Study On Corrosion Behavior And Mechanism Of Bridge Weathering Steel Q345qNH

Weathering steel is widely used in bridge construction due to its strong corrosion resistance,low economic loss and low manufacturing and assembly costs.At present,due to the complex and changeable atmospheric environment,the development of weathering bridge steel poses a higher challeng.In addition,most schoolars have studied the single,typical corrosive environment and the real atmospheric environment,lacking the analysis of corrosion beheavior and corrosion mechanism under the complex environment of practical engineering applications.Based on this,this paper takes weathering bridge steel Q345qNH as the research object and the influence of Nb element on the structure and corrosion resistance of weathering bridge steel Q345qNH is systematically investigated through the dry-wet alternating accelerated corrosion test.The corrosion resistance and corrosion behavior of weathering bridge steel Q345qNH under different environments are emphatically studied.The corrosion mechanism under the complex deicing salt environment,NaHSO₃ environment and the mixed environment is discussed,which provides a theoretical basis for the research,application and development of corrosion resis tance in the complex environment.Through analysis,the main results and conclusions of this thesis are as follows:1.The addition of trace amounts of Nb in the weathering bridge steel Q345qNH can improve the corrosion resistance.It mainly refines the grain of the structure,precipitates NbC on the grain boundary,reduces the depositing of chromium carbide content at the grain boundary,and effectively reduces the intergranular corrosion.In the simulated deicing salt corrosion environment,the rust layer formed by the Nb-containing Q345qNH steel is more stable and cpmpact than that of the Nb-free Q345qNH steel.2.Corrosion laws vary in different corrosive environments.The polished weathering bridge steel Q345qNH has the least corrosion loss in deicing sa lt solution and the largest corrosion loss in NaHSO₃ solution.At the initial stage of corrosion,the corrosion rate of Q345qNH steel in three kinds of solutions increased rapidly,and the corrosion rate decreased significantly after 144 h.During 288 h and 480 h,the corrosion rate in NaHSO₃ solution showed a significant downward trend,which was1.5 times of that of mixed solution and 3.8 times as much as that of deicing salt solution.However,the initial corrosion rate of Q345qNH steel with oxide scale in the three kinds of solutions is contrary to that of bare steel,the corrosion rate is relatively slow in the initial stage,and the corrosion rate increased significantly after 192 h.Meanwhile,the rising trend in deicing salt solution and mixed solution is significantly faster than NaHSO₃ solution.3.The rust layer vary in different corrosive environments.With the prolongation of corrosion time,the corrosion of weathering bridge steel Q345qNH in NaHSO₃environment has the highest compactness and stability of the rust layer,while the rust layer formed by corrosion in deicing salt environment is the most loose and porous.This is mainly related to the active mechanism of different corrosive solution and the phase composition of the rust layer.In the simulation of corrosion behavior in different environment,the rust layer components of the bare steel Q345qNH contain three phases ofα-FeOOH,γ-FeOOH and Fe₃O₄.Since the deicing salt environment and the mixed environment belong to a high Cl^-environment,a large amount of unstableβ-FeOOH phase is also formed in two corrosive environments,which reduces the stability of the rust layer.The phase composition of corrosion products of Q345qNH steel with oxide scale is similar to that of the bare steel,and the rust layer has more Fe₂O₃ phase in each corrosion solution.4.Corrosion mechanisms vary in different corrosive environments.In th e simulated deicing salt environment,the corrosion behavior of polished weathering bridge steel Q345qNH is mainly affected by the corrosion of Cl^-and the mutual coupling of multiple ions,which tends to form unstable and loose lamellar rust layer;In the simulated SO₂ corrosion environment,the corrosion behavior of Q345qNH steel is mainly affected by HSO₃^-,which is easy to form“SO₄^2-cycle regeneration mechanism”.In the simulated mixed corrosion environment,the corrosion behavior is the result of the interaction of the first two corrosive environments.The corrosion behavior of weathering bridge steel Q345qNH with oxide scale is more complicated,which is not only affected by different corrosive solution,but also affected by the oxide scale of the material itself.At the initial stage of corrosion,the scale plays a protective role in Q345qNH steel,and the corrosion rate is relatively slow.In the deicing salt environment,the corrosion beheavior is dominated by Cl^-in oxide scale defects.In the simulated SO₂ corrosion environment,the corrosion beheavior is dominated by the overall corrosion of slowly dissolved oxide scale.In the middle and late stages of corrosion,the galvanic corrosion of the“large cathode and small anode”formed by Q345qNH steel accelerates the corrosion rate.In the mixed environment,corrosion beheavior is affected by two media,and the corrosion rate change trend is consistent with the corrosion of the deicing salt environment.