Understanding corrosion protection mechanisms of nanocomposite polymer coatings on carbon steel : A combination of in-situ AFM and electrochemical studies

Abstract: Corrosion protection of carbon steel is crucial in today’s world and there is an urgent need to replace the hazardous compounds used with environmentally friendly anticorrosion coatings. In this European collaborative research project, new types of functional composite anticorrosion coatings for steel were developed with properties beyond the state-of-the-art using nanotechnology. The corrosion protection properties of waterborne (WB) acrylic and solventborne (SB) alkyd anticorrosion coatings are combined with special ceria nanoparticles (CeNPs), nanoclay and polyaniline (PANI). The corrosion protection mechanisms of these new coating systems in a corrosive environment of 3 wt. % NaCl solution have been explored. In this work, a WB acrylic coating with high hydrophobicity and low emission of volatile organic compounds (VOC), and a WB composite coating with the addition of acetic acid stabilized CeNPs were developed. An advanced surface functionalization was used to make the CeNPs compatible with the WB matrix and to achieve improved corrosion protection for carbon steel. A SB compositealkyd coating with high solids content and the addition of dimethyl dehydrogenated tallow ammonium chloride modified nanoclay, and another one with sulfonic acid doped emeraldine salt (ES) form of PANI were also developed, taking into account of the compatibility between the additives and the polymer matrix.Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were utilized for characterization of the these coatings. Ex-situ and in-situ atomic force microscopy (AFM) was employed to study the stability and activity of the nanoparticles in the nano-structured latex in these coatings. Electrochemical controlled (EC)-AFM and cyclic voltammetry (CV) were used to study the occurrence of active redox reactions in the nanoparticle additive in the acrylic and alkyd composite coatings. In combination, open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) were used to study the long-term corrosion protection behavior of these coatings. The corrosion protection mechanisms of the developed coating systems were revealed by a combination of AFM and electrochemical techniques. The key results and conclusions can be summarized as follows:     The WB acrylic coating with crosslinking was shown to be a stable barrier protection against corrosion for carbon steel in the 3 wt. % NaCl solution, which can be attributed to its high level of hydrophobicity and the compact structure with densely packed latex nanoparticles.   The improved corrosion protection of the WB composite acrylic coating with 1 wt. % CeNPs in the NaCl solution was attributed to the uniform distribution of the CeNPs in the composite coating and the re-deposition of the insoluble cerium compounds, impeding the penetration of water and corrosive ions into the metal surface.    The enhanced corrosion protection of the SB composite alkyd coating with 3 wt. % nanoclay was attributed to the intercalation of the nanoclay with the laminar structure in the polymer matrix that decreased water penetration.   The redox reaction peaks in the CV and the changes of the PANI aggregates under the applied potentials observed in the EC-AFM revealed the active corrosion protection mechanism of the 1 wt. % PANI in the SB composite alkyd coating. The PANI appears to have passivated the metal surface by a redox reaction between the ES and leuco emeraldine base (LB) forms of PANI during exposure to the NaCl solution.   In short, our study has demonstrated the improved corrosion protection properties and revealed the protection mechanisms of the new nanocomposite coatings for carbon steel. Thus, with the environmentally-friendly anticorrosion coatings developed in this project it will be possible to reduce the use of toxic and hazardous substances in and to reduce the release of VOCs from anticorrosion coatings. Furthermore, this work has opened up the possibility for using nanoparticles in the development of organic coatings for corrosion protection of metal.

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