# Effect of Thermo-Mechanical Processing on the Corrosion Behavior of Fe-30 Mn-5 Al-0.5C TWIP Steel
## 1 Introduction  
In order to stabilize the austenitic phase in a twinning-induced plasticity (TWIP) steel at room temperature (RT), a Mn content higher than 27 wt.% is required [1; 2; 3; 4]. The obtention of a stable carbide-free fully austenitic microstructure on TWIP steel with manganese contents below 25 wt.% Mn, requires a low-carbon content (<0.6 wt.% C), carbon is an interstitial element which acts as an austenite stabilizer [5]. However, increased C content promote the formation of carbides (Fe\({}_{3}\)C and Mn\({}_{3}\)C) [6; 7; 8]. The main alloying element in TWIP steels is considered to be Mn due to its crucial roles in preserving the austenitic structure as well as controlling the stacking fault energy (SFE) of the ternary Fe-Mn-Al alloy [9; 10]. Depending on the magnitude of the SFE, different phase transformation processes can be promoted. For instance, martensitic transformation is favored for SFE values below 12 mJ/m\({}^{2}\); predominant twinning and gliding dislocation mechanisms are found between 12 and 35 mJ/m\({}^{2}\); and above 35 mJ/m\({}^{2}\) dislocation glide is the dominant deformation mechanism [11; 12; 13]. With superior formability and excellent tensile strength, high-Mn TWIP steel offers improved mechanical properties through strain hardening. Consequently, the use of high-Mn TWIP steel is of paramount important to decrease structural components' weight, while improving the press forming behavior. These benefits are highly sought in the automotive industry for decreasing fuel consumption and reducing greenhouse gas emissions [14; 15]. High-Mn TWIP steels (15-30 wt.% Mn) present a wide range of applications, for instance, structural elements in the automotive industry. This is due to thehigh tensile strength (above 1 GPa) as well as a great tensile-strength-to-ductility ratio. Furthermore, TWIP steels present higher energy absorption properties (>55 kJ/kg), as compared to other advance high-strength steel (AHSS) grades such as transformation induced plasticity (TRIP) steel TRIP700, and also the dual phase (DP) steel DP600 [16; 17; 18; 19]. Due to their impact resistance, effectiveness, and low environmental impact, TWIP steels have become of major interest in the automotive industry. However, the corrosion of TWIP steels presents a significant hurdle for their practical use [20; 21; 22]. It has been reported that cold plasticity deformation of high-Mn TWIP steels enhances the susceptibility of corrosion [23; 24; 25]. Furthermore, the higher Mn content promotes the electrochemical activity of the alloy in chloride and acidic solutions [26]. An increase in the electrochemical dissolution kinetics of TWIP steel has been observed in acid media due to the presence of alkaline Mn oxide (MnO) in the corrosion products. This increased anodic dissolution is attributed to the presence of Mn\({}^{2+}\) cations in solution, and the subsequent hydrogen evolution reaction (HER) on the cathodic half-cell electrochemical reaction [27]. Increased Al content in high-Mn TWIP steel has been shown to improve corrosion performance, while additions of Mn reduce the corrosion resistance [28; 29]. Despite the more active standard reversible potential of Al than that of Mn, \(E_{\text{Al3+/Al}}=-1.66\) V\({}_{\text{SHE}}\) and \(E_{\text{Mn2+/Mn}}=-1.18\) V\({}_{\text{SHE}}\), respectively, where SHE is standard hydrogen electrode, the higher Mn content governs the anodic dissolution reaction on TWIP steels [30]. Al has been found to impart corrosion protection on TWIP steels by the formation of a stable aluminum oxide (Al\({}_{2}\)O\({}_{3}\)) passive surface layer [31]. Moreover, Al addition decreases the corrosion current density (\(i_{\text{corr}}\)) while increasing the corrosion potential (\(E_{\text{corr}}\)) towards nobler values, whereas increased Mn content produces the opposite effect [32]. Lastly, the corrosion resistance was shown experimentally to increase with Al additions and decrease with Mn additions by polarization curves of Fe\(-\)Mn\(-\)Al alloys in sodium hydroxide (NaOH) solution [26].