Study On Environmental Friendly Passivation Process For Batch Hot Dip Galvanizing

Zinc is one of the most widely used hot-dip materials.Due to cheap price,excellent corrosion resistance and mechanical properties,galvanized sheet is widely used in automobiles,home appliances,containers,building materials,transportation,energy and other fields.Since the electrode potential of Zn is relatively low,it is necessary to improve the corrosion resistance of the galvanized layer by passivation treatment.As people pay more attention to environmental protection,traditional Cr6+ has been banned due to highly toxic and carcinogenic properties.It is a general trend to find an environmental protection passivation technology to replace hexavalent chromium passivation.However,complex structural parts produced by the solvent method do not match the existing trivalent chromium and chromium-free passivation solutions due to limitations of the surface quality and shape.Therefore,it is full of value to develope environment-friendly passivation treatment technology for batch hot-dip galvanized steel.In this paper,the formulation and process of trivalent chromium and chromium free passivation solution for batch hot-dip galvanized steel were studied.The microstructure,corrosion resistance and film forming process of the passive layer were investigated and optimized by lead acetate drop test,neutral salt spray test(NSS),scanning electron microscope(SEM),Tafel polarization curve,EIS impedance spectrum,open circuit potential/time curve(OCP)and NaCl solution immersion test.The orthogonal test is used to screen and optimize the formula.The main components of trivalent chromium passivation solution were 70 wt.%Cr3+,4 wt.%complexing agent A,2 wt.%complexing agent B,0.6 wt.%F-,0.4 wt.%Co2+,3 wt.%organic blocking agent.The best film formation process parameters are:the dilution ratio is 1:7,the passivation time is 60 s,the curing temperature is 120?,and the curing time is 90 s.Through SEM and roughness test,it was found that the surface roughness of the Cr3+passivation samples reduced and the surface became more flat.The electrochemical process and NSS were used to study the film formation process and corrosion resistance of the passivated samples.It was found that the OCP curve had an inflection point at about 45 s,indicating that the corrosion resistance of the passivated sample was the best.Ecorr of the passivated sample is shifted forward,Icorr is decreased by two orders of magnitude,and |Z|LF increased by two orders of magnitude.This indicates that the coverage of the passivation film effectively inhibits the penetration of the corrosive medium,resulting in a decrease in the charge migration rate,a suppression of the reaction process between the cathode and anode,a decrease in the corrosion rate,and an improvement in the corrosion resistance.The effects of reactive film-forming agents on the corrosion resistance of passivation film in chromium-free passivation solution were investigated by orthogonal experiments and single-factor tests.The optimal film-forming ratio of the chromium-free passivation solution was determined to be:5 wt.%molybdic,8 wt.%phosphate,0.4 wt.%vanadium,0.3 wt.%rare earth salt,3 wt.%KH560.The optimal film-forming process for the chromium-free passivation film was obtained:the passivation solution dilution ratio is 1:7,pH is 4.5,passivation time is 60 s,the curing temperature is 150? and the curing time is 120 s.Through SEM and surface roughness testing,it was found that compared with the substrate,the number of pits and the diameter of the pits on the surface of the chromiumfree passivation film decreased,and the surface roughness decreased.The roughness of the passivation film added with KH560 is smaller,indicating that silane has a certain filling effect on the passivation film.The electrochemical performance and NSS test found that Ecorr of the passivation film compounded with silane was significantly shifted,|Z|LF increased,the salt spray time was longer.Thus the physical shielding performance and corrosion resistance of the sample was greatly improved.By comparing the fitting results of electrochemical tests and NSS test,it is found that Ecorr of trivalent chromium passivation shifted more positively,and Icorr is lower than that of chromium-free passivation samples.The low-frequency impedance value is more than two orders of magnitude higher than that of the substrate,and-? of the high-frequency region of Cr3+ passivation layer is close to 80°,indicating that the passivation film is denser,and the physical shielding effect of blocking the corrosive medium is more significant,and the corrosion resistance performance is greatly improved.The 72 h NSST results of trivalent chromium passivation layer and chromium-free passivation layer both meet the industrial application indicators.