Corrosion of armor wire steel in the annulus of flexible pipes at near neutral pH
Journal article, Peer reviewed
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2019Metadata
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International Corrosion Conference Series. 2019, paper 13302Abstract
Stress corrosion cracking (SCC) of carbon steel may occur at near neutral pH in deoxygenated solutions with high bicarbonate concentration. Similar conditions are encountered in the annular space of flexible pipes when it is filled with CO2 containing condensed water or seawater. The CO2 diffuses from the bore into the annulus through the polymer sheets. Corrosion of the armor wires in contact with the water will result in high levels of Fe2+ and bicarbonate (HCO3-). Several studies have indicated that SCC may occur in solutions with high HCO3- concentration by production of elemental hydrogen during the corrosion reaction. The objective of the present study was to investigate if such conditions can persist for sufficiently long time in the annulus of flexible pipes to induce SCC of armor wire steel. It implies that the nucleation and growth of siderite (FeCO3) is so slow that the solution remains supersaturated for days to weeks. The results show that it is possible. At temperature lower than 40 °C and CO2 partial pressure of 10-5 kPa the saturation ratios (SR) of siderite can remain much higher than 1 and maintain a near-neutral pH for several hundred hours. However, the corrosion rate of armor wire steel at these conditions is low. Siderite precipitates at the steel surface and the cathodic reaction rate becomes diffusion controlled. Paper reproduced with permission from CORROSION/2019 Annual Conference and Exhibition. www.nace.org