In situ study of phosphate diffusion in epoxy coatings using attenuated total reflection infrared spectroscopy

Abstract

Attenuated total reflection infrared spectroscopy (ATRIR) has been used to monitor the ingress of water and other low molecular weight, infrared active, species into an epoxy polymer coated onto zinc selenide (a high refractive index, infrared transparent substrate). Such a situation effectively models a coated metal and allows the probing of water migration to the coating/substrate interface entirely in situ. Results of diffusion transients for water were validated with gravimetric data and show two stage diffusion at lower temperatures and single stage Fickian diffusion at higher temperatures. The diffusion coefficients (2×10⁻⁹-6×10⁻⁹cm²s⁻¹) for water migration obtained in this way are consistent with previous literature values generally obtained using gravimetric methods. The ATRIR method was then applied to study the diffusion of infrared active inhibitor species in epoxy. Although, many such species have a poor or obscured response between 4000 and 400 cm⁻¹, phosphate absorbs sufficiently, and identifiably, in the 1075-1090 cm⁻¹ region to be detectable. The results demonstrate for the first time unambiguous evidence of the diffusion of phosphate from an external medium to a coating/substrate interface. Importantly, the diffusion transient for phosphate is identical to that for water. Thus, phosphate, and presumably other ionic species, are transported as hydrated entities within epoxy and migrate at the same rate as water. Presumably, such species also accumulate at the interface at a similar rate. This finding has profound implications on the initiation and progression of substrate corrosion.