Study On Corrosion Resistance Of Zn-Al Composite Coatings With Different Al Content By Low Pressure Cold Spraying

In the field of long-term anti-corrosion of large steel structure,Zn coating has excellent cathodic protection due to its high chemical activity;Al coating is prone to produce passivation film,and the extremely high self-healing properties of oxide film greatly reduce the uniform corrosion rate.At the same time,the corrosion products produced by the corrosion process also play a self-sealing role.During the cold spraying process,the Zn-Al composite coating maintained the properties of the materials itself,because the material particles did not melt,so it has advantages in marine anti-corrosion.In this paper,Zn-Al composite coating was sprayed by cold spraying technology on the surface of Q235 steel substrate.The microstructure,element distribution,phase composition and valence state of the coating were analyzed by scanning electron microscope(SEM),energy dispersive spectrometer(EDS),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS).The hardness of the coating was tested by Vickers microhardness.The corrosion resistance of the composite coating is studied by static-immersion and neutral salt spray test.The results are as follows.The composite coatings prepared by surface of the low-pressure cold spraying technology have a flat surface and uniform structure.The coatings are mainly composed of black-gray Al phase and white Zn phase,which hardly oxidize during the spraying process.The hardness of the Zn-Al composite coatings is low that in the range 66-76 HV_0.25,and the porosity are less than 1%.when immersed in 3.5wt.%Na Cl solution for 1 hour,the Zn-Al composite coating containing 55wt.%Al has the best corrosion resistance,that with the increase of Al content,coating has the lowest corrosion current(I_corr=1.206×10^-4 A/cm²)and the lower open circuit potential(-1.008 V)and in the immersion experiment,the corrosion current density of the composite coating with higher Al content(?35wt.%)hardly change with the prolongation of immersion time,and coating with the Al coating has excellent electrochemical protection.The corrosion products(Zn₅(OH)₈Cl₂H₂O and Zn(Cl O₄)₂(H₂O)₆)of the composite coating with low Al content(<35wt.%)are relatively loose and can not form dense oxide film.The coating with higher Al content(?35wt.%)produces stable and dense oxide films(Al₂O₃,Al(OH)₃ and Al OOH).The impedance value of Zn-55Al composite coating after immersion in 1 hour(3.235×10⁴?·cm²)is lower than the 1440 h(2.608×10³?·cm²),and Zn-55Al composite coating still has the best corrosion resistance during long-term immersion compared with the other four coatings.In the neutral salt spray test,with an increase in the Al content,the coating has better corrosion resistance.In particular,Zn-55Al composite coating has the lowest corrosion rate(0.00528 mg/cm²·h)and the best corrosion resistance,which reaches the level 8 neutral salt spray evaluation standard.The corrosion morphology of the coating with high Al content is mainly corrosion crack,for others,the coatings mainly appeared a large area of corrosion pits distributed in the surface with a mountains shape,and a large number of flocculent of flocculent spicules are produced inside.The passivation current value of the coatings with low-Al content coatings(J_b=0.00179 A)are much greater than that of the high-Al content coatings(J_b=0.000278 A),which is not prone to passivation and produces less oxide film.