Coatings to Resist High Temperature Corrosion

Monday, April 30, 2001 10:30 AM in Room Royal Palm Salon 1-3

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10:30 AM A1-1-1 The Effects of Platinum Concentration on the Oxidation Resistance of Superalloys Coated with Single Phase Platinum Aluminide
A.L. Purvis, B.M. Warnes (Howmet Thermatech Coatings)
Superalloy samples were coated using a low activity platinum aluminide applied by chemical vapor deposition (CVD). Different platinum thickness values were obtained on each sample by electroplating prior to CVD aluminizing and resulted in an array of platinum concentrations in the ensuing coating. The aluminide coating applied was a single phase, low activity platinum aluminide, with platinum concentrations varying from 10-40 weight percent.@paragraph@ The samples were then subjected to cyclic oxidation testing at 1175@super o@C until failure. As expected, it was discovered that higher concentrations of platinum resulted in a coating more resistant to degradation during the cyclic oxidation testing. The dependence of platinum displayed an approximately linear relationship to cyclic oxidation life in the test cases cited. Compositional traces of coating and alloying elements were tracked before and after the oxidation testing. The results were compared to previous research on oxidation resistance of bare alloys and of those in previous classical literature on two-phase platinum aluminide coatings. The ramifications of diffusion of many of the alloying elements are discussed with regard to their relative effects on oxidation resistance of the coating, and for gas turbine hardware in service conditions. }
10:50 AM A1-1-2 Reactive Element Modified CVD Low Activity Platinum Aluminide Coating
B.M. Warnes (Howmet Thermatech Coatings)
CVD co-deposition of aluminum, hafnium, zirconium and silicon has been used to make reactive element modified low activity platinum aluminide coatings for gas turbine hardware. The controlled addition of reactive elements produces diffusion coatings with more than four times greater cyclic oxidation resistance than traditional two phase type platinum aluminide coatings. In addition the CVD co-deposition process removes harmful substrate impurities during aluminizing. The co-deposition process is outlined, the optimum coating structure and composition are discussed, bulk chemical analysis results from coated super alloy foils are summarized and the cyclic oxidation test data is presented.
11:10 AM A1-1-3 A Comparative Study of Nickel Aluminide Coatings by PVD Techniques.
J.L. He, A. Leyland, A.D. Wilson, A. Matthews (University of Hull, United Kingdom)
Blades used in the hot sections of gas turbines usually have a thermal barrier coating (TBC) of partially yttria-stabilised zirconia (PYSZ). To help protect against high temperature oxidation of the blade an intermediate bond coat is applied, typically of an MCrAlY-type metal alloy. An alternative bond coat material is nickel-aluminium alloy. Various processing routes have been studied for both bond coat and TBC deposition. A potentially attractive processing route is to deposit both the bond coat and the TBC by a PVD method; this would have the advantage of permitting sequential deposition in the same coating cycle. Whilst much research has been carried out on PVD MCrAlY and PYSZ coatings, relatively little work has been carried out on PVD NiAl, which represents a simpler intermetallic alloy with less critical composition control requirements. Thus, we have investigated NiAl deposition by three ion-assisted coating routes – arc, electron beam and sputter deposition. Coatings were deposited on a nickel-based alloy (Inconel 600) and an AISI 304 stainless steel. The differences in structure and phase composition from each deposition method are reported, together with data on the cyclic oxidation performance. The influence of process parameters on coating characteristics and degradation mechanisms is discussed.
11:30 AM A1-1-4 Synthesis and High-Temperature Corrosion Behavior of CVD Aluminide Coatings on Iron-Base Alloys
Y. Zhang (Tennessee Technological University); B.A. Pint, J.A. Haynes, I.G. Wright, P.F. Tortorelli (Oak Ridge National Laboratory)
Iron aluminide coatings produced by chemical vapor deposition (CVD) are being studied as one method to improve the high temperature oxidation resistance of certain Fe-base alloys. The goal of this program is to evaluate whether aluminide coatings have sufficient long-term durability for applications in advanced fossil energy systems. A hot wall CVD reactor was used to synthesize high quality and high purity coatings on different types of iron base alloys. The CVD coating parameters are being optimized in order to fabricate a standard coating for performance evaluation in a variety of corrosion conditions, such as air-H@sub 2@O and H@sub 2@-H@sub 2@O-H@sub 2@S environments. Preliminary results indicate that iron aluminide coatings on Fe-9Cr-1Mo and type 304L stainless steel have excellent short-term resistance to oxidation and water vapor attack at 800°C, due to the formation of a protective alumina scale. The effects of coating composition, microstructure and exposure cycle frequency on high temperature oxidation resistance are being evaluated.