In situ electrochemical studies of forming-induced defects of organic coatings on galvanised steel

A new electrochemical setup is presented for in situ measurements during uniaxial forming of thin film coated metal substrates. This approach allows the formability analysis of a zinc pigmented organic coating on a galvanised steel substrate. The aim is to monitor the formation of defects during the forming process. The setup comprises an electrochemical microcapillary cell in a three-electrode arrangement and a miniaturised linear stretching device. The development of forming-induced defects is monitored in situ by applying electrochemical impedance spectroscopy (EIS) and also microscopically analysed by means of field emission scanning electron microscopy (FE-SEM). The studies were supported by GOM (R) grid measurements and finite element simulations of model sample forming degrees. The established technique enables the evaluation of the correlation between forming degree and degradation of the barrier properties of organic coatings. Finally a phosphating process on the unformed and formed specimen is electrochemically and microscopically analysed to correlate the respective defect size with its local reactivity. The results show that stretching-induced defects occur at the interface between spherical Zn particles and the epoxy binder matrix. The defect size increases with increasing strain values. The phosphating process leads to the nucleation of phosphate crystals especially in the forming-induced defects and thereby reduces the free zinc in the defect area. The kinetic of the phosphating is accelerated with increasing size of the defect. (C) 2009 Elsevier Ltd. All rights reserved.