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Abstract: Thick corrosion scales form within carbon steel oilfield pipelines in sweet (CO2 saturated) environments. The morphology and the extent of the resultant pseudo-protective nature of these scales has been seen to be dependent on multiple factors including solution pH, temperature, flow rate, and partial pressure of CO2 present. Different techniques (SEM, XRD, FIB, etc.) have been used in the past to characterise these corrosion scales. However, limitations in these techniques occur due to the fact that only small regions of the scale can be characterised in a feasible time. Further limitations may result in difficulty to relate local scale features with corrosion morphology on the evolving metal surface (i.e. localised corrosion) as the morphology of the substrate surface can only be characterised once the scale is removed. The aim of this work is to address this issue through the use of high resolution x-ray absorption tomography to characterise the internal morphology of the corrosion scales from ex situ specimens exposed to CO2 environments as a function of pH and temperature. X-65 pipeline steel, high purity 99.99 Fe and low purity 99.0 Fe pins were used in these experiments. Specimens were exposed to CO2saturated solutions using two different methods, notably open circuit potential (OCP) immersion for 7-12 days and electrochemical polarisation for two hours at 200 mV(vs OCP). After the scaling experiments, samples were characterised using XRD and SEM. X-ray tomography was subsequently performed at the University of Manchester X-ray Imaging Facilities to characterise the corrosion morphology. Transmission X-ray microscopy (TXM) was used in absorption mode and phase contrast mode to obtain three dimensional reconstructions of the specimens, where the different features of the scale and the internal corroded substrate were imaged and characterised simultaneously. Results indicate differences in the morphology of the corrosion scale depending on the method that was used to corrode the samples. As a consequence of the supersaturation in Fe2+, polarised samples present a thicker scale than the scales obtained by immersion at OCP conditions. The polarisation process also produces a less uniform corroded substrate than when the sample is immersed for 7 days at open circuit in the corroding environments used. In this work high resolution X-ray tomography has been proven to be a very powerful technique to study the scale morphology as well as the features of localised corrosion occurring under scale in the substrate.

Open Access

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Abstract: We report major advances in the analysis of synchrotron 3D datasets acquired from human healthy and carious dental enamel. Synchrotron tomographic data for three human carious samples and a non-carious reference tooth sample were collected with the voxel size of 325 nm for a total volume of 815.4 × 815.4 × 685.4 μm3. The results were compared with conventional X-ray tomography, optical microscopy, and focused ion beam-scanning electron microscopy. Clear contrast was seen within demineralised enamel due to reduced mineral content using synchrotron tomography in comparison with conventional tomography. The features were found to correspond with the rod and inter-rod structures within prismatic enamel. 2D and 3D image segmentation allowed statistical quantification of important structural characteristics (such as the aspect ratio and the cross-sectional area of voids, as well as the demineralised volume fraction as a function of lesion depth). Whilst overall carious enamel predominantly displayed Type 1 etching pattern (preferential demineralisation of enamel rods), a transition between Type 2 (preferential inter-rod demineralisation) and Type 1 was identified within the same lesion for the first time. This study does not intend to give extensive results on the different lesions studied, but to illustrate a new method and its potential application.

Open Access

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Abstract: The three-dimensional (3D) distribution of individual atoms on the surface of catalyst nanoparticles plays a vital role in their activity and stability. Optimising the performance of electrocatalysts requires atomic-scale information, but it is difficult to obtain. Here, we use atom probe tomography to elucidate the 3D structure of 10 nm sized Co2FeO4 and CoFe2O4 nanoparticles during oxygen evolution reaction (OER). We reveal nanoscale spinodal decomposition in pristine Co2FeO4. The interfaces of Co-rich and Fe-rich nanodomains of Co2FeO4 become trapping sites for hydroxyl groups, contributing to a higher OER activity compared to that of CoFe2O4. However, the activity of Co2FeO4 drops considerably due to concurrent irreversible transformation towards CoIVO2 and pronounced Fe dissolution. In contrast, there is negligible elemental redistribution for CoFe2O4 after OER, except for surface structural transformation towards (FeIII, CoIII)2O3. Overall, our study provides a unique 3D compositional distribution of mixed Co-Fe spinel oxides, which gives atomic-scale insights into active sites and the deactivation of electrocatalysts during OER.

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Abstract: Cellular Automata integrated with Finite Elements (CAFE) have been used to develop a method to account for the effect of microstructure on quasi-brittle damage development. The microstructure is simulated explicitly by subdividing a finite element into smaller cells. A heterogeneous structure is created from key cells (seeds) using defined characteristics; the influence of the initial finite element mesh is effectively removed during the development of the microstructure. Graded microstructures, textures, particle anisotropy and multiple phases can be readily simulated, such as those in composites and porous materials. A mesh-free framework has been developed to compute the damage development through the microstructure, using cellular automata. With this method, we can study the development of discontinuous cracking and damage coalescence, and its sensitivity to microstructure. Experiments have been carried out to observe the three-dimensional development of damage, using high-resolution synchrotron X-ray computed tomography and digital volume correlation to observe Hertzian indentation of a SiC-SiC fibre composite, quantifying damage by measurement of the displacement fields within the material. The results demonstrate the applicability of the modelling strategy to damage development, and show how model input data may be obtained from small specimen tests, which could be performed at elevated temperatures with irradiated materials.