| Summary: | Accurate control of the aluminium concentration of Hot Dip Galvanising (HDG) baths is of critical importance to produce galvanised steel with the optimum coating microstructure and hence desired chemical and physical characteristics. As HDG is a continuous process, a means of continuously monitoring the aluminium level is required. Through on-line monitoring, for example by electrochemical sensors, the aluminium content can be maintained at the desired level at all times and hence the coating composition controlled continuously. A brief study of sensors based upon NaAlCl<SUB>4</SUB>(I)/NaCl(s) electrolytes was conducted to develop "best practice" procedures and conditions for the testing of novel, electrochemical sensors in small Zn-Al melts. Using the determined, optimum, experimental conditions, two contrasting novel approaches for sensing aluminium concentrations in molten zinc have been investigated. The first approach examined the possibility of <I>in-situ</I> formation of a high conductivity, molten salt electrolyte from a readily available pre-cursor - sodium chloride. High conductivity electrolytes were produced by DC electrolysis of the pre-cursors in Zn-Al melts, the acquired EMFs from which were found to be in good agreement with those predicted by theory. However, problems in terms of the deviation of EMFs from theory on extended testing were observed, and these rationalised in terms of varying component activity. In the second approach, a trivalent-aluminium-ion conducting, solid electrolyte system was investigated and its potential for sensing aluminium concentrations assessed. High-density, aluminium tungstate-alumina composites were produced using novel, preparation techniques and the composites found to exhibit Al<SUP>3+</SUP>, ionic conductivity enhancements, over pure aluminium tungstate, of over an order of magnitude. The conductivities of the composites were found to be critically dependent upon the sintering temperature, and this related to alumina phase transformations within the samples during sintering. On testing of the electrolytes in sensors in molten zinc, problems were encountered in terms of both the reaction of aluminium tungstate with zinc, and the loss of oxygen from the electrolytes' structures resulting in electronic conduction and associated deviations of sensor EMFs from theory.
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