A Study On Corrosion Behavior Of Q345q Bridge Steel In Typical Atmospheric Environment In Northwest China

In recent years,for medium and long-span highway bridges,the construction of steel structure bridges in northwest China has received widespread attention,which not only conforms to the concept of resource-saving development,but also meets the needs of economic construction in western China.However,the dry and rainless atmospheric environment with strong wind and radiation in northwest Gansu makes the corrosion of iron and steel materials not to be neglected.Faced with the salinity brought by wind and sand,the residual salt of snow melting agent in winter and the pollution of industrial atmosphere,the aging problem of bridge steel structure seriously affects the service life.Based on this,this paper is based on the ordinary bridge steel Q345q,deicing salt,NaHSO₃ and deicing salt+NaHSO₃ were used to simulate three typical northwest atmospheric environments.The dry and wet alternating accelerated corrosion test was used to systematically study the corrosion behavior of Q345q in with or without oxide scale in three different environments.The corrosion mechanism of polished and oxidized scale samples was discussed.The acceleration and correlation of indoor simulation tests were verified according to the corrosion data of one year of atmospheric exposure,and atmospheric corrosion time model was established.The main results and conclusions are as follows:1.The corrosion kinetics of polished samples in three media were analyzed by weight loss method.The corrosion rate decreases with time.The order of corrosion rate is:NaHSO₃>deicing salt+NaHSO₃>deicing salt.After fitting the corrosion data with?W=Atⁿ,the value of n is greater than 1 under 144h,indicating that the rust layer is unprotective.The value of n is less than 1 between 144h and 480h,indicating that the rust layer has certain protection.By means of XRD,SEM and electrochemistry,the phase,corrosion morphology and polarization curves of the rust layer were compared and analyzed.In the deicing salt solution,due to the self-catalytic effect of Cl^-in the pores,unstable corrosion products such as?-FeOOH and FeOCl were formed,resulting in increased corrosion current density,less self-corrosion potential and the lack of of protection of layer of rust.The"acid-generating circulation mechanism"occurs in NaHSO₃ solution,which inhibits the formation of?-FeOOH,and the self-corrosion current density decreases gradually.Combined with the morphology of the rust layer,the rust layer has strong protection.In deicing salt+NaHSO₃ solution,the coupling effect of corrosion behavior is not a simple addition of two media.The partial occlusion micro-cell of Cl^-is weakened,and its protection is between the two.2.The corrosion kinetics of the samples with oxide scale in three solutions were analyzed by weight gain method.The order of corrosion rate was:NaHSO₃>deicing salt+NaHSO₃>deicing salt.The value of n in deicing salt and mixed medium was less than 1 under 192h.The corrosion rate was almost unchanged,indicating that the oxide sscale had certain protective effect on the matrix.After 192h,the value of n is greater than 1,the corrosion rate is accelerated,and the scale is gradually destroyed.Corrosion occurred in NaHSO₃ solution for 48 hours,and then the corrosion rate increased.Combined with the phase,corrosion morphology and polarization curve of samples of oxide scale with rust layer,the local corrosion mainly takes place in the deicing salt solution,which results in the corrosion of micro-batteries with large cathode and small anode at the defect of oxide scale.The autocatalytic effect of Cl^-causes the rust layer to be loose and porous.The section has local cracks,and the self-corrosion current density of the rust layer increases.In NaHSO₃ solution,the"acid-generating cycle mechanism"is dominant,and the self-corrosive current density of the rust layer first increases and then decreases.The rust layer has a certain protective effect.In deicing salt+NaHSO₃ solution,“pitting corrosion”+overall corrosion occurs,and the synergistic effect of Cl-and HSO₃^-is reduced.3.The corrosion kinetics of bare steel and steel with oxide scale exposed to air for one year was analyzed by weight gain method.Combining with corrosion kinetics of bare steel samples and samples with oxide scale in simulated industrial atmospheric environment?NaHSO₃ solution?in the laboratory,the corresponding time points of accelerated corrosion and atmospheric exposure corrosion were obtained,namely 72 h and 144 h correspond to 5 months and 1 year of atmospheric exposure respectively,and 144 h and 288 h correspond to 7 months and 1 year of atmospheric exposure respectively.Under the condition of atmospheric exposure corrosion,the rust layer of bare steel exposed to atmosphere for one year is more protective according to the density of surface morphology and cross-section morphology,the change of n value fitted by kinetic curve and the size of self-corrosion potential.However,the sample with oxide scale has the effect of delaying corrosion in a short time.With the dissolution of oxide scale,corrosion microbatteries with large cathode and small anode are easily formed between oxide scale and steel matrix,which accelerates corrosion.In 0.01 mol/L NaHSO₃ solution,there is a certain correlation between dry-wet cyclic accelerated corrosion and natural atmospheric corrosion test in Lanzhou.But there are also some differences,which are manifested in the fact that the rust layer formed on bare steel under atmospheric environment is denser than that formed by accelerated corrosion.Local pitting corrosion is the main corrosion mode of samples with oxide scale in atmospheric environment,while total and pitting corrosion is the main reason for accelerated corrosion,which is mainly due to the presence of a large number of dust and corrosive particles falling on the surface of samples in atmospheric environment,which is prone to pitting corrosion.