Performance Evaluation and Life Prediction Modeling of Industrial Gas Turbine Blade Coatings Used in Saudi Arabia

The service life of industrial gas turbines (IGTs) blades and vanes operating at high temperatures is governed by their response to thermo-mechanical deformation and degradation caused by extended exposure to complex aggressive environments. New development in the components’ base materials primarily aims at improving the high temperature mechanical properties of the blade materials such as creep strength and low and high cycle fatigue behavior. For example, modern IGTs employ directionally solidified and single crystal nickel-based superalloys for high pressure blades and vanes. A great variety of coating compositions and application techniques are used in the hot sections of IGTs to improve their durability against environmental degradation. Many IGTs manufacturers use metallic based-coatings for high pressure turbine blades and vanes. Ceramic thermal barrier coatings may be employed but would increase the cost significantly.

Most Ni-based superalloy turbine blades are protected by MCrAIY type overlay coatings which utilize low pressure plasma spraying (LPPS) to provide protection against oxidation and hot corrosion. The composition of the coating can be varied to optimize the resistance to the dominant mode of attack.

There are essentially three modes of temperature-dependent attack for turbine hot section components: Type II hot corrosion (650-800°C), Type I hot corrosion (800-950°C) and oxidation (>950°C) (Pomeroy, 2005). Khanna and Jha (1998) investigated the chemistry of formation of salts during the combustion of coal/fuel. The sulphur present in coal and fuel oils yields sulphur dioxide (SO2 ) on combustion, which is then partially oxidized to sulphur trioxide (SO3 ). Sodium chloride salt (NaCl), either as an impurity in the fuel or accompanying intake air, reacts with SO3 and water vapor at the combustion temperature to yield sodium sulphates (Na2 SO4 ), which deposit on the base metal surfaces. During combustion, vanadium present in the fuel oxidizes to V2 O5 which subsequently reacts with Na2 SO4 to form low melting sodium vanadates, highly corrosive molten salt.

Type II hot corrosion involves the formation of base metal (nickel or cobalt) sulphates which require high levels of partial pressure of SO3 for their stabilization. The base metal sulphates react with Na2 SO4 to form low melting compounds which subsequently prevent the formation of a protective oxide film, thereby leading to accelerated oxidation. Silicon containing and/or chromium rich diffusion coatings offer improved resistance for this type of attack. Further, CoCrAlY generally out performs NiCrAlY-based coatings with best performance associated with increased chromium content.

Type I hot corrosion occurs when the sulphates deposited on the base metal are in the liquid state, react with the base metal, and form internal sulfides with low melting compounds (mainly chromium sulfides) with catastrophic consequences. The most suitable coatings to resist Type I hot corrosion are platinum aluminide diffusion coatings and MCrAlY coatings containing up to 25 wt.% Cr and 6 wt.% Al.

For oxidation resistance at high temperatures, overlay coatings of classic design with 18–22% Cr and 8–12% Al perform well due to the formation of highly protective alumina scales. In general, NiCrAlY and NiCoCrAlY outperform the CoCrAlY-based systems when comparing their oxidation resistances.

It is obvious that no single coating system can offer optimum protection over a wide range of turbine operating conditions especially if frequent startup/shutdown cycles are encountered. In addition, the use of low grade fuels with high pollutant level make the design of optimized coating systems more illusive. Damaging contaminants in fuel and intake air include sulfur, sodium, chloride, and vanadium. For instance, the sulfur in the fuel is generally limited to 0.3 wt.% for commercial jet engines and to 1.0 wt.% for marine gas turbines, sodium chloride is present as airborne salt near coastal areas, and vanadium is considered as an impurity in heavy fuel grades.

Coastal power generation sites are common in Saudi Arabia and represent great challenges to efforts intended to improve the reliability and service life of IGTs hot section components. The main objective of this proposal is to address the reliability and service life limitations for existing IGTs blades and vanes coating systems used by the local power generation community in Saudi Arabia. The scope of the proposed work includes

(*) Experimental characterization of the coating failure modes associated with the use of low grade fuels commonly used by the local power producers.
(*) Improving the understanding of the mechanisms of attack to enable mechanism-based life prediction modeling.
(*) Developing a scientific approach and a methodology for mechanism-based theoretical and numerical modeling of coating performance.