High velocity oxy-fuel (HVOF) coatings for steam oxidation protection

Abstract

Introduction European COST Actions 522 (completed in 2003) and 536 (ongoing) have concen-trated on designing and producing steels for steam power plants capable of operating at 600–625 °C in order to increase effi ciency and reduce emissions [1–4]. However, efforts to produce ferritic steels for turbine components capable of operating at temperatures of 625 °C or higher have not yet been successful [5]. Materials such as P92 and COST-developed CB2 (9 wt.% Cr) with high creep strength up to 625 °C, have unacceptable oxidation resistance (Fig. 6.1), whereas materials with higher Cr content such as COST-developed FT4 (11 wt.% Cr) have better oxidation resistance but lower creep strength. When exposed to high-pressure steam at these tempera-tures, ferritic steels develop thick oxides which spall after relatively few hours of exposure [6]. Cross-section reduction, blockage and component damage due to erosion caused by the spalled oxides are some of the possible consequences. A similar situation occurred with power generation and aeronautical gas turbines 45 years ago, when efforts to develop superalloys with the required mechanical properties as well as very low oxidation rates resulted in failure at higher operating temperatures. The solution was to employ coatings on superalloys with the required mechanical properties and presently, all new generation gas turbines require high-temperature oxidation resistance coatings as well as thermal barriers [7]. In 1998, efforts to examine the feasibility of applying coatings to steam turbine components were carried out within the context of COST 522 [8]. The results were very promising and the work continued within the framework of the European Commission project 'Coatings for Supercritical Steam Cycles' (SUPERCOAT) in which eight partners from different organisations across Europe participated [9,10]. A number of coatings, applied by means of slurry deposition, pack cementation and High Velocity Oxy-Fuel (HVOF) thermal spray were subjected to a variety of tests, including steam oxidation, creep strength, thermo-mechanical fatigue, etc. The project has recently been completed and some of the explored coatings were down-selected as candidates for industrial scale application on real components and for validation. * Reprinted from A. Agüero et al.: HVOF coatings for steam oxidation protection. High velocity oxy-fuel (HVOF) coatings for steam oxidation protection 53 Among the explored coating techniques, chosen on the basis of being potentially appropriate for coating large steam turbine components, HVOF thermal spray has emerged as one of the most successful. Abe and co-workers have also studied a number of HVOF deposited materials for this application [11]. This paper describes the steam oxidation behaviour of several alloyed materials deposited by this technique including the characterisation of the protective oxides formed on each material.