Nitriding Treatments in Nickel–Chromium-Based Superalloy INCONEL 718: A Review

Abstract

This literature review focuses on the nitriding treatments of nickel-based superalloy UNS N07718, also called INCONEL 718. This alloy was selected due to the relevance of this alloy in the aerospace and oil and gas industries. The referenced studies reflect a clear trend towards improving superalloys through advanced nitriding processes, highlighting plasma nitriding as one of the most promising techniques to enhance alloy 718 against corrosion and wear. The importance of optimizing nitriding parameters, such as temperature and time, is emphasized to increase wear and corrosion resistance while minimizing adverse effects like delamination or the formation of undesirable phases. Finally, the most appropriate alternatives for future research are recommended.

1. Background

Nickel alloys hold great importance in materials engineering due to their exceptional combination of mechanical, chemical, and overall physical properties, which make them highly versatile and in demand across various industries [1]. Compared to stainless steels, Ni-based alloys can be utilized at extremely low temperatures (cryogenic) and temperatures exceeding 500 °C, maintaining their high strength and corrosion resistance in many environments [2]. This combination of properties made them suitable for industrial applications such as aerospace, oil and gas, power generation, and the chemical and petrochemical industries [3]. Strengthening mechanisms such as oxide dispersion, solid solution, and precipitation are used to improve mechanical properties like tensile strength and hardness, which are essential for ensuring integrity in applications subjected to high loads [2].

This review is focused on one of the most popular precipitation-strengthened Ni-based alloys [3]: UNS N07718, commercially known as alloy 718, INCONEL718, or INCONEL alloy 718. INCONEL is a registered trademark of Special Metals, developed in 1962 as part of the second generation of precipitation-hardened Ni–Cr alloys; they are frequently called “superalloys” because they offer excellent performance at extreme service temperatures, from below freezing to temperatures nearly 700 °C [1,3].

Special Metals [3] states that INCONEL718 is the most widely used age-hardenable aerospace alloy. Initially, this alloy was extensively used in aerospace applications and turbines and later in nuclear engineering and oil and gas extraction. In the latter industry, it is widely used in completion, wellhead, and production equipment, where the ability to resist wear and aggressive environments and maintain a maximum hardness is necessary when subjected to constant abrasion, erosion, or friction. This industrial necessity led to the development of surface treatments to enhance these properties in the alloys. Examples include nitriding processes, which can improve the surface properties of nickel alloys [4,5,6,7,8,9,10,11,12,13] and in stainless steels [14]. Different nitriding methods depend on the nitriding medium: gas, liquid, or plasma. Nitriding treatments aim to introduce nitrogen into the crystal structure of the alloy through mechanisms such as interstitial solid solution or the formation of new nitrided phases. However, compared to stainless steels, the solubility of nitrogen in nickel-based alloys is lower, which limits the formation of interstitial solid solutions [1].

Additionally, these alloys contain significant concentrations of nitride-forming elements such as chromium, titanium, aluminum, and niobium, which can adversely affect corrosion resistance. For these reasons, nitriding alloy 718 is a challenging surface treatment. The purpose of this review paper is to summarize the current state of knowledge of nitriding treatments in UNS N07718. Additionally, the goal is to identify and highlight the aspects that need further research, as the study of nitriding treatments for this alloy has been limited and only recently explored (within the last 30 years).

2. Research Methodology

The main tool used to identify the relevant literature for this review was the SCOPUS search engine, a large database of abstracts and citations covering more than 25,000 journals from all disciplines. The query used in the search engine was: ‘nitriding’ AND ‘INCONEL 718’ OR ‘N07718’. The search was conducted within ‘Article title, Abstract, Keywords’, and included all document types. The 24 records returned by the queries (sent on 18 July 2024) were carefully reviewed and filtered based on their content and relevance to the objective of the review. Five (5) records were discarded as they were not directly related to the application of nitriding on the alloy 718.

The analysis of the search results provided by Scopus shows that 13 of the 19 works were published in the last 10 years, as shown in Figure 1a. This reveals that research into the use of nitriding treatments has recently been promoted. The metrics of the remaining nineteen articles were exported as an RIS file to be analyzed using VOSVIEWER software (version 1.6.20) [15]. The technique for science mapping was co-word analysis, derived from the “abstract”, “author keywords”, and “index keywords”. The fractional counting methodology was employed to construct the bibliometric network, as recommended by [16]. The result of the bibliometric network is shown in Figure 1b for keywords with a minimum of three occurrences.

Two clusters were obtained in the co-word analysis of Figure 1b:

• Green: ‘Nitriding’, ‘Plasma Nitriding’, ‘INCONEL 718’, and ‘Corrosion’ as the main keywords with 14, 13, 12, and 11 occurrences, respectively. Those terms are associated with general aspects of nitriding, such as ‘surface treatment’, ‘scanning electron microscopy’, ‘treatment temperature’, and ‘hardness’.
• Red: ‘Nickel alloys’, ‘Wear’, and ‘Chromium nitrides’ as the main keywords with 11, 11, and 10 occurrences, respectively. Those keywords were linked to related terms such as ‘friction’, ‘tribology’, ‘surface roughness’, ‘nitrogen’, and ‘nitrided layer’.

The keywords “corrosion” and “wear”, each with 11 occurrences, constitute the most studied central aspects in both clusters. This result reflects the importance of their evaluation in nitriding research on INCONEL 718. In fact, out of the 19 references, a total of 16 directly evaluated these aspects, with five evaluating both corrosion and wear [6,11,12,18,19]. Three studies evaluated only corrosion [9,20,21], while five focused solely on wear [4,8,22,23,24]. Finally, only three works assessed tribocorrosion conditions [5,10,13].

The nitriding conditions applied to the INCONEL 718 (N07718) alloy in the references found in the literature review are summarized in

Table 1. Summary of the nitriding conditions for INCONEL 718 (UNS N07718) alloy.

Ref.	Year	Thermal Treatment of the Alloy UNS N07718//Hardness	Type of Nitriding (Reactor Characteristics)	Nitriding Conditions
[23,24]	1997	Solubilized for 2 h at 1025 °C, and aged at 780 °C for 6 h	High-temperature plasma nitriding (Cold wall)	t: 1, 4 and 16 h T: 550 to 750 °C M: H2:N2 = 1:1 V: Not specified
[18]	2006	Annealed//480 HV0.05	Intensified plasma-assisted nitrided (Cold wall)	t: 3 h T: 450 y 490 °C M: N2:Ar = 4:1 V: −1000 V
[22]	2016	Not informed//482 HV0.01	Direct-current plasma nitriding (Not specified)	t: 1 h y 4 h T: 400, 500, and 600 °C M: H2:N2 = 1: 1 V: 500 V
[13]	2016	Not informed//~450 HV0.05	Salt bath nitriding	t: 4 h, 8 h y 16 h T: 425, 450, 475, 500 °C M: K2CO3, Na2CO3, (CO(NH2)2), others.
[11]	2017	Not informed//250 HV0.05	Low-temperature gas nitriding	t: 5 h T: 460 °C M: Ammonia
[10]	2020	Not informed//~450 HV0.01	Liquid nitriding	t: 16 h T: 500 °C M: KCNO, NaCNO, KCl, NaCl, K2CO3, Na2CO3, and Li2CO3
[12]	2020	Not informed//~450 HV0.05	Hot-wall plasma nitriding	t: 6 h T: 400, 450 y 500 °C M: N2:H2 = 75%:25% V: Not specified
[8]	2020	Annealed cast grade and additively manufactured//253 HV0.05	Arc-enhanced glow discharge (With heating-assisted system)	t: 1, 1.25, 1.5 and 2 h T: 450 °C M: N2/Ar/H2 = 79/14/7 in % V: −250 V
[9]	2022	Age-hardened//490 HV0.025	Triode plasma nitriding (Auxiliar heating)	t: 20 h y 4 h T: 400, 425, 450 °C and 700 °C M: N2:Ar at 7:3 V: −200 V
[6]	2023	Solubilized for 1 h at 1089 °C, and aged at 788 °C for 7 h//450 HV0.1	Low-temperature plasma nitriding (Cold wall)	t: 4 h T: 400 y 450 °C M: N2/H2/Ar at 70/20/10 in % V: 500 V
[5]	2023	Solubilized for 1 h at 1090 °C, and aged at 788 °C for 7 h//530 HV0.005	Low-temperature plasma nitriding (Active screen)	t: 20 h T: 400 °C M: N2:H2 = 75%:25%

. References [19,20] are not found in Table 1 since it was not possible to access those works. The works [4,21] do not report the nitriding conditions.

3. Review of the Properties and Characteristics of Nitrided UNS N07718

The analysis of the literature results was organized into three fundamental aspects: hardness and nitrided layer thickness, corrosion, and wear, which will be presented in the following subsections.

3.1. Hardness and Nitrided Layer Thickness

Figure 2 presents a comparative graph of the hardness of the nitrided samples from different studies as a function of treatment temperature and time. The studies referenced in Figure 2 employed various nitriding methods: liquid [10,13], gaseous [11], and plasma [5,6,8,9,12,18,22,23,24].

As a general trend in all the investigations, an increase in treatment temperature leads to an increase in the surface hardness after nitriding. For example, observed in Figure 2, the hardness increased of the nitriding treatments performed by Maniee et al. [12] at 400, 450, and 500 °C or by Kovaci et al. [22] at 400, 500, and 600 °C. Similarly, at the same temperature, an increase in treatment time produces the same effect, as seen in Figure 2 for the work of Kovaci et al. [22] comparing 1 h versus 4 h duration at temperatures of 400, 500, and 600 °C.

In Figure 3, a comparative graph of the nitrided layer thickness as a function of treatment temperature is presented. Similar to the hardness (Figure 2), there is a direct relationship between treatment time and temperature with the increase in layer thickness. We recommend that the reader review the impact of treatment time or temperature on layer thickness (in Figure 3) as detailed in the studies by Maniee et al. [12], Kovaci et al. [22], and Jing et al. [13].

Given that nitriding is a thermochemical treatment, thermally activated treatments are expected to be controlled by solid-state atomic diffusion [14]. Consequently, the higher the temperature or the longer the treatment duration, the greater the nitrogen diffusion into the surface. This results in increased residual stresses in the crystal lattice due to the formation of more expanded structures and high nitrogen supersaturation, thereby increasing the surface hardness after nitriding treatments [9,12,13,22]. In the case of nickel alloys, particularly UNS N07718, surface hardening has been confirmed due to the formation of nitrogen-expanded austenite (γ_N) [7,8,9,11,12,13,18], as well as in nitrided austenitic or duplex stainless steels at low temperatures [14]. Additionally, the use of high temperatures, specifically ≥450 °C, has shown the precipitation of chromium nitride (CrN), known for its high hardness and influence on corrosion resistance, in nitriding treatments of UNS N07718 [6,9,10,12,13,18,22].

Figure 2 and Figure 3 show that most nitriding treatments were conducted in the 400–500 °C range, resulting in hardness increases of two to four times that of the substrate (age-hardened ~450–500 HV, see Table 1), with layer thicknesses mainly between 3 and 13 µm. This reveals a considerable discrepancy in the hardness and layer thickness results among the studies. For example, at a fixed temperature (such as 450 °C), different values are observed without a direct correlation to treatment time. However, in many cases, liquid nitrided specimens exhibit higher hardness and thicker layers compared to plasma nitrided samples, even at the same temperature. This is because liquid nitriding promotes the formation of layers primarily composed of CrN, which is harder than the expanded austenite layers [10,13].

Although the base material is the same (UNS N07718), it undergoes two main heat treatments: annealing and age-hardened (see Table 1). Note that the lowest hardness values (Figure 2) were obtained by Zhang et al. [11] and Mondragón-Rodríguez et al. [8], whose nitriding treatments (gas and plasma, respectively) were conducted on annealed substrates (~250 HV), in contrast to the other studies, which used hardened substrates (450 to 500 HV; see Table 1). This suggests that the substrate’s thermal treatment affects the nitriding samples’ surface hardness. However, when looking only at the works that used hardened substrates, there are still huge differences between the hardness results. Therefore, there are other causes of the wide variability of results among the studies.

The authors acknowledge that the discrepancies in hardness (Figure 2) and thickness (Figure 3), particularly among plasma treatments, may be related to the different characteristics of the reactors and treatment parameters, such as gas mixtures, pressure, and voltage (Table 1), which influence the effectiveness of nitriding on nitrogen diffusion. However, establishing cause-and-effect relationships between these factors and the nitriding results is a challenging task, as these aspects are not fully described in the studies.

In hard coatings on soft substrates, the hardness measurement of the substrate-free coating occurs when the indentation depth is less than 10% of the coating thickness, according to Bückle’s Law [25]. This principle has also been applied to hard films produced by thermochemical treatments on softer substrates [6,9,14]. When penetration exceeds 10%, the measured hardness value is not exclusive to the coating but is influenced by the plastic deformation of the substrate. This principle is particularly relevant if we consider, for example, that a reported hardness of 1500 HV_0.05, measured with an indentation load of 0.5 N (the most commonly used, see Table 1), can generate an indentation depth of around ~1.1 µm. In cases where a load of 0.1 N was used, the depth is approximately ~0.5 µm.

These results imply that the indentation penetrated more than 10% of the layer thickness in most studies (Figure 2), considering thicknesses between 2 and 10 µm. Consequently, the measured hardness corresponds to a compound hardness of the system nitrided layer–substrate, and they are not the surface hardness of the nitrided layer (substrate-free). In other words, most of the hardness results in Figure 2 are, to a greater or lesser extent, influenced by the deformation of the substrate during the measurement due to the thin nitrided layers obtained. This is probably the leading cause of the dispersion of results in Figure 2, given that different indentation loads applied to different layer thicknesses were used in the investigations. Recently, ref. [25] has determined that this limit may be less than 10%, as the measured hardness depends on factors such as the ratio between the hardness of the coating and the substrate and the ratio of the thickness of the coatings to the indentation depth.

3.2. Corrosion

The study of corrosion in nitrided INCONEL 718 alloys is a common and highly relevant topic, employing various approaches to evaluate corrosion resistance. As mentioned in Section 2, out of the nineteen studies on nitrided INCONEL 718, eight assess corrosion resistance [6,9,11,12,18,19,20,21], while three evaluate erosion–corrosion [5,10,13]. All these studies were conducted in NaCl-based solutions (3.5%, 0.1 N or 0.6 M).

The approaches used included a combination of electrochemical techniques such as potentiodynamic polarization [5,6,9,11,12], open circuit measurements [5,6,9,11,12,18], electrochemical impedance spectroscopy (EIS) [11,12], and potentiostatic polarization for crevice corrosion tests [6].

The low-temperature plasma nitriding processes applied by Singh and Meletis [18] (450 °C—3 h, intensified plasma-assisted nitriding) and by Oikawa et al. [5] (400 °C—20 h, Active Screen Plasma Nitriding) increased the open circuit potentials (OCP) of the nitrided samples compared to untreated ones. Benefits in corrosion behavior due to low-temperature treatments (≤460 °C) were also demonstrated through potentiodynamic polarization tests in NaCl solutions [5,6,9,11,12]. In most cases, the best resistance in nitrided conditions was observed within the Tafel region, characterized by an increase in corrosion potential (E_corr), which was either accompanied by or independent of a reduction in the corrosion rate. These improvements are explained by the presence of γ_N in the nitrided layer, formed both in gas nitriding [11] and plasma nitriding [5,6,9,12].

However, Tao et al. [9] and Nuñez et al. [6] state that nitrided conditions tend to increase the passivation current (i_pass) and decrease the potential range of the passivation region compared to the untreated material [6,9]. Another common aspect among the studies is that the presence of CrN in the nitrided layer along with γ_N is not sufficient to significantly impair corrosion resistance [6,9,11]. Nevertheless, when the nitrided layer is composed exclusively of CrN, the loss of corrosion resistance is evident due to the formation of Cr-depleted regions, as observed by [6,9,12] when nitriding at temperatures ≥ 450 °C. An example of these effects of nitriding temperature on the polarization curve is presented in Figure 4a, taken from [12].

On the other hand, electrochemical impedance studies, although less frequent in research, provide important information about the nature of the electrochemical system formed by the material under study and the medium. Maniee et al. [12] and Zhang et al. [11] found that the Nyquist curves of both untreated and nitrided INCONEL 718 have the characteristic shape of a capacitive impedance arc, as represented in Figure 4b. According to [12], the equivalent circuit consists of the solution resistance (Rs), the charge-transfer resistance (Rct), and a constant phase element (CPE) (imperfect capacitor). This circuit corresponds to the imperfect nature of species transfer between the electrolyte and the metal surfaces. The higher Rct of the plasma nitrided conditions (at 400 °C and 450 °C), according to [12], represents a higher barrier that the ion needs to overcome to cross the electrode/electrolyte interface, compared to the untreated electrode/electrolyte interface. In the case of the nitrided condition at 500 °C, the value of Rct is lower than the untreated condition, a result that agrees with the lower general corrosion resistance evidenced through the Tafel curves in the same study [12].

It is widely known that precipitation-hardened INCONEL 718 has a heterogeneous microstructure composed of an austenite (γ) matrix and different phases such as gamma prime (γ′-Ni₃Al, Ni₃Ti, and Ni₃(Ti,Al)) and gamma double prime (γ″-Ni₃Nb), as well as carbides and nitrides of elements like Nb, Fe, Cr, and Ti, among others. However, only the work of Tao et al. [9] mentioned the effect of nitriding on these phases (γ′ and γ″) and their participation in corrosion. The authors state that the different response to low-temperature nitriding between γ′/γ″ and the matrix did not affect the corrosion behavior due to the absence of long-range elemental migration of substitutional atoms (such as Fe, Cr, Ni, etc.) [9].

Localized corrosion resistance from pitting and crevice corrosion was only evaluated by Nuñez et al. [6] in INCONEL 718 plasma nitrided at 400 °C and 450 °C. The treatment at the higher temperature had negative effects on both types of localized corrosion, showing increased susceptibility compared to the untreated condition. On the other hand, their work demonstrated how low-temperature plasma nitriding (400 °C) was effective in enhancing resistance to crevice corrosion, as observed in the lower corrosion depth in Figure 5. Additionally, this nitriding condition showed lower pitting density compared to the untreated condition in potentiodynamic polarization tests [6].

The results indicate that nitriding, especially when conducted at low temperatures, shows a tendency to enhance corrosion resistance in Cl-containing environments compared to untreated INCONEL 718. This improvement is attributed to the formation of a homogeneous nitrided layer composed of nitrogen-expanded austenite, γ_N, which acts as a protective barrier against corrosion. Changes in microstructure, including the formation of chromium nitride (CrN) and the distribution of alloying elements, are crucial in controlling the corrosion properties of the alloy. Unlike nitrided stainless steels, CrN precipitation in the expanded austenite layer does not impair corrosion resistance [6,9,11]. However, nitriding at temperatures ≥450 °C led to the formation of a layer of CrN. With this layer, the samples became susceptible to general pitting and crevice corrosion attacks, as demonstrated by [6,9,12].

It is worth remembering that corrosion evaluation through Tafel tests, such as those carried out by [5,11,12], barely reflects the performance against generalized corrosion. Therefore, studies of other types of corrosion, such as those carried out by Nuñez et al. [6], emphasize the importance of these treatments in improving resistance to localized corrosion, aspects essential for the durability and reliability of materials under adverse conditions. Thus, the studies reflect a clear trend toward improving superalloys through advanced nitriding processes, with plasma nitriding as one of the most promising techniques for strengthening INCONEL 718 against corrosion in chloride-containing environments. Thus, the results indicate that careful control of nitriding parameters, mainly temperature, can significantly improve corrosion resistance, which is essential for extending the material’s service life in severe industrial applications.

3.3. Wear

The INCONEL 718 alloy has been the subject of numerous studies to improve its tribological properties (wear resistance and coefficient of friction) through various nitriding techniques, modifying its surface at a chemical and microstructural level. This objective is reflected in the bibliographic search results: of the nineteen (19) scientific publications on nitrided INCONEL 718, wear resistance was evaluated in ten (10) of them. The dry pin-on-disk was the most commonly used wear test, as exemplified by the works of [8,10,12,18,22]. The results of this type of test agree that the nitriding treatment decreases the wear rate and, in some cases, the coefficient of friction, in addition to promoting changes in the wear mechanisms compared to untreated INCONEL 718. In the particular case of plasma nitriding, the research by Kovací et al. [22] and Maniee et al. [12] demonstrates that the untreated specimens exhibit the highest wear rate, which decreases upon growth in the time and temperature of the nitriding process. This results from forming the nitrided layer with greater hardness and thickness as the treatment time and temperature increase.

The effect of test temperature (25 °C, 100 °C, and 200 °C) on tribological performance was evaluated by Xue et al. [10] in dry pin-on-disk tests. The results show that the wear rate remains unchanged with increasing test temperature for both INCONEL 718 with and without liquid nitriding. The untreated’ wear tracks are more profound and broader, with many deep grooves and spalling (Figure 6a,c,e), which, according to the authors [10], corresponded to abrasive and adhesive wear. The wear tracks of nitrided samples are smoother and show a characteristic adhesive wear mechanism (Figure 6b,d,f). As a general result, liquid nitriding can significantly improve the wear resistance of INCONEL 718 alloy, reducing the wear rate by three orders of magnitude compared to untreated samples [10]. Changes in the wear mechanism were also reported in the research of Núñez et al. [6] in scratch resistance tests. The micro-plowing mechanism was predominant in the untreated material, while in the nitrided surfaces, it was micro-cutting with increased scratch resistance and decreased wear coefficient and scratch depth.

Aw, Batchelor, and Loh conducted wear tests using the pin-on-drum configuration, under which they observed that despite the reduction in the coefficient of friction of the nitrided surfaces, material loss due to wear was significant (up to 60%) due to the brittleness of the nitrided layer and its low adhesion to the substrate [23]. This was a consequence of the use of high plasma nitriding temperatures (550 to 750 °C) that promote the formation of a hard and brittle CrN layer instead of a layer mainly composed of expanded austenite (γ_N), as identified in the low-temperature nitriding investigations (<500 °C) mentioned above [5,6,8,9,10,12,18,22]. In liquid nitriding, ref. [13] also reported losses in tribocorrosion resistance of INCONEL 718 nitrided at 500 °C due to high residual compressive stresses induced by CrN precipitation. In contrast, in nitriding at 425, 450, and 475 °C, the strength was superior to the untreated material [13].

On the other hand, Zhang et al. [11] observed that the ultrasonic nanocrystal surface modification (UNSM) process before gas nitriding harmed wear resistance (dry high-frequency reciprocating rig) due to the increased surface roughness. This effect was compensated for by the hardness gain due to nitriding. The coefficient of friction was the same for all conditions with or without nitriding. According to [11], this was due to the test configuration. However, in pin-on-disk tests, Singh and Meletis [18] justify that the same COF between the untreated and nitrided conditions is explained by the increase in roughness (by more than six times) due to the plasma nitriding treatment performed.

To analyze the tribocorrosive properties of this alloy, Jing et al. [13] and Xue et al. [10] studied the erosion–corrosion resistance of liquid nitrided INCONEL 718. Despite using different experimental setups and abrasive solutions, both studies demonstrated that liquid nitriding increased pure erosion–corrosion resistance. Jing et al. [13] achieved this result as long as the nitrided layer was mainly composed of expanded austenite at treatment temperatures of 425, 450, and 475 °C for 1, 4, and 16 h. However, the samples nitrided at 500 °C (4, 8, and 16 h) in [13] were composed of a CrN layer, whose erosion–corrosion performance was worse than the untreated material.

In contrast, although the nitrided layer in the study by [10] was also solely composed of CrN (nitriding at 500 °C for 16 h), its better performance was attributed to its superior hardness (1810 HV) compared to the hardness of the SiO₂ abrasive particles (1100–1200 HV). Conversely, in the tests by [13], the hardness of the Al₂O₃ abrasive particles ranged from 2500 to 3000 HV, which was higher than the hardness of the samples nitrided at 500 °C for 1 h (~1550 HV), for 4 h (~1800 HV), and for 16 h (~2100 HV). These effects of counterbody properties in tribocorrosion tests were studied in depth by Oikawa et al. [5] on INCONEL 718 nitrided by active screen plasma nitriding (ASPN). The authors [5] discovered that selecting the counterbody material (considering its chemical nature and hardness) influences the wear mechanism and the overall tribological response. For example, when using a steel ball, the nitrided samples showed better tribological performance than the untreated ones. In contrast, the tribological response was the same with or without treatment when using a ceramic ball.

The study of [5] also reveals that plasma nitriding at low temperatures improves tribocorrosion resistance in certain conditions and corrosion resistance, making low-temperature plasma nitriding of INCONEL 718 an even more viable option for various industrial applications. Of the three investigations that addressed tribocorrosion, only Xue et al. [10] highlighted the beneficial effect of nitrogen in controlling the pH during corrosion and in repassivation in the presence of Cl-ions [10].

4. Final Remarks

The nitriding of the UNS N07718 alloy (INCONEL alloy 718) is a thermochemical treatment that has been little explored in the last 30 years. This demonstrates that it is a recent area of research. The literature search shows that 13 of the 19 works were published in the previous ten years. Plasma treatments stand out from liquid and gas nitriding as the most used (in more than 60% of the research).

Temperature and nitriding time are the variables that predominantly influence the composition, thickness, and performance of the nitrided layer since it is a solid-state diffusion-controlled process. As a general trend, treatments at temperatures ≤ 450 °C have demonstrated the formation of γ_N along with the precipitation of nitrides (CrN), accompanied by increases in wear resistance and general corrosion resistance in the presence of chlorides. Improvements were also achieved in localized pitting and crevice corrosion conditions thanks to the nitriding process.

However, layers composed of CrN are obtained when treatments are performed at temperatures ≥450 °C. Under these conditions, both general and localized corrosion resistance (in NaCl solutions) are adversely affected due to the formation of chromium-depleted regions. This nitride layer demonstrates a hardness up to five times that of the substrate but exhibits poor tribological performance. Various wear tests conducted in the works revealed that the hard and brittle CrN layer weakly adheres to the substrate and eventually delaminates, thereby increasing the wear rate.

The results of low-temperature nitriding (≤450 °C) appear promising for this alloy as they show the possibility of forming expanded phases despite the alloy’s significant content of nitrogen-affine, nitride-forming elements [1,9]. Low-temperature nitriding’s hardness, wear, and corrosion results are particularly relevant in applications where the balance between mechanical strength and corrosion resistance is crucial, such as in aerospace and oil and gas applications. UNS N07718 is an alloy that must meet stringent chemical composition and property standards. The requirements for aerospace applications are outlined in the AMS5662 standard [26], and for oilfield applications in the API 6A718 standard [27]. Mention of these standards is absent in the nitriding studies. Furthermore, the limited information available in many studies about the heat treatments performed on the base material is noteworthy (Table 1).

On the other hand, alloy 718 is expected to maintain adequate performance over a wide range of temperatures, from freezing to around 700 °C [2,3,28]. In particular, in oilfield environments, alloy 718 must meet the requirements outlined in NACE MR0175-2021/ISO 15156:2020 [29]) and more specifically in ANSI/NACE TM0284 [30] and ANSI/NACE TM0177 [31]. These standards cover testing metals for their resistance to cracking caused by the combined effects of tensile stress and corrosion in water-based environments containing hydrogen sulfide (H₂S). At room temperature, this issue is typically known as sulfide stress cracking (SSC), whereas at higher temperatures, it is referred to as stress corrosion cracking (SCC). Also, in this sour environment, hydrogen absorption generated by corrosion may be caused by hydrogen stress cracking (HSC).

None of the studies conducted evaluations according to the aforementioned standards. Moreover, most of the corrosion and wear tests were performed at room temperature and under aerated conditions, which are entirely different from the service conditions: the presence of chloride, hydrogen sulfide (H₂S), carbon dioxide (CO₂), low pH, and sometimes free sulfur, along with mixtures of light and heavy oil, methane, sand, and water [28].

Given this situation, the nitriding of alloy 718 is far from being a viable and suitable treatment until standardized performance evaluations are conducted on the nitrided material (AMS5662, API6A718, NACE MR0175/ISO 15156-1:2020, ANSI/NACE TM0284, and ANSI/NACE TM0177, among others). This may seem discouraging to some, but considering the recency of this research area, it clearly indicates the correct path forward, representing significant opportunities for future studies.

The progress achieved thus far enhances the scientific understanding of surface treatment effects. It opens new possibilities for applying INCONEL 718 in industrial environments that require durable, high-performance materials. Finally, the research and development of the nitriding treatment for INCONEL 718 continues to reveal the complexity and importance of complying with the standards for aerospace and oil and gas applications.