A local electrochemical impedance study of the self-healing properties of waterborne coatings on 2024 aluminium alloy

In the present work the processes involved in the degradation of a coated 2024 aluminium alloy from an artificial scratch were investigated by local electrochemical impedance spectroscopy (LEIS). The aim was to assess the self-healing process linked to the release of inhibitive pigments from the coatings. A comparison is provided for the local behaviour of a chromated coating, currently used in the aeronautic industry, and a Cr-free system. Local impedance diagrams were obtained above the scratched areas for different exposure times to the electrolyte (1 mM NaCl). For the chromated sample, the impedance values were high and increased with time, clearly showing a significant inhibition in the scratch. In contrast, for the Cr-free coating, impedance values were lower and in several areas, corrosion products appeared in the scratch. In these corroded zones, after several days of immersion, the diagrams have similar shape as that observed on bare 2024 aluminium alloy. Mappings, performed at 3 Hz, for different exposure times to the electrolyte also clearly showed the self-healing process in the presence of chromates or the development of the corrosion for the unchromated coating. At 1 kHz, similar mappings were observed for the two systems providing support to the fact that the coatings presented good adhesion to the substrate. It must be emphasized that LEIS gives a fast response on the efficacy of the inhibitive pigments and thus, complements the information obtained from the conventional EIS technique. (C) 2016 Elsevier Ltd. All rights reserved.