Kinetics of the zinc anodic dissolution reaction in near neutral EDTA solutions

Polarization curves of the anodic dissolution reaction of zinc were determined in EDTA solutions of different total molar concentrations (0.05 0.10, 0.15 and 0.20 mol dm-3), the pH values of which were systematically varied (pH 3.0 ? 10.0). The Tafel slopes of the anodic polarization curves are close to 40 mV dec-1 at lower current densities (10-5 ? 5x10-4 A cm-2) while at higher current densities (5x10-4 ? 10-2 A cm-2) the slopes are in the range of 60 ? 120 mV dec-1. The order of the anodic reaction determined from the anodic polarization curves at lower current densities are: z+(H+) ?? 1/2 for pH < 8 and z+(H+) ??1 for pH > 8, while z+(H4Y) ??1 for all pH values of the examined EDTA solutions. On the basis of these results, two mechanisms of the zinc anodic dissolution reaction are proposed: at pH < 8 and at pH > 8. In both cases the relevant EDTA species directly participate as reactants in the anodic reaction. The dependences of the corrosion potential on pH and on total molar EDTA concentration indicate that the relevant EDTA species take part as reactants in both the cathodic (hydrogen evolution) and anodic (zinc dissolution) reactions of the zinc corrosion process. Odredjene su polarizacione krive anodnog rastvaranja cinka u rastvorima razlicitih ukupnih molarnih koncentracija EDTA(0,05, 0,10, 0,15 i 0,20 mol dm-3) u kojima su sistematski varirane pH vrednosti (pH=3,0 ? 10,0). Tafelovi nagibi anodnih polarizacionih krivih imaju vrednosti bliske 40 mV dek-1 pri manjim gustinama struje (priblizno 10-5 ? 5x10-4 A cm-2) i 60 ? 120 mV dek-1 pri vecim gustinama struje (priblizno 5x10-4 ? 10-2Acm-2). Redovi anodne reakcije odredjeni iz polarizacionih krivih pri manjim gustinama struje su: z+(H+)?? 1/2 za pH8, dok je z+(H4Y)?1 za sve pH vrednosti ispitivanih rastvora. Na osnovu ovih rezultata predlozena su dva mehanizma anodnog rastvaranja cinka ? za pH 8. U oba slucaja relevantne vrste EDTA ucestvuju kao reaktanti. Odredjene vrednosti potencijala korozije u funkciji pH i ukupne molarne koncentracije EDTA indiciraju da relevantne vrste EDTA ucestvuju kao reaktanti kako u anodnoj (rastvaranje cinka) tako i katodnoj (izdvajanje vodonika) reakciji procesa korozije cinka.