Cr₂O₃ (surface)<br>Minor: α-(Al,Cr)₂O₃ (nodules)</td></tr></tbody></table>

The results of the Raman analyses gathered in Table 3 show that Cr₂O₃ forms preferably at high P₀₂ while Al₂O₃ occurs at low P₀₂ even in the presence of H₂O. The very rare small Cr₂O₃ nodules formed
at 950°C in Ar-10%H₂ may have developed upon heating of the sample to the oxidation temperature but the Raman intensity is nevertheless very low. In all cases, the surfaces were poorly covered with oxides after 48h of exposure to the low P>O₂ atmospheres at 950 and 1100°C. The observations under the SEM and the EDS analyses were far more informative.

**Figure 4** shows the SEM surfaces of the additively manufactured 699XA exposed to the three atmospheres for 48h at 950 and 1100°C. It can be observed that small polygonal oxide grains typical of those of Cr₂O₃ [12] grow over the surface of the additively manufactured 699XA when oxidized in air for 48h. When increasing the temperature from 950 to 1100°C, the surface is covered more extensively with oxides whose grain size also increases with temperature. In Ar-10H₂, the oxide nodules identified as made of Al₂O₃ by Raman spectroscopy develop over the surface that displays a decoration of the grain boundaries. Interestingly, at 1100°C such nodules are metallic (Cr) and barely contain any oxygen according to the EDS analysis suggesting that the Cr₂O₃ nodules grown upon heating in Ar to 1100°C have been reduced under Ar-10%H₂ upon the 48h at 1100°C. In addition, small round pores (marked with black arrows) have developed at the metal surface. With the addition of H₂O to the gas, different oxide features appear as a function of temperature. Indeed, small polygonal crystals of Cr₂O₃ and some porous nodules develop at the surface at 950°C. The latter grow and cover the surface at the expense of the small grains at 1100°C. According to the EDS analysis and the Raman results, the nodules are mainly composed of α-(Al,Cr)₂O₃ while the tiny crystals contain mostly Cr and some Ni suggesting the formation of Cr₂O₃ and NiCr₂O₄.

<figure><img src="image_7.png" /></figure>
**Figure 4.** SEM surfaces of the additively manufactured 699XA exposed to the three atmospheres for 48h at 950 and 1100°C. The arrow marks highlight surface porosity.

In the cross-sections (Figure 5), it can be noticed that the oxidation in air leads to the formation of an external scale that is thicker at 1100°C than at 950°C (see Table 4). Underneath the scale, the internal oxides grow along what appears to be the grain boundaries. Such internal oxides have a different contrast in the backscattered electron mode. In particular, they appear grey close to the oxide scale but they are much darker deeper in the substrate. The EDS analyses indeed show that the former are rich in Cr and Ni oxides but the latter are exclusively composed of Al. Decreasing the $P_{O_2}$ of the gas results in much thinner external oxide scales, in particular under $Ar-10\%H_2$ (note the different scale bars).

At 1100°C, the external scale thickens when exposed to air and it does not appear compact, like with $Ar-10\%H_2-10\%H_2O$ gas. However, the external scale thickness grown in $Ar-10\%H_2$ is well below 1 µm. In all cases, the most distinctive feature is the significant internal oxidation front, whose thickness increases with decreasing $P_{O_2}$ and increasing temperature as shown in Table 4. It can be noticed that the greatest increase of internal oxidation occurs in air while the exposures to the $H_2$-containing gas though significant, are more moderate between 950 and 1100°C.

<figure><img src="image_8.png" /></figure>

**Figure 5.** SEM cross-sections of the additively manufactured 699XA exposed to the three atmospheres for 48h at 950 and 1100°C. Note the different scale bars for $Ar-10\%H_2-10\%H_2O$ at 950°C.
<table><caption>Table 4.- Thickness of internal oxidation of the additively manufactured 699XA after 48h of exposure to the three atmospheres for 48h at 950 and 1100°C. The "increase" refers to the times the internal oxidation extended between 950 and 1100°C.</caption><thead><tr><th></th><th>950°C</th><th>1100°C</th><th>increase</th></tr></thead><tbody><tr><td>Air</td><td>12±2 µm</td><td>29±4 µm</td><td>x 2.4</td></tr><tr><td>Ar-10%H<sub>2</sub></td><td>18±5</td><td>25±4</td><td>x 1.4</td></tr><tr><td>Ar-10%H<sub>2</sub>-10%H<sub>2</sub>O</td><td>18±2 µm</td><td>33±5 µm</td><td>x 1.8</td></tr></tbody></table>

The X-ray maps shown in **Figure 6** confirm that the outer scales grown in air and water vapour are mostly composed of $Cr_2O_3$ while those grown in the $H_2$ are rich in $Al_2O_3$, in particular at 1100°C. In addition, the extent of the internal oxidation along the defects is significantly much greater in air than in the $H_2$-containing atmospheres (note the different scale bars) and in all cases, the Al content of the internal oxides increases with the distance to the scale/substrate interface.

<figure><img src="image_9.png" /><figcaption>Figure 6.- X-ray maps of the distribution of Cr and Al in the cross-sections of the additively manufactured AM 699XA exposed to the three atmospheres for 48h at 950 and 1100°C. Note the change in colour of the Al and Cr maps between air and the low $P_2O_5$ atmospheres to better highlight the differences in composition of the scales.</figcaption></figure>

## 4.- Discussion

The composition of alloy 699XA improves significantly the metal dusting and oxidation resistance in the high temperature range [1] as already proven in the recent works of Schlereth et al. with [3] and without [2] surface treatments. This alloy can be considered a marginal $Al_2O_3$ former according to its
Al/Cr/Ni ratio whenever the P<sub>O2</sub> is sufficiently low [15]. The formation of Al<sub>2</sub>O<sub>3</sub> can procure resistance to carburization to Ni-Cr [16] and Fe-Cr [17] alloys in particular at very high temperatures by fostering the Al diffusion from the bulk [16]. Therefore, it appears interesting from the scientific perspective to understand how different gas atmospheres may affect the establishment of the oxide scales and the overall behaviour of a brand new class of additively manufactured 699XA.

It has been shown in **Figure 5** that the major uptake recorded in the TGA curves (Figure 2) is related to the internal oxidation of the additively manufactured material. Such internal oxidation of additively manufactured Ni-based alloys is a well-known phenomenon that is typically related to the linear and 2D defects shown in **Figure 1** that ensues from the process itself and contributes significantly to the overall mass gains shown in **Figure 2** [e.g., 18,19]. In our particular case, the samples contained a significant number of grain and twin boundaries and a very high dislocation density (Figure 1) through which diffusion of the reacting species readily occurs. This is particularly true in the case of air whenever the oxide scales are not sufficiently continuous as Juillet et al. demonstrated in the comparative studies between an additively manufactured and a wrought IN-718 (~20 wt.% Cr) oxidized in air and Ar between 600 and 800°C for 48h [18]. However, the reasons that lead to the very significant specific mass gains in the presence of water vapour are to be understood. Similarly, one could have expected no mass gain under Ar-10%H<sub>2</sub>, since it is virtually free of oxidising species (P<sub>O2</sub> = 2.6 10<sup>-27</sup> bar). To elucidate the origin of such internal oxidation, various complementary tests at 1100°C were conducted with both the additively manufactured and the hot rolled counterpart (Figure 7).

<figure><img src="image_10.png" /></figure>

**Figure 7.**- Evolution of the specific mass with time of additively manufactured (AM) and hot rolled 699XA exposed at 1100°C under different testing conditions: (a) with flowing Ar conditions and (b) with static Ar (1 bar) conditions. Note the dramatic increase of specific mass gain when heating till 1100°C was conducted under vacuum, hence showing the strong oxidizing power of Ar. The results are compared with the AM specimens exposed to Ar-10%H<sub>2</sub> (P<sub>O2</sub> = 2.6 10<sup>-27</sup> bar).
First, a blank (pure alumina) specimen underwent the same 48h exposure in static Ar (Figure 7a). Neither buoyancy effects nor oxidation of the Pt suspensions were recorded. This ensures that the only contribution to the mass variations comes from the samples themselves. Subsequently, additively manufactured specimens with two surface polishings (SiC P180 and SiC P2400) were oxidised in static Ar and no major difference in mass variation between the samples was found. This indicates that the potential deformation and the surface roughness effects induced upon the metallographic preparation can be excluded in the oxidation of alloy 699XA at 1100°C. The same test conducted with the hot rolled samples (SiC P180) showed a dramatic decrease in the specific mass gain (Figure 7a). Interestingly, the greater mass gain in the additively manufactured alloy can readily be associated with the absence of a continuous external scale and the marked internal oxidation in comparison with the hot rolled counterpart (Figure 8).

<figure><img src="image_11.png" /></figure>
**Figure 8.** SEM micrographs of the samples surfaces and cross-sections of the (a) and (c) additively manufactured and (b) and (d) the hot rolled 699XA alloy after oxidation in flowing Ar (~10⁻⁶ bar) at 1100°C.

The low mass gain of the hot rolled material also occurred with static Ar atmosphere (Figure 7b) whereby the system had been first heated under Ar flow, the chamber filled in with Ar till 1 bar once the 1100°C reached and finally closed for 24 h to maintain Ar in the chamber. Within uncertainty limits, this test indicates that the main mass uptake occurs upon heating and that the O₂ and H₂O impurities (2 and 3 vpm, respectively in the bottle) are sufficient to oxidize the hot rolled 699XA even in the absence of a continuous supply of Ar. In contrast, a dramatic increase of the mass gain occurs when barely any oxide scale is allowed to form upon heating from room temperature to 1100°C (vacuum) and Ar is then introduced in the chamber at high temperature. This indicates that the establishment of the oxide scales upon heating is key to lower the oxidation kinetics and that Ar is a strong supplier of oxidising species. Although the kₚ values are calculated from the overall mass gain that also include internal oxidation (see internal oxidation thickness, Table 4), such heating stage in Ar and the subsequent different oxygen partial pressures may explain the increased kₚ values with temperature in Ar-10%H₂ and Ar-10%H₂-H₂O but not in air. In air, the oxygen partial pressure is very high (P₀₂ = 2.1 10⁻¹ bar) and allows the quick establishment of Cr₂O₃ (and NiCr₂O₄ in our case) scales like P. Berthod found on the oxidation of model Ni₃₀Cr in air at 1000 and 1100°C [13].

An intermediate situation seems to occur under flowing Ar-10%H₂ because the very low P₀₂ (2.6 10⁻²⁷ bar assuming 2 ppm of technical Ar) may not authorise the growth of any continuous external scale upon heating in Ar to block the internal oxidation and this contributes to the overall mass gain observed for the whole 48h of exposure. Once, Ar-10%H₂ is introduced in the reaction chamber, oxidation becomes thermodynamically possible. This can be better understood by adding our experimental data to the plot proposed by Schiek and collab. for the dependence of P₀₂ against temperature for various water vapour containing gas mixtures and of the stability of major oxides [20] (Figure 9). It can be observed that only Al₂O₃ can form under the Ar-10%H₂ atmosphere whether at 950 or 1100°C, which is indeed the only oxide found in the fluorescence region of the Raman spectra. This also demonstrates that the additive manufactured material is permeable to the Ar-10%H₂ gas mixture. Since the mass of oxygen is 16 times greater than that of hydrogen, most of the mass gain is attributed to oxidation, let alone that molecular diffusion appears to be promoted in the presence of hydrogen [21].

As opposed to the dry Ar-10%H₂, the P₀₂ in the Ar-10%H₂-10%H₂O atmosphere is sufficiently high (P₀₂= 5.2 10⁻¹⁸ bar) to oxidise Cr and Al but not Fe and Ni [20,22]. This was also confirmed by Esuman
et al. in the oxidation at 1050°C for 72h of model Ni-Cr alloys with Ar-10%H2O (10⁻⁶ bar) who proposed that the decreased growth rate of NiO also decreases critical Cr content required for protective scale formation at reduced P₂O₅ [5]. This helps in explaining why Cr₂O₃ grows at the surface of our samples oxidised under Ar-10%H₂-10%H₂O at 950°C (Figures 3 and 4 and Table 3) although the major contribution of the external nodules is α-(Al,Cr)₂O₃. Our results also appear close to the work of Nijdam and collab., who found exclusive growth of α-Al₂O₃ on NiCrAl alloys by increasing the Al flow to the surface by cold deformation (grinding and polishing) and/or by reducing the P₂O₅ [23], both conditions occurring in the additively manufactured material notably at 1100°C and under the low pressures of our Ar-10%H₂-10%H₂O atmosphere. Mathieu et al. also indicated that water vapour enhanced chromia growth, internal oxidation of Al and protrusion formation in the oxidation of four commercial Ni-(25-30)Cr-(0-2.3)Al (wt%) alloys [4]. Such protrusions like the ones observed in Figure 4 appear to be related to the internal oxidation of Al pushing out metal (that may subsequently oxidize) hence causing local damage of the scale through which molecular gas transport may occur [20]. Since the P₂O₅ is sufficiently low in Ar-10%H₂-10%H₂O (P₂O₅= 5.2 10⁻¹⁸ bar), once the external Cr₂O₃ scale is established at the alloy/gas interface, only α-Al₂O₃ can develop further (Figures 5 and 6). Overall, the idea that alloy 699XA is a marginal alumina former is confirmed here provided the P₂O₅ is sufficiently low and the flow of Al maintained.

<figure><img src="image_12.png" /></figure>
**Figure 9.-** The dependence of $P_{O_2}$ with the reciprocal temperature for various gas compositions and oxide systems. Adapted from the work of Schiek and collab. [20].

**5.- Conclusions**

The additively manufactured NiCrAl alloy (699XA) exposed to dry air, Ar-10%H₂ and Ar-10%H₂-H₂O for 48h at 950 and 1100°C shifts from being a Cr₂O₃ former to an Al₂O₃ forming material at reduced $P_{O_2}$. In all atmospheres but especially in air, significant internal oxidation occurs at these temperatures. The high number of grain boundaries in the additively manufactured material compared to the hot rolled material promotes the Al flow to develop exclusively Al₂O₃ under Ar-10%H₂, in particular at 1100°C. In contrast, the oxides below the Cr₂O₃ layer are rich in Cr and Ni while Al₂O₃ appears further inwardly thereby suggesting a sufficiently high $P_{O_2}$ in the material. The quick establishment of the internal alumina in the low $P_{O_2}$ conditions (Ar-10%H₂ and Ar-10%H₂-H₂O) hampers further internal attack of the material.

The potential hydrogen incorporation should be assessed to elucidate whether it assists molecular transport of the oxidizing species or whether the grain boundaries are the major contributor to internal oxidation. The comparison of the hot rolled against the additively manufactured material tends to confirm that only the latter is responsible for the degradation.

**6.- Acknowledgements**

This work has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 958192 (top-AM), which is gratefully acknowledged. Special thanks to the Digital Additive Production (S. Reich, P. Rob, B. Verma) of RWTH Aachen for the fabrication of the additive manufactured counterparts, to the Lukasiewcz Institute of Technology of Krakow (W. Polkowsky, T. Dudziak) for HIPing the samples, and to the Dechema Institute (C. Schlereth, E. White and M. Galetz) for the calculation of the $P_{O_2}$.

**7.- Data availability**

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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