Study On The Corrosion Resistance Of Dopamine/Polyetherimide Composite Coatings For Magnesium Alloys

Magnesium and its alloys have excellent physical and mechanical properties.However,among the currently known metal electrode potentials,the potential of the magnesium alloy is extremely low,which means that magnesium and its alloys are easily subjected to electrochemical corrosion.In addition,the loose and porous oxide film formed on the surface of magnesium alloy undoubtedly increases its corrosion sensitivity.And now,the relatively low corrosion resistance of magnesium alloy strictly limits their extensive use and look forward to an early breakthrough as soon as possibile.In order to address the above problems,this paper creatively introduces the polydopamine intermediate layer to construct a polydopamine/polyetherimide composite coating system.Compared with a single coating(organic,inorganic)system,the composite coating has the advantages of simple preparation process,environmental protection,energy saving,and long-term corrosion resistance,and can be easily promoted in industrial applications.First,the polished AZ31 magnesium alloy sheet was placed in dopamine hydrochloride solution to obtain a polydopamine(PDA)coating.A large number of irregular cracks exist on the surface of the coating,with a thickness of 2±0.1 ?m,and the coating is tightly bonded to the substrate.The polarization curve test results show that the corrosion potential and corrosion current density of Mg-PDA samples were positively shifted by 20 mV and decreased by 2 orders of magnitude compared with the corrosion potential and corrosion current density of the matrix,respectively.The corrosion inhibition rate of the Mg-PDA sample reaches 93.07%.In addition,the EIS results show that the resistance of Mg-PDA samples was maintained at 103? cm2 after immersing in 3.5 wt.%NaCl medium for 1 h,which was slightly higher than that of the substrate(102.75 ?cm2).The results of the tape test showed that the PDA coating and the matrix had a good binding force.Second,the Mg-PDA sample was placed in a polyetherimide(PEI)solution to induce formation of a polyetherimide coating on the surface of the PDA coating.The surface of the coating is dense and nonporous,with a thickness of 28.6 ±1.1 ?m.The polarization curve test results show that the corrosion potential and corrosion current density of Mg-PDA@PEI samples are positively shifted by 10 mV and decreased by 3 orders of magnitude compared with the corrosion potential and corrosion current density of the matrix,respectively.The corrosion inhibition rate of the Mg-PDA@PEI sample reaches 98.45%.In addition,EIS test results show that the resistance of Mg-PDA@PEI samples after immersing in 3.5 wt.%NaCl medium for 2 h and 35 days was 107 ? cm2 and 104 5 ? cm2,respectively.The results of the tape test showed that the PDA coating and the matrix had a good binding force.Finally,Zview 2 software was performed to fit the EIS experimental data to obtain electrochemical element parameters at various time monitoring points,and equivalent circuits in different corrosion stages was established to reveal the corrosion mechanism of the coating.The results show that the resistance of Mg-PDA samples immersed in 3.5 wt.%NaCl medium for 7 days was 103 4 ? cm2,which was slightly higher than that of Mg-PDA samples.The resistance of Mg@PEI samples immersed in 3.5 wt.%NaCl medium for 21 days was close to the initial resistance of the Mg matrix.The resistance of Mg-PDA@PEI sample immersed in 3.5 wt.%NaCl medium for 35 days was close to the initial resistance of Mg matrix.