Identification of degradation mechanisms in coatings for supercritical steam applications

Abstract

The development and qualification of coatings for materials used in modern steam power plants stems from the increased demand for higher efficiency, and hence higher operating temperatures. Within the EU funded project 'SUPERCOAT', several coatings, both overlay and diffusion type, were investigated. Seven different coatings are presented in this work. They included two commercially available HVOF coatings (Ni-20Cr and Ni-50Cr), an aluminium-based slurry coating (IPCOTE), together with two further variations of this slurry coating containing sputter-coated interlayers. An overlay slurry coating consisting of silica particles embedded in a matrix of alumina and chromia was also examined. The final coating to be investigated was a pack-aluminised sample of P92.All the coating systems examined showed superior oxidation resistance compared to the 9%Cr steel substrate (P91 or P92) in extended exposures to a steam environment at 650 8C. However, in service component lifetime will be limited by degradation of the coating, therefore it is essential that the mechanisms controlling this behaviour are understood. This paper reviews several degradation mechanisms that have been observed during long-term exposure of these coatings. The mechanisms that have been observed include depletion of active alloying elements, diffusion of aluminium into the substrate from the coating, formation of Kirkendall porosity and mechanical failure of the coatings. Examples of each of these mechanisms will be presented. Possible processing routes to avoid these degradation mechanisms will also be discussed.

Surface oxide formation on IN625 and plasma sprayed NiCrAlY after high density and low density supercritical water testing

“…A commonly used method for improving corrosion resistance, without sacrificing mechanical or irradiation properties, is by the application of a corrosion resistant coating to the surface of the material . Many iron, nickel, and aluminum‐based coatings have been tested in steam environments and have shown good adhesion, thin stable oxides and resistance to cracking.…”

Behaviour of Ni-based coating alloys in supercritical water

“…One coating that shows particularly good properties in the literature is Ni20Cr. Since HVOF deposited Ni20Cr shows no coating degradation or substrate attack and a very thin protective Cr203 layer on the coating surface after 20000h of exposure to steam at 650°C [38,39]. Pack aluminizing of a nickel base alloy IN625, showed an increased corrosion resistance after 50h of being oxidized in air at 1000°C and 1100°C [40], whereas pack aluminizing and slurry coatings on steel substrates were found to be detrimental due to 40 the inward diffusion of aluminum towards the substrate rather than outwards during the diffusion heat treatment in argon at 700°C for lOh [39].…”

Steam oxidation resistance of plasma sprayed chromium‐containing coatings at 1200 °C

“…Jin et al deposited a 75Cr 3 C 2 ‐25NiCr (wt.%) coating on a Zr‐2.5Nb substrate by high‐velocity oxy‐fuel spray (HVOF) technique and found that the coated samples gained less weight compared to the uncoated Zr‐2.5Nb after 1 h oxidation in 700–1000 °C dry air environment. Sundararajan et al compared the steam oxidation resistance of APS and HVOF Ni50Cr (wt.%) coatings at 600–750 °C and found that the HVOF‐Ni50Cr coating could protect the underlying 9Cr‐1Mo substrate from oxidation for 1000 h. Durham et al reported that the HVOF Ni20Cr (wt.%) coating had an excellent performance under the steam oxidation at 650 °C for 1000 h, for a dense and adherent chromia scale with a thickness about 1 µm was formed on the surface of the coating. In general, compared with the Cr 3 C 2 ‐NiCr coating, the NiCr coatings prepared by thermal spray are denser and have higher bonding strength with the substrates due to the excellent ductility of the NiCr alloy.…”