| Magnesium alloy is a potential ideal bipolar material with low density,light weight,high strength,excellent electrical and thermal conductivity,good mechanical and machining performance,low price and commercial conditions.However,magnesium alloys have active chemical properties and are prone to corrosion failure in the environment of proton exchange membrane fuel cell(PEMFC),which has become a key bottleneck restricting the application and commercial development of Mg-based bipolar plates for PEMFC.In recent years,scholars at home and abroad have carried out a lot of research on the corrosion of magnesium based bipolar plate surface coating,but so far the problem has not been well solved.As a new and improved technology with corrosion resistance,superhydrophobic surface of magnesium alloy provides an effective way to solve its corrosion problem.However,the existing superhydrophobic surface preparation techniques of magnesium alloys have many shortcomings that are difficult to overcome,such as complex preparation process,high cost,environmental pollution and unstable preparation structure.Therefore,in order to better solve the corrosion failure of magnesium alloy bipolar plate in PEMFC environment,this paper adopts hydrothermal treatment,wire-cutting and wire-cutting-hydrothermal treatment to prepare nanometer and micro-nano composite hydrophobic and superhydrophobic structures on the surface of AZ91 magnesium alloy.The main research contents are as follows:(1)The surface nano-rough structure of AZ91 alloy was prepared by hydrothermal treatment,and then the surface morphology and phase of the sample were studied.The formation and hydrophobic mechanism of stearic acid modified film with low surface energy were discussed,and the hydrophobic modification process was optimized.Finally,the influence mechanism of the surface wettability and nano-rough structure characteristics on the surface wettability was analyzed.The results show that after 7 h of hydrothermal treatment,the rough nano-structure is basically formed and tends to be stable,and the nano-flake structure is mainly composed of Mg(OH)2 phase.The reason for the hydrophobicity of the modified film is that the surface energy of the covalent bonding part decreases obviously due to the existence of the long chain of hydrophobic alkanes,and the hydrophobicity modification effect is the best after the alloy sample is soaked in stearic acid solution for 12 h and dried at 100℃ for 2 h.Water contact angle reaches a maximum of 146.8° after heat treatment 7 h,hydrophobic effect is enhanced,but all were showed on the droplet surface preparation of high viscosity,this is because the hydrothermal preparation of nano rough classification structure shows the characteristics of two levels of structure is not obvious,the air cushion effect enhancement is limited,the droplet adhesion is bigger,so is always difficult to observe droplet scrolling effect.(2)The surface graded micro nano structure of AZ91 alloy was prepared by wire cutting hydrothermal treatment.Then,the influence mechanism of surface structure morphology,wettability and micro nano structure characteristics on surface wettability was analyzed,and the mechanical stability of the prepared surface was tested.The results show that the surface of the prepared sample is composed of micron square cone nano flower cluster composite structure.After modification,the static contact angle of water droplets in the composite structure is as high as 173.5°,and the rolling angle is only 3.5°,showing obvious superhydrophobic effect.This is due to the synergistic effect of the two-stage structure,which increases the surface roughness and presents the enhanced Cassie interface characteristics of solid gas liquid three-phase contact,which increases the air phase captured by the surface structure and forms an obvious "air cushion" effect,so that the viscous resistance to the liquid almost disappears and shows excellent and different superhydrophobic characteristics.In addition,the macro performance of the prepared hierarchical structure is that the fragile nano sheet structure is filled between the interconnected micron bosses,forming the "armor"protective effect of hard materials on the fragile nanostructures,so as to obtain excellent mechanical stability.(3)The electrochemical test and characterization of the surface corrosion behavior of micro nano structure are carried out.Further combined with the analysis of the interaction relationship between the preparation surface and electrolyte interface and the impedance spectrum in the process of electrode preparation,the equivalent circuit model reflecting the corrosion interface behavior of micro nano structure is established.Based on the fitting solution of circuit element parameters,the evolution law of the preparation surface corrosion characteristic parameters with the structural characteristics is clarified,the quantitative description of the charge transfer and material diffusion behavior between the sample and the corrosion medium is realized,the decisive relationship between the superhydrophobic surface of the composite micro nano structure and the realization of the corrosion resistance functional characteristics of magnesium alloy is clarified,and its corrosion resistance mechanism is revealed.The following conclusions are obtained:for the hydrophilic AZ91 alloy substrate,due to the direct contact between the corrosion medium and the substrate,the active anions dominated by chloride ions diffuse from the bulk solution it is adsorbed on the metal surface and reacts with cations in the substrate,which directly leads to surface corrosion dissolution and surface perforation erosion damage,so its corrosion resistance is poor.For the composite prepared samples,the anti-corrosion mechanism is mainly manifested in the physical isolation of the modified film and the super hydrophobic dynamic displacement of the composite micro nano structure surface to the corrosive medium,so the corrosion resistance is significantly improved. |
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