Effect of seawater flow velocity on the damage tolerance of thermally sprayed aluminium coatings

This work focuses on the effect of seawater flow velocity on the corrosion performance of Thermally Sprayed Aluminium (TSA) Coating obtained by twin-wire arc spraying. Hollow cylindrical coupons were machined from an S355 steel rod and coated with 300 μm-thick TSA (AA 1050). An intentional defect was machined on the outer surface to expose the substrate. Polarisation Resistance (PR) measurements were carried out to calculate the corrosion rate. Open Circuit Potential (OCP) was also monitored. An analytical rotator was employed for testing TSA-coated steel cylindrical coupons without and with damage at 50 rpm (3 cm/s), 600 rpm (38 cm/s) and 1000 rpm (63 cm/s) for 10 days in synthetic seawater (SSW) at 18 ± 2 °C. Tests under stagnant conditions were also conducted. SEM examination and Raman spectroscopy identified calcareous deposits on the exposed steel and Al oxides/hydroxides on the coating. Removal of aluminium corrosion products from the coupon surface due to the SSW flow exposes fresh coating surfaces to the electrolyte, increasing the coating corrosion rate. Hydrodynamic conditions strongly influenced the composition and precipitation rate of the calcareous deposits on the exposed steel in TSA-coated steel with intentional damage. After testing, the exposed steel had limited clusters of calcareous deposits. Aluminium is consumed to protect the exposed steel, while the flow enhances oxygen availability at the coating surface. • OCP variations in TSA without damage result from seawater flow removing aluminium corrosion products. • The removal of aluminium corrosion products exposes a fresh coating surface, which accelerates the corrosion rate. • Seawater flow velocity, intentional damage, and limited calcareous deposits resulted in a high corrosion rate in TSA. • The damage tolerance of TSA relies on the precipitation rate, stability, and type of calcareous deposits on the exposed steel. • TSA consumption by galvanic action will continue until calcareous deposits form, limiting oxygen reduction in the defect.