Effect of Inclusions on the Corrosion Properties of Nickel Based Alloys 718 and EP718

Inclusions in steels and alloys are known as factors that could lower deformation, mechanical, corrosion and other properties. Study of inclusions in nickel based alloys is important since these materials could suffer from corrosion degradation in harsh operational conditions, where inclusions could lead to a pitting initiation. For estimation of a harmful effect of different inclusions on corrosion resistance of Ni-based alloys, inclusion characteristics (such as composition, morphology, size, number and location) on film filter and dissolution of metal matrix around different inclusions on surfaces of metal samples after electrolytic extraction (EE) were investigated in two samples of industrial Ni-based alloys (alloy 718 and EP718). It was found that both Ni-based alloys have various inclusions: carbides (large size NbTi-C and small multicomponent carbides), nitrides TiNb-N and sulfides (TiNb-S in EP718 alloy). The higher harmful effect on corrosion resistance of metal was detected around sulfides and small carbides containing Mo, W, Cr. Dissolution effect was also observed around large carbides and nitrides, especially around inclusions having size more than 10 µm. Moreover, the dissolution of metal matrix around inclusions and clusters located on the grain boundaries is 2.1-2.7 times larger compared to that for those inclusions inside of grains of the given alloy samples.


Introduction
Many investigations show that inclusions (incoherent precipitates) that are formed during production [1,2,3] can significantly affect mechanical, corrosion, deformation, machining properties of steels and alloys that were shown for low alloyed steels and carbon steels [4,5,6], stainless steels [7,8,9,10] and alloys [ 11,12]. Main reasons of the effects concerned to voids or additionally stressed zones between matrix and inclusions that lead to the formation of microcracks and their propagation in a metal matrix. Another reason is that around inclusion are formed zones free or with lower concentrations of precipitated phases and elements contained in inclusions or metal zones depleted by alloying elements. As a result, the metal zones adjacent to inclusions are characterized by higher level of defects and higher stress [1]. Therefore, the mechanical properties and corrosion resistance of steels and alloys depend on the amount of these harmful inclusions, their size, morphology and distribution.
Ni-based alloys have a high corrosion resistance, mechanical properties and widely spread in industrial application as a material for wellhead component and downhole equipment for corrosive service at high temperatures and pressures in the presence of aggressive components (such as H2S, CO2, chlorides) [14]. Since Ni-based alloys have complicated chemistry, it leads to form of various deleterious inclusions and phases such as carbides, sulphides, delta-phase, etc. Some inclusions can initiate a pitting corrosion and could lead to environmentally assisted cracking, corrosion fatigue, hydrogen embrittlement. Therefore, the study of corrosion and mechanical properties of Ni-based alloys and their dependence on different inclusions became the object of attention in many studies [15,16,17,18,19,20].
For instance, it was reported in [21,22] that stable pittings started at Nb-C rich particles, whereas nitrides of Ti and Nb have a lower effect on corrosion [23]. Precipitation of carbides of chromium and tungsten (such as M23C6, M6C) on the grain boundaries are also possible precursor sites for pitting in Ni-base alloys [24,25]. Mo-rich phases are also a reason of sensitization and poor corrosion properties on the grain boundaries [26]. In [27] it was shown, that delta phase has a detrimental effect on passivation and potential breakdown of alloys. Moreover, delta phase is also a trapping site for hydrogen, which lowered resistance to hydrogen embrittlement and dependent upon the morphology of second phase particles [28,29]. Despite of high velocity of crystallization additive technologies also give specific inclusion, so in [30,31] it was founded coarse carbides of Ti, Nb, Cr, Mo, Nb/Mo-rich particles.
Thus, the Ni-based alloys contain various inclusions. However, at the moment there are unclear and contradictory data on the influence of composition, size, location and morphology of different inclusions on the corrosion properties. Therefore, the main goals of the present study are to test an application of electrolytic extraction for precise three-dimensional investigations of different inclusions and clusters (their composition, morphology, size, number and location in metal matrix) in two industrial Ni-based 718 and EP718 alloys and to evaluate the effect of various inclusions and clusters on corrosion resistance of these alloys by investigation of dissolution metal matrix around typical inclusions during electrolytic extraction process.

Materials and Methods
In order to investigate the effect of inclusions on corrosion properties two commercially available nickel based alloys were studied. The materials were produced by VIM+VAR processes, then forged and heat treated according corresponding specifications. The chemical composition of the studied materials is given in Method of electrolytic extraction (EE) was used for 3D investigation of inclusions. Extraction was done in 10%AA electrolyte (10% acetylacetone -1% tetramethyl-ammonium chloride-methanol) at the following parameters: electric current 0.04-0.06 A and voltage 2.9-3.8 V According to previous studies [32], this electrolyte for extraction was selected so as not to dissolve the detected inclusions during intensive dissolution of metal matrix. Therefore, it can be assumed that the inclusions were not directly involved in the electrochemical dissolution of the metal matrix around the inclusion. 0.08-0.10 g of alloy samples were dissolved during each electrolytic extraction, which correspond to a dissolved metal layer of 70-95 µ m. After dissolution, the inclusions were collected on a surface of the polycarbonate film filter having a hole diameter of 0.4 µ m.
Characteristics of non-metallic inclusions (such as composition, morphology, size, number and location) were analyzed by means of scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The inclusions were investigated on a surface of film filters (FF) as well on a surface of metal samples (MS) that were subjected to dissolution. The SEM images of typical inclusions investigated on a film filter and metal surface after EE are shown in Figure 1. For evaluation of the effect of inclusions on the corrosion resistance of alloy, a dissolution extent of metal matrix around different inclusions after electrolytic extraction was determined by measurement of diameters of the crater (Dcr) around inclusion and calculation of relative coefficient of metal dissolution (KD), which can be calculated as follows: where Acr and Aincl are the area of the crater around an inclusion and the area of this inclusion measured on an SEM image, respectively.

Characterization of inclusions after electrolytic extraction of metal samples
Based on the 3D investigations of inclusions on film filter and on the surface of metal samples after electrolytic extractions, it was found that the total number of inclusions per unit volume in EP718 is about 6.5 times larger than that in 718 alloy. However, in most cases, a determination only of the total content of all inclusions cannot explain the difference in corrosion resistance of similar steels or alloys. It is well known that different inclusions (having various characteristics such as compositions, morphology, size and location in metal matrix) can significantly effect on the corrosion properties of steels and alloys. Therefore, this study was focused on estimation of the effect of these characteristics of different type inclusions on dissolution extend of metal matrix around these inclusions during electrolytic extraction, which may be similar to corrosion processes in pipeline steels and alloys.
Different inclusions observed in 3D on film filter and metal surface after electrolytic extraction of EP718 sample were conditionally divided into four groups depending on their composition and morphology. Table 2 shows typical SEM images, contents of the main elements and size ranges of different types of inclusions. Since the contents of such light elements as C, N and O cannot be accurately determined by EDS, their contents did not give in the Table 2. It was found that the sample of EP718 alloy contains the following typical inclusions: 1. Irregular or regular carbides of Nb and Ti contained also up to 1-6% of W (NbTi-C type) having sizes of 3-40 µ m. A ratio of Nb and Ti contents of these carbides (RNb/Ti = %Nb / %Ti) in these inclusions varied from 1.2 up to 2.4 and had an average value of 1.6±0.2. 2. Regular nitrides of Ti and Nb (TiNb-N type) contained up to 5% of W and had a size of 4-26 µ m.
Moreover, these inclusions contained sometimes also a small amount of carbides of these elements. The value of a RNb/Ti in these nitrides was from 0.2 up to 0.6 (0.3±0.2 on average) depending on the fraction of NbTi-C inclusions precipitated on nitrides. 3. Irregular sulphides of Ti and Nb (TiNb-S type) contained 7-21% of S, ≤ 4% of W and ≤ 3% of Mo.
These sulfides having size of 2-37 µ m were observed mostly as clusters on grain boundaries of a metal sample along the deformation direction in combination with carbides ( Table 2). The value of a RNb/Ti in these sulfides was from 0.2 up to 0.7 (0.5±0.2 on average) depending on the fraction of carbide inclusions. Moreover, it was found that these types of sulfides, that were detected as acicular inclusions by using a conventional 2D investigations on polished surface of metal samples, have a plate or petal-like shape with thickness 1-2 µ m, as can be seen in Table 2. 4. Small size multicomponent carbides (0.5-4 µ m) contain Nb, Ti, Mo, W and Cr (NbTiMoWCr-C type) and have different morphologies (such as spherical, irregular and acicular shapes). Length of acicular inclusions can be up to 12 µ m. The contents of main elements in these inclusions, may vary (RNb/Ti = 1.4±0.5), as can be seen in Table 2. This type of inclusions was observed mostly on grain boundaries of metal matrix and partially on surfaces of different inclusions. Based on the location, morphology and compositions, it can be safely assumed that these inclusions precipitated in a solidified metal matrix during cooling and heat treatment.
It should be noted that the NbTi-C and TiNb-N inclusions were observed on film filter and metal surface after electrolytic extraction as separate particles as well as clusters or group of particles located very close to each other, which summarized size is significantly larger (in most cases) than that of separate particles. Moreover, sometimes the inclusions of different types were observed together in one group or located close to each other.
Based on the composition and morphology, different inclusions observed in alloy 718 were conditionally divided into the three following groups (as shown in ): 1. Regular and irregular carbides of Nb and Ti (NbTi-C type) contained also 0-7% of Cr and having sizes 2-30 µ m. The ratio of Nb and Ti contents of these carbides (RNb/Ti) in these inclusions was varied in the wide range (from 3.4 up to 23) and has an average value of 9.9±3.7. These NbTi-C inclusions were observed on film filter and metal surface as separate particles as well as clusters or group of particles located very close to each other, as shown in . 2. Large size irregular nitrides of Ti and Nb (TiNb-N type) having size of 9-27 µ m. The value of a RNb/Ti in these nitrides was from 0.2 up to 0.9 µ m (0.3±0.2 on average). These nitride inclusions are usually located in metal matrix as separate particles. 3. Small size carbides (0.5-8 µ m) contain mostly Nb, Ti and 1-3% Cr (NbTi-C) and can have spherical, irregular or acicular shapes, as shown in . The value of a RNb/Ti in these nitrides varied in the wide range from 5.6 up to 14.9 µ m (on average 8.6±2.1). These small size carbides were located usually on grain boundaries of metal matrix and sometimes on surfaces of different inclusions, as can be seen in . It was assumed that these small carbides precipitated in a solidified metal matrix during heat treatment.

Effect of inclusions on corrosion resistance
After electrolytic extraction, the surfaces of metal samples were investigated for evaluation of characteristics and location of different inclusions in a metal matrix. It was found that the small size inclusions (such as NbTiMoWCr-C in EP718 alloy and NbTiCr-C in alloy 718) and sulfides (TiNb-S and TiNb-SC in the EP718 alloy) are located mostly on grain boundaries of metal matrix though large size nitrides and carbides in these Ni-based alloys can be observed both on grain boundaries as well inside metal grains.
Also, it was detected that the metal matrix was not evenly dissolved during the electrolytic extraction process around different inclusions. As can be seen in Figure 3, the dissolution of metal matrix around some inclusions was almost uniform (Figure 3, a) as in most areas of metal sample, whereas large size "craters" of metal dissolution was observed around other inclusions and clusters (Figure 3, b). Moreover, significant metal dissolution as "ditches" was observed on grain boundaries of the EP718 sample. These formed craters and ditches around some inclusions and on grain boundaries can be explained by the easier electro-chemical dissolution of the weakened metal matrix around these corrosion-active inclusions and on grain boundaries during electrolytic extraction. However, according to publications of Inoue et al. [13], the 10%AA electrolyte used in this study for electrolytic extraction does not dissolve the inclusions observed in these alloy samples. In other words, these inclusions are not involved directly in dissolution process of the metal matrix around these inclusions due to some additional electro-chemical reactions. As a result, it can allow us to assess the corrosion resistance of metal matrix around different inclusions and to detect the inclusions having most harmful effect on weakened of metal matrix around these inclusions.  Table 4.  According to the obtained results given in Table 4 and Figure 4, the largest size of craters (Dcr up to 40 μm) has the NbTi-C and TiNb-S inclusions of titanium carbides, niobium, and titanium sulfides located along grain boundaries in EP718 sample. The average dissolution coefficients of metal matrix around these inclusions (KD) are 4.9 and 8.8, respectively, which suggests that sulfides are more active in terms of corrosion. Titanium sulfides, which are rarely mentioned in the literature, were also detected in the EP718 alloy. Nevertheless, it is known that sulfides are corrosive inclusions that significantly reduce pitting resistance (from 3 to 100 times) and are preferred centers for pitting, especially in chloride environments [5,9,35].
It should be pointed out that the NbTi-C inclusions located inside of grains have 2 times smaller values of Dcr and KD compared to those for inclusions located on grain boundaries. Moreover, it can be seen that the small amount of NbTiMoWCr-C inclusions located at the grain boundaries have the highest values of dissolution coefficient KD (from 14 up to 32) compared to the other types of inclusions. It can be explained by the depletion of the metal matrix around of these inclusions by the alloying elements (such as chromium, molybdenum and tungsten), which are contained in these inclusions and responsible for corrosion resistance of metal matrix. However, the size of the craters/ditches around these inclusions is relatively small (less than 15 µ m) compared with other types of inclusions due to their smaller size.
The TiNb-N inclusions were observed inside the grains and, judging by the low dissolution coefficient (KD~2.8) and small diameter of craters (9-10 µ m), have a lower effect on corrosion, which confirms literature data [21]. As it can be seen in Table 4 and Figure 5, the largest craters in the alloy 718 were observed around TiNb-N (Dcr = 24-51 μm) and NbTi-C (Dcr = 16-40 µ m) inclusions located at the grain boundaries. At that the size of craters around these types of inclusions tends to increase significantly with an increasing of equivalent size (Deq) of inclusions. Though the KD coefficient for these inclusions can be varied in a wide range (from 2 up to 13), the dissolution coefficient of metal matrix around these inclusions located on grain boundaries is on average 2.5 times larger compared to those located inside of grains.
The values of KD for small size NbTiCr-C inclusions, which are located mostly on grain boundaries, can be varied in a wide range (from 2 up to 12) although the size of craters/ditches around these inclusions in this 718 alloy usually is much smaller (Dcr = 2-7 µ m) compared to that of other types of inclusions. It is interesting to point out that these NbTiCr-C inclusions, which also contained сhromium as a corrosion resistance alloying element, did not show practically any particular effect on corrosion dissolution of metal matrix in the 718 alloy. It may be explained by much lower content of Cr (1-3%) in these small carbides in the 718 alloy compared to the contents of alloying elements (3-25% Cr, 0-38% Mo and 0-26% W) in NbTiMoWCr-C inclusions in EP718 alloy. Therefore, the metal matrix around these small NbTiCr-C inclusions has no (or has too small) concentration depletion by the alloying elements (such as Cr). As a result, the dissolution of metal matrix (KD) around these small inclusions on grain boundaries in the 718 alloy is much smaller (~4 times) compared to that in the EP718 alloy.

Discussion
Based on comparative analysis of inclusions and dissolution of metal matrix around these inclusions after electrolytic extraction of EP718 and 718 alloys, the inclusions harmful for corrosion properties of alloys were determined.
Similar types of inclusions (such as NbTi-C and TiNb-N) which correspond primary MC carbides based on the chemistry and morphology and are formed when solidifying [33,34] were found in both Ni-based alloys, For both studied nickel based alloys, the dissolution of metal matrix around of NbTi-C inclusions and clusters have very similar dissolution parameters as for inclusions located inside the grains as well for inclusions located on grain boundaries, as it can be seen in Table  4. However, it should be pointed out that these carbides in the 718 alloy contain a much larger amount of Nb (RNb/Ti = 9.9±3.7) compared to that in the EP718 alloy (RNb/Ti = 1.6±0.2), which can be explained by the larger content of Nb in the alloy 718 (up to 5.2%).
It was found that compositions of TiNb-N inclusions that are MN nitrides with formed even with insignificant amount of nitrogen in both samples are very similar (RNb/Ti = 0.3±0.2) [33,34]. However, despite the fact that the average dissolution coefficients (KD) for nitrides (TiNb-N) in both alloys are similar (2.8 in EP718 alloy and 2.4 in alloy 718), the equivalent diameter of "craters" in alloy 718 is significantly larger (Dcr = 17-26 µ m) compared to that in EP718 alloy (Dcr = 9-10 µ m). It can be explained by a larger amount of nitrides observed in the alloy 718 samples, though some number of large sized nitride clusters was also found in the EP718 sample. Moreover, most of the large size TiNb-N inclusions in the 718 alloy are located on grain boundaries of metal matrix, for which dissolution parameters are much higher (KD = 6.5±5.5 and Dcr = 24-36 µ m). Thus, despite the fact that nitrides are considered the most neutral from the point of view of corrosion, but with their large sizes (10-27 µ m), they can harmfully effect the corrosion resistance by increasing the area of the cathode.
Both alloys are also characterized by a more active harmful effect of all types of inclusions located along grain boundaries. The values of the dissolution coefficient KD for inclusions located at grain boundaries are from 2.1 to 2.7 times larger than that for inclusions located in the grain for both investigated alloys. The dissolution coefficient of metal matrix around TiNb-S inclusions and clusters (KD = 8.8±3.6), which were found on grain boundaries in the EP718 alloy, is considerably larger compared to that for NbTi-C and TiNb-N inclusions in this alloy. Therefore, this type of sulphides should be considered as very harmful inclusions, which can significantly increase a pitting corrosion of the EP718 alloy.
The dissolution coefficient for small chromium-containing carbide inclusions located on grain boundaries is much lower for alloy 718 (KD = 5.2±3.2) compared to that for EP718 alloy (KD = 21.3±7.7). In contrast to the EP718 alloy, the KD value for these small Cr-containing inclusions is relatively low and comparable with the dissolution coefficients for other inclusions observed in the alloy 718. This difference in behavior can be explained by the lower contents of Cr (1-3%) small carbides in 718 alloy and by the more complex composition of these carbide inclusions of the EP718 alloy ( Table 2) contained 3-25% of Cr, 0-38% of Mo and 0-26% of W, that are М6С carbides according to chemical composition that are formed in the temperature range 850 до 1100С [33,34], which are responsible for pitting resistance.
Based on the revealed differences in the composition and amount of inclusions in EP718 and 718 alloys, it can be concluded what inclusions reduce the corrosion resistance of EP718 alloy. 4. Titanium sulfides have the most negative effect on matrix dissolution; 5. Carbides, especially at the boundaries, containing chromium, molybdenum and tungsten in their composition cause a significant dissolution of the matrix around the inclusions, 6. Large (more than 10 µ m) carbides and nitrides of titanium and niobium are also able to reduce corrosion resistance. Thus, an application of the electrolytic extraction in combination with 3D investigations of different inclusions and clusters and an evaluation of electrolytic dissolution of metal matrix around these inclusions in the EP718 and 718 alloys allowed detect more harmful inclusions regarding corrosion of metal matrix in these Ni-based alloys. As a next step of the future study, the reasons of formations and mechanisms of effects of harmful inclusions on different stages of the Ni-based alloy production can be considered.

Conclusions
In this study, main characteristics of different inclusions (such as composition, morphology, size, number and location) and dissolution of metal matrix around these inclusions were investigated on film filter and on the surfaces of metal samples after electrolytic extraction (EE) of two industrial Nibased alloys (alloy 718 and EP718). The obtained results can be summarized as follows: 1. The electrolytic extraction technique can be successfully applied for three-dimensional (3D) investigations of different inclusions on film filter and on the surface of metal samples after extraction of Ni-based alloys. It was shown that the morphology of inclusions is much more Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 19 July 2020 doi:10.20944/preprints202007.0428.v1 complicated than can be seen on a flat section. For instance, a thin plate-like sulphides of Ti and Nb (with a thickness of 1-2 µ m) were detected in clusters (up to 37 µ m) are located mostly on grain boundaries in the EP718 alloy, which were detected only as separate acicular sulfides by conventional two-dimensional (2D) investigations on polished surface of this metal sample. 2. Evaluation of different extent of dissolution of metal matrix around different inclusions after EE by determination of equivalent diameter of "crater" (Dcr) and relative coefficient of metal matrix dissolution (KD) allows estimate a metal weakening around investigated inclusions, which correlates to a corrosion resistance of metal. 3. In the EP718 alloy, 4 types of inclusions were found, which are ordered according to a higher degree of influence on corrosion resistance of the alloy: TiNb-S sulfides, NbTi-C carbides, small size multicomponent carbides (NbTiMoWCr-C) and TiNb-N nitrides. Three types of typical inclusions were found in alloy 718: NbTi-C carbides, TiNb-N nitrides and small size carbides (NbTiCr-C). In addition to separate inclusions, clusters (up to 40 µ m) consisting of different inclusions were found in both alloys. 4. Harmful effect of inclusions on dissolution of metal matrix decreases from the sulfides (TiNb-S) and small carbides (NbTiMoWCr-C) located on grain boundaries to the large carbides (NbTi-C) and nitrides (TiNb-N) located as on grain boundaries as well inside of grains. The nitride inclusions (TiNb-N) having the size larger than 10 µ m can also significantly reduce the corrosion resistance of Ni-based alloys, although in the literature they are described as the most neutral with respect to the matrix. 5. In addition to the composition of the inclusion, their location (at the boundary or in the grain) and size also affect the corrosion resistance and the propensity to pitting. For instance, the dissolution parameters (KD and Dcr) for inclusions located on grain boundaries are from 2.1 to 2.7 times larger than that for inclusions located inside the grains for both investigated alloys. The inclusions affect differently: the composition, size, position that inclusions located at the grain boundaries are more significant reduce corrosion resistance. Large inclusions of more than 10 µ m affect the corrosion resistance more significantly.