High anti-corrosion barrier of poly(methyl methacrylate))-silica coatings explained: a thousand-days study

Abstract

Poly(methyl methacrylate) (PMMA)-silica coatings form a few micrometers thick anti-corrosive barrier that blocks the permeation of water and ions when exposed to harsh environments. Their excellent anti-corrosive performance stands out for protecting steel and aluminum alloys immersed in seawater for long periods (> 2 years), making them compliant to several applications in the marine, aeronautical, and automotive industries, for example. A key approach to understanding the degradation of high-performance polymer and hybrid coatings over time consists of analyzing their water uptake-induced structural changes. This work is the first to examine in detail the uptake and structural modification of PMMA-silica coatings on AA2024 and AA7075 Al alloys immersed over a period of more than 1000 days in 0.6 M NaCl solution, using several complementary methods. Gravimetry, thermal analysis, infrared spectroscopy and electrochemical impedance spectroscopy (EIS) were employed to monitor the evolution of coated samples. The structural analysis performed using nuclear magnetic resonance, X-ray photoelectron spectroscopy, electron and atomic force microscopies before and after immersion indicate a slight leaching-induced surface roughening due to Si-O-Si hydrolysis. These findings comply with water uptake profiles calculated using the coating permittivity extracted from EIS data. The remarkable barrier property with impedance modulus in the GΩ range is associated with low uptake values (~ 0.6 vol.%) that can be explained by the highly cross-linked hybrid structure, which results in a very low permeation rate of the electrolyte towards the coating/alloy interface. The applied methodology is of crucial importance for establishing a standardized analysis for high-performance barrier coatings used for technological applications.