Thermally sprayed aluminum (TSA, 99.5%Al min) coating has been used increasingly for the protection of carbon steel offshore structures, topside equipment and flowlines/pipelines exposed to marine atmospheres, splash and tidal zones, and complete immersion conditions. This paper investigates the effectiveness of TSA coatings in preventing localized corrosion of 25Cr superduplex stainless steel (SDSS), such as pitting and crevice corrosion, for subsea applications. Both uncoated welded 25Cr SDSS and surfaces coated with epoxy paint were examined in the present work.
Pitting and crevice corrosion tests, on welded 25Cr SDSS specimens with and without TSA/epoxy coatings, were conducted in recirculated, aerated, artificial seawater at 90°C for 90 days. The tests were carried out at both free corrosion potentials and an applied CP of -1100mVSCE using a potentiostat. The pH of the test solution was monitored daily and adjusted to between 7.5 and 8.2, using dilute HCl solution or dilute NaOH, depending on the pH of the solution measured during the test.
The test results demonstrated that TSA provided sufficient protection to welded 25Cr SDSS to prevent pitting and crevice corrosion in artificial seawater at 90°C, even when a 10mm diameter holiday (~2% area) was present.