The aim of this research was to investigate the cause of the severe localisedcorrosion that sometimes occurs at welds in carbon steel pipelines carryinghydrocarbons and inhibited brines saturated with carbon dioxide. A rotatingcylinder electrode (RCE) apparatus was designed so that electrodes machined fromthe weld metal, heat affected zone (HAZ) and parent material of welded X65pipeline steel could be galvanically coupled and tested in high shear stressconditions. The galvanic currents flowing between the weld regions were recordedusing zero-resistance ammeters and their self-corrosion rates were found bypolarisation resistance measurements. The total corrosion rate of each weldregion was obtained from the sum of the self-corrosion and galvaniccontributions. In uninhibited conditions, the weld metal and HAZ were bothcathodic to the parent material and localised corrosion was prevented. However,when an oilfield corrosion inhibitor was present a current reversal took place,which resulted in accelerated weld corrosion. Electrochemical impedancespectroscopy (EIS) showed that the inhibitor film had lower electricalresistance and was less protective on the weld metal than on the parentmaterial. At the highest shear stress, a second current reversal could occurwhen the inhibitor was removed from all regions of the weld and there was areturn to the original galvanic behaviour. It was concluded that preferentialweld corrosion was caused by unstable conditions in which the inhibitor film wasselectively disrupted on the weld metal but remained effective on the other weldregions.
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