Impedance study of the influence of chromates on the properties of waterborne coatings deposited on 2024 aluminium alloy
Résumé
Coatings formulated with either SrCrO4or a mixture of Cr(VI)-free pigments were compared in animpedance study. In addition to classical EIS measurements carried out in the metal/coating/electrolyteconfiguration, the study involved measurement of the impedance of the coatings in the dry state, bothbefore exposure to the electrolyte (0.5 M NaCl solution) and after a 1-month exposure followed by a12-month drying. The results were analysed with reference to models that assumed resistivity varia-tions along the coating thickness. The resistivity-position dependence was close to a power law for drycoatings and close to exponential for wet coatings. In the latter, and for short immersion times, the expo-nential variation was located in the outer coating region, at shorter distances from the coating/solutioninterface, and then, for longer immersion times (>1 week), extended to the whole coating thickness. Theeffect of SrCrO4leaching on the properties of the coating is discussed. For both coatings, the barrier effectremained high for a 1-month exposure.
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