Interactions of Different Redox States of Phenyl-Capped Aniline Tetramers with Iron Oxide Surfaces and Consequences for Corrosion Inhibition

Abstract

The phenyl capped aniline tetramer (PCAT) is known for its redox properties and is being studied for its ability to inhibit corrosion of iron and steel in addition to being of interest for sensors and molecular electronics. Here we investigate the interactions, orientation and corrosion inhibition ability of all three oxidation states of the free base form of PCAT with iron oxide surfaces. Raman spectroscopy demonstrates interconversion of these molecules to one another due to charge transfer to the surface. Polarized mid-IR spectroscopy and atomic force microscopy were used to elucidate the molecular orientations on the surface. Electrochemical impedance spectroscopy shows the corrosion resistance of fully reduced PCAT coatings on low carbon steel to be higher than that for half-oxidized and fully oxidized PCAT coatings. A weight loss test, laser line measurements and Raman spectroscopy reveal that even though half-oxidized PCAT initially shows a lower corrosion resistance due to transformation into the fully oxidized form, with time it transforms back into the half-oxidized form and protects the surface. Fully oxidized PCAT molecules show opposite behavior, causing the degradation of the surface over time. We thus attained a deeper insight into the interplay of the different oxidation states for corrosion control. Polyaniline (PANI) is a prominent member of the conducting polymer family because of its diverse application in biosensors, 1 electrochromic devices, 2 fuel cells, 3,4 etc. One of the most studied applications of this polymer is corrosion inhibition of metals. DeBerry showed in 1985 that PANI can inhibit corrosion of iron and steel by passivating the surface in acidic media. [5][6][7] Similarly to the native oxides, the passive oxides on iron or steel maintain a layered structure of a fully oxidized outer layer, followed by an intermediate mixed iron (II) and iron (III) oxide layer transitioning to a very thin inner iron (II) oxide layer next to the bulk metal. PANI is mainly found in three different stable oxidation states (fully reduced or leucoemeraldine, half-oxidized or emeraldine, and fully oxidized or pernigraniline) and several protonation states. The complete mechanism behind the corrosion inhibition ability of this polymer is still unclear due to its complex chemistry. PANI coatings can break down catastrophically, which is the reason why their commercialization has failed. Recent work demonstrated that this breakdown happens due to a high conductivity and long-range electronic interactions.8 Even though historically it had been assumed that conductivity is essential to the inhibition mechanism, recent work has shown that conductivity of pure PANI coatings and PANI containing coatings can be interrupted by sparse addition of small PANI particles without hurting corrosion inhibition efficiency.9-11 Oligoanilines do not share certain drawbacks intrinsic to PANI coatings (e.g. catastrophic failure, poor solubility). Very little work has been done on oligoanilines, which have related physical a...

Semiconducting and electrical properties of thin hybrid films from pyrrolyl- and anilynyl-silicon precursors

“…12) correlates with the interconversion between different redox states due to changes in local pH and to metallo-complexation of the aniline moieties [3][4][5]10,11], being assisted by the easier penetration of Cl − through the p-type semiconducting film. Inductive loops have been reported for electrochemically deposited polyaniline on Pt in 1 M HCl [32] and, more recently, for iron oxide covered with phenyl-capped aniline tetramers in 3.5 wt% NaCl [33].…”

Progress in Corrosion Research on Alternative Liquid Fuels

“…Because of their specificity, their range of application is very fixed, and they are being used less and less with the advent of the low-carbon era. Typical polymer inhibitors include water-based Acrylic terpolymer [181] and butyl-capped Aniline Tetramers [182].…”