ABSTRACT
SIMS has been used to study the nature and composition of oxide films on metals and alloys ranging in thickness from ~1 nm-thick electrochemically-formed films to μm-thick oxides produced at high temperature. Emphasis is placed on thicker oxides where 18O/SIMS has been used to investigate the extent of oxygen diffusion in Cr2O3,γ-Al2O3 and NiO, and the role of ‘reactive elements’ such as Ce in improving high temperature corrosion resistance.
Cr2O3 is found to grow primarily by cation diffusion, although a small amount of inward oxygen diffusion occurs down oxide grain boundaries. On pure Al, γ-Al2O3 grows by oxygen transport via local pathways through an outer amorphous oxide layer and subsequent oxygen incorporation at the periphery of the underlying growing γ-Al2O3 islands. Data for γ-Al2O3 scales formed on Fe-20Al at 800°C demonstrate oxide growth by outward cation diffusion. NiO is confirmed to grow predominantly by cation diffusion.18O/SIMS results for oxidized Fe-Cr alloys sputter coated with ~−4 nm of ceria suggest that when Ce is located within the oxide scale there has been a change in oxide growth mechanism from predominantly cation to predominantly anion diffusion. TEM and EELS imaging microscopy have been used to complement the SIMS data, and have confirmed the presence within the scale of particles of CeCrC>3 which likely impede cation diffusion while allowing oxygen anion diffusion to proceed.
