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A low temperature (677°C [1250°F]) 500-h burner rig test using a fuel with 2% by weight of sulfur was made of IN939, a standard cobalt, chromium, aluminum, yttrium (CoCrAlY) coating, and a nickel, chromium, aluminum, yttrium (NiCrAlY) coating. Both the IN939 and the CoCrAlY coating were found to hot-corrode readily. The CoCrAlY coating hot-corroded by a Type 2 mechanism (low-melting point mixed sulfate), while the IN939 showed evidence of both Type 1 (sulfidation) and Type 2 hot corrosion. The NiCrAlY coating was relatively unattacked. A change in the mode of attack was noted in the CoCrAlY coating when the rig temperature dropped to 650°C (1200°F) for 24 h. The corrosion mode may have changed from one in which solid cobaltous oxide is the stable phase at the gas/scale interface to one in which solid cobalt sulfate is the stable phase.
MetallurgistMember of ASTM, U.S. Navy, David Taylor R&D Center, Annapolis, MD
Stock #: JTE10269J