Electro-galvanized Silicate Passivation Process And Mechanism

Enormous economic loss and society hazard are caused by metal corrosion.Even though electroplating zinc coating is an efficient means to improve the corrosion resistance ability of steel and iron,the corrosion is extremely easy to take place when the galvanized zinc coating is in the moist environment,thus the coating loses the protection effect.In order to enhance the corrosion resistance,improve the surface quality and extend the service life, the galvanized film must be carried on passivation processing.The traditional passivation processed by chromate is high toxicity,and it can easily cause the environment pollution and harm people's health.So,in this thesis,the non-toxic and non-pollution silicate (Na₂SiO₃)passivation technique was well studied.The passive film's anticorrosion mechanism and forming mechanism has been preliminary discussed.Therefore,the primary coverage of this thesis as follows:The optimum composition of the passivation solution was gained by the orthogonal experiment and the single factor test as follows:Na₂SiO₃·9H₂O 15g/L,H₂SO₄ 5ml/L, H₂O₂ 10ml/L,HNO₃ 5 ml/L,additive 3 g/L.Three primary factors,which influence the corrosion resistance of the silicate films determined by the orthogonal experiment,were the solution temperature,the pH value and the immersion time.And their effects on the corrosion resistance of silicate films were studied by the single factor test.Finally,the best technological conditions were gained as follows:the solution temperature 15～40℃,the pH value 2.0～2.8,the immersion time 90s around.The corrosion resistances of passive film and galvanized film were studied by the neutral salt spray test(NSS),dipping in salt solution and CuSO₄ solution drop test.The test results indicat that the silicate passive films can obviously improve the anti-corrosive performance and anti-white rust ability.Compared with the low chromate passive film,the silicate passive film's corrosion resistance is better.Tafel polarization curve,EIS and Open circuit potential-time curve on passive film and galvanized film were investigated.The electrochemical parameters of electrochemistry were gained by computer fitting.And,the electrochemistry parameters of silicate passive films are:E_corr-0.85736 V,I_corr0.00012623 Amp/cm²,R_t 580Ω·cm².But the low chromate passive film's electrochemistry parameters are:E_corr-0.9456 V,I_corr0.00034980 Amp/cm²,R_t 150Ω·cm².According to the results from experiments,the silicate passive film's anticorrosion mechanism has been analyzed.It is thought that the silicate passive film has three aspects affects to enhance the corrosion resistance performance of galvanized Zinc coating,which are electrochemical effect,isolation effect and corrosion inhibitive effect.The microscopic appearance and the component analysis of the silicate passive film have been investigated by SEM,EDAX and XPS techniques.The passivation process in silicate passivation solution has been investigated by electrochemistry methods.So the mechanism of forming the silicate passive film has been discussed.It is considered that the process of forming passive film has two steps:1)dissolved Zinc coating;2)the process of forming the membrane by polymer deposition.After scale-up experiment,the stability and corrosion resistance of silicate passivation solution have been investigated.The results show that the stability of the solution is good, the film is even,and the corrosion resistance is good.When the silicate applies in the low-iron galvanized iron alloy coating,the anticorrosion performance of the formed silicate passive film has been studied.The results indicate that the silicate passive film can improve the corrosion resistance of the Zn-Fe alloy coating effectively.Compared with the Zn coating,the corrosion resistance silicate passive film on Zn-Fe coating is better.In conclusion,the silicate passivation technique of electrogalvanized coating has many advantages,for example:Good stability,better corrosion resistance than the low chromate passivation,simple operation,low cost,non-toxic and non pollution,etc.The present situation of the environment pollution can be improved if the silicate passivition technique can be used in electro-plating industry.The silicate passivition technique has broad application prospect,because it can replace the traditional chromate passivation and promote a realization of green production in electro-plating industry.