Control of Preferential Weld Corrosion of X65 Pipeline Steel in Flowing Brines Containing Carbon Dioxide

This paper investigates the effectiveness of typical oilfield corrosion inhibitors in controlling preferential weld corrosion of X65 pipeline steel in brines saturated with carbon dioxide at one bar pressure.  A rotating cylinder electrode apparatus was designed so that weld metal, heat affected zone (HAZ) and parent material could be tested in high shear stress conditions.  The galvanic currents flowing between the weld regions were recorded using zero-resistance ammeters and their self-corrosion rates were found by linear polarisation resistance measurements.
 The galvanic currents and self-corrosion rates both increased with shear stress.  In uninhibited conditions, weld metal and HAZ were cathodic to parent material and no localised corrosion occurred.  However, 30 ppm of oilfield inhibitor caused a current reversal at low shear stress, resulting in accelerated weld corrosion. A second current reversal could occur at higher shearer stress by removing inhibitor from all regions of the weld, with a return to the original galvanic behaviour.  It was concluded that preferential weld corrosion was caused by unstable conditions where the inhibitor film was selectively removed from weld metal but remained effective on other weld regions.  The paper will compare the performance of inhibitors containing different functional groups.