Evaluation of the anticorrosive behaviour of organic coatings applied on galvanised steel surfaces

Abstract

The study of the anticorrosive behaviour of the four systems of organic coatings — two with coal tar epoxy layer and two with zinc rich epoxy layer and external coating with epoxy paint — which were applied to galvanised steel specimens and were exposed to NaCl solution leads to the following conclusions: ▀ All four systems investigated provide higher protection than simple galvanising during the exposure time of the present work, exhibiting corrosion current densities lower than those of the uncoated galvanised steel even after 4 months of exposure. ▀ The systems comprising coal tar epoxy demonstrate higher anticorrosive protection than the systems with zinc rich epoxy and external coating with epoxy paint. The former maintain lower corrosion curent densities, almost constant phase angle (80°–85°) in the whole frequency range of the measurements, and high values of coating resistance, resistance of pores, and exponent of the CPE of the bulk of the coating. These elements indicate a homogeneous and resistant system with no considerable change of its electrical characteristics which retains its capacitive behaviour, a fact which is also verified by the slight increase in the water uptake(%v) and by the visual observations. On the contrary, the latter demonstrate higher corrosion curent densities, a step-by-step decrease of the phase angle, a significant reduction of the coating resistance, the resistance of pores and the exponent of CPE, and the increase in the constant of CPE, clues which characterize non-homogeneous materials where an increased penetration of polar water molecules occurs. This is also confirmed by the tendency for an increased water uptake, the high values of dielectric losses and the appearance of conductive pathways. ▀ Independent of the coating system, the existence of a wash primer layer leads to an increase of the anticorrosive protection.