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

reactive elements

nitridation

High temperature materials

nitrides

alumina

hydrides

microstructure

chromia

oxidation

Lecture hall Kollektorn, MC2, Kemivägen 9, Chalmers
Opponent: Professor Gordon Tatlock, Liverpool University, UK

Författare

Nooshin Mortazavi Seyedeh

Chalmers, Fysik, Materialens mikrostruktur

Effect of thermal cycling on protective properties of alumina scale grown on thin Haynes 214 foil

Corrosion Science,; Vol. 98(2015)p. 688-698

Artikel i vetenskaplig tidskrift

In Situ ESEM Investigation of KCl-Induced Corrosion of a FeCrAl and a Model FeNiCrAl Alloy in Lab Air at 450 degrees C

Journal of the Electrochemical Society,; Vol. 162(2015)p. C744-C753

Artikel i vetenskaplig tidskrift

N. Mortazavi, C. Geers, M. Esmaily, V. Babic, M. Sattari, K. Lindgren, P. Malmberg, B. Jönsson, M. Halvarsson, J.-E. Svensson, I. Panas and L.-G. Johansson. Hydrogen in alumina scales – Unravelling the roles of water and reactive elements in high temperature oxidation. Under review in Nature Materials.

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.

Drivkrafter

Hållbar utveckling

Ämneskategorier

Materialteknik

Kemiteknik

Fundament

Grundläggande vetenskaper

Infrastruktur

Chalmers materialanalyslaboratorium

Styrkeområden

Materialvetenskap

ISBN

978-91-7597-664-8

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4345

Utgivare

Chalmers tekniska högskola

Lecture hall Kollektorn, MC2, Kemivägen 9, Chalmers

Opponent: Professor Gordon Tatlock, Liverpool University, UK