Proyecto de Investigación: DESARROLLO DE METODOLOGIAS DE ENSAYO PARA EVALUAR LA EROSION-CORROSION Y MONITORIZACION DE CORROSION EN PLANTAS DE GENERACION DE ENERGIA ELECTRICA POR VAPOR
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ENE2008-06755-C02-01
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Effects of a Steam Pre-treatment on the Formation and Transformation of Alumina Phases on Fe Aluminide Coatings
(Springer Nature Link, 2012-12-25) Agüero, Alina; Hernández, M.; Santaballa, A.; Ministerio de Economía y Competitividad (MINECO)
Several researchers have studied the transformation of metastable aluminas (γ- and θ-) to α-Al2O3 but very little is known regarding alumina scales formed under water vapour and their transformation to α-Al2O3. Some results have indicated that water vapour increases the oxidation rate of alumina-scale forming coatings but others have found the opposite, that is, that under water vapour the oxidation rates decrease as either transition aluminas do not form or the transformation to α-Al2O3 is accelerated. In addition, it was found that χ-Al2O3 is the only oxide that forms at the initial stages of oxidation under 100 % steam on Fe–Al coatings at 650 °C. Under these conditions, this oxide is very protective, and slowly transforms onto α-Al2O3. A preliminary study of the transformation of χ- to α-Al2O3 at 900 °C under laboratory air was carried out. χ-Al2O3 was generated by a steam pre-treatment on slurry Fe aluminide coatings deposited on P92.
Anomalous steam oxidation behavior of a creep resistant martensitic 9 wt. % Cr steel
(Elsevier, 2013-06-09) Agüero, Alina; González, Vanessa; Mayr, Peter; Spiradek Hahn, Krystina; Ministerio de Economía y Competitividad (MINECO)
The efficiency of thermal power plants is currently limited by the long-term creep strength and the steam oxidation resistance of the commercially available ferritic/martensitic steel grades. Higher operating pressures and temperatures are essential to increase efficiency but impose important requirements on the materials, from both the mechanical and chemical stability perspective. It has been shown that in general, a Cr wt. % higher than 9 is required for acceptable oxidation rates at 650 °C, but on the other hand such high Cr content is detrimental to the creep strength. Surprisingly, preliminary studies of an experimental 9 wt. % Cr martensitic steel, exhibited very low oxidation rates under flowing steam at 650 °C for exposure times exceeding 20,000 h. A metallographic investigation at different time intervals has been carried out. Moreover, scanning transmission electron microscopy (STEM) analysis of a ground sample exposed to steam for 10,000 h at 650 °C revealed the formation of a complex tri-layered protective oxide comprising a top and bottom Fe and Cr rich spinel layer with a magnetite intermediate layer on top of a very fine grained zone.
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.
