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

摘要

This paper investigates the effectiveness of two typical oilfield corrosion inhibitors in controlling preferential weld corrosion of X65 pipeline steel in artificial seawater saturated with carbon dioxide at one bar pressure. A rotating cylinder electrode apparatus was designed so that weld metal, heat affected zone 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 obtained by linear polarization resistance measurements. In uninhibited conditions, the weld metal and heat affected zone were cathodic to parent material and no localised corrosion occurred. However, 30 ppm of inhibitor caused a current reversal at low shear stress, resulting in preferential corrosion of the weld metal. At high shear stress, the currents increased and further reversals occurred. The inhibitors were more effective in controlling the self-corrosion rates of the parent material than weld metal and heat affected zone. Total corrosion rates were calculated from the sum of the galvanic and self-corrosion contributions and the efficiencies of the two inhibitors were compared. It was concluded that preferential weld corrosion was caused by unstable conditions in which the inhibitor film was removed selectively from weld metal and heat affected zone but remained effective on the parent material.