Abstract
The hydrogen permeation experiment, performed with a stepwise permeation sequence involving “1st permeation-desorption-2nd permeation under loading, demonstrates that fine blister cracks are frequently observed on the steel surface in hydrogen charging side after the 2nd permeation under the load over 95% of yield strength of the steel. To accommodate the experimental phenomena under the loading conditions, a numerical model is developed for determination of hydrogen diffusion parameters of the sour-resistant ferritic steel evaluated under tensile stress in plastic ranges. To solve the modified diffusion equation, a numerical finite difference method (FDM) is employed. The diffusion parameters determined by curve-fitting with the newly proposed diffusion equation indicates that, with the transition of mechanical domain from local-plasticity to generalized-plasticity, a big increase in the crack formation rate and hydrogen capture rate per irreversible trap are observed. It suggests that the transition probability for hydrogen transport from interstitial lattice site to irreversible trap site increases with the stress level.
Highlights
The high-strength ferritic steels used in the petrochemical industry suffer frequently from hydrogen assisted cracking (HAC) problem when they are used in a sour environment containing H2S.1–3 Atomic hydrogens which result from the reduction of H+ ions dissociated from H2S become the hydrogen molecule by the recombination reaction (H + H → H2)
The typical hydrogen induced blister cracks (HIBC) and stepwise cracking shown in Fig. 5a and 5b, respectively, indicate that the failure mode for the steel is type I stress cracking (SSC) which is generally termed as the stress-oriented hydrogen induced cracking (SOHIC).[28]
This study provides a new means to determine the hydrogen diffusion parameters for the high-strength steel subjected to the tensile stress even in plastic ranges
Summary
In order to perform the former desorption procedure without difficulty, a large circular drain depicted in Fig. 2 was made at the bottom of the charging cell. This is an important step in a sense that the procedure makes it possible to prevent the specimen surface from corrosion process and to ensure that any corrosion products acting as barrier against the hydrogen diffusion are not formed on the surface. When the permeation current in the desorption step reached the background current level, meaning that the diffusible hydrogen atoms were completely removed from the steel, the 2nd permeation test was subsequently conducted on the steel
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