Preparation And Properties Of Yttrium Salt Doped Silane Films On AZ31B Magnesium Alloy

For decades,silane has been used as an environmentally friendly alternative to chromate as a protective metal pretreatment.However,under the continuous electrochemical erosion of corrosive chemicals,silane can not provide adequate long-term protection for metal surface.Degradation of silane near low cross-linking density areas in cracks,micropores and coatings can lead to metal corrosion,especially at the silane metal interface.By adding corrosion inhibitors and/or nanoparticles,the performance of silane coatings in metal protection has been improved.The doping modification of BTESPT and KH560 silane coupling agent was studied by single factor variable method.The anti-corrosion ability of silane film was improved by adding Y（NO₃）₃.Firstly,Y（NO₃）₃ doped modified BTESPT silane films were prepared by impregnation method.Tafel test and EIS test confirm that the optimum addition amount is 12g/L.In the room temperature brine immersion test,the corrosion resistance of single-layer silane film is better than that of blank sample,but a small amount of white matter and pitting corrosion are also produced.The Yttrium salt doped modified sample has almost no pitting corrosion,and only a small amount of white sediment is produced on the surface.The corrosion mechanism of bare Mg、Mg/BTESPT and Mg/BTESPT/dope in 3.5% Na Cl solution was tested.Tafel test results show that the self-corrosive current density of BTESPT silane film is 3 orders lower than that of magnesium alloy blank sample at the initial stage of immersion,and the polarization resistance is 1 order higher than that of magnesium alloy blank sample.The modified silane film doped with yttrium salt is 2 orders lower than that of traditional silane film.At 72 h,the I_corr of BTESPT silane film is still 2 orders lower than that of magnesium alloy sample,and the polarization resistance is 1 order higher.However,the I_corr of yttrium salt doped modified silane film is four orders of magnitude lower than that of traditional silane film,and the polarization resistance is two orders higher.Experiments show that the corrosion resistance of the sample obtained by doping the optimum amount of Y³⁺ is excellent.EIS results show that the low-frequency impedance of BTESPT silane film is 2 orders higher than that of magnesium alloy sample after 5 minutes immersion,while that of yttrium salt-doped silane film is 2 orders higher than that of traditional silane film.After 48 hours immersion,The low-frequency impedance of yttrium salt-doped silane films decreased from6.2×10⁵Ω·cm²to 0.96×10⁵Ω·cm² from 1 h to 48 h.After soaking to 72,the low frequencyimpedance value of the traditional silane film is 2 orders higher than that of the magnesium alloy sample,and the yttrium salt doped modified coating is 2 orders higher than that of BTESPT silane coating,which indicates that yttrium salt doping improves the corrosion resistance of the composite silane film.FTIR measurements show that there are Si-O-Si and Si-OH in the coatings.The network formed by them leads to a denser film.The presence of Y³⁺indicates that Y is successfully embedded in the gap of the silane film.XRD measurements showed that Y³⁺ was oxidized into silane film to form Y₂O₃,Mg（OH）₂ and Y（OH）₃.Secondly,Y（NO₃）₃ doped modified KH560 silane film was prepared.Potentiodynamic electrochemical Tafel curves and AC impedance spectroscopy showed that the optimum addition amount was 5g/L.In the salt water immersion experiment at room temperature,after 24 hours of immersion,there was a little pitting corrosion on the surface of the sample treated with KH560 silane,and a little white substance was produced on the surface.However,there was almost no pitting corrosion and white precipitate on the sample doped with 5g/L Y³⁺ and 10g/L Y³⁺.After 72 hours,there was no pitting corrosion and white precipitate on the surface of the sample treated with KH560 silane.There were a lot of corrosion products on the surface of samples treated with KH560 silane,and the surface corrosion is very serious.The surface of 5g/L Y³⁺doped modified sample is mostly intact,but there are also a few pitting corrosion and white products.There are many pitting corrosion and white precipitates on the surface of 10g/L Y³⁺ doped modified sample,and only a few parts of its surface are intact.SEM showed that there were a few micropores and defects in the KH560 silane film,but there were almost no micropores and defects on the surface of the sample doped with yttrium salt,which indicated that the addition of Y³⁺ filled the micropores in the silane film.EDS results showed that Y element was successfully embedded in the silane film,and the content of Mn2+ decreased,and the content of Y³⁺ increased,which indicated that yttrium oxide was formed in the silane film,which made the silane film become yttrium oxide.More compact and corrosion resistant.Tafel test results show that the self-corrosive current density of 5g/L Y³⁺ doped modified silane film is three orders of magnitude higher than that of traditional KH560 silane film,and the polarization resistance is two orders higher than that of traditional KH560 silane film.Compared with 10g/L Y³⁺ doped modified silane film,the self-corrosive current density is in the same order of magnitude,the polarization resistance is one order higher than that of 10g/L Y³⁺ doped modified silane film,and 10g/L Y³⁺ doped silane film is10g/L Y³⁺ doped at 72 h.The corrosion current density of doped modified silane film is three orders of magnitude higher than that of traditional KH560 silane film,which is basically the same as that of 5g/L Y³⁺ doped modified silane film.The polarization resistance of doped modified silane film is four orders of magnitude higher than that of KH560 silane film,and even more than half of that of 5g/L Y³⁺ doped modified silane film.Therefore,when testing the corrosion mechanism of silane film,excessive Y³⁺ addition will lead to precipitation with OH-in solution.The silane film is self-healing,denser and more anti-corrosion.EIS results showed that the low-frequency impedance values of 5g/L Y³⁺ doped modified silane films and 10g/L Y³⁺ doped modified silane films were 3 orders higher than those of KH560 silane films after 5 minutes immersion.After 72 hours immersion,the low-frequency impedance values of 5g/L Y³⁺ doped modified silane films and traditional silane films were in the same order of magnitude,but the 10g/L Y³⁺ doped modified silane films were higher than those of them by 3 orders of magnitude.FTIR measurements showed that the formation of Si-OH in the composite silane film was slow due to the addition of excessive Y³⁺ doped silane film,which made it difficult to form film.It also confirmed that carboxylate was produced in the solution.XRD measurements showed that Y₂O₃ was oxidized by the composite silane film and precipitated by the reaction of Mg²⁺ and Y³⁺ with OH-in solution.