Corrosion Protection Application of Liquid-Infused Surface with Regional Growth of LDH Films on Al Alloys
Authors: Shuyue Jiang , Haifeng Zhang , Yanjing Tuo , Wanting Rong , Liu Xiaowei 
Published: February 2020

Al alloys with their easy accessibility and good workability have been extensively used in oceanic engineering area [1]. However, corrosion failure of Al alloys is a major challenge. Although Al alloys are easily oxidized to protect themselves from corrosion, reactive anions can erode metallic substrates thereby leading to corrosion failures in wet conditions [2]. There are three different strategies that can be exploited to address the challenge. First, LDH films on Al substrate and their alloys demonstrate better corrosion protection effect because the film itself also works as a strong physical barrier [3]. Second, when the superhydrophobic surface was immersed in solution, an “air valley” film was generated on the surface, acting as a protective layer against water molecules and aggressive ions in wet conditions. According to the study of By Yuekun Lai [4], a discrete TCL is energetically advantageous to drive a droplet off a superhydrophobic surface, showing lower surface adhesion. Hence, to further decrease the surface adhesive forces, we proposed the regional growth of LDH films to reduce the continuity of the three-phase (solid-air-liquid) contact line (TCL). Finally, Nepenthes pitcher survives by using micro-structure to lock in an intermediary liquid [5], so we can exploit lubricant-infused roughness surface acts as an effective self-protective layer. Here, we design a combination of these above strategies and fabricate the slippery liquid-infused surface with regional growth of LDH films on Al alloys (denoted as sample LR).
The chemical compositions and morphologies of sample LR were characterized by energy-dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). Fig. 1 shows the composition and distribution of elements after hydrothermal reaction for 3h. In Fig 1(a), we can conclude that the surface consists of Zn (58.6%), O (29%), Al (11.77%), and C (0.63%), respectively. To further study the flower-like substance on the top of surface, the SEM-EDS plan scan analysis is used in Fig.1(b)~(f). From the mapping of sample, the elements concentrated on the top are Zn and O, so the flower-like structure is Zn-Al LDH layer [6]. The preparation schematic of sample LR is showed in Fig.2(a). The morphologies of the sample LR after laser engraving and hydrothermal synthesis were characterized by SEM in Fig.2(b~d), which shows the regional growth of Zn-Al LDH layer. After chemically modified by FAS, the surface exhibits an excellent superhydrophobic property. Then the krytox100 is impregnated on the hydrophobic surfaces to form lubricant-infused surface. Polarization curves obtained by electrochemical experiments are applied to investigate the anti-corrosion ability. In Fig.3, the Tafel curves are tested after the sample LR were immersed in 3.5% NaCl aqueous solution for 10 min, 5 h, 10 h and 24 h. The corrosion current density of the sample LR reduces from 3.6´10-8 A/cm2 to 1.15´10-8 A/cm2 with the corrosion time, which means that sample LR has the stronger anti-corrosion ability owing to the existence of lubricant. With the increases of corrosion time, the lubricant spreads to form a protective film on the surface. On the contrary, the corrosion current density of Al alloys increases from 1.09´10-6 A/cm2 to 1.74´10-7 A/cm2 when the corrosion time varies from 10 min to 24 h. To further explore the corrosion inhibition capability of the sample, the electrochemical impedance spectra (EIS) is measured by an electrochemical method. As shown in Fig.4, although the corrosion time increases from 10 min to 24 h, the impedance spectra of the sample LR always show a large impedance semicircle whose diameter is around several thousands of W×cm2, which is bigger than the untreated Al alloys (200~400 W×cm2). In addition, the impedance of sample LR increases and Al alloys decreases with the corrosion time. After immersed in seawater for 24 h, the Zre of sample LR augments from 1088 W×cm2 to 1411 W×cm2. Conversely, the Zre of Al alloys decreases from 401 W×cm2 to 132 W×cm2. The results are consistent well with Tafel curves. These results indicate that sample has excellent corrosion resistance in simulated seawater.

Reference

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Figure 1