Corrosion in Dense Phase CO2 – the Impact of Depressurisation and Accumulation of Impurities

Abstract

When dense phase CO2 is depressurised and forms a two phase gas/liquid system, impurities will partition between the two phases and go preferentially to the phase where their solubility is highest. Partitioning and depressurisation experiments run at 4 and 25°C showed that water, H2S and SO2 accumulated while O2 was depleted in the remaining liquid CO2 phase when the system was depressurised via the gas phase. When the water solubility is exceeded, a third aqueous phase can form. The accumulation of impurities increased the corrosivity of the remaining liquid phase and carbon steel specimens exposed in the autoclaves were corroded after 3 days exposure. Lowest corrosion rates (< 0.1mm/year) were measured in a system with CO2 and water (489 and 1222ppmv) only. The corrosion rate is reasonably low because the water phase quickly becomes saturated with corrosion products that reduce the corrosivity. The situation was different when the CO2 contained SO2 (138ppmv) and NO2 (191ppmv) as these gases accumulate in the water phase, reduce the pH and increase the reactivity of the aqueous phase. The specimens in the SO2 experiments got covered with a black film and the corrosion rates were about 0.1mm/year while the specimens in the NO2 experiment developed a brown rusty layer of corrosion products and the corrosion rate was about 1mm/year. These high corrosion rates only last until the impurities are consumed, the replenishment and availability of impurities becomes therefore an important issue. The accumulation of impurities (including water) was maximum 5–10 times the original concentration in the experiments where the liquid phase was reduced to 10–20% of the original volume. The ratio of the remaining to the original liquid phase volume can be much lower in a long pipeline and a much larger accumulation can be foreseen. If the corrosion rate consumes most of the available impurities in the pipeline, the corrosion rate can be much higher than the corrosion rate measured in the present experiments.