Cavitation erosion behavior of super-hydrophobic coatings on Al5083 marine aluminum alloy

Al5083 alloy is widely used in marine and ship building industries for the construction of ship structures due to its high strength, fatigue resistance, and relatively good corrosion resistance against salty water. However, cavitation is one of its limiting factors in some important parts such as propellers used for marine applications. In the current research, the cavitation erosion behavior of super-hydrophobic coatings deposited on Al5083 aluminum was studied. The super-hydrophobic coating process included anodizing the surface in sulfuric acid followed by the surface chemical modification process with two Triethoxy Octylsilane (KH-832) and 1 H, 1 H, 2 H, 2H-Perfluoro Octyl-Trichloro Silane (PFOTS) classes. The cavitation test was conducted according to ASTM-G32 standard using the vibration amplitude of 20 mu in distilled water. The surface damage on the superhydrophobic coatings was investigated by using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Cavitation erosion caused the surface roughness of the as-anodized sample to increase from 85.9 nm to 153 nm, whereas for the coated samples, the cavitation process resulted in surface smoothing. In fact, cavitation erosion decreased the surface roughness of KH-832 and PFOTS coated samples from 269 nm to 119 nm and from 251 nm to 167 nm respectively. The number of the cavities formed on the surfaces of KH-832 and PFOTS coatings was more than that in the as-anodized sample due to their rougher surfaces. However, the superhydrophobic nature of the coatings resulted in the formation of small bubbles. Hence, the depth of the generated cavities in KH-832 and PFOTS samples was lower than that in the as-anodized sample. In fact, the cavities on the coated surface did not penetrate into the substrate and this enhanced the cavitation resistance of the sample. Finally, a model for the cavitation erosion behavior of PFOTS and the anodized samples was presented.