Electrochemical studies of coating degradation

Abstract

The polymer coatings considered in this thesis work principally by creating a barrier, in order to prevent a corroding medium such as water and / or oxygen from contacting the surface of the underlying metal. Such coatings are subject to attack from the environment in which they are placed. This attack can lead to failure of the coating and corrosion of the underlying metal. In this thesis, three principle means of coating degradation, leading to subsequent corrosion of the substrate have been considered: Mechanical damage of the coating, the effect of ultra-violet light weathering and filiform corrosion. Electrochemical measurements have been made in order to attempt to quantify both the degree of coating breakdown and the extent of corrosion of the substrate. The principle method for measuring coating breakdown and substrate corrosion utilised in the work for this thesis was Electrochemical Impedance Spectroscopy (EIS). EIS, in principle, allows both changes in coating porosity resulting from coating breakdown, and the extent of corrosion of the substrate to be measured. The extent of delamination under polymer coatings on defects of different sizes and on different substrates has been measured. The effect of ultra-violet light weathering of polymer coatings was measured using EIS and correlated with measurements of light reflection of the coating. Filiform corrosion was induced on two different substrates, coated with various coatings. The effect of substrate and coating on filiform corrosion growth rate and mechanism has been considered.