The Effect of Temperature on The Corrosion Behavior of API 5L-X65 Steel Pipes Under The Influence of Carbonate / Bicarbonate Ion Concentration in The Presence of Chloride Ion

Temperature effect on the corrosion behavior of API 5L-X65 in the presence of 0.05, 0.1, and 0.5M concentration of Carbonate, Bicarbonate and Carbonate/Bicarbonate was investigated. Potentiodynamicpolarization and Mott-Schottky tests at varying temperatures were used to investigate the corrosion behavior and “the semiconductor” properties of X65 steel’s passive layer. Following the addition of 3% NaCl these tests were repeated and data compared. The polarization curves showed that the passive layer in the carbonate solution was more stable than in the bicarbonate solution: as the passive current density was lowered. It was also revealed that in the bicarbonate solution, passivation happened (for which lowering the temperature was more effective). Mott-Schottky test was more temperature dependent than the polarization test (in carbonate and carbonate/bicarbonate solutions), as the temperature increase did not show any significant effect on the passivation current of steel. On the other hand, the Mott-Schottky test showed that in a solution without chloride ion the increase in temperature resulted in increase of the electron donor density (ND). This change was related to an increase in the number of spot defects of the passive film. The semiconductor behavior of passive layer was n-type, and the electron donor densities in the order of 1021-1022 cm-3. At concentrations of 0.05 and 0.1 M of carbonate and carbonate/bicarbonate solutions (in the presence of chloride ion) the increase in temperature increased the ND. But as the solution concentration increased to 0.5 M, the temperature increase had less effect on the increase of ND. Two sets of results in carbonate and carbonate/bicarbonate solution concentration, (in the presence of chloride ion) by Mott-Schottky test were in good agreement with that of polarization test (where the temperature increase did not alter the passivation current density).