Proyecto de Investigación: DEGRADACION Y PROTECCION DE MATERIALES EN ATMOSFERAS DE OXI-COMBUSTION
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ENE2011-29203-C02-01
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Oxidation under pure steam: Cr based protective oxides and coatings
(Elsevier, 2013-09-20) Agüero, Alina; González, Vanessa; Gutiérrez del Olmo, Marcos; Muelas Gamo, Raúl
At temperatures of 900 °C and higher, the formation, transformation and failure of protective oxides in air have been deeply studied. However, there is significantly less available information of these processes when they take place under pure steam and in the lower temperature range pertinent to steam power plants. New designs for these plants are expected to operate at 625–700 °C, at which the candidate ferritic/martensitic steels exhibit very low steam oxidation resistance. In this paper, available knowledge of the behavior of Cr based protective oxides formed under steam at 650 °C will be presented. It is already known that on ferritic/martensitic steels with a Cr content lower than ~ 9 wt.% such as P92, a nonprotective, thick, dual layer composed of Fe3O4 and (Fe, Cr)3O4 forms. However, significantly higher steam oxidation resistance has been recently found when exposing NPM, a 9 wt.% Cr martensitic steel rich in W and Co, to pure steam at 650 °C. In this case a protective, very thin multilayer forms, with alternating Fe3O4 and (Fe, Cr, Mn)3O4 layers. Different oxides formed after 10,000 h of exposure to steam at 650 °C, on Cr containing coatings. In the case of Fe based, Cr rich coatings, both diffusion and overlay, a protective spinel was observed. However, Cr containing coatings based on Ni develop a very stable, protective thin Cr2O3 layer. Results show that along with the Cr content, other factors such as the grain size below the scale appear to determine the formation of thin protective scales. The steam pressure was also found to significantly and negatively affect the stability of protective Cr based oxides. Chromia former steels and coatings may not be the best solution for 650 °C new generation steam power plants.
Corrosion Resistance of Novel Coatings on Ferritic Steels for Oxycombustion–Supercritical Steam Boilers: Preliminary Results
(Springer Nature Link, 2015-07-23) Agüero, Alina; Baraibar, Ignacio; González, Vanessa; Muelas Gamo, Raúl; Plana, Daniel; European Commission; Ministerio de Economía y Competitividad (MINECO)
Increasing the efficiency of coal fired steam power plants is an important contribution towards clean coal power. In fact, new ferritic steels are expected to withstand 325 bar and 650 °C. Moreover, in order to facilitate CO2 capture oxygen can be used instead of air for combustion (oxycombustion) so that no NOX emissions are produced. Boiler components, such as superheater tubes, are exposed to both steam and fireside corrosion and at higher temperatures, ferritic steels corrode at very fast rates under both atmospheres. A solution can be found in the use of protective coatings, a number of which, applied by techniques capable of depositing said coatings both on the inner and outer surfaces of tubes, are being studied within nationally and European funded projects. In particular, two new Ni and Cr modified aluminide coatings deposited on P92 by non-line-of-sight hybrid processes have been produced and the preliminary results of on-going laboratory testing, both under oxycombustion model atmospheres as well as under pure steam at 650 °C are promising, in particular those exhibited by the Cr enriched aluminide coating. Moreover, results obtained in a pilot oxycombustion boiler operated by CIUDEN in Leon, Spain are also shown.
Overview of steam oxidation behaviour of Al protective oxide precursor coatings on P92
(Taylor and Francis online, 2016-03-04) Agüero, Alina; Gutiérrez del Olmo, Marcos; Muelas Gamo, Raúl; Spiradek Hahn, K.
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 coating's 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.
