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

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).

Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry 369

Fig. 19. Elongated austenite grains of 26Mn-3Si-3Al-Nb steel after cold deformation and

Fig. 20. Regions of hydrogen failures and hydrogen microcracks in 25Mn-3Si-1.5Al-Nb-Ti

After bending and 100 hours immersion in 1N H2SO4 both steels show a meaningful percentage mass decrement, among 47 and 49% and 2 orders of magnitude lower in chloride solution (Table 3). Cold deformation rises slightly the mass decrement in acidic medium, in comparison with the specimens investigated in undeformed state (Table 2). The opposite is true for chloride solution. However, the differences are not significant. Comparable mass loss of two steels both in non-deformed and plastically deformed states indicates that the

Steel grade 1N H2SO4 3.5% NaCl 26Mn-3Si-3Al-Nb 47.5 ± 1.6 0.33 ± 0.01 25Mn-3Si-1.5Al-Nb-Ti 49.5 ± 2.4 0.37 ± 0.12 Table 3. Mean percentage mass loss of cold-deformed samples after the immersion tests, %

Corrosion medium

steel, plastically deformed and immersed in 1N H2SO4 (transverse section).

initial structure does not have meaningful impact on the corrosion progress.

some places of hydrogen accumulation (transverse section).

Hydrogen failures were usually observed to the depth of about 0.3 mm. Places of hydrogen accumulation are also visible on samples revealing the steel structure after cold deformation. Usually, these places are elongated non-metallic inclusions, grain boundary areas and/or twin boundaries (Fig. 19). The 26Mn-3Si-3Al-Nb steel keeps after plastic deformation the austenitic structure, whereas a fraction of martensite in 25Mn-3Si-1.5Al-Nb-Ti steel increases (Fig. 20).

Besides non-metallic inclusions, especially privileged to hydrogen accumulation are lamellar areas of martensite. Absorbed atomic hydrogen penetrating the steel, accumulates in places with non-metallic inclusions, lamellar precipitations of the second phase, microcracks and other structural defects, where convenient conditions for recombining of atomic hydrogen to molecular H2 exist. The recombination of atomic hydrogen to molecular state is a very exothermic reaction, which provides a pressure increase in formed H2 bubbles as well as nucleation and growth of microcracks in a surface region of the sample (Fig. 20).

Fig. 17. Wide corrosion pits on the surface of 26Mn-3Si-3Al-Nb steel immersed in 1N H2SO4 and probable places of hydrogen penetration (transverse section).

Fig. 18. Regions of hydrogen accumulation around elongated sulfide-type non-metallic inclusions in 25Mn-3Si-1.5Al-Nb-Ti steel (transverse section).

Hydrogen failures were usually observed to the depth of about 0.3 mm. Places of hydrogen accumulation are also visible on samples revealing the steel structure after cold deformation. Usually, these places are elongated non-metallic inclusions, grain boundary areas and/or twin boundaries (Fig. 19). The 26Mn-3Si-3Al-Nb steel keeps after plastic deformation the austenitic structure, whereas a fraction of martensite in 25Mn-3Si-1.5Al-Nb-

Besides non-metallic inclusions, especially privileged to hydrogen accumulation are lamellar areas of martensite. Absorbed atomic hydrogen penetrating the steel, accumulates in places with non-metallic inclusions, lamellar precipitations of the second phase, microcracks and other structural defects, where convenient conditions for recombining of atomic hydrogen to molecular H2 exist. The recombination of atomic hydrogen to molecular state is a very exothermic reaction, which provides a pressure increase in formed H2 bubbles as well as nucleation and growth of microcracks in a surface region of the sample (Fig. 20).

Fig. 17. Wide corrosion pits on the surface of 26Mn-3Si-3Al-Nb steel immersed in 1N H2SO4

Fig. 18. Regions of hydrogen accumulation around elongated sulfide-type non-metallic

and probable places of hydrogen penetration (transverse section).

inclusions in 25Mn-3Si-1.5Al-Nb-Ti steel (transverse section).

Ti steel increases (Fig. 20).

Fig. 19. Elongated austenite grains of 26Mn-3Si-3Al-Nb steel after cold deformation and some places of hydrogen accumulation (transverse section).

Fig. 20. Regions of hydrogen failures and hydrogen microcracks in 25Mn-3Si-1.5Al-Nb-Ti steel, plastically deformed and immersed in 1N H2SO4 (transverse section).

After bending and 100 hours immersion in 1N H2SO4 both steels show a meaningful percentage mass decrement, among 47 and 49% and 2 orders of magnitude lower in chloride solution (Table 3). Cold deformation rises slightly the mass decrement in acidic medium, in comparison with the specimens investigated in undeformed state (Table 2). The opposite is true for chloride solution. However, the differences are not significant. Comparable mass loss of two steels both in non-deformed and plastically deformed states indicates that the initial structure does not have meaningful impact on the corrosion progress.


Table 3. Mean percentage mass loss of cold-deformed samples after the immersion tests, %

Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry 371

Fig. 21. Deep corrosion decrements and banding-like arrangement of corrosion products in

Fig. 22. Cracked layer of corrosion products with banding-like arrangement in 25Mn-3Si-

Fig. 23. Numerous craters formed due to corrosion pitting and probable hydrogen penetration in 26Mn-3Si-3Al-Nb steel, plastically deformed and immersed in 1N H2SO4.

1.5Al-Nb-Ti steel, plastically deformed and immersed in 1N H2SO4.

26Mn-3Si-3Al-Nb steel, plastically deformed and immersed in 1N H2SO4.

Figures 21-23 present the SEM microstructures of plastically deformed samples immersed in acidic solution. It is characteristic that corrosion cracks were not observed, whereas deep corrosion damages and band arranged corrosion products can be perceived (Fig. 21). The corrosion products layer is not continuous and has many cracks (Fig. 22). Besides remaining corrosion products, a numerous number of craters, created due to intensive corrosion pitting and probably as a result of hydrogen impact, is characteristic. Craters forming is accompanied by local cracking of corrosion products layer (Figs. 21, 22), uncovering the metal surface and causing further penetration of the corrosive medium and finally the intensive progress of general and pitting corrosion.

Hydrogen Induced Cracking (HIC) is a problem in carbon steels and especially in highstrength low-alloy steels. Typical examples are hydrogen failures of gas pipelines containing hydrogen sulfide (Cottis & Newman, 1995; Ćwiek, 2009). Conventional Cr-Ni austenitic steels are not usually liable to such damages. One of the reasons is relatively low diffusion coefficient of hydrogen in austenite as distinguished from steels with ferritic or martensitic structures (Kumar & Balasubramaniam, 1997; Xu et al., 1994). However, enhanced permeation of hydrogen was observed in cold worked austenitic steels what was attributed to strain-induced martensitic transformation leading to promote hydrogen diffusion as the diffusivity is much higher in the bcc martensite lattice (Kumar & Balasubramaniam, 1997). The hydrogen induced surface cracking at the high hydrogen concentration places, i.e. grain and twin boundaries, / interface was also observed in Cr-Ni steels during hydrogen effusion from the supersaturated sites (Yang & Luo, 2000). Additionally, hydrogen mobility is enhanced by the presence of high-dislocation density due to cold working (Ćwiek, 2009). It is important to note that hydrogen impact occurs both in diphase (Figs. 18, 20) and single phase (Figs. 10, 23) structures of the steels.

In the investigated high-Mn austenitic steels the high corrosion progress and uncovering of metal surface by formed successively corrosion pits (Figs. 17, 23) should be taken into account. Uncovered active metal inside of expanding pits reacts with the acidic solution with hydrogen emission. In this regard hydrogen impact can influence the corrosion behaviour of the investigated steels. Indirect confirmation of this fact are numerous craters formed due to corrosion pitting and probably hydrogen impact (Figs. 10, 23). The effect of hydrogen can be further enhanced by the presence of increased sulphur concentration (for example present as sulfide non-metallic inclusions).

On the surface of specimens dipped in NaCl solution, a layer of corrosion products, which protects the metal against continuous penetration of corrosive media, is forming. Created scaled layer strongly adheres to the base, though numerous surface cracks (Fig. 24). There are many corrosion cracks running from the specimen surface with a maximum value of inner stresses in the steel with martensite plates (Fig. 25). Rectilinear course of cracks, shows the transcrystalline cracking character, to which austenitic steels in media with chloride ions are sensitized. Corrosion cracks were not present in the steel with single-phase austenitic matrix. A few microcracks along martensite lamellas were also revealed (Fig. 26). The microcracks were usually nucleated on elongated non-metallic inclusions and were spread along martensitic plates with a hardness higher compared to the austenitic matrix (Grajcar et al., 2010a).

Figures 21-23 present the SEM microstructures of plastically deformed samples immersed in acidic solution. It is characteristic that corrosion cracks were not observed, whereas deep corrosion damages and band arranged corrosion products can be perceived (Fig. 21). The corrosion products layer is not continuous and has many cracks (Fig. 22). Besides remaining corrosion products, a numerous number of craters, created due to intensive corrosion pitting and probably as a result of hydrogen impact, is characteristic. Craters forming is accompanied by local cracking of corrosion products layer (Figs. 21, 22), uncovering the metal surface and causing further penetration of the corrosive medium and finally the

Hydrogen Induced Cracking (HIC) is a problem in carbon steels and especially in highstrength low-alloy steels. Typical examples are hydrogen failures of gas pipelines containing hydrogen sulfide (Cottis & Newman, 1995; Ćwiek, 2009). Conventional Cr-Ni austenitic steels are not usually liable to such damages. One of the reasons is relatively low diffusion coefficient of hydrogen in austenite as distinguished from steels with ferritic or martensitic structures (Kumar & Balasubramaniam, 1997; Xu et al., 1994). However, enhanced permeation of hydrogen was observed in cold worked austenitic steels what was attributed to strain-induced martensitic transformation leading to promote hydrogen diffusion as the diffusivity is much higher in the bcc martensite lattice (Kumar & Balasubramaniam, 1997). The hydrogen induced surface cracking at the high hydrogen concentration places, i.e. grain and twin boundaries, / interface was also observed in Cr-Ni steels during hydrogen effusion from the supersaturated sites (Yang & Luo, 2000). Additionally, hydrogen mobility is enhanced by the presence of high-dislocation density due to cold working (Ćwiek, 2009). It is important to note that hydrogen impact occurs both in diphase (Figs. 18, 20) and single

In the investigated high-Mn austenitic steels the high corrosion progress and uncovering of metal surface by formed successively corrosion pits (Figs. 17, 23) should be taken into account. Uncovered active metal inside of expanding pits reacts with the acidic solution with hydrogen emission. In this regard hydrogen impact can influence the corrosion behaviour of the investigated steels. Indirect confirmation of this fact are numerous craters formed due to corrosion pitting and probably hydrogen impact (Figs. 10, 23). The effect of hydrogen can be further enhanced by the presence of increased sulphur concentration (for

On the surface of specimens dipped in NaCl solution, a layer of corrosion products, which protects the metal against continuous penetration of corrosive media, is forming. Created scaled layer strongly adheres to the base, though numerous surface cracks (Fig. 24). There are many corrosion cracks running from the specimen surface with a maximum value of inner stresses in the steel with martensite plates (Fig. 25). Rectilinear course of cracks, shows the transcrystalline cracking character, to which austenitic steels in media with chloride ions are sensitized. Corrosion cracks were not present in the steel with single-phase austenitic matrix. A few microcracks along martensite lamellas were also revealed (Fig. 26). The microcracks were usually nucleated on elongated non-metallic inclusions and were spread along martensitic plates with a hardness higher compared to the austenitic matrix (Grajcar

intensive progress of general and pitting corrosion.

phase (Figs. 10, 23) structures of the steels.

example present as sulfide non-metallic inclusions).

et al., 2010a).

Fig. 21. Deep corrosion decrements and banding-like arrangement of corrosion products in 26Mn-3Si-3Al-Nb steel, plastically deformed and immersed in 1N H2SO4.

Fig. 22. Cracked layer of corrosion products with banding-like arrangement in 25Mn-3Si-1.5Al-Nb-Ti steel, plastically deformed and immersed in 1N H2SO4.

Fig. 23. Numerous craters formed due to corrosion pitting and probable hydrogen penetration in 26Mn-3Si-3Al-Nb steel, plastically deformed and immersed in 1N H2SO4.

Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry 373

The automotive industry still requires steel sheets with higher strength, ductility and technological formability. Recently, special pressure is put to the need of increasing the passive safety of passengers what can be met by using specially designed controlled crash zones absorbing the energy during crash events. High-manganese austenitic alloys satisfy these requirements. However, the main disadvantage is their relatively poor corrosion resistance.

The results presented in this study focused on the evaluation of corrosion resistance of two high-Mn steels of the different initial structure in acidic and chloride media. The investigations were carried out on the specimens after the thermo-mechanical rolling and after cold deformation. The results of immersion and potentiodynamic tests as well as structural analysis prove that both examined steels, independent of initial structure, have very low corrosion resistance in acidic medium and low corrosion resistance in chloride

 the mass decrement of specimens immersed in 1N H2SO4 for 100 hours is equal about 40% and is about 100 higher compared to the specimens immersed in 3.5wt% NaCl; The percentage mass loss of plastically deformed specimens is slightly higher compared to non-deformed specimens in acidic solution and slightly lower in chloride medium; the percentage mass decrement in the steel with single-phase austenitic structure is slightly lower than in the steel with martensite lamellas. However, any significant

impact of the second phase on corrosion process acceleration was not observed; the decisive impact on the corrosion resistance of examined steels has their chemical composition, which determines the high rate of manganese and iron dissolution in acidic solution. The oxygen depolarization process results in formation of corrosion products layer on the surface of the steel examined in chloride medium. Therefore, the

mass decrement of steels in 3.5wt% NaCl is much lower than in 1N H2SO4;

non-metallic inclusions and growing along hard martensite plates;

 both steels are liable to general and pitting corrosion, especially intensively in the sulfuric acid solution. A very adverse influence on corrosion pitting initiation has a relative large fraction of non-metallic inclusions, especially of these forming local aggregations. In chloride solution it also results in occurring local microcracks, nucleating at elongated

 the surface layer of band-arranged corrosion products located accordingly to the deformation direction has many cracks, especially in surroundings of corrosion pits and non-metallic inclusions. In case of acidic solution, the cracks are also formed round craters

 the craters identified in both steels examined in acidic medium are combined effect of various corrosion damages in high-Mn austenitic steels. Hydrogen impact is the additional effect accompanying general corrosion and corrosion pits forming. Its influence is enhanced by numerous corrosion pits, sulfide non-metallic inclusions and

 mechanical twins formed during cold working in the single-phase austenitic steel accelerate the corrosion progress. However, the special care should concern the steels containing martensite plates (in the initial structure, strain-induced or hydrogeninduced), which are especially susceptible to surface cracking in hydrogen containing

**5. Summary** 

solution. In particular it was found that:

formed due to corrosion pitting;

martensite plates of high hardness;

solution and to corrosion cracking in chloride solution.

Fig. 24. Scaled and cracked layer of corrosion products in 25Mn-3Si-1.5Al-Nb-Ti steel, plastically deformed and immersed in 3.5wt% NaCl.

Fig. 25. Deep corrosion cracks in 25Mn-3Si-1.5Al-Nb-Ti steel after bending and the immersion in 3.5wt% NaCl.

Fig. 26. Microcracks running from non-metallic inclusions along hard martensitic plates in 25Mn-3Si-1.5Al-Nb-Ti steel, plastically deformed and immersed in 3.5wt% NaCl.
