## 3 Results
### Corrosion analysis of the container  
After the test, the containers were cut to see whether there had been corrosion in the container material, stainless steel, that could have damaged it.  
As shown in Figure 10, a greenish coloration appeared on the PCM's surface, which may be due to a degradation of the stainless-steel container forming chromium oxide.  
Figure 9: PCM work scheme.  
To verify this, a micrographic analysis was carried out. This analysis consists of the container cutting and its embedding in a resin for later observation in an optical microscope (NIKON brand, EPIPHOT model). Figure 11 shows a micrograph of the oxide layer present in the AISI 304 container after the muffle tests. It presents a 100 \(\upmu\)m-thick, non-homogeneous and without continuity layer, which is detached in fragments. This indicates that the material used as a container (AISI 304 stainless steel) has been affected by its contact with the PCM (lithium carbonate).  
Given the importance of selecting a container material resistant to contact with carbonates at high temperature, a corrosion test was run for two stainless steels, AISI 304 and AISI 310, whose chemical compositions are listed in Table 2. The test consisted of immersing the stainless steel in Li\({}_{2}\)CO\({}_{3}\) at 800 \({}^{\circ}\)C.  
Figure 11: Micrograph corresponding to cross section of the container after tests: oxide layer.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Type of Stainless Steel** & **Cr** & **Ni** & **Si** & **C** & **Mo** & **Mn** & **S** & **P** & **Fe** \\ \hline
**304** & 18-20 & 8-11 & \textless{1} & \textless{1} & \textasci{1} & \textasci{1} & \textasci{1} & \textasci{1} & \textasci{1} \\ \hline \hline
310 & 24-26 & 19-22 & \textless{1.75} & \textless{0.25} & - & \textless{2} & \textless{0.03} & \textless{0.45} & balance \\ \hline \end{tabular} Specimens of 80 mm \(\times\) 15 mm in metal sheet (AISI 304 and AISI 310, respectively) were prepared. These were immersed for 240 h at 800 \({}^{\circ}\)C in Li\({}_{2}\)CO\({}_{3}\), Figure 12, after which they were evaluated under an optical microscope.  
\end{table}
Table 2: Chemical composition of stainless steels (wt%) [19].  
Figure 10: Cutting of the sample after testing.  
The thickness, continuity and homogeneity of the oxide layer were the parameters evaluated in each sample.  
Figures 13 and 14 show the micrographs taken in the optical microscope of the oxide layers formed in the specimens of AISI 304 and AISI 310.  
Figure 14: Oxide layer in AISI 310 sample.  
Figure 12: (a) Samples immersed in Li\({}_{2}\)CO\({}_{3}\) at 800 \({}^{\circ}\)C in oven; (b) sample before testing.  
Figure 13: (a) Heterogenous thickness of the oxide layer in AISI 304 sample. (b) Lack of continuity; oxide layer detached in some areas.  
Table 3 shows the percentages in atomic weight of the main elements of the oxide layer present in both steels.  
As can be seen, the percentages of Cr and Ni in the AISI 310 oxide layer are greater than those of the AISI 304. These high percentages of Cr and Ni favor the homogeneity and continuity of the oxide layer, avoiding its detachment and improving the steel's protection against high-temperature corrosion [20,21]. The technique used does not allow us to analyze the content of Li due to its low atomic weight. However, this element is also present in the oxide layer, as noted by other authors [19].