Research On Surface Modification And Corrosion Resistance Mechanism Of Ferromagnetic Absorbent

With the vigorous development of the information age,the fifth generation（5G）communication technology based on gigahertz electromagnetic wave has brought ultra-high information transmission efficiency in compact mobile devices and industrial equipment.However,the application of new technology provides great efficiency and convenience for our life,but also brings serious electromagnetic pollution.Therefore,the research and development demand of high-performance microwave absorption materials is increasing.However,in the complex and changeable practical application environment（ocean,acid rain,high temperature,etc.）,ferromagnetic absorbent may cause serious corrosion problems due to long exposure.Most importantly,corrosion will not only reduce the microwave absorption performance of materials,but also cause huge environmental burden and economic losses.For example,in the marine environment,coating peeling caused by corrosion of ferromagnetic absorbers may deteriorate the stealth performance of warships and submarines,and cause marine pollution problems of great harm.Therefore,it is of great significance to develop new materials with high efficiency of microwave absorption and excellent corrosion resistance.In this dissertation,the research object is about ferromagnetic absorbers FeSiAl alloy（FSA）and carbonyl iron powder（CI）.And combined with the change of microstructure and composition after the corrosion of ferromagnetic absorbers,the connection between corrosion and wave absorption was established.Based on the principles of barrier protection and corrosion inhibitor protection by surface modification technology,the coupling mechanisms of multistage interface protection based on surface ceramics,high efficiency absorption-long term corrosion protection and hydrophobic self-healing protection were proposed,which expanded the application space of traditional ferromagnetic absorbers.The relevant research content of this dissertation is as follows:1.After the corrosion of FSA alloy in acidic salt solution,the phase and structure change obviously.The corrosion products are mainly FSA alloy,accompanied by a certain amount of Fe₃O₄.The formation of Fe₃O₄ may be due to the exposure of more Fe⁰ after the alloy structure is damaged.Under the action of dissolved oxygen,Fe0 or Fe²⁺is oxidized into Fe³⁺.The regular structure with smooth surface of FSA alloy becomes irregular structure with dense nanoparticle surface.The influence of corrosion on the microwave absorption performance of ferromagnetic absorbers is mainly reflected in the reflection loss（RL）and effective absorption bandwidth（EAB）.For example,when the thickness is 3.0 mm,the FSA alloy without corrosion can achieve a minimum reflection loss(RL_min)value of about-35.0 d B at 9.6 GHz and EAB up to 11.7 GHz,showing excellent broadband strong absorbing properties.After corrosion about 360 h,RL_mindecreased to-28.4 d B and EAB declined 1.7 GHz.Although a certain degree of corrosion optimized the impedance matching characteristics in some bands,the attenuation characteristics in corresponding bands deteriorated significantly due to the change of phase and morphology.2.By atomic layer deposition and high temperature induced phase transformation technology,the nano-ceramic protective structure on the surface of ferromagnetic absorbers was accurately constructed.Finally,the FSA@Zn O@Al₂O₃ multistage structure was successfully prepared,and the dual-function coupling mechanism of ceramic enhanced absorption-corrosion on the surface of ferromagnetic absorbers was also proposed.It is found that after the optimization and regulation of the dielectric/magnetic loss coordination mechanism,the microwave absorption performance of the multistage structure in the L and S bands has been significantly improved.For example,the RL_min value at 3.4 GHz was as high as-50.6 d B,and the EAB was 1.5 GHz,covering 75%of the S-band.By adjusting the thickness of the test sample,it can realize ultra-wide-band absorption of 1.8～18.0 GHz,namely L-Ku.It is found that high temperature annealing can induce the FSA phase transition into DO₃ phase with high order degree,which can increase the saturation magnetization and permeability.In addition,the elimination of lattice defects and recrystallization enhanced the conductivity loss,and the multi-interface structure design enriched the polarization loss mechanism.Under the synergistic action of dielectric/magnetic loss mechanism,the low frequency absorption performance was optimized.The electrochemical corrosion test results showed that the corrosion resistance of FSA alloy was enhanced significantly after surface ceramic.Compared with FSA alloys,the E_corr and I_corr value of multilevel structures shifted to the right and decrease,respectively,which means that the electrochemical corrosion thermodynamics and kinetics of FSA alloys can be significantly inhibited after surface ceramics.The corrosion thermodynamics and kinetics potential of FSA alloys were inhibited by high impedance and high barrier characteristics originated from ceramic protective structure.3.Sol-gel technology was used to construct the surface hydrophobic structure of ferromagnetic absorbers,and the surface hydrophobic and efficient absorption-long-term anticorrosion mechanism of ferromagnetic absorbers was proposed.After prolonged corrosion,the CI@Si O₂/HMDS core-shell structure exhibited stable microwave absorption properties.When the thickness was 2.5 mm,the RL_min and EAB of the uncorroded sample at 7.8 GHz were about-21.7d B and 4.8 GHz,respectively.The RL_minand EAB of the corroded sample at 7.3 GHz are about-21.8 d B and 3.8 GHz,respectively.The low dielectric and high wave transmittance of Si O₂/HMDS protective layer were very important to maintain the high efficiency of carbonyl iron absorber.After surface hydrophobic treatment,the corrosion resistance of CI was significantly enhanced,and its E_corr and I_corr shifted to the right and decreased significantly,respectively,indicating that the thermodynamic and kinetic inhibition effect of CI was significant.The electrochemical anticorrosion mechanism was mainly reflected in the concept of"green fluorine-free organic-inorganic protection".On the one hand,the protective layer has the characteristics of high density,strong hydrophobicity and low conductivity as if a layer of ultra-thin and hard"armor"effectively shields the corrosive medium and improves the long-term corrosion resistance of CI under the acidic salt solutions.On the other hand,Fe₃O₄ produced by micro-corrosion may plug the small holes formed by erosion and continuously enhance the long-term corrosion resistance of ferromagnetic absorbers.4.Inspired by the unique underwater adhesion ability of marine mussels,polydopamine and benzotriazole were introduced into the organic-inorganic hydrophobic ferromagnetic absorbers system to prepare the ferromagnetic absorber（CPBSH）with the characteristics of hydrophobic passive protection and self-healing active protection.A novel concept of self-healing ferromagnetic absorbent and its absorption-self-healing mechanism were proposed.The results showed that the absorption performance of CPBSH self-healing ferromagnetic absorbent was close to CI on the whole,or even slightly enhanced.At the thickness of 1.82 mm,the RL_min value of CPBSH at 10.7 GHz was about-55.4 d B,and the EAB was up to 7.6 GHz,which effectively covered 7.7～15.3GHz.After treated by surface hydrophobic and self-healing inhibitor,the corrosion resistance of CI was significantly improved and showed electrochemical self-healing behavior.The corresponding electrochemical anticorrosion mechanism was mainly embodied in the concept of"hydrophobic self-healing protection".In the first instance,we analyzed the changes of the microwave absorption performance of ferromagnetic absorbers in the corrosive environments,then proposed three effective anti-corrosion modification methods,finally studied the performance of analyzed the coupling mechanism of microwave absorption and anti-corrosion.The research work in this dissertation has a guiding role in the development of salt spray corrosion resistant ferromagnetic absorbers,and provides theoretical basis and experimental support for ferromagnetic absorbers to play the advantages of microwave absorption efficiently and stably in the marine environment,and has a reference role for the research and development of new dual-function with absorption-anticorrosive stealth materials.