Effect of tensile stress in elastic and plastic range on hydrogen permeation of high-strength steel in sour environment

Determination of Hydrogen Diffusion Parameters of Ferritic Steel from Electrochemical Permeation Measurement under Tensile Loads

“…It results in an unstable background current and unreliable permeation data in the permeation test and this can be clearly demonstrated in our previous works. 14,15 For this reason, Pd coating cannot be applied for the experiment involving the stepwise sequence designed in this study. Instead of Pd, Ni-coating having much higher elongation level (>10%) was used in this study since higher elongation level of the Ni-coating layer can endure higher level of loading condition.…”

“…14,15,44 Particularly, under the tensile stress condition, the thin layer of Pd coating deposited on the high strength steel membrane can be plastically deformed and damaged. It results in an unstable background current and unreliable permeation data in the permeation test and this can be clearly demonstrated in our previous works.…”

“…The longer break-through time and slower permeation rate under the tensile stress in plastic range have also been discussed in our previous works. [14][15][16] In general, under the generalized-plastic domain, applied stress can concentrate mostly at the 2 nd phase particles and hydrogen atoms can be preferentially trapped at those sites like the case of the local-plastic range. Garet et al 43 have reported that the plastic deformation is concentrated at the inclusion, leading to cracking in the bulk of the inclusion, either at the inclusion matrix interface or at sharp extremities.…”

“…15 The radius of the circular area exposed to the electrolyte was 10 mm and the membrane thickness was 1 mm, so that the ratio of radius to the membrane thickness met the requirement specified in ISO 17081 standard method. Although there has been a controversy as to whether the thin steel membrane can be controlled by the volume diffusion through the steel or by the surface reaction, our preliminary test result has revealed that the steel membrane under applied cathodic current density of 2 mA cm −2 is controlled by the volume diffusion.…”

“…9,10 Although failure of the palladium (Pd) coating under the high tensile stress condition has raised controversy on the reliability of the permeation data, our previous works have demonstrated clearly that the tensile stress in plastic range results in a considerably slower permeation rate by excluding other side effects such as the reduced rate of hydrogen oxidation caused by disruption of the Pd layer. [14][15][16] When the stress level becomes close to the yield strength (YS), however, the effect of tensile stress on the hydrogen permeation does not follow the general effect of elastic stress which induces the increase in permeation flux due to the lattice expansion, but often results decrease in the permeation flux. 17 A slight decrease in the apparent diffusivity of hydrogen is also observed.…”