Fundamental Study On The Pickling Process For Austenitic Stainless Steel And Photocatalytic Oxidation Treatment Of Nitrogen Oxides Waste Gas

In the production and molding process,due to high temperature and oxygen-containing environment,a layer of oxide skin will be generated on the surface of austenitic stainless steel,and the nitric-basic acid pickling is the most commonly used removal method,but a large amount of nitrogen oxides are produced during the pickling process,and the environment would be polluted.So,it is the meaningful topic of research to optimize the pickling process and reduce NO_x production.The production mechanism of NO_x in the nitric-basic acid pickling and the effects of inhibitor and accelerator on the pickling efficiency and exhaust emission were investigated detailedly.The polarization curve and electrochemical impedance spectroscopy were used to study the effect of the composition in sulfuric-basic acid pickling on oxide dissolution.At the same time,the NO_x produced in nitric-basic acid pickling was treated by photocatalytic oxidation and absorbtion on the TiO₂nanotubes/foam titanium.The main conclusions are as follows:?1?In the nitric-basic acid pickling,it is concluded that NO_x comes from two parts:one is the reaction of oxides with the acid,the amount of NO_x generation is about 70%,and the other is from the reaction of exposed matrix metal with the acid,the amount is about 30%.The amount of NO_x generation increases with the increasing of HNO₃ concentrations and HF concentrations.The NO_x generation is reduced by the addition of small amount of inhibitor into the pickling solution.The addition of accelerator can reduce the time of pickling oxides efficiently,and it can improve the pickling efficiency.So,the amount of NO_x in the presence of additive is only 40%of that generated by the additive-free cleaning solution.A new process for optimizing pickling of austenitic stainless steels is made as follows:the composition of the pickling solution contains 90¹10 g/L nitric acid,40 g/L hydrofluoric acid,30 g/L inhibitor and 1² g/L accelerant,and the appropriate pickling temperature is 20⁵0.?2?In the sulfuric-basic acid pickling,as the concentration of sulfuric acid and pickling time increases,the corrosion current density of the oxides increases,and the charge transfer resistance gradually decreases.In single sulfuric acid solution,there exists stable passivating region,and it does not change obviously with the increase of sulfuric acid concentration.The inductive arc appears on the electrochemical impedance spectra in sulfuric acid solution,which is the evidence of the adsorption products on the surface of stainless steel,and it impede the oxide pickling.The addition of HF will expand the range of active dissolution zone and shrink the passivating zone,so that the rate of oxide scale shedding is accelerated.It is found that the passivating zone disappears when the hydrofluoric acid concentration is more than 12 g/L.With the increase of Cl^-concentration,the passivating zone of the stainless steel matrix shrinks.The passivating zone will disappear when the Cl^-concentration is more than 40.7 g/L,and the pitting corrosion occurs on stainless steel surface after pickling.The smaller the ratio of n_[SO4^2-]/n_[F^-]and n_[SO4^2-]/n_[Cl^-],the higher the pickling rate,so the suitable ratio range is 0¹,too high concerntration can affect the surface quality after pickling.The inductive arc in the electrochemical impedance spectra disappears with the addition of H₂O₂.It is beneficial to the dissolution and removal of stainless steel oxide scale,and the pickling rate increases.?3?The process for photocatalytic oxidation treating NO_x was explored.The hydrothermal reaction is metal titanium reacting in 10 mol/L NaOH at 110?for 24 h.Therewith tack the metal out to pickle using the hydrochloric acid,and finally calcinate at 400?.The freely growing and intercrossing TiO₂ nanotube array with good photocatalytic performance are made on the surface of titanium matrix.TiO₂nanotube array/titanium foam is prepared using optimized hydrothermal technology,and is used as photocatalyst in the self-made photocatalytic device to catalyze and oxidize NO_x.It is found that the NO_x content in the waste gas is reduced from 35 mg to about 10 mg after photocatalytic degradation treatment under nitrogen environment and UV-visible light irradiation,and the degradation rate reached 72%.