Magnesium alloys have been widely used in aerospace, ship transportation, automobile parts, electronic equipment, and other fields because of their high specific strength, good specific stiffness, good machinability, and castability. However, owing to the extremely low electrode potential of magnesium alloy, it is easily corroded in a humid environment. Therefore, its poor corrosion resistance is one of the problems that prevent magnesium alloy from being further promoted and used, and improving the corrosion resistance of magnesium alloys is extremely important. The traditional methods to improve the surface corrosion resistance of magnesium alloys are micro-arc oxidation, rapid solidification, organic coating, etc. The production of superhydrophobic surfaces on magnesium alloy substrates is a simple and efficient method that has been used in recent years. However, the fabrication of superhydrophobic surfaces is often accompanied by etching with chemical reagents and modification with low-surface-energy substances, which increases the complexity and cost of the process. In this study, we used nanosecond laser processing of AZ91D magnesium alloy surfaces to form laser surface texturing, create periodic pits and dotted projections, and then supplemented them with low-temperature heat treatment to successfully modulate the surface of magnesium alloy from hydrophilic to hydrophobic to superhydrophobic. We investigated the effect of laser-processing-induced transformation of surface wettability on the salt spray corrosion resistance of magnesium alloys.
Magnesium alloys were used for this study. First, regular micro- and nanostructures were created using a nanosecond laser on the surface of magnesium alloy. Subsequently, they were placed in an ethanol solution, cleaned with ultrasonic waves, and placed in an oven for one hour of heat treatment at 150 ℃. Next, static contact angle and rolling angle measurements were performed on individual specimens using a contact angle measuring instrument to determine the wettability of the different specimens. Second, scanning electron microscopy was used to photograph the surface morphology and test the surface elemental composition of the magnesium alloys with different wettabilities, and salt spray corrosion experiments were performed on the magnesium alloys with different wettabilities, with corrosion times of 6, 12, 20, and 30 h. In addition, the surface morphology of the specimens after 30 h of salt spray corrosion was photographed using a confocal microscope, and the surface height and average height of corrosion layer were measured to calculate the thickness of the corrosion layer, which was used as a criterion to determine the corrosion resistance.
The surface contact angles of the prepared samples with different wettabilities are clearly differentiated (Table 2), among which the contact angle of the superhydrophobic Cassie state surface is 153.33° and the rolling angle is 3.51°. The elemental composition of the surface of the superhydrophobic specimens of magnesium alloy was tested before and after heat treatment, and it was found that the elemental content of C had an effect on the surface wettability (Table 3), and salt spray corrosion experiments were performed on each sample for different periods. The corrosion resistance of the superhydrophobic surface improved significantly with the increase in corrosion time (Fig. 4). The confocal surface morphology of the samples with different wettabilities after 30 h corrosion was photographed. Then, the average height of the corrosion layer was measured in combination with the contour method to calculate the thickness of the corrosion layer on the surface of the magnesium alloy (Fig. 8). The corrosion layer of the superhydrophobic Cassie state was the thinnest, measuring 20 μm, whereas the corrosion layer thickness of the original sample at this time was over 158 μm.
In this study, a nanosecond laser is used to texture the surface of AZ91D magnesium alloy, supplemented by low-temperature heat treatment, to obtain surfaces with different wettability states. After heat treatment, a superhydrophobic surface with excellent performance was obtained, with a contact angle of 153.33° and a roll angle of 3.51°. The laser modulation of wettability on the surface of magnesium alloys, as well as the mechanism of corrosion resistance improvement, were investigated by testing and characterizing the surface morphology and composition, static contact angle, and corrosion resistance of magnesium alloys with different wettabilities. The following are the main conclusions:
(1) Laser process parameters have an important influence on surface topography; laser processing and subsequent low-temperature heat treatment contribute to an increase in surface C element content; and surface topography and surface C element content can regulate surface wettability.
(2) The surface corrosion effect is determined by the corrosion capacity of the salt spray, corrosion resistance of the surface, and residence period of the salt spray on the surface. Laser processing can induce the formation of oxide films on the surface of magnesium alloys, which helps to improve their resistance to salt spray corrosion. The corrosion resistance of the laser-woven hydrophilic surfaces is higher than that of the original surface. Cassie surfaces with a small rolling angle and a superhydrophobic state have the best resistance to salt spray corrosion because of the short residence period of salt droplets on them. The dense and intact surface of the superhydrophobic Wenzel state with a contact angle greater than 150° and a large roll is one of the most important reasons for the good corrosion resistance of the surface.
