Study On Damage Characteristics Of Rust Layer On Weathering Steel And Their Influences On Corrosion Behavior

The excellent atmospheric corrosion resistance of weathering steel originates from the dense protective rust layer on its surface,but in the service process of steel structure,due to various natural and man-made factorss,the local damage of rust layer will be caused.In the atmospheric environment,this may lead to two different results:(1)corrosion develops along the damage and eventually leads to the overall failure of the steel structure;(2)a new dense rust layer was formed at the damaged area,which recovered the protection of the rust layer.As for the specific result,it should be related to the damage characteristics,composition and structure of weathering steel and environmental factors,but it has not been specially studied.In this work,the damage to the rust layer on weathering steel and carbon steel was artificially introduced by thermal shock and extrusion,and the effect of damage on their corrosion behavior in laboratory accelerated environment and coastal atmospheric exposure was studied.With the help of optical microscope,scanning electron microscope,X-ray diffraction,electrochemical measurement and self-made ethanol adsorption/dehydration method,the damage characteristics of rust layer and their evolutions in the subsequent corrosion process were mainly characterized.The ethanol adsorption/dehydration method described in this paper is a non-destructive analysis for quantitative characterization of defects in the rust layer.The theoretical model for calculating the specific surface area,and porosity of the rust layer were established based on the experimental data of ethanol adsorption/dehydration.This method can obtain the porosity of the rust layer which was unavailable by the traditional N2 adsorption.Furthermore,it can avoid the disadvantage of disturbing the rust layer by N2 adsorption,and also eliminate the deficiency of water adsorption which will lead to further corrosion.After the thermal shock,the adsorption/dehydration rates and porosity of the complete rust layer on weathering steel increased significantly,and the rust resistance also decreased significantly,which indicated that the protective ability of rust layer decreased.It is found that the thermal shock resistance of rust layer on weathering steel is better than that of plain carbon steel.The inner rust layer on weathering steel has higher thermal shock resistance than the outer rust layer.In the rust layer on weathering steel,the crack propagation induced by thermal shock is basically parallel to the interface between rust layer and steel matrix,while for plain carbon steel,the crack propagation direction is random.The rust layer onweathering steel is composed of light-shade band structure,which is almost parallel to the interface between rust layer and steel matrix.In the light band,the Fe/O ratio is higher,the alloying element content is lower,and the metallicity is stronger.It is speculated that the formation of the bands structure is due to the alternation of in-depth development and transverse extension of corrosion.The difference of compositions between light bands and shade bands makes the bands interfaces preferred paths for crack propagation so that most of cracks in rust layers on weathering steel are approximately parallel to the interface.The compactness,rust resistance and stability index of the damaged samples decreased first and then increased;Meanwhile,the rust layer appeared delamination,which was more obvious in the indoor accelerated corrosion,indicating that the new rust was formed preferentially in the damage area during the subsequent corrosion process,which not only filled the defects,but also caused secondary damage to the rust layer in the early stage of corrosion.After the indoor accelerated corrosion test followed by the atmospheric exposure test,the rust layer of the undamaged samples also appeared delamination,and the rust resistance first decreased and then increased,accompanied by the corrosion current first increased and then decreased.It shows that the stable composition of corrosion products is related to the corrosion environment,and the proportion of corrosion products formed in the accelerated corrosion process will change in the subsequent atmospheric environment.The repair of damage to the rust layer is faster in the accelerated corrosion process,while the quality of repair is better in the real atmosphere.