5th International High Manganese Steel Conference - HMNS 2022

INTRODUCTION
Herein, the structural and chemical features of passive films formed on a high Cr-added high-Mn 18 (wt%) twinning-induced plasticity (TWIP) steel and on two different austenitic stainless (304L and 316L) steels were revealed via Cs-corrected scanning transmission electron microscopy (STEM) and atom probe tomography (APT).

EXPERIMENTAL
With a goal of increasing the corrosion resistance of typical high-Mn TWIP steel series, high amount of Cr (17wt%) was added to typical 18 (wt%) TWIP steel, i.e., HCr-HMnS. While comparing the passivation abilities of the HCr-HMnS and two 304L and 316L stainless steels by potentiodynamic polarization curves, the thickness and chemical compositions of the material passivation films were examined via Cr-corrected STEM and APT.

RESULTS AND DISCUSSION
Although the passive films of all the samples with similar Cr concentrations had the same thickness, unprecedented hexagonal wurtzite MnO inside the passive film of HCr-HMnS specimen was susceptible to corrosion cracking; this was not observed in the passive films of typical stainless steels. This MnO caused crack formation during potentiodynamic polarization test, suggesting that reducing the harmful MnO by adding Mo and Ni facilitates the development of high-Mn base stainless steels. Furthermore, higher MoO2 composition of the passive films on 316L stainless steels than 304L type series might would result in primarily the improved pitting resistance.

CONCLUSION
We explored the origin of less passivation ability of the high Cr-added TWIP steel sample than the typical stainless steels. Furthermore, based on the chemical difference of passive films between the 316L and the 304L stainless steels, we suggested the primary cause of the improved pitting resistance of 316L stainless steels than 304L type series. We expect that alloying the Mo and Ni, which are capable of reducing the MnO-density, may be beneficial for developing high-Mn base stainless steel materials.