| Popis: |
The market for polymer laminated packaging steel is currently growing due to the improved barrier performance, reduced processing by the can maker and increasingly as a result of environmental concerns associated with solvents and health concerns with bisphenol A (BPA), a major component in epoxy phenolic lacquers. Despite the benefits of these laminated systems a concern exists around environmental stress cracking/ crazing (ESC) of the polymer, a phenomenon that accounts for 15% of bulk polymer failures [1]. The primary aim of this paper was to examine the effect ESC has on the corrosion performance of Electro Chromium Coated Steel (ECCS) laminated with polyethylene terephthalate (PET). Electrochemical Impedance Spectroscopy (EIS) was used to characterise the ESC/corrosion phenomena. EIS is a recognised system for assessing organic coatings however it gives a global measure of coating performance thus dye penetrant and scanning vibrating electrode technique (SVET) have been used to spatially characterise the EIS signal. EIS measurements were carried out over a 4 day period in aqueous 5 wt % NaCl to measure the drop in coating impedance as the coating degrades. This was fitted to an equivalent circuit that is correlated back to the coated system. Dye penetrant (figure 1) and SVET (figure 2) were subsequently used to characterise the influence the crazing has on the failure of the coating. In brief, an argument is presented that two main features exist; initially there is the openness of the crazes at the surface that provide diffusion routes into the surface although a significant time delay occurs due to the complex diffusion routes offered by the crazes. After the time delay the second feature occurs as diffusion increases and then in turn the conductivity of the coating also increases until conditions become sufficient at the metal/polymer interface for the steel to be attacked. Due to the poor protection offered by the nano scale chromium coating corrosion then ensues. [1] D. C. Wright, Environmental stress cracking of plastics Shrewsbury: Rapra Technology, 1996. Figure 1 |
