The effect of stress state on the hydrogen embrittlement of nickel

The ductility of nickel sheet subjected to in situ cathodic hydrogen charging has been investigated over a range of multiaxial stress states including uniaxial, plane-strain, and equibiaxial tension. The data show that the extent of ductility loss due to the presence of hydrogen increases as the stress state tends from uniaxial to equibiaxial tension. In all instances, the hydrogen embrittlement is characterized by intergranular fracture with failure occurring due to microcrack formation, microcrack link-up, and macrocrack growth. The increased susceptibility of nickel to intergranular hydrogen embrittlement with increasing biaxiality of stress state is shown to be a consequence of an enhanced rate of the link-up of strain-induced intergranular microcracks.     

Influence of sulfur,phosphorus, and antimony segregation on the intergranular hydrogen embrittlement of nickel

The effectiveness of sulfur, phosphorus, and antimony in promoting the intergranular embrittlement of nickel was investigated using straining electrode tests in IN H₂SO₄ at cathodic potentials. Sulfur was found to be the critical grain boundary segregant due to its large enrichment at grain boundaries (10⁴ to 10⁵ times the bulk content) and the direct relationship between sulfur coverage and hydrogen-induced intergranular failure. Phosphorus was shown to be significantly less effective than sulfur or antimony in inducing the intergranular hydrogen embrittlement of nickel. The addition of phosphorus to nickel reduced the tendency for intergranular fracture and improved ductility because phosphorus segregated strongly to grain interfaces and limited sulfur enrichment. The hydrogen embrittling potency of antimony was also less than that of sulfur while its segregation propensity was considerably less. It was found that the effectiveness of segregated phosphorus and antimony in prompting intergranular embrittlementvs that of sulfur could be expressed in terms of an equivalent grain boundary sulfur coverage. The relative hydrogen embrittling potencies of sulfur, phosphorus, and antimony are discussed in reference to general mechanisms for the effect of impurity segregation on hydrogen-induced intergranular fracture.     

Hydrogen embrittlement of ultra-pure alloys of the inconel 600 type: Influence of the additions of elements (C,P, Sn,Sb)

The mechanical behavior of very high purity nickel base alloys of the Inconel 600 type that were simultaneously charged with hydrogen and deformed in tension was investigated. Experimental results show that this procedure decreases markedly the fracture strain of the pure 76 pct Ni-16 pct Cr-8 pct Fe alloy; cracks are observed after two to four pct elongation, and the fracture is completely intercrystalline. Hydrogen embrittlement appears as an intrinsic property of the Ni-Cr-Fe system in the sense that the grain boundary cohesion decreases when the purity of the alloy increases. The presence of carbon or phosphorus in the alloys increases grain boundary cohesion. The addition of metallic elements such as antimony or tin has relatively little effect on intergranular embrittlement.     

The effect of aging on hydrogen embrittlement of a nickel alloy

The effect of aging at 500° C on the hydrogen embrittlement tendency of a cold worked Ni-base superalloy was investigated in a series of experiments which included hydrogen charging studies, mechanical tests in hydrogen and in air, and fractographic and slip line investigations. Embrittlement tendency increased (time-to-failure decreased) markedly during the first hour of aging and then remained constant until about 1000 h aging time, whereupon it increased rapidly again. The short-time embrittlement could be accounted for either by a mechanism involving segregation of P to grain boundaries or by one involving planar slip induced by short-range order. The hydrogen charging studies indicated that hydrogen uptake decreases during aging, a result which is not consistent with the P segregation hypothesis. The increase in embrittlement at long aging times is most readily explained in terms of planar slip induced by long range order. Tensile tests over a range of strain rates suggested that accelerated transport of hydrogen by dislocation dragging of hydrogen atmospheres is involved in embrittlement. Formerly Graduate Assistant, Dept. of Met. Eng. & Mat. Sci., University of Notre Dame     

An investigation of grain-boundary embrittlement in Fe−P,Fe−P−S,and Fe−Sb−S alloys

Grain-boundary embrittlement of pure iron due to phosphorous, antimony and sulfur is studied using fracture appearance transition temperature measurements and Auger electron emission spectroscopy chemical analysis of fractured surfaces. Phosphorus and sulfur are found to segregate to grain boundaries in the entire ferrite range. Segregation to grain boundaries of these elements in the austenite appears to be negligible. It is shown that the segregation of these elements to grain boundaries in iron does not conform to the Gibb's equilibrium segregation model. Sulfur appears to be a more severe embrittler of iron than does phosphorous. P. V. RAMASUBRAMANIAN formerly was Graduate Research Assistant, University of Minnesota. D. F. STEIN, formerly Professor, Department of Mechanical and Chemical Engineering, University of Minnesota     

The effect of fe additions on the embrittlement of Cu-Bi alloys

The effect of iron additions on the embrittlement of Cu-Bi alloys was studied by monitoring the ductility and grain boundary chemistry of embrittled specimens as a function of iron content. Mechanical properties improved for the same embrittling heat treatment as the bulk iron level increased, and this was correlated with a decrease in bismuth segregation to the grain boundaries. No iron was detected segregated to the boundaries, and several possible mechanisms were proposed to explain the beneficial effect of the iron additions. It was also found that approximately 70 min at 530°C is required to attain equilibrium for segregation of bismuth to the grain boundaries, and a diffusion coefficient derived from this data was found to be reasonable for bulk diffusion of bismuth in copper.     

The effect of fe additions on the embrittlement of Cu-Bi alloys

The effect of iron additions on the embrittlement of Cu-Bi alloys was studied by monitoring the ductility and grain boundary chemistry of embrittled specimens as a function of iron content. Mechanical properties improved for the same embrittling heat treatment as the bulk iron level increased, and this was correlated with a decrease in bismuth segregation to the grain boundaries. No iron was detected segregated to the boundaries, and several possible mechanisms were proposed to explain the beneficial effect of the iron additions. It was also found that approximately 70 min at 530°C is required to attain equilibrium for segregation of bismuth to the grain boundaries, and a diffusion coefficient derived from this data was found to be reasonable for bulk diffusion of bismuth in copper.     

Effects of B,C, and Zr on the structure and properties of a P/M nickel base superalloy

The boron and carbon levels of a P/M nickel base superalloy were systematically varied in order to determine the mechanisms by which these elements strengthen the alloy, and their optimum concentration. Carbon levels were reduced to 20 ppm while the boron level was varied from 0.02 to 0.10 wt pct. Carbon levels of 0.002 and 0.05 wt pct were also studied, while maintaining a boron concentration of 0.02 wt pct. Zirconium levels were maintained at 0.06 wt pct. The resulting alloys were subjected to identical heat treatments and examinedvia SEM, TEM, and STEM microscopy. The alloys were also subjected to tensile, creep, stress-rupture, and fatigue crack growth tests. Results show that both carbon and boron have a strong influence on the formation of grain boundary precipitates, as expected. Carbon was present as the MC and M₂₃C₆ type carbides, while boron combined to form an intergranular M₃B₂ boride. Boron and zirconium were observed to be critical to the alloys' mechanical properties, although boron levels above the solubility limit resulted in no further improvement or debit in strength. Carbon additions resulted in no improvement in properties, indicating the feasibility of a carbon-free P/M superalloy. The role of the minor element additions is discussed in terms of both microstructural features and related strengthening mechanisms.     

The intergranular embrittlement of nickel by hydrogen: The effect of grain boundary segregation

The mechanical behavior of polycrystalline nickel specimens that were deformed in tension and cathodically charged with hydrogen simultaneously was investigated with particular emphasis on the fracture of such electrodes. This procedure leads to definite, if, however, weak serrated yielding and also markedly reduces the elongation at fracture compared to polycrystals unexposed to hydrogen. Moreover, in contrast to hydrogenated nickel monocrystals which neck down to give a chisel-edge fracture typical of ductile metals, hydrogenated polycrystal fractures are brittle and intergranular. The embrittlement of nickel by hydrogen is shown by means of Auger electron spectroscopy to be associated with the segregation of hydrogen recombination poisons to the grain boundaries. In essence, it is suggested that the entry of hydrogen into the nickel specimens occurs preferentially in the proximity of grain boundary intersections with the free surface, due to the presence therein of Sb and Sn which act as hydrogen recombination poisons and stimulate the absorption of hydrogen by the metal. The presence of such impurities in the grain boundaries suggests that a pressure mechanism is not involved in the intergranular cracking.     

An example of the effect of hydrogen trapping on hydrogen embrittlement

The role of internal hydrogen in reducing the tensile reduction of area of iron-titanium alloys is examined. The population of hydrogen at potential crack nucleii is shown to be controlled by its dynamic interaction with mobile dislocations and its subsequent transport to fixed traps. Expressions are developed for both the number of hydrogen atoms at a given irreversible trap, as well as the time necessary to reach such a number. Reducing the number below the critical value to nucleate a crack, or increasing the time to reach this value will improve an alloy’s performance, and this improvement is related to controllable external and metallurgical variables. These predictions of the model are shown to be consistent with companion experimental data, and with the trap theory of hydrogen embrittlement.     

Effects of zinc coatings on the stress corrosion cracking and hydrogen embrittlement of low-alloy steel

The effects of electroplated and hot-dip zinc coatings on the fracture of low-alloy steel AISI 4140 bars tempered to hardnesses in the range Rc 33 to 49 were studied. Either electroplated or hot-dip zinc coatings decrease resistance to stress corrosion cracking,i.e., they reduceK _sc, the threshold stress intensity for stress corrosion cracking in 3.5 wt pct NaCl solution. AboveK _scelectroplated-zinc coatings do not appear to affect the crack-growth rate, although the incubation period prior to the onset of crack growth is reduced. Hot-dip zinc coatings increase stress corrosion crack growth rates slightly because of the additive effect of internal dissolved hydrogen. Hot-dip zinc coatings reduce the critical stress intensity for fracture in the absence of a corrosive environment because of embrittlement by internal hydrogen which is released from traps during hot-dip coating and confined by the inter metallic coatings which form on the steel surface in the hot dip bath. A simple fracture mechanics analysis indicates that either increasing diameter or the presence of a zinc coating lowers the critical hardness at which the stress corrosion cracking of structural bolts can occur.     

A comparison of gaseous hydrogen embrittlement,slow-strain-rate hydrogen embrittlement,and stress-corrosion cracking InTi-8Ai-1Mo-1V

Metallographic, fractographic, and acoustic-emission studies have been carried out on the near-α commercial alloy, heat treated to produce the Widmanstätten structure and in some cases aged to precipitate α₂. Both aged and unaged material underwent GHE in ～ 10 kPa gaseous hydrogen, failure occurring along the α-β interface. Electron-diffraction studies established the presence of a layer of fee titanium hydride at the fracture surfaces, and acoustic-emission and fractographic observations indicated that propagation was discontinuous. The aged alloy underwent SSRHE in inert environments and SCC in 3 pct aqueous NaCl and, in contrast to GHE, failure occurred across the α-plates in both cases, producing indistinguishable cleavage-like fracture surfaces. Again, titanium hydride was detected at the fracture surfaces and, from acoustic-emission studies, crack propagation appeared to be discontinuous. Based on these observations, it is suggested that the three forms of failure occur by a common mechanism, namely by the repeated formation and rupture of the hydride phase.     

Effects of zinc coatings on the stress corrosion cracking and hydrogen embrittlement of low-alloy steel

The effects of electroplated and hot-dip zinc coatings on the fracture of low-alloy steel AISI 4140 bars tempered to hardnesses in the range Rc 33 to 49 were studied. Either electroplated or hot-dip zinc coatings decrease resistance to stress corrosion cracking,i.e., they reduceK _sc, the threshold stress intensity for stress corrosion cracking in 3.5 wt pct NaCl solution. AboveK _scelectroplated-zinc coatings do not appear to affect the crack-growth rate, although the incubation period prior to the onset of crack growth is reduced. Hot-dip zinc coatings increase stress corrosion crack growth rates slightly because of the additive effect of internal dissolved hydrogen. Hot-dip zinc coatings reduce the critical stress intensity for fracture in the absence of a corrosive environment because of embrittlement by internal hydrogen which is released from traps during hot-dip coating and confined by the inter metallic coatings which form on the steel surface in the hot dip bath. A simple fracture mechanics analysis indicates that either increasing diameter or the presence of a zinc coating lowers the critical hardness at which the stress corrosion cracking of structural bolts can occur.     

Effects of grain-boundary segregation and precipitation on the hydrogen susceptibility of nickel

The straining electrode tensile test was employed to study hydrogen embrittlement in three grades of nickel. Aging at intermediate temperatures (400 to 800 °C) after solution annealing caused sulfur segregation to grain boundaries in the materials. This sulfur segregation was found to influence intergranular hydrogen embrittlement, and the effect was most prominent when the surface supply of hydrogen was low and when the grain boundaries were free of graphite precipitates. Aging also induced graphite precipitation at the grain boundaries of one grade of nickel (Nickel 200). The presence of these grain-boundary particles reduced susceptibility to hydrogen embrittlement, but at the same time caused a form of aging embrittlement. The effect of the second-phase precipitate, when present, masked the effect of sulfur segregation on hydrogen susceptibility. Formerly with Bell-Northern Research, Ottawa, ON, Canada.     

Hydrogen embrittlement of amorphous alloys based on iron and nickel

Hydrogen embrittlement of amorphous alloys based on iron and nickel was examined in constant extension rate tests during cathodic polarization. Tests were carried out at a current density of 50 A·m⁻² in 0.1 M H₂SO₄ with and without addition of NaAsO₂ and NaCl. Hydrogen permeation and polarization curves were measured. Differences in the degree of hydrogen embrittlement were ascribed mainly to differences in the entry of hydrogen into the alloys. It was suggested that a low degree of embrittlement resulted largely from a hindrance of hydrogen entry. For alloys containing phosphorus, this might be associated with phosphorus-containing oxyanions and salts. For the alloy with higher silicon content it may be associated with the formation of oxides of silicon.     

溶解氧变化对阴极极化下10Ni5CrMo钢氢脆敏感性的影响

摘要：为研究溶解氧质量浓度对10Ni5CrMo钢在阴极极化条件下氢脆敏感性影响规律，对10Ni5CrMo钢进行了阴极极化下的电化学交流阻抗谱测试﹑并采用慢应变速率拉伸实验和断口分析方法研究了海水中溶解氧质量浓度变化和不同阴极极化下10Ni5CrMo钢的氢脆敏感性。结果表明：溶解氧质量浓度变化对10Ni5CrMo钢强度几乎没有影响；同一溶解氧质量浓度下，随极化电位负移，断裂时间、伸长率、断面收缩率明显降低，氢脆系数增加，氢脆敏感性显著提高，极化电位达到-1000mV时，氢脆系数已超过安全区允许的最高值25%，进入危险区；同一极化电位下，随着海水中溶解氧质量浓度减少，材料塑性变差，断裂时间、伸长率和断面收缩率不断降低，氢脆系数增加，氢脆敏感性提高。     

Effects of hydrogen and carbon on thermally activated deformation in nickel

Deformation experiments were performed to determine the effects of hydrogen and carbon on the activation parameters for dislocation slip in nickel. The techniques used were isothermal stress relaxation and differential temperature measurements. These methods allowed determination of the activation enthalpy and activation area for dislocation motion in nickel and the effects of hydrogen and carbon on these parameters. The results show that hydrogen increases the dislocation mobility in Ni and in NiC alloys by reducing the activation enthalpy for dislocation motion, while carbon reduces the dislocation mobility by increasing the activation enthalpy. Hydrogen solutes decrease the activation area for dislocation motion in both pure Ni and in NiC alloys.     

Kinetics of solution of hydrogen in liquid iron,nickel, and copper containing dissolved oxygen and sulfur

An investigation of the effect of surface active oxygen and sulfur on the rate of hydrogen solution in inductively stirred liquid iron, nickel, and copper was made using a modified constant-volume Sieverts’ method. The overall and initial rates of hydrogen solution in liquid iron and nickel were found to decrease with increasing oxygen content in concentration ranges found in the commercial refining of these metals. The results were consistent with a change in the rate controlling step from mass transport of hydrogen atoms in the metal phase to mixed control of a surface chemical reaction and a diffusion controlled process in the melt. For liquid copper the overall rate of hydrogen solution decreased with increasing oxygen content, but the initial rate could not be determined. It was believed that condensation of water vapor and melt surface turbulence interfered with measurement of the initial rate of hydrogen solution in liquid copper. The addition of sulfur to liquid iron, nickel, and copper lowered the overall rate of hydrogen solution, but the effect was not as pronounced as that for oxygen. Formerly Graduate Student, The University of Michigan, is Research Engineer, Noranda Research Center, Pointe Claire, Quebec, Canada