Abstract
Cyclic voltammetry, potentiodynamic and potentiostatic anodic polarization techniques complemented by SEM investigation are used to study the initiation and inhibition of pitting corrosion of nickel in NaOH solutions. The presence of Cl- ions locally destroys the passive film formed on the nickel electrode at the pitting potentials, E-p, which are more active the higher the concentration of the Cl- ions. E varies with the logarithm of Cl- ions concentration according to sigmoidal S-shaped curves.
The change in the integrated anodic charge amount, Delta q(a), in the presence and absence of Cl- ions, is taken as a measure of pitting corrosion likelihood to take place. The logarithm of Delta q(a) changes linearly with the C-Cl- according to:
log Delta q(a) = a(1) + b(1) log C-Cl-, where a(1) and b(1) are constants.
Addition of monomethylamine, monoethylamine, and dimethylamine up to a certain threshold concentration, which depends on amine type, causes complete depression of the dissolution current peak and a marked shift of the critical pitting potential into the noble direction. Following this threshold concentration, the critical pitting potential, E-p, changes with the inhibiting agent concentration according to the following equation:
E-p = alpha(1) + beta(1) log C-inh, where alpha(1) and beta(1) are constants.
Hydrazine enhances the active dissolution and pitting corrosion. The pitting potential is shifted into the active direction at all its concentrations. This behavior is attributed to the formation of hydrazine Ni-complexes. Urea behaves bifunctionally, inhibiting pitting corrosion of nickel when present in low concentrations, and enhancing the attack at higher concentrations. (C) 2011 Elsevier B.V. All rights reserved.