Long term durability and corrosion resistance of hydrothermally etched slippery AA5083 surfaces under brine water up to 4 months

Abstract

Aluminium alloys are widely used in the construction of marine vessels owing to their low density, high corrosion resistance, low cost, high weldability and ease of availability. Previously, superhydrophobic Al alloys were shown to improve anti-biofouling properties, corrosion resistance and reduced frictional drag. However, their long-term durability in underwater is limited. This research successfully showcases the slippery dewetting properties of silicone oil coated hydrothermally etched Al 5083 alloys under 3.5 wt. % NaCl brine solution for up to 122 days. The etched surfaces comprise of polygonal, spherical microfeatures and nano grass/flakes upon etching for 1 h, 5 h, 10 h, and 20 h using 1 vol. % NH4OH aqueous solution. The mean surface roughness (Ra) increased from 40 nm (polished) to 1.3-3.6 µm (etched) due to the formation of Al(OH)3 layer as revealed by X-ray diffraction analyses. Water droplets readily slide over specimens coated with 25 μm thick silicone oil (100,000 cSt), exhibiting contact angle hysteresis (CAH) of <5° and slide-off angles (SAs) of <15°. When submerged in stagnant 3.5 wt. % NaCl brine solution, ≥60 % of the oil thickness (i.e. ∼15 µm) was retained even after 122 days. Among all, the 10 h etched specimens consistently exhibited excellent retention of slippery properties up to 122 days of submersion. Additionally, oil coated 10 h etched AA 5083 showed excellent anticorrosion, with corrosion current densities reduced by 3 orders of magnitude compared to polished AA 5083 and oil coated AA 5083.