Evaluation of Flow Enhanced Preferential Weld Corrosion of X65 Pipeline Steel Using a Novel Submerged Impingement Jet

Abstract

Abstract Preferential weld corrosion (PWC) remains a major operational challenge that jeopardizes the integrity of oil and gas carbon steel pipelines. Corrosion inhibition is conventionally used to mitigate this problem however several indications suggest that some corrosion inhibitors may increase PWC. This work therefore, seeks to assess the suitability of a novel submerged jet impingement technique for the study of flow enhanced preferential weld corrosion and inhibitor evaluation in a typical offshore production system. A novel jet-impingement target was constructed from samples of parent material, heat affected zone and weld metal, and subjected to flowing brine at velocities up to 10 ms-1, to give a range of hydrodynamic conditions from stagnation to high turbulence. The submerged jet-impingement flow loop was used to investigate corrosion control of X65 steel weldment in flowing brine, saturated with carbon dioxide at 1 bar, and containing a typical oilfield corrosion inhibitor. Galvanic currents between the electrodes in each hydrodynamic zone were recorded using zero-resistance ammeters and their self-corrosion rates were measured using the linear polarisation technique. At low flow rates, the galvanic currents were small and in some cases the weld metal and heat affected zone were partially protected by the sacrificial corrosion of the parent material. However, at higher flow rates the galvanic currents increased but some current reversals were observed, leading to accelerated corrosion of the weld region. The results are explained in terms of the selective removal of the inhibitor film from different regions of the weldment at high flow rates and the corrosion mechanism in the presence of oxygen is discussed.