Study On The Coloring Mechanism And Properties Of Molybdate Passivation In Zinc Coating

Electrogalvanizing is an efficient anti-corrosion method,which can effectively reduce the erosion of steel parts.However,its galvanized layer is easy to interact with the components in the air under the atmospheric environment to form white,loose needle-shaped rusts.In order to further improve the corrosion resistance of the galvanized layer,the galvanized layer is usually passivated after the galvanizing process.With the development of passivation technology,high-contamination chromium salt passivation solutions have gradually been replaced by other non-pollution passivation processes.At present,titanium salts,silicates,organics,molybdates,rare earth salts,and organic-inorganic composites,etc.The system is several important systems for green type galvanizing passivation.A molybdate passivation system is selected in this paper.Mo and Cr both belong to Group VI B elements and have similar chemical properties.Relatively,the molybdate passivation system has low toxicity,and the molybdate passivation system does not change the environmental temperature and p H.Sensitive,is an ideal chromium-free passivation element,which is most likely to replace chromate passivation system.In this paper,Q235A galvanized low carbon steel is used as the carrier,and sodium molybdate solution is used as the base liquid.The effects of different processes on the passivation effect and the process of forming the best passivation film are discussed.By designing and implementing single-factor and orthogonal experiments,the molybdate passivation component and the best passivation process were finally determined.The formula was:20g/L Na₂Mo O₄,5.3g/L Na NO₃,5.3g/L KF;the passivation time was 90s;the passivation temperature is room temperature;the p H is adjusted with nitric acid,and the p H is controlled within the range of 6.1?6.3.In this process,a bright and bright golden passivation film can be obtained,with good corrosion resistance,and the neutral salt spray test reaches 84h.According to the analysis of inverted metallographic microscope,it can be seen that the passivation film formed is bright in color,network structure and dense;according to SEM/EDS analysis,it can be seen that the golden passivation film does not contain fluorine ions,and the surface is dense and smooth.The experimental results show that only The passivation film is only golden yellow when the fluoride salt is added.According to analysis,in the color reaction,the fluoride ion only acts as an active agent to activate zinc,so that the color reaction can proceed;combined with XRD analysis,we can know that the passivated surface The main phases of the phase are H₂Mo O₄·H₂O,Zn Mo O₄,Fe(OH)₃,and Zn(OH)₂.There are two reasons for the excellent corrosion resistance of the passivation film studied in this subject:one is that the passivation film obtained by passivation has a dense network structure on the surface,which can effectively isolate metal materials from the corrosive medium;The molybdate in the passivation film can form a competitive relationship with the chloride ions in the corrosive medium,thereby reducing the possibility of pitting corrosion.Other elements have a certain positive effect on enhancing the corrosion resistance of the passivation film.