Preparation,Properties And Mechanism Of Long-turn Corrosion Resistant Chromium-Free Insulating Coating For Oriented Silicon Steel

Oriented silicon steel is a key material for many fields such as military industry,electronics industry,etc.The insulating coating on its surface is still chromium-containing coating,which is harmful to human body and environment.With the promotion of green transformation in steel and other industries during the"14th Five-Year Plan"period,the research and development of chromium free insulating coatings for oriented silicon steel is imminent.However,the chromium-free coating faces the problem that it cannot simultaneously achieve the technical effect of chromium-containing coating and withstand the high temperatures（≥800℃）required for oriented silicon-steel rolling process.To break through this bottleneck,it is necessary to deepen the research on the reaction behavior of coating components,the evolution behavior of film structure during curing and the mechanism of interaction between coating and substrate.This study used the aluminum dihydrogen phosphate（Al（H₂PO₄）₃）solution and silica sol（Si O₂·n H₂O）as the main film-forming substances,modified bentonite,magnesium oxide（Mg O）and molybdate as functional fillers to prepare chromium-free inorganic coating for oriented silicon steel,revealed the component reaction behavior and structure evolution during curing process of the coating,explained promotion effect of Mg O on film densification during curing,influence of molybdate valence state regulation characteristics on passivation mechanism and corrosion resistance mechanism of the coating.It has great significance for overcoming the technical bottleneck of the industrialization of chromium-free insulating coatings for oriented silicon steel and provides a new theoretical basis for research on long-turn corrosion resistant chromium-free insulating coatings.The specific work contents are as follows.（1）The film-forming material composition consisted of Si O₂·n H₂O as the main component and Al（H₂PO₄）₃ solution as the auxiliary component,and modified bentonite addition method and coating preparation process were studied.Acidic Si O₂·n H₂O with gradation particle size was used,and the mass ratio of Si O₂·n H₂O to Al（H₂PO₄）₃ was kept at 2.5:1-3.0:1,and the mass fraction of modified bentonite was 1%,which can obtain a coating with better storage stability and corrosion resistance.The neutral salt spray resistance test（NSS）without corrosion lasted for 36 h,and the micromorphology was stacked spherical particles,with some micropores and cracks,reaching 158 nm at the widest point.After the low-temperature curing stage,the coating will complete the evaporation of water;in the high-temperature annealing stage,the structure evolution of the coating is phase transition.And the main structure of the coating is amorphous aluminum phosphate（Al PO₄）and silicon dioxide（Si O₂）,containing a small amount of crystalline Al PO₄ and Si O₂,whose space group are C2221（20）and Fd-3m（227）,respectively.（2）The mechanism of Mg O promoting film formation and densification during coating curing was studied.Mg O reacts with Al（H₂PO₄）₃ solution to consume free phosphoric acid（H₃PO₄）,accelerates and deepens polycondensation of Al（H₂PO₄）₃ in curing stage,and promotes crystallization of Al PO₄ in high-temperature annealing stage.As Mg O mass fraction increases,coating corrosion resistance first increases then decreases.When Mg O mass fraction is 0.1 wt%,coating corrosion resistance is optimal.The NSS test without corrosion lasts for 84 h and pore diameter decreases to 100 nm-130 nm.Excessive addition of Mg O causes coating to crystallize excessively,and Al PO₄ and Si O₂ space group are different which causes stress accumulation,increase of cracks increase and decrease of corrosion resistance.（3）The relationship between molybdate valence properties regulated with zinc（Zn）and film passivation reaction was studied,and a method for quickly characterizing molybdate valence properties with polarization curve testing was explored.In molybdate and phosphomolybdate solutions,Zn reduced hexavalent molybdenum(Mo⁶⁺)to trivalent molybdenum(Mo³⁺)and pentavalent molybdenum(Mo⁵⁺)respectively,obtaining mixed valence state molybdate solution（PMZ）and phosphomolybdate solution（PMNZ）.Mo³⁺can make coating have better corrosion resistance.When PMZ mass fraction is 10%,it achieves no corrosion for 156 h NSS test,corrosion area less than 5%for 204 h.The results also show that polarization curve is a reasonable effective method for rapid characterization Mo composition characteristics in PMZ.The more lower valence molybdenum content,the lower corrosion potential is.The error between actual measurement potential and calculated potential of potential function constructed by Nernst equation is-1.34%-1.63%.（4）The change rules of oriented silicon steel substrate micro-morphology,element composition and electrochemical characteristics during coating failure process were explored,and corrosion kinetics model was established by weight method.Anti-corrosion mechanism of coating is shielding effect,and coating failure originated from coating peeling caused by corrosion product accumulation.The initial corrosion product is Fe（OH）₂,part corrosion product transformed iron oxide thereafter,and ratio of iron and oxygen increased.During electrochemical characteristic observation experiment,the fitting circuit already appeared two time constants when immersion 0.5 h,representing electrolyte solution penetration in coating;Warburg diffusion coefficient appeared when immersion 3 h,representing electrolyte solution lateral diffusion in coating;when immersion 192 h,electrolyte solution completely penetrated coating,penetrated magnesium silicate（Mg Si O₃）surface layer,and provided third time constant,and the low frequency impedance modulus sharply decreased;electrolyte solution penetrated Mg Si O₃layer,and provided fourth time constant thereafter,representing complete failure of the coating.When using PMZ solution,the fitting circuit still keeps three time constants after immersion 216 h.This indicates that PMZ addition can prolong shielding effect.Corrosion kinetics model established by weight gain method for the early corrosion stage of coatings agrees with the quintic function.The intercept reflects surface porosity.Each coefficient of the function reflects the ability of the coating to resist corrosion at different stages.The PMZ coating gained weight quickly in the early stage and reached a weight gain balance,while the weight gain in the middle stages was slow,indicating that the coating can effectively prevent the diffusion of corrosive media and the occurrence of corrosion reactions.（5）Compared with the performance of the chromium-containing coating,the chromium-free coating has excellent corrosion resistance,and the corrosion area of the204 h NSS experiment is less than 5%and interlayer resistance is more than 100Ω·cm²,which is superior to the chromium-containing coating.The adhesion is qualified,measured by the copper rod bending method as C,and measured by the grid method as 0.The tensile and pencil hardness are similar to those of the chromium containing coating,which are 64.16 J·m^-2 and 2H,respectively.The water contact angle is 47.58°greater than the chromium-containing coating.The gloss is 14 Gu less than that of the chromium containing coating.