Fabrication and Corrosion Performance of a Superhydrophobic Stainless Steel Surface
Authors: Amiriafshar Mona , Duan Xili , Nasiri Ali 
Published: October 2019

Abstract
Fabrication of surfaces with hydrophobic and superhydrophobic property has drawn extensive interests as a solution to protect metal surfaces from corrosion attacks, with potential applications in cooling devices for electronics, microfluidic systems for controlled drug delivery, as well as anti-icing, and self-cleaning techniques. This study addresses the impact of surface wettability, i.e., hydrophobicity and superhydrophobicity, on corrosion resistance improvement of metal materials. Hydrophobic and superhydrophobic metal surfaces are desirable to minimize the adhesion between water droplets and the surface. This study aims to fabricate and investigate 17-4 PH stainless steel surfaces with lowered surface energies and modified wetting properties. Various micro- and sub-micro scale finished surfaces with different surface roughness, namely as-received, sandblasted, ground, and polished, were employed, followed by applying a low energy superhydrophobic coating to fabricate hydrophobic and superhydrophobic surfaces on 17-4 PH stainless steel base material. The specific impacts of the surface roughness on wettability and corrosion resistivity of the manufactured surfaces were examined. The ground and polished surfaces followed by applying a 30–50 μm thick superhydrophobic dip coating resulted in steady-state contact angles of up to 152° and 146°, respectively, while the non-engineered coated base metal exhibited the contact angle of 140°. The ground surface with the average surface roughness (Ra) of ∼ 0.03 μm has the optimal roughness. According to the Cassie-Baxter model, the coated ground surface can retain the entrapped air within its interstices more adequately than the other surfaces with either lower surface roughness, such as the polished surface with the roughness of 0.02 μm, or higher surface roughness, such as as-received and sandblasted surfaces with the Ra value of 5.52 μm and 11.98 μm, respectively.
To study the corrosion performance and electrochemical stability of the fabricated surfaces, cyclic polarization testing (CPT) and electrochemical impedance spectroscopy (EIS) were performed in an aerated 3.5 wt.% NaCl solution that mimics seawater environment. The electrochemical measurements confirmed that the water-repelling property of the surface contributes to the anti-corrosion capability of the substrate. Data from the corrosion tests indicate that the lowest corrosion current density, highest corrosion potential, and highest pitting potential, were found for the coated ground surface followed by the coated polished surface. The EIS results also highlighted the significantly greater absolute value of impedance for the coated ground and coated polished surfaces even after 240 hrs of immersion in the electrolyte solution than the other fabricated surfaces at lower frequency ranges. The improvement in the 17-4 PH stainless steel corrosion performance was contributed to the size of the fabricated surface micro- and sub-micro scale features, capable of retaining the entrapped air within the roughened surface structure when fully immersed in a corrosive environment. This work demonstrates the effectiveness of a simple fabrication process to create hydrophobic and superhydrophobic stainless steel surfaces with improved corrosion resistivity.