Experimental Study On Flow Corrosion Behavior Of Stainless Steel Pipelines In Nitric Acid

Nitric acid is a kind of important industrial raw material which is widely used in various important industrial fields.In recent years,with the continuous expansion of the nitric acid production,the corrosion leakage events of stainless steel nitric acid devices and pipelines have occurred frequently,especially the phenomenon of local erosion corrosion.Therefore,the corrosion prevention of nitric acid devices and pipelines is the problem which always bothers the design and use of nitric acid devices.In this thesis,experiment and simulation study of the flow erosion about nitric acid was carried out by developing the jet flow corrosion test device of nitric acid,using polarization curves,EIS,and surface microtopography analysis methods to explore the influences of impact velocity,impact angle and temperature on corrosion law of stainless steel.Besides,the simulation of the fluid mechanics factors such as velocity field,pressure field and shear force was performed on the test device,and compared with the test results.The main conclusions are as follows:(1)304 stainless steel under flowing nitric acid flushing still has obvious passivation process.The corrosion mechanism of 304 stainless steel in the medium is not converted by the factors such as impact velocity,impact angle and temperature,while the kinetic process of intergranular corrosion of metal in the medium is influenced,which,in turn,affects the corrosion behavior.(2)The test results show that with the increase of temperature and velocity of flow nitric acid medium,the corrosion rate of the metal was significantly accelerated,the charge transfer between the interfaces was also sped up,and the stability of the passivation process of the metal matrix was reduced,which were more likely to transpassivation and then intergranular corrosion occurred.In addition,when the impact angle of the flowing nitric acid medium on the surface of stainless steel was 45°,the corrosion resistance of the metal was the strongest and the stability of the surface film layer was the best.When the impact angle was less than 45°,with the increase of the impact angle,the stability and protection of the film were continuously improved,and the corrosion resistance was continuously enhanced.When the impact angle was greater than 45°,the corrosion rate of the metal continued to increase and the corrosion resistance gradually deteriorated as the impact angle increased.When the impact angle was 90°,the corrosion rate of stainless steel was the fastest,and the resistance to flowing nitric acid erosion was the worst.(3)The corrosion resistance of the metal in the flowing nitric acid was significant influenced by the average static pressure and shear stress on the surface of the test metal.When the static pressure generated during the flow erosion process was large,the corrosion rate of the test metal was mainly determined by the impact of static pressure.As the impact of static pressure continued to increase,the corrosion rate accelerated.While in the single-phase flow impact test,The shearing effect of the shearing force on the metal surface is not sufficient to destroy the passivation film layer,mainly by changing the fluid boundary layer,so that the concentration of oxygen in contact with the metal surface is changed,thereby affecting the corrosion rate.What’s more,the effect of shear force would be obvious only when the static pressure value was small.However,when the static pressure value was large,the corrosion rate was still mainly affected by the impact of static pressure.