Effect Of K₂SiF₆Sealing On The Corrosion Resistant Of K₂FeF₆Conversion Coating On Steel

In order to improve the corrosion resistance and the adhesion with the organiccoatings of iron and steel, at present, the most common methods are phosphating andchromate chemical conversion treatment after galvanization. But the use of chromatecarcinogen and the problem of eutrophication caused by wastewater of phosphatingdischarge cause a serious impact on the environment, human health and sustainabledevelopment of society. Therefore, phosphorus-free and chromium-free conversioncoating technology research and application on iron and steel is great significance.On the basis of our potassium fluoferrite conversion coating on the surface of ironand steel, in order to improve its anti-corrosion performance, the effect of sealing withsodium silicate and fluosilicate on the corrosion resistance of K2FeF6conversioncoating was researched by contrast and single factor experiment, respectively. Theeffect of sealing was further improved by adding the etching agent, oxidant andaluminium salt after confirming the main salt of potassium fluosilicate, then optimizedthe process conditions. The results showed that under the condition of K₂SiF₆11g/L,（NH₄）₂S₂O₈5g/L, NaF20g/L, Al₂（SO₄）₃2g/L, pH4,35°C and30minutes, salt spraytime of the conversion coating after sealing was up to18h, was three times more thanthat without sealing.The results from appearance, metallographic microscope, SEM, electrochemicaltesting and EDS analysis showed that the conversion coating was more homogeneousand denser, continuous and complete after sealing treatment; the corrosion potential ofspecimen than the bare iron was shifted41mV positively, and the corrosion currentdensity was reduced by nearly an order of magnitude; the elements of Si, O, Na wereadded to conversion coating, the formation of SiO2enhanced the corrosion resistance.