THE STUDY OF COAL-TAR EPOXY PROTECTIVE COATINGS BY IMPEDANCE SPECTROSCOPY

Steel panels coated with a coal tar epoxy composition were studied by impedance spectroscopy in the dry condition as well as during exposure to synthetic seawater and to 10% NaOH solution. In the temperature ra nge from 0 to 20-degrees-C the dry coating behaves as an almost ideal capacitor with a dielectric constant ranging from 3.0 to 3.2. The coal tar epoxy coating immersed in synthetic seawater showed capacitative behaviour over a long period of exposure, in the frequency range 100 H z to 100 kHz. However, the thinner coatings (approximately 50 mum) wer e subject to a permanent slow degradation and their resistance decreas ed to approximately 10(7) OMEGAcm2 after 7 months of continuous immers ion, whereas the degradation process for thicker coatings (approximate ly 125 mum) was slower and the resistance was still above 10(8) OMEGAc m2 after 9 months of continuous immersion. The water uptake showed an initial rapid rise to between 9 and 13%. The coating degradation in a 10% NaOH solution is rather more rapid and after only one day of immer sion a coating resistance of 10(6) OMEGAcm2 and a water uptake of 30% was found.