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Methods

The consumption of oxygen that occurs when an object corrodes can be measured accurately to provide information about corrosion rates.

The consumption of oxygen that occurs when an object corrodes can be measured accurately to provide information about corrosion rates.

This project will measure the individual corrosion rates of approximately 300 archaeological iron nails at controlled relative humidity (RH) and temperature. Archaeological iron nails with metallic iron cores and attached mineralized layers will be placed in individual sealed reaction vessels, whose interior RH is controlled by conditioned silica gel that is monitored by matchbox-sized RH/temperature data loggers. Ambient temperature will be controlled to 20oC. The consumption of oxygen within the reaction vessels will be measured remotely, using a fluorescence-based oxygen sensor. The influence of corrosion on the physical integrity of the object will be assessed and upon completion of the tests each nail will be digested to determine its chloride content. The quantified corrosion rates at specified RH values can then be assessed as a function of chloride content of iron. Linking this data to the effect of corrosion on the heritage value of the object is possible by recording the physical change produced by corrosion. A corrosion scale linking chloride, RH and physical integrity of archaeological iron objects will be produced. This can then be used to assess object lifespan according to physical change (heritage value) rather than the loss of metal.

Weeping of iron is indicative of high chloride content and leads to cracking of the corrosion products. Recording the progression of these signs of corrosion is an important part of the project.

Weeping of iron is indicative of high chloride content and leads to cracking of the corrosion products. Recording the progression of these signs of corrosion is an important part of the project.

Corrosion rate scales and monitoring corrosion

Monitoring the cumulative effect of corrosion on archaeological iron, while it is in storage or on display is never carried out as there is no direct method for doing this. Although the RH within the storage environments is often recorded to determine if conditions meet target values,  this offers no feedback on cumulative corrosion of iron for fluctuations above the no-corrosion RH. Developing low resistance corrosion sensors, which reproducibly record corrosion scaled to chloride-infested archaeological iron,  will facilitate recording of cumulative corrosion that informs on the overall effectiveness of the storage environment. 

 

As corrosion progresses, the iron metal is consumed and corrosion products build up, increasing the resistance of the sample. This is the basis for the ERCM sensors being developed in this project. (Image credit: Eleni Kapatou)

As corrosion progresses, the iron metal is consumed and corrosion products build up, increasing the resistance of the sample. This is the basis for the ERCM sensors being developed in this project. (Image credit: Eleni Kapatou)

Research in Manchester has already developed and tested pre-corroded electrical resistance corrosion sensors (ERCMs) that emulate the corrosion profile of archaeological iron. The focus of this research will be to improve the manufacturing reproducibility and recording of the sensors so they can be reliably used in corrosion monitoring. Once this is achieved sensor corrosion rate can be scaled using data from the Cardiff tests, thereby allowing them to record how archaeological iron objects would have corroded in the surrounding environment. This monitoring of storage and display environments will provide accurate information about the corrosion of iron and facilitate predictions on its longevity.

Further reading