A mechanistic approach to predicting long-term performance of thermal spray coatings

Cost-effective corrosion mitigation of offshore steel structures can be achieved by Thermal Spray Coatings (TSCs). These coatings, when comprised of Al, Zn and their alloys, provide a physical barrier against the environment when intact, and cathodic protection to underlying steel when damaged. As a result of the cathodic polarisation and subsequent increase in localised pH, calcareous matter (comprised of Mg(OH)₂ and CaCO₃) deposits on top of steel decreasing its corrosion. The study of thermal spray coatings onto steel with defects, such as those caused by mechanical damage or erosion remains largely unexplored. Electrochemical and surface characterization techniques were used in this PhD thesis to investigate the corrosion protection provided by TSCs on carbon steel substrates with and without defects in artificial seawater. Laboratory results showed that the presence of defects accelerates the formation of a protective corrosion product layer on the TSA coating. It was also revealed by Voltammetry and Electrochemical Impedance Spectroscopy (EIS) that both calcareous deposits and aluminium corrosion products hindered diffusion of dissolved O₂. The influence of the composition of seawater on the corrosion mechanism of TSCs was also studied. It was observed how, in order to recreate marine environments, it is crucial to prepare electrolyte solutions containing at least Ca²⁺, Mg²⁺ and SO₄², as these revealed by means of polarization resistances values to have high impact on the formation of corrosion deposits and calcareous matter. Thus, affecting the kinetics of both anodic and cathodic reaction. Lastly, validation of laboratory results was achieved by performing electrochemical measurements in situ. From the results here presented, it is expected to improve the design of corrosion protection systems with TSCs for offshore structures. This also translates in reduced costs and improved safety in marine environments.