A Study On Crevice Corrosion Behavior Of Q370qNH Bridge Weathering Steel In Simulated Different Atmospheric Environments Under Dry And Wet Alternation

With the popularization of large-span steel structure bridges in China,bridge weathering steels have shown good development prospects in the field of bridge construction due to their excellent resistance to atmospheric corrosion.While the extensive use of welding,riveting and other connection methods in bridge structural parts has made the crevice corrosion of bridge weathering steel widely occurs,which becoming a weak part of bridge members.In addition,the hidden nature of the crevice position further increases the safety hazard of the steel bridge.At present,there is insufficient understanding of the crevice corrosion mechanism of weathering steels in different corrosive atmospheres,especially the crevice corrosion research in dry and wet alternating environments is scarce.The dry-wet alternation accelerated corrosion method is a simulation of real atmospheric corrosive environment with rain and sun.Therefore,the differences in the corrosion behavior of the specimens with and without crevices,and the mechanism of crevice corrosion under alternating wet and dry conditions which will be of great significance to the corrosion and protection of bridge structural parts.In this paper,the dry-wet alternation cycle corrosion method was used to simulate three typical corrosion environments in Northwest China,such as deicing salt,NaHSO₃ and deicing salt+NaHSO₃ alternate action.Scanning electron microscopy?SEM?and laser confocal microscopy were used to observe the corrosion morphology,and the composition and electrochemical characteristics of the rust layer were characterized by XRD and electrochemical workstation.The conclusions are as follows:1.In the deicing salt environment,the corrosion weight loss of the creviceless sample was always greater than the total corrosion weight loss of the crevice sample during the whole cycle;The rust layer of the creviceless sample was loose and porous,and the corrosion products were mainly?-FeOOH,?-FeOOH,?-FeOOH,FeOCl,Fe₂O₃,and Fe₃O₄.With the extension of the corrosion time,the unstable?-FeOOH and FeOCl contents gradually increased,resulting in a lower self-corrosion potential and poor protection of the rust layer;The corrosion products inside and outside the crevice were similar to those without a crevice,but the content of?-FeOOH and FeOCl was small,and the content of dense and stable?-FeOOH and Fe₃O₄ was large.and there was more content outside the crevicep than inside the crevice.Combined with the self-corrosion potential of the rust layer,the protective order of the three rust layers was:outside the crevice>in the crevice>creviceless.The corrosion types of creviceless and crevice samples were mainly general corrosion and pitting corrosion,but the range of pits in the crevice was wide and shallow.Therefore,The degree of corrosion of creviceless samples>out-crevice samples>in-crevice samples.2.In the NaHSO₃ environment,the corrosive solution showed strong acidity.The weight loss of creviceless specimens was always larger than the total weight loss of crevice specimens.The corrosion products of the three rust layers were composed of?-FeOOH,?-FeOOH,Fe₂O₃,and Fe₃O₄.The reductive?-FeOOH content in creviceless samples increased continuously,resulting in a decrease in the rust layer stability;And the corrosion products inside and outside the crevice were similar to the creviceless samples,but the order of?-FeOOH and Fe₃O₄ content of the dense structure was:outside the crevice>inside the crevice>creviceless,so the order of the self-corrosion potential was the same.The type of corrosion in all three locations was general corrosion+pitting.Among them,the size of the etch pit of the creviceless sample was the largest,which was easy to develop in depth,and it was difficult to stabilize the formation of rust layer,while etch size of crevice sample was relatively small,so the corrosion degree of the three samples was:creviceless samples>out-crevice samples>in-crevice samples.3.In the synergistic environment of deicing salt and NaHSO₃,corrosion was more complicated.The weight loss of creviceless samples was always higher than the total weight loss of crevice samples.The corrosion products of the three rust layers were composed of?-FeOOH,?-FeOOH,?-FeOOH,FeOCl,Fe₂O₃,and Fe₃O₄.The corrosion products of creviceless samples were mainly unstable?-FeOOH and FeOCl,and the rust layer was loose and easy to fall off.It increased with the number of dry and wet cycles,which led to a reduction in self-corrosion potential,making the rust layer less protective;The content of?-FeOOH and Fe₃O₄ was outside the crevice>inside the crevice>creviceless,so the self-corrosion potential of the rust layer was outside the crevice>inside the crevice>creviceless.The three samples were all pitting+general corrosion,but the creviceless samples had the most corrosion,and the corrosion inside and outside the crevice was relatively light.4.The mechanism of crevice corrosion under the alternating wet and dry environment was different from the immersion environment,and different alternating wet and dry environments had different corrosion behaviors.Cl^-will promote the conversion of FeOCl??-FeOOH to form a loose and porous rust layer with poor protection;SO₄^2-promotes the formation of?-FeOOH,makes the rust layer dense and uniform,and increases the protection.During the wet phase of the liquid film,Cl^-and SO₄^2-,etc.diffused into the crevice to form a closed cell;however,during the dry phase of the liquid film evaporation,the diffusion of Cl^-,SO₄^2-into the slit was stopped.Corrosion autocatalytic effect was difficult to occur due to low Cl^-,SO₄^2-concentration in the crevice,and due to the lack of oxygen in the crevice,Fe₃O₄ with dense structure was formed.On the other hand,the diffusion of the corrosive medium into the crevice during the wet cycle caused the concentration of Cl^-and SO₄^2-outside the crevice to decrease,and the degree of corrosion outside the crevice was lighter than that of the creviceless specimen.In the deicing salt environment,the pitting effect of Cl^-was obvious,however,due to the limited diffusion of Cl^-,pitting in the crevice was not easy to develop in depth;Under the industrial atmospheric environment,the longitudinal corrosion tendency of creviceless samples was relatively large,which was not conducive to the formation of stable rust layers;Under the synergistic effect of the two environments,due to the common influence of Cl^-and SO₄^2-,the corrosion type was pitting corrosion+general corrosion.