Assessment of enamel rating testing as a quality inspection tool for food can applications

The Enamel Rating Test (ERT) is commonly used as a tool to check the quality of polymeric coatings applied to the insides of food and beverage cans. This work addresses the sensitivity of the ERT and detrimental effects of the ERT on subsequent coating performance. Electrochemical Impedance Spectroscopy (EIS), which is assumed to be nondestructive, was used to assess the quality of coatings before and after the ERT. Most of the coatings passed the ERT with a current value less than 10 mA even though EIS could distinguish a wide range of coating protectiveness. Furthermore, the impedance drop found by EIS testing after 1 week immersion in 1 % NaCl solution was higher for samples previously tested by ERT than for those not tested by ERT prior to immersion. The ERT was not only unable to distinguish between good and bad coatings but also negatively affected the performance of the coatings, which was confirmed by visual inspection observations after 20 days.