Resistance to corrosion of galvanized steel covered with an epoxy-polyamide primer coating

The resistance to corrosion provided by an epoxy-polyamide primer attached to a galvanized steel panel was assessed by electrochemical impedance spectroscopy. Three different coating thicknesses were tested in this work, namely 100, 200 and 500 mum, which were applied on panel sections cut-off from overhead electrical transmission-line towers. Corrosion tests were performed by exposing the painted coupons to a 3.08 NaCl aqueous solution in a horizontal flat-cell. Changes in the impedance characteristics of the systems were found to occur as a function of the exposure time in all the three cases, though their evolution with time showed marked differences which derived directly from the different thicknesses of the coatings. With the thinnest coating considered, the primer film could not provide an effective protection for the metal that underwent corrosion asa result of the delamination process. On the contrary, when the coating film was sufficiently thick, effective protection of the metallic substrate was provided by the epoxy-polyamide film. (C) 2001 Elsevier Science B.V. All rights reserved.