Post-testing measurement of freely movable and diffusible hydrogen in context of WEC formation at cylindrical roller thrust bearings from 100Cr6

Sub-surface crack networks in areas of altered microstructure are a common cause for bearing failures.Due to its appearance under light microscopy,the damage pattern is referred to as White Etching Cracks(WEC).The root causes leading to the formation of WEC are still under debate.Nevertheless,it has already been shown that atomic hydrogen can have an accelerating effect on the formation and propagation of WEC.In addition to hydrogen pre-charging,hydrogen can be released and absorbed during rolling/sliding due to the decomposing of the lubricant and water.The current work focuses on the analysis of the hydrogen content of cylindrical roller thrust bearings after testing in a FE8 type test rig using two different lubricants.Within the framework of this work,two different hydrogen analysis methods were used and assessed regarding their applicability.The results show that the so-called Hydrogen Collecting Analysis(HCA)is more suitable to investigate the correlation between lubricant chemistry and hydrogen content in the test bearings than the Local Hydrogen Analysis(LHA).The measurements with the HCA show a continuously increasing freely movable and diffusible hydrogen content under tribological conditions,which leads to the formation of WEC.Comparative tests with an oil without hydrogen showed that the tendency of the system to fail as a result of WEC can be reduced by using a lubricant without hydride compounds.

Hydrogen embrittlement of X80 pipeline steel in H2S environment: Effect of hydrogen charging time, hydrogen-trapped state and hydrogen charging–releasing–recharging cycles

This study investigated the susceptibility of X80 pipeline steel to hydrogen embrittlement given different hydrogen pre-charging times and hydrogen charging–releasing–recharging cycles in H2S environment.The fracture strain of the steel samples decreased with increasing hydrogen pre-charging time;this steel degradation could almost be recovered after diffusible hydrogen was removed when the hydrogen pre-charging time was<8 d.However,unrecoverable degeneration occurred when the hydrogen pre-charging time extended to 16–30 d.Moreover,nanovoid formation meant that the hydrogen damage to the steel under intermittent hydrogen pre-charging–releasing–recharging conditions was more serious than that under continuous hydrogen pre-charging conditions.This study illustrated that the mechanical degradation of steel is inevitable in an H2S environment even if diffusible hydrogen is removed or visible hydrogen-induced cracking is neglected.Furthermore,the steel samples showed premature fractures and exhibited a hydrogen fatigue effect because the repeated entry and release of diffusible hydrogen promoted the formation of vacancies that aggregated into nanovoids.Our results provide valuable information on the mechanical degradation of steel in an H2S environment,regarding the change rules of steel mechanical properties under different hydrogen pre-charging times and hydrogen charging–releasing–recharging cycles.

Molecular mechanics and dynamics simulation of hydrogen diffusion in aluminum melt

The main impurities in aluminum melt are hydrogen and Al_2O_3,which can deteriorate melt quality and materials performance.However,the diffusion process of H atoms in aluminum melt and the interactions among Al atoms,Al_2O_3 and hydrogen have been studied rarely.Molecular mechanics and dynamics simulations are employed to study the diffusion behaviors of different types of hydrogen,such as free H atoms,H atoms in H_2 and H^+ions in H_2O using COMPASS force field.Correspondingly,force field types h,h1h and h1o are used to describe different types of hydrogen which are labeled as H_h,H_(h1h) and H_(h1o).The results show that the adsorption areas are maximum for H_(h1o),followed by H_(h1h) and H_h.The diffusion ability of H_(h1o) is the strongest whereas H_h is hard to diffuse in aluminum melt because of the differences in radius and potential well depth of various types of hydrogen.Al_2O_3 cluster makes the Al atoms array disordered,creating the energy conditions for hydrogen diffusion in aluminum melt.Al_2O_3 improves the diffusion of H_h and H_(h1o),and constrains H_(h1h) which accumulates around it and forms gas porosities in aluminum.H_(h1o) is the most dispersive in aluminum melt,moreover,the distance of Al-H_(h1o) is shorter than that of Al-H_(h1h),both of which are detrimental to the removal of H_(h1o).The simulation results indicate that the gas porosities can be eliminated by the removal of Al_2O_3 inclusions,and the dispersive hydrogen can be removed by adsorption function of gas bubbles or molten fluxes.

Location-dependent effect of nickel on hydrogen dissociation and diffusion on Mg(0001)surface:Insights into hydrogen storage material design

Density functional theory(DFT)calculations have been performed to investigate the hydrogen dissociation and diffusion on Mg(0001)surface with Ni incorporating at various locations.The results show that Ni atom is preferentially located inside Mg matrix rather than in/over the topmost surface.Further calculations reveal that Ni atom locating in/over the topmost Mg(0001)surface exhibits excellent catalytic effect on hydrogen dissociation with an energy barrier of less than 0.05 eV.In these cases,the rate-limiting step has been converted from hydrogen dissociation to surface diffusion.In contrast,Ni doping inside Mg bulk not only does little help to hydrogen dissociation but also exhibits detrimental effect on hydrogen diffusion.Therefore,it is crucial to stabilize the Ni atom on the surface or in the topmost layer of Mg(0001)surface to maintain its catalytic effect.For all the case of Ni-incorporated Mg(0001)surfaces,the hydrogen atom prefers firstly immigrate along the surface and then penetrate into the bulk.It is expected that the theoretical findings in the present study could offer fundamental guidance to future designing on efficient Mg-based hydrogen storage materials.

Diffusion of Hydrogen along the Grain Boundaries in Ni_3Al Alloys

The diffusivity of hydrogen in two Ni3Al alloys (No.1 and No.2) has been measured in the temperature range of 100’C to 420’C using an ultrahigh vacuum gaseous permeation technique. The diffusivity data fall into two segments, in which the hydrogen diffusivity adheres to the Arrhenius form, respectively. From the hydrogen diffusivity, it is conjectured that the hydrogen diffusivity reflects the hydrogen transportation along the grain boundaries at lower temperature and the hydrogen transportation in the lattice at higher temperature. The intergranular fracture of L12-type intermetallics induced by hydrogen at relative low temperature results from hydrogen transportation along the grain boundaries and not in the lattice.

Hydrogen Diffusion in Amorphous Ti_(0.88)Ni_(1.00)Film

Hydrogen diffusion coefficients in amorphous Ti0.88Ni1.00 film were measured using electrochemical permeation technique. Diffusion coefficients increased with increasing hydrogen concentration. Activation energy of hydrogen diffusion was determined through measurement of the steady state anodic diffusion current density as a function of temperature, and an equation was derived to calculate the activation energy.

Study on hydrogen diffusion behavior in MlNi_(3.75) Co_(0.65) Mn_(0.4) Al_(0.2) alloy electrode by chronoamperometry

Hydrogen diffusion coefficients in MlNi 3.75 Co 0.65 Mn 0.4 Al 0.2 alloy electrode as a function of state of charge (SOC) or temperature were determined by chronoamperometry. It is found that hydrogen diffusion coefficient decreases with the increase of SOC or the decrease of temperature. The activation energy for hydrogen diffusion in the alloy electrode with 50%SOC is evaluated to be 19.9?kJ/mol.

Hydrogen Permeation and Diffusion in Amorphous Alloy Ni_(68)Cr7_Si_8B_(14)Fe_3 at Elevated Temperature

The behaviour of hydrogen permeation and diffusion in amorphous alloy Ni68Cr7Si8B14Fe3 hasbeen investigated by an ultrahigh vacuum gas permeation technique. A comparison experimentwas carried out between the as-quenched and annealed States (400℃/2h) of the amorphousalloy. The results show that, for both states of the amorphous alloy in the temperature rangeof 200～350℃, the diffusivity and permeability of hydrogen are in agreement with Arrheniusrelationship, there does not exist H-trapping effect, and the activation energies of diffusion andpermeation almost keep the same.

Underwater Laser Welding/Cladding for High-performance Repair of Marine Metal Materials:A Review

With the rapid developments of marine resource exploitation,mounts of marine engineering equipment are settled on the ocean.When it is not possible to move the damaged equipment into a dry dock,welding operations must be performed in underwater environments.The underwater laser welding/cladding technique is a promising and advanced technique which could be widely applied to the maintenance of the damaged equipment.The present review paper aims to present a critical analysis and engineering overview of the underwater laser welding/cladding technique.First,we elaborated recent advances and key issues of drainage nozzles all over the world.Next,we presented the underwater laser processing and microstructural-mechanical behavior of repaired marine materials.Then,the newly developed powder-feeding based and wire-feeding based underwater laser direct metal deposition techniques were reviewed.The differences between the convection,conduction,and the metallurgical kinetics in the melt pools during underwater laser direct metal deposition and in-air laser direct metal deposition were illustrated.After that,several challenges that need to be overcame to achieve the full potential of the underwater laser welding/cladding technique are proposed.Finally,suggestions for future directions to aid the development of underwater laser welding/cladding technology and underwater metallurgical theory are provided.The present review will not only enrich the knowledge in the underwater repair technology,but also provide important guidance for the potential applications of the technology on the marine engineering.

Review of Hydrogen Embrittlement in Metals: Hydrogen Diffusion, Hydrogen Characterization, Hydrogen Embrittlement Mechanism and Prevention

Hydrogen dissolved in metals as a result of internal and external hydrogen can affect the mechanical properties of the metals, principally through the interactions between hydrogen and material defects. Multiple phenomena such as hydrogen dissolution, hydrogen diffusion, hydrogen redistribution and hydrogen interactions with vacancies, dislocations, grain boundaries and other phase interfaces are involved in this process. Consequently, several hydrogen embrittlement(HE) mechanisms have been successively proposed to explain the HE phenomena, with the hydrogen-enhanced decohesion mechanism, hydrogenenhanced localized plasticity mechanism and hydrogen-enhanced strain-induced vacancies being some of the most important. Additionally, to reduce the risk of HE for engineering structural materials in service, surface treatments and microstructural optimization of the alloys have been suggested. In this review, we report on the progress of the studies on HE in metals, with a particular focus on steels. It focuses on four aspects:(1) hydrogen diffusion behavior;(2) hydrogen characterization methods;(3) HE mechanisms;and(4) the prevention of HE. The strengths and weaknesses of the current HE mechanisms and HE prevention methods are discussed, and specific research directions for further investigation of fundamental HE mechanisms and methods for preventing HE failure are identified.

Relationship between Hydrogen Diffusion and Blistering Nucleation and Growth

The formation condition of hydrogen blister in 18 Ni maraging steel without any inner or external stress was investigated.The results show that the critical diffusible hydrogen concentration of a blister forming is about 1.4×10^(-5),which is corresponding to the current density of 30mA/cm^2 during cathodic charging in a sodium hydroxide solution.For a 0.1cm thick sample,no matter the current density is equal to or much larger than the critical value,it spends at least about 132 hto form a hydrogen blister when hydrogen charging in single direction.It is approximately equal to the time for hydrogen atom to diffuse throughout the sample,which exactly depends on the hydrogen diffusion coefficient and the penetration depth.The very first clear suggestion was reported that the incubation period for hydrogen blister nucleation was necessary.According to the Fick′s laws,calculations show that the normalized hydrogen concentration in the escaping surface almost reaches 0.96 times of the charging surface,which means that the diffusion almost reaches a dynamic balance.A model was illustrated to describe the competitive relationship between hydrogen diffusion and blister formation.

Hydrogen embrittlement and failure mechanisms of multi-principal element alloys:A review

Multi-principal element alloys exhibit excellent physical,chemical and mechanical properties,and they are used as novel structural materials for potential applications in nuclear energy,hydrogen energy,and petrochemical fields.However,exposing components made of the alloys to service conditions related to the mentioned applications may induce hydrogen embrittlement(HE)as one of the typical failure mechanisms.In this review,we report and summarize the progress in understanding HE in multi-principal element alloys,with a particular focus on high-entropy alloys(HEAs).The review focuses on four aspects:(1)hydrogen migration behavior(hydrogen dissolution,hydrogen diffusion,and hydrogen traps);(2)factors affecting HE(hydrogen concentration,alloy elements and microstructure);(3)tensile mechanical properties in the presence of hydrogen and micro-damage HE mechanisms;(4)the design concept for preventing hydrogen-induced mechanical degradation.The differences in the HE behavior and failure mechanisms between HEAs and traditional alloys are compared and discussed.Moreover,specific research directions for further investigation of fundamental HE issues and a strategy for a simultaneous improvement in strength and HE resistance are identified.

Adsorption and Diffusion of Hydrogen Atoms on Stepped Ni (115) Surface

We have introduced a 5-parameter Morse function to simulate the pairwise poten-tial and studied the adsorption and diffusion of hydrogen atoms on the Ni low indexsurfaces by pairwise method and satistying results were obtained. In this letter,we further investigate the properties of the adsorption and diffusion of hydrogen at-oms on the Ni (115) stepped surface by the same method and the optimumparameters.