Corrosion protection of aluminium by a cataphoretic epoxy coating V.B. Mis Ïkovic Â-Stankovic  a,* , M.R. Stanic  a , D.M. Draz Ïic  b a Faculty of Technology and Metallurgy, University of Belgrade, P.O. Box 3503, 11120 Belgrade, Yugoslavia b ICTM-IEC, P.O. Box 815, Njegos Ïeva 12, 11001 Belgrade, Yugoslavia Received 27 October 1998; accepted 11 February 1999 Abstract The corrosion behavior and thermal stability of epoxy coating electrodeposited on aluminium have been investigated during exposure to 3% NaCl. Electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption experiments, thermogravimetric analysis (TGA) and polarization measurements were used. From the results obtained from EIS (pore resistance, coating capacitance, relative permittivity of coating, charge-transfer resistance, double-layer capacitance), gravimetric liquid sorption experiments (diffusion coef®cient, energy of activation of water diffusion) and TGA (water content inside the coating and thermal stability), it can be concluded that the electrochemical, transport and thermal properties of an epoxy coating on aluminium are signi®cantly improved with respect to the same epoxy coating on other substrates (steel, phosphatized steel, steel modi®ed by Zn±Ni alloy). The better protective properties can be explained by less porous structure of the coating on aluminium, caused by lower rate of H 2 evolution, while the prolonged corrosion protection is due to the passive, mainly barrier ®lm of Al 2 O 3 on its surface. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Corrosion; Epoxy coatings; Electrodeposition; Aluminium; EIS 1. Introduction Polymer coatings provide corrosion protection by acting as a barrier layer between the substrate material and the environment [1±3]. All polymer coatings are to some degree permeable to water, oxygen and ions, depending on the characteristics of the polymer coating, the type of substrate and the surface treatment and the metal/coating interface. Aluminium and aluminium alloys have been recently introduced in car industry and substitute partially or even totally the steel due to its high strength/stiffness to weight ratio and good corrosion resistance [4±7]. When exposed to aggressive environments, aluminium and its alloys react especially with oxygen and chloride ions to form complex interface. It is well known that the oxide ®lm which forms on aluminium is responsible for the high corrosion resistance of aluminium [8]. The aim of this work was to investigate the corrosive- protective properties of an epoxy coating electrodeposited on aluminium and the effect of aluminium surface on transport properties, electrochemical behaviour and thermal stability of the coating, since it was shown that the substrate can signi®cantly in¯uence the characteristics and stability of the polymer/metal system against corrosion [9±11]. 2. Experimental Protective epoxy coatings were formed during cathodic electrodeposition of an epoxy resin modi®ed by amine and isocyanate on aluminium Al 99.5, using a constant voltage method (CATOLAC emulsion 543.052, produced by Indus- trie Vernici Italiane under Pittsburgh Paint and Glass (PPG) license: solids content, 36 wt.%; density at 258C, 1020± 1060 g dm 3 ; basic groups content, 50 mmol per 100 g solid resin; acid groups content, 25 mmol per 100 g solid resin; solvent content, 5±7 wt.%; electrolytic conductivity at 258C for 15 wt.% resin concentration, 1150 S cm 1 ). The resin concentration in the electrodeposition bath was 10 wt.% solid dispersion in water at pH 5.7, the temperature was 278C and the applied voltage was 250 V. The test panels (20  20  0.23 mm) were pre-treated by degreasing in 10% NaOH at 608C, pickling with a 50% HNO 3 and rinsing Progress in Organic Coatings 36 (1999) 53±63 *Corresponding author. Fax: +381-113370-387; e-mail: vesua@elab.tmf.bg.ac.yu 0300-9440/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII:S0300-9440(99)00024-7