Microstructure Study of NiCrAlY and FeCrAlY Exposed to Superheated Steam at 800 °C

Abstract

Supercritical water-cooled reactor (SCWR) was chosen as Generation IV reactor concept in Canada to utilize Canada's expertise and technical capabilities from past research and designs. The conceptual design of Canadian SCWR has a core outlet temperature of 650 °C at 25 MPa and a peak cladding temperature as high as 800 °C. Corrosion/oxidation resistance is an important factor in material selections and also coating considerations. Most of the reported supercritical water (SCW) test data have been obtained at temperatures up to 700 °C as no autoclave exists that can operate above 700 °C at supercritical pressures and temperatures. Superheated steam (SHS) is used as a surrogate fluid to SCW in this study to evaluate two coating materials, FeCrAlY and NiCrAl, at 800 °C. The results showed that the FeCrAlY became discolored rapidly while NiCrAl still maintained some metallic sheen after 600 h. The weight change results suggest that more oxide formation took place on FeCrAlY than NiCrAl. In particular, grain boundary oxide (Al2O3) formed on FeCrAlY surface upon exposure to steam after 300 h. Further exposure caused more intragranular Al2O3 to form, in addition to magnetite formation on the grain boundary regions. For NiCrAl samples, NiO formed after steam exposure for 300 h. Spinel and (Cr,Al)2O3 were also found after 300 h along with very limited amount of Al2O3. After 600 h, Al2O3 became well developed on NiCrAl and the coverage of spinel and Cr2O3 on the surface reduced.

Steam Oxidation of Aluminide-Coated and Uncoated TP347HFG Stainless Steel under Atmospheric and Ultra-Supercritical Steam Conditions at 700 °C

“…Considerable efforts have been dedicated to the development of coatings for super-critical power plants [12][13][14][15][16][17][18][19] but little work has been published regarding the behaviour and coatings under high pressure steam. Coatings such as MCrAlY overlays (M = Ni, Fe) [20,21] ceramics such as Al 2 O 3 [22], and diffusion aluminides deposited by slurry application [23][24][25] or by pack cementation [26] have been studied over the past few years and in most cases these coatings displayed a substantial improvement in terms of high pressure steam oxidation resistance. For instance, specimens of P92, both uncoated and coated with a slurry aluminide were exposed to real operating conditions in a bypass of a steam power plant and the results were compared to those obtained in the laboratory under atmospheric steam [23].…”

Oxidation Study of Incoloy 800H Tubes Exposed to Super-Heated Steam and Supercritical Water

“…Bsat et al [2018-1] tested NiCrAlY coated Alloy 214 specimen coupons in SCW (700 o C, 25 MPa, 150 ppb O2, pH 7, 7 mL/min) for 1,000 hours and found a thin oxide scale likely containing Cr2O3 and Ni(Al,Cr)2O4. Saez-Maderuelo et al [2018] when testing NiCrAl coupons in SHS (800 o C, 0.1 MPa) noted that an oxide scale formed containing a mixture of NiO,(Cr,Al)2O3 spinel, chromia, and alumina after 300 hours, and alumina and chromia after 600 hours. In general, both in this study and in previous work, the oxide scales observed were thin and without distinct phase separation, no matter the test condition in SHS and SCW.…”