INVESTIGATING THE CORROSION BEHAVIOUR OF ZX10 MAGNESIUM ALLOY IN VARIOUS CORROSIVE MEDIA AND THE CORROSION RESISTANCE OF MAGNESIUM OXIDE COATINGS
.ii ACKNOWLEDGMENT..iv TABLE OF CONTENTS..v LIST OF TABLES..vii LIST OF FIGURES..viii LIST ABBREVIATIONS AND SYMBOLS..x CHAPTER ONE: INTRODUCTION..1 1.1 Overview..1 1.2 Motivation..4 1.3 Research objectives..5 1.4 Thesis arrangement..6 CHAPTER TWO: LITERATURE REVIEW..7 2.1 Magnesium and its alloys..7 2.1.1 Zinc containing and Calcium containing magnesium alloy..7 2.1.2 The challenge of magnesium alloy..8 2.2 The corrosion of magnesium and its alloy..10 2.2.1 The corrosion types of magnesium and its alloy..10 2.2.2 The reactions of magnesium in aqueous solution..11 2.3 The surface film on magnesium and its alloy..13 2.4 Corrosion protection of magnesium alloy .16 2.4.1 Electrophoretic coating..17 vi 2.4.2 Previous research on MgO coating on magnesium..21 CHAPTER THREE: MATERIALS AND METHODOLOGY..23 3.1 Reagents and materials..23 3.2 Materials and material preparation..23 3.3 Corrosion tests..24 3.4 MgO coating preparation procedure on magnesium substrate..25 3.5 Characterization of MgO coating. 26 3.6 Evaluation of the corrosion resistance of MgO coating on magnesium substrates. 27 CHAPTER FOUR: RESULTS AND DISCUSSION..28 4.1 Degradation behaviour of bare magnesium alloys .28 4.1.1 Electrochemical analysis..28 4.1.2 Weight loss measurement. 31 4.1.3 Surface analytical evaluation. 33 4.1.4 Corrosion at the grain boundaries..36 4.1.5 Analysis of adhering corrosion products. 38 4.2 Investigation of effect of calcination temperature on corrosion resistance of MgO coating.40 4.2.1 MgO coatings on magnesium alloy: coating formation mechanism. 40 4.2.2 Characterization of MgO coatings: XRD and XPS analyse..42 4.2.3 Measurement of protective performance of MgO coating by Tafel polarization..44 4.2.4 Measurement of corrosion resistance by impedance spectroscopy..47 4.2.5 Correlating thermal calcination mechanism and corrosion resistance of coatings..51 4.3 Proposing corrosion mechanism..53