Hydrogen-enhanced Decohesion Research Articles

Discrete dislocation modeling of stress corrosion cracking in an iron

Abstract Material strengthening and embrittlement are controlled by interactions between dislocations and hydrogen that alter the observed deformation mechanisms. In this work, we used an energetics approach to differentiate two fundamental stress corrosion mechanisms in iron, namely, hydrogen-enhanced localized plasticity and hydrogen-enhanced decohesion. Considering the small-scale yielding condition, we use a discrete dislocation framework with line dislocations to simulate the crack-tip plastic behavior. The crack growth was modeled using the change in surface energies (cohesive zone laws) due to hydrogen segregation. The changes in the surface energies as a function of hydrogen concentration are computed using atomistic simulations. Results indicate that, when hydrogen concentrations are low, crack growth occurs by alternating mechanisms of cleavage and slip. However, as the hydrogen concentrations increased above some critical value, the crack grows predominately by the cleavage-based decohesion process.

Hydrogen damage of steels: A case study and hydrogen embrittlement model

Many efforts have been made to understand the effects of hydrogen on steels, resulting in an abundance of theoretical models and papers. However, a fully developed and practically applicable predictive physical model still does not exist industrially for predicting and preventing hydrogen damage. In practice, it is observed that different types of damages to industrial boiler components have been associated with the presence and localization of hydrogen in metals. In this paper, a damaged boiler tube made of grade 20 – St.20 (or 20G, equivalent to AISI 1020) was investigated. The experimental research was conducted in two distinctive phases: failure analysis of the boiler evaporator tube sample and subsequent postmortem analysis of the viable hydrogen embrittlement mechanisms (HE) in St.20 steel. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack (HTHA) during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, tube wall thickness measurement, tensile testing, hardness measurement, impact strength testing (on instrumented Charpy machine), analysis of the chemical composition of corrosion products – deposit and the microstructural characterization by optical and scanning electron microscopy – SEM/EDX. The HTHA damage mechanism is a primary cause of boiler tube fracture. Based on the multi-scale special model, applied in subsequent postmortem investigations, the results indicate a simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. The model is based on the correlation of mechanical properties to the SEM fractography analysis of fracture surfaces.

Effect of pre-strain on susceptibility of Indian Reduced Activation Ferritic Martensitic Steel to hydrogen embrittlement

The role of pre-strain on hydrogen embrittlement susceptibility of Indian Reduced Activation Ferritic Martensitic Steel was investigated using constant nominal strain-rate tension test. The samples were pre-strained to different levels of plastic strain and their mechanical behavior and mode of fracture under the influence of hydrogen was studied. The effect of plastic pre-strain in the range of 0.5–2% on the ductility of the samples was prominent. Compared to samples without any pre-straining, effect of hydrogen was more pronounced on pre-strained samples. Prior deformation reduced the material ductility under the influence of hydrogen. Up to 35% reduction in the total strain was observed under the influence of hydrogen in pre-strained samples. Hydrogen charging resulted in increased occurrence of brittle zones on the fracture surface. Hydrogen Enhanced Decohesion (HEDE) was found to be the dominant mechanism of fracture.

Factors affecting hydrogen-assisted cracking in a commercial tempered martensitic steel: Mn segregation, MnS, and the stress state around abnormal cracks

The purpose of this paper is to reveal the dominant factors affecting tensile fracture under a hydrogen gas atmosphere. Tensile tests were conducted in hydrogen gas with circumferentially-notched specimens of a commercial tempered martensitic steel. Two specimens were exposed to hydrogen gas for 48h before tensile testing; the other two specimens were not pre-charged. Longitudinal cracks along the loading direction and a transverse crack perpendicular to the loading direction were observed on a cross section of the non-charged specimen, but there was only one small crack on a cross section of the pre-charged specimen. Electron back scatter diffraction, energy dispersive X-ray spectrometry and finite element method analyses were applied to clarify the relationships among the longitudinal crack, Mn segregation, microstructures of martensitic steel and hydrogen. As a result, it has been demonstrated that Mn segregation and MnS promote hydrogen-assisted cracking in the tempered martensitic steel, causing the longitudinal cracking which is a mechanically non-preferential direction in homogeneous situations. More specifically, we have shown that the role of the Mn segregation is to promote the hydrogen-enhanced decohesion effect (HEDE), which is particularly important for crack propagation in the present case. These considerations indicate that the presence of Mn is crucially important for hydrogen-assisted cracking associated with hydrogen-enhanced localized plasticity (HELP) as well as HEDE.

Effect of pre-strain on hydrogen embrittlement of high strength steels

The effect of the pre-strain on hydrogen embrittlement (HE) of high strength steels was investigated by slow strain-rate tensile (SSRT). The specimens were electrochemically hydrogen-charged in 0.5mol/L H2SO4 with 1g/L CH4N2S at room temperature for 24h. It is found that the amount of hydrogen increases linearly with the exponent of pre-strain, i.e., CH=11.963+0.00385exp(1.144εP). The HE susceptibility of the alloy initially decreases and then increases with increasing pre-strain. The ultimate tensile strength for the hydrogen-charged specimen is highest in the 3% pre-strain, which is interpreted by competition between strain hardening and Hydrogen Enhanced Decohesion (HEDE) mechanism. The fracture mode shows a mixed mode fracture of quasi-cleavage and intergranular fracture for hydrogen-charged specimens while the fracture mode exhibits dimples for hydrogen-free specimens.

Hydrogen-assisted decohesion and localized plasticity in dual-phase steel

Hydrogen embrittlement affects high-strength ferrite/martensite dual-phase (DP) steels. The associated micromechanisms which lead to failure have not been fully clarified yet. Here we present a quantitative micromechanical analysis of the microstructural damage phenomena in a model DP steel in the presence of hydrogen. A high-resolution scanning electron microscopy-based damage quantification technique has been employed to identify strain regimes where damage nucleation and damage growth take place, both with and without hydrogen precharging. The mechanisms corresponding to these regimes have been investigated by employing post-mortem electron channeling contrast imaging and electron backscatter diffraction analyses, as well as additional in situ deformation experiments. The results reveal that damage nucleation mechanism (i.e. martensite decohesion) and the damage growth mechanisms (e.g. interface decohesion) are both promoted by hydrogen, while the crack-arresting capability of the ferrite is significantly reduced. The observations are discussed on the basis of the hydrogen-enhanced decohesion and hydrogen-enhanced localized plasticity mechanisms. We discuss corresponding microstructure design strategies for better hydrogen-related damage tolerance of DP steels.

Cr effect on hydrogen embrittlement of Fe3Al-based iron aluminide intermetallics: Surface or bulk effect

The objective of this work is to reduce the susceptibility of binary Fe3Al intermetallics to hydrogen embrittlement with the addition of chromium as the ternary element. The Young’s modulus, Gibbs free energy for homogeneous dislocation nucleation (HDN) and velocity of dislocations were evaluated by an in situ nanoindentation method in air and under cathodic and anodic charging. The results show the influence of hydrogen on the reduction of Young’s moduli of alloys, in agreement with the hydrogen-enhanced decohesion model, whereas measurements of the pop-in load indicate a drastic decrease after cathodic charging in samples with low Cr content. This is thought to be due to the decrease of the energy needed for HDN in the dislocation-free samples based on the defect-acting agents concept. Additionally, the effect of hydrogen on the mobility of dislocations and also the influence of Cr on the characteristics of passive oxide layer were analysed in detail.

An Overview of the Hydrogen Embrittlement of Iron Aluminides

New technologies often lead to the development of new materials with better mechanical, thermal and chemical behaviors. Recently, transition metals (TM), or aluminide intermetallics, for example TiAl, NiAl, FeAl, and Fe3Al, have attracted considerable interest due to their unique properties such as high melting points, enhanced corrosion resistance, relatively low density, and utility as soft magnetic materials [Baker and Munroe (1997); Noebe et al. (1993); Sundararajan et al. (1995)]. The main problem of these alloys, however, is their limited ductility at ambient temperature especially in presence of water vapor, and limited strength and creep resistance at high temperatures. In order to evaluate the reasons of the embrittlement in iron aluminides at room temperature, the brand new method of in-situ nanoindentation is used for the Fe3Al intermetallic with a DO3 structure.The results of nanoindentation, under cathodic and anodic charging, show the influence of hydrogen on the reduction of Young's moduli of alloys in agreement with the hydrogen-enhanced decohesion (HEDE) model; whereas measurements of the pop-in load indicate a drastic decrease after cathodic charging in samples with low Cr content. This is thought to be due to the decrease of the energy needed for homogeneous dislocation nucleation (HDN) in the dislocation free samples based on the Defect Acting Agents (defactants) concept. Additionally, the effect of hydrogen on hardness or the mobility of dislocations was noted. This paper explains in detail some aspects that are necessary for the performance of in-situ nanoindentation.

Hydrogen Embrittlement of Low Carbon Structural Steel

Hydrogen embrittlement (HE) of steels is extremely interesting topic in many industrial applications, while a predictive physical model still does not exist. A number of studies carried out in the world are unambiguous confirmation of that statement. Bearing in mind multiple effects of hydrogen in certain metals, the specific mechanism of hydrogen embrittlement is manifested, depending on the experimental conditions.In this paper structural, low carbon steel, for pressure purposes, grade 20 - St.20 (GOST 1050-88) was investigated. Numerous tested samples were cut out from the boiler tubes of fossil fuel power plant, damaged due to high temperature hydrogen attack and HE during service, as a result of the development of hydrogen-induced corrosion process. Samples were prepared for the chemical composition analysis, hardness measurement, impact strength testing (on instrumented Charpy machine) and microstructural characterization by optical and scanning electron microscopy - SEM/EDX.Based on multi-scale special approach, applied in experimental investigations, the results, presented in this paper, indicate the simultaneous action of the hydrogen-enhanced decohesion (HEDE) and hydrogen enhanced localized plasticity (HELP) mechanisms of HE, depending on the local concentration of hydrogen in investigated steel. These results are consistent with some models proposed in literature, about a possible simultaneous action of the HELP and HEDE mechanisms in metallic materials.

Coupling aspects in the simulation of hydrogen-induced stress-corrosion cracking

Modelling of hydrogen-induced stress-corrosion cracking (HISCC) has to consider coupling effects between the mechanical and the diffusion field quantities. Four main topics are addressed: i) surface kinetics, ii) diffusion, iii) deformation and iv) crack growth. Surface kinetics is realised by a chemisorptions model, hydrogen diffusion is formulated by an enhanced diffusion equation including effects of plastic deformation, deformation rate and hydrostatic pressure, deformation is described by von Mises plasticity, and crack growth is simulated by a cohesive model, where both yield and cohesive strength depend on the hydrogen concentration. The effect of atomic hydrogen on the local yield strength is modelled by the so-called HELP (Hydrogen-Enhanced Localised Plasticity) approach, and the influence on the cohesive strength is taken into account by the so-called HEDE (Hydrogen-Enhanced DEcohesion) model. As the two models predict contrary effects of atomic hydrogen on the material behaviour, namely a decrease of the local yield strength resulting in larger plastic deformations and a reduction of the cohesive strength and energy inducing lower ductility, respectively, the coupling phenomena are studied in detail. The model is verified by comparing experimentally measured and numerically simulated CTOD R-curves of C(T) specimens.

Hydrogen embrittlement phenomena and mechanisms

Mechanisms of hydrogen embrittlement in steels and other materials are described, and the evidence supporting various hypotheses, such as those based on hydride formation, hydrogen-enhanced decohesion, hydrogen-enhanced localised plasticity, adsorption-induced dislocation emission, and hydrogen-vacancy interactions, are summarised. The relative importance of these mechanisms for different fracture modes and materials are discussed based on detailed fractographic observations and critical experiments.

Recent developments in the study of hydrogen embrittlement: Hydrogen effect on dislocation nucleation

In this paper, the intrinsic complexities of the experimental examination of hydrogen embrittlement are discussed. On the basis of these complexities, an experimental approach, in situ electrochemical nanoindentation, is proposed and performed on different materials. This technique is capable of registering the onset of plasticity in extremely small volumes, namely perfect crystals in hydrogen-free and charged conditions. It is shown that hydrogen reduces the required stress for the onset of plasticity, i.e. homogeneous dislocation nucleation by reduction in the shear modulus, dislocation line energy and stacking fault energy. The change in the shear modulus can be related to reduction in crystal cohesion whereas the reduction in dislocation line energy and stacking fault energy are explained by the defactant concept, i.e. reduction in the defect formation energy in the presence of hydrogen. Thus, neither hydrogen-enhanced decohesion nor hydrogen-enhanced plasticity, but the reduction in the cohesion and defect formation energy are responsible for hydrogen embrittlement.

Proven Solutions for Corrosion, Hydrogen Embrittlement, and Excessive Brightener Consumption

Mechanisms of hydrogen embrittlement in steels and other materials are described, and the evidence supporting various hypotheses, such as those based on hydride-formation, hydrogen-enhanced decohesion, hydrogen-enhanced localised plasticity, adsorption-induced dislocation- emission, and hydrogen-vacancy interactions, are summarised. The relative importance of these mechanisms for different fracture modes and materials are discussed based on detailed fractographic observations and critical experiments.

Stress Corrosion Cracking (SCC) in Mg‐Al Alloys Studied using Compact Specimens

Transgranular Stress Corrosion Cracking (TGSCC) in Mg-Al alloys was studied using compact specimens. The mechanism for TGSCC of Mg alloys is a form of Hydrogen Embrittlement (HE) with Hydrogen coming from the cathodic partial reaction of Mg corrosion reaction. The test materials were the Mg-Al alloys AZ91 and AZ31 that were machined from as-cast ingots or large extrusions. Control tests were carried out in laboratory air and the specimens were immersed in the environment to just above the machine notch. SCC in AZ91 was characterized by superficial stress corrosion crack branches extending from the fatigue crack tip along the maximum principle stress contours in the region. The mechanisms that were proposed are Hydrogen Enhanced Decohesion (HEDE), Hydrogen Enhanced Localized Plasticity (HELP), Adsorption Induced Dislocation Emission (AIDE) and Delayed Hydride Cracking (DHC).

In situ electrochemical nanoindentation: A technique for local examination of hydrogen embrittlement

Nanoindentation combined with AFM (NI-AFM) has been used to study the effect of electrochemically in situ charged hydrogen on the deformation of small volumes of nickel and copper single crystals. Hydrogen reduces the unstable elastic plastic transition load (pop-in) in nickel, but does not have any effect on copper. It has been shown that the activation energy for the onset of plasticity (dislocation nucleation) is reduced by dissolved hydrogen. This is because hydrogen reduces shear modulus and stacking fault energy in nickel, whereby the former results in hydrogen-enhanced decohesion (HEDE) and the latter in the hydrogen-enhanced plasticity (HELP) mechanism.

Modeling of hydrogen embrittlement in single crystal Ni

A large-scale molecular dynamics simulation by the embedded atom method was carried out on hydrogen embrittlement of a single crystal containing 1,021,563 nickel atoms. The details of the deformation in the specimen were identified by a new method of the deformation analysis. Plenty of slip deformation occurred around the crack tip and in the bulk of the hydrogen-free specimen. Hydrogen embrittlement was most serious in the specimen hydrogen-charged in the notched area. Serious embrittlement was also observed in the specimen hydrogen-charged in the slip planes, in which dislocation emission was localized at the crack tip and enhanced on the planes where hydrogen atoms were located. It is considered that the fracture process is due to the hydrogen-enhanced decohesion mechanism.

Microstructure-based modeling of hydrogen assisted cracking in pearlitic steels

This paper analyzes the process of hydrogen assisted cracking in pearlitic steels, which are increasingly cold drawn in the course of manufacturing to produce prestressing steel wires used in prestressed concrete. A microstructure-based modeling of the environmentally-assisted fracture phenomenon is proposed to rationalize the results for diferent degrees of drawing. The approach is based on two items — (i) the model by Miller and Smith of shear cracking (SC) in pearlite; (ii) the mechanism of hydrogen enhanced decohesion (HEDE) proposed by Gerberich. There is an evolution from a predominant micromechanism of SC in slightly drawn steels to a predominant micromechanism of HEDE in heavily drawn steels.

Plastic deformation: a major factor in hydrogen embrittlement of low alloy steel

Abstract Hydrogen embrittlement of a low alloy steel has been studied using tensile tests and fracture mechanics techniques. Embrittlement was found to be caused by hydrogen-enhanced decohesion at internal interfaces. Hydrogen degradation of the relatively soft material requires that hydrogen is accumulated on a localized scale by dislocation sweep-in.

Low energy cleavage fracture associated with the hydrogen embrittlement of refractory metal alloys

Three dislocation models are investigated for explaining the hydrogen embrittlement of refractory (b.c.c.) metal alloy single crystals and large-grain polycrystals on the basis of cleavage initiation. The models considered are: (1) stress-induced dislocation pile-ups and dislocation reactions; (2) hydrogen-enhanced decohesion of slip plane pile-ups and planar dislocation arrays; and (3) slip and hydrogen interactions with lineage subgrain boundaries. Emphasis is given in each case to the crystallographic features of low strain cleavage initiations in the presence of hydrogen as observed in three refractory alloy systems. Electron microscope observations, including the occurrence of deformation twinning, are presented for the hydrogen-charged materials as a function of strain. Cumulative (low energy) stress fields are described for the dislocation pile-ups and the lineage boundaries. The enhancement of hydrogen concentration in the presence of the stress fields of these dislocation configurations is determined. The slip-twinning relationship and cleavage associated with hydride microprecipitates are considered.

The influence of manganese sulphide inclusions on environmentally assisted crack growth behaviour: fractographic aspects: Bulloch, J.H.Theor. Appl. Fract. Mech. Oct. 1988, 10, (2), 89–95

Three dislocation models are investigated for explaining the hydrogen embrittlement of refractory (b.c.c.) metal alloy single crystals and large-grain polycrystals on the basis of cleavage initiation. The models considered are: (1) stress-induced dislocation pile-ups and dislocation reactions; (2) hydrogen-enhanced decohesion of slip plane pile-ups and planar dislocation arrays; and (3) slip and hydrogen interactions with lineage subgrain boundaries. Emphasis is given in each case to the crystallographic features of low strain cleavage initiations in the presence of hydrogen as observed in three refractory alloy systems. Electron microscope observations, including the occurrence of deformation twinning, are presented for the hydrogen-charged materials as a function of strain. Cumulative (low energy) stress fields are described for the dislocation pile-ups and the lineage boundaries. The enhancement of hydrogen concentration in the presence of the stress fields of these dislocation configurations is determined. The slip-twinning relationship and cleavage associated with hydride microprecipitates are considered.