Corrosion Behavior of Interstitially Hardened Marensitic Stainless Steels

Case hardening with carbon has been used to produce wear resistant surfaces in steels.  However, when traditional case hardening processes are applied to Cr containing alloys such as stainless steel, corrosion performance is degraded because of chromium carbide formation.  In recent years, it has been shown that surface modification of stainless steels (SS) using interstitial hardening (IH) under paraequilibrium conditions enhanced surface hardness without compromising ductility or corrosion resistance. Paraequilibrium refers to the fact that diffusion kinetics of substitutional solutes such as Cr and Ni in a 316 type composition, for example, are many orders of magnitude slower than interstitial solutes, such that they are effectively immobile, whereas carbon can diffuse to depths of 20-30 µm. Thus carbon can be introduced without the formation of carbides. Plasma processing with nitrogen has also been used to produce IH surfaces on stainless steels. A growing body of work performed at NRL on IH-treated stainless steel alloys shows that IH-treatment improved corrosion, fatigue, galling, and wear resistance of the treated materials compared to the untreated alloys. In this communication, the electrochemical and corrosion behavior of IH-treated martensitic stainless steels (17-4, 15-5 and 13-8) in salt water and seawater environments will be reported and discussed as function of treatment temperature and surface concentration.