| With the development of deep-sea petroleum exploitation,more and more high-strength steel was adopted in engineering structures.However,hydrogen induced cracking(HIC)was observed in these structures under cathodic protection.Hydrostatic pressure was considered as one of the most important factors in deep sea environment.Nevertheless,few research revealed the fact that whether deep-sea hydrostatic pressure induced HIC in high-strength steel.Observing the mechanism of the effect of hydrostatic pressure on HIC could contribute to establish the standard of cathodic protection in deep sea environment.The hydrogen envolution reaction(HER)on the surface of metal and hydrogen diffusion in the bulk of the metal are first and second steps of HIC.Thus,the deep-sea hydrostatic pressure effect on HER and hydrogen permeation were studied in this research,the conclusions are listed below:(1)A514 offshore structural steel was exposed to hydrostatic pressure ranging from 0.1 to 40 MPa to examine hydrogen permeation during galvanostatic hydrogen charging.Hydrostatic pressure decreases the energy barrier for hydrogen absorption and desorption,while increasing hydrogen adsorption.These effects are induced by both steady-state current density and the apparent diffusivity slight increase with hydrostatic pressure.Taking surface effect of hydrogen permeation into account,the intrinsic diffusivity of(3.2 ± 0.04)×10-6cm2/s is obtained,which is independent of the hydrostatic pressure.(2)Armco iron samples are exposed to pressures of 0.1-30 MPa to study hydrogen permeation during potentiostatic charging.Electrochemical impedance spectroscopy(EIS)is performed to analyze the electrode/electrolyte interface reaction.According to EIS and first-principles calculations,the hydrostatic pressure decreases the distance between the adsorbed hydrogen atoms and the sample surface,which induces hydrogen atom diffusion into the subsurface by decreasing the diffusion barrier energy.Under high pressure,hydrogen molecules gather at the sample entry side and inhibit the hydrogen recombination reaction.Thus,the amount of adsorbed hydrogen and the hydrogen subsurface coverage increase.(3)A new electrochemical impedance spectroscopy(EIS)model,which considers both the Tafel recombination and the Heyrovsky reaction under permeable boundary conditions,was developed to characterize the kinetic parameters of the hydrogen evolution reaction(HER)under hydrostatic pressure.The effect of the hydrostatic pressure on the kinetic parameters of the HER and the permeation of A514 steel in alkaline solution were measured using potentiodynamic polarization,the Devanathan cell hydrogen permeation,and EIS.The hydrostatic pressure accelerates the Volmer reaction and inhibits the Tafel recombination,which increases the number of adsorbed hydrogen atoms.On the other hand,the pressure accelerates the Heyrovsky reaction,which decreases the amount of adsorbed hydrogen atoms.At 10 to 40 MPa hydrostatic pressure within the-1.0 to-1.1 VSSE cathodic potential region,the HER is controlled by hydrogen partial pressure,and hydrogen adsorption is the Langmuir type.Within the-1.2 to-1.3 VSSE cathodic potential region,the HER is controlled by the potential,and hydrogen adsorption gradually transfers from the Langmuir type to the Temkin type with increasing hydrostatic pressure.(4)The effect of hydrostatic pressure on cathodic depostion and long term hydrogen permeation in 0.2 mol/L NaOH,3.5%NaCl and artificial sea water were characterized by Devanathan cell hydrogen permeation,EIS,and EDS tests.It is founded that the hydrostatic pressure accelerated Volmer reaction and inhibited Tafel reaction,which induced the hydrogen atoms permeate into steel.On the other hand,the entry hydrogen induced more iron particles disintegrated from metal surface.Also,the hydrostatic pressure increases the interfacial pH.Thus,more cathodic deposition produced on metal surface under high pressure.These two competitive effects determined the process of hydrogen entry into steel... |
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