An Evaluation of the Corrosion and Mechanical Performance of Interstitially Surface

A surface hardening technique called “interstitial hardening” has been developed to introduce interstitial carbon atoms into stainless steel surfaces without the formation of carbides. Surface hardening of machine elements such as impellors or fasteners would improve performance regarding cavitation and galling resistance, and has intensified interest in this process. The interstitial hardening technique involves an activation step where the protective oxide film is removed from the surface in order to allow appreciable diffusion of the interstitial atom into the material at temperatures below which precipitate phases will form. Commercial processes have been developed to do this using halogen gases or plasma. However, there remains a need to characterize and validate the specific performance characteristics of the hardened materials. The stability of the hardened surface and the reproducibility of the process on various substrates need to be verified. In particular, the process parameters for which the corrosion resistance of the stainless steel is retained, rather than degraded, is of particular interest for marine applications. This project incorporated experimental testing conducted on 316L stainless steel that has been surface hardened using available commercial techniques, using both carbon and nitrogen as the interstitial atom. The hardness and thickness of the surface hardened layer is characterized and compared using metallography and microhardness profiling. The corrosion performance of the hardened surface is assessed using electrochemical potentiodynamic testing to determine the pitting potential in 3.5 wt. % NaCl solution. Corrosion fatigue and slow strain rate testing of untreated, hardened and damaged, hardened surfaces exposed to ASTM seawater is conducted. Finally, critical galling stresses are determined and compared. Post-test examination of damage attempts to identify mechanisms of material failure and characterize how corrosion-assisted cracks initiate and grow in surface-hardened materials.