High Temperature Oxidation Performance of Aluminide Coatings

In order to determine the potential benefits and limitations of aluminide coatings, coatings made by chemical vapor deposition (CVD) on Feand Ni-base alloy substrates are being evaluated in various hightemperature environments. Testing of coatings on representative ferritic (Fe-9Cr-1Mo) and austenitic (type 304L stainless steel) alloys has found that high frequency thermal cycling (1h cycle time) can significantly degrade the coating. Based on comparison with similar specimens with no thermal cycling or a longer cycle time (100h), this degradation was not due to Al loss from the coating but most likely because of the thermal expansion mismatch between the coating and the substrate. Several coated Ni-base alloys were tested in a high pressure (20atm) steam-CO2 environment for the ZEST (zero-emission steam turbine) program. Coated specimens showed less mass loss than the uncoated specimens after 1000h at 900°C and preliminary characterization examined the post-test coating structure and extent of attack. INTRODUCTION 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 chromiaand 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 CVD coatings 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 CVD processes and the short-term performance of these coatings.5,14,15 The current work is focusing on the long term behavior in both diffusion tests16 and 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 CVD for testing in a high pressure (20atm) steam-CO2 environment for the ZEST (zero-emission steam turbine) program. Such alloys would be needed as