Smart coating with ph-responsive microcapsules containing green inhibitors for corrosion resistance of magnesium alloys / Nurul Shuhada Binti Haji Mohamed

Faculty of Mechanical and Automotive Engineering Technology

Thesis (Doctor of Philosophy) -- Universiti Malaysia Pahang - 2025

Includes bibliographical references

Metal corrosion is a major loss in various sectors, including transportation, civil and marine engineering, and biomedical engineering. Conventional protective coatings, particularly epoxy-based systems, demonstrate inherent limitations due to microcrack formation during service, leading to localized corrosion and premature material failure Therefore, to overcome these limitations, this study introduces a novel pH-responsive multilayer smart coating system designed to provide autonomous corrosion protection through dual-function self-healing and inhibitor release. This work aims to investigate the pH-responsive multilayer smart coatings, their self-healing ability, and the corrosion performance of multilayer smart coatings containing the corrosion inhibitor benzotriazole (BTA) at 5, 10, and 15 wt.% concentrations. These coatings were synthesized with multilayer biopolymer chitosan at 5 and 10 wt.% concentrations, and their corrosion performance was evaluated. In this study, dual-functional poly(urea-formaldehyde) (PUF) microcapsules were created using an in-situ polymerization technique. These microcapsules contained the corrosion inhibitor BTA in the multilayer shell of pH-responsive biopolymer chitosan and linseed oil (LO) as a healing agent. Microcapsules were used to develop an epoxy-based smart coating. Fourier transform infrared spectroscopy (FTIR), UV-Vis spectroscopy, optical microscopy (OM), scanning electron microscopy with energy dispersive X-ray analysis (SEM with EDX), and zeta potential measurements were used to characterize the PUF microcapsules, multilayer microcapsules, and epoxy-based smart coatings. The corrosion performance of coated magnesium alloy samples was evaluated using open circuit potential (OCP), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS). The average diameter of the spherical PUF microcapsules was 92.34 μm, whereas the multilayer microcapsules measured between approximately 11.25 μm and 12.03 μm. Zeta potential measurements showed successful assembly of the cationic and anionic layers. The percentage release of BTA in acidic medium was over 40%, while for pH 10, it was less than 25%, due to the protonation and deprotonation of H+ ions. Additionally, self-healing tests in buffer solutions with pH 4 and pH 10 revealed successful healing of the multilayer samples. The multilayer smart coatings exhibited OCP values in the range of -1.74 V to -1.48 V. The PDP indicated that the multilayer smart coating decreased thecorrosion rate compared to the epoxy coating, PUF coating, and bare Mg alloy, asevidenced by an inhibition efficiency of 90 to 99%. The EIS results demonstrated that thepresence of multilayer smart coatings offered more protection than the PUF and epoxycoatings, where sample C10B5 exhibited the highest film resistance (Rfilm) value of 7.211x 104 Ω·cm² at pH 4. In an alkaline solution at pH 10, sample C10B10 showed the highestRfilm value of 7.964 x 104 Ω·cm². In conclusion, the innovative multilayer smart coatingexhibited superior self-healing and anti-corrosion properties on AZ31 Mg alloy,demonstrating an active response mechanism with controlled BTA release in variousenvironments. This study is recommended as a solution for proper smart coatingtechniques to enhance the corrosion performance of AZ31 Mg alloy. Potentialapplications include extending the lifespan of structural components in harshenvironments, protecting medical implants, and improving the durability of marine andtransportation infrastructure.