Influence of the mechanical deformation on the water uptake and failure mechanisms of polyester coil-coatings

The Erichsen cupping test is widely used to evaluate formability and adhesion of coil-coated materials, as it produces strains comparable to industrial forming. In the present work, a hot-dip galvanized steel with a polyester/melamine primer and topcoat was selected. Mechanical deformations were applied using a spherical indenter at penetration depths of 1-6 mm. Coating thickness along the deformation profile was measured on cross-sections by optical microscope. The effective contact area of the coating with the electrolyte in the electrochemical cell was determined by 3D scanning. Electrochemical impedance spectroscopy in a 0.5 M NaCl solution over 72 h was used to assess water uptake and anticorrosion performance, following methodologies developed in previous studies. The influence of coating thickness and effective contact area on the EIS-calculated water uptake was examined. A deformation threshold (≥5 mm) was identified, above which blister initiation was strongly accelerated. Failure mechanisms such as cracks in the Zn layer and loss of coating adhesion leading to blistering were characterized using electrochemical measurements and post-mortem SEM-EDS observations. The approach provides relevant dielectric and electrochemical markers for understanding how mechanical deformation influences water uptake and failure in coil-coatings. • Erichsen deformation (1–6 mm) causes non-uniform coating thinning, most severe at the dome apex and intermediate zone. • Variations in coating thickness and effective surface area strongly influence the calculated water uptake. • EIS-calculated water uptake remains within experimental uncertainty and does not vary with mechanical deformation. • EIS dielectric analysis suggests faster molecular mobility with deformation, implying increased water uptake. • Deformation ≥5 mm causes early blistering and coating failure, confirmed by OCP, EIS, SEM-EDS, and QCT tests.