Effect of Nitrogen on Semiconducting Properties of Passive Films of a High Nitrogen Face-Centered-Cubic Phase Formed on Austenitic Stainless Steel

Abstract

Passive films of a high nitrogen face-centered-cubic (f.c.c.) phase (γN) formed on the plasma-based low-energy nitrogen ion implanted AISI 316L austenitic stainless steel with no pitting corrosion in 3.5% NaCl aqueous solution were investigated by using electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis. The protective passive films on the γN phase layer were obtained as n-type and p-type semiconductors in the potential range above and below the flat band potential, respectively, which corresponded to the two regions structure: the iron hydroxide/oxides in the outer region and the chromium hydroxide/oxides and iron oxides accompanying the chromium and iron nitrides in the inner region. The γN phase layer has a maximal phase angle of 83° with an impedance slope of about −0.91 over a wider frequency range approximately from 10−2 Hz to 105 Hz. The donor and acceptor densities of the γN phase layer decreased approximately from 2.4×1021/cm3 to 6.9×1018/cm3 and from 2.9×1021/cm3 to 5.2×1019/cm3, and the flat band potential decreased approximately from −547 mV(SCE) to −669 mV(SCE), relative to that of the original austenitic stainless steel. The nitrogen beneficial effect in the γN phase layer on the pitting corrosion resistance contributed to inhibiting the adsorption of aggressive chloride ions in the outer n-type iron hydroxide/oxides region, to restricting the migration of space charges as a complete barrier from the n-type and p-type semiconductors regions, and to neutralizing of the protons due to the formation of ammonia in NaCl solution.