In situ electrochemical impedance spectroscopy of non-BPA food contact coatings on electrolytic tinplate under retort conditions

This study presents an experimental setup to evaluate in situ changes in the corrosion protection performance of food can coatings under retort conditions (121 °C for 90 min; 204,774 Pa). Bisphenol A (BPA)-based epoxy phenolic coating is compared to non-BPA polyester phenolic coatings. Non-BPA coatings were formulated with three levels of phenolic crosslinker to control the glass transition temperature (T_g). Coated tinplate was exposed to 1% NaCl solution and monitored before, during, and after retorting using electrochemical impedance spectroscopy (EIS). Charge-transfer resistance (R_ct), pore resistance (R_pore), and coating capacitance (C_c) from equivalent circuit modeling of impedance data served as quantitative metrics for coating performance and mapped to T_g. There were no clear trends in coating performance correlated with polymer properties before retorting. During retorting, however, R_pore increased with T_g such that coatings could better resist the migration of corrosive species. This strong positive correlation between R_pore and T_g was also observed after 28 d of storage in 1% NaCl solution at 50 °C post-retorting, suggesting potential service life prediction capability. In situ EIS measurements during retorting could serve as a useful tool to screen coating formulations to accelerate the design and development of novel non-BPA coatings.