Effect of Flow Velocity on Cathodic Protection of DH36 Steel in Seawater

doi:10.11902/1005.4537.2013.183

Journal of Chinese Society for Corrosion and protection

2014, Vol. 34

Issue (6): 550-557 DOI: 10.11902/1005.4537.2013.183

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Effect of Flow Velocity on Cathodic Protection of DH36 Steel in Seawater

FAN Fengqin¹, SONG Jiwen², LI Chengjie¹, DU Min¹(

)

1. Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China
2. CNOOC Energy Technology & Services Limited Beijing Branch Information Technology Development Center, Beijing 100027, China

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Abstract

A series of cathodic polarization experiments for DH36 steel has been performed with seawater flow in a range of flow velocities: 0.20, 0.40, 0.60, 0.80, 1.00, 1.20, 1.40 and 2.00 m/s in a pipe flow circulating seawater device. For each flow velocity, at least three different polarization current densities were chosen to perform galvanostatic cathodic polarization for 7 d. The results showed that the current density demand for an adequate cathodic protection (CP) increased with the flow velocity; the potential could be also polarized to achieve -800 mV vs the silver/silver chloride (seawater) reference electrode (Ag/AgCl [sw]) when the velocity was up to 1.00 m/s; however when the velocity was above 1.20 m/s, erosion-corrosion probably could occur even the polarization potential has achieved the protective potential; the calcareous deposits formed on the steel surface were most single magnesium-rich layers. Exceptionally, calcium-rich deposits could form on top of the magnesium-rich layer only when a very high current density was applied.

Key words: cathodic protection flow velocity protection current density calcareous deposit galvanostatic polarization

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Cite this article:

FAN Fengqin, SONG Jiwen, LI Chengjie, DU Min. Effect of Flow Velocity on Cathodic Protection of DH36 Steel in Seawater. Journal of Chinese Society for Corrosion and protection, 2014, 34(6): 550-557.

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https://www.jcscp.org/EN/10.11902/1005.4537.2013.183 OR https://www.jcscp.org/EN/Y2014/V34/I6/550

Fig.1 Schematic diagram of the experimental setup

Fig.2 Schematic diagram of the experimental cell: (a) lateral view, (b) cross section profile

Fig.3 Evaluations of potential at different current densities in sea water with the flow rate of 0.20 m/s (a), 0.40 m/s (b), 0.60 m/s (c), 0.80 m/s (d), 1.00 m/s (e), 1.20 m/s (f), 1.40 m/s (g) and 2.00 m/s (h)

Table 1 Final potentials after polarization for 7 d under different conditions of sea water flow rate and current density

Fig.4 Macroscopic photographs of the cathode surface with different velocities and current densities after polarization for 7 d under the different conditions of sea water flow rate and current density: (a1, a2, a3) 0.20 m/s; 300, 400, 600 mA/m²; (b1, b2, b3) 0.80 m/s; 400, 600, 800 mA/m²; (c1, c2, c3) 1.00 m/s; 800, 1000, 1200 mA/m²; (d1, d2, d3) 1.20 m/s; 800, 1000, 1200 mA/m²; (e1, e2, e3) 1.40 m/s; 1000, 1200, 1600 mA/m²; (f1, f2, f3) 2.00 m/s; 1000, 1200, 1600 mA/m²; respectively

Fig.5 SEM photographs of the calcareous deposit formed at the sea water flow rate of 0.20 m/s and different current densities: (a) 300 mA/m², (b) 400 mA/m², (c) 600 mA/m², (d) 800 mA/m²

Fig.6 EDX analysis of the deposit layer formed at the current density of 800 mA/m²: (a) inner layer, (b) outer layer

Table 2 Contents of main elements in calcareous deposit formed at different current densities and 0.20 m/s flow rate

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