| The proven vanadium titano-magnetite resource in Panzhihua region is as high as 9.39 billion tons,which is the greatest vanadium titanium concentrate supply base in China.At present,the strong magnet-reselection-flotation-electric separation process is adopted for separation;15 million tons of vanadium titanium concentrate at the grade of 55-57%can be obtained every year,which will produce over 7 million tons of ilmenite tailings,resulting in the comprehensive resource utilization rate of lower than 40%.T here are over 10%Ti and Fe elements,as well as small amounts of noble metal elements(such as Co,Ni,Sc and Ga)in the tailings that have not been sufficiently utilized,due to the relatively lagged behind technique of preparing,vanadium-titanium recovery technology and equipment,together with the poor economic and technical indexes,leading to great waste of resources.Moreover,the tailing pond has occupied a large amount of land,and destroyed the surrounding ecological environment;typically,the tailing pond volume pressure is increasing yearly,which has caused potential risks to the production and life of the downstream people.Therefore,exploring a new path and new approach to comprehensively utilize the ilmenite tailing waste resources in Panzhihua conforms to the principle of the comprehensive utilization of vanadium-titanium resources and circular economy development.Enamel coating is the one-layer or multi-layer of amorphous glass inorganic coating formed through fusing the enamel glaze-coated metal substrate surface at high temperature,which possesses favorable metal wettability and affinity.At the time of firing,the enamel can form the oxidation transition layer with the metal surface,and fuse with the metal surface,which can prevent the corrosion of metal in mineral acids,salts,organic weak acids and organic solvents at various concentrations,exert the buffering protection effect,and reduce the corrosion of metal substrate.To sufficiently utilize the valuable metal elements,including Si,Ti,V,Al,Co,Ca and Mg,in the ilmenite tailings in Panzhihua,to develop an enamel coating with acid and alkali corrosion resistance on Q235 carbon steel substrate surface that contains the vanadium-titanium elements can provide a new research thinking and direction for the comprehensive utilization of ilmenite tailings in Panzhihua.In this paper,the ilmenite tailings in Panzhihua were used as the raw materials to prepare the enamel glaze through adding chemical raw materials,such as quartz,feldspar,borax,Na2CO3,NaNO3,CoO,Ni(NO3)2 and Li2CO3,after high temperature(1250-1300?)melting.Afterwards,the enamel coating with acid and alkalicorrosion resistance was prepared on the Q235 carbon steel surface after thermal treatment.Besides,the physical properties of the enamel coating were characterized by XRD,SEM and DSC,while the acid and alkali corrosion resistance of the enamel coating was investigated through electrochemical test methods like EIS,Tafel and SECM.The XRD spectrum of the enamel glaze prepared through adding the ilmenite tailings was dominated by the SiO2 and NaAlSiO4 phases,which conformed to the composition of enamel glaze crystal phase.When the addition amount of ilmenite tailings was increased from 8%to 14%,the prepared enamel coating had reduced resistance to sudden temperature change accordingly.The corrosion potential of the enamel coating in the 30%H2SO4 solution tested by the Tafel polarization curve ranged from-0.61 V to-0.79 V,the coating corrosion rate was increased from 8.36 mg.(cm2.d)-1 to 18.1 mg.(cm2.d)-1,and the corrosion degree was aggravated.The Nyquist curve shows two time constants in the high frequency region,with a large radius of the capacitive reactance arc.The glaze structure corroded by H+ was unstable,and a Warburg impedance straight line with the angle of tilt of 45° occurred in the low frequency region.In SECM microcorrosion morphology test,a current feedback spike of the probe at a small extent could be seen at the X-axis direction,and the current feedback value was 1.5?1.8×10-11A.With the increase in the addition amount of tailings,the coating was subjected to a greater corrosion degree by H+,which had formed the corrosion active site.The 3D corrosion morphology plotted by the SECM was basically consistent with the SEM macrocorrosion morphology.To further enhance the adherence performance and the acid and alkali corrosion resistance of the enamel coating,equivalent amount of sodium silicate titanate was added into the glaze to replace the borax,the glaze melting temperature was reduced to 1200-1250?,the melt shrinkage stress and viscosity were reduced,the bubbles produced in the enamel layer could easily flow out,and the prepared coating had smooth glaze.With the increase in the addition amount of sodium silicate titanate,more island-anchor dendrite structures were formed at the carbon steel substrate interface,and the adherence performance between the enamel coating and the carbon steel substrate was increased.In DSC test,the ?TTS and Kgl values were decreased,and the possibility of crystallization of the prepared enamel coating was increased.The microcrystal enamel coating prepared through burning at 850? for 15 min and micro-crystallization insulation for 30 min,the diffraction peaks of SiO2,NaAlSiO4 and titanophosphate silicate(Ti4Si2P6O25)crystallization content occurred in the XRD spectrum.The microcrystal enamel coating was subjected to Tafel polarization curve test in 30%H2SO4 solution,and the results suggested that,the Ecorr value was reduced with the increase in the addition amount of sodium silicate titanate,and the corrosion rate was reduced from 1.85 mg.(cm2.d)-1 to 0.35 mg.(cm2.d)-1.As for the Nyquist curve,a capacitive reactance arc with a large radius occurred in the high frequency region,while diffusion impedance was detected in the low frequency region,and the log|Z|?logf curve approached 90° with the middle frequency region range,with the slope of nearly 1.The coating equaled an isolation layer with great resistance and small capacitance,which could effectively prevent H+ from permeating into the interior coating.In SECM,an obvious current feedback spike occurred at 140-160?m in the X-axis direction,and the current feedback value was 1.84?3.26×10-9A,suggesting that the acid corrosion resistance of the enamel coating added with sodium silicate titanat was improved after the micro-crystallization process.Glass is a kind of amorphous inorganic material with high SiO2 content,in which the material points are disorderly arranged.Its properties,such as expansion coefficient,hardness,mechanical strength and melt software temperature,are close to those of enamel glaze.Adding glass powder into the enamel glaze during melting allows for better fusing and permeation of the glass with the glaze,thus adding to the integrity of the[-SiO4-]tetrahedron skeleton structure in the enamel layer.For the prepared enamel coating,the corrosion current density icorr of the Tafel polarization curve in the 30%H2SO4 solution was 2.29×10-3?A.cm-2,and the corrosion rate was 0.639 mg/cm2.d-1.In EIS test,the Nyquist curve had a great capacitive reactance arc radius in the first quadrant of the high frequency region,which corresponded to the capacitive reactance of the coating and the capacitance Rs in the acid solution,while an inductive reactance arc accompanying with diffusion phenomenon occurred in the fourth quadrant of the low frequency region.The corroded coating glaze was annular,the capacitive reactance arc radius in the high frequency region was gradually increased with the increase in the addition amount of glass powder,meanwhile,the corrosion degree on the coating glaze was reduced,the enamel layer structure was integral,and the enamel coating had manifested favorable acid corrosion resistance.The addition of sodium silicate titanate can effectively increase the adherence performance between the enamel coating and the carbon steel substrate,while the addition of glass powder can markedly enhance the acid and alkali corrosion resistance of the enamel coating,but the adherence performance of the coating is reduced.To prepare the enamel coating and carbon steel substrate with marked adherence performance and favorable acid and alkali corrosion resistance,22-25%sodium silicate titanate was added into the raw material to prepare the ground enamel layer with the carbon steel substrate;then,20-35%glass powder and 0.4-0.6%Li2CO3 were added to prepare the enamel glaze,and the enamel-glass coating could be prepared by meas of secondary enameling.With the increase in the addition amount of glass powder,the Tafel fitted corrosion potentials of the prepared glass enamel coating had shifted positively in 2 mol/L NaOH solution,20%HCl solution and 30%H2SO4 solution,respectively;additionally,the corrosion current density was reduced,the corrosion rate was reduced from 0.086 mg.(cm2.d)-1 to 0.005 mg.(cm2.d)-1,and the coating had strong acid and alkali corrosion resistance.In the EIS test on the glass enamel coating using 50%H2SO4 solution,the Nyquist curve had manifested two time constants after immersion corrosion for 4-8 h,and the resistance value had reached 1.12?1.21×106?.cm2,which was the initial stage of coating corrosion.When the corrosion time was increased to 48-72 h,the capacitive reactance arc radius of the Nyquist curve was gradually reduced,and the 45° Warburg diffusion sign could be seen in the low frequency region,with the resistance value being reduced to 7.8?8.2×105?.cm2.When the corrosion time was increased to 96-120 h,the capacitive reactance arc radius of the Nyquist curve was further decreased,and the corrosion process was at the middle and late stage.In the Bode phase diagram,the initial phase angle was 62°,and the low frequency region was gradually reduced and approached 0°,which was equivalent to a shielding layer with great resistance and small capacitance;the corrosion process was at the relatively stable and slow late stage,the resistance value was stabilized at 3.8?4.0×105?.cm2,and the coating had manifested strong acid corrosion resistance. |
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