**4.1.1 Results of immersion tests**

The results of the immersion tests in two media are given in Table 2. After 100 hours immersion in 1N H2SO4 both steels showed a significant percentage mass decrement, among 38 and 41%. Mass loss of samples dipped in 3.5wt% NaCl is about 100 times lower. The difference is due to different corrosion mechanisms. When the solution is acidic, the corrosion process is running according to hydrogen depolarization, whereas in chloride media the specimens are corroding with oxygen depolarization.


Table 2. Mean percentage mass loss of samples after the immersion tests, %.

In 26Mn-3Si-3Al-Nb steel immersed in 1N H2SO4 many deep corrosion pits along the whole specimen surface were observed (Fig. 5). Moreover, in places with higher density of nonmetallic inclusions, microcracks locally occur. Similar pits are present in the steel with lower aluminum content. Slightly smaller corrosion pits are formed in specimens after the immersion test in 3.5wt% NaCl, regardless of a steel type. Places privileged to creation of corrosion pits are pointwise aggregations and chains of non-metallic inclusions (Fig. 6).

Characteristically for the structure of 25Mn-3Si-1.5Al-Nb-Ti steel dipped in 3.5wt% NaCl solution are small microcracks located along martensite lamellas (Fig. 7). They are propagated from significantly elongated in a rolling direction, sulphuric non-metallic inclusions. In specimens with the single-phase austenitic structure, microcracks were not observed.

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.

Investigation of the electrochemical corrosion behaviour was done in a PGP 201 potentiostat using a conventional three-electrode cell consisting of a saturated calomel reference electrode (SCE), a platinum counter electrode and the studied specimen as the working electrode. To simulate the corrosion media, 0.5N H2SO4 and 0.5N NaCl solutions were used. The solution temperature was 23°C±1°C. The corrosion behaviour was studied first by measuring the open circuit potential (OCP) for 30 min. Subsequently, anodic polarization curves were registered. The curve started at a potential of ~100 mV below the corrosion potential. The potential has been changed in the anodic direction at the rate of 1 mV/s. After the anodic current density being equal 1mA/cm2 was achieved, the direction of polarization has been changed. Thus, the return curve was registered. The corrosion current densities

The results of the immersion tests in two media are given in Table 2. After 100 hours immersion in 1N H2SO4 both steels showed a significant percentage mass decrement, among 38 and 41%. Mass loss of samples dipped in 3.5wt% NaCl is about 100 times lower. The difference is due to different corrosion mechanisms. When the solution is acidic, the corrosion process is running according to hydrogen depolarization, whereas in chloride

Steel grade 1N H2SO4 3.5% NaCl 26Mn-3Si-3Al-Nb 38.4 ± 5.2 0.40 ± 0.03 25Mn-3Si-1.5Al-Nb-Ti 41.3 ± 9.6 0.48 ± 0.03

In 26Mn-3Si-3Al-Nb steel immersed in 1N H2SO4 many deep corrosion pits along the whole specimen surface were observed (Fig. 5). Moreover, in places with higher density of nonmetallic inclusions, microcracks locally occur. Similar pits are present in the steel with lower aluminum content. Slightly smaller corrosion pits are formed in specimens after the immersion test in 3.5wt% NaCl, regardless of a steel type. Places privileged to creation of corrosion pits are pointwise aggregations and chains of non-metallic inclusions (Fig. 6).

Characteristically for the structure of 25Mn-3Si-1.5Al-Nb-Ti steel dipped in 3.5wt% NaCl solution are small microcracks located along martensite lamellas (Fig. 7). They are propagated from significantly elongated in a rolling direction, sulphuric non-metallic inclusions. In specimens with the single-phase austenitic structure, microcracks were not

Corrosion medium

and the polarization resistance were obtained on the basis of the Tafel analysis.

**3.3 Potentiodynamic polarization tests** 

**4. Results and discussion** 

observed.

**4.1.1 Results of immersion tests** 

**4.1 Corrosion behaviour in the initial state** 

media the specimens are corroding with oxygen depolarization.

Table 2. Mean percentage mass loss of samples after the immersion tests, %.

Fig. 5. Corrosion pits and microcracks in 26Mn-3Si-3Al-Nb steel after the immersion test in 1N H2SO4.

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

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

Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry 363

Fig. 8. Bursted corrosion products layer on 26Mn-3Si-3Al-Nb steel surface after the

Fig. 9. Craters created as a result of corrosion pitting and bursted corrosion products layer in

Fig. 10. Craters created as a result of corrosion pitting and corrosion products residues in

25Mn-3Si-1.5Al-Nb-Ti steel after the immersion test in 1N H2SO4.

26Mn-3Si-3Al-Nb steel after the immersion test in 1N H2SO4.

immersion test in 3.5wt% NaCl.

corrosion products is occurring, protecting the metal against further corrosive medium penetration. Created layer includes many cracks, especially in surroundings of non-metallic inclusions (Fig. 8). In case of the specimens dipped in H2SO4 solution, the number of created surface cracks is much higher (Fig. 9). Apart from corrosion products residues, many craters formed as a result of corrosion pitting. They are occurring both in the steel with martensite lamellas (Fig. 9), as well as in the steel with a single-phase austenitic structure (Fig. 10).

The results confirmed the low corrosion resistance of high-manganese steels in acidic and chloride media. Especially low corrosion resistance the investigated steels show in 1N H2SO4, where the mass decrement is about 40%, what is about 100 times higher than for specimens dipped in 3.5wt% NaCl (Table 2). The similar order of magnitude of corrosion progress was observed for the Fe-0.05C-30Mn-3Al-1.4Si steel (Kannan et al., 2008).

The high difference in corrosion resistance is because of different corrosion mechanisms in both environments. The big mass loss in the H2SO4 solution is due to the hydrogen depolarization mechanism, which is typical for corrosion in acidic media. Hydrogen depolarization is a process of reducing hydrogen ions (from the electrolyte) in cathodic areas by electrons from the metal, to gaseous hydrogen, resulting in continuous flow of electrons outer the metal and consequently the corrosion progress. Due to this process, numerous corrosion pits occur in examined steels (Figs. 5, 6). Corrosion pits are occurring most intensively in the places containing non-metallic inclusions. They are less precious than the rest of material, fostering potential differences and galvanic cell creation. This causes the absorption of hydrogen ions, which, due to increasing pressure and temperature can recombine to a gaseous form and get out of the metal accompanying formation of corrosion pits (Fig. 10). This process is accompanied by local cracking of corrosion products layer (Figs. 9, 10), uncovering the metal surface and causing further penetration of the corrosive medium and the intensive corrosion progress.

In chloride solution, the corrosion process is running according to the oxygen depolarization. In this mechanism, oxygen included in the electrolyte is being reduced by electrons from the metal to hydroxide ions. On the surface of the alloy appears a layer of corrosion products (Fig. 8), protecting the material before further penetration of the corrosion medium. This is why the mass loss in chloride solution is much lower compared to acidic medium. At less corrosion-resistant places (e.g. with non-metallic inclusions) potential differences are occurring. This enables the absorption of chloride ions, which are forming chlorine oxides of increased solubility. This results in local destructions of corrosion products layer (Fig. 8) and the initiation of corrosion pits. Further pit expansion is running autocatalytic.

As a consequence of small steel softening during static recrystallization (Grajcar & Borek, 2008; Grajcar et al., 2009) after finishing rolling, the state of internal stresses in examined steels can be increased. In specimen areas with internal stresses, crevices are occurring. Due to limited oxygen access and a lack of possibility of corrosion products layer forming, they become susceptible to corrosion. As a result of chloride ions adsorption on the crevice bottom, a concentrated electrolyte solution is forming, fostering the corrosion progress (Cottis & Newman, 1995). As a result, stress corrosion cracking can take place. Microcracks were observed along martensite lamellas in 25Mn-3Si-1.5Al-Nb-Ti steel. The microcracks initiation proceeds in places with elongated non-metallic inclusions (Fig. 7), while their propagation runs along plates of the second phase.

corrosion products is occurring, protecting the metal against further corrosive medium penetration. Created layer includes many cracks, especially in surroundings of non-metallic inclusions (Fig. 8). In case of the specimens dipped in H2SO4 solution, the number of created surface cracks is much higher (Fig. 9). Apart from corrosion products residues, many craters formed as a result of corrosion pitting. They are occurring both in the steel with martensite lamellas (Fig. 9), as well as in the steel with a single-phase austenitic structure (Fig. 10).

The results confirmed the low corrosion resistance of high-manganese steels in acidic and chloride media. Especially low corrosion resistance the investigated steels show in 1N H2SO4, where the mass decrement is about 40%, what is about 100 times higher than for specimens dipped in 3.5wt% NaCl (Table 2). The similar order of magnitude of corrosion

The high difference in corrosion resistance is because of different corrosion mechanisms in both environments. The big mass loss in the H2SO4 solution is due to the hydrogen depolarization mechanism, which is typical for corrosion in acidic media. Hydrogen depolarization is a process of reducing hydrogen ions (from the electrolyte) in cathodic areas by electrons from the metal, to gaseous hydrogen, resulting in continuous flow of electrons outer the metal and consequently the corrosion progress. Due to this process, numerous corrosion pits occur in examined steels (Figs. 5, 6). Corrosion pits are occurring most intensively in the places containing non-metallic inclusions. They are less precious than the rest of material, fostering potential differences and galvanic cell creation. This causes the absorption of hydrogen ions, which, due to increasing pressure and temperature can recombine to a gaseous form and get out of the metal accompanying formation of corrosion pits (Fig. 10). This process is accompanied by local cracking of corrosion products layer (Figs. 9, 10), uncovering the metal surface and causing further penetration of the

In chloride solution, the corrosion process is running according to the oxygen depolarization. In this mechanism, oxygen included in the electrolyte is being reduced by electrons from the metal to hydroxide ions. On the surface of the alloy appears a layer of corrosion products (Fig. 8), protecting the material before further penetration of the corrosion medium. This is why the mass loss in chloride solution is much lower compared to acidic medium. At less corrosion-resistant places (e.g. with non-metallic inclusions) potential differences are occurring. This enables the absorption of chloride ions, which are forming chlorine oxides of increased solubility. This results in local destructions of corrosion products layer (Fig. 8) and the initiation of corrosion pits. Further pit expansion is running

As a consequence of small steel softening during static recrystallization (Grajcar & Borek, 2008; Grajcar et al., 2009) after finishing rolling, the state of internal stresses in examined steels can be increased. In specimen areas with internal stresses, crevices are occurring. Due to limited oxygen access and a lack of possibility of corrosion products layer forming, they become susceptible to corrosion. As a result of chloride ions adsorption on the crevice bottom, a concentrated electrolyte solution is forming, fostering the corrosion progress (Cottis & Newman, 1995). As a result, stress corrosion cracking can take place. Microcracks were observed along martensite lamellas in 25Mn-3Si-1.5Al-Nb-Ti steel. The microcracks initiation proceeds in places with elongated non-metallic inclusions (Fig. 7), while their

progress was observed for the Fe-0.05C-30Mn-3Al-1.4Si steel (Kannan et al., 2008).

corrosive medium and the intensive corrosion progress.

propagation runs along plates of the second phase.

autocatalytic.

Fig. 8. Bursted corrosion products layer on 26Mn-3Si-3Al-Nb steel surface after the immersion test in 3.5wt% NaCl.

Fig. 9. Craters created as a result of corrosion pitting and bursted corrosion products layer in 25Mn-3Si-1.5Al-Nb-Ti steel after the immersion test in 1N H2SO4.

Fig. 10. Craters created as a result of corrosion pitting and corrosion products residues in 26Mn-3Si-3Al-Nb steel after the immersion test in 1N H2SO4.

Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry 365

interesting to note the fast increase of corrosion current after the initiation of pitting

Fractographic analyses of sample surface after the corrosion tests allowed to evaluate the type and the degree of corrosion damages. On the surface of samples investigated in 0.5N NaCl numerous relatively small corrosion pits and micropores were revealed (Figs. 13, 14). Damaging of a superficial layer occurred around the pits. Cracked passive layer was also observed, what could be a result of rapid penetration of corrosive medium into interior of investigated specimens (Fig. 14). Similar corrosion effects, i.e. pitting, cracked interfacial layer and scaled surface were identified in the specimens after electrochemical tests in 0.5N H2SO4 solution (Figs. 15, 16). The results correspond well with those obtained after

Fig. 11. Anodic polarization curve registered for the sample of 25Mn-3Si-1.5Al-Nb-Ti steel in

Fig. 12. Anodic polarization curve registered for the sample of 25Mn-3Si-1.5Al-Nb-Ti steel in

corrosion. The escalation of corrosion current usually is more mild.

immersion tests.

0.5N NaCl.

0.5N H2SO4 .

The mass decrement in two steels is comparable (Table 2) both in acidic and chloride media. It indicates that a small martensite fraction does not have meaningful impact on the corrosion progress. The observed corrosion products are related rather to the chemical composition than to the phase structure of investigated steels. That confirms a slightly higher mass decrement in the steel with lower Al and somewhat higher Si content. In general, the low corrosion resistance of high-manganese steels is from the fact, that Mn in steels forms unstable manganese oxide due to low passivity coefficient and hence reduces their electrochemical corrosion resistance (Kannan et al., 2008). It leads consequently to the high dissolution rate of manganese and iron atoms both in H2SO4 and NaCl solutions (Ghayad et al., 2006; Hamada, 2007; Kannan et al., 2008; Zhang & Zhu, 1999).

The high mass decrement of steels examined in H2SO4 solution is a result of fast general corrosion progress and corrosion pits formation (Figs. 5, 6). Much lower mass loss in steels examined in NaCl solution is connected mainly with corrosion pits forming (Fig. 6). The presence of corrosion pits in chloride medium in steels of the type Fe-25Mn-5Al and Fe-0.2C-25Mn-(1-8)Al was also confirmed by other authors (Hamada, 2007; Zhang & Zhu, 1999). However, significant participation of pitting corrosion was not observed in the studies on Fe-0.05C-29Mn-3.1Al-1.4Si steel (Kannan et al., 2008) and on Fe-0.5C-29Mn-3.5Al-0.5Si steel (Ghayad et al., 2006). Localized corrosion attack in the presently investigated steels is enhanced by the lower aluminium and higher silicon concentrations. It means that a character of corrosion damages in high-manganese steels in chloride medium is a complex reaction of the chemical composition and structural state related with a phase composition and the degree of strain hardening.
