THE APPLICATION OF IMPEDANCE METHODS TO STUDY THE EFFECTS OF WATER-UPTAKE AND CHLORIDE-ION CONCENTRATION ON THE DEGRADATION OF PAINT FILMS .1. ATTACHED FILMS

Impedance and gravimetric methods have been applied to study the uptake of water for paint films on metallic surfaces (referred to as attached films) exposed to aqueous solutions covering a range of chloride ion concentrations. The paint films were 25-mu-m experimental silicone modified polyester applied to a Zn-55 Al alloy coated steel substrate and contained different levels of flatting agent particles. Water uptake, calculated from gravimetric tests, increases as the amount of flatting agent increases. This relationship could not be resolved from the impedance tests. Gravimetric water uptake increases with time, and with decreasing solution chloride ion concentrations, in agreement with water uptake at 2 h calculated by the 1 kHz capacitance method. However, 1 kHz capacitance calculated water uptake at 1 d was unreliable due to other processes, such as paint film degradation, occurring in the strongest chloride ion solution (5% NaCl). These other processes affected the 1 kHz capacitance values and therefore could be detected by impedance, but not by gravimetric methods. Sample performance life has been obtained in different ways from the impedance tests, and is shown to decrease as solution chloride ion concentration increases. Impedance spectroscopy demonstrates that the corrosion resistance of the substrate drops soon after immersion and is controlled predominantly by the paint film resistance, which is related to the state of degradation of the paint film.