**2. Superalloys**

52 Efficiency, Performance and Robustness of Gas Turbines

800 and 9500 C and type II that takes place from 600 to 7500 C. At higher temperatures, there is no hot corrosion problem as the salt evaporates. Unlike oxidation, hot corrosion is highly detrimental. In fact, hot corrosion is a limiting factor for the life of components for marine gas turbines. Vanadium that is present in the fuel makes the marine environment further corrosive by forming low melting point chemical compounds. Therefore, selection of appropriate materials is paramount importance. An ideal construction material should be able to survive this harsh corrosive environment. Thus, in order to improve the efficiency of marine gas turbine engines significantly, either the existing materials / coatings which can exhibit very good hot corrosion resistance or the advanced materials with considerably improved properties are necessary. Efforts made in this direction made it possible to develop a new superalloy which exhibits excellent high temperature strength properties [1].

Fig. 2. Failed gas turbine blade due to type I and II hot corrosion

The majority of nickel based superalloy developmental efforts have been directed towards improving the alloy high temperature strength properties with relatively minor concern The selected superalloys for the investigation are presented in Table 1. It is to be noted that SU 263, SU 718, IN 738 LC and IN 792 superalloys contain no rhenium but sufficient amount of chromium. However, SU 263 contains 6% molybdenum and 20% cobalt, iron content is very high in SU 718 with 6% tungsten, 6.5% tantalum and reduced molybdenum 3%. Good amount of tantalum and cobalt 8.5% each and further reduction in molybdenum 1.75% make IN 738 LC. IN 792 contains very low content of tungsten, molybdenum, more amount of aluminium 7.6% and tantalum 5% while CMSX-4 superalloy has 3% rhenium and reduced chromium. The newly developed alloy contains 6.5% rhenium and a very small amount of chromium. The modified chemistry with 6.5% rhenium, 8.5% tantalum and 5.8% tungsten makes the new superalloy to exhibit very good high temperature strength properties [1].



The Selection of Materials for Marine Gas Turbine Engines 55

Fig. 4. As hot corroded superalloy SU-718 in marine and vanadium containing

Fig. 5. As hot corroded superalloy IN 738 LC in marine and vanadium containing

environments under type I and type II conditions

environments under type I and type II conditions

As hot corroded superalloys like SU 263, SU 718, IN 738 LC in marine and vanadium containing environments under both type II and type I conditions are presented in figures 3- 5, while figures 6 and 7 show the hot corroded IN 792, CMSX-4, new superalloy under type II and type I conditions. As can be seen, all the selected superalloys were severely corroded under both the conditions. However, the corrosion is more severe under type I when compared to type II conditions. It indicates that all the superalloys are highly susceptible to hot corrosion. Among them, the new superalloy is more vulnerable to hot corrosion. The new alloy degrades at a very faster rate making it difficult to recognize over a period of time as evidenced from the experiments (Fig.8). It is clearly indicating that the modified chemistry of the new superalloy could not improve its hot corrosion resistance. However, it exhibits very good high temperature strength characteristics as mentioned earlier.

Fig. 3. As hot corroded superalloy SU-263 in marine and vanadium containing environments under type I and type II conditions

As hot corroded superalloys like SU 263, SU 718, IN 738 LC in marine and vanadium containing environments under both type II and type I conditions are presented in figures 3- 5, while figures 6 and 7 show the hot corroded IN 792, CMSX-4, new superalloy under type II and type I conditions. As can be seen, all the selected superalloys were severely corroded under both the conditions. However, the corrosion is more severe under type I when compared to type II conditions. It indicates that all the superalloys are highly susceptible to hot corrosion. Among them, the new superalloy is more vulnerable to hot corrosion. The new alloy degrades at a very faster rate making it difficult to recognize over a period of time as evidenced from the experiments (Fig.8). It is clearly indicating that the modified chemistry of the new superalloy could not improve its hot corrosion resistance. However, it

exhibits very good high temperature strength characteristics as mentioned earlier.

Fig. 3. As hot corroded superalloy SU-263 in marine and vanadium containing

environments under type I and type II conditions

Fig. 4. As hot corroded superalloy SU-718 in marine and vanadium containing environments under type I and type II conditions

Fig. 5. As hot corroded superalloy IN 738 LC in marine and vanadium containing environments under type I and type II conditions

The Selection of Materials for Marine Gas Turbine Engines 57

Fig. 8. Before and after hot corrosion in marine environment under type I conditions

Fig. 9. The effect of marine environment on superalloys under type I conditions

Fig. 6. As hot corroded superalloys in marine environment under type II conditions

Fig. 7. As hot corroded superalloys under type I conditions in marine environment

Fig. 6. As hot corroded superalloys in marine environment under type II conditions

Fig. 7. As hot corroded superalloys under type I conditions in marine environment

Fig. 8. Before and after hot corrosion in marine environment under type I conditions

Fig. 9. The effect of marine environment on superalloys under type I conditions

The Selection of Materials for Marine Gas Turbine Engines 59

superalloys like Co, Cr, W, Ti, Ta, Re etc. Typical surface morphology of SU 718 (Fig.10) demonstrates the impact of marine environment by forming big cracks. The cross sections of hot corroded superalloys revealed that the corrosion-affected zone is large for all the superalloys (Fig.11). Among them, the affected zone is more for the new superalloy indicating that severe corrosion took place during the hot corrosion process under marine

Fig. 11. Cross sections of typical hot corroded superalloys in marine environment

The elemental distributions of hot corroded IN 792 and CMSX-4 superalloys under type I conditions showed that IN 792 superalloy, which contains good amount of chromium (13.5%) could form continuous chromia scale on its surface. It also promoted alumina as well as titania scales. However, extensive diffusion of sulphur and oxygen into the superalloy was clearly observed. While CMSX-4 that contains about 6.5% chromium and 3% rhenium could not form continuous chromia scale. Thin alumina scale was observed on the superalloy surface. Good amount of rhenium was present in the corrosion products. Small

environmental conditions.

Figure.9 shows the hot corrosion behavior of few more superalloys like Nimonic-75, Nimonic-105, CM 247 LC etc. corroded in the presence and absence of marine environments under type I conditions. In the absence of marine environment, the corrosion was less for all the superalloys [9]. Appreciable corrosion was observed for all the superalloys in the presence of marine environment. It indicates that marine environment plays a significant role in causing severe corrosion, thereby reducing the superalloy life considerably. Among the superalloys, CM 247 LC was corroded severely indicating that this superalloy is highly susceptible to hot corrosion. In fact many cracks were developed on the scale and subsequently spallation took place. However, there were no cracks and no spallation of oxide scales was reported for other superalloys. In case of CM 247LC alloy, no material was left after exposure of 70 hours to the marine environment and only corrosion products with high volume of corrosion products was observed [9].

Fig. 10. A typical hot corroded superalloy under type I conditions in marine environment

The surface morphologies of various hot corroded superalloys are revealed that the surface morphology is different for various superalloys under the selected environmental conditions. Electron Dispersive Spectroscopy (EDS) measurements revealed that the corrosion products contain mainly sulphides and oxides of nickel and alloying elements of

Figure.9 shows the hot corrosion behavior of few more superalloys like Nimonic-75, Nimonic-105, CM 247 LC etc. corroded in the presence and absence of marine environments under type I conditions. In the absence of marine environment, the corrosion was less for all the superalloys [9]. Appreciable corrosion was observed for all the superalloys in the presence of marine environment. It indicates that marine environment plays a significant role in causing severe corrosion, thereby reducing the superalloy life considerably. Among the superalloys, CM 247 LC was corroded severely indicating that this superalloy is highly susceptible to hot corrosion. In fact many cracks were developed on the scale and subsequently spallation took place. However, there were no cracks and no spallation of oxide scales was reported for other superalloys. In case of CM 247LC alloy, no material was left after exposure of 70 hours to the marine environment and only corrosion products with

Fig. 10. A typical hot corroded superalloy under type I conditions in marine environment

The surface morphologies of various hot corroded superalloys are revealed that the surface morphology is different for various superalloys under the selected environmental conditions. Electron Dispersive Spectroscopy (EDS) measurements revealed that the corrosion products contain mainly sulphides and oxides of nickel and alloying elements of

high volume of corrosion products was observed [9].

superalloys like Co, Cr, W, Ti, Ta, Re etc. Typical surface morphology of SU 718 (Fig.10) demonstrates the impact of marine environment by forming big cracks. The cross sections of hot corroded superalloys revealed that the corrosion-affected zone is large for all the superalloys (Fig.11). Among them, the affected zone is more for the new superalloy indicating that severe corrosion took place during the hot corrosion process under marine environmental conditions.

Fig. 11. Cross sections of typical hot corroded superalloys in marine environment

The elemental distributions of hot corroded IN 792 and CMSX-4 superalloys under type I conditions showed that IN 792 superalloy, which contains good amount of chromium (13.5%) could form continuous chromia scale on its surface. It also promoted alumina as well as titania scales. However, extensive diffusion of sulphur and oxygen into the superalloy was clearly observed. While CMSX-4 that contains about 6.5% chromium and 3% rhenium could not form continuous chromia scale. Thin alumina scale was observed on the superalloy surface. Good amount of rhenium was present in the corrosion products. Small

The Selection of Materials for Marine Gas Turbine Engines 61

The elemental distribution of hot corroded new superalloy under type I and type II conditions are presented in figures.14 and 15 respectively. The results showed extensive presence of oxygen, sulphur and sodium in the corrosion products. Considerable diffusion of sulphur into the superalloy was clearly observed under type I conditions while oxygen under type II conditions. Rhenium and tungsten were present in the corrosion products under type I and they were present in the corrosion affected zone of new superalloy under type II. Ta and Hf were seen in the corrosion affected region. It is important to mention here that neither alumina nor chromia formation was observed on the superalloy. It is due to the fact that chromium content in the new superalloy is considerably low. At the same time,

Fig. 13. Elemental distribution of CM 247 LC superalloy after hot corrosion under type I

conditions in marine environment

other alloying elements could not form any protective oxide scales.

amounts of sodium and chlorine were also present in the corrosion products but not diffused into the superalloy. However, significant diffusion of sulphur and oxygen into the superalloy was noticed (Fig.12). Extensive diffusion of sulphur was observed in case of hot corroded CM 247 LC alloy (Fig.13). It is to be noted that neither chlorine nor sodium was diffused into the superalloy.

Fig. 12. Elemental distribution of CMSX-4 superalloy after hot corrosion under type I conditions in marine environment

amounts of sodium and chlorine were also present in the corrosion products but not diffused into the superalloy. However, significant diffusion of sulphur and oxygen into the superalloy was noticed (Fig.12). Extensive diffusion of sulphur was observed in case of hot corroded CM 247 LC alloy (Fig.13). It is to be noted that neither chlorine nor sodium was

Fig. 12. Elemental distribution of CMSX-4 superalloy after hot corrosion under type I

conditions in marine environment

diffused into the superalloy.

The elemental distribution of hot corroded new superalloy under type I and type II conditions are presented in figures.14 and 15 respectively. The results showed extensive presence of oxygen, sulphur and sodium in the corrosion products. Considerable diffusion of sulphur into the superalloy was clearly observed under type I conditions while oxygen under type II conditions. Rhenium and tungsten were present in the corrosion products under type I and they were present in the corrosion affected zone of new superalloy under type II. Ta and Hf were seen in the corrosion affected region. It is important to mention here that neither alumina nor chromia formation was observed on the superalloy. It is due to the fact that chromium content in the new superalloy is considerably low. At the same time, other alloying elements could not form any protective oxide scales.

Fig. 13. Elemental distribution of CM 247 LC superalloy after hot corrosion under type I conditions in marine environment

The Selection of Materials for Marine Gas Turbine Engines 63

Fig. 15. Elemental distribution of new superalloy after hot corrosion under type II conditions

in marine environment

Fig. 14. Elemental distribution of new superalloy after hot corrosion under type I conditions in marine environment

**BSE Cr** 

Fig. 14. Elemental distribution of new superalloy after hot corrosion under type I conditions

in marine environment

Fig. 15. Elemental distribution of new superalloy after hot corrosion under type II conditions in marine environment

The Selection of Materials for Marine Gas Turbine Engines 65

a) The tungsten present in the new superalloys reacts with the oxide ions present in the

WO3 + O 2- = WO4 2-

b) As a result, the oxide ion activity of the environment decreases to a level where acidic

Al2O3 = Al 3+ + O 2-

Cr2O3 = Cr 3+ + O 2- A similar reaction mechanism occurs if the superalloys contain other refractory elements

The following section describes an electrochemical phenomenon that explains the new

Hot corrosion of new superalloy takes place by oxidation of base as well as alloying elements like nickel, cobalt, chromium, aluminium, tantalum, rhenium etc. at the anodic site

SO32- or S or S2- and oxygen to O2-. Since the metal ions i.e. Ni2+, Co3+, Cr3+, Al3+, Re4+, Ta5+ ions etc. are unstable at the elevated temperature and therefore reacts with the sulphur ions to form metal sulphides. The metal sulphides can easily undergo oxidation at elevated temperatures and form metal oxides by releasing free sulphur (MS + 1/2 O2 = MO + S). As a result, sulphur concentration increases at the surface of superalloy and enhances sulphur diffusion into it and forms sulphides inside the superalloy. The practical observation of sulphides in hot corroded superalloy specimens clearly indicates that the electrochemical reactions took place during the hot corrosion process. Simultaneously, the metal ions react with oxide ions that are evolved at the cathodic site leading to the formation of metal oxides. The metal oxides dissociate at elevated temperatures to form metal ions and oxide ions. As a result, oxygen concentration increases at the surface and thereby diffuses into the superalloy. Practical observation of oxides in hot corroded superalloys is a clear indication that the electrochemical reactions took place during the

Therefore, the hot corrosion of new superalloy is electrochemical in nature and the relevant

Fig.16 illustrates an electrochemical model showing the new superalloy degradation is electrochemical in nature. Similar mechanism is applicable to other superalloys and their families. The motivation behind suggesting an electrochemical model is to show that the degradation of superalloys in marine environments at elevated temperatures is electrochemical in nature and hence, the electrochemical techniques are quite helpful not only in evaluating them for their hot corrosion resistance but also for understanding their hot corrosion mechanisms. In fact, the electrochemical evaluation of superalloys with and

2- reduces to

fluxing reaction with the protective alumina and chromia can occur

superalloy degradation process in detail under hot corrosion conditions:

and forms Ni2+, Co3+, Cr3+, Al3+, Re4+, Ta5+ions etc. while at the cathodic site, SO4

environment and forms tungsten ion

like vanadium and molybdenum [9].

**2.2.1 Electrochemical mechanism** 

hot corrosion process.

electrochemical reactions are shown below:

without coatings is more reliable and fast.

Sulphur diffusion and formation of metal sulphides preferentially chromium and nickel sulphides was reported to be the influential factor. When sulphide phases are formed in superalloys, Ni- based alloys are inferior to cobalt and iron based alloys, which are especially effective in destroying the corrosion resistance of alloys [12]. In essence, the alloying elements play a significant role and decide the life of superalloys under hot corrosion conditions [12].
