Characterization of the water uptake and electrolyte uptake of organic coatings and the consequences by means of electrochemical impedance spectroscopy and UV–vis spectroscopy

•We measure the electrolyte uptake of coatings.•We develop a description of EIS spectra (phase model) without simulation with an equivalent circuit.•We show that the electrolyte uptake initiates the corrosion not the water uptake.•We describe a new kind of swelling that solidifies a coating to improve the barrier properties.

A new method to the investigation of the electrolyte uptake in a coating is presented. By means of UV–vis spectroscopy without nitrogen atmosphere in the region of 189 nm the concentration change of the overlaying electrolyte over a coating could be defined. In combination with the analysis of the water/electrolyte uptake by means of the electrochemical impedance spectroscopy (EIS) the resulting electrolyte uptake could be defined on a qualitative level. Furthermore a simple analysis of EIS spectra by a graphical interpretation of the Nyquist spectra has been developed. With the EIS spectra analysis in the same timeline as the water/electrolyte uptake a model for the processes inside a coating during corrosive stress has been obtained. It shows that the fast water uptake is followed by a slow ion uptake and this causes the initiation of a corrosion process at the interface.