ABSTRACT: Aluminide coatings are of interest for many high temperature applications because of the possibility of improving the oxidation resistance of structural alloys by forming a protective external alumina scale.1-5 Steam and exhaust gas environments are of particular interest because alumina is less susceptible to the accelerated attack due to hydroxide formation observed for chromia- and silica-forming alloys and ceramics.6-13 For water vapor testing, one ferritic (Fe-9Cr-1Mo) and one austenitic alloy (304L) have been selected as substrate materials and CVDcoatings have been used in order to have a well-controlled, high purity coating. It is anticipated that similar aluminide coatings could be made by a higher-volume, commercial process such as pack cementation. Previous work on this program has examined as-deposited coatings made by high and low Al activity CVDprocesses and the short-term performance of these coatings.5,14,15 The current work is focusing on the long term behavior in both diffusion tests16and oxidation tests of the thicker, high Al activity coatings. For long-term coating durability, one area of concern has been the coefficient of thermal expansion (CTE)mismatch between coating and substrate.5 This difference could cause cracking or deformation that could reduce coating life. Corrosion testing using thermal cycling is of particular interest because of this potential problem and results are presented where a short exposure cycle (1h)severely degraded aluminide coatings on both types of substrates. To further study the potential role of aluminide coatings in fossil energy applications, several high creep strength Ni-base alloys were coated by CVDfor testing in a high pressure (20atm) steam-CO2
environment for the ZEST(zero-emission steam turbine) program. Such alloys would be needed as
structural and turbine materials in this concept. For Ni-base alloys, CVD produces a ≈50μm β-NiAl outer
layer with an underlying interdiffusion zone. Specimens of HR160, alloy 601 and alloy 230 were tested
with and without coatings at 900°C and preliminary post-test characterization is reported.

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SOURCE: Pint, B. A., Y. Zhang, J. A. Haynes and I. G. Wright. “High Temperature Oxidation Performance of Aluminide Coatings.” Annual report to U. S. Department of Energy. Contract DE-AC05-00OR22725. 2003.