Iron Calcium Carbonate Formation in CO₂ Environments – Effects of A/V Ratio in Autoclave Experiments Available to Purchase

In aqueous carbon dioxide (CO₂) corrosion of carbon steels, the presence of calcium ions influences the kinetics of corrosion product formation and can affect both general and localized forms of attack. When conducting laboratory experiments, understanding the transient solution chemistry due to the corrosion process is critical when evaluating the effects of calcium on corrosion behavior. Autoclave experiments were conducted at 80°C and 5 barg with 3 brines containing 0, 1000 and 5000 mg/l of Ca²⁺. Iron flux into the brine was varied using high and low area to volume (A/V) ratios. Results from mass loss and electrochemical measurement techniques show that the presence of calcium reduces initial corrosion rates but slows down the formation of more protective corrosion product layers. A higher A/V ratio led to much faster corrosion product formation and altered the composition of the resulting corrosion products in the presence of calcium ions. The results demonstrate how conflicting views on the role of calcium can be observed depending on the A/V ratio and test duration.

Carbon dioxide (CO₂) saturated brines containing high levels of calcium are commonly encountered across the energy sector: from hydrocarbon recovery to the harvesting of geothermal energy and re-deposition of CO₂ for permanent storage. These brines originate in deep underground reservoirs at elevated pressures and temperatures.¹ Despite susceptibility to corrosive attack under these conditions, carbon steels are the preferred choice of pipeline materials for such processes, attributable to their low cost, availability and ease of manufacture.²

When carbon steels are exposed to calcium rich brines, the combination of an electrolyte rich in calcium ions (Ca²⁺) and carbonate ions (CO₃^2-), with the flux of Fe²⁺ from the actively corroding carbon steel, can quickly create conditions for scale and/or corrosion product precipitation and build up. The rate of accumulation, morphology and tenacity of these precipitates on the steel surface is pertinent to pipeline integrity and maintenance planning. The right environment can enable formation of compact layers that provide sustained protection to the steel substrate. Conversely, lower coverage can promote localized attack while excessive precipitation can lead to pipe clogging.^3,4