# Effect of Thermo-Mechanical Processing on the Corrosion Behavior of Fe-30 Mn-5 Al-0.5C TWIP Steel
## 3 Results and Discussion
Table 1 summarizes the values for the \(E_{\text{corr}}\), \(E_{\text{pit}}\), \(i_{\text{pass}}\), \(E_{\text{prot}}\), and \(\Delta E=E_{\text{pit}}-E_{\text{prot}}\) electrochemical parameters obtained from the CPP curves in Figure 4. More noble \(E_{\text{corr}}\) values were exhibited by the recrystallized specimens than by the TMP specimens, with a small difference of 0.09 V for the recrystallized  
Figure 4: Cyclic potentiodynamic polarization CPP of TMP and recrystallized TWIP steels in deaerated 1.0 wt.% NaCl solution. specimen. Similarly, more positive \(E_{\text{pit}}\) values were shown for the recrystallized than for TMP specimens, thus indicating a higher resistance to pitting corrosion, which may be interpreted as a less protective passive layer formed on the TMP specimen than that of the recrystallized specimen. In general, both type of specimens (TMP and recrystallized) exhibited the ability to repassivate. The hysteresis loop (\(\Delta E\)), defined by \(\Delta E=E_{\text{pit}}-E_{\text{prot}}\), allows the evaluation of the susceptibility to localized corrosion [35]. Thus, smaller values of \(\Delta E\) mean higher resistance to pitting. Though, the differences of \(\Delta E\) for the TMP and recrystallized specimen were found to be 0.147 and 0.059 V\({}_{\text{VCE}}\), respectively. Figure 5 shows the IFM images of TMP TWIP steel (see Figure 5a,b) and recrystallized TWIP steel specimens (see Figure 5c,d) both after the CPP test in deaerated 1.0 wt.% NaCl solution. Figure 5a,b reveal a higher pit density for the TMP TWIP steel compared to the recrystallized TWIP steel sample [29]. The recrystallization of the grains of TMP TWIP, seen in Figure 3, decreased the locations for pits to nucleate, as well as the recrystallization made for a greater alumina layer which enhanced the corrosion protection of the samples (see Figure 5c,d). Despite the addition of the recrystallization process, a similar pit depth of \(\sim\)6 \(\upmu\)m was found on both specimens, while wider pits were shown on the TMP TWIP specimen, therefore a small pit aspect ratio is depicted for the recrystallized TWIP steel specimen. The high resolution XPS spectra of the TMP TWIP steel is presented in Figures 6 and 7 for Fe 2P\({}_{3/2}\) and Al 2p, respectively. Four peaks were assigned to iron, metallic Fe at 707.2 eV, Fe\({}^{2+}\) oxide (FeO) at 709.2 eV, Fe\({}^{3+}\) oxide (Fe\({}_{2}\)O\({}_{3}\)) at 711.2 eV, and Fe\({}^{3+}\) oxyhydroxide (\(\alpha\)-, \(\beta\)- or \(\gamma\)-FeOOH) at 713.3 eV \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \(E_{\text{corr}}\) & \(E_{\text{pit}}\) & \(E_{\text{prot}}\) & \(i_{\text{pass}}\) & \(\Delta E\) \\  & (**V\({}_{\text{VCE}}\)**) & (**V\({}_{\text{VCE}}\)**) & (**V\({}_{\text{VCE}}\)**) & (**u\(\Delta\)/cm\({}^{2}\)**) & (**V\({}_{\text{VCE}}\)**) \\ \hline TMP TWIP & \(-0.820\pm 0.052\) & \(-0.677\pm 0.056\) & \(-0.530\pm 0.042\) & \(9.44\pm 0.61\) & \(0.147\pm 0.011\) \\ Recrystallized TWIP & \(-0.811\pm 0.065\) & \(-0.596\pm 0.031\) & \(-0.537\pm 0.054\) & \(18.03\pm 0.97\) & \(0.059\pm 0.013\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrochemical parameters obtained from the CPP tests of TMP and recrystallized TWIP steels in deaerated 1.0 wt.% NaCl solution (see Figure 4). Figure 5: Infinite focus microscopy (IFM) images after CPP in deaerated 1.0 wt.% NaCl: (**a**) TMP TWIP sample, (**b**) TMP TWIP topography, (**c**) recrystallized TWIP, and (**d**) recrystallized TWIP topography. as observed in Figure 6[34; 38; 39]. Figure 7 shows high resolution XPS spectrum of TMP TWIP steel specimen for Al 2p containing two peaks assigned to metallic Al at 72.0 eV and Al\({}^{3+}\) oxide (Al\({}_{2}\)O\({}_{3}\)) at 74.5 eV. This Al\({}_{2}\)O\({}_{3}\) layer present in the surface is related to the passive behavior observed in the CPP tests performed.