## 3 Results and Discussion
### SEM and FTIR Observation  
Based on Table 3, the maximum inhibition efficiency of the SM solution for each zone is WM 0.4 g/L, HAZ 0.2 g/L, and BM 0.2 g/L. An immersion test was carried out between the specimens with the SM solution and without the SM solution, which aims to obtain an overview of the surface morphology of each specimen. The surface morphology of the specimens after 5 days of immersion are shown in Figure 12. On the metal surface, several cracks and deposits occurred after being immersed in HCl and HCl + SM. However, the layer of sediment formed in HCl + SM is more even than when immersed in HCl (arrows indicate the areas with minimal sediment). This observation shows how the inhibitory effect of the SM solution restrains the corrosion process. Figure 13 depicts the FTIR spectra of the SM [60] and all the specimens after 5 days of immersion in 1 M HCl and 1 M HCl + SM. Different bonds appeared in the spectra of the weldment specimens when compared to the original SM spectra. There was a peak of about 1630 cm\({}^{-1}\) after immersion in HCl, showing a bending vibration of O-H, and weak absorption around 680 cm\({}^{-1}\), indicating a link with Fe (hematite). The intensity of these two peaks decreased after immersion in HCl + SM, indicating the development of hematite groups (stable iron oxide, Fe\({}_{2}\)O\({}_{3}\)) rather than OH bonds (iron hydroxide). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Inhibitor Concentration (g/L)** & **WM** & **HAZ** & **BM** \\ \hline
0 & 5.05 \(\pm\) 0.26 & 2.44 \(\pm\) 0.12 & 2.81 \(\pm\) 0.14 \\ \hline
0.1 & 3.09 \(\pm\) 0.16 & 2.13 \(\pm\) 0.11 & 2.27 \(\pm\) 0.11 \\ \hline
0.2 & 3.06 \(\pm\) 0.15 & 1.58 \(\pm\) 0.08 & 1.38 \(\pm\) 0.07 \\ \hline
0.3 & 3.08 \(\pm\) 0.16 & 2.15 \(\pm\) 0.11 & 2.57 \(\pm\) 0.13 \\ \hline
0.4 & 2.47 \(\pm\) 0.12 & 2.30 \(\pm\) 0.16 & 2.63 \(\pm\) 0.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The measured volume of hydrogen gas that evolved during PP test (\(\times 10^{-4}\) mL/mm\({}^{2}\).s). Figure 11: Hydrogen evolution rate and number of bubbles detected compared with corrosion rate of (a) WM, (b) HAZ, and (c) BM for different concentrations of sodium molybdate. The examination through an EDS analysis demonstrated a rise in the oxygen content as shown in Table 7. This suggests the development of certain oxides that have enveloped the surface of the metal, potentially in the form of Fe(OH)\({}_{2}\)[7]. The hardness test conducted on the three zones of the weldment indicates that the BM zone exhibits the highest level of hardness. This can be attributed to a microstructure \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Specimen** & \multicolumn{5}{c}{**Element (wt \%)**} \\ \cline{2-7}  & **C** & **O** & **Si** & **Cl** & **Mn** & **Fe** \\ \hline WM in HCl & 2.31 & 1.15 & 0.55 & 0.20 & 0.94 & 94.85 \\ WM in HCl + SM & 2.40 & 30.07 & 1.57 & 0.16 & 1.03 & 43.1 \\ HAZ in HCl & 2.28 & 1.16 & 0.56 & 0.22 & 0.75 & 95.02 \\ HAZ in HCl + SM & 2.34 & 25.62 & 1.94 & 0.20 & 1.45 & 56.76 \\ BM in HCl & 2.68 & 1.02 & 1.00 & 1.88 & 1.03 & 92.40 \\ BM in HCl + SM & 2.30 & 21.79 & 1.99 & 0.98 & 1.29 & 64.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Detection of elements on the specimen surface by using EDS. Figure 12: SEM photograph of metal weldment after being immersed for 5 days in 1 M HCl and 1 M HCl + Sodium Molybdate: (**a**) WM, (**b**) HAZ, and (**c**) BM. Figure 13: FTIR spectra of (**a**) WM, (**b**) HAZ, and (**c**) BM zone after being immersed for 5 days in 1 M HCl and 1 M HCl + sodium molybdate (0.4 g/L for WM, and 0.2 g/L for HAZ and BM). characterized by a smaller grain size, which leads to a bigger volume of grain boundaries [17]. The microstructure of the HAZ consists of acicular ferrite and a fine-coarse grain structure. Meanwhile, the primary microstructure of the WM zone is predominantly determined by the grain boundary of ferrite (as depicted in Figure 5). This is associated with a low cooling rate and the process of ferrite formation facilitated by carbon diffusion.