Calcareous deposition on thermally sprayed aluminum surfaces

The formation of calcareous deposits on thermally sprayed aluminum (TSA)-coated surfaces immersed in natural seawater and connected to a cathodic protection system is a recurring issue. Calcareous deposition has occurred even though TSA coatings have a lower cathodic over potential compared with carbon and stainless steels, which should reduce the thermodynamic precipitation driving force. There is at present limited knowledge on the calcareous deposits’ formation kinetics on TSA in natural seawater following prolonged exposure at different temperatures. In this study, bare 25Cr super duplex stainless steel (SDSS) and TSA-coated SDSS samples (with and without an silicone-based sealer) were exposed to natural seawater under potentiostatic cathodic protection (CP) and open circuit potential (OCP) conditions at different temperatures (i.e., 20, 35, 60 and 80°C) for 1.5 years. Samples were extracted at various intervals to construct growth kinetics curves under the various test conditions. The deposits were subsequently examined using scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Lastly, recommended CP protection potentials and current density values were obtained based on the findings of this study. Key words: calcareous deposits, cathodic protection, thermal spray coatings