Chloride induced pitting corrosion of nickel in alkaline solutions and its inhibition by organic amines

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, Ep, which are more active the higher the concentration of the Cl− ions. Ep varies with the logarithm of Cl− ions concentration according to sigmoidal S-shaped curves.The change in the integrated anodic charge amount, Δqa, in the presence and absence of Cl− ions, is taken as a measure of pitting corrosion likelihood to take place. The logarithm of Δqa changes linearly with the CCl−CCl− according to:log Δqa=a1+b1 log CCl−,   where   a1   and   b1   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, Ep, changes with the inhibiting agent concentration according to the following equation:Ep=α1+β1 log Cinh,   where   α1   and   β1   are   constants.Ep=α1+β1 log Cinh,   where   α1   and   β1   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.

► The cyclic voltammgrams show that the integrated anodic charge amount, Δqa in the presence of Cl− ions is taken as a measure of pitting corrosion likelihood, Δqa varies linearly with log CCl−.log CCl−.
► The pitting corrosion potential, Ep, varies with the log CCl−.log CCl−.
► Organic amines inhibit the pitting corrosion.
► Hydrazine enhances the pitting corrosion of nickel, due to the formation of hydrazine Ni-complexes, [Ni(N2H4)n]Cl2.
► Urea behaves bifunctionally, inhibits pitting corrosion and enhances it.