Relationship between Metallurgical States and Corrosion Resistance of Nitrided Martensitic Steels in Marine Environment

The objective is focused on the comprehension of the relationship between the metallurgical state and the functional properties of nitrided martensitic stainless steels. Nitriding is performed by a low temperature plasma treatment (Direct Current mode) in various conditions. Metallurgy of the nitrided layer was characterized at different scales, from the macroscopic to the nanometric ones. An increase of the thickness of the nitrided layer with temperature of the plasma treatment is observed. Precipitation of CrN and others nitrided phases in relation with crystallographic defects is observed. Moreover, the TEM analyses on FIB samples extracted from the surface highlight the formation of a succession of different nitrided layers depending on the nitrogen content : expanded martensite, iron-nitride phases [2]. The electrochemical properties of nitrided samples are also assessed by voltammetry test and extended immersion test. Results highlight differences on the corrosion behaviour of the nitrided X17CrNi16-2 steels with temperature of the plasma treatment. The reactivity evolution is linked to the formation of a “composite” system composed of different layers. SECM was used to understand the reactivity evolution through these layers, and successive surface grinding permits to corroborate the reactivity evolution and to understand the origin of the corrosion resistance evolution after nitriding process. [1] H.E. Boyer, T.L. Gall. Metal Handbook, Desk edition, ASM, Ohio, USA, 1986 [2] J. Yang &al. Materials and Design, Vol. 32 (2011) 808-814