Study On The Performance Of A Double Layer Cr-Al Composite Coating

Industrial electrodeposition of Al is often prepared using non-aqueous systems,in which ionic liquids are widely used as plating solutions to obtain a dense,well-bonded Al layer at room temperature,and ionic liquids are influenced by viscosity and conductivity,the lower the viscosity the higher the conductivity of the system,the better the deposition of Al layers.Aluminide coatings have not only oxidation resistance,but also thermal erosion resistance,thus becoming a hot research topic.The effect of temperature and organic solvents on the viscosity of the system and the effect of temperature,organic solvents and alkali metal halides on the conductivity of the system were measured in an AlCl₃-quaternary ammonium salt ionic liquid with a molar ratio of 2:1.Due to more cracks on the surface of single Cr plating layer leading to poor corrosion resistance,while Al has better corrosion resistance,covering the surface of Cr plating layer to form a Cr-Al double-layer composite layer,thus achieving the requirements of protecting the substrate and improving its corrosion resistance.X-ray diffraction（XRD）,scanning electron microscopy/energy spectrometry（SEM/EDS）and other technical means were used to characterize the physical composition and apparent morphology of the plated layers,and electrochemical corrosion resistance of the substrate,Cr plating and Cr-Al double-layer composite plating was analyzed by electrochemical corrosion performance testing.The conclusions of the experimentally measured studies are as follows.（1）In the AlCl₃-quaternary ammonium salt ionic liquid with a molar ratio of 2:1,the viscosity decreases with increasing temperature.Organic solvents（benzene,toluene and xylene）can effectively reduce the viscosity of the system,with benzene having the most obvious effect on the viscosity of the system.The conductivity of the system increased with temperature,and organic solvents（benzene,toluene and ethylbenzene）all increased the conductivity,with the highest conductivity(14.5 mS/cm^-2)at a temperature of 25℃with 50 vol.%benzene.The solubility of alkali metal halides was tested in TMPAC-AlCl₃ ionic liquid with a molar ratio of 1:2 for and the change of conductivity,and the solubility relationship was:LiCl>KI>LiF,and when the temperature>60℃,LiCl in the electrolyte was close to saturation,and the conductivity increased.（2）Compared with TMPAC-AlCl₃ ionic liquid,the addition of 50 vol.%benzene can improve the grain size of Al plating,and the Al plating is flat and smooth.Heat treatment can improve the grain size of Al plating,resulting in reduced grain size,dense crystallization and no boundary.By changing the process parameters,it can be concluded that:the bonding,thickness,ductility and coverage of Al plating and substrate are relatively good under the condition of electrodeposition time of 2 h.Under the condition of plating solution temperature of 50℃,the migration ability between ions is also enhanced,and the Al plating layer is more dense in crystallization,with the best ductility and thickness;under the condition of current density of 10 mA/cm²,the Al plating layer has good continuity and better bonding,and its heat treatment can make the grain more refined and borderless.（3）The corrosion resistance of the substrate,Cr plating,and Cr-Al double-layer composite plating was tested,and the relationship between them was strong and weak:substrate<Cr plating<Cr-Al double-layer composite plating.Cr-Al double-layer composite plating has the highest reaction resistance,the corrosion potential is more positive,the corrosion current is lower,and the radius of capacitive arc resistance is larger.Therefore,the electrodeposition of Cr-Al double-layer composite coating can improve the corrosion resistance.