Optimization Of Si And Ni Content In 690MPa Weathering Bridge Steel And Its Service Behavior In Industrial Atmosphere

High strength,corrosion resistance and low cost have become the development trend of bridge steel.The use of high strength weathering steel can reduce the weight of bridge structure,increase the span capacity and bearing capacity,extend the service life,and is also very friendly to the environment.Si and Ni are common additions to bridge steel.Si is an alloying element with low cost to improve strength,but too much addition will deteriorate the impact toughness and welding performance.Ni can improve strength and low temperature impact toughness,but when the content is too high,the efficiency of Ni will slow down and greatly increase the production cost.In addition,it is necessary to investigate the influence mechanism of Si and Ni on corrosion resistance of steel in industrial atmospheric environment,so as to provide a basis for the development of weathering bridge steel without coating under industrial atmospheric environment.This paper aims at the development and service safety of the latest generation of690MPa resistant bridge steel（Q690qENH）.The effects of Si and Ni elements on corrosion resistance,corrosion resistance of base metal and welded joints in industrial atmosphere were systematically studied.The corrosion resistance mechanism of Si and Ni elements,corrosion kinetics law of Q690qENH steel,corrosion resistance of base metal and welded joints were clarified,which provided data support and theoretical guidance for the first demonstration application of Q690qENH steel in China（Wuhan Jianghan Seven Bridge）.The main research progress and conclusions are as follows:（1）In the industrial atmosphere,the corrosion resistance of Q690qENH steel will be deteriorated if Si element is added too much.Si mainly exists in the rust layer in the form of loose porous Fe₂SiO₄,which hinders the formation of Fe₃O₄ and can not well prevent the corrosive media such as O₂ and HSO₃^-from entering the steel substrate,resulting in a low content of dense and stableα-FeOOH in the rust layer.The content of Fe₂SiO₄ in the rust layer depends on the content of Si.With the increase of Si content,the corrosion rate of Q690qENH steel gradually increases,the protection indexα/γ*of the rust layer decreases,and the corrosion resistance decreases gradually.（2）The addition of Ni can improve the corrosion resistance of Q690qENH steel in industrial atmosphere.The addition of Ni can improve the self-corrosion potential of Q690qENH steel,slow down the anodic dissolution,reduce the corrosion current density and increase the resistance of rust layer.Ni is distributed uniformly in the rust layer,mainly in the form of NiFe₂O₄.On the one hand,NiFe₂O₄ improves the ion selectivity of the rust layer and prevents SO₂ from penetrating into the substrate.On the other hand,it promotes the transformation ofγ-FeOOH toα-FeOOH in the rust layer.With the increase of Ni content from 0.3%to 0.5%,the enrichment of S in the rust layer decreases obviously,the protection indexα/γ*value of the rust layer of Q690qENH steel increases,the corrosion rate decreases,and the corrosion resistance improves.（3）In the industrial atmosphere,the corrosion resistance of Q690qENH steel is 40%higher than that of Q235,which follows the corrosion dynamics rule of W=1.330t^0.55.At the early corrosion stage,the corrosion resistance of Q690qENH with bainite is better than that of Q235 with ferrite and pearlite structure.At the later corrosion stage,the corrosion rate of Q690qENH gradually decreases and is much lower than that of Q235.The enrichment of Cu,Cr and Ni alloy elements in the rust layer of Q690qENH improves the density and stability of the rust layer,and the resistance to corrosive media is stronger,the protection indexα/γ*of the rust layer is larger,and the resistance and linear polarization resistance of the rust layer are larger.It’s more protective.（4）The welding joint of Q690qENH has better corrosion resistance in industrial atmosphere.At the early corrosion stage,the corrosion resistance of the weld zone dominated by ferrite is better than that of the base metal zone dominated by bainite.Due to the coarse grain and uneven distribution,the corrosion resistance of the heat affected zone of bainite structure is the worst,and there is no obvious galvanic corrosion in different micro-regions of the welded joint.At the later corrosion stage,Cu and Cr are obviously enriched in the rust layer in different areas of the welded joint.Because the content of Ni in the welded material is higher than that in the base material,the concentration of Ni in the rust layer in the weld zone is much higher than that in the base material and heat affected zone.The higher content of alloying elements in the weld zone makes the rust layer more flat and compact,and has higher polarization resistance and impedance value,resulting in better corrosion resistance of the whole welded joint than the base material.