# Effect of Thermo-Mechanical Processing on the Corrosion Behavior of Fe-30 Mn-5 Al-0.5C TWIP Steel
## 2 Materials and Methods  
The elemental composition of as-cast high-Mn TWIP steel ingots containing 30 wt.% Mn, 5 wt.% Al, 0.5 wt.% C, 0.01 wt.% S, \(<\)0.01 wt.% P, and balance Fe, was determined by X-ray fluorescence (XRF) using a S8 Tiger (Bruker, Middleton, WI, USA) wavelength dispersive spectrometer. Specimens were obtained from ingots of TWIP steel prepared in a high temperature furnace using an alumina crucible and argon as a protective gas environment. The main constituents of the alloy are Fe and Mn, while Al and C are the main alloying elements. Figure 1 shows a typical microstructure of as-cast specimens with dendrites and interdendritic spaces of a solidification microstructure. The ingots were subjected to a thermo-mechanical processing (TMP), which consists of a hot-rolling process at 1100 \({}^{\circ}\)C. Strips with a final thickness of 2.5 mm were obtained. Subsequently, the strips were subjected to a cold-rolling process to decrease the thickness to 1.5 mm. Followed by recrystallization at 900 \({}^{\circ}\)C for 30 min, obtaining a microstructure characterized by equiaxial grains of \(\sim\)20 \(\upmu\)m in diameter. For the cyclic potentiodynamic polarization (CPP) test, the specimens were mounted in epoxy resin and the surface was prepared with silicon carbide (SiC) paper, grinding up to 1200 grade, then cleaned with ethanol and deionized water followed by air-drying. Specimens were immediately tested after sample preparation. Furthermore, a protective lacquer was used to prevent crevice corrosion in the resin/alloy interface. The electrolyte used was a deaerated 1 wt.% NaCl solution with 30 min of nitrogen bubbling to avoid oxygen depolarization by oxygen reduction. A 30 min open circuit potential (OCP) monitoring was performed prior to testing until steady-state was reached. CPP tests were performed starting from a potential of \(-0.03\) V\({}_{\rm{OCP}}\) and scanned anodically until the current density limit of \(1.0\times 10^{-3}\) A/cm\({}^{2}\) was achieved, after that, the scan was reversed in the cathodic direction and the scan rate was 0.1667 mV/s according to ASTM G61-86 standard [33]. All experiments were performed at room temperature (\(\sim\)25 \({}^{\circ}\)C) and in triplicate, ensuring reproducibility. The polarization tests were performed using a three-electrode electrochemical cell with the TWIP specimen acting as the working electrode, a platinum mesh with a large surface area as the counter electrode, and a saturated calomel (SCE) reference electrode. The CPP tests were performed using a potentiostat/galvanostat Gamry Reference 600 (Gamry Instruments, Warminster, PA, USA). Specimens for metallographic analysis were prepared by grinding to 2000 grit SiC paper and polishing to a mirror finish using 500 nm non-crystalline colloidal silica. Etching of the polish specimens was done by immersion in Nital solution (10 wt.% HNO\({}_{3}\) in ethanol) for 30 s. The microstructure was characterized by optical microscopy (OM) Nikon Eclipse MA 100 (Nikon Metrology Inc., Brighton, MI, USA), scanning electron microscopy (SEM) Hitachi-TM3030 (Hitachi High-Tech America Inc, Schaumburg, IL, USA), and infinite focus microscopy (IFM) in an Alicona infinite focus G5 microscope (Bruker, Itasca, IL, USA). X-ray photoelectron spectroscopy (XPS) was conducted after a passive layer was generated by potentiostatic polarization for 1 h at \(-0.750\) V\({}_{\rm{SCE}}\) in 1.0 wt.% NaCl solution and under deareated conditions. This passive layer formation procedure has been shown in previous works to alter the surface layer [28; 34]. In order to minimize surface contamination, the samples were introduced in a nitrogen saturated glove bag to eliminate oxygen contamination immediately after CPP tests.