# Dynamic Corrosion Test Using LiNO3 Containing Molten Salt for CSP Applications
## 2 Methodology  
Molten salts mixtures used for this research are KNO\({}_{3}\), NaNO\({}_{3}\) (SQM refined 99%, Antofagasta, Chile) and LiNO\({}_{3}\) (Todini 99%, Milan, Italy). Molten salts were dried in an oven at 100 \({}^{\circ}\)C for 1 h before the preparation of the ternary salt and a stepwise heating was applied to reduce the water content and obtain a homogeneous mixture. Three samples of carbon steel A516 were prepared from bulk materials with dimensions of \(25\times 10\times 2\) mm, providing a hole for hanging in the sample holder. The samples were polished using abrasive papers (SiC) with different granulometry (from #320 to #1200) to remove any existing oxide layer before the corrosion test. As it was mentioned before, A516 was the selected steel since it presents a suitable commercial solution for CSP plants and it has been used in parabolic trough solar plants at storage temperatures below 400 \({}^{\circ}\)C. Its composition is showed in Table 3.  
An autoclave of 4 L of volume was employed to test the corrosion induced by the salts under dynamic conditions. The autoclave components in contact with the molten salt, as well as the stirrer and the thermocouple sheath were made of Hastelloy C276. The stirrer was placed at the end of a hollow tube, employed to cool the stirrer down with water during operation. An amount of 2.7 kg of the lithium nitrate mixture was charged on the autoclave (Figure 1 left) and A516 coupons were placed concentrically to the mixer on a rack specially designed to this experiment (Figure 1, right). With this configuration, the molten salt flowed over the specimens at the same speed. The angular velocity of the mixer was 1250 rpm, which is equivalent to a lineal speed of 0.30 m/s. The temperature was set up at 150 \({}^{\circ}\)C for two hours to achieve the complete melting of the salt and then it was increased slowly until the testing temperature of 390 \({}^{\circ}\)C was reached. Figure 1: Automelave filled with the lithium ternary molten salt (**left**) and detail of the rack with alloys specimens and the mixer (**right**). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Alloy** & \multicolumn{6}{c}{**Weight (\%)**} \\ \hline \multirow{3}{*}{A516} & Si & Mn & P & C & S & Fe \\ \cline{2-7}  & 0.25 & 1.1 & 0.025 & 0.29 & 0.03 & Balance \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemical composition of carbon steel A516. Test coupons were removed from the molten salt at 200, 600 and 1000 h, cooled slowly and cleaned with hot distilled water. The formula (Equation (1)) used to calculate the mass gain over time is:  
\[\frac{\Delta m}{S_{0}}=\frac{m_{f}-m_{i}}{S_{0}} \tag{1}\]  
where \(m_{i}\) is the initial mass of the specimen, \(m_{f}\) is the mass of the same at time t, and \(S_{0}\) is the initial area of the specimen. Coupons were weighed in a precision balance (\(\pm\)0.0001 g) AG135 from Mettler-Toledo. Morphology and elemental composition of the exposed surfaces and their cross sections were inspected by SEM, model JEOL JSM-7500F, operated in high vacuum mode at 20 keV equipped with a backscattered electron detector, used for the microstructural analysis, and coupled to energy dispersive X-ray spectrometer (EDS) analyzer with a recommended working distance of around 10 mm. The top surfaces were inspected without further surface manipulation. For cross section studies, coupons were mounted in a polymer resin prepared by mixing Remet Hardrock 554 resin and the hardener catalyst, in 2:1 proportion. After the resin cured (24 h at room temperature), the samples were grinded and polished using SiC for a proper preparation. Crystallographic phase composition was determined by X-Ray diffraction using a PANalytical X'Pert PRO MDP and powder diffraction database. For XRD analysis, 6/20 Bragg-Brentano powder diffractometer of 240 millimeters of radius was used with a copper anode with k\(\alpha\) radiation of \(\lambda\) = 1.5418 A.