Aluminum reduction pots at the Pittsburgh ReductionCompany's (Alcoa's) plant i n 1889. Adapted from a photograph, courtesy of Alcoa. DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor Battelle Memorial Institute, nor any of their employees, makes any WUIUI~~, expnsrcd or implkd, or assumes any kgal liability or nsponribi#tr for the accuracy, c o f n~, or uwfulmss of any information, apparlbs, produd, or process dirdortd, or rtprtsents that its use wwM not infdnge privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government of any agency thereof, or Battelle Memorial Institute. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
Activities in support of the Sensors Development Program for FY 1989 were conducted under three major tasks: Digital Signal Analysis (DSA), Development of the Reference Anode, and Field Test of the Reference Anode. DSA of current and voltage signals colleCted from PNL bench-scale laboratory cells suggest there may be correlations between certain quantification parameters derived from these data and the current density of the working anode. Although some data show a dependence on alumina concentration, additional effort is required to determine whether a correlation of DSA with this important cell operating parameter can be unequivocally shown. If successful, such a correlation would form a viable basis for a DSA-based alumina sensor. Digital current and voltage signals collected during the Prototype Inert Anode Test gave correlations involving current density that were similar to those obtained using data from the PNL bench-scale laboratory cells. This suggests a DSA-based sensor may work in a large-scale aluminum reduction cell if it can be shown to work in a bench-scale test. In other words, there may be no serious effects on DSA data resulting from scale-up considerations. Analog current and voltage data were generally stable throughout the Prototype Inert Anode Test and therefore did not signal the onset of the corrosion of the prototype anode that was observed in post-test analysis. This indicates that monitoring de signals may not be a suitable approach to determining, in every case, when an inert anode is undergoing degradation during its operation. Despite the magnitude of effort expended in FY 1989 to develop the reference anode, serious issues regarding its operation and durability remain. Problems were also encountered in trying to use the reference anode in the Prototype Inert Anode Test. Consequently, PNL recommended that further development of the reference anode be suspended. Future tests, including the Pilot Cell Test, will use an Al/Alz03 reference electrode. The Pilot Cell Test was postponed to FY !99D because of delays in obtaining the required materials and manufacture of the anodes from commercial vendors. v Electrochemical impedance data collected on inert anodes in PNL benchscale laboratory cells during the evaluation of the reference anode show a dependence on current density and alumina concentration that is consistent with the formation of a resistive reaction layer. The formation of a resistive reaction layer is not the only possible mechanism for the observed results. Alternative mechanisms are being considered based on separate experimental studies. However, an alumina sensor based on the dependency of the impedance data is not expected to viable because significant hysteresis was observed in these data. vi ACKNOWLEDGMENTS The authors wish to acknowledge the technical assistance of N.
_2_ r • SUI_RY Results of post-test microscopic and elementdl analysis of the reaction zone on polarized cermet inert anodes, over a range of current densities and alumina concentrations, suggest that an alumina film does not form to protect the anode from dissolution. Rather, significant morphological and compositional changes occur at or near the anode surface. These changes and the • chemical reactions that cause them involve the cermet material itself and appear to be responsiblefor propertiesthat were previouslyassigned to an " alumina film. In particular,a reaction layer formed from the cermet material may have protectiveproperties,while changes in roughness and porosity may contributeto the electrochemicalimpedance. iii ACKNOWLEDGMENTS The authorsacknowledge the assistance of the staffat the PacificNorthwest Laboratory (PNL),in particular the managerial guidanceprovidedby L. G. Morgan,the technical contributions of D. M. Strachan, the microscopy work done by N. T. Saenzand J. M. McCarthy, and the secretarial supportby D. L. Schneider.We are also gratefulfor the programmatic assistance pro--videdby M. J. McMonigle, Officeof .Conservation and Renewable Energy_,CE), U. S. Department of Energy(DOE),Washington, DC, and the RichlandOperations -Office,DOE.
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor B<~ttellc Memori<~l Institute, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to <~ny specific commercial product, process, or service by tr<~de n<~me, tr<~demark, manufacturer, or otherwise does not necess<~ri ly constitute or imply its endorsement, recommendation, or f<~voring by the United States Government or <~ny <~gency thereof, or B<~ttellc Memorial Institute. The views <~nd opinions of authors expresserl herein do not necess<~rily state or reflect those of the United States Government or any agency thereof.
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