Mapping water uptake in organic coatings using AFM-IR

Abstract

The long-term failure of seemingly intact corrosion resistant organic coatings is thought to occur via the development of ionic transport channels, which spontaneously evolve from hydrophilic regions on immersion, i.e., as a result of localized water uptake. To this end, we investigate water uptake characteristics for industrial epoxy-phenolic can coatings after immersion in deionized water and drying. Moisture sorption and the changing nature of polymer-water interactions are assessed using FTIR for dry and pre-soaked films. More water is found to be absorbed by the pre-soaked coatings on exposure to a humid environment, with a greater degree of hydrogen-bonding between the polymer and water. Furthermore, morphological changes are then correlated to localized water uptake using the AFM-IR technique. Nanoscale softened regions develop on soaking, and these are found to absorb a greater proportion of water from a humid environment.

Corrosion of carbon steel in the [P 14666 ][Br] ionic liquid: The effects of γ-radiation and cover gas

“…Water is not reactive with organic radicals and hence its transport is not directly affected be simulated by addition of H2O2 to the water [8], as H2O2 is the corrosion rate controlling water radiolysis product [35,36]. We speculate that the formation of H2O2, or organic peroxy radicals (•RO2) formed via radiolytic decomposition of IL cations in presence of dissolved oxygen and/or water (reaction 3), may be responsible for the radiation-induced corrosion behaviour observed in this study.…”

Advances in corrosion protection by organic coatings: What we know and what we would like to know

“…Recently, however, advances in sub-diffraction limit infra-red analysis using atomic force microscopy have permitted mid-infra chemical imaging at the nanometer scale 9 . Such work has shown that water uptake into epoxy-phenolic can coating systems is highly heterogeneous 10 and is correlated to areas unreacted epoxy that are presumed to be associated with local cross-link density and polymer network free volume. Related work has also, for the first time, confirmed a chemically and physically heterogeneous nanostructure at the same length scale within an epoxy-phenolic network 11 .…”

The degradation mechanism of an epoxy-phenolic can coating

“…However, at no point during ion permeation experiments was a sudden increase in conductivity measured, as would be expected if a channel capable of supporting rapid ionic transport were to spontaneously form. Furthermore, it is important to note that prior to any deformation due to water sorption (detectable only after several days immersion [15]), ion transport was already found to occur across the intact coatings, indicating that ion penetration is not wholly dependent on water-induced degradation.…”

“…Indeed, we have previously reported water plasticisation of these epoxy-phenolic can coatings, where the measured T g was depressed by 4 ºC (from 89 ºC to 85 ºC) after 7 days immersion in deionised water and drying. [15] Since the penetration depth of the evanescent wave for a germanium internal reflection element at 2550 cm -1 can be calculated to be <500 nm [32], a further notable result for FTIR experiments is the rapid penetration of water through the entirety of the (nominally 8 µm thick) coatings (<60 min), demonstrating that water can freely diffuse through the intact coating structure. To assess whether this could be accompanied by ion transport when the immersion medium is electrolyte, permeation experiments were undertaken.…”

“…[13,14] In addition, we recently reported the first direct observations of heterogeneous water sorption into epoxy-phenolic coatings using the newly developed AFM-IR technique. [15,16] It was found that water diffusion into films is initially heterogeneous, and that prolonged immersion in deionised water yields localised deformation and swelling. This supports a picture of water-induced damage as the first stage of coating deterioration.…”