Studies on the pitting corrosion of lead in carbonate media

Cyclic voltametry and potentiodynamic single sweep techniques are used to study the electrochemical behaviour of lead in Na2CO3 solutions containing various concentrations of ClO4- as aggressive anion. The effects of different concentrations, in terms of destruction of passivity and initiation of pitting corrosion, were monitored with reference to the change in integrated anodic charge. It was found that Deltaq(a), (taken as a measure of the extent of pitting) varies linearly with log CClO4-. The pitting corrosion potential, E-pitting, varies with log C-ClO4(-) according to sigmoidal curves. These curves are explained on the basis of formation of passive, active and continuously propagating pits. Additions of aliphatic amines shift the pitting corrosion potential, E-pitting. into the noble (positive) direction, indicating the inhibition action of the added amines on the pitting attack. E-pitting varies with the logarithm of the inhibitor concentration according to: E-pitting = a + b log C-inh The inhibition of pitting corrosion by the aliphatic amines is assumed to be due to either competitive adsorption between the CO32- with ClO4- anions, and/or the chemisorption of the amine on the metal with the formation of a metal-nitrogen coordination bond. The efficiency of these compounds as pitting corrosion inhibitors increases with the increase in the chain length of the alkyl group.