## 3 Results
### Corrosion Resistance of the HTO-Treated 304/6061 Joint
6061 alloy (Figure 7d). Figure 7e,f show the corrosion morphology of the interface after polarization of the weld joint, while Figure 7g,h show the corrosion morphology of the 6061 side of the weld after polarization. Obvious galvanic corrosion was found in the 304/6061 joints (whether HTO treated or not). The 6061 aluminum alloy and weld were corroded as anodes, while 304 stainless steel was protected as cathodes. In addition, the corrosion of the sample after high-temperature oxidation was more serious than that of the weld joint sample without high-temperature oxidation. The corrosion phenomenon is similar to that of a single base metal; there were some local corrosion pits on the welded joint sample without the HTO film (Figure 7g), while large corrosion areas and local corrosion pits were found on the welded joint sample with the HTO film (Figure 7h). The above phenomena indicate that the HTO treatment will reduce the corrosion resistance or even accelerate the corrosion of the base metals and weld joints. Because there were tiny cracks in the oxide film (Figure 5a,e), the corrosion solution was immersed in it and reacted with the base metal to corrode the metal. The composition of the base metals treated by HTO was measured by an EDS point scan and is listed in Table 6. Point 1 has an atomic composition of 39.23% Fe, 10.63% Cr and 41.04% O, while point 2 has an atomic composition of 62.66% Fe, 17.52% Cr and 11.43% O. The results of point 4 show that it is an aluminum matrix, and the results of points 3, 5 and 6 show that it is aluminum oxide. From the results of the EDS point scan, it is confirmed that the corrosion products of the 304 stainless steel base metal are mainly iron and chromium oxides, while the corrosion products of the aluminum alloy base metal are mainly aluminum oxides. Variations in the aluminum, iron and oxygen contents at the interface of the weld joint were measured by an EDS line scan along the yellow line marked in Figure 7e,f, as shown in Figure 8. This change indicates that the oxygen element distribution on the aluminum side of the weld treated by HTO is uneven, and its fluctuation is larger than that of the sample without HTO treatment. The uniformity of the oxide distribution is related to the corrosion resistance of the sample; the more uneven its distribution is, the worse its corrosion resistance is. A total of 360,000 data points were obtained through electrochemical noise measurements both in current and potential vs. time. Figure 9 displays the time dependence of the galvanic potential (a) and current density (b) for the 304-6061 and 304-6061 (HTO) couples in 3.5 wt.%NaCl solution. From these plots, both the galvanic potential and current density of the 304-6061 couple oscillate sharply; the maximum amplitude for potential is 0.6 V, while that for current density is 0.32 \(\times\) 10\({}^{-6}\) A/cm\({}^{2}\). The galvanic potential of the 304-6061 (HTO) couple has no obvious fluctuation, except that the potential fluctuation amplitude is approximately 0.01 V between 24,374 s and 31,254 s. High frequency fluctuation is observed for the galvanic current density of the 304-6061 (HTO) couple after 30,000 s and then trends toward stable. From the above analysis, it can be inferred that the galvanic corrosion of the 304-6061 couple was more active than that of the 304-6061 (HTO) couple. The average galvanic current density of the 304-6061 couple (1.571 \(\times\) 10\({}^{-5}\) A/cm\({}^{2}\)) is slightly higher than that of the 304-6061 (HTO) couple (1.044 \(\times\) 10\({}^{-5}\) A/cm\({}^{2}\)), which means that the samples after HTO treatment have a lower tendency of galvanic corrosion. However, according to the data analysis after polarization corrosion (Figures 6 and 7), the samples with HTO treatment have more serious corrosion, which may be due to the following two reasons: on the one hand, the HTO treatment is carried out in air, and the oxygen content is not sufficient, resulting in an uneven oxide film (Figure 3).