| Owing to its easy smelting,low cost,high plasticity and welding performance,Q235 carbon steel is widely used in the parts of vehicle.Welding is necessary in the process of manufacturing and maintenance of vehicle.In the process of welding,nonuniform heating and cooling will lead to distinct changes of structure,composition and consequent in the weld joint,which will cause the pit corrosion,the crevice corrosion,the galvanic corrosion and the other local corrosion.The local corrosion is hard to be discovered and it will make great threat for the reliability and security of the welding joint.Therefore,the quality of the weld joint concerns anti-corrosion quality,degree of safety,and service life of vehicle.Thus,study on the welded joints Corrosion behavior and monitoring technique of Q235 carbon steel are significant for corrosion protection and the security of vehicle.In recent years,with the development of technology for the local corrosion study,there are various novel electrochemical techniques with temporal / spatial resolutions applied to weld corrosion research.In this work,we emphasized to use scanning reference electrode technique and self-made Ag/AgCl flexible array reference electrode,combined with SEM,EDS,metallographic microscope and EMPA to study the corrosion behavior of Q235 carbon steel in different micro area,researched the corrosion resistance of different forms of welding and different method of welding to see how corrosion works.Trying to develop an flexible array reference electrode techniques with spatial resolution,which is no limitations on weld shapes,were performed to study the corrosion process of Q235 steel welded joints in situ.The main results as following:(1)By using Scanning Reference Electrode Technique to test the potential distribution of the weld zone,the heat-affected zone and the base metal in the p H = 9,0.01 mol/L Na Cl solution in different time.Tracking the STM topography images of the weld zone,the heat-affected zone after different immersion time in-situ,furthermore,explore the different concentration ofchloride ions made effect for the potential distribution.It was shown that,the weld zone and the heat-affected zone were more prone to corrosion than base metal,Corrosion developed along the grain boundary,topography images of the weld zone and the heat-affected zone irregularity,with the development of corrosion,height difference grow,finally developed into the uniform corrosion.The Cl~-will lead the passivation coating susceptible to be damaged.So,corrosion becomes more serious in the higher concentration of Cl~-.(2)An Ag/AgCl flexible array reference electrode has been developed to the monitor dynamic corrosion process of different forms of Q235 steel welding in situ.The results indicated the prepared Ag/AgCl flexible array electrodes were acutely responsive to the concentration of chlorine ion and well stable.It clearly pointed out the local corrosion sites and monitor the entire corrosion process of the weldments with irregular surface.(3)Using self-made Ag/AgCl flexible array reference electrode to test the potential distribution of 3 kinds of Q235 steel welding of the argon arc welding,the arc welding,the Acetylene-oxygen welding,compared potential difference value and averaged potentials in different time.It shown that,the corrosion resistance of 3 kinds of Q235 steel welding were,in order,the argon arc welding,the arc welding,the Acetylene-oxygen welding.(4)Explored the internal relations of Cl~-and potential distribution by Cl~--potential comprehensive probe,combined with metallographic microscope,SEM,EDS and EMPA to study metallographic structure,morphology and composition.Results shown that,Mn S inclusions could cause pitting,desorption of Cl~-lead the passivation coating to be destroed.Metallographic structure in the weld zone,the heat-affected zone is different with the base metal,the Widmanstatten structure will lead to weld zone and heat-affected zone which are more prone to corrosion than base metal. |
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