Influence of the stress generated during an ageing cycle on the barrier properties of cataphoretic coatings

Abstract

The ageing of organic coatings is generally associated with an increase of stresses leading to a coating degradation: loss of adherence, increase of the porosity and irreversible changes during exposure to climatic parameters such as humidity and temperature. In this work, electrochemical impedance spectroscopy (EIS) and the cantilever method (stressmeter) were used to establish a correlation between the loss of barrier properties and the increase of the stress of electrocoated paints. The stress was generated during hygrothermal ageing cycles in a climatic chamber (20 h at 55 °C–84% RH and 4 h at 23 °C–40% RH). Phosphatised steel plates were used for the electrochemical measurements and calibrated steel strips for the stress measurements. Two electrocoated paints differing by their composition were tested. For both electrocoatings, the plates and the calibrated steel strips were submitted to 0, 1, 2 and 4 hygrothermal cycles in the climatic chamber. The generated stress was determined after each cycle. The strong influence of humidity on the stress was underlined during humidity cycles at ambient temperature. The sensitivity of the organic coatings to humidity increases with the number of humidity cycles and finally induces an irreversibility of the system. The barrier properties were evaluated after the different hygrothermal cycles and after 0, 2, 7, 14 and 21 days of immersion in 0.5 M NaCl electrolyte solution by EIS on the electrocoated steel samples. The stress generated during ageing increases with the number of cycles and at the same time a decrease of the barrier properties is observed. For the electrocoating whose stress evolution is more sensitive to humidity, a good correlation is observed between the increase of the internal stress and the increase of the coating capacitance due to water uptake by the coating.