In order to mitigate the preferential corrosion of welded joints, the mechanism of the preferential corrosion needs to be clear. For this purpose, electrochemical tests and immersion tests of weldedjoints of line pipes in CO2 saturated synthetic sea water were carried out. In addition to these tests, electrochemical potential distributions on the inner surface of two types of linepipes that have different chemistries were analyzed using a two dimensional boundary elements method. Corrosion rate distributions in the inner surface were also estimated by using the electrochemical potential distributions analyzed and polarization curves of the inner surfaces. These test results show that the preferential corrosion of Welded joints occurs in the manner of galvanic corrosion caused by the differences in electrochemical potential of weld metals and base metals. These differences are mainly caused by difference in their chemical compositions. Corrosion in the macro-cathod portion was also observed. The addition of Cu, Ni and Mo to weld metals in amounts larger than that in the base metals is effective in preventing the preferential corrosion of the weld metals.
Because of the buffeting effect and its resulting larger capacity to provide IT, higher corrosion rate is expected in CO2 containing environment (Kodama, 1983). General corrosion and preferential corrosion are reported as typical corrosion types in this environment. As weld metal is generally different from base metal in chemical composition and microstructure, it is believed that these differences cause preferential corrosion of welded joints galvanically when these two dissimilar materials are coupled in corrosive environment. Furthermore, the area of weld metal is much smaller than that of the base metal, andas a result, enhanced corrosion of the weld metal may occur. Corrosion problems have occurred in pipelines and OCTG which are used in CO2 containing fluids(Ikeda,1983, Ikeda, 1984, Videm, 1987).