Oxidation Behavior of Non-Modiﬁed and Rhodium- or Palladium-Modiﬁed Aluminide Coatings Deposited on CMSX-4 Superalloy

: Rhodium-modiﬁed as well as palladium-modiﬁed and non-modiﬁed aluminide coatings on CMSX-4 Ni-based superalloy were oxidized in air atmosphere at 1100 ◦ C. Uncoated substrate of CMSX-4 superalloy was also oxidized. The microstructure of coatings before oxidation consists of two layers: an additive and an interdiffusion one. The NiAl intermetallic phase was found in the microstructure of non-modiﬁed coatings, while the (Ni,Rh)Al intermetallic phase was observed in the microstructure of rhodium-modiﬁed aluminide coatings before oxidation. The (Ni,Pd)Al phase of palladium-modiﬁed aluminide coatings in the additive layer was observed before oxidation. The microstructure of the oxidized non-modiﬁed coatings consists of the γ ’-Ni 3 Al phase. The oxide layer (10 µ m thick) consists of the NiAl 2 O 4 phase and porous Ni-rich oxide. The oxide layers (5 µ m thick) formed on the surface of rhodium or palladium-modiﬁed coatings consist of the α -Al 2 O 3 phase and the top layer of the NiAl 2 O 4 phase. Al-depleted (30 at. %) β -NiAl grains besides the γ ’-Ni 3 Al phase were found in the rhodium-modiﬁed coating, while only the γ ’-Ni 3 Al phase region was revealed in the palladium-modiﬁed coating, Rhodium-modiﬁed coatings with small rhodium content (0.5 µ m rhodium layer thick) can be an alternative for palladium-modiﬁed ones with bigger palladium content (3 µ m thick palladium layer). analysis performed at Phase composition of the was investigated using the X’TRA X-ray diffractometer, equipped with a ﬁltered copper lamp with a and


Introduction
Nickel-based superalloys are widely used on turbine blades and vanes in the hot section of aircraft engines [1]. Aluminum content in the nickel-based superalloys is kept at a level below 6% wt. [2]. Exposure of superalloys to an environment of high-pressure turbine engines leads to their degradation from oxidation, hot corrosion and thermal fatigue [3]. Usage of nickel-based superalloys covered by diffusion aluminide coatings ensures high-temperature oxidation resistance of turbine blades and vanes [4]. Platinum modification of aluminide coatings improves their performance, and resistance to oxidation and hot corrosion [5]. However, usage of platinum-modified aluminide coatings has two main limitations: poor ductility, due to the presence of the PtAl 2 brittle phase, and the high price of platinum. These reasons motivate the replacement of platinum with palladium [6].
According to Li et al. [7] palladium stabilizes the β-(Ni,Pd)Al phase, retards degradation of coatings and increases oxidation resistance. Hong et al. [8] showed that the addition of palladium accelerates the θ-Al 2 O 3 to α-Al 2 O 3 transition and simultaneous diffusion of titanium from the substrate to the coating's surface. He et al. [9] proved that palladium stabilizes protective alumina scale and restrains outward diffusion of substrate elements to the β-(Ni,Pd)Al phase. Consequently, it increases the lifetime of the β-NiAl phase and hot corrosion resistance of coatings. Hong et al. [8] alleged that a great number of pores present in the substrate during annealing and aluminizing of palladium-coated superalloy limits coatings usage and oxidation resistance.

Aluminide Coatings Before Oxidation
The microstructure of non-modified and rhodium-or palladium-modified aluminide coatings consisted of two layers: an additive and an interdiffusion one (Figure 1a,c,e). The NiAl intermetallic phase was observed in the microstructure of non-modified coatings (Figure 1a). The (Ni,Rh)Al intermetallic phase was observed in the microstructure of rhodium-modified aluminide coatings (Figure 1c). The non-modified and rhodium-modified aluminide coatings are about 30 µm thick (Figure 1a-d). The palladium modification increased the coating's thickness to 37 µm (Figure 1e,f). The distribution of Ni and Al in the cross-section confirms the presence of the NiAl phase ( Figure 1b). Rhodium content between the additive and interdiffusion layers is low (2-4 atom %). No rhodium-rich particles were observed in the additive layer, only Kirkendall-like porosity and few particles of Topologically Closed-Pack phases (Figure 1c). Distribution of palladium in the additive layer is quite regular (Figure 1d). This phenomenon indicates the presence of the (Ni, Pd)Al phase in the additive layer. The interdiffusion layer and the substrate/interdiffusion interface are rich in inclusions containing alloying elements. The analysis of the chemical composition of these phases suggests the presence of Topologically Closed-Pack µ and σ phases. Phase composition and aluminide coatings thickness before oxidation are shown in Table 1.

Aluminide Coatings Before Oxidation
The microstructure of non-modified and rhodium-or palladium-modified aluminide coatings consisted of two layers: an additive and an interdiffusion one (Figure 1a,c,e). The NiAl intermetallic phase was observed in the microstructure of non-modified coatings (Figure 1a). The (Ni,Rh)Al intermetallic phase was observed in the microstructure of rhodium-modified aluminide coatings (Figure 1c). The non-modified and rhodium-modified aluminide coatings are about 30 µm thick (Figure 1a-d). The palladium modification increased the coating's thickness to 37 µm (Figure 1e,f). The distribution of Ni and Al in the cross-section confirms the presence of the NiAl phase (Figure 1  b). Rhodium content between the additive and interdiffusion layers is low (2-4 atom %). No rhodium-rich particles were observed in the additive layer, only Kirkendall-like porosity and few particles of Topologically Closed-Pack phases (Figure 1c). Distribution of palladium in the additive layer is quite regular (Figure 1d). This phenomenon indicates the presence of the (Ni, Pd)Al phase in the additive layer. The interdiffusion layer and the substrate/interdiffusion interface are rich in inclusions containing alloying elements. The analysis of the chemical composition of these phases suggests the presence of Topologically Closed-Pack µ and σ phases. Phase composition and aluminide coatings thickness before oxidation are shown in Table 1.

Oxidation Resistance of Aluminide Coatings and Bare Superalloy
Oxidation kinetics curves ( Figure 2) were prepared to show resistance to oxidation under cyclic conditions of coatings and bare superalloy. Each one of the coatings exhibits a rapid weight growth during first cycles of oxidation which was caused by quick oxide layer formation. The obtained results indicate that the best oxidation resistance exhibits rhodium-modified aluminide coatings, which failed after 13 cycles (Figure 2a). Weight change that reaches up to 0.2 mg/cm 2 is during the 3 cycles of oxidation. Afterwards, the weight change of coating is subtle until the 9th cycle when the weight of the sample starts to decrease slowly, which indicates the onset of oxide-scale spallation. Eventually, the coatings weight fell below its initial value in the 14th cycle. The palladium-modified aluminide coating reaches a maximum weight gain of 0.2 mg/cm 2 after 1 cycle of oxidation ( Figure  2b). Afterwards, until approximately the 4th cycle, the weight change is subtle, probably due to repeated oxide-scale spallation and formation. After the 4th cycle the weight change of the coating decreases. Non-modified aluminide coating reaches the maximum weight change of 0.7 mg/cm 2 in the 3rd cycle ( Figure 2c). Such weight change is due to very fast oxide layer formation. Afterwards, the weight change is almost subtle and is about 0.6-0.5 mg/cm 2 , which is probably due to repeated oxide-scale formation and spallation. After the 7th cycle of oxidation, weight change quickly decreases, which is probably due to an inability of the oxide layer to heal after spallation. The weight change of bare CMSX-4 superalloy decreases after just 1 cycle of oxidation ( Figure 2d). This is due to by-bed protective properties of the oxide layer that formed on its surface.

Oxidation Resistance of Aluminide Coatings and Bare Superalloy
Oxidation kinetics curves ( Figure 2) were prepared to show resistance to oxidation under cyclic conditions of coatings and bare superalloy. Each one of the coatings exhibits a rapid weight growth during first cycles of oxidation which was caused by quick oxide layer formation. The obtained results indicate that the best oxidation resistance exhibits rhodium-modified aluminide coatings, which failed after 13 cycles (Figure 2a). Weight change that reaches up to 0.2 mg/cm 2 is during the 3 cycles of oxidation. Afterwards, the weight change of coating is subtle until the 9th cycle when the weight of the sample starts to decrease slowly, which indicates the onset of oxide-scale spallation. Eventually, the coatings weight fell below its initial value in the 14th cycle. The palladium-modified aluminide coating reaches a maximum weight gain of 0.2 mg/cm 2 after 1 cycle of oxidation ( Figure 2b). Afterwards, until approximately the 4th cycle, the weight change is subtle, probably due to repeated oxide-scale spallation and formation. After the 4th cycle the weight change of the coating decreases. Non-modified aluminide coating reaches the maximum weight change of 0.7 mg/cm 2 in the 3rd cycle ( Figure 2c). Such weight change is due to very fast oxide layer formation. Afterwards, the weight change is almost subtle and is about 0.6-0.5 mg/cm 2 , which is probably due to repeated oxide-scale formation and spallation. After the 7th cycle of oxidation, weight change quickly decreases, which is probably due to an inability of the oxide layer to heal after spallation. The weight change of bare CMSX-4 superalloy decreases after just 1 cycle of oxidation ( Figure 2d). This is due to by-bed protective properties of the oxide layer that formed on its surface.

Non-Modified Aluminide Coating after the Oxidation Test
The surface of non-modified aluminide coating after oxidation is presented in Figure 3. The results of chemical composition analysis in microareas are presented in Table 2. The surface consists of the oxide layer formed during oxidation and visible as gray-scale areas and bright areas of the substrate after oxide layer spallation.
Chemical composition in microareas indicates that three types of oxides may coexist. Chemical composition of microareas 1 and 3 corresponds to (Ni,Cr)Al2O4 oxides. Above these microareas is slightly darker oxide layer, marked 2. The predominant elements present in this area are Al, O and Ni. There are also small amounts of Cr, Co and W. However, the concentration of the main elements indicates the presence of NiAl2O4 spinel, which was also detected using XRD ( Figure 4). There are also some porous oxides (marked 4). Chemical composition of these regions indicates Ni-rich oxide occurrence. Chemical composition of the brightest and distinct areas marked 5 is very similar to that of the substrate alloy, which indicates that it was exposed by oxide layer spallation. A Ni3Al phase was identified by XRD analysis.
Swadźba et al. [15] identified also Al2O3 oxides after oxidation of aluminide coatings, but in the presented research, Al2O3 oxides were not found.

Non-Modified Aluminide Coating after the Oxidation Test
The surface of non-modified aluminide coating after oxidation is presented in Figure 3. The results of chemical composition analysis in microareas are presented in Table 2. The surface consists of the oxide layer formed during oxidation and visible as gray-scale areas and bright areas of the substrate after oxide layer spallation.
Chemical composition in microareas indicates that three types of oxides may coexist. Chemical composition of microareas 1 and 3 corresponds to (Ni,Cr)Al 2 O 4 oxides. Above these microareas is slightly darker oxide layer, marked 2. The predominant elements present in this area are Al, O and Ni. There are also small amounts of Cr, Co and W. However, the concentration of the main elements indicates the presence of NiAl 2 O 4 spinel, which was also detected using XRD ( Figure 4). There are also some porous oxides (marked 4). Chemical composition of these regions indicates Ni-rich oxide occurrence. Chemical composition of the brightest and distinct areas marked 5 is very similar to that of the substrate alloy, which indicates that it was exposed by oxide layer spallation. A Ni 3 Al phase was identified by XRD analysis.
Swadźba et al. [15] identified also Al 2 O 3 oxides after oxidation of aluminide coatings, but in the presented research, Al 2 O 3 oxides were not found.

Non-Modified Aluminide Coating after the Oxidation Test
The surface of non-modified aluminide coating after oxidation is presented in Figure 3. The results of chemical composition analysis in microareas are presented in Table 2. The surface consists of the oxide layer formed during oxidation and visible as gray-scale areas and bright areas of the substrate after oxide layer spallation.
Chemical composition in microareas indicates that three types of oxides may coexist. Chemical composition of microareas 1 and 3 corresponds to (Ni,Cr)Al2O4 oxides. Above these microareas is slightly darker oxide layer, marked 2. The predominant elements present in this area are Al, O and Ni. There are also small amounts of Cr, Co and W. However, the concentration of the main elements indicates the presence of NiAl2O4 spinel, which was also detected using XRD ( Figure 4). There are also some porous oxides (marked 4). Chemical composition of these regions indicates Ni-rich oxide occurrence. Chemical composition of the brightest and distinct areas marked 5 is very similar to that of the substrate alloy, which indicates that it was exposed by oxide layer spallation. A Ni3Al phase was identified by XRD analysis.
Swadźba et al. [15] identified also Al2O3 oxides after oxidation of aluminide coatings, but in the presented research, Al2O3 oxides were not found.     After oxidation, non-modified aluminide coating consists of the aluminum depleted γ'-Ni3Al phase (Figure 5a-c). Swadźba et al. [15] allege that presence of γ'-Ni3Al phase is a result of β→ γ' transformation which occurred due to the aluminum depletion from the β phase. The γ' phase contains about 11 at. % Al, 66 at. % Ni, 9 at. % Cr, 11 at. % Co and 5 at. % W (Table 3). Aluminum concentration in the γ'-Ni3Al phase is too low to form an oxide layer consisting of the Al2O3, so oxides containing nickel, chromium and aluminum are being formed [16].  After oxidation, non-modified aluminide coating consists of the aluminum depleted γ'-Ni 3 Al phase (Figure 5a-c). Swadźba et al. [15] allege that presence of γ'-Ni 3 Al phase is a result of β → γ' transformation which occurred due to the aluminum depletion from the β phase. The γ' phase contains about 11 at. % Al, 66 at. % Ni, 9 at. % Cr, 11 at. % Co and 5 at. % W (Table 3). Aluminum concentration in the γ'-Ni 3 Al phase is too low to form an oxide layer consisting of the Al 2 O 3 , so oxides containing nickel, chromium and aluminum are being formed [16].   After oxidation, non-modified aluminide coating consists of the aluminum depleted γ'-Ni3Al phase (Figure 5a-c). Swadźba et al. [15] allege that presence of γ'-Ni3Al phase is a result of β→ γ' transformation which occurred due to the aluminum depletion from the β phase. The γ' phase contains about 11 at. % Al, 66 at. % Ni, 9 at. % Cr, 11 at. % Co and 5 at. % W (Table 3). Aluminum concentration in the γ'-Ni3Al phase is too low to form an oxide layer consisting of the Al2O3, so oxides containing nickel, chromium and aluminum are being formed [16].   The surface of the rhodium-modified aluminide coatings after oxidation is presented in Figure 6. The results of chemical composition in microareas are presented in Table 3 (Figure 7). There are no porous oxides as in the non-modified aluminide coatings. Chemical composition of the brightest areas marked 3 is very similar to that of the substrate alloy, which indicates that it was exposed by oxide layer spallation. It was identified as Ni 3 Al phase by the XRD analysis, similar to the non-modified coating.  The surface of the rhodium-modified aluminide coatings after oxidation is presented in Figure  6. The results of chemical composition in microareas are presented in Table 3. The surface consists mainly of gray areas of oxides and bright areas of substrate formed by oxide layer spallation. The predominant kind of oxides are the darkest oxide layer (Point 1) containing 45.6 at. % Al and 54.4 at. % O (Table 4). Such chemical composition indicates the presence of Al2O3. The presence of Al2O3 was confirmed by the XRD analysis (Figure 7). Another type of oxide (light gray color) contains 10.1 at. % Ni as well as Al and O and small content of W, Co and Cr. XRD results reveal the presence of NiAl2O4 spinel (Figure 7). There are no porous oxides as in the non-modified aluminide coatings. Chemical composition of the brightest areas marked 3 is very similar to that of the substrate alloy, which indicates that it was exposed by oxide layer spallation. It was identified as Ni3Al phase by the XRD analysis, similar to the non-modified coating.
(a) Figure 6. Surface of rhodium-modified aluminide coating after oxidation at 1100 °C for 21 cycles with marked microareas analyzed using EDS (b) after oxidation. Table 4. Chemical composition in microareas 1-3, Figure 6b, at. %.   Intensity, imp/s Figure 6. Surface of rhodium-modified aluminide coating after oxidation at 1100 • C for 21 cycles (a) with marked microareas analyzed using EDS (b) after oxidation. The cross-section of coating consists of light-gray and dark-gray grains (Figure 8a-c). The light-gray γ'-Ni 3 Al grains, marked 2 in Figure 8, contain 18.7 at. % Al, 65.4 at. % Ni and among others 0.1 at. % Rh (Table 5) formed due to Al depletion from the β-NiAl phase. The Al-depleted β-NiAl phase grains contain 30.2 at. % Al, 56.9 at. % Ni and higher amount of rhodium 0.5 at. %. The aluminum content indicates that this phase has not yet transformed into the γ' phase. Considering the presence of the β-NiAl phase grains, it is evident that this type of coating is more stable during oxidation in comparison to the non-modified one.   The cross-section of coating consists of light-gray and dark-gray grains (Figure 8a-c). The light-gray γ'-Ni3Al grains, marked 2 in Figure 8, contain 18.7 at. % Al, 65.4 at. % Ni and among others 0.1 at. % Rh (Table 5) formed due to Al depletion from the β-NiAl phase. The Al-depleted β-NiAl phase grains contain 30.2 at. % Al, 56.9 at. % Ni and higher amount of rhodium 0.5 at. %. The aluminum content indicates that this phase has not yet transformed into the γ' phase. Considering the presence of the β-NiAl phase grains, it is evident that this type of coating is more stable during oxidation in comparison to the non-modified one.

Palladium-modified Aluminide Coating after the Oxidation Test
The surface of the palladium-modified aluminide coating after oxidation is presented in Figure  9. The results of chemical composition in microareas are presented in Table 6. The surface consists of gray areas of oxides and bright areas of substrate formed by oxide layer spallation. The gray oxide layer contains 48.3 at. % Al and 51.7 at. % O ( Table 6). Such chemical composition indicates the presence of Al2O3, confirmed by the XRD (Figure 10). Chemical composition of the brightest areas, marked 1, is very similar to that of the substrate alloy, which indicates that it was exposed by oxide layer spallation. It was identified by the XRD analysis as the Ni3Al phase, similar to the non-modified and rhodium-modified aluminide coatings. NiAl2O4 spinel above Al2O3 was also identified (Table 6, Figure 10).
The cross-sectional microstructure of the investigated coating consists of the γ'-Ni3Al phase in the outer zone (Figure 11a, b, Point 1, Table 7). Chemical composition of oxidized coating below outer zone (Point 2) is similar to Point 1 (Figure 11 a, c). Therefore, phase composition in these two points is the same. Significant porous areas below the outer zone of coating was also observed.

Palladium-modified Aluminide Coating after the Oxidation Test
The surface of the palladium-modified aluminide coating after oxidation is presented in Figure 9. The results of chemical composition in microareas are presented in Table 6. The surface consists of gray areas of oxides and bright areas of substrate formed by oxide layer spallation. The gray oxide layer contains 48.3 at. % Al and 51.7 at. % O ( Table 6). Such chemical composition indicates the presence of Al 2 O 3, confirmed by the XRD (Figure 10). Chemical composition of the brightest areas, marked 1, is very similar to that of the substrate alloy, which indicates that it was exposed by oxide layer spallation. It was identified by the XRD analysis as the Ni 3 Al phase, similar to the non-modified and rhodium-modified aluminide coatings. NiAl 2 O 4 spinel above Al 2 O 3 was also identified (Table 6, Figure 10).
The cross-sectional microstructure of the investigated coating consists of the γ'-Ni 3 Al phase in the outer zone (Figure 11a,b, Point 1, Table 7). Chemical composition of oxidized coating below outer zone (Point 2) is similar to Point 1 (Figure 11a,c). Therefore, phase composition in these two points is the same. Significant porous areas below the outer zone of coating was also observed.   Intensity, imp/s Ni3Al NiAl2O4 Al2O3 Figure 9. Surface of the palladium-modified aluminide coating after oxidation at 1100 • C for 21 cycles (a) with marked microareas analyzed using EDS (b) after oxidation.  Figure 9. Surface of the palladium-modified aluminide coating after oxidation at 1100 °C for 21 cycles (a) with marked microareas analyzed using EDS (b) after oxidation.

Discussion
The non-modified aluminide coating's formation is a result of two processes [14]: diffusion of nickel, cobalt, chromium, titanium and other superalloys' elements from the substrate to the surface, leading to formation of the interdiffusion layer, reaction of nickel with aluminum supplied by the gas phase in the CVD process and formation of the additive layer.
The total coating thickness includes two layers. Both consist of the β-NiAl phase. As solubility of alloying elements in the β-NiAl phase is very low, these elements precipitate in Topologically Closed-Pack phases (µ and σ).
Rhodium content between additive and interdiffusion layers is 4 at. %. According to the Al-Ni-Rh phase diagram, it is too low to form rhodium-rich precipitations. Therefore, rhodium dissolves in the β-NiAl phase.
The formation of the rhodium-modified aluminide coating seems to take place in several steps: rhodium probably dissolves in γ + γ' phases near the surface of CMSX-4 alloy during heating of the alloy with 0.5 µm of rhodium; aluminum during aluminizing in the CVD process arrives at the surface where Rh is diluted by Ni and since aluminum has more affinity to nickel than rhodium, it forms the (Ni,Rh)Al phase layer.
The formation of the palladium-modified aluminide coating seems to take place in the next steps [13]: Palladium dissolution in γ + γ' phases near the surface of CMSX-4 alloy during heating of alloy with 3 µm of palladium; reaction of dissolved in the substrate palladium with aluminum; and (Ni,Pd)Al phase formation.
The oxidation of the non-modified aluminide coating probably takes places as follows: initial development of continuous Al2O3 oxides; diffusion of aluminum from coating to Al2O3 oxides; and to substrate. As a result of diffusion of aluminum a Ni3Al phase is being formed and propagates inwards. Prolongation of oxidation leads to scale spallation with subsequent formation of poorly-protective oxides such as NiAl2O4 and (Ni,Cr)O. A schema of evolution of non-modified aluminide coating before oxidation and after oxidation is presented in Figure 12.

Discussion
The non-modified aluminide coating's formation is a result of two processes [14]: diffusion of nickel, cobalt, chromium, titanium and other superalloys' elements from the substrate to the surface, leading to formation of the interdiffusion layer, reaction of nickel with aluminum supplied by the gas phase in the CVD process and formation of the additive layer.
The total coating thickness includes two layers. Both consist of the β-NiAl phase. As solubility of alloying elements in the β-NiAl phase is very low, these elements precipitate in Topologically Closed-Pack phases (µ and σ).
Rhodium content between additive and interdiffusion layers is 4 at. %. According to the Al-Ni-Rh phase diagram, it is too low to form rhodium-rich precipitations. Therefore, rhodium dissolves in the β-NiAl phase.
The formation of the rhodium-modified aluminide coating seems to take place in several steps: rhodium probably dissolves in γ + γ' phases near the surface of CMSX-4 alloy during heating of the alloy with 0.5 µm of rhodium; aluminum during aluminizing in the CVD process arrives at the surface where Rh is diluted by Ni and since aluminum has more affinity to nickel than rhodium, it forms the (Ni,Rh)Al phase layer.
The formation of the palladium-modified aluminide coating seems to take place in the next steps [13]: Palladium dissolution in γ + γ' phases near the surface of CMSX-4 alloy during heating of alloy with 3 µm of palladium; reaction of dissolved in the substrate palladium with aluminum; and (Ni,Pd)Al phase formation.
The oxidation of the non-modified aluminide coating probably takes places as follows: initial development of continuous Al 2 O 3 oxides; diffusion of aluminum from coating to Al 2 O 3 oxides; and to substrate. As a result of diffusion of aluminum a Ni 3 Al phase is being formed and propagates inwards. Prolongation of oxidation leads to scale spallation with subsequent formation of poorly-protective oxides such as NiAl 2 O 4 and (Ni,Cr)O. A schema of evolution of non-modified aluminide coating before oxidation and after oxidation is presented in Figure 12. Rhodium modification of aluminide coatings effectively improves oxidation resistance of superalloy. Moreover, deposition of thin rhodium layer (0.5 µm thick) on the superalloy followed by aluminizing ensures better oxidation resistance than deposition of thicker palladium layer (3 µm thick) on the superalloy followed by aluminizing.
The oxide layer formed on the surface of the rhodium or palladium-modified coatings consists of the Al2O3 phase matrix with top layer of the NiAl2O4 phase. On the surface of non-modified coating NiAl2O4 oxides and some porous oxides with chemical composition of (Ni,Cr)O phase were found. According to Swadźba et al. [15] NiAl2O4 forms mainly above the γ'-Ni3Al regions where not enough Al is available for Al2O3 exclusive formation. It was also found that modification of rhodium (0.5 µm rhodium layer thick) and of palladium (3 µm palladium layer thick) decreases the oxide layer growth rate. The oxides layer thickness of the non-modified coating was about 10 µm, while the oxides layer thickness of the rhodium or palladium-modified coatings was about 5 µm ( Figure  13).  -modified; b). rhodium-modified (0.5 µm rhodium layer) and c). palladium-modified (3 µm palladium layer).
Cross-sectional microstructure of the rhodium-modified coatings after oxidation consists of three regions: Al-depleted (30 at. %) β-NiAl phase grains, the γ'-Ni3Al phase region and the refractory-rich precipitates region. In comparison to the non-modified and the palladium-modified, the rhodium-modified coating exhibits β-NiAl phase stability, and stable α-Al2O3 layer. It seems that rhodium produces at the same time an increase of the diffusivity of aluminum and a decrease of that Rhodium modification of aluminide coatings effectively improves oxidation resistance of superalloy. Moreover, deposition of thin rhodium layer (0.5 µm thick) on the superalloy followed by aluminizing ensures better oxidation resistance than deposition of thicker palladium layer (3 µm thick) on the superalloy followed by aluminizing.
The oxide layer formed on the surface of the rhodium or palladium-modified coatings consists of the Al 2 O 3 phase matrix with top layer of the NiAl 2 O 4 phase. On the surface of non-modified coating NiAl 2 O 4 oxides and some porous oxides with chemical composition of (Ni,Cr)O phase were found. According to Swadźba et al. [15] NiAl 2 O 4 forms mainly above the γ'-Ni 3 Al regions where not enough Al is available for Al 2 O 3 exclusive formation. It was also found that modification of rhodium (0.5 µm rhodium layer thick) and of palladium (3 µm palladium layer thick) decreases the oxide layer growth rate. The oxides layer thickness of the non-modified coating was about 10 µm, while the oxides layer thickness of the rhodium or palladium-modified coatings was about 5 µm ( Figure 13). Rhodium modification of aluminide coatings effectively improves oxidation resistance of superalloy. Moreover, deposition of thin rhodium layer (0.5 µm thick) on the superalloy followed by aluminizing ensures better oxidation resistance than deposition of thicker palladium layer (3 µm thick) on the superalloy followed by aluminizing.
The oxide layer formed on the surface of the rhodium or palladium-modified coatings consists of the Al2O3 phase matrix with top layer of the NiAl2O4 phase. On the surface of non-modified coating NiAl2O4 oxides and some porous oxides with chemical composition of (Ni,Cr)O phase were found. According to Swadźba et al. [15] NiAl2O4 forms mainly above the γ'-Ni3Al regions where not enough Al is available for Al2O3 exclusive formation. It was also found that modification of rhodium (0.5 µm rhodium layer thick) and of palladium (3 µm palladium layer thick) decreases the oxide layer growth rate. The oxides layer thickness of the non-modified coating was about 10 µm, while the oxides layer thickness of the rhodium or palladium-modified coatings was about 5 µm ( Figure  13).  -modified; b). rhodium-modified (0.5 µm rhodium layer) and c). palladium-modified (3 µm palladium layer).
Cross-sectional microstructure of the rhodium-modified coatings after oxidation consists of three regions: Al-depleted (30 at. %) β-NiAl phase grains, the γ'-Ni3Al phase region and the refractory-rich precipitates region. In comparison to the non-modified and the palladium-modified, the rhodium-modified coating exhibits β-NiAl phase stability, and stable α-Al2O3 layer. It seems that rhodium produces at the same time an increase of the diffusivity of aluminum and a decrease of that Cross-sectional microstructure of the rhodium-modified coatings after oxidation consists of three regions: Al-depleted (30 at. %) β-NiAl phase grains, the γ'-Ni 3 Al phase region and the refractory-rich precipitates region. In comparison to the non-modified and the palladium-modified, the rhodium-modified coating exhibits β-NiAl phase stability, and stable α-Al 2 O 3 layer. It seems that rhodium produces at the same time an increase of the diffusivity of aluminum and a decrease of that of other superalloy elements, so that the rhodium containing coating acts as a filter enabling aluminum to diffuse more easily than the other elements, resulting in the formation of aluminum oxide layer. This also helps to form aluminum oxide layers more rapidly and under lower aluminum concentrations as well as to maintain the β-NiAl phase at the coating surface for longer periods. Chromium, cobalt and tungsten, as well as nickel, rhodium and aluminum, are found in the β-NiAl phase of rhodium-modified coatings after oxidation. However, contents of chromium, cobalt and tungsten in the β-NiAl phase are smaller than in the γ'-Ni 3 Al phase. All these indicate that rhodium tends to be solid solutioned in the β instead of the γ' phase. The rhodium-modified aluminide coatings perform better than the non-modified and palladium-modified ones. In particular, protective oxide layer keeps for longer periods of time on the rhodium-modified coatings compared to the non-modified and palladium-modified ones. β phase remains in the rhodium-modified coatings while γ' phase is both non-modified and palladium-modified after oxidation. It is possible that the major function of rhodium is to produce a large initial reservoir of aluminum and to reduce the amount of aluminum lost by diffusion processes. Rhodium probably has similar effects on the aluminide coating, since rhodium belongs to the platinum group of metals. Rhodium, similar to platinum, may accumulate near the thermally grown oxide due to the selective oxidation of other elements. Then, the aluminum activity gradient is increased, and the flux of aluminum to the thermally grown oxide/intermetallic coating interface is increased as well. This phenomenon may promote alumina scale formation. Some inclusions enriched in substrates elements have been observed in the additive layer of non-modified aluminide coatings (Figure 1a), whereas far fewer incursions have been observed in the additive layer of rhodium-modified ones. Therefore, it may be assumed that increasing of the lifetime of the β-phase is possible by impeding the outward diffusion of substrate elements to the additive layers of coating. The amount of the γ'-Ni 3 Al phase formed under oxide layer in the rhodium-modified coatings is much smaller than in the palladium-modified and non-modified ones. It indicates that transformation from the β-NiAl phase to the γ'-Ni 3 Al phase is decelerated due to the presence of the (Ni,Rh)Al phase in the as-deposited coating [13]. Because the γ'-Ni 3 Al phase has a poor oxidation resistance, the deceleration of the γ'-Ni 3 Al phase formation leads to improvement of the oxidation resistance. The aluminum depletion in the palladium-modified aluminide coating was lower than in the non-modified one, nevertheless aluminum content was too low to maintain stable β-NiAl phase. McMinn et al. [17] allege that palladium-modified aluminide coating is a poor protector due to formation of pores by the Kirkendall effect. Hydrogen that dissolves in the coating during the aluminizing process results in blistering of the coating and subsequent rapid pitting at high temperature [18].
The oxide scale formed on the surface of the rhodium-modified aluminide coatings is more adherent than on the non-modified one. According to Zhang et al. [19] oxide adherence at coating grain boundaries may be influenced by several mechanisms: chemical effects (short-circuit diffusion pathways for alloying elements and sulfur impurities), geometric effects and preferential void nucleation and coalescence sites. Refractory-rich particles were observed at the grain boundaries of the β-NiAl phase in the as-deposited non-modified aluminide coating, but were not observed at the grain boundaries of the β-(Ni,Rh)Al phase in the as-deposited rhodium-modified aluminide coating [13]. According to Zhang et al. [20] the presence of refractory elements on the coating grain boundaries may accelerate oxide spallation by degrading the oxide-metal bond strength, stabilizing the γ'-Ni 3 Al phase formation or increasing the growth rate of the oxide layer once oxide layer spallation and reformation begin.
The analysis of chemical composition in microareas proves that rhodium is easy to solidify into solution in the β-NiAl phase. The higher content of rhodium in the β-NiAl phase sustains aluminum concentration. Addition of rhodium helps to stabilize the β-NiAl phase of high aluminum content and to delay the degradation of the aluminide coating.

Conclusions
The microstructure of non-modified and rhodium or palladium-modified aluminide coatings consist of two layers: an additive and an interdiffusion one before oxidation. The NiAl intermetallic phase was found in the microstructure of a non-modified coatings, while the (Ni,Rh)Al intermetallic phase was observed in the microstructure of rhodium-modified aluminide coatings. The (Ni,Pd)Al phase of palladium-modified aluminide coatings in the additive layer was observed before oxidation.
The oxide layer (10 µm thick) formed on the surface of the non-modified coating consists of the NiAl 2 O 4 phase, moreover porous oxide with chemical composition of Ni-rich oxide was found. After oxidation, the microstructure of the non-modified coating consists of the γ'-Ni 3 Al phase. The oxide layer (5 µm thick) formed on the surface of rhodium or palladium-modified coatings consists of the α-Al 2 O 3 phase and the top layer of the NiAl 2 O 4 phase. Al-depleted (30 at. %) β-NiAl phase grains as well as the γ'-Ni 3 Al phase region were found in the rhodium-modified coating, while only γ'-Ni 3 Al phase region was revealed in the palladium-modified coating. The rhodium-modified coating shows lower weight change than the non-modified and palladium-modified ones. Therefore, rhodium-modified aluminide coatings with small rhodium content (0.5 µm rhodium layer thick) can be an alternative for palladium-modified aluminide ones with bigger palladium content (3 µm palladium thick layer).