1987 Volume 36 Issue 3 Pages 134-141
Degradation of coated steels were studied with ac impedance method for the frequency rage of 1mHz to 100kHz. The impedances at a non-deteriorated area, an anodic blistering area, and cathodic blistering areas with film delaminations were measured separately, then compared with those at the areas where the films were artificially deteriorated by anodic or cathodic polarization. The results revealed that the impedance measured for the whole area was nearly equal to that for cathode area. It was found that the cathode area was about one hundred times larger than the anodic blister. Using model blisters with free films and acidic solution as anolyte and basic solution as catholyte, the film resistances in unit area for anodic and cathodic blisters showed a similar value and they decreased in an almost similar rate by the polarization. From discussions on the equivalent circuits of these three different types of area, the impedance for the whole area is expressed by a parallel connection of these circuits so it will correspond to the lowest impedance of these in the measured frequency range. In practice, large area of cathodic deterioration is mostly responsible for the impedance characteristics measured for the whole area. For the evaluation of deteriorated coating films, it was proposed to use the break point frequency appeared in the high frequency range since this should be proportional to the area of cathodic delaminations and blisters.