Effect of seawater constituents on the performance of thermal spray aluminium (TSA) in marine environments

Thermally sprayed coatings are often used to mitigate corrosion of offshore structures. They act as a physical barrier to the aggressive marine environment and as a sacrificial distributed anode for low carbon steel. In such environments, the severity of material degradation depends on many factors. The effect of temperature, exposure time or the presence of microorganisms are the focus of many studies for example, however the effect of the different ions present in seawater remains largely unexplored. The chemical composition of the water changes considerably depending on the location; industrial, glacial, estuarine, etc. In addition, when Thermal Spray Aluminium (TSA) protects steel in seawater, calcareous matter precipitates as a result of the cathodic polarisation and subsequent localized increase in pH. Therefore, understanding how ions such as magnesium (II), calcium (II) or carbonates alter the coating properties in marine environment is important. This paper reports the experimental work carried out with TSA coated steel samples with defects in order to simulate mechanical damage or erosion of the coating. The combination of electrochemical tests and surface characterization provided evidence of the efficiency of the calcareous bilayer that forms on top of steel reducing the TSA degradation.