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Heritage Wrought Iron: Towards the Development of Evidence Based Standards for Coating

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Heritage Wrought Iron: Towards the Development of Evidence Based Standards for Coating Heritage wrought iron: towards the development of evidence based standards for coating Nicola J. Emmerson Thesis submitted to Cardiff University in candidature for the degree of PhD September 2015 i Summary Effective management of heritage assets relies on decision-making which is underpinned by empirical evidence of impact of treatments on long term survival prospects of materials. Historic wrought iron presents a particular problem for conservation. It occupies a niche position between heritage and engineering, is frequently exposed to outdoor atmospheric corrosion and, in the case of bridges, gates and similar structures, may be required to perform a distinct function. Sector guidance to direct practices is based on anecdotal evidence and established methods. British Standards relate to modern steels hence application to historic ferrous metals is complicated by differences in metallurgy and lack of concession to conservation ethics. This study generates empirical evidence of the effects of five surface preparation methods and three protective coating systems on the corrosion rate of historic wrought iron samples. Immersion in sodium hydroxide solution and blasting with crushed walnut shells are found to reduce corrosion rates of uncoated wrought iron. Aluminium oxide and glass beads blasting increase corrosion rate but offer removal of contaminants and a keyed surface for coating adhesion. Flame cleaning increases corrosion rate by almost four times the uncleaned wrought iron corrosion rate. A two-pack epoxy resin coating system with polyurethane topcoat applied over substrate surfaces blasted to Sa2.5 (near white metal) and a surface tolerant single- pack alkyd coating applied over coherent oxide layers successfully prevented corrosion for almost two years in high static relative humidity environments. An alkyd system applied over Sa2.5 blasted surface does not significantly reduce corrosion rate of the uncoated substrate. A cost benefit approach to interpreting the empirical results in relation to practicalities of applying the treatments is advocated. The methods developed for standardising historic sample material and measuring oxygen consumption of coated samples as proxy corrosion rate offer scope for further work in this area. A standardised approach to testing permits correlation of test data between workers in this area to generate a database of empirical data to inform decision-making. i Acknowledgements Many thanks to Historic Scotland for funding this study, in particular Craig Kennedy (now Herriot Watt University), David Mitchell, Alick Leslie and all of the Technical Conservation team. This practitioner focused study could not have taken place without the participation of conservation and heritage ironwork practitioners. Thanks to Pete Meehan for advice, sample material and access to blasting shops and foundries, to Eura Conservation for coatings, to Alex Coode for his insight and to all practitioners involved in the flame cleaning. Thanks also to Dave Thickett at Historic England for his support and advice. At Cardiff University, thanks to the Conservation team of Jane Henderson, Yiota Manti and especially to Phil Parkes for his training on the SEM. Thanks to Paul Nicholson and John Hines for their support during the PhD annual reviews and to Helen Szewczyk for her caring and efficient administration of postgraduate study. At Amgueddfa Cymru, I am grateful to Tom Cotterell for his assistance with XRD. Within the Cardiff Ferrous Metals Research Group, thanks to Melanie Rimmer for her invaluable advice and assistance and to Eric Nordgren for his companionship, experience and anecdotes. Thanks to Amber Lawson for sharing the images of paint layer cross sections. The biggest debt of gratitude is owed to David Watkinson for his supervision and friendship over the course of this PhD. Finally, thanks to my family for their unfailing support in all things. ii Dedication For James Donald Emmerson, Christopher John Mannion and Reuben Jung Karki. iii Contents 1 Introduction ......................................................................................................... 1 1.1 The nature of historic wrought iron .............................................................. 1 1.1.1 Production and standardisation............................................................. 1 1.1.2 Composition and properties .................................................................. 3 1.1.3 Form and function .................................................................................. 5 1.2 Research in heritage wrought iron ................................................................ 7 1.3 The study in context ...................................................................................... 9 1.4 Aim and objectives ...................................................................................... 10 1.5 Thesis structure ........................................................................................... 10 2 Corrosion Principles: Heritage wrought iron and its contexts ........................... 12 2.1 Principles of corrosion ................................................................................. 12 2.1.1 The corrosion process .......................................................................... 12 2.1.2 Corrosion of iron: an overview ............................................................ 14 2.2 Corrosion prevention by protective coatings .............................................. 18 2.2.1 Mechanisms of protection ................................................................... 19 2.2.2 Factors affecting coating performance and failure ............................. 21 2.3 Corrosion variables ...................................................................................... 25 2.3.1 Water.................................................................................................... 25 2.3.2 Climate ................................................................................................. 26 2.3.3 Oxygen .................................................................................................. 28 2.3.4 Intrinsic factors ..................................................................................... 33 2.3.5 Pollutants and contaminants ............................................................... 35 2.4 Corrosion products and their properties .................................................... 38 iv 2.4.1 The importance of corrosion products ................................................ 38 2.4.2 αFeOOH (Goethite) .............................................................................. 39 2.4.3 βFeOOH (Akaganeite) .......................................................................... 39 2.4.4 γFeOOH (Lepidocrocite) ....................................................................... 41 2.4.5 Fe3O4 (Magnetite) ................................................................................ 41 2.4.6 α-Fe2O3 (Haematite) ............................................................................. 41 2.4.7 ƴFe2O3 (Maghemite) ............................................................................ 42 2.4.8 Ferrous and ferric chlorides ................................................................. 42 2.5 Corrosion context ........................................................................................ 43 3 Corrosion Prevention Treatments ..................................................................... 48 3.1 Traditional treatment of wrought iron ........................................................ 48 3.2 Current practice ........................................................................................... 50 3.2.1 Recording and dismantling .................................................................. 51 3.2.2 Cleaning and surface preparation ........................................................ 51 3.2.3 Repairs .................................................................................................. 53 3.2.4 Protective coatings ............................................................................... 54 3.2.5 Application of protective coatings ....................................................... 55 3.2.6 Maintenance ........................................................................................ 55 3.3 Evidence from paint layers in section ......................................................... 56 3.4 Decision-making in conservation of historic wrought iron ......................... 60 3.4.1 Decisions and decision-makers ............................................................ 60 3.4.2 Guidance .............................................................................................. 64 3.4.3 Use of standards in heritage contexts ................................................. 65 4 Experimental ...................................................................................................... 69 4.1 Ethos and rationale ..................................................................................... 69 4.1.1 Research ethos ..................................................................................... 69 v 4.1.2 Related work ........................................................................................ 69 4.1.3 Rationale for methods and parameters ............................................... 71 4.2 Investigating the impact of surface preparation method on corrosion of historic wrought iron .............................................................................................
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