Intergranular corrosion of austenitic industrial stainless steels in oxidizing nitric media: influence of microstructure and chemical composition
Résumé
Intergranular corrosion (IC) of low carbon austenitic stainless steels in oxidizing nitric media is a long-known industrial matter [1]. However, a special stainless steel Uranus S1N, with 4 wt.% of Si, can resist to this localized corrosion. Therefore, this work aims to better understand the origins of IC in industrial austenitic steels and to explain the resistance of Uranus S1N to find a way to improve the composition and, thus, properties of future steels.
The corrosion behavior of industrial steels of different chemical compositions sensitive to IC corrosion in oxidizing nitric media (304L, 316L, Uranus 65) was investigated in this work and compared to that of Uranus S1N. The microstructure of all steels before corrosion was systematically compared in terms of grain size, crystallographic texture and grain boundary misorientation distribution. Then all the steels were submitted to the same corrosion tests, and the results were compared with the microstructural study results.
It appears that the resistance to IC of Uranus S1N should be related to the differences in chemical composition. Previous works showed the negative influence of chemical impurities (N, S, P or B) on IC [2] and the ambivalent role of Si, which can either enhance IC or protect the steel against corrosion [3]. Those elements are known to segregate at grain boundaries [4]. Using the analysis of grain boundary composition, this study aims to confirm or infirm this possible explanation of the origin of IC.
[1] H. Coriou, Corrosion et anticorrosion, vol. 14, 1966.
[2] I. Ioka, «Influence of impurities on intergranular corrosion of extra high purity austenitic stainless steels,» 17th International conference on Nucelar Engineering, 2009.
[3] P. Fauvet, «corrosion mechanisms of austenitic steels in nitric media used in reprocessing plants,» Journal of Nuclear Materials, vol. 375, n° %11, p. p52, 2008.
[4] L. Beaunier, «Corrosion of grain boundaries : initiation processes and testing,» Journal de physique Colloques, vol. 43, 1982.