Land-based gas turbine exhaust stacks are typically made of low carbon steels, and when used in very harsh industrial environments, they are subjected to corrosion attack leading to material degradation and loss of structural integrity. To maintain acceptable condition, various types of corrosion resistant paints and coatings are used. These include aluminum-based polysiloxane organic paints, as well as wire arc sprayed stainless steel and aluminum coatings. Aqueous corrosion protection and thermal stability up to 538°C are required. Development and testing of various coating systems was conducted and presented in this paper. Laboratory and engine field-testing demonstrated the best performance for sealed aluminum wire arc sprayed coatings

Ceramic-based thermal spray coatings have been lately introduced as abradable materials for applications in the hot section of gas turbine engines. This paper reports abradable characteristics and high-temperature oxidation resistance of plasma sprayed yttria-stabilized zirconia (YSZ) and agglomertated yttria-stabilized zirconia with Polyester (YSZP). Unlike conventional blended coatings, agglomerated YSZP plasma-sprayed coatings are more forgiving to technique and processing parameter variations during the thermal-spray operation. The abradability studies were conducted on a test rig at temperatures of 600, 900 and 980 °C using coated (hardened) and uncoated blades. The oxidation resistance of the coatings was measured as a function of the cycles to failure and was conducted at temperature ranges of 930 to 1150°C in still air. Results showed that the ceramic coating, YSZ-Pl, was capable of withstanding both mid- and high-temperature rubs with little deleterious effects to the uncoated blade-tip across the temperature range, and presented a cyclic oxidation resistance comparable to that of YSZ coating.

Various low-temperature abradable coatings for applications reaching 482°C were evaluated with the purpose of being applied onto the compressor spacers of an 11 MW land-based gas turbine engine. The coating is intended to reduce the compressor rotor tip-clearance, and is expected to increase the compressor efficiency by a range of 0.5 to 1%. This study evaluated physical and metallurgical characteristics, abradable and oxidation-resistance properties of plasma-sprayed AlSi-BN, NiCrFeAl-BN and quasicrystal AlCuFeCr, and flame-and plasma-sprayed NiCrFeAl-BN.

Tungsten carbide-12 wt.% cobalt (WC-12Co) coatings and chromium plating are used to provide wear resistant surfaces in gas turbine applications. These treatments provide surfaces with hardnesses greater than 60 Rockwell C. In addition, a surface finish better than 8 microinches RMS is required for optimum performance. To achieve this surface finish, diamond grinding is required. The diamond grinding step adds considerable cost to the product and economical benefits could be achieved if more conventional grinding techniques were incorporated. A program was initiated to develop an alternative thermal spray coating, with a target hardness lower than 60 Rockwell C, but high enough to provide the wear resistance required. Spray development was conducted on five commercially available materials using the Diamond Jet 2600 high velocity oxy-fuel process. Laboratory evaluation included coating microstructure, macro- and microhardness, bond strength, salt spray corrosion, and cyclic compression tests.