Study On Marine Atmospheric Corrosion Behavior Of Welded Joints Of 690 MPa Grade Bainitic Weathering Steel

In this paper,the corrosion kinetics of Q690 bainitic bridge steel and its welded joints in simulated marine atmospheric environment and the mechanism of galvanic corrosion behavior at the initial stage of corrosion were studied by means of cyclically wetting and drying method,electrochemical method,combined with modern surface analysis techniques such as X-ray diffraction(XRD),field emission electron probe microscope(FE-EPMA),atomic force microscope(AFM)and self-made capillary microcell.It provided data accumulation and theoretical support for the development of welded joints of 690 MPa grade bainitic weathering steel and its application in actual service environment.The main research progress and results are as follows :(1)The corrosion processes of Q690 steel and its welded joint may be differentiated as two stages,namely,the accelerated corrosion stage and the deceleration corrosion stage.The rate transition point is between 72h-96 h,and the corrosion rate of welded joint is slightly larger than that of base metal.With the increase of corrosion time,Cr,Cu and Si are enriched in the inner rust layer of Q690 steel and its welded joint,but the enrichment degree of the inner rust layer of Q690 base metal is higher than that of its welded joint,and finally shows that the corrosion rate of the two tends to be stable in the later stage of corrosion,and Q690 base metal is slightly better than its welded joint.(2)A micro-galvanic corrosion cell is formed between the base metal(BM),heat affected zone(HAZ)and weld metal(WM)of the welded joint of Q690 bainitic weathering steel.The dynamic behavior depends on the microstructure of the sample and the formation and stabilization of the surface rust layer.It can be divided into four stages :the initiation and development of micro-galvanic corrosion,the decrease of galvanic corrosion rate,the polarity reversal of galvanic couple and the disappearance of galvanic effect.During the whole corrosion cycle,the corrosion rate of Q690 bainitic weathering steel welded joints is higher than that of the base metal,and the difference between the two increases first and then decreases gradually until it is stable,which is closely related to the micro-galvanic corrosion kinetic behavior of welded joints.(3)The galvanic corrosion behavior induced by inclusions in the BM is the weakest,and the local corrosion anode radius formed by inclusions is the smallest,follows by the fine grained heat affected zone(FGHAZ),and the local corrosion anode radius formed by inclusions in the coarse grained heat affected zone(CGHAZ)is the largest.Although there is no obvious local corrosion anode in the WM,there is still a slight difference between the Volta potential of the inclusions and the matrix,which makes the ferrite(F)in the WM zone preferentially dissolve,resulting in the overall corrosion resistance of the WM zone between CGHAZ and FGHAZ.At the initial stage of corrosion,the corrosion resistance of the four regions of the Q690 welded joint is: CGHAZ < WM < FGHAZ <BM.