The reductive deposition of quaternary pyridinium corrosion inhibitor multilayers on glassy carbon electrodes

The redox behaviour of several quaternary pyridinium corrosion inhibitors ("Quats") at glassy carbon (GC) electrodes has been investigated in neutral aqueous solutions. The primary emphasis is on n-butyl-3-(N-octylcarbamyl)pyridinium bromide, one of the Quats determined to have very good corrosion inhibition properties based on parallel weight loss experiments. It is demonstrated that the reduction of the Quats, which occurs in a two-electron reaction at potentials negative of -1.0 V vs. SSCE, results in the formation of a neutral product. This forms a porous film on the electrode surface, with clusters that expand and thicken as more Quat deposits, as confirmed by atomic force microscopy. The film charge density reaches a maximum of ~4 mC/[cm.sup.2], equivalent to more than 100 monolayers of Quat, after which no further Quat can be deposited, suggesting full surface coverage by a nonconducting product. The Quat multilayer can then be redissolved at potentials positive of ~ -0.2 V, during its oxidation back to its original solution-soluble form. Key words: quaternarized pyridinium compounds, reductive film deposition, glassy carbon, multilayer, corrosion inhibitors, cyclic voltammetry, AFM.