Surface Analysis Techniques

Several surface and subsurface analysis techniques were utilised to investigate the mechanisms of erosion, corrosion and erosion-corrosion and the significance of the degradation on the carbon steel surfaces used in

erosion-corrosion tests. The methodology for each of the main types of analysis used in this thesis are shown in this section.

4.6.1 White Light Interferometry

Profiling surfaces of samples was an essential technique used to measure wear of surfaces caused by particle impingements in SIJ tests. 3D profiles of the surface were measured using a Bruker NPFLEX white light interferometer. The Bruker NPFLEX uses white light interferometry to scan the surface and produce a detailed profile. This non-contacting technique was the preferred method of surface profiling as it provided a very fast and accurate method of measuring a 3D profile of the entire sample surface.

Profiles of the samples used in SIJ tests were measured using the NPFLEX by positioning the sample on the microscope and adjusting the vertical height of the microscope to locate the interference fringes on the sample. A 2.5x magnification objective was used to scan the surfaces. After the fringe was located, the measurements over the surface were started. A 3D profile was obtained by starting the measurements in the centre of the wear scar on the sample and moving the stage in two directions in a spiral motion from the centre of the sample toward the edge of the sample. A minimum diameter of 12 mm across the sample was scanned and a 25% overlap was specified between the measurements so that a continuous profile could be produced across the surface. The measurements were then stitched together to give a full 3D profile of the surface. The surface was filtered to remove the form and tilt of the sample so that the 3D profiles could be extracted from a flat surface to distinguish between the form of the sample and the mechanical damage as a result of particle impacts on the surface of the sample.

4.6.2 SEM Analysis

SEM provides high resolution, high magnification images of surfaces and can be used to analyse a variety of effects on carbon steel surfaces after erosion and erosion-corrosion tests. Barker [35] and Gnanavelu [105] used an SEM to analyse carbon steel surfaces for evidence of erosion damage on surfaces after SIJ erosion and erosion-corrosion tests. A Hitachi TM3030 Benchtop SEM was used to analyse carbon steel surfaces in this thesis in the secondary electron mode with an operating voltage of 15 kV. The Hitachi benchtop SEM was used as it provided a convenient method of analysis surfaces at high magnification with sufficient resolution and quality.

4.6.3 Hardness Testing

Micro-indentation of the X65 samples was completed using a Mitotoyo HM- 122 micro-indenter to measure the Vickers hardness of samples. Micro- indentation can be used to determine the Vickers hardness (𝐻_𝑉) of a metal specimen by loading a pointed diamond of a known geometry into the test specimen, Figure 4.11. The deeper the indenter penetrates into the surface, the softer the material is and the larger the area of the indentation would be. To determine the hardness of the specimen, the indentation load applied is divided by the area of the indentation, Equation (4.7) [165].

𝐻_𝑉 =𝐹𝑖

𝐴 (4.7)

where 𝐹𝑖 is the indentation load and 𝐴 is the area of the indentation. An

indentation load of 4.9 N was used to indent the carbon steel samples in this work to determine the hardness of samples after erosion and erosion- corrosion testing using the SIJ.

Figure 4.11 Method of hardness testing showing (a) applied load, 𝑭𝒊, from a

pyramid indenter and (b) indentation of area, 𝑨, produced on the surface adapted from [165]

4.6.4 Analysis of Sand Particle Size

Sand particles were analysed to measure their size using a Malvern Mastersizer 2000 particle sizer. A sample of sand particles was added to deionised water prior to measurements and recirculated through the Mastersizer to disperse the sand particles. After dispersion, the sand-water solution was recirculated through a glass chamber and measurements of size were made using a laser diffraction technique. The sizes of the particles were measured by determining the angular variation in intensity of scattered light as the laser beam passed through the dispersed sample [206]. The scattering

pattern could then be used to determine the particle size. The refractive index of the sand particles was required to be input in order to calculate particle size. A refractive index of 1.544 and an imaginary component of refractive index, commonly referred to as absorption of the material, of 0.1 was used. Five repeated measurements of particle size were completed for each sample of sand particles measured in this thesis and three separate samples of the same particle batch was analysed to determine an average particle size distribution.

4.6.5 Subsurface Analysis Using a Focused Ion Beam (FIB)

Subsurface analysis was completed using a FEI Helios G4 CX DualBeam high resolution Field Emission Gun Scanning Electron Microscopy (FEGSEM) with a precise 30 kV liquid gallium ion beam. The FIB was used to mill sections from metal samples to analyse the subsurface of samples after erosion and erosion-corrosion tests. A Pt layer was deposited onto the sample first to prevent damage at the top surface in the region analysed. Bulk milling of the sample using the ion beam was completed at an operating current 21 nA to a depth of approximately 15 μm and width of approximately 10 μm. The milled surface was cleaned and polished using the ion beam at an operating current of 0.79 nA. The FIB could also be used to etch the surface so that the microstructure could be analysed. The surface was etched at an operating current of 7.7 pA. SEM analysis of the surface was completed to image the surface using secondary emission at an operating voltage of 5 kV and 10 kV. Final imaging using the ion beam was completed at operating currents of 7.7 pA and 24 pA.

4.7 Using CFD to Predict Fluid Flow, Particle Trajectories and