Study on the Preparation and Corrosion Properties of Hydrotalcite Composite Permeation Layer via Plasma Electrolytic Carbonitriding
In order to improve the corrosion resistance of steel materials treated by plasma electrolytic infiltration,annealed 20 steel was used as the experimental substrate.Micro-nanopores formed during the plasma electrolytic carbonitriding process were utilized as containers to load magnesium-iron layered double hydroxides(Mg-Fe-LDHs)as a corrosion inhibitor,and the effects of Mg-Fe-LDHs on the corrosion perform-ance of 20 steel were investigated.First,formamide was used as the carbon and nitrogen source,and KCl was used as the conductive salt.These components were mixed with deionized water to form an electrolyte.Then,the samples were treated at a constant DC pulse voltage of 220 V for 9 min to produce a carbonitrided layer(denoted as PEC/N samples).Subsequently,the PEC/N samples were immersed in a Mg-Fe-LDHs precursor solution containing urea,MgSO4 and Na2CO3,and hydrothermally crystallized at 140 ℃ for 10 h to obtain a composite infiltration layer(denoted as PEC/N-LDHs samples).The performance of the composite infiltration layers was analyzed using a microhardness tester,scanning electron microscopy(SEM),X-ray diffraction(XRD)and electrochemical testing.Results showed that the microhardness of the composite infiltration layer reached approximately 900 HV,which was 4.6 times higher than that of the substrate.The carbonitrided layer of the PEC/N samples was about 20 μm,with a smooth and flat cross-section but a rough,porous and honeycomb-like surface.After hydrother-mal synthesis of LDHs,the surface became smooth,lacking a visible honeycomb structure,and a large number of flake-like crystalline depos-its appeared on the surface.The XRD spectra of the PEC/N samples showed absorption peaks corresponding to Fe3C,Fe5C2,α-Fe and Fe3N phases.After the in-situ loading of Mg-Fe-LDHs,new absorption peaks corresponding to the(003),(006)and(009)crystal planes of Mg-Fe-LDHs appeared in the XRD spectra of the PEC/N-LDHs samples.Combined with SEM morphology analysis,it was confirmed that Mg-Fe-LDHs were successfully loaded,presenting a flake-like structure.Compared with 20 steel,the PEC/N samples exhibited improved corrosion potential(Ecorr)and corrosion current density(Jcorr),reaching-0.451 V and 75.22 μA/cm2,respectively,although the low-frequency impedance modulus(|Z|0.01 Hz)showed no significant increase,reaching only 998.5 Ω·cm2.After loading Mg-Fe-LDHs,the PEC/N-LDHs samples demonstrated further improvements,with Ecorr,Jcorr and|Z|0.01 Hz reaching-0.355 V,39.11 μA/cm2 and 6 586.0Ω·cm2,respectively,showing significant enhancement compared to both 20 steel and PEC/N samples.These results indicated that plasma electrolytic carbonitriding treatment of 20 steel could produce a carbonitrided layer with high hardness.Additionally,the hydrothermal loading of LDHs further improved the corrosion resistance of the composite infiltration layer without affecting its hardness.The PEC/N-LDHs composite infiltration layer thus achieved both high hardness and superior corrosion resistance.
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