Evidence-based Condition-Monitoring Strategy for Preservation of Heritage Iron

From archaeological objects through to ships, iron is a ubiquitous heritage metal. Unfortunately, iron readily corrodes and iron objects are thus eventually destroyed during storage and display, especially when contaminated with chloride from burial or marine contexts. Only desiccation below 15% RH or de-oxygenation can stop corrosion in these instances, as attempts to remove chloride are unsatisfactory. Mechanical desiccation is energy-hungry and expensive, while passive use of desiccants to control small spaces is difficult to manage long-term. Currently, management must either adopt precise and costly no-corrosion desiccation, or allow partial desiccation with unknown preservation outcomes.

This research identifies and tests new ideas for managing the preservation of heritage iron via the concept of 'corrosion control' rather than 'corrosion prevention'. In a world of dwindling resources, not all objects can justifiably merit indefinite preservation. It may be possible to define and assign lifespans to lower preservation costs and energy expenditure.

The project will:

- measure the corrosion rate of chloride contaminated heritage iron;
- define 'object lifespan' in relation to heritage value;
- relate this to relative humidity
-test novel ways of monitoring corrosion rate by developing 2 types of sensor;
-produce a management model for optimising conservation based on controlled corrosion, cost and energy expenditure.

Experimental work will quantitatively examine long-term corrosion rates of up to 300 samples of heritage iron. These results will underpin subsequent field-testing of heritage iron objects, and the development of sensors and management guidelines.

The amount of oxygen consumed by the corrosion process is measured and related to the chloride in each object, as a function of atmospheric moisture and physical changes to the samples. Thus examination of links between corrosion rate, RH and loss of heritage value will generate a preservation scale that can be used to predict object 'lifespan' as a function of humidity. We will introduce options for using corrosion control to preserve objects for predicted time periods at specified RH values.

It is necessary to monitor corrosion rates in controlled environments to support this approach. Monitoring the rather low corrosion rate occurring on heritage iron is experimentally challenging. Consequently, two indirect methods of measuring corrosion will be explored, developed, tested and scaled against heritage iron using corrosion rate data and object 'lifespan' as generated and defined by this research:

- a novel corrosion monitor that dynamically records electrical resistance change to measure corrosion;
- a passive sampler that utilises chemical reaction, weight and colour change.

The resulting corrosion control model and its monitoring methods will be tested in-situ using Brunel's iconic and multiple conservation award winning ship ss Great Britain. Monitoring the controlled environment at selected points on the ship will identify corrosion rates and these may then be linked to preservation costs and the carbon footprint of environmental control. A cost-benefit analysis can then be extrapolated to offer options for acceptable corrosion rates and object longevity. A similar test will be run in the metal stores at the Mary Rose Trust but on smaller objects held in different conditions.

The outcomes will be of direct practical use to the wide range of organisations that store and conserve large and small iron objects. The project will deliver a holistic management system for the first time. This will enable the development of clear guidelines on the preservation of chloride contaminated iron, based on predictions of lifespan from accurate corrosion rate data. The transparent decision making process will allow managers to link corrosion control to preservation outcomes and use of resources.

Who will benefit?
This project produces multiple impacts on the heritage sector of which the main beneficiaries will be organisations responsible for the preservation of iron. There is also potential application to industrial contexts, especially for impacting on industrial corrosion control.

The Project Partners are ss Great Britain and Mary Rose Trust (fieldwork testing of both sensors), English Heritage (testing passive sensors) and Eura Conservation and Dorothea Restorations (providing on-going commercial and industrial input and advice). Their established profiles will attract attention and publicity that will embed our research into the heritage sector.

How will they benefit?
Heritage Sector practice: An obvious impact of this study is to further knowledge and understanding of corrosion science within the heritage sector and benefit future research there. It also influences preservation strategies for ferrous heritage by challenging current practice and its underpinning rationale. The potential impact is significant. Introducing the concept of quantitative corrosion control and object lifespan to offer new preservation options could revolutionise long-term storage of heritage iron. Our dissemination routes and diverse partnerships will generate high profile impact. Producing best practice guidelines will be the first step towards benchmarking preservation and producing conservation standards for desiccated storage of heritage iron.

Resource Management: UK heritage is a multi-million pound industry. It generates income from tourism, but consumes money in caring for this heritage. Managing preservation should balance outcomes against financial and environmental costs. Perhaps the biggest impact of this project could be a broad-ranging questioning of conservation concepts and ethics that currently focus on complete prevention of corrosion - but this would be a very long-term outcome Generating quantified preservation options will facilitate a move towards evidence based pragmatism in conservation decision making. Adopting these new ideas and practical outcomes will impact on the tourist economy and public sector by reducing the cost of preserving heritage iron. The biggest impact will be in the public sector, where responsibility for much of our heritage lies. Our case studies at ss Great Britain and Mary Rose Trust will provide practical and adaptable examples here. We expect other nations to utilise our research and ideas, which will be disseminated through prestigious publication and international conferences. Impact within corrosion control contexts outside of the heritage sector could contribute to increased longevity of materials; where maintaining low humidity is the preferred corrosion control method a reassessment of RH control standards is a possibility for chloride contaminated ferrous metals.

Commercial exploitation: The active and passive sensors developed in the project have commercial relevance for measuring the aggressiveness of environments towards chloride contaminated ferrous metals. In the long-term, there is obvious potential for the application of research findings to industrial iron usage on an international scale - particularly in chloride rich marine atmospheres where the corrosion sensors may offer opportunity for recording corrosion when cables/structures are chloride infested (e.g. shipping, bridges, transportation, road and rail infrastructure). There is also potential for application to dry storage of equipment that has been used in marine contexts (MoD, shipping, rescue and recovery sectors).

Potential industrial applications and connections with industrial sectors will be assessed after the first year, when some results from examining corrosion rates and sensor development are available. This will allow us to contextualise against industry needs identified from literature