## 3 Results and Discussion
### Effect of DB and Heat Treatment on Corrosion Resistance of 304 SS  
The electrochemical performance of the 304 SS in the as-received and initially heat-treated states, before and after DB, was determined by open circuit potential (OCP) measurements, followed by potentiodynamic (PD) tests. The PD polarisation curves are depicted in Figure 19, and the electrochemical parameters are shown in Table 3.  
Figure 18: The effect of the steel’s initial state on the fatigue behaviour of the non-DB specimens (AR1 and HT7). The polarisation curves of all samples in the as-received state (Figure 19a) showed similar behaviour, with the formation of a pseudopassive region (from \(-500\) up to approximately \(500\) mV vs. Hg/HgSO\({}_{4}\)) in the anodic part and a posterior transpassive region (\(800\)-\(1200\) mV vs. Hg/HgSO\({}_{4}\)) around the same potentials. Similar behaviour was described in a previous report [33]. The subsequent increase in current density (corresponding to approximately \(\mathrm{E}=1300\) mV) may correlate to pitting potential. The current deflection after the transpassive region corresponds to oxygen evolution. The polarisation diagrams of the as-received specimens show that, in contrast to AR1 and AR3, the passivation current density (j\({}_{\mathrm{pass}}\)) of the AR2 specimen was reduced over two orders of magnitude. The pits on the surface of the AR2 DB sample were characterised by smaller sizes compared with those observed on the AR1 and AR3 sample surfaces (Figure 20). The smaller percentage content of \(\alpha^{\prime}-\)martense in the AR2 and AR5 samples after smoothing DB, and especially after subsequent heating at \(350\)\({}^{\circ}\)C for three hours (AR5), may reduce the corrosion current density and corrosion rate (CR) compared with the turned (AR1) and DB hardening (AR3) specimens. Additionally, the lower corrosion current and CR may be related to the greatest relaxation of the residual stresses (see Figure 15), which was introduced by smoothing DB after heating at \(350\)\({}^{\circ}\)C for three hours (AR5). The lack of residual stress relaxation in the martensitic phase introduced by hardening DB after heating at \(350\)\({}^{\circ}\)C (AR6 sample) resulted in increased values of jcorr, jpass, and CR in the AR6 sample. Figure 19: Representative anodic PD polarisation curves of the samples immersed in \(0.5\) M H\({}_{2}\)SO\({}_{4}\) at ambient temperature: (**a**) as-received initial state and (**b**) heat-treated initial state. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Initial State**} & \multirow{2}{*}{**Specimens**} & **Ess** & **Ecorr** & \multirow{2}{*}{**Jcorr**} & **Rp** & **CR** \\  & & **(mV vs.** & **(mV vs.** & **(mA\(\cdot\)cm\({}^{-2}\))** & **(f1)** & **(mm/yr)** \\  & & **Hg/HgSO\({}_{4}\))** & & & & \\ \hline \multirow{6}{*}{as-received} & AR 1 & \(-818\) & \(-808\) & \(0.392\) & \(318.9\) & \(0.0041\) \\  & AR 2 & \(-857\) & \(-827\) & \(0.311\) & \(338.7\) & \(0.0033\) \\  & AR 3 & \(-851\) & \(-827\) & \(0.518\) & \(238.2\) & \(0.0055\) \\  & AR 5 & \(-851\) & \(-833\) & \(0.098\) & \(1359\) & \(0.001\) \\  & AR 6 & \(-764\) & \(-770\) & \(1.814\) & \(156\) & \(0.019\) \\ \hline \multirow{6}{*}{heat treated} & HT 7 & \(-401\) & \(-432\) & \(1.103\) & \(229.4\) & \(0.0116\) \\  & HT 8 & \(-426\) & \(-456\) & \(1.620\) & \(290\) & \(0.0171\) \\ \cline{1-1}  & HT 9 & \(-389\) & \(-419\) & \(0.366\) & \(2190\) & \(0.0039\) \\ \cline{1-1}  & HT 11 & \(-22\) & \(-159\) & \(0.0004\) & \(411,000\) & \(0.000004\) \\ \cline{1-1}  & HT 12 & \(-773\) & \(-779\) & \(0.876\) & \(257\) & \(0.009\) \\ \hline \end{tabular}
\end{table} 
Table 3: Representative electrochemical parameters derived from the respective polarisation curves of the samples. tion curves, meaning that the passivation process was hindered. In addition, the HT7 and HT8 samples showed a high amount of pitting with larger sizes (Figure 20) on their surfaces, indicating the susceptibility of the passive film to localised damage.