Corrosion Evaluation of Painted Steel Panels by Electrochemical Measurement

Paint is applied to effectively protect steel structures from corrosion. Coatings act as a barrier to a corrosive solution and transfer the electrical charge from the corrosive solution to the metal underneath the coating. The diffusion path of coatings may vary owing to the composition of the coatings. High-performance organic paint systems with two different primers and the same intermediate and top coats were used for the experiments. Both uncontrolled long-term testing and controlled laboratory testing were conducted to evaluate the performance of the coatings. Salt spray, alternating current (AC) impedance, and direct current (DC) corrosion were carried out on the fresh as well as exposed coated samples. Polarization resistance, coating capacitance, water absorption, and corrosion potential were studied to understand the causes of deterioration and to have faster assessment of durability of these paint formulations. To characterize the corrosion product, scanning electron microscopy (SEM) and Raman spectra investigations were carried out. The performance of an epoxy Zn phosphate base primer is very much comparable with an epoxy Zn-rich base primer as evident from the various controlled laboratory and atmospheric tests.