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Abstract
Metal substrates that are used in oil and gas transportation pipeline made from carbon steel grade API-5L X52 with composition from a maximum of 0.28, 1.25, 0.03 and 0.03 wt % of carbon, magnesium, phosphorus and sulfur, respectively (balance in iron) are examined in citrate and phosphate buffers in pH range 4.0 to 6.5 before and after coating the substrate with silica gel film prepared by Sol Gel method and in absence and presence of chloride ions. The electrochemical behavior of the electrode made of the alloy was tested using open circuit potential, Tafel polarization, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The surface was examined using field emission scanning electron microscopy (FE-SEM). The electrochemical measurements showed that the corrosion rate increases with decrease of pH of citrate buffer, while tests in phosphate buffer showed a reversed behavior which attributed to the corrosion inhibition characteristic of the phosphate ions. The metal substrate also tested after coating with different formulations of silica coating prepared by sol gel method, and the electrochemical, measurements showed a decrease of general corrosion rate of coated substrate with respect to bare substrate, but only the coating prepared by sol gel partial hydrolysis method can protect the surface from pitting corrosion