# Effect of Thermo-Mechanical Processing on the Corrosion Behavior of Fe-30 Mn-5 Al-0.5C TWIP Steel
## 3 Results and Discussion  
OM images of TMP and recrystallized TWIP steel samples are presented in Figure 2. Figure 2a shows the TMP specimen microstructure with oriented grains as a consequence of deformation during the hot-rolling process. In contrast, Figure 2b includes the recrystallized TWIP steel specimen showing a homogeneous \(\gamma\)-austenite monophasic microstructure with equiaxial grains and the presence of annealing twin formation on individual grains during recrystallization and grain growth processes. In addition to the annealing twin formation, an increase in the grain size of the recrystallized TWIP samples is seen, similar to what it can be seen in the literature at that temperature range [35; 36]. While the recrystallization decreases the dislocation density, the twin density is not affected as much, most of the twins remaining previously formed [35]. To further study the microstructure of the recrystallized specimen, SEM micrographs were taken. Figure 3 shows a typical austenitic monophasic microstructure for the recrystallized specimens, confirming the homogeneous \(\gamma\)-austenite monophasic microstructure with equiaxial grains and the presence of annealing twin formation on individual grains. Figure 3: SEM micrograph of the recrystallized Fe–30 Mn–5 Al–0.5C TWIP specimens. Figure 2: Optical microscopy (OM) micrographs: (**a**) thermo-mechanical processed (TMP), and (**b**) recrystallized Fe–30 Mn–5 Al–0.5C TWIP steel specimens. CPP curves for TMP and recrystallized TWIP steel specimens immersed in the deaerated 1.0 wt.% NaCl solution at RT are presented in Figure 4 to compare the electrochemical behavior. A passive-like behavior can be observed in both samples. The passive current density (\(i_{\text{pass}}\)), which indicates the passivity plateau, can be observed with values around 18.04 mA/cm\({}^{2}\) for both the TMP and recrystallized specimens. Furthermore, an increase in the pitting potential (\(E_{\text{pit}}\)) which is defined by an abrupt increase in the current density, can be observed. It should be noted that in the absence of oxygen in the deaerated 1.0 wt.% NaCl solution, the primary cathodic reaction is HER. If an activation-controlled kinetics is assumed, the behavior can be explained via an increase in the reversible potential for the HER (\(E_{\text{rev,H}}\)) due to a pH decrease in the solution as well as an increase in the catalytic properties of the surfaces towards the HER. Differences can be observed between the TMP and recrystallized specimens. The TMP TWIP steel showed higher values for the cathodic current density than the recrystallized specimen. Differences can also be observed in the anodic kinetics. For the TMP TWIP specimen, a small passivity plateau was drawn with an \(i_{\text{pass}}\) value of 9.44 mA/cm\({}^{2}\), this low \(i_{\text{pass}}\) may be related to the protective properties of the thin and stable Al oxide layer formed. This Al imparts passivity via the formation of an Al\({}_{2}\)O\({}_{3}\) layer [28]. Comparatively, Al displays a more active behavior than Mn, hindering the electrode reaction [28]. It should be noted that Al content not only increases the SFE, but also stabilizes the \(\gamma\)-austenite against phase transformation [6; 37]. After reaching the anodic current density limit (\(1.0\times 10^{-3}\) A/cm\({}^{2}\)), the polarization was reversed in the cathodic direction in order to assess the repassivation behavior of the high-Mn TWIP steels. This repassivation is reached once the recorded current density intersects the forward scan line, defining the protection potential (\(E_{\text{prot}}\)). This behavior was showed in both TWIP steel alloys studied after a positive hysteresis loop was observed. The larger this loop, the higher the resistance to repassivation of the alloy and, thus, greater susceptibility to local corrosion is observed.