**3.1 Material**

356 Corrosion Resistance

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

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

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

Results of corrosion tests described above concern steels in the annealed or supersaturated state. The influence of cold plastic deformation on corrosion behaviour in 3.5wt% NaCl was studied in Fe-0.5C-29Mn-3.5Al-0.5Si steel (Ghayad et al., 2006). It was found on the basis of potentiodynamic tests, that the steel shows no tendency to passivation, independently on the steel structure after heat treatment (supersaturated, aged or strain-aged). Higher corrosion rate of deformed specimens than that of specimens in supersaturated state, was a result of faster steel dissolution caused by annealing twins, which show a different potential than the matrix. The highest corrosion rate was observed in strain-aged samples, as a result of ferrite formation, which creates a corrosive galvanic cell with the austenitic matrix. The enhanced corrosion attack at the boundaries of deformation twins was also observed in Fe-

being five orders of magnitude lower compared to the Fe-22Mn-0.6C steel.

shows no passivation in 3.5wt% NaCl solution.

corrosion resistance (e.g. Al, Cr).

**2.2 Effect of deformation** 

22Mn-0.5C steels (Mazancova et al., 2010).

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.


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

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).

Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry 359

Fig. 4. Austenitic matrix with martensite of 25Mn-3Si-1.5Al-Nb-Ti steel after the thermomechanical rolling and immersion in 1N H2SO4 (a) and X-ray diffraction pattern (b).

The immersion tests were used to assess the corrosion resistance of the steels at the initial state (after the thermo-mechanical rolling) and after cold deformation. The corrosion resistance was investigated in two solutions: 1N H2SO4 and 3.5wt% NaCl. Prior to the corrosion tests, three samples of the each steel with the size 3.2x10x15 mm were ground to the 1000-grit finish and then they were washed in distilled water, ultrasonically cleaned in acetone and finally rinsed with ethanol and dried. The specimens were weighed with the accuracy of 0.001g and put into the solution for 100 hours at the temperature of 23±1°C. After the test the specimens were weighed and analysed using optical microscopy and SEM. Corrosion loss was calculated in a simple way as the difference between final and initial mass of the samples. Percentage mass decrement was also calculated. Cold deformation was applied by bending at room temperature. Samples with a size of 10x15 mm and a thickness

Metallographic observations of non-metallic inclusions and corrosion pits were carried out on polished sections, whereas the microstructure observations on specimens etched in nital. The investigations were performed using LEICA MEF 4A light microscope, with magnifications from 100 to 1000x. Fractographic investigations were carried using scanning

of 3.2 mm were bent to an angle of 90°, with a bending radius of 3 mm.

**3.2 Immersion tests** 

The steels were delivered in a form of sheet segments of 340x225x3.2 mm, obtained after the thermo-mechanical rolling. The thermo-mechanical processing consisted of:


The microstructures of the steels after the thermo-mechanical treatment are shown in Figs. 3 and 4. The 26Mn-3Si-3Al-Nb steel exhibits a homogeneous austenite structure with grains elongated in the rolling direction (Fig. 3a). The susceptibility to twinning confirms the presence of a great number of annealing twins. The single-phase structure of the steel is confirmed by X-ray diffraction pattern in Fig. 3b. The lower SFE of the 25Mn-3Si-1.5Al-Nb-Ti steel results in the presence of the second phase with a lamellar shape, distributed in the austenite matrix (Fig. 4a). The number of annealing twins is much lower. The X-ray diffraction analysis confirms the presence of martensite (Fig. 4b).

Fig. 3. Austenitic structure with annealing twins of 26Mn-3Si-3Al-Nb steel after the thermomechanical rolling and immersion in 1N H2SO4 (a) and X-ray diffraction pattern (b).

Fig. 4. Austenitic matrix with martensite of 25Mn-3Si-1.5Al-Nb-Ti steel after the thermomechanical rolling and immersion in 1N H2SO4 (a) and X-ray diffraction pattern (b).

#### **3.2 Immersion tests**

358 Corrosion Resistance

The steels were delivered in a form of sheet segments of 340x225x3.2 mm, obtained after the

 heating the charge up to the temperature of 1100°C and austenitizing for 15 minutes, rolling in a range from 1050°C to 850°C in 3 passes (relative reduction: 20, 15 and

holding of the rolled sheet segments at the temperature of finishing rolling for 15s,

The microstructures of the steels after the thermo-mechanical treatment are shown in Figs. 3 and 4. The 26Mn-3Si-3Al-Nb steel exhibits a homogeneous austenite structure with grains elongated in the rolling direction (Fig. 3a). The susceptibility to twinning confirms the presence of a great number of annealing twins. The single-phase structure of the steel is confirmed by X-ray diffraction pattern in Fig. 3b. The lower SFE of the 25Mn-3Si-1.5Al-Nb-Ti steel results in the presence of the second phase with a lamellar shape, distributed in the austenite matrix (Fig. 4a). The number of annealing twins is much lower. The X-ray

Fig. 3. Austenitic structure with annealing twins of 26Mn-3Si-3Al-Nb steel after the thermomechanical rolling and immersion in 1N H2SO4 (a) and X-ray diffraction pattern (b).

thermo-mechanical rolling. The thermo-mechanical processing consisted of:

solution heat treatment of the flat specimens in water.

diffraction analysis confirms the presence of martensite (Fig. 4b).

15%),

The immersion tests were used to assess the corrosion resistance of the steels at the initial state (after the thermo-mechanical rolling) and after cold deformation. The corrosion resistance was investigated in two solutions: 1N H2SO4 and 3.5wt% NaCl. Prior to the corrosion tests, three samples of the each steel with the size 3.2x10x15 mm were ground to the 1000-grit finish and then they were washed in distilled water, ultrasonically cleaned in acetone and finally rinsed with ethanol and dried. The specimens were weighed with the accuracy of 0.001g and put into the solution for 100 hours at the temperature of 23±1°C. After the test the specimens were weighed and analysed using optical microscopy and SEM. Corrosion loss was calculated in a simple way as the difference between final and initial mass of the samples. Percentage mass decrement was also calculated. Cold deformation was applied by bending at room temperature. Samples with a size of 10x15 mm and a thickness of 3.2 mm were bent to an angle of 90°, with a bending radius of 3 mm.

Metallographic observations of non-metallic inclusions and corrosion pits were carried out on polished sections, whereas the microstructure observations on specimens etched in nital. The investigations were performed using LEICA MEF 4A light microscope, with magnifications from 100 to 1000x. Fractographic investigations were carried using scanning

Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry 361

Fig. 5. Corrosion pits and microcracks in 26Mn-3Si-3Al-Nb steel after the immersion test in

Fig. 6. Corrosion pits in 25Mn-3Si-1.5Al-Nb-Ti steel after the immersion test in 3.5wt% NaCl.

Fig. 7. Austenitic matrix containing plates of martensite and microhardness test results of

A precise definition of the character of corrosion damages was possible on the basis of SEM observations. On the surface of specimens of both steels dipped in NaCl solution, a layer of

1N H2SO4.

25Mn-3Si-1.5Al-Nb-Ti steel.

electron microscope SUPRA 25 (Zeiss) at the accelerating voltage of 20kV. In order to remove corrosion products, the specimens were ultrasonically cleaned before the analysis.
