In this work we evaluate the influence of silicon on the high-temperature oxidation of austenitic stainless steels and propose a mechanism that explains the Reactive-Element Effect (REE) in terms of a synergistic action between the reactive element and the silica layer that forms in the innermost areas of the scale. To do this we have studied the oxidation at 900°C of austenitic commercial alloys (AISI-304, AISI-316 and AISI-310S) and a laboratorydesigned high-silicon stainless steel (AISI-304). Lanthanum was selected as the reactive element which was surface deposited by means of ion interchange. Results obtained in this work allowed us to state that the reactive element would enhance the formation of a silica layer that shows diffusion through the scale. The reactive element also changes the expansion coefficient at the scalealloy interface, increasing the adherence of the oxide layer to the metal.

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Nuclear power plants have been an important source of energy in many countries for the last fifty years. The Canadian supercritical water cooled reactors (SCWR), among the Generation IV reactors, are currently being designed in Canada and many other countries. It presents challenges for many scientists and engineers due to the harsh environment in which the reactor must operate and the lack of knowledge of material's performance in this corrosive conditions of high temperature and pressure. This thesis entails the corrosion testing of four candidate materials, two nickel based alloys (IN 625 and A-286) and two stainless steels (AISI 304 and AISI 310), and microstructure evaluation of samples after being exposed to supercritical water (SCW), subcritical water (SubCW), and superheated steam. Samples were ground to 600 grit finish and placed in an autoclave and a superheated steam rig to a maximum of three thousand hours and one thousand hours respectively. Procedures, based on the manufacturer's manuals, were created for the setup and operation of all the equipment. The temperature of all three tests was set to 625 o C with a pressure of 29 MPa (SCW), 8 MPa (subcritical water) or ambient (superheated steam). The surface morphology and composition were analyzed using scanning electron microscope (SEM) imaging and energydispersive X-ray spectroscopy (EDS). Results from this study show a poor corrosion performance, in terms of surface oxide formation and weight change, for A-286 and AISI 304 in all conditions due to their low chromium content. AISI 310 and IN 625, with higher chromium content, exhibit an excellent corrosion resistance in almost all conditions, except for the superheated steam test where oxide spallation may have

In this study, an Al-containing alloy 214 was evaluated in superheated steam at 800 °C for a duration of 600 h. The purpose of using superheated steam was to simulate the supercritical water (SCW) condition at higher temperatures where no commercial SCW rig is currently capable of reaching (800 °C and beyond). After exposure to superheated steam, the weight change and surface oxidation were analyzed. Alloy 214 experienced greater weight gain than IN 625 and Ni20Cr5Al, due to its low Cr content. Formation of both Cr2O3 and Al2O3 was observed on the surface after 300 and 600 h of exposure. However, as exposure progressed, more Ni and Mn-containing spinel started to form, signaling Cr and Al depletion on the metal substrate surface.

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