**4. Summary**

68 Efficiency, Performance and Robustness of Gas Turbines

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

Given below are the proposed mechanistic steps that degrade titanium alloy, IMI 834 under

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

TiO2 + 2 Cl- = TiCl2 + 2 O2- (1)

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

Al2O3 + 6Cl- = 2 AlCl3 + 3 O2- (3)

 AlCl3 = Al3+ + 3 Cl- (4) The Al3+ ions react with oxygen ions to form a loose and non-protective alumina scale,

 Al3+ + 3 O2- = Al2O3 (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

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,

formation of oxygen dissolved region in the titanium alloy subsurface.

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

(2)

environments at elelvated temperatures to form volatile TiCl2

TiCl2 = Ti2+ + 2 Cl-

due to high oxygen solubility in titanium alloys. 2. Al2O3 reacts with Cl- ions to form aluminum chloride

which spalls very easily, as in the case of titania

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

titanium alloys compared to other environments.

hot corrosion conditions in marine environment:

**3.1 Degradation mechanism** 

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 effective protection.
