The application of impedance methods to study the effects of water uptake and chloride ion concentration on the degradation of paint films—II. Free films and attached/free film comparisons

Impedance and gravimetric methods have been applied to study the uptake of water over a range of solution chloride ion concentration in free films (i.e. paint films not attached to a substrate). The paint films were 25 μm experimental silicone modified polyester. Comparisons have been made between the impedance and gravimetric methods to highlight advantages/disadvantages of each technique, and between the attached and free film results. Water uptake on the free films, calculated from gravimetric tests, increases with immersion time and with decreasing solution chloride ion concentration, in agreement with similar tests on attached films. The calculation of water uptake by the 1 kHz capacitance method was unreliable due to other processes occurring, such as paint film degradation and changing modes of distribution of water uptake at longer times. Sample failure time calculated by the single frequency (1 kHz) impedance test is a better measure of performance life than that calculated from wide frequency range tests. Both the free and attached film results show failure time decreasing with increasing solution chloride ion concentration. This suggests that the paint film degradation is caused predominantly by the solution and not by secondary effects such as build-up of alkalinity in the accumulated water layer at the metal/film interface on the attached films caused by cathodic reduction of oxygen. The marked drop in paint film resistance or the large rise in the 1 kHz capacitance with time is thought to be a measure of the extent of degradation of the paint film, caused by hydrolysis of the silicone modified polyester paint film. Water uptake, calculated by either the 1 kHz capacitance or gravimetric method, is greater but sample failure time is longer for the attached films than for the free films. Initially, water appears to enter the paint film rapidly, but slows down after a few hours when paint degradation becomes increasingly significant, and dependent on the solution chloride ion concentration.