EXECUTIVE SUMMARY: Aluminide coatings are of interest for many high temperature applications
because of the possibility of improving the oxidation of structural alloys by forming a
protective external alumina scale. In order to develop a comprehensive lifetime
evaluation approach for aluminide coatings used in fossil energy systems, some of the
important issues have been addressed in this report for aluminide coatings on Fe-based
alloys (Task I) and on Ni-based alloys (Task II).
In Task I, the oxidation behavior of iron aluminide coatings synthesized by
chemical vapor deposition (CVD) was studied in air + 10vol.% H2O in the temperature
range of 700-800°C and the interdiffusion behavior between the coating and substrate
was investigated in air at 500-800°C. Commercial ferritic (Fe-9Cr-1Mo) and type 304L
(Fe-18Cr-9Ni, nominally) austenitic stainless steels were used as the substrates. For the
oxidation study, the as-deposited coating consisted of a thin (<5μm), Al-rich outer layer
above a thicker (30-50μm), lower Al inner layer. The specimens were cycled to 1000 1-h
cycles at 700°C and 500 1-h cycles at 800°C, respectively. The CVD coating specimens
showed excellent performance in the water vapor environment at both temperatures,
while the uncoated alloys were severely attacked. These results suggest that an aluminide
coating can substantially improve resistance to water vapor attack under these conditions.
For the interdiffusion study, the ferritic and austenitic steels were coated with relatively
thicker aluminide coatings consisting of a 20-25μm outer layer and a 150-250μm inner
layer. The composition profiles before and after interdiffusion testing (up to 5,000h)
were measured by electron probe microanalysis (EPMA). The decrease of the Al content
at the coating surface was not significant after extended diffusion times (≤ 5,000h) at
temperatures ≤ 700oC. More interdiffusion occurred at 800oC in coatings on both Fe-
9Cr-1Mo and 304L alloys; a two-phase microstructure was formed in the outer coating
layer on 304L after interdiffusion of 2,000h at 800°C. The interdiffusion behavior was
simulated using a computer model COSIM (Coating Oxidation and Substrate
Interdiffusion Model), which was originally developed for MCrAlY overlay coatings by
NASA. Complimentary modeling work using a mathematic model from Heckel et al.
also was conducted. Reasonable agreement was observed between the simulated and
experimental composition profiles, particularly for aluminide coatings on Fe-9Cr-1Mo
ferritic steels.
In Task II, the research focused on the CVD aluminide bond coats for thermal
barrier coatings (TBC). The martensitic phase transformation in single-phase β-NiAl and
(Ni,Pt)Al coatings was studied and compared. After isothermal exposure to 1150°C for
100 hours, the β phase in both types of coatings was transformed to a martensite phase
during cooling to room temperature. Martensitic transformation also was observed in the

(Ni,Pt)Al bond coat with and without the ceramic top layer after thermal cycling at
1150°C (700 1-h cycles). Such transformation resulted from Al depletion in the coating
due to the formation of the Al2O3 scale on coating surface and interdiffusion between the
coating and superalloy substrate. The volume changes associated with the martensitic
transformation could affect the coating surface stability ("rumpling") and thus
contributing to TBC failure. To elucidate the effect of Hf levels in the superalloy
substrate on the oxidation performance, directionally-solidified René 142 superalloys
containing three different Hf contents with and without aluminide coatings were
cyclically oxidized at 1100 and 1150°C in air. Poor scale adhesion was observed for all
bare and NiAl-coated René 142 superalloys, as compared with single-crystal superalloys
such as René N5. Spallation occurred at relatively early stages disregarding the Hf
contents in the superalloys. Finally, a platinum plating system has been set up at
Tennessee Technological University to carefully control the Pt plating process for
synthesizing (Ni,Pt)Al and other Pt-containing coatings. The effects of the Pt
electroplating parameters such as plating current density on Pt adhesion and uniformity
were examined. The plating rate increased nearly linearly with the increased current
density from 0.2 to 0.6A/dm2. More Pt was built up near the specimen edges/corners
than on the flat surface at high current density; the thicker Pt near specimen edges/corners
over the flat surfaces led to localized spallation of the Pt layer in these areas.

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SOURCE: Ying Zhang. “Aluminide Coatings for Power-Generation Applications.” Report for Oak Ridge National Laboratory for the U. S. Department of Energy under contract DE-AC05-00OR22725. December 18, 2003.