This study reports the mechanism of formation of calcareous deposits on TSA-coated BS EN 10027-1 S355 J2G4 steel in boiling synthetic seawater. Steel specimens were arc sprayed with commercially pure aluminum (99.5%) and a 5% holiday was drilled to expose the underlying steel. These were then immersed in boiling (~101ºC) ASTM D1141 seawater solution (pH8.2 at 20ºC) for ~5000 hours. The potential was monitored and linear polarisation resistance (LPR) technique was used to obtain the corrosion rate. After completion of the test the pH of the solution was measured and the specimens were analysed first using XRD and then using SEM. The solution pH was found to be 6.29 at 20 ºC and the holiday region was covered with a deposit with no visible rust. A thin layer was observed in cross-section which seemed to be divided into three parts viewed at a higher magnification. Energy dispersive x-ray spectroscopy (EDS) showed presence of Mg with no Ca. XRD confirmed the presence of brucite [Mg(OH)2] in the calcareous deposit.