Influence of methylcellulose-coated paper separators on the corrosion, polarization and impedance characteristics of zinc in concentrated ammonium chloride solution. II. Comparison of data obtained in flooded electrolyte and battery environments

Abstract

An attempt has been made to determinein situ the corrosion rate of the zinc anode in Leclanché batteries devoid of ZnCl₂ in the electrolyte using steady-state polarization and a.c. impedance techniques. Previously obtained corrosion data for zinc immersed in flooded NH₄Cl electrolyte has been used as a point of reference with which to compare thein situ data and hence gauge its legitimacy. It is shown that in the case of cells containinguncoated paper separators, good agreement between the two sets of data is obtained, thus demonstrating thatin situ corrosion rate measurements can be made. This allows the effects of ionic composition, zinc alloying constituents, amalgamation level, electrolyte impurities, and the role of oxygen, etc, to be determined in the battery environment. However, the anode polarization and impedance characteristics of ZnCl₂-free cells containing methylcellulose-coated paper separators do not reflect corrosion processes. In fact the electrochemical data is almost indistinguishable from that obtained in ZnCl₂-containing cells. It is postulated that the presence of the methylcellulose layer permits the zinc ion concentration to build up adjacent to the anode causing the cathodic hydrogen evolution process to be electrochemically masked by zinc deposition. The electrochemical characteristics therefore reflect zincexchange processes only. Neither the presence of the separator coating itself nor the restricted volume of electrolyte appear to be unilaterally responsible for the anomalous behaviour of ZnCl₂-free cells, but rather a combination of these two conditions is necessary. Thus considerably restricted diffusion and the near absence of convection would appear to be the important factors which dictate the observations.