Non-Linear Evaluation of Coatings Performance: Evaluation of Polyester/Melamine Coil Coating Hydrolysis in NSS Test

Abstract

Coating is one of the most common and effective ways to protect metallic elements from corrosion. The evaluation of a coating’s performance is related to its quality and durability. Neutral salt spray (NSS) is a common accelerated test used for coatings. Commercial polyester/melamine coil coatings applied on aluminum specimens were exposed to NSS and recorded at 0, 200,400, 600 and 1000 h of exposure. Coating performance changed with time of exposure. The main degradation process is due to the hydrolysis of the coating. A decrease in corrosion rate was observed due to the sealing of the coating, probably due to the deposition of corrosion products, salts, or a mixture of both. EN (electrochemical noise) was a more sensitive technique comapred to EIS (electrochemical impedance spectroscopy). The first changes in the coating were detected after 400 h of exposure by EIS and after 200 h by EN. EIS and EN showed equivalent results in studying the corrosion evolution of coated aluminum. Non-linear analysis using recursive plots (RP) showed the chaotic dynamics occurring with time and the interaction of micro electrochemical cells that formed on the metallic surface; it offered information similar tothe traditional EIS technique and adds complementary data to understand the corrosion phenomenon.