Browsing by Author "Alawadhi, Khaled"
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Item Open Access Inhibition of Weld Corrosion in Flowing Brines Containing Carbon Dioxide(Cranfield University, 2009-01) Alawadhi, Khaled; Robinson, M. J.The aim of this research was to study the effectiveness of a typical oilfield corrosion inhibitor, which is considered to be a green inhibitor (non toxic to the environment) in controlling internal corrosion of welded X65 pipeline steel in brines saturated with carbon dioxide at one bar pressure, under dynamic flowing conditions, over a range of temperatures. Several experimental configurations were used ranging from a simple flat plate design to a novel rotating cylinder electrode, to allow high shear stress to be achieved. The rotating cylinder electrode (RCE) apparatus was designed to allow steel from the weld metal, heat affected zone (HAZ) and parent material to be galvanically coupled and tested in high shear stress conditions. In producing the RCE, the three regions of the weld were identified by optical metallography and samples of each were machined to produce cylindrical electrodes, which were mounted on a motor driven shaft. Electrical connections were made to the three electrodes via a high quality slip-ring assembly. The galvanic currents flowing between the regions of the weld were recorded using zero-resistance ammeters and their self-corrosion rates were found by uncoupling the electrodes and performing polarization resistance measurements. For static conditions the inhibitor had an effective performance and after a short initial period during which film formation took place, at longer exposure times a dramatic reduction of corrosion rate was obtained. Under flowing conditions, both the galvanic currents and the self-corrosion rates were found to increase with the shear stress, as the rotational speed of the RCE was increased. The total corrosion rate of each weld region was assessed from the sum of the self-corrosion and galvanic contributions. In most cases, the weld metal and HAZ were shown to be cathodic to the parent material and this was considered to be a desirable situation as localised corrosion of the weld was minimised. However, in some circumstances, including inhibition of pre-corroded steel surfaces, a current reversal took place, which resulted in accelerated corrosion of the weld. These findings are explained in terms of the protective nature of the films that form on each region of the weld.Item Open Access Preferential weld corrosion of X65 pipeline steel in flowing brines containing carbon dioxide(Maney, 2011-06-01T00:00:00Z) Alawadhi, Khaled; Robinson, M. J.The aim of this research was to investigate the cause of the severe localised corrosion that sometimes occurs at welds in carbon steel pipelines carrying hydrocarbons and inhibited brines saturated with carbon dioxide. A rotating cylinder electrode (RCE) apparatus was designed so that electrodes machined from the weld metal, heat affected zone (HAZ) and parent material of welded X65 pipeline steel could be galvanically coupled and 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 polarisation resistance measurements. The total corrosion rate of each weld region was obtained from the sum of the self-corrosion and galvanic contributions. In uninhibited conditions, the weld metal and HAZ were both cathodic 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 impedance spectroscopy (EIS) showed that the inhibitor film had lower electrical resistance and was less protective on the weld metal than on the parent material. At the highest shear stress, a second current reversal could occur when the inhibitor was removed from all regions of the weld and there was a return to the original galvanic behaviour. It was concluded that preferential weld corrosion was caused by unstable conditions in which the inhibitor film was selectively disrupted on the weld metal but remained effective on the other weld regions.