Effect of Al(NO3)3 on the Corrosion Resistance of LDH Steam Coatings on Magnesium Alloy Surface
Magnesium and its alloys,due to their remarkable biodegradability and mechanical properties,particularly their specific strength,have gained widespread attention and are being increasingly utilized as a green engineering material.However,their reactive chemical nature makes them susceptible to corrosion,limiting their widespread application.Surface modification techniques now serve as an effective means of enhancing the corrosion resistance of magnesium alloys.Layered double hydroxide(LDH),a layered structural material,has the ability to adsorb corrosive ions,thus protecting the magnesium alloy substrate.Reports indicate that the Al content in magnesium alloys can affect the LDH content in steam coatings,which in turn affects the corrosion resistance of these coatings.However,the impact of introducing Al outside the magnesium alloy substrate on corrosion resistance remains unclear.The work aims to investigate the in-situ steam synthesis mechanism of LDH and assess how the Al(NO3)3 affects the corrosion resistance and mechanism of LDH steam coatings on AZ91D magnesium alloy surfaces.An in-situ steam method was used to deposit LDH steam coatings on AZ91D magnesium alloy surfaces.This process involved a steam reaction at 150℃for 5 hours,during which different concentrations of Al(NO3)3 were added to the steam source solution to generate Al3+.The LDH steam coatings were characterized by XRD,FT-IR,SEM and EDS.Potentiodynamic polarization,electrochemical impedance spectroscopy,and salt spray tests were utilized to evaluate their growth mechanism,corrosion resistance,and corrosion mechanism.The results indicated that LDH steam coatings were successfully deposited on the surface of AZ91D magnesium alloy with different concentration gradients of Al(NO3)3.The main constituent phases of the coatings were Mg(OH)2,Mg-Al-NO3-LDH,and Mg-Al-CO32-LDH.When compared to LDH obtained without Al(NO3)3,the growth of Al(NO3)3/LDH was uniform and the structure was dense.The order of corrosion resistance of LDH-100,LDH-200,LDH-50,LDH-20,LDH,AZ91D magnesium alloy was determined through potentiodynamic polarization,electrochemical impedance,and salt spray tests.The corrosion products of the LDH steam coatings primarily consisted of Mg(OH)2 and MgCO3.When a 100 mmol/L concentration of Al(NO3)3 was used as the steam source,sufficient Al3+ participated in the nucleation and growth of LDH,leading to the synthesis of LDH with the best corrosion resistance.This improvement was attributed to the appropriate concentration of Al3+ promoting the formation of LDH steam coatings with fewer by-products and a dense structure.A discussion on the growth mechanism and corrosion mechanism of LDH steam coatings was also provided.The LDH steam coatings prepared with Al(NO3)3 as a steam source demonstrates great potential for application on the surface of magnesium alloys as green engineering materials.
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