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Atmospheric Corrosion of AA2024 in Ocean Water Environments

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Atmospheric Corrosion of AA2024 in Ocean Water Environments ATMOSPHERIC CORROSION OF AA2024 IN OCEAN WATER ENVIRONMENTS BY SARAH JANE MARIE GLANVILL A thesis submitted to the University of Birmingham for the degree of DOCTOR OF PHILOSOPHY School of Metallurgy and Materials College of Engineering and Physical Sciences University of Birmingham February 2018 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. ABSTRACT Observations of atmospheric corrosion on aluminium alloy AA2024-T3 were made within droplets of NaCl solution and simulated ASTM ocean water. High speed in-situ synchrotron X-ray tomography has been used to observe the initiation and propagation of corrosion sites. Tomographic scans had a temporal resolution of 300 s per full tomographic scan, providing a non-destructive visualisation of corrosion. Sites initiate at numerous surface morphologies, including at intermetallic inclusions, under surface deposits and salt crystals, and also at sites with no obvious micron-scale microstructural features. It was observed that corrosion sites grow discontinuously and inhomogeneously when conditions are constant, consistent with bursts of dissolution followed by some repassivation. The nature of corrosion products was investigated with Raman spectroscopy and EDX. The dominant corrosion product was Dawsonite for both droplet solutions, however the distribution of corrosion products differed between the two types of droplet. NaCl droplets spread more, resulting in fewer, larger corrosion sites. ASTM ocean water droplets showed multiple small corrosion sites within the droplet owing to the presence of Hydrotalcite around the droplet perimeter. Corrosion development under wet-dry cycling conditions was different for NaCl and ASTM ocean water droplets. Under NaCl droplets, no growth or initiation was observed during “dry” phases. However, ASTM ocean water remains partially wet in “dry” periods as some of the salt constituents in ASTM ocean water remain hydrated. As a consequence, corrosion site growth is able to continue. Dedicated to my family and friends. ‘An understanding of the natural world and what's in it is a source of not only a great curiosity but great fulfilment.’ David Attenborough ‘When life gets your down, you know what you gotta do? Just keep swimming.’ Dory ACKNOWLEDGEMENTS First of all I would like to thank my supervisor Professor Alison Davenport who has shown a tremendous amount of patience and belief in me, and without whom this PhD may never had been started, let alone finished. Words are not enough to express my gratitude for everything she has done for me. I would also like to thank my co-supervisor Professor Trevor Rayment, who has given me invaluable advice and guidance, especially with work on Tomography. Thank you to Mark Basham my Diamond Light Source supervisor for his time and technical input on tomography analysis. To all my colleagues; Steven, George, Sophie, Angus, Berenika, Andy, Flaviu, Qi, Haval, Liya and Majid. The help, support and memories I have gained from you will stay with me forever. Without you all as friends, as well as colleagues I have no doubt I would not have obtained the level of knowledge I have today. Thank you. Thank you to Andrew du Plessis for the initial guidance and data to analyse when I first started this PhD. I'm particularly thankful to my parents for their constant and undying support in everything that I've done over my time at Birmingham. And finally to my grandad Keith B. Glanvill in honour of his memory. Thank you for teaching me curiosity. This project was funded by Diamond Light Source. i Contents 1. INTRODUCTION .......................................................................................... 1 2. LITERATURE REVIEW ................................................................................ 5 2.1 Aircraft Management.............................................................................. 5 2.2 Maintenance Intervals ............................................................................ 5 2.3 Aluminium Alloy (AA)2024 ..................................................................... 6 2.3.1 Introduction ..................................................................................... 6 2.3.2 The Metallurgy of AA2024 ............................................................... 8 2.4 Corrosion of AA2024 Under Full Immersion ........................................ 11 2.4.1 Introduction ................................................................................... 11 2.4.2 Stability of the Passive Film in Wet Environments......................... 11 2.4.3 Localised Corrosion of AA2024 ..................................................... 13 2.4.4 Intergranular Corrosion (IGC) ........................................................ 17 2.4.5 Trenching and Grooving ................................................................ 19 2.4.6 Corrosion Product Precipitation ..................................................... 20 2.5 Atmospheric Corrosion ........................................................................ 26 2.5.1 Deposition of Salts ........................................................................ 26 2.5.2 Deliquescence ............................................................................... 27 2.5.3 Wet Dry Cycling............................................................................. 28 2.5.4 Secondary Spreading .................................................................... 32 2.6 Atmospheric Corrosion of AA2024 ....................................................... 34 2.6.1 Introduction ................................................................................... 34 2.6.2 Early Methods of Testing ............................................................... 34 2.6.1 Summary of Atmospheric Corrosion .............................................. 36 2.7 X- ray Microtomography (XMT) ............................................................ 37 2.7.1 Introduction ................................................................................... 37 2.7.2 Synchrotrons Sources for X-ray Tomography ............................... 37 2.7.3 In-Situ Visualisation of Corrosion .................................................. 41 2.8 Summary ............................................................................................. 43 3. EXPERIMENTAL METHOD ....................................................................... 45 3.1 Material Overview ................................................................................ 45 3.1.1 Characterisation of AA2024-T3 Plate ............................................ 46 3.2 Salt Droplets ........................................................................................ 48 3.3 Synchrotron Experiments ..................................................................... 49 3.3.1 Material ......................................................................................... 49 ii 3.3.2 Tomography Sample Preparation.................................................. 50 3.3.3 Tomography Experimental Conditions .......................................... 51 3.3.4 Beam Line Set Up ......................................................................... 55 3.3.5 Tomography Data Analysis ........................................................... 55 3.4 Lab Based Experiments ....................................................................... 60 3.4.1 Material ......................................................................................... 60 3.4.2 Lab Based Experimental Conditions ............................................. 62 3.4.3 Sample Preparation for Imaging .................................................... 63 3.4.1 Sample Imaging ............................................................................ 63 3.4.1 Raman Microscopy ........................................................................ 63 3.4.2 SEM, EDX and Optical Procedures ............................................... 66 3.4.3 XRD Sample Preparation and Measurements .............................. 66 3.4.4 FTIR Sample Preparation and Measurements .............................. 66 3.4.5 Salt Droplet Quantification ............................................................. 67 4. THE INITIATION OF ATMOSPHERIC CORROSION ................................ 68 4.1 Introduction .......................................................................................... 68 4.2 Results ................................................................................................. 70 4.2.1 Preliminary Observations of Corrosion in Tomographic Projections 70 4.2.2 Initiation and Growth ..................................................................... 76 4.2.3 Current Density ............................................................................. 84 4.3 Discussion ........................................................................................... 86 4.3.1 Corrosion Initiation Sites ..............................................................
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