Surface defects and corrosion in electrostatically deposited powder films

The corrosion resistance of electrostatic powder coatings depends upon obtaining a smooth, nonporous film deposited uniformly over the surface of the entire substrate. The film must be free of voids, pinholes, and other surface defects. Two common defects on powder coated film arise from: (1) Faraday Cage effect causing uneven film thickness of recessed surface and (2) back corona that results in pinholes and craters. The effects of surface defects on corrosion were studied using an industrial grade thermosetting powder electrostatically sprayed onto grounded aluminum alloy substrates using a corona gun. The film thickness was varied from 25 to 75 /spl mu/m to represent the effect of Faraday Cage problem in recessed areas. Similarly the effects of back corona were studied by subjecting the powder layer to corona ion bombardment. For both the cases, additional surface defects in the form of scratch and pinholes were introduced on the cured film and tested for corrosion resistance. Corrosion testing of the coated substrates was performed by immersion in neutral salt solution as per standard method recommended in ASTM G 44-99. The corrosion resistance was estimated by electrochemical impedance spectroscopy (EIS) and Tafel plots. Corrosion resistance decreased from 700 kohms/cm/sup 2/ to 350 kohms/cm/sup 2/ when film thickness decreased from 75 /spl mu/m to 25 /spl mu/m, but with induced back corona, the resistance for the same film thickness changed from 300 kohms/cm/sup 2/ to 200 kohms/cm/sup 2/.