**4.1.2 Results of potentiodynamic polarization tests**

Electrochemical corrosion resistance in potentiodynamic tests was carried out on the steel characterized by two-phase structure after the thermo-mechanical rolling. The change of current density as a function of potential for the sample investigated in 0.5N NaCl solution is presented in Fig. 11. The value of corrosion potential was equal -796 mV and the corrosion current density was 8.4 A/cm2. Determination of pitting potential was impossible due to the fast course of corrosion processes. It is clear in Fig. 11 that the passivation did not occur. The factors which precluded repassivation inside pits being formed on the surface of the sample were probably the increase of chloride ions concentration as a consequence of their relocation along the corrosion current, what made the contribution to the formation of a corrosion cell inside the pit as well as difficult supply of oxygen into the interior of the pit because of its low solubility in the electrolyte. The change of polarization of samples did not cause any decrease of anodic current.

The corrosion potential of the 25Mn-3Si-1.5Al-Nb-Ti steel investigated in 0.5N H2SO4 is equal to –574 mV (Fig. 12). It is shifted towards the more noble direction, as compared to chloride solution. However, the corrosion current density is equal to about 3400 A/cm2, what is over two orders of magnitude higher compared to chloride solution.

The similar values of the corrosion potential and corrosion current density both in chloride and acidic media are reported by other authors (Ghayad et al., 2006; Kannan et al., 2008). The sample gains no passivation and the pitting potential was about 57 mV (Fig. 12). It is

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

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

Electrochemical corrosion resistance in potentiodynamic tests was carried out on the steel characterized by two-phase structure after the thermo-mechanical rolling. The change of current density as a function of potential for the sample investigated in 0.5N NaCl solution is presented in Fig. 11. The value of corrosion potential was equal -796 mV and the corrosion current density was 8.4 A/cm2. Determination of pitting potential was impossible due to the fast course of corrosion processes. It is clear in Fig. 11 that the passivation did not occur. The factors which precluded repassivation inside pits being formed on the surface of the sample were probably the increase of chloride ions concentration as a consequence of their relocation along the corrosion current, what made the contribution to the formation of a corrosion cell inside the pit as well as difficult supply of oxygen into the interior of the pit because of its low solubility in the electrolyte. The change of polarization of samples did not

The corrosion potential of the 25Mn-3Si-1.5Al-Nb-Ti steel investigated in 0.5N H2SO4 is equal to –574 mV (Fig. 12). It is shifted towards the more noble direction, as compared to chloride solution. However, the corrosion current density is equal to about 3400 A/cm2,

The similar values of the corrosion potential and corrosion current density both in chloride and acidic media are reported by other authors (Ghayad et al., 2006; Kannan et al., 2008). The sample gains no passivation and the pitting potential was about 57 mV (Fig. 12). It is

what is over two orders of magnitude higher compared to chloride solution.

(Ghayad et al., 2006; Hamada, 2007; Kannan et al., 2008; Zhang & Zhu, 1999).

and the degree of strain hardening.

cause any decrease of anodic current.

**4.1.2 Results of potentiodynamic polarization tests** 

interesting to note the fast increase of corrosion current after the initiation of pitting corrosion. The escalation of corrosion current usually is more mild.

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 immersion tests.

Fig. 11. Anodic polarization curve registered for the sample of 25Mn-3Si-1.5Al-Nb-Ti steel in 0.5N NaCl.

Fig. 12. Anodic polarization curve registered for the sample of 25Mn-3Si-1.5Al-Nb-Ti steel in 0.5N H2SO4 .

Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry 367

Fig. 15. Scaled and partially cracked surface of the specimen after electrochemical tests in

Fig. 16. Scaled surface and corrosion pits of the specimen after corrosion tests in the 0.5N

In both steels, immersed after deformation in 1N H2SO4 many corrosion pits of various size were observed (Fig. 17). The amount and the size of pits are very high and they are formed along the entire surface of specimens. Privileged places to pits forming are surface concentrations of non-metallic inclusions, which are also probable place of hydrogen penetration. Hydrogen also penetrates deeper into the steel – probably by martesite plates – accumulating in a surroundings of elongated non-metallic sulfide inclusions (Fig. 18).

**4.2 Corrosion behaviour in the cold-worked state** 

the 0.5N H2SO4.

H2SO4..

Fig. 13. Numerous corrosion pits on the surface of the specimen after electrochemical tests in 0.5N NaCl.

Fig. 14. Corrosion pitting on the surface of the specimen after electrochemical tests in 0.5N NaCl.

Fig. 13. Numerous corrosion pits on the surface of the specimen after electrochemical tests in

Fig. 14. Corrosion pitting on the surface of the specimen after electrochemical tests in 0.5N

0.5N NaCl.

NaCl.

Fig. 15. Scaled and partially cracked surface of the specimen after electrochemical tests in the 0.5N H2SO4.

Fig. 16. Scaled surface and corrosion pits of the specimen after corrosion tests in the 0.5N H2SO4..
