Ageing of marine coating in natural and artificial seawater under mechanical stresses

Abstract

In order to study the effect of a visco-elastic stress (tension and compression mode) onto the performances of a thick marine organic coating, free films and coated panels were immersed in natural seawater and in NaCl 3wt.% solution at room temperature (20°C), fixed temperature (45°C) or under cyclic temperatures. Free films were analysed using uniaxial elongation and Dynamic Mechanical Analysis (DMA) and the degradation of coated samples was investigated using Electrochemical Impedance Spectroscopy (EIS).In natural and artificial ageing, the glass transition temperature Tg and the Young modulus were found to increase with immersion time but different kinetics were observed depending on the ageing solution. For coated panels, the effect of tension was found to be harmful for the coating whereas the compression seemed to have a slight effect onto the coating degradation independently to the ageing environment.By comparing the degradation of coatings in natural and artificial sea water, it was shown that the kinetic of degradation was faster in artificial medium while the water absorption was larger in natural seawater. The results highlight the harmful impact of seasonal temperature variation in natural medium which could be explained by the development of internal stresses into the coating.