Electrochemical Modification of the Passive Oxide Layer on a Ti Film Observed by In Situ Neutron Reflectometry

Abstract

Anodization and the effect of subsequently applying cathodic potential to a thin‐film Ti electrode in an aqueous NaCl solution have been studied with in situ neutron reflectometry. This new technique provides further insight into the processes underlying anodic oxide formation and hydrogen absorption under cathodic polarization. The results (Pilling‐Bedworth ratio, anodization ratio, the onset of fluctuations in electrode current under cathodic potential, etc.) are generally in agreement with the literature, but this new technique provides further insight into the electrochemical processes. The anodized oxide is observed to be not porous, and has the density of the rutile structure for its entire thickness. However, it comprises two distinct regions: an inner region similar in thickness and composition to the original air‐grown oxide, and an outer region containing a significant amount of hydrogen. The similarity of the inner region to the original oxide suggests that the underlying oxide growth mechanism for Ti is the “point‐defect model.” Under applied cathodic potentials the overall oxide thickness remains constant, but the inner region is gradually converted to a material similar in hydrogen content to the outer region. The onset of massive hydrogen penetration seems to occur when the conversion is complete, or when the inner region has been reduced to only a few atomic layers. © 1999 The Electrochemical Society. All rights reserved.