| Magnesium alloys are widely used in aerospace and transportation due to their high specific strength,high specific stiffness and excellent damping capabilities.However,the high chemical activity limits their application,requiring surface treatment to prevent corrosion.Although micro-arc oxidation can greatly improve the corrosion resistance of the substrate,the resulting porous microstructure may reduce the corrosion protection ability.To address this problem,rare earth salt was added to a silicate-based electrolyte containing phytic acid to prepare films on AZ31 magnesium alloy surface by micro-arc oxidation treatment.The effects of adding rare earth salts and modulating positive/negative pulses on the microscopic morphology,phase composition,wettability,bonding and corrosion resistance of the ceramic film were investigated.Additionally,the corrosion resistance of the films was evaluated by electrochemical and neutral salt spray tests,and physical models were established to analyze the corrosion behavior.This technique offers practical implications for enhancing the corrosion resistance of magnesium alloys and promoting their application in engineering materials.Further details of the study are provided below.To investigate the effects of rare earth elements on the corrosion resistance of ceramic films,yttrium nitrate,lanthanum nitrate,cerium nitrate,and neodymium nitrate were added separately to the silicate-based electrolyte.The results showed that the addition of rare earth element increased plasma discharge density on the sample surface and growth rate of the film,while reducing local instantaneous discharge intensity.Phase analysis revealed that the film was mainly composed of MgO and Mg2SiO4.Moreover,the rare earth additives effectively reduced the porosity,roughness,and wettability of the film.The Bode plot was used to determine the impedance modulus of the film,which indicated that the film prepared by adding lanthanum nitrate exhibited the highest impedance modulus(|Z|f = 0.1 Hz)at low frequencies,reaching 2.53×106 Ω·cm2,which showed the best corrosion resistance.On the basis of determining the optimal corrosion resistance of the film with the addition of lanthanum nitrate,the molar ratio of phytic acid to lanthanum nitrate was controlled at 24:1,19:1,14:1,and 9:1,respectively,and the total amount of the two substances was fixed to investigate their effects on the corrosion resistance of the ceramic films.The results showed that as the molar ratio decreased,the surface roughness of the films decreased first and then increased,while the contact angle showed the opposite trend,in accordance with the Wenzel model.Potentiodynamic polarization tests showed that the film prepared with a molar ratio of 14:1 exhibit the highest corrosion potential and the lowest corrosion current density of-1.315 V and 2.412 × 10-8 A/cm2,respectively.In addition,the Nyquist plot showed that the sample always had a larger capacitive arc radius in different immersion times,indicating better longterm corrosion resistance in the corrosive solution.An electrolyte with a 14:1 molar ratio of phytic acid to lanthanum nitrate was selected,with controlled positive/negative pulses of 12:1,10:1,8:1 and 6:1 respectively,to investigate its effect on the corrosion resistance of the ceramic film.The results showed that as the pulse ratio decreased,the corrosion current density first decreased and then increased.During 120 h salt spray corrosion test,the sample with a pulse ratio of 8:1 exhibited the latest corrosion initiation time and the lightest corrosion.The microscopic corrosion morphology showed that cracks of varying degrees appeared on the surface of all samples,and the size of micropores decreased significantly,and even some micropores were completely closed.It was found that larger discharge channels were tended to form crack sources,and cracks extended through multiple micropores,while closed micropores were beneficial in preventing further crack propagation. |
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