Studio dell’ossidazione a caldo di materiali altolegati per caldaie industriali

When speaking about power plants, both emission and efficiency are to be taken into account. For thermoelectrical plants, these concepts are strictly correlated since, the highest efficiency, the lowest CO2 emissions. Increase in efficiency calls for improvement of combustion parameters. For instance, higher flame temperature leads to increase of operating steam pressure and temperature. This increase is limited by the structural materials used for the most critical sections of the plant, such as boiler tubes and turbine. Nowadays, the most used material for boiler tubes is Type P91 carbon steel, though the use of high alloyed SS has increased in the last 15 years. As a result, increment in vapor parameters from 540°C, 270 atm (P91) up to 600°C, 300 atm (austenitic stainless steels) has been experienced. For temperatures close to 700°C, Ni-alloys have been used as materials for steam tubes. The main reason for temperature and pressure limitations is the occurrence of hot corrosion phenomena on tube surfaces both at steam and flame side. Chromium is the key element for the SS oxidation behavior, the corrosion resistance being correlated to its ability to form a chromia- based protective layer. In oxidant environment, the highest the Cr content and surface availability, the densest the oxide layer, which makes the material more resistant to aggressive environment and higher temperatures (sulphur, chlorine, molten salts, carburizing atmosphere, etc.). In this respect, grain size, surface state, intermetallic phases and alloy composition are very important. Apart carbon steels, as SSs are used, negligible corrosion is observed at the flame side if combustion is performed in air excess using a clean fuel (low content of contaminants). It follows that a correct material selection is often sufficient. At the steam side, problems are more serious as the working conditions along the tube are different, thus leading to the presence of liquid water at the pipe inlet and overheated steam at the outlet. The oxide scale usually grows more rapidly to higher thickness as temperature increases. This is not always good, since as the oxide scale grows a corresponding thickness reduction is observed which leads to higher creep susceptibility. The latter is however usually considered during design. Actual problems associated with steam side oxidation are related to oxide scale thermal insulation and spalling in exercise, which can damage the turbine. The aim of this work has been the study of corrosion and oxidation behavior of some conventional and more recent SSs for high temperature applications, to verify their possible use under higher operative conditions than usual (temperature up to 900°C). The experimental work was addressed to the following commercial alloys: AISI 304 HCu, 347 HFG (FG stands for "fine grain") and 310N stainless steels. All materials were supplied in the form of solution annealed seamless tubes. 304 HCu was also furnished as shoot peened. The first activity was characterization of the microstructure of the alloys by optical microscopy (OM) and scanning electron microscopy (SEM). Thermal stability checked by thermogravimetry in the range 700-900°C. Morphology and composition of the oxide scales formed at high temperature were investigated by SEM-EDS, while the protective behavior of oxides was tested by electrochemical techniques.onment, the highest the Cr content and surface availability, the densest the oxide layer, which makes the material more resistant to aggressive environment and higher temperatures (sulphur, chlorine, molten salts, carburizing atmosphere, etc.). In this respect, grain size, surface state, intermetallic phases and alloy composition are very important. Apart carbon steels, as SSs are used, negligible corrosion is observed at the flame side if combustion is performed in air excess using a clean fuel (low content of contaminants). It follows that a correct material selection is often sufficient. At the steam side, problems are more serious as the working conditions along the tube are different, thus leading to the presence of liquid water at the pipe inlet and overheated steam at the outlet. The oxide scale usually grows more rapidly to higher thickness as temperature increases. This is not always good, since as the oxide scale grows a corresponding thickness reduction is observed which leads to higher creep susceptibility. The latter is however usually considered during design. Actual problems associated with steam side oxidation are related to oxide scale thermal insulation and spiting in exercise, which can damage the turbine. The aim of this work has been the study of corrosion and oxidation behavior of some conventional and more recent SSs for high temperature applications, to verify their possible use under higher operative conditions than usual (temperature up to 900 degrees C). The experimental work was addressed to the following commercial alloys: AISI 304 HCu, 347 HFG (FG stands for "fine grain") and 310N stainless steels. All materials were supplied in the form of solution annealed seamless tubes. 304 HCu was also furnished as shoot peened. The first activity was characterization of the microstructure of the alloys by optical microscopy (OM) and scanning electron microscopy (SEM). Thermal stability checked by thermogravimetry in the range 700-900 degrees C. Morphology and composition of the oxide scales formed at high temperature were investigated by SEM-EDS, while the protective behavior of oxides was tested by electrochemical techniques.