Application of the Hydrogen Diffusion Model to the Hydrogen Permeation

The model of hydrogen diffusion formerly de-veloped [1] has been applied successfully to thehydrogen permeation experiment results of threekinds of materials,α—Fe,Fe—Ti alloy and Fe—Ti—Calloy by the mathematical fitting method.From thefitting results it was shown that the model can re-fiect well the diffusion of hydrogen in the materialswith trapping.The obtained trapping parameters(αand β)can be used to explain well the diffusion ofhydrogen in the samples with trapping.     

Hydrogen transport in metals: Integration of permeation,thermal desorption and degassing

A modelling suite for hydrogen transport during electrochemical permeation, degassing and thermal desorption spectroscopy is presented. The approach is based on Fick's diffusion laws, where the initial concentration and diffusion coefficients depend on microstructure and charging conditions. The evolution equations are shown to reduce to classical models for hydrogen diffusion and thermal desorption spectroscopy. The number density of trapping sites is found to be proportional to the mean spacing of each microstructural feature, including dislocations, grain boundaries and various precipitates. The model is validated with several steel grades and polycrystalline nickel for a wide range of processing conditions and microstructures. A systematic study of the factors affecting hydrogen mobility in martensitic steels showed that dislocations control the effective diffusion coefficient of hydrogen. However,they also release hydrogen into the lattice more rapidly than other kind of traps. It is suggested that these effects contribute to the increased susceptibility to hydrogen embrittlement in martensitic and other high-strength steels. These results show that the methodology can be employed as a tool for alloy and process design, and that dislocation kinematics play a crucial role in such design.     

Effects of deformation on hydrogen degradation in a duplex stainless steel

Hydrogen embrittlement of annealed, 20 and 40% cold worked 2205 duplex stainless steels has been evaluated using electrochemical permeation measurement, hydrogen microprint technique and tensile test in this study. Due to hydrogen transport in 2205 duplex stainless steel is mainly lattice diffusion in ferritic phase, more hydrogen distribution, higher permeation rate and effective diffusion in ferritic phase were detected. Hydrogen trapping and mechanical property effects were studied for cold worked specimens. Fractographic investigation revealed that hydrogen absorption promoted transgranular fracture in cold worked specimens. These results exhibits that the cold worked duplex stainless steels are more susceptible to hydrogen embrittlement.     

氢在钢中晶格间隙和氢陷阱之间的扩散模式

氢在晶格间隙和陷阱点的扩散包括：氢在晶格间隙之间的扩散,氢从晶格间隙扩散到陷阱点,氢从陷阱点逃逸到晶格间隙的过程。本文运用气体在金属中的扩散理论,分析了氢在钢中晶格间隙和氢陷阱之间的扩散模式,以及通过扩散在两者之间建立的平衡状态。氢陷阱中氢浓度随时间的变化率等于晶格间隙的氢扩散到陷阱点引起的氢浓度变化率减去陷阱点中氢逃逸到晶格间隙引起的氢浓度变化率,其数学关系式符合McNabb和Foster建立的氢陷阱模型。氢在晶格间隙和陷阱点之间的平衡,实质是氢在晶格间隙的化学势μL和氢在陷阱点的化学势μT之间达到局部平衡,氢在陷阱中的占据分数很低的情况下（θT《1）,氢的有效扩散系数Deff的表达式是与陷阱结合能EB有关的温度函数。     

Influence of nitrogen ion implantation on hydrogen permeation in an extra mild steel

This paper presents the first results on the effect of nitrogen implantation on hydrogen permeation in steels. Nitrogen can modify superficially the steel's chemistry and/or microstructure depending on the fluence and thereby affect the processes of hydrogen diffusion and trapping. The implantations were performed on low carbon steel specimens with different nominal doses (1% to 10% and 33% nitrogen in a superficial layer of approximately 100 to 120 nm). The corresponding microstructures were characterized and permeation tests were conducted at room temperature in a double electrolytic cell. The nitrogen implanted layers on iron affects the electrochemical behaviour of the surface and the permeation in the material. This effect depends on the nitrogen concentration in the layer and on the corresponding microstructure. A continuous Fe₂N layer acts as an efficient barrier to hydrogen entry and permeation when the layer is located on the entry face of the permeation membrane. This effect is stronger when the implanted layer is on the downstream face of the membrane. The low permeability values are mainly attributed to a lower hydrogen solubility in the implanted layer, whereas hydrogen trapping on defects and nitride precipitates delay hydrogen penetration.     

Visualization of hydrogen diffusion in steels by high sensitivity hydrogen microprint technique

Hydrogen diffusion in steels was examined by both a high sensitivity hydrogen microprint technique (HMT) and an electrochemical hydrogen permeation method. The main diffusion path in an extremely low carbon steel was lattice within grains; grain boundaries were not accelerated diffusion paths. In the case of a hypo-eutectoid steel, hydrogen diffused through proeutectoid ferrite and ferrite in pearlite under steady-state of hydrogen diffusion. The diffusion paths, however, were carbide/ferrite interfaces when hydrogen charging was interrupted before achievement of the steady state. This is probably ascribable to the reversible trapping effect of the interface. The detection efficiency of the high sensitivity HMT was 75% for the low carbon steel and 40% for the hypo-eutectoid steel.     

Visualization of hydrogen diffusion in steels by high sensitivity hydrogen microprint technique

Hydrogen diffusion in steels was examined by both a high sensitivity hydrogen microprint technique (HMT) and an electrochemical hydrogen permeation method. The main diffusion path in an extremely low carbon steel was lattice within grains; grain boundaries were not accelerated diffusion paths. In the case of a hypo-eutectoid steel, hydrogen diffused through proeutectoid ferrite and ferrite in pearlite under steady-state of hydrogen diffusion. The diffusion paths, however, were carbide/ferrite interfaces when hydrogen charging was interrupted before achievement of the steady state. This is probably ascribable to the reversible trapping effect of the interface. The detection efficiency of the high sensitivity HMT was 75% for the low carbon steel and 40% for the hypo-eutectoid steel.     

Effect of prestraining on the behavior of hydrogen in structural steel

We study the influence of cold working on the behavior of hydrogen in 0.25C-1Cr-1Mn-1Si-1Ni high-strength low-carbon low-alloy steel after various types of thermal treatment. The results of measuring the permeation of hydrogen are used to determine the lattice diffusion coefficient, the efficiency of trapping of hydrogen, and the formation of internal cracks for specimens deformed by shot peening, cyclic tension-compression, and uniform tension. The mode of deformation does not affect the value of the lattice diffusion coefficient for the bainite structure. All types of deformation increase the efficiency of traps corresponding to the deformation of the bainite lattice by cold working. The type of deformation specifies the morphology of internal hydrogen-induced cracks along the local slip strips in the metal. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 65–70, September–October, 2007.     

Hydrogen behaviour in aged low activation martensitic steel F82H for fusion reactor applications

Abstract

A time dependent permeation method is used to measure the permeability, diffusivity, and solubility of hydrogen in the low activation martensitic steel F82H, based on 7–10%Cr steel agedfor 2000 h under vacuum at 773 K. The measurements cover the temperature range 373–723 K which includes the onset of hydrogen trapping effects on diffusivity and solubility. The results are interpreted using a trapping model. The number of trap sites and their average energies for hydrogen in the aged F82H steel are determined. These data are compared with those obtainedfor deuterium in F82H steel.     

Influence of microstructure on hydrogen embrittlement behaviour of 2·25Cr–1 Mo steel

Abstract

The hydrogen embrittlement of Cr–Mo steel has been studied. The effects of tempering temperature on the hydrogen diffusivity and hydrogen assisted cracking, without external stress, are investigated. Hydrogen permeation and trapping, for the various microstructural conditions, were measured using electrochemical equipment. The microprecipitate distribution in the steel was observed using transmission electron microscopy. The steel, which was heat treated to give a variety of microstructures, was cathodically hydrogen charged and the critical microstructural sites for hydrogen induced cracking examined. Cracks initiated by the charging treatments were found to nucleate at MnS interfaces. Hydrogen diffusivity and trapping are strongly dependent on the tempering parameters.

MST/1947     

Improved resistance to hydrogen embrittlement by tailoring the stability of retained austenite

ABSTRACT

The susceptibility of hydrogen embrittlement (HE) in GCr15 bearing steel under two types of heat treatments: quenching and tempering (QT) and pre-quenching and austempering (PQA) were investigated. Results showed that PQA-treated specimen have higher mechanical stability of the retained austenite (RA) compared with QT-treated specimen. The experiment of indentation test and hydrogen bubbles suggested that PQA-treated specimen was less susceptible to hydrogen than the QT-treated sample. Subsequently, hydrogen permeation tests revealed significant differences in diffusion coefficient and the number of hydrogen trapping sites between QT and PQA specimens. It was demonstrated that PQA is an appropriate heat treatment to tailor the stability of the RA and enhances the resistance to HE of GCr15 bearing steel.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

THE TRAPPING PHENOMENA OF HYDROGEN STUDIED BY THE ELECTROCHEMICAL PERMEA—TION TECHNIQUE AND THE DISCUSSION OF SEVERAL EXPERIMENTAL PROBLEMS

本文运用电化学渗透技术研究了18Ni 马氏体时效钢的氢扩散和捕集现象。结果表明,时效试样内存在析出相颗粒,使表观氢扩散系数减小,其原因是析出相界面捕集氢,阻滞了氢输运过程的进行;并发现500℃时效3h 表观扩散系数最小,扩散激活能最大,因为该制度时样品内析出相颗粒的数量最多,分布最弥散,此时的析出相界面捕集能为10.0kJ/mol,陷阱密度(室温时)为3.4×10~(26)/m~3。最后就常规双电解池式的电化学渗透实验,分析和讨论了几个值得注意的问题。     

A review on diffusion modelling in hydrogen related failures of metals

Modelling of hydrogen embrittlement in order to prevent engineering failures requires a characterization of transport phenomena in the bulk metallic as a first step. Interstitial solid state diffusion can be described as a random phenomenon, however there will be also some drift forces biasing this behaviour so a modification in Fick's laws is needed. The potential energy landscape of the metal lattice characterizes the influence of imposed fields and microstructural defects in transport kinetics. Thus, considering the chemical potential as a driving force, the physical basis of diffusion will be translated into continuum equations. Finally the two-level models that take into account lattice and trapping sites for hydrogen will be reviewed.     

Control of hydrogen cracking in the welded steel using microstructural traps

Hydrogen diffusion into steel can embrittle the material in H₂S environments, but this effect can be offset by suitable hydrogen trapping sites in the microstructure. Fine Ti(C,N) inclusions have been studied as the trapping sites in high strength low alloy (API X-70) welds, with Ti additions ranging from 0.004 to 0.16?wt.%. The trapping sites were investigated by electron microscopy and thermal desorption spectroscopy. Manganese sulphide particles were the main initiation sites for hydrogen induced cracking as expected. The optimum Ti addition was around 0.02% with no evidence of cracking in the weld. The estimated values of trapping activation energy for dislocations, microvoids, MnS and Ti(C, N) were approximately 25.9, 34.6, 65.1 and 87.6?kJ?mol^?1, respectively.     

Hydrogen permeation in plastically deformed steel membranes

We study the influence of plastic deformation on the electrochemical permeability of hydrogen. For this purpose, membrane specimens of FeE690T steel were deformed in the cold state by rolling or tension, and the diffusion of hydrogen in these specimens was measured by the classical Devanathan-Stachursky technique. The effective diffusion coefficients are determined according to the time dependences of the intensity of hydrogen flow from the back side of the membrane. They are characterized by the well-pronounced dependence on the level of strain. Moreover, the diffusion of hydrogen is simulated by taking into account the phenomenon of seizure of hydrogen atoms by traps formed in the presence of plastic strains. The linear diffusion equation is used to compute the time dependence of permeation of hydrogen for various densities of the traps. The effective coefficients of hydrogen diffusion are determined from the permeation curves. The accumulated results are in good agreement with the experimental data. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 1, pp. 77–81, January–February, 2006.     

Analysis of Hydrogen Diffusion in Metals with Trapping under Local Equilibrium Assumption

The assumption of the local equilibrium ofhydrogen distribution in metals[1]was used in themodel formerly developed[2]to describe the diffu-sion of hydrogen in metals.From the assumption adirect relationship between the hydrogen diffusivityand the hydrogen concentration in metals is estab-lished asD=D_o/{1+N_x(k/p)/[1+C(k/p)]}The comparison between the two results drawnfrom the assumption of equilibrium and the dynam-ics of hydrogen trapping[3]was also presented.Thecomputation results well explained the scatteringphenomenon existed in hydrogen diffusion data andsuggested that the experimental conditions shouldbe identical for the study of hydrogen permeation inmetals.     

A New Analysis of Hydrogen Diffusion in Metals with Trapping

A new model of hydrogen diffusion in metalshas been developed,it is more efficient todescribe the hydrogen diffusion with trappingin metals.In the model newly developed an impli-cit dependence on time of hydrogen diffusioncoefficient in metals with trapping was firstlybuilt and it is shown that hydrogen diffusioncoefficient will be different at different posi-tions in a dynamic process of hydrogen diffusionin a metal.Numerical solutions of the present modelwere obtained by finite difference method.Bychanging the parameters in the model the diffusionof hydrogen in a metal and the effect of trappingwere described and discussed.And the comparisonbetween the well known McNabb and Foster's modeland the present model was also made.     

A computational simulation for H-dislocated BCC Fe interaction

A model was performed to simulate hydrogen diffusion and trapping on an Fe₃₅ cluster with a (100) dislocation over three different paths, to obtain the relative amount of hydrogen atoms in different sites along each path. The results indicate that hydrogen locates preferentially on the surface zone and on the void produced by the dislocation. For hydrogen energies greater than certain values that depend on the path, the hydrogen concentration predominates on the void region.     

Behaviour of hydrogen in pure aluminium,Al-4 mass% Cu and Al-1 mass% Mg2Si alloys studied by tritium electron microautoradiography

The effect of lattice defects and microstructures on hydrogen distribution in pure aluminium, Al-4 mass% Cu and Al-1 mass% Mg₂Si alloys has been studied by tritium electron microautoradiography. It was found that in pure aluminium and both the alloys, dislocations and grain boundaries act as short-circuiting diffusion paths and also as trapping sites for hydrogen. It was found that the Guinier-Preston (GP) zones in the Al-1 mass% Mg₂Si alloy do not act as trapping sites but as repellants for hydrogen, in contrast to the GP zones in the Al-4 mass% Cu alloy in which they act as trapping sites for hydrogen. In the Al-1 mass% Mg₂Si alloy, the interfaces between the matrix lattice and the metastable precipitate and between the matrix lattice and the equilibrium precipitate, were proved to be strong trapping sites for hydrogen. In the Al-4 mass% Cu alloy, the equilibrium precipitate itself has been found to be able to trap hydrogen.