HIGH TEMPERATURE OXIDATION PHENOMENA'S STUDY OF HIGH ALLOYED MATERIALS FOR POWER GENERATION PLANTS

The aim of the doctorate project is the study of the corrosion and oxidation behavior of some commercial and more recent SS and Ni based alloys for high temperature applications in order to verify their possible use either at operative conditions higher than conventional (temperature up to 900°C) or in different environment (oxygen, air, carburizing atmospheres). During the first year, the following aspects were investigated: - Microstructural characterization of solution annealed AISI 310N, 347HFG and 304 HCu (the latter supplied also in the shoot peened condition) was carried out. An homogeneous austenitic fine grain structure accompanied with Nb segregation was observed for 347HFG and 304HCu. In the case of 310N the grains were coarser and the presence of Si segregation were randomly detected beside Nb. - Microhardness profiles (HV50) were conducted to evaluate the penetration effects of shoot peening treatment at depths as high as 150-200 m. - Discontinuous oxidation testing were performed at temperatures in the range of 700-900°C under 15 Nm3/h flowing moist air atmosphere for 7 days. The preliminary results seem to confirm an oxidation susceptibility in the following order: 310N<347HFG<304HCu. During the second year, the following activities have been carried out: - Oxidation kinetic study (thermogravimetric tests) of as received AISI 310N, 347HFG, 304HCu and Alloy 617 in oxygen atmosphere at 900°C; 310N, 347HFG and Alloy 617 show a protective oxidation kinetic, while 304HCu suffers from spalling. - Investigation of grown oxide layer nature of all SS by microscopic techniques revealed an average oxide thickness in the range of 5 – 15 m for 347HFG; about 5 m until spallation for 304HCu and less than 5 m for 310N. In the case of 347HFG a three layer oxide was clearly detected consisting of a first layer of Cr oxides followed by a mixed Fe and Cr oxide and finally an external layer of FeOx. For 310N only a Cr oxide layer was detectable; for 304HCu the layer being essentially Fe-Cr oxides. Result confirmed also by GDOES. - Study of the electrochemical behaviour of as received and oxidized SS specimens by means of polarization curves. All “as received” specimens exhibit a similar passive behaviour in terms of both potentials and current densities, apart the point corresponding at the critical passive current (icr). For oxidized specimens a statistical approach should be followed. During the third year, in addition to the accomplishment of the previous topics, new electrochemical approaches were tested during a period spent by the candidate following Dr. Digby Macdonald's research labs at The Pennsylvania State University. Activities were focused on the passivity study of 347HFG stainless steel. Starting from the results of the potentiodynamic studies, an electrochemical impedance spectroscopy procedure was setted up; results were optimized by applying the “point defect model” developed by the same Dr. Macdonald revealing kinetic parameters of the passive layer and is semiconductive nature that is essentially n-type for all anodic potential values except near the breakdown in which assume a p-type character.