Study on Construction of Biomimetic Super-hydrophobic Coatings on Magnesium Alloys and Corrosion Resistance Behavior

Author:Li Da Wei

Supervisor:wang hui yuan liu yan


Degree Year:2019





As the lightest engineering metal material,magnesium(Mg)alloy has excellent properties such as high damping,good thermal conductivity,high specific stiffness and specific strength,high recovery and anti-electromagnetic interference.Therefore,magnesium alloy has great application prospects in the fields of aerospace,automotive industry and electronic communication.However,magnesium and magnesium alloys have high chemical activity and loose surface oxide film,resulting in poor corrosion resistance of Mg alloys.In view of the above key problems,Mg–Al–Zn/Sn alloys was selected as the research object,improved the corrosion resistance of Mg alloy through the microtexture evolution and surface treatment,and can further expands the application of Mg alloy in actual production.The effects of deformation and heat treatment on the mechanical properties and corrosion resistance of Mg alloys were studied.The superhydrophobic surface with controllable adhesion was constructed on the surface of Mg alloy by combining laser processing with chemical etching and the corrosion mechanism was studied.The superhydrophobic coating with excellent comprehensive properties was prepared by spraying method.The long-term electrochemical corrosion behavior and mechanism of the coating were investigated.The main conclusions are summarized as follows:(1)The mechanical properties and corrosion behavior of AZ31 extruded alloy sheet after rolling and annealing treatment were investigated.It was found that the main microtexture evolution was remarkably refined,low dislocation density and weak texture intensity.The ultimate tensile strength(UTS)of the alloy increased from 232 MPa to 273 MPa,the yield strength(YS)increased from 124 MPa to 205 MPa and the fracture elongation(FE)increased from 12.5%to 26.4%.(2)Based on the static and electrochemical corrosion behaviors of AZ31 magnesium alloy sheet after rolling and annealing,the average corrosion rate of the sheet was reduced from 2.35 mmy-1 to 1.05 mmy-1.Compared with the bare Mg alloy,the icorr of the super-hydrophobic surface samples is decreased from 2.49×10-4 A·cm-2 to 3.21×10-5 A·cm-2,approximately one order of magnitude;The corrosin potential(Ecorr)incresed from-1.552 V(vs.SCE)to-1.491 V(vs.SCE).To provide a theoretical basis for simultaneously improving the mechanical properties and corrosion resistance of Mg alloys.(3)Superhydrophobic surface was successfully prepared on Mg alloy by laser processing combined with chemical etching,the maximum CAs was up to 158.2°.By adjusting the laser processing parameters to change the surface microstructure,super-hydrophobic surface achieved the transition from low adhesion(Cassie mode)to high adhesion (Wenzel mode).The results of the potentiodynamic polarization curves shows that the icorr of the superhydrophobic surface is reduced from 5.63×10-4 cm-2 to 6.55×10-5 cm-2 compared with the substrate,approximately one order of magnitude.Compared with Wenzel state,the corrosion potential(Ecorr)of the superhydrophobic surface with Cassie state move to a more positive direction.It is revealed that the surface treatment further improved the corrosion resistance of Mg alloy sheets,the corrosion resistance of super-hydrophobic micro-structured Mg alloy surface can be controlled by controlling the adhesion of droplets.Meanwhile,in the pH range of 4-14,exposed to the air for 1 month,the surface remains superhydrophobicity.(4)A robust,repairable and corrosion-resistant superhydrophobic coating was prepared on the surface of the Mg alloy by one-step spray method,the water contact angle CAs reached 159.5°and the sliding angle SAs was 3.8°.The addition of surface-modified nano-SiO2 filler plays a crucial role in the construction of surface roughness(Ra),and the Ra increased from 16.1 nm to 35.2 nm.The superhydrophobic coating exhibited an excellent comprehensive property,such as immersed in boiling water for 2 h,soaked in liquid nitrogen for 1 h,irradiated with UV light for 48 h and exposed to strong acid and alkali solution can always maintain original surface wettability.In addition,the coating can withstand a variety of mechanical durability tests,such as tape peeling after knife scraping,sandpaper rubbing 100 cycles,and so on.Mg alloy super-hydrophobic coatings exhibited excellent self-cleaning properties in both air and oil.The fabricated coatings exhibit a self-healing ability against O2 plasma etching,the number of cycles is up to 18 times.(5)The long-time immersion electrochemical corrosion behavior revealed that the icorr of the super-hydrophobic coating is greatly decreased by approximately two orders of magnitude,and the coating exhibited superior corrosion inhibition efficiency(ηp=98.9%).After soaking in 3.5%wt%NaCl solution for 14 days,the surface CAs of the coating is still above 135°,and the electrochemical impedance modulus(|Z|)is still significantly higher than that of the Mg matrix,indicating that the coating can resist long time corrosion of corrosive liquids.The super-hydrophobic coating combined the corrosion resistance with the excellent comprehensive performance,it is beneficial to expand the practical application field of Mg alloy and prolong the service life.Overall,In this paper,the effects of microstructure evolution and surface treatment on corrosion behavior of Mg–Al–Zn/Sn alloys were investigated,and established a mechanism for simultaneously improving mechanical properties and corrosion resistance.By combining laser processing with chemical etching,the superhydrophobic surface with controllable adhesion was constructed and its influence mechanism on corrosion resistance was studied.A fluorine-free superhydrophobic coating with excellent comprehensive properties such as mechanical durability,chemical and corrosion resistance stability were proposed by spraying method,provided a new idea for the integration of structural and functions of superhydrophobic Mg alloy surface in the application field.