| Magnesium alloys have broad application prospects in automobile,aerospace,electronics,medical and other fields owning to their low density and high specific strength.However,the active electrochemical properties of magnesium alloys and the PB ratio of oxide film on the surface is less than 1,it can easily cause corrosion when humid environment,which greatly restricts its further development.Although a variety of surface treatment methods for magnesium alloy have been developed at home and abroad,used for improving corrosion resistance of magnesium alloys,but an effective and environmentally friendly chemical conversion treatment method for magnesium alloy should be further explored.This paper according to the formation characteristics of the layered double hydroxides(LDHs)of magnesium alloy existing,and AZ91D magnesium alloy for commercial use at present was used as the research object,the CO2 pressurization method was forward for the first time,and it was introduced into the preparation process of the LDHs conversion coating,in order to further study the environmentally friendly chemical conversion treatment method,the conversion coating formation process and mechanism as well as corrosion resistance.The OM microstructure observation,SEM test,XRD component and microstructure test,TEM microstructure test,potentiodynamic polarization curve test,hydrogen evolution and immersion test methods were used to study the influence of CO2 pressurization on the microstructure and corrosion resistance of the LDHs conversion coatings on AZ91D magnesium alloy;The influence of CO2 pressure formation of the LDHs conversion coating was also studied based on thermodynamics and kinetics,and the corrosion behavior and corrosion resistance of LDHs conversion coating prepared under CO2 pressurization in Cl-containing medium were studied.In addiction,combined with microstructure analysis,starting from the special micro structure of fine-grained crystal by hot extrusion,the coating formation mechanism of AZ91D magnesium alloy in second phase morphology was studied,and the coating formation control theory of coarse-grained and fine-grained materials of AZ91 series magnesium alloys under CO2 pressurization was preliminarily establishedCO2 pressurization method saves the time of coating preparation and complex adjustment process,and greatly improves the preparation efficiency of the LDHs conversion coating while ensuring the stability of formation process.The CO2 pressurization method is 48 times the preparation efficiency of one step immersion method and 8 times that of two-step immersion method.Compared with the one step immersion method and two-step immersion method,the microstructure of the coating prepared by CO2 pressurization method is more compact,the thickness of the conversion coating is also relatively large.Further,the mechanical properties and corrosion resistance are also better.CO2 pressure changes the characteristics of solution chemical of the conversion solution.CO2 pressure promoted the electrochemical reaction and ionization reaction of the solution in the forward direction,while the electrochemical reaction and ionization reaction together promoted the formation reaction in the forward direction.Under the same time and temperature conditions,the thickness of the coating formed increases along with CO2 pressurization.When the pressurization of CO2 is smaller than 3 MPa,the influence of ?-Mg17Al12 on the coating formation is greater.The thickness of the conversion coating at ?-Mg17Al12 phase is quite small.When the CO2 pressurization is greater than 3MPa,severe cracks and voids appear inside the coating.When the CO2 pressurization is 3MPa,the corrosion resistance of the LDHs conversion coating is relatively good with the optimal thickness and compact.At a CO2 pressurization of 3 MPa,the thickness of the LDHs conversion coating increases along with the treatment time.During formation initiation,the conversion coating is formed preferentially on the ?-Mg phase,and then on the ?-Mg17Al12 phase,because of the potential difference between the ?-M phase and the ?-Mg17Al12 phase.The conversion coating grows layer by layer with time,the most even and compact coating appear at 30 min.As the treatment time increases,the coating becomes loose and porous,and the corrosion resistance decreases accordingly.For AZ91D magnesium alloys,the kinetics process of the LDHs conversion coating growth can be divided into rapid growth stage 0?10 min,slow growth stage 10 min?20 min,and periodic growth stage 30 min?1 h.The apparent activation energies of these three stages are 21.78 KJ·mol-1,31.86 KJ·mol-1,and 34.92 KJ·mol-1,respectively.AZ91D magnesium alloy cast in coarse-grain state,the ?-Mg17Al12 phase is distributed in bulk and lamellar forms on the edge of the matrix ?-Mg phase grain in a grid format,with the starting point of the ?-Mg17Al12 phase of the lamellar closely connected to the ?-Mg17Al12 of the bulk.During the coating forming reaction,the ?-Mg17Al12 phase as cathode accelerates the dissolution of the anode ?-Mg phase as a cathode,with the pH value required for the LDHs conversion coating deposition being preferentially reached.Under the CO2 pressurization,the thickness of the LDHs conversion coating between the two phases was uneven,and the thickness fluctuated between 1.2 ?m and 7.5 ?m.AZ91D magnesium alloy in fine-grain extruded state,the bulk and lamellar ?-Mg17Al12 phases are crushed,and then it precipitates out again.In the transversal section of the extruded alloy,the ?-Mg17Al12 phase is isolated on the ?-Mg phase matrix in the shape of small dots;In the longitude of the extruded alloy,the ?-Mg17Al12 phase distributes on the ?-Mg phase matrix with a streamlined appearance,Refining and rearranging the ?-Mg17Al12 phase results in the formation of numerous microgalvanic on the alloy surface,and promotes the effect of the microgalvanic in the formation process.The refining and rearranging of?-Mg17Al12 phase in the transversal section of the extruded alloy can hinder the formation reaction,while in the longitude of the extruded alloy,the formation reaction can be accelerated.Meanwhile,the microstructure difference results in a significant difference in the electrochemical activity of the magnesium alloy substrate surface.High electrochemical activity accelerates the dissolution of magnesium alloys and makes it easier to achieve the pH required for the LDHs conversion coating deposition.Its electrochemical activity can be arranged in increasing order:as-cast<transverse<longitude.When the assisted by the enhancement of microgalvanic effect and electrochemical activity,a thicker and more compact LDH conversion coating is formed on the longitudinal section of the extruded alloy,indicating the highest corrosion resistance. |
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