High temperature alloys play a crucial role in many important applications, e.g., land based gas turbines, jet engines, petro-chemistry, thermal solar power, solid oxide fuel cells, CO2 sequestering and materials processing. The ability of high temperature alloys to resist corrosion due to reactions with a corrosive environment (oxygen, water vapour, chlorides, sulphides, etc.) relies on the spontaneous formation of a protective surface oxide layer, the so-called "oxide scale". Thus, if unimpeded, these reactions quickly destroy the materials’ properties. The foundations of the oxidation science were laid many years ago by Carl Wagner who described oxide scale growth as an electrochemical process with a cathode at the scale/gas surface and an anode at the alloy/scale interface. Despite decades of research in this "mature" field, there remains several "mysteries" in the field, that need to be addressed in order to produce better materials in the future.
The aim of this thesis is to generate new knowledge about high temperature corrosion through employing careful exposures in combination with modelling and state-of-the-art materials analysis. Method-wise, this thesis concerns the capability of environmental scanning electron microscopy (ESEM) in studying the very early stages (up to 1 h) of oxidation in an in-situ manner (live). Also, the work presents a successful implementation of the transmission Kikuchi diffraction (TKD) in the corrosion research through designing a novel thin foil holder and fully optimizing the acquisition parameters. Corrosion-wise, the thesis deals with a number of issues including the effect of thermal cycling on the oxidation behaviour of a thin Ni-based alloy, oxidation mechanism of an advanced commercial FeCrAl alloy (Kanthal APMT) in low-oxygen activity environments, and the role of reactive element particles on the oxidation behaviour of high temperature alloys. Importantly, this creates new generic insights into the mechanism of formation of protective oxide scales, at odds with the established scenario for oxide scale growth (i.e., the Wagner’s theory). The main discovery of this thesis, the interplay of water and reactive elements, is a new concept for the field of high temperature oxidation and nitridation of complex alloys. It is hoped that the results presented in this thesis could fine-tune the oxidation properties of iron- and nickel-based (super)alloys in years to come.