**3. Titanium alloys**

66 Efficiency, Performance and Robustness of Gas Turbines

= SO32- + O2-

= S + 4O2-

= S2- + 4O2-

At the anode At the cathode Ni = Ni2+ + 2e- 1/2 O2 + 2e- = O2-

SO42- +8e-

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

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

Cr = Cr3+ + 3 e- SO42- + 2e-

Co = Co3+ + 3e- SO42- +6e-

Al = Al3+ + 3e-

Re = Re4+ + 4e-Ta = Ta5+ + 5e-

electrochemical phenomenon

**2.3 Development of smart coatings** 

etc.

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 life of gas turbine engines significantly.

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

The Selection of Materials for Marine Gas Turbine Engines 69

diffuse into the titanium alloys, react with alloys constituents to destroy atomic-binding forces and cause cracking. These detrimental observations clearly stress the need to protect titanium alloy components from hot corrosion and thereby enhance their life by avoiding failures during service. These studies also focus on the development of coatings, which can protect titanium alloys both from oxidation as well as hot corrosion, since both the processes

Different smart coatings based on a variety of elements and their combination were designed and developed on titanium alloy, IMI 834. The extensive investigations revealed that the smart coatings based on aluminium that were developed by innovating a new pack composition showed an excellent resistance both under hot corrosion as well as oxidation conditions [15]. The elemental distribution showed a protective, continuous and adherent alumina scale over the coating. It indicates that an excellent protection was provided by the developed smart coating to the titanium alloys from hot corrosion. Further, the developed coatings can be prepared by a simple technique, easy to coat large components and moreover highly economical. Hence, it is recommended to use the developed smart

The chapter presented hot corrosion results of selected nickel based superalloys for marine gas turbine engines both at high and low temperatures that represent type I and type II hot corrosion. The results have been compared with a new alloy under similar conditions in order to understand the characteristics of the selected superalloys. It is observed that the nature and concentration of alloying elements mainly decide the resistance to type I and type II hot corrosion. CM 247LC and the new superalloy are extremely vulnerable to both types of hot corrosion. Relevant reaction mechanisms that are responsible for degradation of various superalloys under marine environmental conditions were discussed. The necessity to apply smart coatings for their protection under high temperature conditions was stressed for the enhanced efficiency as the marine gas turbine engines experience type I and type II hot corrosion during service. Further, the hot corrosion problems experienced by titanium alloy components under marine environmental conditions were explained along with relevant degradation mechanisms and recommended a developed smart coating for their

coatings for the modern marine gas turbine engine titanium alloy components.

[2] M.R. Khajavi and M.H. Shariat, Engineering Failure Analysis, 11 (2004) 589

[4] N. Eliaz, G. Shemesh and R.M. Latanision, Engineering Failure Analysis, 9 (2002) 31

[3] J.M. Gallardo, J.A.Rodriguez and E.J. Herrera, Wear, 252 (2002) 264

[5] M. Konter and M. Thumann, J. Mater. Process Technol., 117 (2001) 386

are experienced by gas turbine engine components.

**3.2 Smart coatings development** 

**4. Summary** 

effective protection.

[1] N. Das, US patent 5,925,198, July 1999

[6] J. Stringer, Mater.Sci.Technol., 3 (1987) 482 [7] A.S.Radcliff, Mater. Sci. & Tech., 3 (1987) 554 [8] R.F.Singer, Mater. Sci. & Tech., 3 (1987) 726

**5. References** 

region varies with the temperature at which the titanium alloys were corroded. It is important to mention that the depth of the titanium alloys affected in marine environment is about 100 times more than that of the alloys corroded in other environments at the same temperature [16]. It clearly indicates the greater aggressiveness of marine environments to titanium alloys compared to other environments.

### **3.1 Degradation mechanism**

Given below are the proposed mechanistic steps that degrade titanium alloy, IMI 834 under hot corrosion conditions in marine environment:

1. The oxide scale that forms on the surface of IMI 834 is predominantly TiO2 in association with Al2O3. The TiO2 reacts with chloride ions present in the marine environments at elelvated temperatures to form volatile TiCl2

$$\text{TiO}\_2 + 2\text{ Cl}\_1 = \text{TiCl}\_2 + 2\text{ O}\_2 \tag{1}$$

The TiCl2 dissociates at elevated temperatures to form Ti2+ and Cl - ions

$$\text{TiCl}\_2 = \text{Ti} 2^\* + 2 \text{ Cl} \cdot \tag{2}$$

The titanium ions then react with oxygen ions present in the environment to form a non-adherent and non-protective TiO2 scale which spalls very easily. Chloride ions penetrate into the alloy to form volatile chlorides. This process continues until titanium in the alloy is consumed. In other words, the reaction is autocatalytic. The oxygen ions that form in reaction (1) diffuse into the alloy and form an oxygen-dissolution region due to high oxygen solubility in titanium alloys.

2. Al2O3 reacts with Cl- ions to form aluminum chloride

$$\text{Al}\_2\text{O}\_3 + 6\text{Cl}^\cdot = 2\text{ AlCl}\_3 + 3\text{ O}^{2\cdot} \tag{3}$$

The AlCl3 that formed in the above reaction dissociates to form Al3+ and Cl<sup>−</sup> ions

$$\text{AlCl}\_3 = \text{Al}^{3+} + \text{\textasciicircum} \tag{4}$$

The Al3+ ions react with oxygen ions to form a loose and non-protective alumina scale, which spalls very easily, as in the case of titania

$$\rm Al^{3+} + 3\,\rm O^{2-} = Al\_2O\_3 \tag{5}$$

As mentioned above, the chloride ions penetrate into the titanium alloy to form volatile chlorides and the reaction is autocatalytic. The oxygen ions that formed in reaction (3) diffuse into the alloy and react with titanium. The reactions (1) and (3) contribute to the formation of oxygen dissolved region in the titanium alloy subsurface.

As a result of the above reactions, the degradation of titanium alloys takes place at a faster rate [16] and situation can easily make the components fabricated from titanium alloys, susceptible to failure under normal service conditions of gas turbines. Even in actual jet engines, cracking was reported on salted Ti–6Al–4V alloy discs . Logan *et al [17]* were proposed that oxygen ions from the scale and chloride ions from marine environment, diffuse into the titanium alloys, react with alloys constituents to destroy atomic-binding forces and cause cracking. These detrimental observations clearly stress the need to protect titanium alloy components from hot corrosion and thereby enhance their life by avoiding failures during service. These studies also focus on the development of coatings, which can protect titanium alloys both from oxidation as well as hot corrosion, since both the processes are experienced by gas turbine engine components.

### **3.2 Smart coatings development**

Different smart coatings based on a variety of elements and their combination were designed and developed on titanium alloy, IMI 834. The extensive investigations revealed that the smart coatings based on aluminium that were developed by innovating a new pack composition showed an excellent resistance both under hot corrosion as well as oxidation conditions [15]. The elemental distribution showed a protective, continuous and adherent alumina scale over the coating. It indicates that an excellent protection was provided by the developed smart coating to the titanium alloys from hot corrosion. Further, the developed coatings can be prepared by a simple technique, easy to coat large components and moreover highly economical. Hence, it is recommended to use the developed smart coatings for the modern marine gas turbine engine titanium alloy components.
