Kinetics of Formation of Passive Oxide Films on Corrosion‐Resistant Fe‐Cr Thin Films

Abstract

The transient electrochemical behavior of chromium‐containing amorphous and amorphous/microcrystalline iron‐alloy thin films in near‐neutral solutions has been investigated and compared with the behavior of “bulk” amorphous and bulk crystalline specimens of comparable compositions. The films have been deposited onto Ebrite (Fe‐Cr) substrates by the ion‐plating technique. The transient current‐decay behavior upon application of a positive voltage step from the specimen rest potential is reported here. This voltage‐step technique has revealed several distinct regions in the behavior of current with time. Initially, there is an exponential decay region, with current relaxation time constant of 0.5–0.8 sec. Then follows another exponential decay with . The next region of current decay can be described by a power law: , with in the range of 0.6–0.7 for the ion‐plated films and the “bulk” amorphous Fe‐Cr alloy (Metglas 2826A), and 1.0 for the bulk polycrystalline alloy (Ebrite). Finally, there is a relatively slow exponential decay region with time constant of 200 sec to over 1000 sec for low and intermediate values of the voltage step, . For high values of , this region is approximately exponential over limited periods of time, with increasing monotonically from a few hundred seconds to over 6000 sec. The overall results strongly suggest that different mechanisms are operative at various intervals during the formation of the passive oxide layer.