Zeolite/polyaniline based self-healing and silicon oxide coatings for microbially induced corrosion inhibition

Abstract

Microbially induced corrosion (MIC) occurs due to the presence of microorganisms such as bacteria, which form biofilms on the metal surface that can cause corrosion. Among the different methods that have been used to protect against MIC, coating has gained more attention because of its ease of application, low-cost and high effectiveness. Recent research has shown that self-healing coatings concept based on releasing healing agent when micro-cracks are initiated in the coating surface and hydrophobic silicon oxide based organic and inorganic coatings have great potential for use as antifouling coating. The aim of this research is to investigate the effects of self-healing and silicon oxide (SiO) coatings on inhibiting MIC in saline environment. The self-healing coating was prepared via interfacial polymerization of zeolite, polyaniline, and zeolite/polyaniline composite and then encapsulated with urea fomaldehyde as a shell material to form the microcapsules and embedded in epoxy to form coating material which was applied on mild steel substrate. The SiO coating, on the other hand, was deposited on mild steel substrate using radio frequency (RF) magnetron sputtering physical vapor deposition (PVD) method with different parameters of RF power, temperature, pressure and deposition time in order to achieve optimum parameters based on minimum surface roughness and good adhesion. The surface topography and roughness were examined by atomic force microscope (AFM), while the thickness and morphology of the coatings were observed using field emission scanning electron microscope (FESEM) equipped with energy dispersive spectrometer (EDS). The adhesion test was performed using nano scratch test for SiO coating and Pull off test for self-healing coating and supported by Rockwell C test. The corrosion behavior was investigated through salt spray test for 28 days and immersion tests in nutrient rich simulated seawater (NRSS) medium with pseudomonas aeruginosa bacteria for 70 days. The Tafel electrochemical test and electrochemical impedance spectroscopy (EIS) was performed on both bare and coated steel samples. AFM results clearly revealed that by varying the sputtering parameters has a strong influence on the surface roughness of the deposited SiO coating in which its thickness varied between 30 nm to 50 nm. The thickness for self-healing coating was between 50 μm to 175 μm. From the adhesion results, both coating methods produced superior adhesion on steel substrates. Fourier transform infrared spectroscopy (FTIR) results show the successful encapsulation of the three synthesized materials. The total self-healing occurred after the release of the core material when the capsule was ruptured after 21 days left at room temperature. The specimen exposed in salt spray chamber exhibited excellent corrosion resistance for all investigated coating materials, while, the specimens immersed in NRSS medium with pseudomonas aeruginosa bacteria showed varying anti-corrosion properties. Tafel results show that the lowest corrosion rate was observed for SiO coating with a value of 0.219 mm/yr, followed by encapsulated zeolite/polyaniline composite self-healing embedded in epoxy of 0.334 mm/yr. EIS results show that among all the coatings, encapsulated zeolite/polyaniline composite self-healing embedded in epoxy coating has the highest impedance modulus (Z) at a frequency of 0.01 of 7800 ohms. In conclusion, zeolite/polyaniline composite self-healing coating is the best among all the coating materials which shows superior anticorrosive and MIC inhibition property.