ICMCTF2009 Session A2: Coatings for Use in Harsh Environments

In stationary gas turbines, hot gas path components are in contact with combustion product gases containing impurities. The materials and coatings will degrade by gaseous attack and by hot corrosion, if corrosive salts can condense. The kind and the extent of corrosion attack are strongly dependent on the type and quantity of corrosive species. These contaminants have their origin in the fuel, the intake air and the injected water, where the fuel is the main source. The contaminants are varying with the different fuel types. From a corrosion perspective two main type of fuels can be differentiated: fuels with a high amount of impurities and cleaner fuels with a low amount of impurities. The ash-bearing fuels such as crude and heavy oil belong to the first group, whereas natural gas and diesel belong to the second group. Other liquid fuels or syngas from coal / biomass gasification can belong to both groups depending on their impurity content, which is determined by the level of fuel cleaning.

The encountered corrosion mechanism will be described in dependence of the chemical composition and the quantity of the contaminants. The different types of corrosion attack are illustrated by micrographs from ex-service components and from laboratory samples.

To minimise the hot corrosion damages, process- and material-related countermeasures are proposed for the different fuels. The process-related solutions are described shortly and their impact on the gas turbine operation will be evaluated. Their main goal is the change of the corrosive environment through fuel treatment or use of additives. The other option is the use of higher corrosion-resistant materials and coatings in the hot gas path. An overview of the typical gas turbine materials will be given with respect to their corrosion performance. The focus will be put on the behaviour of metallic overlay coatings and thermal barrier coatings in corrosive environment. Consequences will be discussed for the use of corrosion-resistant systems in new power plants with low CO₂ emissions.

The presented work aims to find material solutions for a recently developed process that allows the recovery of Phosphorus, an essential plant nutrient and therefore integral component of many fertilizers, from sewage sludge ashes. The process offers the possibility to overcome the previsible shortage of Phosphorus as a natural resource. It is distinguished by the use of highly chlorine-containing atmospheres at temperatures up to 1000°C. Unfortunately, there are currently no materials commercially available that can withstand such conditions over longer periods of time.

For the development of a sufficiently resistant material system, nickel base alloys were chosen as a base material. They feature outstanding high temperature corrosion resistance and strength. To further advance these qualities, protective diffusion coatings, applied by pack cementation, were developed. The thermodynamic assessment of different coating elements revealed that aluminum and silicon have the best prerequisites to form and maintain slow-growing, stable oxide layers with the highest potential for being protective against corrosive attacks.

The performance of the coated materials was examined in long-term tests under simulated field conditions at high temperatures and under atmospheres of up to 10% Cl₂ in air. In the paper the theoretical considerations with regards to coating design and results of these experiments will be presented and discussed.

Thermally sprayed abradable seals are employed in turbomachinery to reduce leakage gaps between stationary and rotating parts to improve efficiency and stall margin. These seals are commonly composite type coatings which derive their abradability from the use of low shear strength materials or from a porous, friable coating structure. The development of mostly zirconia based thermal shock resistant ceramics for use at high temperatures, allowed for abradable seals for the high pressure turbine stages to be developed. On stages with reduced temperatures, both ceramic and metallic coatings of the MCrAlY type can be used.

This paper reviews the state of the art in compressor and turbine clearance control materials and systems, with a focus on novel thermally sprayed ceramic turbine seals with encouraging property combinations which are achieved by the introduction of alternative stabilizers. Normally ceramic seal surfaces require hard tipping of the rotor blades to allow them to cut properly and recent silicon carbide hard tipping technology will be presented.

It is widely recognised that a good abradable coating can significantly improve the efficiency of a gas turbine engine, with correct design allowing blade wear to be minimised. Thus my understanding the abradable process, its link to abradable coating microstructure and the underlying cutting mechanics, it is possible to optimise coating properties.

In this study, metallographic characterisation of abraded coatings has been carried out for three different blade tip/coating systems. Based on this understanding, a micro-cutting model has been constructed to predict abradable behaviour.

It is demonstrated that for a given abradable system coating performance is highly dependent on parameters that define the abradable process, particularly blade tip velocity and incursion rate, and their interaction with the abradable coating microstructure/properties. Based on this modelling work guidelines are given to aid the optimisation of abradable coatings.