If samples are removed from a controlled environment for measurement, the surface could react with the ambient and give misleading results. Therefore the most accurate way to model real corrosion or other chemical transformations is to examine the samples of interest while they reside within the created conditions. This requires a piece of equipment which allows surface analysis to
occur in situ, such as the eCell [33] which allows simultaneous surface analysis within an electrochemical (or VOC-saturated) environment. The cell window must be transparent to the electromagnetic radiation to be used and the material of the cell must be resistant to the generated environment. Also, the thickness of the electrolyte between the sample and the window must be sufficiently thin (100
!m) to allow penetration of the incident radiation and avoid scattering of signal from the liquid.
In this research we are interested in in situ synchrotron X-ray techniques for tracking corrosion and/or electrochemical behaviour. Spectroelectrochemical cells for use in synchrotron beamlines were first pioneered by Fleischmann, Robinson and co-workers, as they defined these requirements for in situ analytical instrumentation for corrosion studies [34] and spectroelectrochemistry.
The authors applied a SR-XRD technique to a lead-dioxide layer deposited on platinum in a sulphuric acid electrolyte within a Bragg-type cell [34–36]. The X-rays with a wavelength of 1.307 Å were incident at 4° and cyclic voltammetry was performed using the lead-dioxide sample as a working electrode during surface analysis. In addition the cell was used for in situ EXAFS of samples pre-passivated within the electrochemical cell [34]. Since the development of these first cells, in situ cells have been designed by many other researchers for similar purposes [37–41].
There are principally two types of cell: Bragg and Laue cells (Figure 1.4). Bragg cells work in reflection mode whereas Laue cells work in reflection or transmission mode. For a Bragg-type cell, the window of the cell must be parallel to the surface of interest. In a Laue-type cell, the incoming X-rays pass through a window perpendicular to the surface of interest and outgoing X-rays
pass through a second perpendicular window. In order to reduce scattering from the electrolyte in a Laue-type cell it is possible to reduce the thickness of the cell [42].
Figure 1.4 (a) Reflection geometry in a Bragg cell, and (b) transmission geometry in a Laue cell.
Reprinted from reference [42] with permission from Elsevier.
The Electrochemistry and Surface Analysis (ESA) and Analytical Science Projects (ASP) groups at Ghent and Warwick Universities have designed four iterations of an environmental/electrochemical cell (eCell) for the examination of conservation processes simultaneously by electrochemical and synchrotron X-ray methods [33]. The eCell is a Bragg cell used in reflection mode for surface analysis but remote control of the working electrode ensures movement between this position and a “dwell’ position which allows the electrode to be immersed in electrolyte. The eCell environment can be customised depending on the experimental conditions required: gas flow, liquid flow, synchrotron radiation and/or electrochemistry. The first system to be studied was chlorine-contaminated copper [43,44]. Copper-containing marine artefacts become impregnated with chloride ions during their residence in seawater. After removal
from this low-oxygen environment into ambient conditions, the nantokite formed by reaction of copper with chlorine reacts with oxygen and water to form powdery copper hydroxychlorides [45]. These compounds can cause structural damage to the artefact due to loss of metallic structure, a process more commonly known as “bronze disease” [14,46,47]. As previously mentioned, a common method of chloride removal for the prevention of bronze disease is soaking in water or a sodium sesquicarbonate solution [18]. In addition, a weak polarising potential could be applied to reduce copper chloride species thus removing chlorides from the metal [48–50].
Using XRD and X-ray absorption spectroscopy (XAS) in a time-lapse experiment, the growth and disappearance of copper corrosion products were monitored with time [20,51–53]. The corrosion potential of the electrochemical cell (Ecorr) (the copper sample being the working electrode) was also measured using open circuit potential (OCP). The results from the spectroscopy and diffraction were compared with the Ecorr to see if a relationship between corrosion product removal and Ecorr could be established. If such a relationship existed, Ecorr
could possibly be used as a simple monitoring tool for chloride removal during soaking. It was found that Ecorr could only track the removal of chemically bound chlorides. If chlorides were adsorbed to the surface, monitoring by OCP was not possible even though they would still pose a threat to the metallic surface after conservation.
Also using the eCell, the effectiveness of electrolytic reduction of a chloride-impregnated sample was investigated. XRD patterns were extracted while the conservation treatment was taking place [54]. In this way, the transformation of nantokite to cuprite was visualised, thus showing the successful removal of
chlorides using this technique. Other copper corrosion transformations have also been observed by exposing reference materials to reactants of interest and tracking the crystalline transformations with time without electrochemical intervention [11,12].
Lead decanoate (Pb(CH3(CH2)8COO)2) was studied as a possible conservation coating on lead using spectroelectrochemical techniques within the eCell: the growth of the crystalline coating was studied over time using SR-XRD alongside EIS to measure the effectiveness directly of the growing layer [53,55]. This layer was then exposed to acetic acid vapour within the cell and the resulting crystalline corrosion products were recorded over time using XRD [56]. More recently the eCell has also been used to study the growth and deterioration of lead dodecanoate (Pb(CH3(CH2)10COO)2) spectroelectrochemically using cyclic voltammetry (CV) and linear sweep voltammetry with SR-XRD [57].
The eCell is capable of providing a wide variety of environments for many different samples. The portable cell described in this thesis (Chapter 6 and 7) aims to provide multi-sample capability for longer-term experiments.