Powder coating systems have been considered for different purposes since they have many advantages in use. The performance of the different powder coating systems needs to be examined by various methods. In this paper, a polyester powder coating was applied on various pretreated mild steel panels. Electrochemical Impedance Spectroscopy (EIS) measurements were used to study the durability of the coating on pretreated mild steels upon immersion in 3.5% w/ w NaCl solution at ambient temperature for 7 and 50 days. Various elements of the equivalent circuit are proposed for the different metal/ coating systems. The lowest coating and double layer capacitances were measured for acid pickled followed by phosphating pretreated steel panel while the coating and charge transfer resistance were the highest for this system. It revealed an improvement of coating durability on the substrate by phosphating pretreatment. This can be attributed to decreasing ingress of water, dissolved oxygen and ions into the coating.

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