Raúl Muelas

Steam Oxidation of Slurry Aluminide Coatings on Ferritic Steels for Advanced Coal-Fired Steam Power Plants

New steels are being developed to achieve high creep strength as well as high resistance to oxidation in steam, to be used for new generation steam turbine components, which are expected to operate at 600-650oC in order to reach higher power generation efficiency. In particular, components such as steam pipes as well as turbine rotors, casings and blades must be resistant to the growth as well as to the exfoliation of oxides. New materials with very high creep strength have been developed by lowering the Cr content, but preliminary field test has shown an unacceptable high rate of oxidation and spalling. In recent studies carried out by our group within the framework of the COST 522 action, a number of commercially available coatings were explored for steam oxidation protection. These included materials known to have good high temperature oxidation resistance and deposited by techniques that can be employed to coat large steam turbine components either at the plant or at their location of manufacture, and also taking into consideration economical aspects. Promising results were obtained both at the laboratory scale as well as at field testing. For instance slurry aluminide coatings applied on P92 (9wt% Cr) are protective for at least 25,000 h at 650oC (tests are still ongoing). The results presented in this paper explore the behaviour of aluminide coatings applied on different steels (P22, P23 and P92). P22 and 23 are 2 wt. % Cr steels with excellent high temperature mechanical properties and are less expensive than higher Cr materials, but nevertheless exhibit much higher steam oxidation rates. The effect of Cr as well as W on the steam oxidation rate of both coated and uncoated specimens is explored. Laboratory steam oxidation testing as well as characterization of the coatings both before and after exposure will be presented. The results have provided information regarding the mechanism of protection and degradation of these coatings as well as insight for new coating development. Introduction Steam oxidation resistant coatings have so far never been used on European steam turbine components. However, since the operating temperature of these turbines is expected to rise from 550 to 650oC in order to achieve higher efficiencies, key components will require not only high creep strength but also a high resistance to oxidation in a steam environment. As part of the European COST action 522, which was completed in October 2003, new alloy development activities have been very successful in improving creep strength, generally achieved through lowering the chromium content. However, a negative consequence has been the worsening of the resistance to steam oxidation [1]. On ferritic steels with less than 10 w% Cr, very thick oxide scales form at 650oC under steam, consisting of a top layer of Fe2O3 and Fe3O4 and an inner zone mainly (Fe,Cr)3O4 spinels (figure 1) [2]. These scales spall causing metal cross-section loss, component blockage and erosion of components located down-stream resulting as well, in a thermal insulating effect leading to component overheating. Figure 1: Cross section of P92 (a 9wt% Cr ferritic steel) exposed to 5000 h of steam oxidation at 650oC In recent studies carried out by our group also within the framework of the COST 522 action, a number of commercially available coatings have been explored for steam oxidation protection. These included materials known to have good high temperature oxidation resistance and deposited by techniques that can be employed to coat large steam turbine components either at the plant or at their location of manufacture, and also taking into consideration economical aspects. Promising results were obtained both at the laboratory scale as well as at field testing [3, 4]. For instance slurry aluminide coatings applied on P92 (a 9wt% Cr ferritic steel) are protective for at least 25,000 h at 650oC (tests still ongoing). These coatings are applied by depositing an Al slurry followed by a diffusion heat treatment and present several Al-Fe intermetallics phases as seen in figure 2a. Such formed aluminides exhibit stress relieving cracks, present already after the initial heat treatment, probably due to brittleness of the Fe2Al5 phase. On exposure to steam the coating develops a thin protective α-Al2O3 layer and the cracks do no propagate into the base material, nor become sites of preferential attack by steam. However, although failure has not yet been detected after 25,000 h, slow degradation is observed by a reduction of the Al surface concentration caused by Al inwards diffusion and resulting in the formation of Al rich precipitates within the substrate (see figure 2b).

Microstructural Evolution of Slurry Fe Aluminide Coatings during High Temperature Steam Oxidation

Slurry iron aluminide coatings are very resistant to steam oxidation at 600-650º C. These coatings can be used to protect new generation Ultra Super Critical (USC) steam power plant ferritic/martensitic steel components. The microstructure of the initially deposited coating changes as a function of time, mainly due to coating-substrate interdiffusion, going from mostly Fe2Al5 to FeAl, causing the precipitation of AlN in those substrates containing a minimum content of N and moreover, developing Kirkendall porosity at the coating-substrate interface. Steam oxidation at 650º C causes the formation of a protective thin layer of hexagonal χ-Al2O3 phase along with some α- and γ-Al2O3 after the first few hours of exposure. However, despite the relatively low temperature, and after several thousands hours the protective layer was mostly composed of α-Al2O3. A study of the evolution of the microstructure of slurry aluminide coatings deposited on P92 and exposed to steam at 650º C has been carried out by scanning and transmission electron microscopy and X ray diffraction.

Steam Oxidation Testing of Coatings for Next Generation Steam Power Plant Components

To achieve higher power generation efficiency in steam turbines, operating temperatures are expected to rise from 550°C to 650°C. The use of oxidation resistant coatings on currently available materials, with high creep strength but inferior steam oxidation resistance, is being explored in order to accomplish this goal in the context of the European project “Coatings for Supercritical Steam Cycles” (SUPERCOAT). Coating techniques have been chosen on the basis of being potentially appropriate for coating steam turbine components: the application of metallic and ceramic slurries, pack cementation and the deposition of alloyed and cermet materials by thermal spray. The coatings were characterised by metallography, SEM-EDS and XRD and steam oxidation and thermal cycling laboratory testing was carried out at 650º C. In this presentation, the testing results of selected coatings will be shown including those which exhibit the most promising behaviour. For instance, slurry aluminides have been exposed to steam at 650°C for more than 38,000 h (test ongoing) without evidence of substrate attack. Some HVOF coatings such as FeAl, NiCr and FeCr also have shown excellent behaviour. The results have provided information regarding the mechanism of protection and degradation of these coatings as well as insight into new coating development.