This paper discusses the theory behind electrostatic charge decay (ECD), parameters that affect the measurements, and how potential primer adhesion problems can be detected before the application of coatings.

ECD, a technique for detecting flaws in dielectric materials, is examined as a method to detect surface cleanliness of aluminum parts before the application of primer coatings. The ECD device was used to measure the contact potential difference (CPD) of a metal. The CPD is the difference in work function of the specimen and that of a gold reference. It is the measure of the ease with which electrons are released from the surface in the absence of external fields. In addition, a corona charger was used to apply 12 kV onto the surface and the rate of electrostatic charge decay (ECD) was monitored. Together, the CPD and ECD measurements can be used to give important information about the metal surface quality.

CPD and ECD measurements are both affected by surface contamination, surface morphology, nature of the oxide film, adsorbed gases and films, as well as the particular metal alloy. Contaminated surfaces exhibit a different electrochemical behavior. Processed 2219-T87 aluminum was examined. By implementing a process delay between acid deoxidation of the aluminum and rinsing, a significant change was seen in the ECD/CPD readings. ESCA/auger surface analysis indicated only small chemical differences. Scanning transmission electron microscopy (STEM), however, showed that the surface profile nearly doubled in depth on the delayed etch panel. Atomic composition remained relatively the same, but the ECD readings reflected the change in morphology. The delay before rinsing expended the deoxidizer, preferentially etching the metal. The deeper profile provided a better surface for paint adhesion.