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).
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Jan 1, 2010
Takuya Moriya + 2
Open Access
