Dense Phase CO₂ Corrosion and the Impact of Depressurization and Accumulation of Impurities

When dense phase CO₂ is depressurized 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 depressurization 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 depressurized via the gas phase. The experiments also showed that a separate aqueous phase could form in addition to the gaseous and liquid CO₂ phases. The accumulation of impurities increased the corrosivity of the remaining liquid CO₂ phase and carbon steel specimens exposed in the autoclaves were corroded after 3 days exposure. Lowest corrosion rates (< 0.1 mm/year) were measured in a system with CO₂ and water (489 and 1222 ppmv) 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 (138 ppmv) 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 0.1 mm/year while the specimens in the NO₂ experiment developed a brown rusty layer of corrosion products and the corrosion rate was about 1 mm/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.