# Design of the CIEMAT Corrosion Loop for Liquid Metal Experiments
## 2 Corrosion Issue in a Lithium Lead Loop  
The use of lead lithium eutectic alloy as the tritium breeder/carrier provides an attractive option for fusion reactors because of its adequate tritium breeding rate with a reduced lithium inventory thanks to the lead acting as the neutron multiplier and the efficient heat-transfer characteristics. The use of this alloy (PbLi) requires an assessment of its compatibility with structural materials, mainly focusing on the corrosion, mass transfer, and degradation of the mechanical properties of the structural material.  
The principal processes that contribute to the corrosion of materials by liquid metals are dissolution, alloying, intergranular penetration, impurity transfer from or to the liquid metal, and the thermal and concentration gradient for mass transfer. The whole process can bring about a significant wall-thinning wastage and the deposition of corrosion products, which can result in a severe flow change and an accumulation of radioactive material in unshielded regions. The direct consequence is an increase in the pumping power requirement, a decrease in energy conversion efficiency, and the need for extra shielding in the radionuclide accumulation areas. For all these reasons, it is extremely important to have precise control over the evolution of structural material deterioration.  
This variety of possible corrosion phenomena depends on many variables, which include [10]:  
1. the melt itself (the impurity content and the composition of the impurities as well as the lithium content of the liquid metal alloy);
2. the microstructure of the material in contact with lithium lead;
3. the exposure time [33];
4. the flow velocity and profile;
5. the difference in temperature across the loop (\(\Delta\)T in some specific part of the loop, such as cold traps or pump channels);
6. the Pheli temperature profile.  
Although these variables are extremely important for corrosion control, some of them are the result of special interests. This is the case for the flow velocity, which provides the mass transfer of the structural material, and the temperature, considering that thermal-gradient mass transfer occurs in non-isothermal systems because of the temperature-dependence of solubility [9]. The mass transfer rate of an element \(i\) at a determined temperature can be described as:  
\[R_{i}=k(C_{\rm s}-C_{i}) \tag{3}\]  
where \(k\) is the rate constant of the rate controlling dissolution, \(C_{s}\) is the solubility of the element \(i\) in the liquid metal and depends exponentially on temperature, and \(C_{i}\) is the concentration of \(i\) in the liquid metal, which is approximately constant in the liquid metal loop. The magnitudes of \(C_{s}\) and \(C_{i}\) determine whether there is dissolution (\(R_{i}>0\)) or deposition (\(R_{i}<0\)), and the kinetics of the dissolution and deposition reactions represented by the \(k\) factor can deeply influence the overall rate of transfer. The \(k\) factor for corrosion is strictly related to the chemical potential, which can be expressed as the tendency of a component to escape from a certain phase; a higher chemical potential implies a greater tendency of a component to leave its phase (e.g., W \(<\) \(<\) Fe, Cr \(<\) Ni) [6].  
Finally, the flow velocity is strictly related to the liquid metal composition, which must be deeply characterized before and after every experimental campaign. A liquid metal used as tritium breeding and as a coolant requires higher velocity, so the final design of the reactor has a direct relationship with the PbLi dynamics inside the breeder module. Even if the magnitude of the corrosion effects depends on the specific reactor design, the mechanisms of mass transfer will be the same.  
Starting from these assumptions, a thermal-convection pumped loop, CiCLo-C, able to achieve a velocity up to 1 m/s and a temperature of 550 \({}^{\circ}\)C temperature in the test section, was implemented with the aim of providing important results for all lead lithium BB designs.