# Effect of Thermo-Mechanical Processing on the Corrosion Behavior of Fe-30 Mn-5 Al-0.5C TWIP Steel
## 3 Results and Discussion
High resolution XPS spectra for the recrystallized TWIP steel specimen were not included because no additional peaks were revealed, also showing four peaks for Fe 2p\({}_{3/2}\), and two peaks for Al 2p. Figure 6: High resolution XPS spectra for Fe 2p\({}_{3/2}\) of TMP TWIP steel. Figure 7: High resolution XPS spectra for Al 2p of TMP TWIP steel. A small peak was observed for Mn in both the TMP and recrystallized specimens. Thus, the deconvolution of the data could not be performed. Mn oxides are easily formed and dissolved preferentially at the exposed surface due to the lower stability of the Mn in the Fe\(-\)Mn based TWIP steel alloy containing Al [40]. The atomic percentage (at.%) of manganese analyzed was 0.5 and 0.4 at.% Mn for TMP and recrystallized TWIP steel specimens, respectively. The cationic fractions of Fe\({}_{\text{cat}}\), Al\({}_{\text{cat}}\), and Mn\({}_{\text{cat}}\) for Fe, Al, and Mn, respectively, were used to analyze the passive layer composition. The cationic fraction is determined by the relationship between a specific oxide and the addition of each contribution of the oxides that form the passive layer. For instance, the expression for Al would be: Al\({}_{\text{cat}}\) = Al\({}_{\text{ox}}\)/(Fe\({}_{\text{ox}}\) + Al\({}_{\text{ox}}\) + Mn\({}_{\text{ox}}\)), where Fe\({}_{\text{ox}}\), Al\({}_{\text{ox}}\), and Mn\({}_{\text{ox}}\) are the oxides of Fe, Al, and Mn, respectively. The cationic fractions for Fe, Al, and Mn elements for the TMP specimen were determined to be: 0.43, 0.49, and 0.08 for Al, Fe, and Mn, respectively. For the recrystallized TWIP steel specimen the cationic fractions were: 0.06, 0.91, and 0.03 for Fe, Al, and Mn, respectively. The passive layer of both the TMP and recrystallized TWIP steels is mainly composed of Fe and Al oxides. The content of the Al\({}_{2}\)O\({}_{3}\) in the passive layer increased up to 91% for the recrystallized TWIP, compared with 43% for TMP specimen. As indicated above, the passivating properties of Al alloying element are higher than Fe and Mn [26; 31; 41]. These results agree with the information supplied by Pourbaix diagram indicating that Al generates a protective and stable Al\({}_{2}\)O\({}_{3}\) layer in the pH range of 4-9 [42]. Therefore, this corroborates the electrochemical CPP results obtained in the present study. A significant difference in the Gibbs free energy (\(\,\)G\({}_{298}\)) at room temperature of the different oxides determines which element will be preferentially oxidized. In this case, the \(\,\)G\({}_{298}\) for the different oxides are: Al\({}_{2}\)O\({}_{3}\) (-1582.4 kJ/mol), Fe\({}_{2}\)O\({}_{3}\) (-742.0 kJ/mol), Fe\({}_{3}\)O\({}_{4}\) (-1015.5 kJ/mol), Mn\({}_{2}\)O\({}_{3}\) (-881.1 kJ/mol), and Mn\({}_{3}\)O\({}_{4}\) (-1283.2 kJ/mol). This indicates that an Al\({}_{2}\)O\({}_{3}\) enriched surface oxide layer may be present due to the preferential oxidation of Al [40; 43]. Interestingly, despite this enrichment of Al\({}^{3+}\) ions within the oxide layer, other ions were also found in large quantities (Fe\({}^{2+}\), Fe\({}^{3+}\), Mn\({}^{2+}\) and Mn\({}^{3+}\)), indicating that the passive layer should be considered as a mixture of Fe, Al and Mn oxides.