Optically Detected X-ray Absorption Spectroscopy Measurements as a Means of Monitoring Corrosion Layers on Copper

Abstract

XANES and EXAFS information is conventionally measured in transmission through the energy-dependent absorption of X-rays or by observing X-ray fluorescence, but secondary fluorescence processes, such as the emission of electrons and optical photons (e.g., 200-1000 nm), can also be used as a carrier of the XAS signatures, providing complementary information such as improved surface specificity. Where the near-visible photons have a shorter range in a material, the data will be more surface specific. Moreover, optical radiation may escape more readily than X-rays through liquid in an environmental cell. Here, we describe a first test of optically detected X-ray absorption spectroscopy (ODXAS) for monitoring electrochemical treatments on copper-based alloys, for example, heritage metals. Artificially made corrosion products deposited on a copper substrate were analyzed in air and in a 1% (w/v) sodium sesquicarbonate solution to simulate typical conservation methods for copper-based objects recovered from marine environments. The measurements were made on stations 7.1 and 9.2 MF (SRS Daresbury, UK) using the mobile luminescence end station (MoLES), supplemented by XAS measurements taken on DUBBLE (BM26 A) at the ESRF. The ODXAS spectra usually contain fine structure similar to that of XAS spectra measured in X-ray fluorescence. Importantly, for the compounds examined, the ODXAS is significantly more surface specific, and >98% characteristic of thin surface layers of 0.5-1.5-µm thickness in cases where X-ray measurements are dominated by the substrate. However, EXAFS and XANES from broadband optical measurements are superimposed on a high background due to other optical emission modes. This produces statistical fluctuations up to double what would be expected from normal counting statistics because the data retain the absolute statistical fluctuation in the original raw count, while losing up to 70% of their magnitude when background is removed. The problem may be solved in future through optical filtering to isolate the information-containing band, combined with the use of higher input X-ray fluxes available on third-generation light sources.Electrochemical techniques provide a huge potential range of treatments for cultural heritage and other metals. These include the removal and passivation of corrosion layers, the application of protective coatings, and in situ monitoring during storage and stabilization. However, in the area of heritage science, because of the irreplaceable nature of many historical artifacts, each process needs to be thoroughly tested and understood, ideally before first use. Elsewhere we describe an electrochemical cell (eCell) that allows real metal surfaces (rough, heterogeneous) to be studied using X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) in synchrotron beam lines so that coincident time-resolved electrochemical and X-ray data can be gathered (socalled spectroelectrochemistry).1 Structural information from the X-ray technique is then used to identify reaction products...

Real Time Observation of X-ray-Induced Surface Modification Using Simultaneous XANES and XEOL-XANES

“…The protocols used to produce the corrosion layers are described in detail elsewhere, 21−23 as are typical XANES data from these samples. 7,8 …”

Tracking the progression of bronze disease – A synchrotron X-ray diffraction study of nantokite hydrolysis

“…Corrosion starts from an irregularity where the coupon was cut from a sheet. Sodium sesquicarbonate is in fact used as an active conservation treatment for chloride-infected metals [18,19]. It transforms nantokite to cuprite, therefore 'removing' chlorides from the copper artefact.…”

Synthesis and surface characterization of a patterned cuprite sample: Preparatory step in the evaluation scheme of an X-ray-excited optical microscopy system

“…This includes a body of work on copper and cupreous alloys using synchrotron X-ray diffraction (SR-XRD) [8][9][10], X-ray absorption (XAS) [11,12] and X-ray fluorescence (SR-XRF) [13,14]. Previously, we have applied optically detected X-ray absorption spectroscopy (ODXAS) (the use of X-ray excited optical luminescence (XEOL) to acquire X-ray absorption data) to copper corrosion [11,12]. ODXAS potentially allows surface-specific chemical mapping of corrosion phenomena in both gaseous and liquid environments using microprobe [15] and microscopy tools [16][17][18].…”