Technical Note: Corrosion of Cathodically Polarized Thermally Sprayed Aluminum in Subsea Mud at High Temperature

Thermally sprayed aluminum (TSA) is not immune to corrosion when polarized by sacrificial aluminum anodes. However, the corrosion rate is usually low resulting from a protecting oxide on the TSA surface, keeping the TSA passive. A certain cathodic reaction rate is found on the TSA, producing hydroxide. The hydroxide may elevate the pH on the surface of the TSA to a range where aluminum is corroding actively. When the TSA is exposed to seawater, the hydroxide will be transported away from the TSA surface by diffusion and convection, and the pH is maintained within the passive range. However, in mud there is no convection and the diffusion is limited. This may result in accumulation of hydroxide at the TSA/mud interface and activation of the TSA. The cathodic polarization may therefore result in decreased lifetime of the TSA in mud. Corrosion of cathodically polarized TSA in mud has been studied at various temperatures and potentials. It was found that polarization of the TSA to −1.2 V_Ag/AgCl resulted in increasing pH and rapid corrosion of the TSA. However, polarization to −1.1 V_Ag/AgCl gave moderate corrosion rates. The corrosion rate increased with temperature and decreased with time. Cathodic current density on TSA in mud was below the design values given for aluminum-coated surfaces in seawater in DNV-RP-B-401. Cathodic current and corrosion of the TSA was probably limited by a few micrometer thick surface layer of aluminum oxide that was formed during exposure, and not calcareous deposits.