Effect of inhibitors on preferential weld corrosion under sweet conditions

Abstract

Purpose – The aim of this investigation was to compare the performance of three typical oil field carbon dioxide corrosion inhibitors in controlling preferential weld corrosion (PWC) of X65 pipeline steel in artificial seawater (3.5 weight per cent) saturated with carbon dioxide at one bar pressure. Design/methodology/approach – A novel rotating cylinder electrode (RCE) apparatus was used to evaluate the effect of flow on the inhibition for the weld metal (WM), heat-affected zone (HAZ) and parent material. To fulfill this objective, the galvanic currents flowing between the weld regions were recorded using parallel zero-resistance ammeters, and the self-corrosion rates of the couples were obtained by linear polarization resistance measurements. Findings – The results showed that when 30 ppm of green oil field inhibitors were present in the service environment, a current reversal took place, resulting in accelerated weld corrosion. 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 of the WM and HAZ material. It was concluded that PWC was caused by unstable conditions in which the inhibitor film was selectively disrupted from the WM and HAZ, but remained effective on the parent material. Originality/value – Electrochemical corrosion rate measurements were carried out using an RCE produced from the different regions of the weld. An advantage of using the RCE is that the hydrodynamic conditions are very well defined, and it is feasible to translate the conditions that are known to exist in a production pipeline to those used in laboratory tests.