Study On Localized Corrosion Behavior Of Galvanized Coatings And Passive Films Based On SECM

Steel materials are one of the most widely used metal materials at present,However,steel materials are prone to electrochemical corrosion in the use environment,and the annual loss amounts to hundreds of billions of dollars.Hot-dip galvanizing is widely used for corrosion protection of steel as an economical and effective corrosion protection technology.At present,there have been many research reports on the influence of the composition and microstructure,corrosion environment and surface treatment of the galvanized layer on its corrosion behavior.However,these studies mainly used the macroscopic electrochemical method to analyze the corrosion behavior of galvanized layer,which can only provide the macroscopic average corrosion information of the coating.It is difficult to reflect the metal/solution interface,the occurrence of early micro-scale corrosion and the requirements for in-situ research on local corrosion.Scanning electrochemical microscope（SECM）has high spatial resolution and electrochemical sensitivity,which can be used for early micro-scale analysis of corrosion,reproducing the electrochemical reactions at the interface between samples and solutions,the corresponding electrochemical behaviors and surface changes.It plays an important role in the study of localized corrosion of metals.In this paper,polarization curve（Tafel）,electrochemical impedance spectroscopy（EIS）and scanning electrochemical microscope（SECM）are combined to deeply study the effects of corrosive ions,Immersion time and other factors on the corrosion behavior of galvanized steel cross section,galvanized layer surface and trivalent chromium passivation film of galvanized layer,and further explore the corrosion mechanism of galvanized steel.The main conclusions are as follows:Localized electrochemical corrosion behavior of the interface of hot-dip galvanized coating.During the immersion corrosion process,the distribution pattern and reasons of reactive current and oxygen in the interface of Zn coating/steel substrate region were analyzed.The distribution pattern and the formation mechanism of corrosion products in the interface region were explored.The localized electrochemical corrosion mechanism of the Zn coating/steel substrate interface of the hot-dip galvanized steel was revealed.It is found that the corrosion products are mainly distributed on the surface of the steel substrate close to the interface of coating/steel substrate and play a protective role in the corrosion of the coating in the corrosion process,and the corrosion products are less distributed on the surface of Zn coating.The corrosion products are mainly composed of Zn₅（OH）₈Cl₂,Zn₅（OH）₆（CO₃）₂ and Zn O.In addition,oxygen consumption mainly occurs on the steel substrate area during the corrosion process.With the prolonging of corrosion time,the dissolved oxygen content on the coating and steel substrate gradually decreases.Effect of grain orientation on the localized corrosion behavior of galvanized layer.By comparing and analyzing the corresponding relationship between the grain orientations,the grain corrosion morphology and the localized current in galvanized layer,which explored the influence of grain orientation on the microscopic corrosion behavior of galvanized layer.The results show that the Zn grain near<0001>//ND orientation shows a lower corrosion rate compared to that near<01-10>//ND and<-12-10>//ND orientation.The grain corrosion rate of<01-10>//ND and<-12-10>//ND orientation is similar,which mainly depends on the orientation of neighboring grains.The corrosion morphology of<0001>//ND orientation grain appears hexagonal corrosion pit,and the morphology of<01-10>//ND and<-12-10>//ND orientation grain shows striated structures.In-situ localized corrosion behavior of Cr（Ⅲ）passivation film on galvanized layer.SECM was used to in-situ analyze the variation of micro-area current of Cr（Ⅲ）passivation film on galvanized layer in Na Cl solution with time.It is found that the micro-area current was relatively stable in the early immersion corrosion process,and the passivation film dissolved slowly,which effectively hindered the corrosion.After adding polyaniline into the passivation solution,the reaction activity in the scratch area is significantly reduced,the O₂ consumption is reduced,and the corrosion in the scratch area is slight,which can effectively prevent the corrosion of the corrosion medium to the substrate.