Diamond Light Source Synchrotron Radiation

The in-situ SR-XRD experiments detailed in Chapter 6, 7, 8 and 9 were conducted using the powder diffraction beamline (I15) at Diamond Light Source Synchrotron Facility (DLSSF) (Beamline I15) in Oxfordshire, United Kingdom over a total period of 3 days, see Figure 6.16 and Figure 6.17. The electrochemical flow cell previously outlined was designed to allow in-situ SR- XRD patterns to be recorded in real time while the sample was under electrochemical control. Therefore, relative kinetic information regarding the

formation and growth of corrosion products can be obtained without interrupting the electrochemistry, and then can be correlated with any changes observed via the electrochemical measurements. The SR-XRD technique allowed crystalline phases that were present to be determined as well as providing crystallographic information during the growth of the phases present.

Figure 6.16. Birds eye view of the Diamond Light Source Synchrotron Facility, Harlow, Oxfordshire, United Kingdom [189].

The Diamond Light Source (DLS) is a synchrotron; a huge scientific machine, half a kilometre in circumference, designed to produce very intense beams of X-rays, infrared and ultraviolet light. For centuries, scientists have used microscopes to study things that are too small to see with the naked eye. However, microscopes are limited by the visible light that they use. Optical microscopes can be used to study objects that are a few microns (0.001mm) in size, about the size of cells. However, to study smaller objects like molecules and atoms, scientists need to use the special light generated by the synchrotron.

A schematic of the DLS is shown in Figure 6.17. As a third generation synchrotron, the DLS is comprised of several components including the Electron Gun (1), Linear Accelerator (2), Booster Ring (3), and Storage Ring (4). Each of these sections contributes to producing a beam of synchrotron

light (5), which is then harnessed in a beamline (6), using an optics hutch (7), experimental hutch (8) and work stations (9).

Electron Gun: Bursts of electrons are injected into an ultra-high vacuum stainless steel tube. The energy of the electron beam is 0.22 MeV (million- electron volts). They are then fired out into the machine, where they are accelerated up to very high speeds through a series of three particle accelerators. These are called the linear accelerator (known as linac), the booster synchrotron and the large storage ring. The linac uses microwave energy to increase the energy of the electrons to 250 MeV. The electron beam is then transferred into the booster ring where the microwaves further accelerate the electrons to nearly the speed of light (increasing the energy from 250 MeV to 2900 MeV). The electron beam is transferred into the storage ring where insertion devices called wigglers and undalators can bend the beam many times over very short distances. The storage ring is what gives Diamond its iconic doughnut shape with a huge circumference (half a kilometre). The storage ring is, made of 48 straight sections angled together with 48 bending magnets, and this magnetism is used to steer the electrons around the ring. Third generation synchrotrons like Diamond also use special arrays of magnets called insertion devices.

When high-speed, high-energy electrons are accelerated, or their path is bent passing through powerful magnetic fields, a natural phenomenon occurs to produce an extremely brilliant, full spectrum beam of photons known as synchrotron light which can be 10 billion times brighter than the sun. When the path of the electron beam is bent by Diamond’s powerful magnets, the electrons lose energy in the form of light. Beams of synchrotron light are manipulated and channelled out of the storage ring and into the experimental stations, called beamlines.

It is inside these beamlines that scientists carry out their experiments. The beamline contains three different sections; the optics hutch, where the full spectrum beam of synchrotron light is segmented into portions of electromagnetic spectrum by equipment such as monochromators, then focused with specially curved mirror systems; the experimental hutch, where the selected wavelength of synchrotron light is directed onto the sample to and the experiment is carried out; and the control/work station, where the scientists control the experiment and the data is transferred to computers for storage and analysis. The beams of light are so strong that, in the case of

Diamond’s X-ray beamlines, it is not safe to be in the same room whilst they are being directed at the sample.

Figure 6.17. Diamond Light Source Synchrotron and I15 Beamline schematic (modified from [189]).

6.4 In-situ SR-XRD Flow Cell Synchrotron Experimental

Procedure

To fully utilise the opportunity of having access to the Diamond Light Source synchrotron facility, a strategic test methodology was devised and the testing methodology is summarised in Figure 6.18 before the beam-time commenced.

Figure 6.18. Summary of Synchrotron test methodology at the Diamond Light Source Synchrotron Facility.

The main aim of the synchrotron test methodology was to improve the understanding of the nature and kinetics of corrosion product formation in CO2 corrosion of carbon steel. A range of short term tests were completed over the 3 days which gave an insight into the very early stages of FeCO3 formation under a range of different parameters and will be the focus of the remaining chapters in this work.