INITIATION OF PITTING CORROSION ON ZINC IN NEUTRAL INHIBITED CONDITIONS

The effect of the addition of aggressive salts such as LiCl, NaCl, KCl, RbCl and MgCl2 on the steady-state potential of a Zn electrode previously equilibrated in a passivating chromate solution is established. S-shaped curves are obtained for the variation of the steady-state potential with the quantity of aggressive salt added. For each inhibitor concentration, C(inh), the addition of aggressive ions up to a certain concentration has no effect on the passivity of Zn. However, higher Cl- ion concentration causes destruction of the passive film and initiation of pitting corrosion. Destruction of passivity occurs after an induction period which decreases with the increase in the concentration of the attacking ion and/or the decrease in that of the inhibiting ions. The concentration of aggressive ion, C(agg), that can be tolerated by a certain concentration of the inhibiting ions. C(inh), is given by the relation : log C(inh) = K + n log C(Cl)-, where K and n are constants. The efficiency of these salts in initiating pitting corrosion increases in the order RbCl less-than-or-equal-to MgCl2 less-than-or-equal-to KCl < NaCl < LiCl. The change in the degree of aggressivity of these salts could be attributed either to the incorporation of the cations in the passive film or to their effect on pH.