The Effect of Hydrogen Absorption on the Passivity of Oxides on Carbon Steels

Abstract

Pipelines are designed for a wide variety of applications. Their use in the energy sector for the transportation of crude oil, natural gas, and other petrochemical commodities is still regarded as the safest, fastest and most economical mode of product transmission and delivery. The degradation of the structural integrity of pipelines caused by corrosion has been an on-going problem. Depending on in-service use, different forms of corrosion can occur, with the deleterious effects depending on which type of corrosion is occurring, such as stress corrosion cracking (SCC) and hydrogen induced cracking (HIC) due to hydrogen embrittlement (HE). Further, the hydrogen may affect the protectiveness of any FeIII oxide present on the steel surface. This study investigates the effects of H absorption by X60, X65, X70 steels. A range of electrochemical techniques, such as monitoring the corrosion potential (Ecorr), polarization resistance (Rp) measurements, cyclic voltammetry (CV), and H permeation experiments (using the Devanathan-Stachurski (D-S) approach) have been used to investigate the effect of H on the steel and the oxides present on the steel surface. Surface analytical techniques such as X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) were used to determine the effect of H on the composition of the oxide and to identify the distribution of H trapping sites. The apparent diffusivity of H in X65 was found to depend on the composition of the solution on the reduction side of the D-S cell. The steady-state current measured in the oxidation cell was dependent on the solution composition in the reduction cell. The presence of calcium caused a sharp decrease of oxidation current, likely due to the quick formation of calcium carbonate deposits, whereas magnesium caused a gradual decrease in the observed oxidation current, which is likely due to the thickening of the magnesium hydroxide on the surface. The presence of both calcium and magnesium in the solution affects the amount of H that diffuses through the steel, likely by blocking adsorption sites on the steel surface. Blocked adsorption sites reduce the concentration of atomic hydrogen on the surface and lead to a decrease in the observed steady-state current. Thermal desorption data show that, in the X65 steel, a majority of the H trapped within the steel requires activation energy (Ea) of < 50 kJ/mol for its release, indicating that it is located in reversible trapping sites. SIMS data suggest that trapping occurs predominantly along grain boundaries, with the extent of H trapping at these sites possibly affecting the resistance of the surface FeIII oxides present. The RP values measured after cathodic charging for the pearlite-containing steel (X65) were lower than those obtained on the bainite-containing steel (X70). The presence of FeO on the steel surface after cathodic charging suggests partial oxide reduction, with this reduced oxide being more prominent on the X65 than on the X70 steel. Polarization resistance measurements were used to study the role of hydrogen on the passivity of the steel covered by iron oxides. It has been shown that at open circuit conditions the increase in RP value is slower when the steel has been charged with H. The phase of X70 being investigated was predominantly bainite, which is more resistive to hydrogen absorption. X70, after hydrogen charging, was found to have a lesser change in steady-state RP than did the X65. Further, the retardation of the resistance increase at open circuit was more prominent on the X65 than on the X70. XPS confirmed that more FeO was present on the X65 sample after charging than on the X65 after charging followed by potentiodynamic polarization experiments. All this suggests that, at open circuit conditions after the steel was charged with H, hydrogen being released from reversible trapping sites reduces the FeIII oxide to FeO. The reduction of FeIII to FeO, due to either the charging or release of atomic hydrogen trapped within the lattice, is likely causing a decrease in the resistance of the passive oxide.