Novel Insights into the Oxidation of High Temperature Alloys - The Role of Environment, Microstructure and Reactive elements

Doctoral thesis, 2017

The properties of materials can be deteriorated, when they react with an environment at high temperature and form various types of corrosion products, such as oxides and nitrides. In order to perform successfully at elevated temperatures, all these materials are required to be protected by a slow-growing, continuous and adherent surface oxide layer. Despite years of scientific study, there are still many technological and research questions in the field. In this thesis, the high-temperature corrosion behavior of a number of iron (Fe)- and nickel (Ni)-base alloys are studied and some of the long-standing scientific "mysteries" are addressed. In addition, the thesis presents the fruitful development of the newly introduced TKD method, which was extensively employed to study the microstructure and microtexture of fine-grained oxide scales.   The initial stages of KCl-induced oxidation behavior of two alumina formers (alloys Kanthal APMT and TH1) and one chromia former (alloy Sanicro 25) were analyzed in an O2/H2O environment using an in-situ ESEM method, which were complemented by ex-situ exposures. The in-situ oxidation experiments provided an opportunity to view dynamic processes occurring during the oxidation process ′′live′′. Notably, chlorination of the alloys was evidenced by detection of chlorine below the oxide scales already after 1 hour of exposure. In addition, the effects of thermal cycling on the oxidation behavior of an alumina forming Ni-base alloy (HR-214) was studied in air at 1200°C. Vertical cracks due to the thermal cycling caused consumption of Al for re-healing the cracks and sooner occurrence of transition from alumina to chromia scale. Moreover, nitridation resistance of an alumina-forming alloy (Kanthal APMT) was also studied in a mixture of 95% N2 + 5% H2 at 900°C, where probable paths for nitrogen dissociation and diffusion into the alloy were suggested.   The cornerstone of the thesis is indeed the unravelling of the connection between two long-standing enigmas in the field, i.e., the roles of water and the REs. The interplay between water and REs caused the formation of a previously unrecognized (″messy″) transient nanocrystalline alumina layer with yttrium-decorated grain boundaries. A new scenario for high temperature oxidation is presented, in which water diffuses along yttrium-decorated alumina grain boundaries and is cathodically reduced within the scale. This understanding is supported by identification of hydride in the oxide scale using low-loss EELS. The concept of ″critical″ size of the RE particles on the oxidation performance is explored.