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2. Literature review

2.6. Synchrotron Sources

2.6.1. What is a synchrotron?

A synchrotron is an electron accelerator that is used as a very high quality X-ray source, providing both excellent flux and consistency compared to normal laboratory sources [164] (Figure 2-7).

Figure 2-7 Schematic diagram of a third-generation synchrotron [164].

Electrons are generated in a vacuum at an “electron gun” using a process known as “thermiomic emission”[165] that applies an anodic voltage across a cathodic heater, propelling electrons in the synchrotron. Electrons are then fed into a Linear

Accelerator (LinAc) to an energy of ~100 MeV. The electrons then enter a booster ring where they are further accelerated before entering the storage ring.

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The storage ring is not a “ring” but a polygon that uses bending magnets to deflect the electrons. The storage ring is designed to have a constant current, and is topped up from the boosting periodically as electrons are lost by use or hitting residual gas in the vacuum. Electrons are accessed by “insertion devices” that are used to generate X-rays.

Insertion devices have arrays of magnets lined up with opposite poles. As the electrons move passed these arrays of magnets, they begin to “wiggle” and give off energy in the form of X-radiation as they continually change direction. As the electrons are travelling at relativistic speeds, causes the radiation to be directed forwards, creating an X-ray beam of high flux, brightness (intensity per unit area of source) and spectral brightness (intensity per unit area per unit solid angle per unit energy bandwidth).

The beam that comes out of the insertion device is known as “pink beam” as it has a wide range of wavelengths. It is possible to refine and select the wavelength of the radiation in the “optics” hutch, before experimental use, by using a monochromator. Monochromators use two [166] or four [167] Si (111) crystals to select energy by serial diffraction. By aligning the crystal at specific angles, it is possible to select a single wavelength of radiation by using Bragg’s law [168].

2.6.2. X-ray Diffraction on corrosion systems

X-ray diffraction allows the identification of crystalline phases by their diffraction patterns. Crystalline structures are arranged in such a way that their atoms form sheets or layers with fixed spacing. When X-rays interact with these layers, they give

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constructive interference and diffract in discrete patterns, which are determined by Bragg’s law:

𝑛𝑖𝜆 = 2𝑑𝑠𝑖𝑛𝜃 Equation 2-16

Where ni is any integer, λ is the radiation wavelength, d is the layer spacing, and θ is the angle of incidence.

Most salt layers are crystalline (some are molten), and synchrotron X-ray diffraction has been used to observe these salts in situ. Rayment et al. [7] has used artificial pits of Fe and 316L in 1 M HCl to show a FeCl2.4H2O salt exists during dissolution. Information was also able to be found about the morphology of the salt crystals in the pit, with 316L showing significantly more spottiness in the collected diffraction pattern indicating a coarsening of crystal size. Xu [6, 69] also use a similar technique to compare salt layers of Fe in NaCl with and without nitrate additions, finding that nitrate increases anisotropy of salt crystals. Other groups have investigated salt layers on copper [169] and scale in carbon steels [170].

2.6.3. Radiography

X-ray Radiography can be used to view changes in morphology in situ. The X-ray beam can pass through a sample and be attenuated, i.e. lose flux due to absorption or deflection. The amount of flux lost due to attenuation is usually quantified in terms of a ratio of the original beam:

𝐼𝑛𝑡

𝐼𝑛𝑡0 = exp (−𝜇𝑥𝑡) Equation 2-17

Where Int is the transmitted beam intensity, Int0 is the original beam intensity, μ is the absorption coefficient of the material and xt is the thickness of the material. The

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transmitted beam hits a detector, either an undeveloped film or a fluorescent screen in more modern instances, and produces a two-dimensional image.

In its raw state, this image may not accurately represent the experiment as it contains distortions due to changes in the beam (flat field) and heterogeneities in the detector (dark field). Radiographs are typically corrected after collection by subtracting any contribution of the dark field and dividing through any contribution of the white field. The flat field is usually an average of several images of the x-ray beam without a sample present.

𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 𝑑𝑎𝑡𝑎 =𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 𝑑𝑎𝑡𝑎−𝑑𝑎𝑟𝑘 𝑓𝑖𝑒𝑙𝑑

𝑓𝑙𝑎𝑡 𝑓𝑖𝑒𝑙𝑑−𝑑𝑎𝑟𝑘 𝑓𝑖𝑒𝑙𝑑 Equation 2-18

Radiography has been used to investigate the growth of artificial pits in situ, with Ghahari et al.[8, 10, 171] investigating the dissolution kinetics of 304L to inform their kinetics model. Hammons et al. [172] has also used the technique on Ni to characterise consistency in the dissolution of the interface during corrosion.

2.6.4. Effect of X-radiation on electrochemical data

Xu [58] has previous addressed the paucity of work done on the influence of a high flux of radiation on corrosion processes. Nagy and You [173] have reported on the extent of radiolysis found in in situ synchrotron experiments involving solutions. Radiolysis will always occur, with the generation of H2 and H2O2 as a consequence. Particular concern is in cells with narrow geometries, as most radiolysed solution will stay at the working interface. The effect is broadly mitigated by attenuating the radiation beam by use of Al filters [7].

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