**2.1 Effect of alloying elements**

From the metallurgical point of view, it is known that high temperature strength is obtained by maintaining certain phases that are responsible for high temperature strength. Since a main motive for the Metallurgists is to improve the mechanical strength of an alloy at high temperatures, the addition of certain alloying elements is essential with a view to form gamma prime (γ') and solid solution strengthners. Among the alloying elements, the significant reduction of chromium content and the addition of other elements, in particular tungsten, vanadium, molybdenum etc. makes the superalloys more vulnerable to hot corrosion [12]. It is reported that addition of tantalum and titanium produces beneficial effects for hot corrosion [12], while small additions of manganese, silicon, boron and zirconium do not significantly influence the hot corrosion of superalloys. Carbon addition is detrimental to hot corrosion, as the carbide phases provide sites for initiation of hot corrosion [12]. As observed for IN 738 LC with a large amount of chromium and a small amount of titanium, hot corrosion resistance is very good under type I conditions: however the alloy is vulnerable to type II hot corrosion conditions.The addition of molybdenum and large content of iron made the SU 718 less hot corrosion resistant. The addition of large amount of tungsten, tantalum, rhenium and minor other alloying elements and considerably reduced chromium rendered the new superalloy highly susceptible to hot corrosion. It is important to mention that chromium is the most effective alloying element for imporving the hot corrosion resistance of superalloys. In order to obtain good resistance to hot corrosion, a minimum of 15wt% chromium is often needed in nickel based superalloys and a minimum of 25wt% chromium in cobalt based superalloys [4]. However, it is pertinent to note that other alloying elements play a significant role as evidenced from the reported results. Therefore, it is mandatory to test the alloy under simulated environmental conditions in order to select the more corrosion resistant alloy.

### **2.2 Degradation mechanism**

The results clearly revealed that all the studied superalloys are highly vulnerable to hot corrosion. The results further revealed that the new superalloy corrodes much faster when compared to other studied superalloys. It is attributed to the fact that the tungsten which is the alloying element added along with other alloying elements in order to obtain high temperature strength characteristics of the superalloys, forms acidic tungsten oxide (WO3) due to which fluxing of protective oxide scales such as alumina and chromia takes place very easily. This type of acidic fluxing is self-sustaining because WO3 forms continuously that cause faster degradation of superalloys under marine environmental conditions at elevated temperatures. The degradation mechanism is explained in two steps as follows:

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

From the metallurgical point of view, it is known that high temperature strength is obtained by maintaining certain phases that are responsible for high temperature strength. Since a main motive for the Metallurgists is to improve the mechanical strength of an alloy at high temperatures, the addition of certain alloying elements is essential with a view to form gamma prime (γ') and solid solution strengthners. Among the alloying elements, the significant reduction of chromium content and the addition of other elements, in particular tungsten, vanadium, molybdenum etc. makes the superalloys more vulnerable to hot corrosion [12]. It is reported that addition of tantalum and titanium produces beneficial effects for hot corrosion [12], while small additions of manganese, silicon, boron and zirconium do not significantly influence the hot corrosion of superalloys. Carbon addition is detrimental to hot corrosion, as the carbide phases provide sites for initiation of hot corrosion [12]. As observed for IN 738 LC with a large amount of chromium and a small amount of titanium, hot corrosion resistance is very good under type I conditions: however the alloy is vulnerable to type II hot corrosion conditions.The addition of molybdenum and large content of iron made the SU 718 less hot corrosion resistant. The addition of large amount of tungsten, tantalum, rhenium and minor other alloying elements and considerably reduced chromium rendered the new superalloy highly susceptible to hot corrosion. It is important to mention that chromium is the most effective alloying element for imporving the hot corrosion resistance of superalloys. In order to obtain good resistance to hot corrosion, a minimum of 15wt% chromium is often needed in nickel based superalloys and a minimum of 25wt% chromium in cobalt based superalloys [4]. However, it is pertinent to note that other alloying elements play a significant role as evidenced from the reported results. Therefore, it is mandatory to test the alloy under simulated environmental

The results clearly revealed that all the studied superalloys are highly vulnerable to hot corrosion. The results further revealed that the new superalloy corrodes much faster when compared to other studied superalloys. It is attributed to the fact that the tungsten which is the alloying element added along with other alloying elements in order to obtain high temperature strength characteristics of the superalloys, forms acidic tungsten oxide (WO3) due to which fluxing of protective oxide scales such as alumina and chromia takes place very easily. This type of acidic fluxing is self-sustaining because WO3 forms continuously that cause faster degradation of superalloys under marine environmental conditions at elevated temperatures. The degradation mechanism is explained in two

conditions in order to select the more corrosion resistant alloy.

corrosion conditions [12].

**2.1 Effect of alloying elements** 

**2.2 Degradation mechanism** 

steps as follows:

a) The tungsten present in the new superalloys reacts with the oxide ions present in the environment and forms tungsten ion

$$\rm{WO\_3 + O^{\cdot 2} = WO\_4 \cdot 2 \cdot}$$

b) As a result, the oxide ion activity of the environment decreases to a level where acidic fluxing reaction with the protective alumina and chromia can occur

$$\begin{aligned} \mathrm{Al\_2O\_3} &= \mathrm{Al^{3+}} + \mathrm{O^{2-}} \\\\ \mathrm{Cr\_2O\_3} &= \mathrm{Cr^{3+}} + \mathrm{O^{2-}} \end{aligned}$$

A similar reaction mechanism occurs if the superalloys contain other refractory elements like vanadium and molybdenum [9].

### **2.2.1 Electrochemical mechanism**

The following section describes an electrochemical phenomenon that explains the new superalloy degradation process in detail under hot corrosion conditions:

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 and forms Ni2+, Co3+, Cr3+, Al3+, Re4+, Ta5+ions etc. while at the cathodic site, SO42- reduces to 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 hot corrosion process.

Therefore, the hot corrosion of new superalloy is electrochemical in nature and the relevant electrochemical reactions are shown below:

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 without coatings is more reliable and fast.

The Selection of Materials for Marine Gas Turbine Engines 67

forming appropriate protective scales like alumina or chromia depending on the

The titanium alloy components experience hot corrosion problem when they are used for marine gas turbines [16]. It severely limits the high temperature capability of alloys in terms of mechanical properties. It is therefore, desirable to understand the characteristics of titanium alloys under simulated marine gas turbine engine conditions and then apply appropriate coatings, which can prevent hot corrosion and thereby helps in enhancing the

The hot corrosion characteristics of the titanium alloy, IMI 834 in marine environments at 6000 C revealed that the rate constant increases by about six times in marine environment and about seven times in vanadium-containing marine environment. It indicates that the rate of reaction is very high in marine environments, still higher in vanadium containing marine environments and low in other environments [16]. The Scanning Electron Micrograph (SEM) of the alloys corroded in marine environment at 6000 C clearly shows that the oxide scale that formed on the surface of the titanium alloys was cracked due to the presence of NaCl in the environment (Fig.17). The cracks were not observed for the alloys corroded in other environments. It indicates that the chloride ions present in the marine environment causes the oxide scale to crack and facilitates the corrosive species present in the environment to react with the alloy, which is the reason for observing significant increase in corrosion rate [16]. It is known that chloride ions lead to pitting type of attack, which generally initiates at imperfections in the oxide scale. The micro hardness measurements as a function of depth for the alloys corroded at 6000 C, revealed the presence of about 500 μm hardened zone due to dissolution of oxygen, which is sufficient for affecting the mechanical properties of the titanium alloys by forming a highly brittle zone from which crack initiates during service conditions [16]. The depth of oxygen dissolved

Fig. 17. Effect of marine environment on the stability of titanium alloy IMI 834

surrounding environmental conditions [14-15].

life of gas turbine engines significantly.

**3. Titanium alloys** 

Fig. 16. An electrochemical model showing that hot corrosion of new superalloy is an electrochemical phenomenon

#### **2.3 Development of smart coatings**

From the present results, it is concluded that the new superalloy is highly susceptible to hot corrosion, though it exhibits excellent high temperature strength properties. It is clear that other superalloys are also vulnerable to both types of hot corrosion. It stresses the need to apply high performance protective coatings for their protection against hot corrosion both at low and high temperatures i.e. type II and type I as the marine gas turbine engines encounter both the problems during service. The protective coatings allow the marine gas turbine engines to operate at varied temperatures and enhance their efficiency by eliminating failures during service. Research in this direction has resulted in design and development of smart coatings which provide effective protection to the superalloy components for the designed period against type I, type II hot corrosion and high temperature oxidation that are normally encountered in gas turbine engines which in turn enhances the efficiency of gas turbine engines considerably [14-15]. This is a major developmental work in the area of gas turbine engines used in aero, marine and industrial applications. Unlike the conventional / existing coatings, the smart coatings provide total protection to the superalloy components used in aero, marine and industrial applications by forming appropriate protective scales like alumina or chromia depending on the surrounding environmental conditions [14-15].
