Study on weld corrosion of X65QT pipeline steel

Abstract

The aim of this research is based on the welding consumable related issues at the pipe line construction sites which were experienced by the author of this thesis. Weld corrosion describes selective attack of the weld itself. The origin of this attack is usually a combination of a more active Weld Metal (the Weld Metal tends to be more susceptible to corrosion than the Base Metal) together with Galvanic Corrosion coupling to the Base Metal. The influence of coupling is accentuated by the poor area ratio, i.e. a small anode Weld Metal / large cathode Base Metal. Preferential Weld Metal corrosion of C-Mn steels has historically been a major problem. On the basis of various studies, early researchers concluded that the weldments of silicon killed C-Mn steels are the most electronegative (i.e. cathodic) and the unkilled steels with a low silicon content are the most electro positive. Weld Metal Zones of the welds made with the normal basic-coated electrodes exhibit most electro negative potential and with the acid types exhibit most electropositive whilst the potentials of the Weld Metal Zone of the welds made with rutile electrodes are placed in an intermediate position. The Weld Metals are usually more electronegative than the Base Metal depending upon relative silicon content. The operating conditions can change the weld behaviour from anodic to cathodic or vice versa at any stage during the process.