Corrosion resistance of nitrogen implanted iron foils: a conversion electron Mossbauer effect study

Abstract

Conversion electron mossbauer spectroscopy (CEMS) has been employed to study the effect of ion implantation and subsequent annealing on the oxidation kinetics of iron foils. 80 keVN⁺₂ ions were implanted in iron foils at a dose of 2×10¹⁶ ions/cm² and a number of such samples were subjected to thermal oxidation treatment. Both CEMS and the conventional gravimetric techniques bring out the fact that the as-implanted samples exhibit enhanced oxidation while the implanted and annealed samples show considerably reduced oxidation as compared to that of the virgin iron foils. Ion implantation produces point defects and compressive stresses in the implanted layers. Whereas the former enhances the oxidation rate, the latter tends to inhibit it in the case of iron foils. The vacuum heat treatment anneals out the point defects, stabilizes the compressive stresses, and favors migration of nitrogen atoms towards grain boundaries and dislocation pipes leading to their passivation thereby inhibiting the oxidation of the iron foils. Confirmatory support for this analysis is derived by extending these studies to the study of the oxidation of iron foils implanted with Ar⁺ ions and of iron films in which stresses were induced by cooling the films deposited at high temperature on substrates of molybdenum whose thermal coefficient of expansion differs considerably from that of iron.