On the Nature of the Corrosion Resistance of Interstitially Hardened Stainless Steels

Case hardening by carburization has long been recognized to produce wear resistant surfaces in steels.  Historically, when case hardening was applied to Cr containing alloys such as stainless steel, corrosion performance was degraded because of chromium carbide formation.  More recently, it was shown that the interstitial hardening (IH) surface modification treatment of stainless steels (SS) enhanced surface hardness and fatigue resistance without compromising ductility. Carbon penetrates into the alloy at a depth of up to 25µm from all surfaces accessible to the treatment atmosphere. These statements are also true for IH surface modification with nitrogen and the nitriding process. The industrial implementation of IH surface modification process is extremely simple, can be used in non-line of sight configurations, maintains sample dimensions, and is relatively inexpensive. A growing body of work performed at NRL on IH-treated stainless steel alloys shows that IH improved corrosion, fatigue, galling, and wear resistance of the treated materials compared to the untreated alloys. The improved corrosion resistance of these alloys is believed to be as a result of the amount of carbon or nitrogen in the passive oxide film. This research aims to identify and understand the underlying effects of temperature, interstitial concentration, and other factors, on the passive film and mechanism by which the oxide film provided the enhanced corrosion resistance. The experimental work, which includes various surface treatments (carbon or nitrogen at different temperatures and concentrations) for either 316 or 2205 stainless steel, chemical analysis of the surface and passive film by XPS, and electrochemical characterization of the corrosion resistance in salt water and seawater environments, will be reported and discussed.