## 3 Results
### Corrosion Resistance of the HTO-Treated 304/6061 Joint  
To investigate corrosion properties such as corrosion potential, potentiodynamic polarization tests were conducted, as shown in Figure 6. The corrosion potential (Ecorr) and corrosion current density (Icorr), which were obtained from the potentiodynamic polarization curves, are summarized in Table 5. The corrosion resistance of samples with HTO treatment was lower than that of samples without HTO treatment. Whether oxidized \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Point No.**} & \multicolumn{5}{c}{**at.\%**} & \multirow{2}{*}{**Remarks**} \\ \cline{2-2} \cline{4-6}  & **Al** & **Fe** & **Si** & **Cr** & **O** & \\ \hline
6 & 0.36 & 76.6 & 0.56 & 14.59 & 1.61 & spot scan \\
7 & 71.95 & 0.65 & 11.72 & 0.27 & 9.82 & spot scan \\
8 & 78.21 & - & - & - & 1.69 & spot scan \\
9 & 89.22 & 0.30 & 2.22 & - & 7.04 & spot scan \\
10 & 83.36 & 0.45 & 3.48 & - & 7.45 & spot scan \\ \hline \end{tabular}
\end{table} 
Table 4: _Cont._  
Figure 5: SEM cross-section images of the HTO-treated specimen (**(a–e)** with HTO treatment, **(f)** 6061 side of the weld joint without HTO treatment). at high temperature or not, the corrosion potential of the welding samples was situated between those of the two base materials. This result can be explained by the mix potential theory [15], where a new potential is determined by the intersection of the anodic reaction (6061) and cathodic reaction (304) of the unjoined metals, which is similar to the report by [PERSON] et al. [3]. The sequence of the corrosion current density was weld (HTO) >weld>6061 (HTO) >6061>304 (HTO) >304. The higher the corrosion current density is, the worse the corrosion resistance of the materials. Furthermore, from Figure 6 (the purple curve), it is seen that the anodic and cathodic processes for the weld sample are both under activation control. Nevertheless, a short active-passive transition for the weld (HTO) sample (the yellow curve), which is evidenced with a decrease in corrosion current density as potential increased from \(-0.8\) V to \(-0.5\) V vs. SCE, indicates the occurrence of a passivation process that is related to the formation of the protective layer [16; 17] on the surface of the weld (HTO) sample. The corrosion morphologies after polarization are shown in Figure 7. Some deep corrosion pits were observed in the base metals without HTO treatment as well as the base metals with HTO treatment corroded in a large area. There was intergranular corrosion (Figure 7b) in 304 stainless steel (HTO). This is because sensitization occurred in 304 stainless steels during oxidation treatment at 500 \({}^{\circ}\)C. Sensitization is a deleterious phenomenon for austenitic stainless steel when they are heated to a certain temperature around 500 and 800 \({}^{\circ}\)C [18]. During the exposure of austenitic stainless steel in this high temperature range, chromium carbides precipitate near the grain boundary, resulting in chromium deficiency [19; 20]. According to the EDS spot scanning results in Table 6, the Cr element in point 1 (10.63 at.% or 14.45 wt.%) of the severely corroded area is slightly lower than that in point 2 of the non-corroded area, and the Cr element content in point 2 (17.52 at.% or 18.03 wt.%) is similar to that of the base material (17.51 wt.%). Due to reduced Cr, the corrosion resistance of the grain boundaries was reduced. Then, intergranular corrosion will occur when they are exposed to corrosive media. Corrosion cracks were found in the  
Figure 6: Potentiodynamic polarization curves of the specimens in 3.5 wt.% NaCl solution. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Samples** & **Ecorr, V** & **Icorr, A/m\({}^{2}\times\) 10\({}^{-3}\)** \\ \hline
304 & 0.057 & 0.241 \\
304 (HTO) & \(-0.020\) & 1.07 \\
6061 & \(-0.447\) & 1.4 \\
6061 (HTO) & \(-0.946\) & 11.4 \\ weld & \(-0.341\) & 25.3 \\ weld (HTO) & \(-0.813\) & 34.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results of potentiodynamic polarization tests in 3.5 wt.% NaCl solution.