## 3 Results and Discussion
### Effect of DB and Heat Treatment on Corrosion Resistance of 304 SS
These observations suggest a negative influence of these treatments on the surface corrosion resistance of 304 SS. Compared with the as-received state, the initially heat-treated 304 SS had a coarse-grained structure, which may explain this impaired corrosion behaviour. The extent of localised corrosion was relatively less for the hardening DB sample (HT9). The parallel straight bands that can be seen in the corroded surface display distinctive convex geometry in the background of an almost flat surrounding area, with small amounts of pitting (Figure 20). The former effect is reflected by the higher values of the polarisation resistance (\(\mathrm{R_{p}}\)), as shown in Table 3, suggesting enhanced general corrosion resistance. According to [PERSON] and [PERSON] (2014), higher compressive residual stresses can decrease the passive current density of the 2024-T3 AA alloy in 1 M NaCl. In the present study, the significantly higher compressive stresses in austenite and lower stresses introduced into martensite for the hardening DB sample (HT9) may have accelerated the reduction in \(\mathrm{j_{corr}}\) and \(\mathrm{j_{pass}}\). Although it is believed that the increased surface roughness made the austenitic steel surface more susceptible to both general and pitting corrosion ([PERSON] and [PERSON], 2015), in the present study, the HT9 sample subjected to hardening DB exhibited an anodic shift in \(\mathrm{E_{corr}}\) and a decrease \(\mathrm{j_{corr}}\) when compared with HT8. It is not possible to relate the increase in the surface roughness of HT9 to the higher corrosion resistance compared with HT8. The results indicate that, for the initially heat-treated DB samples, the corrosion resistance of the surface greatly improved after subsequent heating at 350 \({}^{\circ}\)C for three hours. During heating, the diffusion of Cr favoured the rapid formation of a Cr-rich protective film with superior corrosion resistance ([PERSON] and [PERSON], 2015). However, the HT12 sample exhibited a cathodic shift \(\mathrm{E_{corr}}\), although \(\mathrm{j_{pass}}\) was reduced compared with the HT7 sample. Therefore, the improved surface passivity observed after DB and heating at 350 \({}^{\circ}\)C cannot be connected with high-temperature oxidation alone. The small amount of \(\alpha^{\prime}-\)martensite after smoothing DB, the lowest surface roughness, and the highest residual stress relaxation in the austenite phase after the subsequent heating at 350 \({}^{\circ}\)C may explain the low CR for the HT11 specimen. Some authors claim that the cathodic shift \(\mathrm{E_{corr}}\), similar to that seen in the HT12 specimen, is due to the higher fraction of martensite ([PERSON] and [PERSON], 2015). The severe surface plastic deformation (HT12) not only induces martensite transformation, but also causes defects, strains, and residual stresses that influence the CR. Similarly to the as-received state, the lack of residual stress relaxation in the martensite and the formation of tempered \(\alpha^{\prime}-\)martensite with high dislocation density and dispersed precipitates may provide a large number of active sites that promote the CR in the HT12 sample (Table 3). For sample HT9, a low value of CR (second in rank for the initially heat-treated state) was obtained, regardless of the high percentage content of the volume fraction of \(\alpha^{\prime}-\)martensite, the relatively high surface roughness, and the significant residual compressive stresses. It follows that clearly expressed correlation of the volume fraction of the \(\alpha^{\prime}-\)martensite phase, surface roughness, and residual stresses of the samples on the corrosion behaviour of DB 304 SS specimens in 0.5 M \(\mathrm{H_{2}SO_{4}}\) was not observed. When the goal is to maximise the surface corrosion resistance, it is appropriate to combine single-pass smoothing DB and subsequent heating at 350 \({}^{\circ}\)C for three hours. For lowering the CR, the smoothing DB was significantly more effective for the initially heat-treated state compared with the as-received material.