HVOF coatings for steam oxidation protection

Abstract

In the context of the European project ‘Coatings for Supercritical Steam Cycles’ (SUPERCOAT), the use of steam oxidation resistant coatings on currently available ferritic materials with high creep strength but poor oxidation resistance was investigated in order to allow increase in the operating temperature of steam power plants from 550 to 650 °C. Among the explored coating techniques for this application, chosen on the basis of being potentially appropriate for coating steam turbine components, High Velocity Oxy Fuel (HVOF) thermal spray has resulted in one of the most successful techniques. Different alloyed materials such as FeCrAl, NiCrSiFeB, FeAl, NiCr and FeCr have been deposited, optimized and tested under flowing steam at 650 °C. Characterization of as deposited and tested samples by metallography, SEM‐EDS and XRD was carried out. Some of these coatings form protective pure chromium or aluminium oxides exhibiting excellent behaviour for at least 15 000 h of exposure, whereas others form less stable complex mixed oxides which nevertheless grow at considerably slower rates than the oxides formed on uncoated P92 (9 wt% Cr ferritic steel).

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

“…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. Ni20Cr coating shows particularly good properties and forms a very thin oxide layer . Pack aluminizing of a nickel base alloy IN625 showed an increased corrosion resistance, whereas pack aluminizing and slurry coatings on steel substrates were found to be detrimental due to the inward diffusion of aluminum towards the substrate rather than outwards diffusion .…”

Performance of HIPIMS deposited CrN/NbN nanostructured coatings exposed to 650 °C in pure steam environment

“…It has also been reported that a critical amount of Cr and/or Al is required to form and maintain a protective oxide scale which will be effective at temperatures above 600 °C [4,5]. Employing a surface layer with higher Cr or Al content [6] or nourish this layer with a constant supply of Cr from the layers beneath can be tricky in steels such as P92, since low Cr improves their creep resistance and thus their employability in the construction of steam plants [3,7].…”

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].…”