| Cr coatings,with the properties of excellent corrosion resistance have been widely used in marine corrosion and protection of metal component.There are many ways to fabricate Cr coating,such as magnetron sputtering,thermal spraying,electroplating technology and electrospark deposition technology.With a combination of high adhesion,low heat input,high efficiency,the possibility of local surface treatment and the relative simplicity of implementation,the electrospark deposition(ESD)process has gained interests as a promising surface technique for engineering materials.Therefore,this paper mainly focused on the microstructure and corrosion resistance properties of Cr coating fabricated by the electrospark deposition method.Cr coatings were deposited on M50 steel by electrospark deposition with different parameters(100 V-60 ?F,100 V-90 ?F,100 V-120 ?F,150 V-60 ?F,150 V-90 ?F,150 V-120 ?F).Influences of the depositing parameters on the law of deposition?surface morphology?phase composition?microstructure?microhardness and porosity of the coatings were experimentally investigated.Results show that the mass increase per area of coating is increased in accordance with a line proportion with the number of deposition until reached the mass limit.Splashes appearance can be observed on the coating surface,which is a typical surface morphology of the ESD coatings.It can be seen that with the increasing of the deposition voltage,the size of single pulse deposition point is bigger,but the effect of deposition capacitance on the size of single pulse deposition point is much smaller than the deposition voltage.The surface layer of the coating is an amorphous structure,the inner surface layer is a(Fe-Cr)with ultra-fine grain and the inner layer is y(Fe-Cr)with fine grain.Compared with the substrate,the microhardness of the coating with different parameters decrease,but it could reach 550 HV.The porosity of coating with different parameters are less than 2%,when the deposition parameters is too large or small,the porosity of the coating is larger,and the porosity of the coating is smaller when the parameters is moderate.In terms of electrochemical experiments of substrate and coatings,the corrosion potential of coatings with different parameters shifted towards positive compared with substrate and the degree of positive shift 150 V-60 ?F of coating is largest,it is indicate that the tendency of corrosion of coating are decrease and corrosion resistance of 150 V-60 ?F of coating is best.The electrochemical impedance spectroscopy of substrate and coatings consisted of a significant widening capacitance arc.In terms of immersion test of substrate and coatings,the corrosion rate of substrate is significantly greater than coatings,it indicates that the corrosion resistance of coatings are better than substrate.The corrosion rate of 150 V-60 ?F of coating is smaller and corrosion resistance of 150 V-60 ?F coating is better.When removal of corrosion products on the surface,the substrate occurred crack phenomenon,but the coatings remain the original splashes appearance.The corrosion rate of the matrix was significantly greater than that of coating,corrosion resisting coating ability is good,after the removal of corrosion products on the surface.the matrix cracking phenomenon,the coating remains the original shape of splash.In terms of salt spray test of substrate and coatings,the mass loss per area of substrate is significantly greater than the coatings with different parameters and the 150 V-60 ?F of coating has the minimum mass loss.it is show that the corrosion resistance of substrate is less than coatings.the corrosion resistance of 150 V-60 pF of coating is best.The surface of coating could form corrosion products to prevent further corrosion of the coating after salt spay test,but corrosion products is very looseon the surface of substrate.After removing the corrosion products,surface of the substrate distributed many corrosion pits,while the coating surface is flat and free of corrosion pits.As seen in sectional morphology,the substrate has a serious corrosion,the coating is only corroded when longitudinal crack runs through the substrate,the corrosion medium is diffused to the substrate through the crack,so that the substrate is slightly corroded.On the basis of the above three experiments,the corrosion resistance of the substrate and the coating were investigated.The results showed that the corrosion resistance of the coating was significantly better than that of the substrate and the 150 V-60 ?F of coating has the maximum corrosion resistance. |
PDF Full Text Request