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Abstract: The mechano-electrochemical effects of deformation and hydrogen on active dissolution of pipeline steel in near-neutral pH groundwater are described. Thermodynamic analysis shows that the free energy increment due to elastic deformation is insufficient to alter active dissolution rate remarkably. The synergistic effect of stress field and dissolved hydrogen on active dissolution (corrosion) is negligible. The effect of plastic deformation on corrosion relies heavily on the heterogeneity of dislocation structures formed in the deformation. The plastic deformation cannot significantly change the corrosion rate at the open circuit potential. There is good agreement between these theoretical predictions and experimental observations, indicating that the stress corrosion cracking of pipeline steel in the anaerobic groundwater of near-neutral pH is unlikely to be related to the hydrogen-facilitated anodic dissolution mechanism. [Copyright &y& Elsevier]

Mott–Schottky and photoelectrochemical measurements were used to explore the effects of hydrogen and chloride ions on the electronic properties of the passive film on X70 micro-alloyed steel in a solution of 0.5 M NaHCO3. Mott–Schottky analyses have shown that hydrogen increases the capacitance and donor density, and decreases the flat band potential and the space charge layer thickness of the passive film. The photocurrent of the film is remarkably increased by hydrogen. The effects of hydrogen become more pronounced with an increase in the hydrogen charging current densities. Hydrogen has no noticeable effect on the band gap energy Eg and the process by which hole-electron pairs are photo-generated in the film. The presence of chloride ions in the solution produces some similar effects on the electronic properties of the passive film to those observed with hydrogen, but reduces the photocurrent and increases the band gap energy of the film. No significant synergistic effects on the electronic properties of the passive film were observed in the presence of hydrogen and Cl-. These results provide very useful information for elucidating the mechanism by which hydrogen changes the properties of passive film and then promotes localized corrosion. [Copyright &y& Elsevier]

Hydrogen promoted initiation and propagation of pitting and crevice corrosion of X70 micro-alloyed steel were characterized by potential dynamic measurements, the scanning reference electrode technique (SRET) and electrochemical impedance spectra (EIS). At open circuit potential, in situ SRET results show that hydrogen accelerates the nucleation and propagation of pitting of X70 steel. The pitting potential Ep of X70 steel gradually decreases with an increase of chloride ion concentration in NaHCO3 solution. Pre-charged hydrogen does not have a significant effect on the pitting potential Ep and open circuit potential Ecorr of the steel in 0.5 M NaHCO3 solution. However, a synergistic effect of hydrogen and Cl− on the anodic dissolution and pitting potential of X70 steel is observed in 0.5 M NaHCO3 solution containing chloride ions. When crevices are present in X70 steel, hydrogen accelerates the initiation and progress of crevice corrosion. The mechanisms by which hydrogen promotes the initiation and propagation of pitting and crevice corrosion are proposed and discussed. [Copyright &y& Elsevier]

It is the first time that the oscillatory electrodissolution of metals is used to study hydrogen-promoted corrosion, and the primary results prove that it is an effective method for investigating the effect of hydrogen on both the formation and dissolution of a passive film. Effects of hydrogen on the electrochemical oscillations of X70 carbon steel are investigated in 5.0 M H3PO4 solution. During the oscillatory electrodissolution of X70 steel electrode, the chemical environment near the surface of the electrode is changed artificially by the oxidation of hydrogen diffused from X70 electrode to surface. With increasing hydrogen pre-charging current density, both the induction time and the ratio of active time to passive time of the current oscillations increase, and the Flade potential also shifts positively. Oxidation of hydrogen decreases the pH value at the interface between the electrode and solution, which retards the formation of a passive film and subsequently promotes its dissolution. This investigation provides further understanding of the effect of hydrogen on the formation and dissolution processes of passive films. It is the first time to observe the periodical changes in the potential at the electrode/electrolyte interface during current oscillations by using scanning reference electrode technology. The scanning reference electrode technology is also used to verify the effect of hydrogen on the current oscillations. [Copyright &y& Elsevier]

Hydrogen-induced pitting active sites on iron were investigated by electrochemical noise (EN) and scanning reference electrode techniques (SERT). The significant increase in the number of current and potential fluctuations in EN measurements indicates that hydrogen increases the number of active sites for formation of metastable pits, which are precursors of stable pits. The potential distributions over the surfaces of hydrogen charged and uncharged iron polarized at 0.07 V (vs. SCE) for different polarization times also show clearly that hydrogen significantly increases the number of pitting active sites on the surface of charged iron. It was also observed that to reach equal numbers of pitting active sites, a higher applied anodic potential is required on uncharged iron than on hydrogen-charged iron. The increase in the number of pitting active sites due to hydrogen was discussed in terms of the effect of hydrogen on the nature of the passive film and the dissolution rate of iron. [Copyright &y& Elsevier]

The effects of hydrogen on the stability of passive films on iron were investigated by electrochemical methods: open circuit potential decay, cathodic galvanostatic reduction, electrochemical impedance spectroscopy, and breakdown potential measurements. The results show that hydrogen decreases the final static open circuit potential, the cathodic charge for reduction and the charge transfer resistance of the passive film, indicating that hydrogen decreases the stability of the passive film. The charge transfer resistance of the passive film formed on the charged specimen does not change with increasing the film formation potentials, suggesting that increasing film formation potentials under hydrogen charging conditions cannot improve the stability of the passive film. Hydrogen decreases the breakdown potential of the passive film, especially at lower chloride ion concentrations, confirming that hydrogen promotes the susceptibility of the passive film on iron to pitting corrosion. The reasons why hydrogen decreases the stability of the passive film were discussed. [Copyright &y& Elsevier]