In oil and gas production, corrosion (due to the presence of CO₂ or/and H₂S) and scale formation (CaCO₃, MgCO₃, etc.) are two very common phenomena having a significant impact on pipeline integrity. Although corrosion and scaling occur simultaneously in a production environment [1], they are often investigated separately in a research setting, rendering potential interaction effects difficult to evaluate.

Scale formation happens because brines, which are omnipresent in hydrocarbon production systems, contain cations such as Ca²⁺ [2]. Consequently, if the aqueous solubility limit of CaCO₃ is exceeded precipitation will occur and scaling results [3]. There is minimal, often contradictory, literature concerning the effect of Ca²⁺ on CO₂ corrosion. This highlights the need for further systematic experimental studies in this frequently ignored area of corrosion research. A key influencing parameter in studying the effect of calcium-containing solutions on CO₂ mechanisms is the saturation degree of test electrolytes with respect to CaCO₃; this greatly influences the precipitation kinetics of CaCO₃. Previous corrosion studies have focused mostly on the concentration of Ca²⁺ as the core parameter and ignored the degree of saturation. In addition, the lack of control or reporting of key experimental parameters, such as mass transfer rate and especially solution chemistry, often blurred the interpretation of the data and rendered the main findings of these studies difficult to extrapolate.

The present work describes the influence of saturated solutions with respect to CaCO₃ on CO₂ corrosion behavior of ferritic-pearlitic UNS G10180 carbon steel. Particular efforts were made to control and report the water chemistry (pH, Fe²⁺, Ca²⁺ concentrations) and the flow characteristics of a glass cell test system. The experimental setup involved several flat, square steel specimens mounted on specially made concentric holders, all facing a central Rushton-type impeller (4.1″ ID). This setup mimicked the mass transfer rate generated by a fluid flowing at 0.5 m/s in a 0.1m ID pipe.

The corrosion behavior was studied in-situ by electrochemical methods (LPR/EIS). Characterization of the surface carbonate layers was carried out by SEM/EDS and XRD.

Two experiments were performed, one in CaCO₃ saturated solution and one without Ca²⁺ ions (baseline); other than that, both tests were conducted at the same conditions (80°C, pH 6.2, pCO₂ 0.53 bar, 1 wt.% NaCl, and ). Fig. 1 shows the average corrosion rate versus exposure time, measured by LPR, for both experiments. The error bars indicate the maximum and minimum values at each average point. Three distinct regions were identified based on corrosion rate for both electrolytes; active corrosion, nucleation and growth of carbonate layers, and a pseudo-passivation region. Corrosion rate increased in the first region, associated with development of residual iron carbide networks. The porous structure of Fe₃C provided more surface area for hydrogen evolution reaction to occur and thus increased the corrosion rate [4]. In the second region, the Fe₃C network reached a thickness of 20-30 μm, for both experiments. The water chemistry within the Fe₃C network was expected to be different from the bulk; high ferrous ion concentration along with high pH favored nucleation and precipitation of carbonate crystals within the porous structure of Fe₃C and adjacent to the steel surface for both sets of tests. In the pseudo-passivation region, the steel potential noticeably increased, indicating formation of a protective layer on the steel surface. Although, the corrosion behavior of both electrolytes (with and without Ca²⁺/CaCO₃) was almost identical, further surface characterization of the specimens surface revealed some differences. The corrosion products for the saturated solutions with respect to CaCO₃ were a combination of Fe₃C and a solid solution of Fe_xCa_1-xCO₃ where 0.9<x<1, while corrosion products for the baseline test were identified as Fe₃C and pure FeCO₃.

In conclusion, this study showed that saturated solutions with respect to CaCO₃ do not have a considerable effect on the surface carbonate layers. In other words, the protectiveness of Fe_xCa_1-xCO₃ is comparable to pure FeCO₃ for 0.9<x<1. Further work will be dedicated to environments with higher Ca²⁺ contents, better mimicking field environments.

References:

[1] H. Mansoori, et al., “Pitting corrosion failure analysis of a wet gas pipeline,” Eng. Fail. Anal., vol. 82, pp. 16–25, 2017.

[2] H. Mansoori, et al., “Case Study: Production Benefits from Increasing C-Values,” Oil & Gas J., vol. 111, no. 6, pp. 64–69, 2013.

[3] H. Mansoori, et al., “Pitting Corrosion Failures of Natural Gas Transmission Pipelines,” IPTC, Beijing, China, IPTC-16750, 2013.

[4] F. Farelas, et al., “Iron carbide and its influence on the formation of protective iron carbonate in CO₂ corrosion of mild steel,” NACE 2013, Paper No. 2291.