Electrochemical Study of the Protective Characteristics of CO₂ Corrosion Product Layers for Steels API X80 and P110

Abstract:

This work evaluate the protective characteristic of the CO₂ corrosion product layers formed on the surface of two types of steels, API 5L X80 used for transportation of oil and gas, and API 5CT P110 used for case tubing and pipe for oil drilling.Electrochemistry evaluations and morphological characterization of the obtained layer were performed. These steels were exposed to a brine solution containing 3% wt of NaCl, in a pressurized autoclave with 55 bar of CO₂ and total pressure of 75 bar at different temperatures (25, 50 and 75°C) and immersion times (7, 15, 21 and 30 days). The corrosion rate was determined by mass loss tests and electrochemical techniques, such as Linear Polarization Resistance and Electrochemical Impedance Spectroscopy. Characteristics of the corrosion product layer such as thickness, morphology, and chemical composition were analyzed by scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS) and X-Ray diffraction (XRD). The corrosion rate decreases with the increase of the immersion time and temperature, and the lower rate of corrosion was obtained for 75°C after 30 days of immersion, for both steels

• Periodical: Applied Mechanics and Materials (Volume 830)
• Pages: 139-146
• DOI: https://doi.org/10.4028/www.scientific.net/AMM.830.139
• Online since: March 2016
• Authors: Andrea H. Rojas, Adriana Forero Ballesteros, Ivani de Souza Bott*
• Keywords: API 5CT P110, API 5L X80, CO₂ Corrosion, FeCO₃ Layer

References

[6] At 50o C after 30 days of immersion three semicircles are formed (Figure 5d) for the steel X80. These semicircle diameters are larger when compared with those from other immersion times, indicating that the value of Rp is higher and, therefore, the rate of corrosion is reduced dramatically. The formation of three capacitive semicircles and the drop in the corrosion rate points out the formation of several layers of corrosion products with a certain level of protection. At 75o C it was observed two capacitive semicircles, this being more evident for steel P110 (Figure 5e). After 21 days of immersion, a continuous and protective layer was grown, reflecting on the corrosion rates values that were significantly reduced when compared with those obtained after 7 and 15 days of immersion. Comparison of the corrosion rates found by mass loss and electrochemical techniques (Figure 6) show a tendency to decrease with the increase of exposure time and temperature. The results obtained for the RPL and mass loss technique at 25 o C for 7 and 15 days are close. However this tendency does not happen for EIS where the values obtained for these same conditions are very different. This effect is more marked for the API P110. However the results obtained for both steels after 21days of immersion showed good correlation for the three techniques used, indicating good test reproducibility. At 50o C for the loss mass technique and for immersion times of 7, 15 and 21 days showed lower values compared with the ones obtained with the other two techniques. However after 30 days there is a good correlation between all three techniques and the corrosion rate was very similar to both steels. At 70o C after seven days of immersion, the obtained values by the different techniques for both steels are very different. However, this difference decreases with increasing.

[20] 40 60 80 100 120 140 160 180 -20.

[20] [40] [60] [80] Z" (ohm) Z' (ohm) API 5CT P110 at 25ºC.

[7] days 15 days 21 days 30 days (a).

[20] 40 60 80 100 120 -20.

[20] [40] [60] [80] -Z" (ohm) Z' (ohm) API 5L X80 at 25ºC.

[7] days 15 days 21 days 30 days (b).

100 200 300 400.

[50] 100 150 200 250 300 350 400 API 5CT P110 at 50ºC Z" (ohm) Z' (ohm).

[7] days 15 days 21 days 30 days (c).

100 200 300 400.

[10] [20] [30] [40] [50] [60] [70] [80] [90] 100 Z" (ohm) Z' (ohm) API 5L X80 at 50ºC.

[7] days 15 days 21 days 30 days (d).

200 400 600 800 1000 1200 1400 1600 1800 (2000).

[50] 100 150 200 250 300 350 400 Z" (ohm) Z' (ohm) API 5CT P110 at 75ºC.

[7] days 15 days 21 days 30 days (e).

500 1000 1500 2000 2500 3000.

[50] 100 150 200 250 300 350 400 Z" (ohm) Z' (ohm) API 5L X80 at 75ºC.

[7] days 15 days 21 days 30 days (f) Figure 5. Electrochemical Impedance for steel P110 at 25ºC(a), 50ºC (b) and 75ºC (c); X80 (d) 25ºC, (e) 50ºC e (f) 75ºC immersion time, showing good correlation between the three techniques after 30 days immersion presenting the same corrosion rate. At 25 o C and 75 o C, the steel X80 always has a higher corrosion rate when compared with steel P110 for the three techniques used, however, at 50o C the P110 has the highest corrosion rate. No layer formation was observed for the test performed at 25o C, however with the increase of temperature it was detected the formation of a film that can be composed of multiple layers of FeCO3. The thickness and morphology of these layers were highly dependent on temperature, time of exposure and metal substrate. The morphology of the layers obtained at 50o C and 75o C are shown in figure 7 showing the presence of a thin straight line. This line divides the film into two layers, an internal and a slim more external and compact layer when compared with the internal structure, which is a less compact layer.

[6] 9 12 15 18 21 24 27 30 0. 5 0. 6 0. 7 0. 8 0. 9 1. 0 1. 1 1. 2 1. 3 1. 4 1. 5 1. 6 1. 7 1. 8 EIS Corrosion Rate (mm/y) Time (days) P110 X80 25ºC mass loss Rp EIS Rp (a).

[5] 10 15 20 25 30.

[2] [4] [6] [8] [10] [12] [14] [16] [18] [20] Corrosion Rate (mm/y) Time (days) EIS Rp Mass Loss EIS Rp P110 X80 50ºC (b).

[6] 9 12 15 18 21 24 27 30.

[3] [6] [9] [12] [15] [18] [21] [24] Rp EIS Corrosion Rate (mm/y) Time ( days) EIS Rp Mass Loss P110 X80 75ºC (c) Figure 6. Comparison of corrosion rate obtained by of mass loss with Rp and EIS for 25oC (a), 50oC (b) and 75oC M Gao [7] studying cross section a layer, formed on the surface of an API X65, obtained after 240h (10 days) with PCO2 of 10 bar at 75o C, also found a two layer morphology separated by a thin straight line. One of the layers is thicker with a network type morphology and the second layer was thinner and continuous. The results obtained under the conditions of this study, agree with the results found in the work of M Gao [7] and Palacios, C and Shadley, J [8] where the formation of a double layer is a function of the solubility of FeCO3 in the solution. According to the literature [5], the temperature is a key factor in the formation of protective layers on metallic surfaces affecting the solubility of FeCO3, which in turn increase with the increase of temperature. As a result, the iron carbonate precipitation on the metallic surface contributes to the formation of a protective layer with morphological characteristics as observed in Figure 8, inhibiting the corrosion process of the material. API P 110, 50oC, 30 days. API X 80, 75oC, 30 days Figure 7. SEM Layer morphology surface and cross section obtained at 50oC and 75o C X-Ray diffraction (XRD) and EDS of the layer formed from 50o C and 75o C for all immersion times studied are shown in Figure 9 (a) and (b) respectively, indicating the presence of carbon, iron and oxygen. The XRD pattern (Figure 9), show the presence of iron carbonate (FeCO3).

[20] 30 40 50 60 70 80.

[8] [9] [10] [11] [12] [13] [14] [15] FeCO3 FeCO.

[3] Surface Region at 50oC X80 P110 Intensity (a. u. ).

[2] Theta (º) FeCO3 NaCl (a).

[20] 40 60 80.

[8] [9] [10] [11] [12] [13] [14] [15] FeCO3 FeCO3 FeCO3 Intensity (a. u. ).

[2] Theta (º) Surface Region at 75oC API X80 P110 FeCO3 (b) Figure 8. X-Ray diffraction (XRD) pattern and EDS spectrum for the surface of the layer after 30 days immersion at 50oC (a) and at 75oC (b). API P110 API X80 API P110 API X80 Conclusions The linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and Tafel plots were used to calculate the corrosion rates, and determine the point at which the corrosion rate had the most representative drop (critical point), thus allowing to determine the time when a stable and protective layer was formed. The corrosion rates values obtained using electrochemical tests showed similar results to those obtained with weight loss. The corrosion rate decreases with increasing immersion time and temperature, and after 30 days of immersion at 75o C the obtained layer presented a lower corrosion rate and greater protection. Morphological analysis of the layer obtained and chemical composition identified a double layer of iron carbonate as the corrosion product formed at 50o C and 75 o C. These inner layers were apparently less compact than the external ones, which were more protective with the increase of temperature and time. Acknowledgments The authors acknowledge the financial support of IBP-ANP Project no. 31/2012. Bibliography.

[1] A.B. FORERO, M. M. G. NUNEZ, I.S. BOTT. Analysis of the Corrosion Scales Formed on API 5L X70 and X80 Steel Pipe in the Presence of CO2. Materials Research (São Carlos. Impresso), v. -, p. -, (2013).

DOI: 10.1590/s1516-14392013005000182

[2] K. VIDEM, A. DUGSTAD. Corrosion of Carbon Steel in an Aqueous Carbon Dioxide Environment. Part 2. Film formation. Corrosion, v. 28, pp.46-50, (1989).

[3] KERMANI, M. B.; MORSHED, A. Carbon Dioxide Corrosion in Oil and Gas Production A Compendium. Corrosion, v. 59, n. 8, pp.659-683, (2003).

DOI: 10.5006/1.3277596

[4] A. DUGSTAD, M. BJORN, C. SIGMUND. Corrosion of transport pipelines for CO2- effect of water ingress. Energy Procedia, v. 4, pp.3063-3070, (2011).

DOI: 10.1016/j.egypro.2011.02.218

[5] C. DE WAARD, U. LOTZ, D.E. MILLIAMS Predictive Model for CO2 Corrosion Engineering in Wet Natural Gas Pipelines v. 47, pp.976-985, (1991).

DOI: 10.5006/1.3585212

[6] E. BARSOUKOV, J.R. Macdonald. Impedance Spectroscopy Theory, Experiment, and Applications. Published by John Wiley & Sons, Inc., Hoboken, New Jersey. (2005).

DOI: 10.1002/jrs.1558

[7] M. GAO, X. PANG, K. GAO. The growth mechanism of CO2 corrosion product films. Corrosion Science, v. 53, pp.557-568, (2011).

DOI: 10.1016/j.corsci.2010.09.060

[8] C.A. PALACIOS, J. R. SHADLEY Characteristics of Corrosion Scales on Steel in a CO2 Saturated NaCl brine Corrosion v, 47, pp.122-127, (1991).

DOI: 10.5006/1.3585227