Influence of methylcellulose-coated paper separators on the corrosion,polarization and impedance characteristics of zinc in concentrated ammonium chloride solution. I. Behaviour in flooded electrolyte

The corrosion characteristics of both pure and amalgamated zinc have been studied in 6.0 M NH₄ Cl using steady-state polarization and a.c. impedance methods, and the influence of methylcellulose-coated and uncoated paper separators have been determined. Irrespective of the type of separator present, the results can be interpreted almost exclusively in terms of charge-transfer effects. In the presence of the base paper the anodic zinc dissolution process is inhibited by 79% whilst the rate of cathodic hydrogen evolution is inhibited by only 43%. Similarly, in the presence of the complete methylcellulose-coated separator formulation the anodic reaction is inhibited by 98–99% whilst the cathodic reaction is inhibited by only 60–74%. In either case it is shown that the anodic inhibition is considerably greater than that expected on the basis of a simple blocking type model, whereas the cathodic inhibition is considerably less. Evidence is presented which suggests that the excessive inhibition of the anodic current is a consequence of the specific influence of the separator on the stability and adsorption behaviour of Zn(I) intermediates in the zinc dissolution reaction. It is also postulated that the deficient inhibition of the cathodic current may result from the small size and high mobility of the proton which allows it to penetrate those regions of the metal-separator interface normally inaccessible to bulkier ions. Overall, the corrosion-inhibiting efficiency of the base paper is 41% whilst that of the complete methylcellulose-coated paper separator formulation is 70–75%. These results reflect a substantial influence of the separator material on the corrosion behaviour of zinc.