**2.1 General and pitting corrosion**

The mean area of studies on high-manganese steels concern their high-temperature deformation resistance (Bleck et al., 2007; Cabanas et al., 2006; Dobrzański et al., 2008; Grajcar & Borek, 2008; Grajcar et al., 2009) and the cold-working behaviour (Dini et al., 2010; Frommeyer & Bruex, 2006; Frommeyer et al., 2003; Graessel et al., 2000; Huang et al., 2006). Much less attention has been paid on their corrosion resistance (Ghayad et al., 2006; Grajcar et al., 2010a, 2010b; Hamada, 2007; Kannan et al., 2008; Mujica et al., 2010; Mujica Roncery et al., 2010; Opiela et al., 2009). The research on Fe-C-Mn-Al alloys (Altstetter et al., 1986) for cryogenic applications that were supposed to substitute expensive Cr-Ni steels was carried out in the eighties of the last century. The role of manganese boils to Ni replacement and obtaining austenitic microstructure, whereas aluminium has a similar impact as chromium. Improvement of corrosion resistance by Al consists in formation of thin, stable layer of oxides. As the result of conducted research it was found that Fe-C-Mn-Al alloys show inferior corrosion resistance than Cr-Ni steels and they can be used as a substitute only in some applications (Altstetter et al., 1986). The addition of 25% Mn to mild steels was found to be very detrimental to the corrosion resistance in aqueous solutions (Zhang & Zhu, 1999).

Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry 357

Generally, increasing the strength of steels, their hydrogen embrittlement susceptibility increases. This is one of the main problem to use AHSS. If hydrogen content reaches the critical value, it can induce a reduction of strength and ductile properties. A critical concentration of hydrogen is various for different steels (Lovicu et al., 2010; Sojka et al., 2010). Hydrogen embrittlement is usually investigated by performing slow strain rate tensile tests on hydrogenerated samples. Austenitic alloys are considered to be immune to this type of corrosion damage. However, the stress- or strain-induced martensitic transformation of austenite taking place in TRIP-aided austenitic alloys can be a reason of their embrittlement. This can happen due to the high difference in solubility and diffusion rate of hydrogen in the BCC and FCC lattice. Austenite is characterized by high solubility and low diffusivity of H in the A1 lattice and thus acts as a sink for hydrogen lowering its mobility and increasing the hydrogen concentration. Due to the slow diffusion rate of hydrogen in austenite, it is hardly to enrich it homogeneously to a hydrogen content causing embrittlement. However, it was shown (Lovicu et al., 2010) that the hydrogen concentration in surface regions of the high-Mn steel is much higher than in the centre zone. It can lead to the intragranular fracture in these regions because of strain-induced or hydrogen-induced martensitic transformation and finally to reduction of strength and

When the formed automotive element is exposed to the air the delayed fracture can occur. The technological formability is usually investigated in cup forming tests (Otto et al., 2010; Shin et al., 2010). It was observed (Shin et al., 2010) that the 0.6C-22Mn steel cup specimen underwent the delayed fracture when exposed to the air for seven days, even though the specimen was not cracked during forming. This is because the strain-induced martensitic transformation occurred during the cupping test in places of stress concentration. When the addition of 1.2% Al was added the steel cup forms with the high share of mechanical twinning instead the ' transformation. It leads to lower stress concentration and finally

The chapter addresses the corrosion behaviour of two high-Mn steels of different initial structures in chloride and acidic media. Their chemical composition is given in Table 1.

The vacuum melted steels have similar C, Mn and Si concentration. Significant impact on the SFE of austenite has the difference in Al and Ti content. The lower SFE of 25Mn-3Si-1.5Al-Nb-Ti steel compared to 26Mn-3Si-3Al-Nb steel is a result of the lower Al content. Moreover, several times higher Ti content in a first steel provides a decrease of phase

stability, as a result of fixing the total nitrogen and some carbon (Grajcar et al., 2009).

Steel grade C Mn Si Al Nb Ti S P N O Structure 26Mn-3Si-3Al-Nb 0.065 26.0 3.08 2.87 0.034 0.009 0.013 0.004 0.0028 0.0006 25Mn-3Si-1.5Al-Nb-Ti 0.054 24.4 3.49 1.64 0.029 0.075 0.016 0.004 0.0039 0.0006 +

**2.3 Hydrogen embrittlement and delayed fracture** 

to improvement in cup formability (Shin et al., 2010).

Table 1. Chemical composition of the investigated steels, wt. %

**3. Experimental procedure** 

ductility.

**3.1 Material** 

The Fe-25Mn alloy was difficult to passivate, even in such neutral aqueous electrolytes as 1M Na2SO4 solution. With increasing Al content up to 5% of the Fe-25Mn-Al steel, the anodic polarization curves exhibit a stable passivation region in Na2SO4 solution, but it shows no passivation in 3.5wt% NaCl solution.

Recently, corrosion resistance of Fe-0.05C-29Mn-3.1Al-1.4Si steel in acidic (0.1M H2SO4) and chloride-containing (3.5wt% NaCl) environments was investigated (Kannan et al., 2008). Moreover, the corrosion behaviour of the tested high-Mn steel with that of IF-type was compared. Performing immersion and polarization tests it was found that Fe-Mn-Al-Si steel has lower corrosion resistance than IF steel, both in acidic and in chloride media. The corrosion resistance of the high-manganese steel in chloride solutions is higher compared to that observed in acidic medium. The behaviour of Fe-0.2C-25Mn-(1-8)Al steels with increased concentration of Al up to 8% wt. in 3.5wt% NaCl was also investigated (Hamada, 2007). Hamada reported that the corrosion resistance of tested steels in chloride environments is pretty low. The predominating corrosion type is the general corrosion, but locally corrosion pits were observed. In steels including up to 6% Al with homogeneous austenite structure, places where the pits occur are casually, whereas in case of two-phase structure, including ferrite and austenite (Fe-0.2C-25Mn-8Al), they preferentially occur in phase. The corrosion resistance of examined steels can be increased trough anodic passivation in nitric acid, which provides modification of chemical composition and constitution of the surface layer (Hamada et al., 2005). This was done by reducing the surface concentration of Mn and enriching the surface layer in elements that improve the corrosion resistance (e.g. Al, Cr).

A better effect was reached by chemical composition modification. It was found that addition of Al and Cr to Fe-0.26C-30Mn-4Al-4Cr and Fe-0.25C-30Mn-8Al-6Cr alloys increases considerably the general corrosion resistance, especially after anodic passivation ageing of surface layers in an oxidizing electrolytic solution (Hamada, 2007). Cr-bearing steels passivated by nucleation and growth of the passive oxide films on the steel surface, where the enrichment of Al and Cr and depletion of Fe and Mn have occurred. The positive role of Cr in obtaining passivation layers in 0.5M H2SO4 acidic solution was recently confirmed in Fe-25Mn-12Cr-0.3C-0.4N alloy (Mujica et al., 2010; Mujica Roncery et al., 2010). The steel containing increased Cr, C and N content shows passivity at the current density being five orders of magnitude lower compared to the Fe-22Mn-0.6C steel.
