Mitigating localised corrosion using thermally sprayed aluminium coatings on welded 25% Cr superduplex stainless steel

摘要

Thermally sprayed aluminium (TSA) coating has been increasingly used for the protection of carbon steel offshore structures, topside equipment and flowlines/pipelines exposed to both marine atmospheres and seawater immersion conditions. In this paper the effectiveness of TSA coatings in preventing localised corrosion, such as pitting and crevice corrosion of 25%Cr superduplex stainless steel (SDSS) in subsea applications has been investigated. Welded 25%Cr SDSS (coated and uncoated) with and without defects, and surfaces coated with epoxy paint were also examined. Pitting and crevice corrosion tests, on welded 25% Cr SDSS specimens with and without TSA/epoxy coatings, were conducted in recirculated, aerated, synthetic seawater at 90℃ for 90 days. The tests were carried out at both the free corrosion potentials and an applied cathodic potential (CP) of -1100mVSCE. The acidity (pH) of the test solution was monitored daily and adjusted to between pH7.5 and pH8.1, using dilute HCl solution or dilute NaOH, depending on the pH of the solution measured during the test. The test results demonstrated that TSA prevented pitting and crevice corrosion of 25%Cr SDSS in artificial seawater at 90℃, even when 10 mm diameter coating defect exposing the underlying steel was present.