Matching display relative humidity to corrosion rate: Quantitative evidence for marine cast iron cannon balls

Cast iron cannon balls excavated from the wreck of King Henry VIII flagship, the Mary Rose, have been actively corroding on mixed material display at 55% relative humidity (RH). A Cardiff University study has examined corrosion rates of cannon balls treated by hydrogen reduction, alkaline sulfite and Hostacor IT. Oxygen consumption of six cannon balls has been measured at five relative humidities between 20% and 60% RH as proxy corrosion rate. Results show a noticeable increase in corrosion rate at 50% to 60% RH and also returned differences in corrosion rates of cannon balls based on previous treatment. Display RH of 55% for showcases containing organic materials and cast iron is found to be unsuitable for the cannon balls and their display in these conditions is not recommended. This has implications for contextual display at the Mary Rose Trust and elsewhere. Further study with greater sample numbers is providing statistical evidence of the effect of treatment regime on corrosion rate to inform decision making for future treatment, display and storage

Heritage wrought iron: towards the development of evidence based standards for coating

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

Preparing historic wrought iron for protective coatings: quantitative assessment to produce evidence-based protocols

Painted historical wrought iron commonly occurs outdoors exposed to the prevailing climate. Maintaining protective paint layers is an interventive process that often involves removing existing paint layers and repainting. Whilst prior surface preparation greatly affects the longevity of any paint layer, its impact on the metal surface and paint performance has received limited research within heritage contexts, making their selection anecdotal or driven by manufacturers’ guidelines. Historic Scotland-funded research at Cardiff University is quantitatively investigating the effect of surface preparation methods on wrought iron corrosion rates prior to paint application. The feasibility of using historic sample material in this research has been investigated and is reported. Testing historic wrought iron samples in quantitative studies of corrosion offers more direct linkage to heritage scenarios thus facilitating interpretation of results and extrapolation to real time heritage contexts. The use of an oxygen consumption technique to quantitatively determine the corrosion rate of five samples of historic wrought iron in controlled conditions of 90% relative humidity and 20 oC is reported. Results returned corrosion rates indicating a level of reproducibility that, with an error calculation, will allow corroded historic wrought iron to be used for production of test samples to be employed in experiments designed to determine the impact of surface cleaning techniques on the corrosion rate of corroded heritage iron

Impact of washed naturally formed and synthetic β-FeOOH on corrosion rate of iron as a function of relative humidity

In high humidity βFeOOH corrodes iron in contact with it, likely due to the mobile chloride on its surface providing an electrolyte. Studies have shown that washing βFeOOH does not entirely remove chloride, which remains occluded in its crystal structure. Since occluded chloride is not mobile in water it follows that washed βFeOOH should not corrode iron in contact with it. This study reports washing naturally formed βFeOOH and synthetic βFeOOH formed by the Fe/ferrous chloride protocol using a Soxhlet wash system and by stirring in cold water until no further chloride extraction occurs. The corrosion of iron powder mixed with βFeOOH is recorded as oxygen consumption in sealed reaction vessels at 60%, 70% and 80% RH and this is compared to corrosion rates of unwashed βFeOOH at the same RH values. Desalination methods are employed within conservation to remove chloride from archaeological and historical iron. The outcomes of this study offer insight into the impact of this washing process on the post-treatment corrosion of iron by washed βFeOOH and provides guidance on whether temperature differences during treatment will impact on the removal of surface adsorbed βFeOOH. It also offers data on the amount of chloride adsorbed onto the surface of naturally formed and synthetic βFeOOH and whether this influences the rate of corrosion of iron

Protective coatings for historic wrought iron: epoxy resins versus oil-based systems

The heritage ironwork community is debating the practical and moral disadvantages of coating wrought iron with two-pack epoxy coating systems versus their potential for corrosion prevention. Their promotion by manufacturers as highly engineered, long-lasting ‘wonder-coatings’ is being called into question and weighed against the issues of cost, requirement for blasting substrates to Sa2.5 (near white metal), a relatively short pot life, reversibility challenges, and the lack of compatibility between the inflexible cured polymer and the dimensionally thermo-responsive ironwork which is reported to cause cracking of the epoxy resin at joints with resultant ingress of water. Currently absent from this discussion is empirical evidence of the impact of epoxy resin coatings on the corrosion rate of wrought iron as compared with other coating types. A project investigating the surface treatment and protective coating of historic wrought iron is under way at Cardiff University with sponsorship from Historic Scotland. The effect of surface preparation methodology prior to coating on the corrosion rate of the uncoated substrate was reported at EUROCORR 2012. Current work aims to provide a cost benefit comparison between the use of two-pack epoxy resin coating systems and oil-based systems. This paper presents the results of corrosion rate investigations and electrochemical impedance spectroscopy (EIS) analyses of samples coated with a commonly used epoxy resin system and two household name oil-based systems. Mid-19th century rolled plate wrought iron samples were prepared by blasting to Sa2.5 (near white metal) with aluminium oxide powder or by wire brushing, as per the coating manufacturers’ recommendations. Similarly prepared samples were contaminated with synthetic seawater (BS3900-F10:1985) to simulate wrought iron exposed to a coastal environment. All three coatings were applied as per the manufacturers’ instructions. The samples were sealed within individual glass reaction vessels containing silica gel conditioned to 90% RH. Depletion of oxygen within the vessels over 12 months was remotely recorded using a World Precision Instruments OxyMini oxygen meter to measure the quenching of fluorescence of an oxygen sensor spot attached to the wall of each vessel. Oxygen consumption of the coatings was determined using controls and subtracted from that of the wrought iron samples to give the corrosion rates of the metal substrates. Comparative performance of the coatings was investigated using EIS to support the real time long-term tests employing oxygen consumption. Interpretation of the outcomes will build towards evidence based advice for contractors choosing coatings for corrosion prevention of heritage wrought iron monuments in coastal and inland contexts

Desiccated corrosion control of the 324 feet long wrought iron hull of ss Great Britain: theory and operational realities 10 years on

Brunel’s ss Great Britain ocean going liner is an iconic historic ship located in Bristol (UK). At its launch in 1843 it embodied multiple steps forward in engineering and shipbuilding. It was the biggest ship in the world, built with an iron hull using an innovative box girder hull design that was the forerunner for all future ship design and was screw driven. After an eventful life as a liner, emigrant vessel, sailing ship and finally a floating warehouse in the Falkland Islands, it was returned to Bristol in 1970 to rest in the dry dock in which it was constructed, where it become a visitor attraction. Traditional painting techniques failed to offer sufficient protection from corrosion for the corroded hull open to the elements. The ss Great Britain Trust explored ways to preserve the ship, finally settling on a procedure that involved glazing the dock at water level to provide an enclosed space for controlling corrosion of the lower hull by desiccation, leaving the less corroded upper hull exposed to the atmosphere and maintained by traditional painting methods. This addressed the problem of the instability of the salt infested lower hull area. Research at Cardiff University determined the impact of relative humidity (RH) on the corrosion rate of iron to inform the design of the desiccation plant. Mass gain experiments examined the impact of ferrous chloride and the corrosion bearing ferric oxyhydroxide akaganéite, which were both identified on the hull, on corrosion rate of iron identifying corrosion would be negligible at a target RH of 20%. Plant was designed to maintain this value by desiccating and channelling air around the dock. This paper reports the performance of the plant by looking at the spatial distribution of RH on the surface of the hull, inside it and within the roofed in dry dock area. Temperature control within the dock is a challenge during summer months due to solar gain through the glass roof and the welfare, safety and comfort of visitors entering this area has to be considered, as well as operation of the plant. Relationships between corrosion rate, plant performance, RH and temperature are discussed and set against the cost of maintaining the desiccation levels and the ethical constructs of preserving heritage

Using quantitative and qualitative analysis to inform management protocols for the preservation of archaeological ironwork

Heritage preservation often seeks to fulfil an unrealistic goal of stopping decay, whereas controlling decay rate is a more realistic option. To develop pragmatic management based on limiting decay of heritage material requires predictive strategies. These can only be developed by quantifying the decay rates of heritage materials as functions of their intrinsic and extrinsic factors. This data facilitates prediction of object longevity and incremental loss of heritage value, which provides management with an evidence base for decision making that maximises use of resources and provides cost benefit. This evidence based management approach and the challenge it faces are discussed by relating the corrosion rate of archaeological iron objects to management strategies for its preservation. A combination of quantitative analytical methods developed a strong evidence base for managing the preservation of archaeological iron. The corrosion rates of over 100 archaeological iron objects were determined as a function of relative humidity by recording the oxygen consumption of individual objects. This data was then qualitatively linked to the physical damage that corrosion produced in objects using criteria anchored assessment and was quantitatively related to object chloride content using digestion. Predictions for loss of heritage value as a function of corrosion could then be made. Prompt Gamma Activation Analysis (PGAA) at the Budapest Neutron Centre was later employed to link chloride distribution within objects to the physical damage produced by corrosion to facilitate refinement of the predictive corrosion model. English Heritage has published sector guidelines for storing archaeological iron using this research

Matching display relative humidity to corrosion rate: Quantitative evidence for marine cast iron cannon balls

Cast iron cannon balls excavated from the wreck of King Henry VIII flagship, the Mary Rose, have been actively corroding on mixed material display at 55% relative humidity (RH). A Cardiff University study has examined corrosion rates of cannon balls treated by hydrogen reduction, alkaline sulfite and Hostacor IT. Oxygen consumption of six cannon balls has been measured at five relative humidities between 20% and 60% RH as proxy corrosion rate. Results show a noticeable increase in corrosion rate at 50% to 60% RH and also returned differences in corrosion rates of cannon balls based on previous treatment. Display RH of 55% for showcases containing organic materials and cast iron is found to be unsuitable for the cannon balls and their display in these conditions is not recommended. This has implications for contextual display at the Mary Rose Trust and elsewhere. Further study with greater sample numbers is providing statistical evidence of the effect of treatment regime on corrosion rate to inform decision making for future treatment, display and storage

Removal of copper corrosion products from archaeological copper alloys using a Q-switch Nd:YAG 1064 laser: impact on selected corrosion products

Corrosion of archaeological copper alloys during terrestrial burial normally produces a corrosion product profile comprised of a mixed layer of either CuCl/Cu2O or SnO2/CuCl/Cu2O for bronzes, which is overlaid by a corrosion product layer that is normally predominantly comprised of CuCO3CuOH2. If CuCl has been hydrolysed post excavation, Cu2OH3Cl polymorphs may also be present. Aesthetically it is important to expose the shape of an object and to either retain or expose the colour and texture of patinas. Mechanical removal (cleaning) of the outer layers to reveal shape and enhance aesthetics can be labour intensive, while chemical methods are difficult or impossible to control. Laser generated energy potentially offers a controlled method for achieving ethical and aesthetic goals but it’s ability to remove or transform commonly encountered copper corrosion products is unknown. The energy a laser produces and its delivery to surfaces is governed by a wide range of variables. Their individual and collective impact on copper corrosion products must be known for laser treatment of archaeological copper alloys to employ evidence-based operating parameters, which deliver predictive outcomes. This project examined the impact of a Q-switched Nd:YAG 1064nm laser on selected individual corrosion products and their mixtures, which typically occur in corrosion profiles found on archaeological copper alloys. The analogues comprised: CuCO3CuOH2; CuCl; Cu2O; 50/50 CuCl/CuCO3CuOH2mix; and metallic copper. Results made it possible to predict the likely impact of laser ablation on patinas found on archaeological copper alloys. During the experiment only the amount of fluence (energy dispersed over cm2) was controlled, as the same fluence can be replicated when using other lasers, by adjusting energy and spot size. Analysis of the experimental outcomes employed Raman and FTIR spectroscopy to detect transformations of copper compounds and colourimetry to assess colour changes. This data offers insight into both physical and aesthetic changes that could occur from the use of lasers on archaeological objects

Flame cleaning of historic wrought iron: practitioner methods and their impact on oxide morphologies and post-treatment corrosion rates

Flame cleaning followed by wire brushing is a common treatment for wrought iron; the flame combusts existing coatings and spalls oxides while wire brushing removes any debris, producing a sound surface for recoating. Although frequently applied, little is known about the effects of the treatment on the substrate material and its post-treatment corrosion rate. This study reports scanning electron microscope backscattered electron imaging (SEM-BEI) and oxygen consumption corrosion rate testing of wrought iron flame cleaned by three practitioners. Wrought iron samples treated by two practitioners corrode up to 4 times faster than uncleaned control samples. Samples cleaned by one practitioner exhibit no increase in corrosion rate. Torch fuel type and temperature attained by the iron are identified as parameters potentially contributing to differences in corrosion rates. Comparing oxide morphology of treated and untreated samples reveals extensive cracking and fragmentation following flame cleaning. This offers multiple pathways for ingress of oxygen and water to the metal core to support corrosion. This data simultaneously calls into question the ethics of flame cleaning and offers evidence that a ‘safe’ method exists, prompting further research into this popular treatment