Structure and electrochemical properties of PMMA-silica hybrid coatings modified with cerium and lithium ions

Author(s)

Andressa Trentin

• Document type: Doctoral Thesis
• Press: Araraquara. 2019-12-17.
• Institution: Universidade Estadual Paulista (Unesp). Instituto de Química. Araraquara
• Defense date: 2019-10-08
• Advisor: Peter Hammer

Abstract

This work reports on a detailed investigation of the structural and electrochemical properties of poly(methyl methacrylate) (PMMA)-silica coatings. Raman spectroscopy and thermal analysis showed that the fine-tuning of the benzoyl peroxide (BPO) amount, as a critical synthesis parameter, improves the polymerization efficiency of methyl methacrylate (MMA), leading to a highly cross-linked hybrid structure. Electrochemical impedance spectroscopy (EIS) showed for coatings deposited on carbon steel (A1020) and Al alloy (AA7075) a quasi-ideal capacitive impedance response, maintaining the low frequency impedance modulus of up to 10 GΩ cm2 essentially unchanged during 19 months of immersion in 3.5% NaCl. Although high performance passive coatings have been developed, pitting can significantly affect their durability. Hence, active corrosion inhibition induced by lithium and cerium ions in PMMA-silica sol-gel coatings was investigated. The addition of lithium carbonate yielded coatings with improved connectivity of nanometric silica cross-link nodes and stronger adhesion to the aluminum substrate. EIS results showed that higher lithium concentrations result in an increased impedance modulus and induce the self-healing ability, extending significantly the service life of the coatings. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) suggest that the regeneration process occurs by means of lithium ions leaching from the adjacent coating towards the corrosion spot, which is restored by a protective layer of Li containing aluminum hydroxide species. Finally, coatings loaded with Ce(III) and Ce(IV) provided long term protection for aluminum alloy, however only Ce(IV) imparts active inhibition due to the passivation of cathodic sites by forming highly insoluble Ce oxides and hydroxides. Intermediate Ce(IV) concentrations proved to be more effective in improving the barrier property combined with active protection of intact and damaged coatings over immersion time. In brief, this study found a close relationship between optimized structure, anticorrosive effectiveness and self-healing activity induced by lithium and cerium ions in PMMA-silica coatings, highlighting their high potential of the material for the effective protection of metallic surface. (AU)

• FAPESP's process: 15/11907-2 - Structure and electrochemical properties of PMMA-silica hybrid coatings modified with cerium and lithium ions.
• Grantee: Andressa Trentin
• Support Opportunities: Scholarships in Brazil - Doctorate