M. Gutiérrez

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.