Evaluation of Candidate Coatings for the Metal Traycan

The purpose of this study was to obtain quantitative data on the protective properties of coated tin-plated steel traycans versus coated tin-free steel traycans. Four interior coating variables and a control were tested using electrochemical techniques. The study is divided into two parts Part A - Investigation of Organic Traycan Coatings, and Part B - Investigation of Corrosion Protection of Coated Tin-Plate versus Coated Tin-Free Steel Substrates. Under Part A, three basic assessments were conducted on the organic coatings 1. electrochemical quantitative assessment of defect, 2. intrinsic protective property assessment using alternating current impedance techniques, and 3. assessment of blister defects in coatings that are indicative of corrosion. The results showed that the candidate coating, Dexter-Midland Matte Sheet, was the superior coating in terms of least number of defects of all the coatings assessed. Based on the alternating current impedance evaluation, the Valspar coating over tin-plate coil stock had the best resistance to penetration by the test corrosion medium. It was determined that blistering occurred after thermoprocessing and that blisters are the sites of future corrosion. Under Part B, accelerated corrosion tests were performed on coated tin-plated steel and coated tin-free steel with intentional defects i.e., scratches in contact with various foods. The galvanic current generated between steel and tin in these foods was also measured. Results showed that the tin-plated substrate provided two to three times more corrosion protection than the tin-free substrate, and that the greater the galvanic current, the greater the relative protection provided by the tin-plate.