Objective Corrosion is considered as the most critical problem in marine engineering, which results in severe life safety issues and tremendous economic loss. The conventional surface treatment laser texturing applies compact coating onto a metal surface to inhibit the aggression of corrosive medium but can be ineffective when electrolyte solution reaches the metal/coating interface. Recently, it has been proven that superhydrophobic surface exhibits impressive anti-corrosion properties. Among different fabrication techniques for superhydrophobic surfaces, ultrafast laser-based surface-texturing methods have been widely used, but still possess deficiencies, such as low processing efficiency and high maintenance costs. In this study, a novel highly efficient laser-based surface micro/nanostructuring technique was developed for the fabrication of multifunctional surfaces. The laser micro/nanostructured surface exhibits combined functionalities of superhydrophobicity, enhanced microhardness, and improved corrosion resistance in underwater and marine atmospheric environments. This technique shows distinct advantages in terms of processing rate and production cost compared with the conventional laser texturing techniques, which can have strong potential to render a series of applications in the area of marine engineering.
Methods Two important engineering alloys—AISI4130 steel and AA6061 alloy—have been employed in this work as the testing materials. During the laser micro/nanostructuring process, the surface was first textured using a laser scanning system equipped with a single-mode high-energy pulsed nanosecond laser and a galvanometer laser scanner. Subsequently, the laser textured surface was immersed in an ethanol solution consisting of 1.5% mass fraction chlorosilane reagent for 3 h. After chemical immersion treatment, the specimens were cleaned and dried using compressed air. For surface characterizations, the surface topography and chemical compositions of the laser micro/nanostructured surface were first examined using laser scanning confocal microscopy/scanning electron microscopy and X-ray photoelectron spectroscopy. Afterward, the contact and roll-off angles of the laser micro/nanostructured surface were determined using a contact angle goniometer equipped with a high-resolution CMOS camera. In the next step, the Vickers microhardness of the laser micro/nanostructured surface was measured using a digital microindentation tester. Finally, the corrosion resistance of the laser micro/nanostructured surface in both underwater and marine atmospheric environments was studied by electrochemical experiments and monitoring of the deliquescence process.
Results and Discussions There are several key findings for this study. 1) The laser micro/nanostructured surface exhibits a unique dual-scale surface pattern comprising various types of random micro/nanoscale structures including protrusions, platelets, pillars and cavities (Fig. 3). All micro/nanostructures have a feature size ranging from a few tens of nm to several μm and are randomly and closely packed in the whole laser-treated area. The surface structure generated in this work is significantly different from the conventional laser-induced periodically arrayed surface structure. 2) The surface chemistry analysis shows that functional groups, including —CH2—, —CF2—, and —CF3, have been attached to the laser-treated surface, which endows low surface energy (Fig. 4). 3) The contact angle measurements show that the laser micro/nanostructuring process renders both AISI4130 steel and AA6061 alloy consistent superhydrophobicity within a wide laser processing window provided the laser power intensity is above a certain threshold value (Fig. 5). 4) Through the laser micro/nanostructuring process, the microhardness of all surfaces processed by different laser power intensities has been increased (Fig. 6). By using a laser power intensity of 8.4 GW/cm 2, the microhardness is enhanced to (211.1 ±11.0) HV with a 32.7% increase for AISI4130 steel and (126.6 ±6.1) HV with a 19.9% increase for AA6061 alloy. The increase in microhardness is mainly attributed to the generation of finer grains and the higher density of micro/nanostructures. 5) Through electrochemical tests, it is found that the corrosion resistance of the laser micro/nanostructured superhydrophobic surface in an underwater environment has been significantly improved, which is mainly due to the strong capability of the superhydrophobic surface to repel corrosive medium (Fig. 7). 6) By monitoring the deliquescence process of salt particles on the surface, it has been discovered that the untreated surface can be easily corroded by salt particles, while the laser micro/nanostructured superhydrophobic surface shows its effectiveness as a barrier to inhibit the marine atmospheric corrosion induced by deliquesced NaCl particle (Fig. 8 and 9).
Conclusions In this work, we develop a novel highly efficient laser-based surface micro/nanostructuring technique for the fabrication of multifunctional surfaces. This technique combines laser surface texturing and chemical etching. First, a nanosecond pulsed laser is used to pre-condition the surface within a specific laser processing window. Second, the laser textured surface is further chemically treated to reveal the micro/nanostructure generated on the surface, and the surface chemistry is also finely tuned to control the surface wettability. The experimental results indicate that the laser-chemical induced micro/nanostructured surface becomes superhydrophobic, and the surface microhardness has also been enhanced. More importantly, the laser-chemical treated surface exhibits significantly improved corrosion resistance in both underwater and marine atmospheric environments. This technique increases the processing rate compared with conventional laser texturing techniques as well as reduces the production and maintenance costs. It is expected that this novel laser surface micro/nanostructuring process will render more practical applications in the area of marine engineering.
