BioGeoCivil Summit 2015
17-19 November
The Art Centre, Delft ,The Netherlands
3
Studying the past, understanding the present, predicting the future -
analysis of corrosion systems in soil surrounding buried ancient metal
M. Afanasyev1, H. Huisman2, L. A. van Paassen1 and T. J. Heimovaara1
1
Department of Geoscience & Engineering, Delft University of Technology, The Netherlands, Stevinweg 1, 2628 CN, Delft, Netherlands
2
Cultural Heritage Agency of the Netherlands, P.O. Box 1600, 3800 BP Amersfoort, The Netherlands
Abstract
Corrosion of metal infrastructure in soil leads to high costs in monitoring and maintenance. It is not only the metal object that degrades due to corrosion, the presence of corroding metal also induces changes in the surrounding soil. We present a case study, where we analyse a complete corrosion system around ancient buried metal objects. The purpose of our research is to analyse, visualise and, where possible, quantify the impact of the complex interactions between the corroding metal and its environment, in our case soil and/or wood.
Sampling the complete corrosion system - the metal and the adjacent soil - from infrastructure objects such as pipelines and sheet piles without disturbing the sample is very difficult. To overcome this limitation, we collected with minimal disturbance small archaeological metal objects, together with the corrosion-influenced "envelope" that surrounds the objects. The objects, construction nails of forged steel, were collected during the excavation of wreck OR 49 in Dronten, Flevoland. This was a 17-th century merchant ship that sank in the former Zuiderzee sea branch in an area reclaimed in the 1950’ies. Samples have been impregnated with epoxy using the acetone-replacement method and thin sections were prepared from selected samples. Additionally, samples were scanned using Micro Computed Tomography (micro-CT) scanner and viewed in an ESEM (Environmental scanning electron microscope), where the chemical composition at selected points was analysed.
Our results show the impact of the gradual dissolution of metal due to corrosion and the partial conversion of steel into more stable minerals during burial. The result is a complex structure, where the metal core is surrounded by layers of corrosion products, wood preserved due to impregnation with dissolved iron and soil particles cemented by corrosion precipitates. In some cases, we see the corrosion predominantly occurring in weak points, between layers within the forged metal nails. In the CT images, colour variation correlates with the density variations in the samples.
We have demonstrated that, by using archaeological artefacts as a case-study, it is possible to sample and analyse the complete corrosion system in soil. The combination of the different analysis methods provides complementary data, generating a 3-D map of the shape, chemical composition and mineralogy of a metallic archaeological artefact with minimal disturbance. Using the right set of tools it is possible to image in great detail and in 3-D the shape and composition of archaeological metal objects, even if they are no longer recognizable as such visually. Some archaeological metal can be completely transformed such as the core of sample 3 that has been converted to siderite (FeCO3). Such samples can have a longer “life” in their transformed state than as a metal and modifying the composition of the artefact may
BioGeoCivil Summit 2015
17-19 November
The Art Centre, Delft ,The Netherlands
4
well be a suitable conservation method. For better understanding of the corrosion processes in soil our results need to be compared to similar studies of modern corrosion systems in soil and verified by experiments in a controlled environment.
A 2-D slice through the CT scan of sample 3, a corroded nail head. Warmer colors indicate denser material. Metal core is in orange, corrosion products are in yellow-green, the dark blue background is the epoxy, light blue are the soil grains.
Thin section of sample 3, a corroded nail head. The upper part of the nail head has been in contact with the ship's wood, which has been partially preserved, being impregnated by iron.
