Cavitation erosion and corrosion resistance of hydrophobic sol-gel coatings on aluminium alloy

Cavitation erosion and erosion-corrosion are the popular failure modes of hydronautics components namely propellers, valves, turbines etc which occurs due to mechanical destructions and electrochemical corrosion. Erosion corrosion is caused due to surge in the number of solid particles affecting the surfaces whereas cavitation erosion is caused due to steady collapse of cavities or bubbles. Aluminium alloys are widely used in marine renewable industries owing to its high strength, light weight and good corrosion resistance. Despite that, cavi-tation and erosion-corrosion are the limiting factors for these alloys. The aim of the present work is to produce a coating system capable of replacing chromate-conversion coatings on aluminium alloy by combining an anodised layer with additional deposition of superhydrophobic sol-gel coatings. Fundamental characteristics that affect the coating's corrosion and cavitation erosion namely adhesion and thickness was evaluated. Hardness and elastic modulus of the coatings was evaluated using a Nanoscratch mechanical tester. Electrochemical behaviour of the coatings was assessed using Potentiodynamic scanning (PDS) and electrochemical impedance spectroscopy (EIS). Prolonged performance was studied using neutral salt spray test (NSS). Cavitation erosion resistance of the coatings was investigated in laboratory using a standard ultrasonic test apparatus according to ASTM G32-16. Erosion rate of the coatings was evaluated based on cumulative mass loss v/s testing time. SEM/EDX was used to evaluate the surface damage caused by erosion-corrosion and cavitation erosion. The analysis was done aiming to decide if the developed coatings was a better alternative to protect the metals from corrosion and cavitation erosion.