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Tuesday, March 18, 2008 - 4:15 PM
Convention Center, Second Level, R07 (Ernest N. Morial Convention Center)
08178
Corrosion Scale Product and its Influencing Mechanism in Welded Joints Between a Ni-Based Alloy and X60 Steel Weld
ABSTRACT C4 Ni-based alloy and the pipeline steel X60 were welded by TIG welding method with the Ni-based alloy welding-wire Cr-Ni-Mo-3, and the corrosion experiment of the welded joint was carried out by high temperature and high pressure autoclave. The microstructure the fuse region (FZ) against X60 steel, the morphology and distribution of second-phase particle at FZ were observed and analyzed by Scanning Electron Microscopy (SEM) and Transmission Electron Microscope (TEM), And the morphologies of the corrosion scale forming on the surface and cross-section of X60 steel were also observed. The cathodic reaction on the each zone in the welded joint was investigated by the cyclic volt-ampere method. The galvanic couple current between the weld metal and X60 steel, the polarization curve and electrochemical impedance spectroscopy (EIS) of the zone in the welded joint with the corrosion scale were measured. The results show that an amount of Fe3C particles exists in fuse region (FZ) in the weld joint against X60 steel. FeCO3 corrosion scale adhering on the heat-affected zone (HAZ ) against the welded joint of X60 steel changes obviously with the increase of corrosion time. After the corrosion time of 96h, there is more corrosion scale forming on the fine grain zone (FGZ) of HAZ of X60 steel than on its FZ and coarse grain zone (CGZ). But the thickness of corrosion scale forming on FZ and CGZ of X60 steel increase and the difference of the thickness of corrosion scale forming on the above two zones decrease obviously after corrosion time of 192h, the mechanism for which is explained in detail.
Key words: Dissimilar metal weld; fusion zone; corrosion scale; galvanic couple corrosion