Electrochemical assessment of spray-applied thermoplastic coating barrier properties

Flame-sprayed thermoplastic powder coatings act as barrier-type coatings for corrosion protection. Characteristic electrical properties for this type of coating are high resistance and capacitance, and low permeability to water and ions. Electrochemical impedance spectroscopy was used to evaluate the corrosion protection performance and electrical resistance of this type of coating during immersion in 3% NaCl solution. The variations with time of impedance and capacitance of two types of this coating, blue and black, were measured at three immersion temperatures (22, 50, and 80 degree C). The impedance of both coatings remained consistently high when the solution temperature was maintained around ambient (22 degree C). At the higher temperature of 50 degree C, both coatings performed well initially, then showed signs of damage by a drop in the impedance. Both samples maintained at 80 degree C failed after 12 and 25 days for the blue and the black coatings, respectively, with blistering occurring on both panels.