## Mathematical modeling studies of electrochemical growth of oxide films on metals

 dc.contributor.advisor Kurt R. Hebert dc.contributor.author Wang, Mei-Hui dc.contributor.department Chemical and Biological Engineering dc.date 2018-08-23T00:56:07.000 dc.date.accessioned 2020-06-30T07:16:04Z dc.date.available 2020-06-30T07:16:04Z dc.date.copyright Thu Jan 01 00:00:00 UTC 1998 dc.date.issued 1998 dc.description.abstract

This dissertation consists of two parts. First, a mathematical model was formulated to investigate the structural change of the oxide film on aluminum due to cathodic charging. The model assumed a duplex film structure composed of a barrier layer on the metal side and a porous layer on the solution side. The processes formulated in the model include all relevant capacitive processes, conduction in both layers of the oxide film, and interfacial reactions, such as pore filling by oxide growth at potentials higher than the open circuit potential. The model was fit with the anodic current transients during subsequent anodic polarization to determine the structural parameters in the model. As a result, the structural changes caused by cathodic charging were found to be described by the growth of pores in the outer portion of the initial film. The pores may be produced by the non-uniform electrochemical dissolution of the oxide during cathodic current flow;Secondly, a defect cluster model has been developed for ionic conduction in amorphous anodic oxide films. The physical processes in the model include the hopping of oxygen vacancies, as the rate limiting step, and metal ion transport within each vacancy-centered cluster. A mathematical model was formulated for the overall ionic conduction across an oxide. Based on the steady-state solution of the modeling equations, the transference number tM and the field coefficient B in the current-field relation i = Aexp(BE) were derived for A12O3, Ta2O5, Nb2O5 and WO3. The model predicts comparable transference numbers for both metal and oxygen ions and the calculated values of tM are very close to the experimental values. Also with the consideration that the hopping of an oxygen vacancy involves the rotation of a stoichiometric unit OMp, the model predicts the values of B to be about 5 x 10-6 cm/V for the oxides, which is close to most experimental values.

dc.format.mimetype application/pdf dc.identifier archive/lib.dr.iastate.edu/rtd/11821/ dc.identifier.articleid 12820 dc.identifier.contextkey 6510308 dc.identifier.doi https://doi.org/10.31274/rtd-180813-10746 dc.identifier.s3bucket isulib-bepress-aws-west dc.identifier.submissionpath rtd/11821 dc.identifier.uri https://dr.lib.iastate.edu/handle/20.500.12876/65121 dc.language.iso en dc.source.bitstream archive/lib.dr.iastate.edu/rtd/11821/r_9826582.pdf|||Fri Jan 14 18:59:18 UTC 2022 dc.subject.disciplines Chemical Engineering dc.subject.keywords Chemical engineering dc.title Mathematical modeling studies of electrochemical growth of oxide films on metals dc.type article dc.type.genre dissertation dspace.entity.type Publication relation.isOrgUnitOfPublication 86545861-382c-4c15-8c52-eb8e9afe6b75 thesis.degree.level dissertation thesis.degree.name Doctor of Philosophy
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