Alina Agüero

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.

Deposition process of slurry iron aluminide coatings

Abstract Diffusion iron aluminide coatings prevent steam oxidation of ferritic/austenitic steels at 650°C for at least 45,000 h. These coatings are deposited by applying Al slurries followed by a diffusion heat treatment at 650°C. The quality of the coatings is very sensitive to a number of factors such as surface preparation, slurry composition and diffusion treatment temperature. A study of the effect of the different processing parameters has been performed in order to optimize the process from an industrial perspective. Moreover, most commercially available Al slurries contain different levels of Cr6+, a highly carcinogenic species, and therefore Cr6+ free slurry formulations have been prepared. In addition, re-coating after exposure has also been developed since it is not clear yet if these coatings will last the 100,000 h which is the life limit for steam power plant design. Based on these studies, processes suitable for coating real size components and re-coating steam exposed components have been developed and are presented in this contribution.

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.

Long Term Diffusion Studies in Fe Aluminide Coatings Deposited by Slurry Application on Ferritic Steel

Diffusion iron aluminide coatings have shown excellent resistance to high temperature oxidation in air, corrosive atmospheres and steam. A study of the diffusion behaviour of slurry applied diffusion aluminide coatings deposited on ferritic steel have been carried out under a 100% flowing steam atmosphere for up to 50,000 h at 650 °C. The results have shown that initially, the coating forms by outward growth possibly including the dissolution of the steel in molten aluminium. At later stages, during exposure to steam at 650 °C, aluminium diffuses inward and moreover, Fe also diffuses outward resulting in the progressive development of Kirkendall porosity. Results have also indicated that in order to form a pure protective Al2O3 scale the Al wt.% has to be > 4. Below this content Al-Fe mixed oxides develop exhibiting a less protective behaviour.

Overview of steam oxidation behaviour of Al protective oxide precursor coatings on P92

ABSTRACT Future designs for steam power plants are expected to operate at 625–750°C, at which the candidate ferritic/martensitic steels exhibit insufficient steam oxidation resistance. Al-based coatings constitute an alternative to prevent or reduce oxidation. For over 50 years this type of coating has been applied on blades and vanes made of Ni- and Co-based alloys used in the hot section gas of turbines which operate at temperatures higher than 900°C. For these coatings, the mechanism of protection from high-temperature oxidation, is based on the formation and maintenance of a thin layer of dense α-Al2O3. Many articles have been written about the nature, formation and failure mechanism of oxide precursor coatings, under air, at over 900°C. [1–6] However, very little is known regarding alumina scales formed under pure steam at lower temperatures, which is the expected scenario for new steam power plants. This paper covers a recapitulation of the behaviour of Al-based protective oxides formed on coatings with various compositions under steam at 650°C, including new data relative to the formation of said oxides under steam and the microstructure of samples exposed to steam for 70 000 h. It has been shown that on Al containing coatings, such as diffusion Fe aluminides and FeCrAls, alumina forms under steam at 650°C. Provided that a critical content of Al is maintained underneath the scale, Al2O3 is very stable, surpassing 70 000 h under steam at 650°C, without evidence of spallation (testing is still ongoing). The industry target for coatings in this cases is 100 000 h. In turn, the critical Al content depends on the coatings Cr content, and if the oxidation takes place at temperatures of 900°C or higher, under air. However, under steam, alumina phases formation and transformations are different: at 650°C χ-Al2O3 forms initially, and appears to slowly transform unto α-Al2O3. General considerations regarding the stability of protective oxides formed under steam as a function of the composition of the subjacent material will be provided.

Aluminide slurry coatings for protection of ferritic steel in molten nitrate corrosion for concentrated solar power technology

Molten nitrates can be employed as heat storage fluids in solar concentration power plants. However molten nitrates are corrosive and if operating temperatures are raised to increase efficiencies, the corrosion rates will also increase. High temperature corrosion resistant coatings based on Al have demonstrated excellent results in other sectors such as gas turbines. Aluminide slurry coated and uncoated P92 steel specimens were exposed to the so called Solar Salt (industrial grade), a binary eutectic mixture of 60 % NaNO3 – 40 % KNO3, in air for 2000 hours at 550°C and 580°C in order to analyze their behavior as candidates to be used in future solar concentration power plants employing molten nitrates as heat transfer fluids. Coated ferritic steels constitute a lower cost technology than Ni based alloy. Two different coating morphologies resulting from two heat treatment performed at 700 and 1050°C after slurry application were tested. The coated systems exhibited excellent corrosion resistance at both te...

Microstructure of an oxide scale formed on ATI 718Plus superalloy during oxidation at 850 °C characterised using analytical electron microscopy

Abstract The microstructure of a scale formed on the ATI 718Plus superalloy during oxidation at 850 °C for up to 1 000 h in air was characterised using various electron microscopy techniques taking advantage of recent development and capabilities of energy dispersive X-ray spectrometry. The study shows that a protective multilayered Cr2O3 scale was formed on this alloy; the outer Cr2O3 layer was coarse-grained, while the inner one was fine-grained. Underneath, a thin layer composed of TiNbO4 as well as an internal oxidation zone that was a dozen or so micrometres thick and consisted of individual Al2O3 precipitates had formed. The growth of the Cr2O3 layer caused the appearance of a chromium depletion zone, in which the γ′ particles were not observed.