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.
In the oil and gas industry, thermally sprayed aluminum (TSA) coatings are commonly used, primarily, to reduce anode demand in cathodic protection systems and impart some degree of sacrificial protection in the topsides and splash zone areas1. The use of TSA coatings has advantages in systems where long service life is required. TSA coatings are also used to reduce the formation of calcareous deposits, normally a combination of CaCO3 and Mg(OH)2, on heat exchanger piping,2. While the formation of calcareous deposits reduce the current demands of the cathodic protection (CP) system3, 4, there are some drawbacks. For example, the formation of calcareous deposits in subsea heat exchanges must be prevented or minimized, due to their low thermal conductivity, which would reduce the heat transfer efficiency.
