Effect of deep drawing on the performance of coil-coatings assessed by electrochemical techniques

The effect of forming on the corrosion performance of coil-coated galvanised steel is studied. The study was made using the deep drawing geometry and the corrosion behaviour of formed and non-formed samples was compared, using electrochemical impedance spectroscopy (EIS). The responses of the various layers (zinc, phosphate and organic film) were observed separately. Formed samples suffer much faster degradation, in the form of blisters or of total delamination for phosphated and non-phosphated systems, respectively. The drop in the resistance values caused by forming in the phosphated samples revealed a transition from passive to active, whereas in the non-phosphated material the loss of resistance corresponds to an acceleration of the corrosion processes. The choice of the best equivalent circuits describing the impedance response is discussed.