Effects of additional elements of Cr and Ni in iron on hydrogen absorption behavior have been investigated by means of conventional electrochemical hydrogen permeation test. Empirical transient curve of hydrogen permeation current density was agreed with theoretical one on the basis of Fick’s second law with a fixed hydrogen concentration at the hydrogen absorption surface, C₀, at any time. The fact suggests quasi-equilibrium condition to the absorption-desorption reaction of hydrogen at the surface, and that correlation between C₀ and hydrogen absorption rate. As an analytical result of the transient, a diffusion coefficient decreased with an increase in a content of additional elements, and the decrement was enhanced much more by Cr than by Ni. C₀ was almost proportional to a square root of the cathodic current density at the hydrogen absorption surface for any specimens. The slope increased with an increase in a content of the additional elements, and the increment was enhanced much more by Cr than by Ni. There was a correlation between the slope and the diffusion coefficient, and the correlation suggested that the additional elements of Cr and Ni attract hydrogen. In addition, the attraction effect of Cr was much more than that of Ni. Under anodic potential region, the effect of additional element on C₀ was not clearly obtained in the solution of pH 2, but it is found that C₀ of the Cr-added steel was larger and that of the Ni-added steel was smaller than that of Fe in the solution of pH 6.

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Hydrogen entry in low alloy steels and effects of alloying elements were investigated using a hydrogen permeation technique under a simulated atmospheric corrosion condition and an acidic solution. In the base steel, the hydrogen permeation coefficient sensitively varied depending upon the wetting and drying processes in the cyclic corrosion condition.
Additions of Mo, Cu and Ni to the base steel were effective for suppressing hydrogen entry under the cyclic corrosion condition. These elements were also effective in the acidic solution with an initial pH of 3.5. The mechanism of the beneficial effects of these elements was considered to be a change in hydrogen overpotential, which leads to a decrease in hydrogen surface coverage.
S was also effective for suppressing hydrogen entry under the cyclic corrosion condition, although it drastically promoted hydrogen entry in the acidic solution. The detrimental effect in the acidic solution was considered to relate the production of H₂S, a catalytic promoter of hydrogen entry, accompanying with the chemical dissolution of soluble sulfide inclusions such as MnS. On the contrary, under the cyclic corrosion condition, dissolution of MnS inclusions would lead to an increase in pH because of the consumption of H⁺ ions in the thin water layer on the steel surface, resulting in the suppression of hydrogen entry.

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Effect of hardness of steels on hydrogen absorption behavior has been investigated by means of conventional electrochemical hydrogen permeation test. Empirical transient curves of hydrogen permeation current density were agreed with the theoretical one on the basis of Fick’s second law with a fixed hydrogen concentration at the hydrogen absorption surface, C₀, at any time. The fact suggests quasi-equilibrium condition to the absorption-desorption reaction of hydrogen at the surface, and that correlation between C₀ and hydrogen absorption rate. As an analytical result of the transient, a diffusion coefficient of hydrogen, D, decreased with an increase in a hardness of the steel. On the other hand, the value of C₀ increased with an increase in the hardness. There was a good positive correlation between C₀ and an inverse of D in spite of difference in the hardness and the additional element. The fact may suggest that the microstructural defects attract hydrogen both in the bulk and on the surface of the steel, and the effect promotes hydrogen ad-atom absorbing into the iron specimen.

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Ammonium thiocyanate is widely used as a reagent for promoting hydrogen absorption by high-strength steels. The effects of the solution concentration, temperature and dissolved oxygen on corrosion film formation, corrosion reactions and hydrogen absorption were investigated in a hydrogen embrittlement test environment using an ammonium thiocyanate aqueous solution. A still bath of a 20 mass% ammonium thiocyanate solution displayed a supply limitation, whereas a solution with flow influenced the corrosion and hydrogen absorption by the tested steel. The concentration of dissolved oxygen that forms an iron oxide film was found to have a small effect on hydrogen absorption. The change in the hydrogen content of the tested steel with elapsed time is explained in terms of changes in the equilibrium concentration of hydrogen on the steel surface and in the corrosion rate. The change in the solution pH during potentiostatic electrolysis tests in an ammonium thiocyanate solution is also discussed.

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Hydrogen embrittlement properties of several stainless steels and Ni based alloys under cathodic charge (CHE) were investigated. Hydrogen concentration in the materials was varied by controlling hydrogen charging conditions. Slow strain rate test (SSRT) under cathodic charge in aqueous solution was carried out to evaluate CHE susceptibility. Mechanical degradation by hydrogen was evaluated by relative fracture elongation (relative fracture El.) against that in air. Critical surface hydrogen concentration (H_C), the maximum hydrogen at which El. was hard to be decreased, was derived from SSRT results under various hydrogen charging levels. H_C strongly depended on Ni equivalent (Ni_eq), which is a parameter consisting of alloy chemical compositions, reflecting stability of austenitic phase.CHE test results were compared to susceptibilities to hydrogen gas embrittlement (HGE) and internal reversible hydrogen embrittlement (IRHE), which are caused by highly pressurized gaseous hydrogen. Materials accepting higher H_C generally showed higher resistance to HGE and IRHE. Comparing H_C to H_E, concentration of hydrogen absorbing from highly pressurized gaseous hydrogen, enables the risk assessment of hydrogen embrittlement in actual service conditions.

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Hydrogen related intergranular fracture in high strength steels and steel welds are often explained by the hydrogen segregation at grain boundary that induces decohesion of the boundary. In this study, distribution of hydrogen in the grain boundary was visualized by a hydrogen microprint technique (HMT), and the effect of dislocations on hydrogen distribution in steel was analyzed. 0.02%C steel with ferrite microstructure was subjected to a plastic deformation up to 20% and HMT analysis, and then quantitative analysis of hydrogen concentration was conducted by measuring the area of silver particles formed by the reductive reaction between AgBr and hydrogen. The grain boundary hydrogen density was defined by an area of silver particle at the grain boundary per unit grain boundary length and diffusible hydrogen charged. And, it was found that the grain boundary hydrogen density decreases with increasing prestrain since hydrogen is trapped by dislocations inside the grain. The effect of dislocations introduced by martensitic transformation was also investigated using 0.1%C steels in as-quenched and quenched and tempered conditions. Silver particles were mainly observed on lath boundary in the as-quenched condition, while hydrogen density at the prior austenite boundary increased after tempering. It can be said that hydrogen segregation at the prior austenite grain boundary is affected by dislocations and lath boundary in martensite microstructure. Effect of dislocations on hydrogen distribution was also discussed based on dislocation densities and hydrogen contents.

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A nanoindentation apparatus with an electrochemical cell for cathodic hydrogen charging and a wide dynamic range of load duration time was developed, and influence of hydrogen on local mechanical properties of Type 316L stainless steel, pure Ni, and three kinds of Ni–Cr bialloy (56Ni-43Cr, 69Ni-30Cr, 79Ni-20Cr) was investigated. For Type 316L stainless steel and pure Ni, nanohardness was increased by about 10% and unchanged, respectively, by hydrogen charging at load duration time between 0.1 s and 10800 s. On the other hand, for Ni–Cr bialloys, degree of nanohardness increase by hydrogen charging was reduced with increase in load duration time. As a result, for 69Ni-30Cr and 79Ni-20Cr, hydrogen caused softening at long load duration time of more than 600 s. Hydrogen embrittlement susceptibility of each sample was also evaluated by slow-strain rate tensile tests, and compared with the observed nanoindentation results. The tendency was found that, for samples with higher hydrogen embrittlement susceptibility, softening is caused by hydrogen charging at long load duration time.

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In this paper, hydrogen absorption behavior into iron was investigated by measuring alternating current responses for hydrogen-entry side and hydrogen-withdrawal side in an electrochemical hydrogen permeation cell. Hydrogen absorption efficiency of iron was evaluated as a function of electrode potentials by using both DC and AC components of the current responses. The hydrogen absorption efficiency of iron increased with decreasing cathodic polarization. Furthermore, frequency response characteristics of the hydrogen absorption efficiency were investigated at various polarization potentials and the results indicated that the hydrogen absorption efficiency decreased with increasing frequency of the perturbation potential. The frequency response characteristics of the hydrogen absorption efficiency was also simulated by using Fick’s second law and the results suggested that hydrogen absorption kinetics can be considered to be faster than diffusion of hydrogen atoms in iron and to be under an equilibrium condition. By analyzing AC responses for hydrogen-entry side and hydrogen-withdrawal side, hydrogen absorption behavior into iron can be successfully evaluated.

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A micro-capillary technique was applied to a Devanathan-Stachurski electrochemical cell for local measurement of hydrogen permeation into a steel sheet. An electrolyte-flowing design for the hydrogen entry side of the Devanathan-Stachurski cell successfully allowed the detection of hydrogen permeation response on hydrogen exit side electrode in a micro-capillary cell with a diameter of 250 µm. Phase shift of the detected permeation current from a sinusoidal perturbation of the electrolyte flow rate in the hydrogen entry cell was strongly dependent on the metallographic structure of the steel sheet. A local structure, in which two single grains form grain boundaries, led to hydrogen permeation more frequently than did a local structure of single grains. The results suggested that the diffusion coefficient of the boundaries was at least two-times larger than that of the grains.

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Electrochemical hydrogen permeation tests were carried out under controlled temperature and humidity using pure Fe sheet specimens outdoor-exposed at Beijing and Chongqing, China and Okinawa, Japan in order to understand the effect of environmental factors on hydrogen uptake behavior. The maximum hydrogen permeation current densities of the exposed specimens were in the order of Beijing > Chongqing >> Okinawa, while the order of the degree of corrosion of the specimens was Okinawa > Chongqing >> Beijing. X-ray fluorescence analysis showed that the surface concentrations of sulfur on the Beijing- and Chongqing-exposed specimens were higher than that of the Okinawa-exposed specimens, whereas chlorine concentrations of the Beijing and Okinawa-exposed specimens were higher than that of the Chongqing-exposed specimen. Nitrate concentrations of Beijing- and Chongqing-exposed specimens evaluated using nitrate test strips were obviously higher than that of the Okinawa-exposed specimens. It is suggested that air pollutants such as SO₂ and NO₂ and particulate matters containing inorganic acid ions, likely surfate and nitrate ions, and possibly organic acids contribute to acidification of rust layer leading to the enhanced hydrogen entry.

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In this study, hydrogen absorption behavior into Zn and Zn-55mass%Al coated steels was investigated both in an immersion test and a wet and dry cyclic corrosion test with NaCl and Na₂SO₄ solutions by Devanathan-Stachurski permeation technique. In the immersion test, the hydrogen absorption into Zn coated steels was observed after the surface of the steel substrate under Zn coating was exposed to corrosion environments. On the other hand, hydrogen absorption into Zn–Al coated steels was not observed for the duration of the tests because of slower dissolution kinetics of Zn–Al coating than Zn coating in the corrosion environments. In the case of a wet and dry cyclic corrosion test, hydrogen permeation current for Zn coated steel was observed during the test duration only under the condition with NaCl solution. This may be because dissolution of Zn coating takes place more locally in NaCl solution than in Na₂SO₄ solution. These results were discussed based on corrosion rate of the coating of Zn and Zn–Al alloy and on corrosion morphology of Zn and Zn–Al alloy coatings in NaCl and Na₂SO₄ solutions.

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The effects of scratch, surface element composition, and relative humidity on the hydrogen permeation behavior of zinc coated steels during wet and dry cycle corrosion tests were investigated. The permeating hydrogen was measured electrochemically, and a size controlled scratch was formed on zinc coated steels with a laser machining technique. The hydrogen permeation current was observed on the zinc coated steels with a formed scratch (S-coated): no significant hydrogen permeation current was observed on the zinc coated steel without the formed scratch (NS-coated) or on steel with zinc coating removed by mechanical grinding (R-steel). The zinc corrosion products effectively reduced the hydrogen permeation current from the S-coated at low relative humidity. The total amount of permeating hydrogen per unit area of formed scratch was independent of the area of the formed scratch in wet and dry cycling.

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Hydrogen absorption into steel under an atmospheric corrosion condition is considered to occur because of the H⁺ ion is reduced to hydrogen atom (H_ads) receiving electron formed by rust that forms by the corrosion reaction on steels. Thus, we investigated whether it is possible to reduce the amount of hydrogen absorption into steels by adding elements that improve corrosion resistance and control the formation of rust to reduce corrosion reaction. It was discovered that the amount of hydrogen absorption into steels can be reduced by adding elements that improve corrosion resistance and control the formation of rust on the surface of steels.

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A wet and dry corrosion test with a constant dew point was applied to investigate the effects of scratches and specimen composition on the hydrogen permeation behavior of Zn and 55 mass% Al–Zn coated steels as well as of uncoated steel. There was an etched size controlled scratch formed on specimens with a laser machining technique. The hydrogen permeation current was observed to be independent of the specimen composition and the area of the formed scratches, the corrosion products were observed after the tests. The hydrogen permeation current of the steels with the formed scratches decreased with number of cycles of the wet and dry tests. Except in the first cycle there were no significant changes in the current with cycle number on steels without formed scratches. It was suggested that the composition, scratch, and corrosion products play an important role in the hydrogen permeation behavior during wet and dry corrosion.

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Numerical calculation for the diffusion problem of hydrogen absorbed in a steel sheet during hydrogen permeation measurement using a double electrochemical cell was carried out. The finite element method (FEM) was applied to obtain the concentration distribution of hydrogen expressed by one- or two-dimensional Fick’s laws in the sheet, assuming that hydrogen concentration at the hydrogen entry interface was perturbed sinusoidally and both the hydrogen entry and exit reactions were in a mass-transport controlled process. From a comparison with experimental results reported previously, in which a phase shift from entry current to exit current waves observed on a single grain of the specimen sheet was at least two-times larger than that on two grains, it was estimated that the diffusion coefficient at a grain boundary located between two grains was five orders in magnitude larger than that on a single grain.

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Surface potential measurement has been applied to visualize hydrogen permeation behavior through steel materials. The measurement system consists of the surface potential measurement device, X-Y stage and personal computer, and the detective probe and X-Y-Z stage are installed to acrylic container. The hydrogen entry was accelerated by galvanostatic polarization in 0.1 M NaOH solution. The hydrogen evolution was observed only at the small exposed area. The surface potential directly beneath the exposed area shifted to a less noble direction compared with the initial potential during hydrogen charging. The surface potential gradually moved to the original potential after stop of the hydrogen charging. No red rust was observed on the surface of hydrogen detective side after surface potential measurement. The area of negative surface potential expanded in the surface potential distribution over time, and the area was gradually reduced after 3 hours. The location was nearly the same as the exposed area of mild steel at the entry side. The behavior of surface potential change was also correspondent with the hydrogen release behavior from the steel materials. It is concluded that the surface potential measurement can be a powerful tool for the visualization of the hydrogen permeation behavior.

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An area-selective technique for electrochemical hydrogen detection with laser local activation (focused pulse YAG laser irradiation) has been newly developed. The developed technique was applied to measure hydrogen permeation behavior of iron at selected areas of various grain structures. The hydrogen permeation current was detected after the laser irradiation, suggesting that indirect activations of hydrogen generation and entry reactions were produced by the focused pulse YAG laser irradiation, which activated dissolution of metals at the irradiated areas. An obtained hydrogen diffusion coefficient, D_H, was in good agreement with previously reported D_H of iron, while a clear dependence of D_H on grain structure at the entry side was not found.

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Surface potential mapping of ferritic steel loaded with hydrogen through scanning droplet cell microscopy was performed with scanning Kelvin probe showing a drop in measured contact potential difference on hydrogen loaded spot. Equipotential areas were defined and used for determination of hydrogen diffusion coefficient parallel and orthogonal to rolling direction, resulting in values of 1.75×10⁻⁷ cm² s⁻¹ and 3.51×10⁻⁷ cm² s⁻¹, respectively. By controlling atmosphere composition and humidity during consecutive Kelvin probe measurements, the influence of oxygen concentration on the hydrogen discharging process was investigated, revealing interaction between surface oxides and hydrogen.

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A chemical imaging sensor is a semiconductor device that can visualize the pH distribution in a solution. In the conventional constant-bias method, in which a photocurrent map is converted into a pH image, the measurable range of pH values is rather limited and the result is interfered by the impedance of the solution. In this study, a novel method of data acquisition and image construction is proposed, which can measure a larger pH change without interference of the impedance.

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A small Devanathan-Stachurski cell was newly designed to ascertain the hydrogen absorption behavior into re-sulfurized carbon steel during pitting corrosion. In 0.1 M NaCl (pH 5.5), a pit was generated on a small area with MnS inclusions, the current of hydrogen permeation current increased sharply when the solution (capillary) was placed on the steel surface, and the current density returned to the background after the capillary was removed. On the other hand, no pitting was observed on the area without the inclusions, and no permeation current was also observed. The time variation of the permeation current density was measured in 0.1 M NaCl (pH 2.0) and boric-borate buffer with 1 mM NaCl (pH 5.5) solutions. The permeation current from a small area with MnS inclusions was large compared with that of an area without the inclusions. After the experiments, a deep pit was generated on the surface with the inclusions. MnS inclusions were likely to accelerate the steel dissolution and the hydrogen absorption into the steel.

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The HLP (High-strength Line Pipe) Research Committee in the Iron and Steel Institute of Japan (ISIJ) has investigated the major issues of line pipes from the viewpoints of corrosion and fracture. Fitness-For-Purpose (FFP) evaluations for Hydrogen Induced Cracking (HIC) have been the subject of considerable study in recent years. FFP is the idea that appropriate materials are chosen depending on the severity of the use environment. Therefore, it is possible that the applicable range of material strength and the flexibility of manufacturing can be expanded by establishing the FFP evaluation approach. This paper reviews the recent activities of the HLP Corrosion Working Group during the past few years.
The current status of the conventional HIC test method and FFP HIC evaluation methods were overviewed. Based on the results of a variety of experiments, the HLP Research Committee proposed a HIC test solution for FFP evaluations in the NACE Task Group. The proposed solution is a highly concentrated acetate buffer solution that is 5%NaCl+0.93N−(CH₃COOH+CH₃COONa), and is expected to be adopted as the optional solution in NACE TM0284. The solution displayed an excellent pH buffering capacity during a long-term HIC test in comparison with the conventional HIC test solutions. The corrosion rate and hydrogen permeation behavior of the steel specimen in the solution were similar to those in the conventional solutions.

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