Role of chlorides on pitting and hydrogen embrittlement of Mg–Mn wrought alloy

The role of chlorides on stress corrosion cracking behavior of Mg–Mn hot rolled alloy was studied in Mg(OH)₂ saturated, 0.01 M and 0.1 M NaCl solutions. The alloy was found to fail by hydrogen embrittlement mechanism both in presence and absence of chlorides. However, the role of chloride has been found to be to damage the passive film, cause pitting and increasing hydrogen embrittlement tendency of the alloy. Crack initiation occurred through pitting and grew in a transgranular manner involving hydrogen.     

Effect of nickel equivalent on hydrogen gas embrittlement of austenitic stainless steels based on type 316 at low temperatures

The effect of nickel equivalent on hydrogen gas embrittlement (HGE) of austenitic stainless steels of Fe–(10–20)Ni–17Cr–2Mo alloys vacuum-melted in a laboratory, based on type 316 stainless steel, was investigated. Tensile tests were conducted in hydrogen and helium at 1 MPa in the temperature range from 80 to 300 K. It was found that HGE of the alloys below a nickel equivalent of 27% increased with decreasing temperature, reached a maximum at 200 K, and then decreased with further decreasing temperature, whereas no HGE occurred above the nickel equivalent of 27%. It was observed that the content of strain-induced α′ martensite increased with decreasing temperature and nickel equivalent, and hydrogen-induced fracture occurred mainly along α′ martensite structure. Thus, the susceptibility to HGE depended on nickel equivalent. It was discussed that HGE was controlled by strain-induced α′ martensite above 200 K, whereas it was controlled by hydrogen transport below 200 K.     

Delayed static failure of twinning-induced plasticity steels

Delayed static failure of high-Mn twinning-induced plasticity (TWIP) steels containing various Al contents (0–3.5 wt.%) was studied in the context of hydrogen embrittlement. The roles of residual stress and texture on hydrogen embrittlement were discussed. In the deformed state, Al-added TWIP steels exhibited much better delayed fracture resistance as compared to 0 Al steel, owing to the decrease in the number of diffusible hydrogen-trapping sites coupled with smaller residual stress and strong 〈1 1 1〉 and 〈1 0 0〉 textures.     

Hydrogen diffusion coefficients through Inconel 718 in different metallurgical conditions

We report hydrogen permeation studies through cold rolled, solutionized, and precipitation hardened Inconel 718 foils. The effective hydrogen diffusion coefficient is considerably higher (5.3–6.8 × 10⁻¹¹ cm²/s) for the solutionized Inconel 718 than for either the cold rolled (3.3–4.2 × 10⁻¹¹ cm²/s) or precipitation hardened (2.1–2.9 × 10⁻¹¹ cm²/s) specimens. Microstructural studies indicate that the reduced hydrogen diffusion coefficients in the latter specimens arise from hydrogen trapping at dislocations and precipitates that are present at much lower concentrations in the solutionized specimens. Also, repeated permeation transients provide evidence for irreversible hydrogen trapping in the cold rolled and precipitation hardened specimens, but such effects are insignificant in the solutionized specimens.     

Impact of selective oxidation during inline annealing prior to hot-dip galvanizing on Zn wetting and hydrogen-induced delayed cracking of austenitic FeMnC steel

The present study discusses the impact of selective oxidation during in-line annealing of Fe–23%Mn–0.6%C–0.3%Si steel on surface and sub-surface properties and is focused on hot-dip galvanizability and susceptibility to hydrogen-induced delayed cracking. Annealing temperature (700–1100 °C) and dewpoint DP (? 15/?30/?50 °C) of the 5%H₂–N₂ annealing atmosphere were varied in order to investigate Zn wetting in dependence on selective oxidation of Mn and Si. Sub-surface microplasticity (hardness, pop-in frequency, pop-in activation load) was examined by electrochemical nanoindentation in-situ to hydrogen charging (ECNI) to assess hydrogen/material interactions. Zn wetting fails if external Mn and Si oxidation is not avoided by performing high reductive bright annealing (1100 °C/DP ? 50 °C). Zn wetting will however turn to increase if a roughly globular MnO layer appears and Si is internally oxidized (700–900 °C/DP ? 15 °C). Selective oxidation further affects hydrogen/material interactions by influencing the local distribution of solid-soluted Mn: ECNI results indicate hydrogen-induced dislocation demobilization (HEDE mechanism) or dislocation mobilization (HELP mechanism) in dependence on the local amount of solid-soluted Mn within the sub-surface. Macroscopic delayed cracking seems to occur earlier if HELP is predominating. The gained results benefit understanding the impact of selective oxidation on galvanizability and susceptibility to hydrogen-induced failure of austenitic FeMnC steel and advance further developments in processing high Mn alloyed steels.     

Hydrogen embrittlement of ferritic steels: Observations on deformation microstructure,nanoscale dimples and failure by nanovoiding

While hydrogen embrittlement of ferritic steels has been a subject of significant research, one of the major challenges in tackling hydrogen embrittlement is that the mechanism of embrittlement is not fully resolved. This paper reports new observations and interpretation of fracture surface features and deformation microstructures underneath the fracture surface, providing a mechanistic view of failure catalyzed by hydrogen. Linepipe grade ferritic steels were tested in air with electrochemically pre-charged hydrogen and in high-pressure H₂ gas. The fracture surface features were studied and compared using high-resolution surface-sensitive scanning electron microscopy, and the deformation microstructures just beneath the fracture surfaces were studied using transmission electron microscopy. Significant dislocation plasticity was observed just beneath both ductile and quasi-brittle fracture surfaces. Further, the dislocation activity just beneath the fracture surfaces was largely comparable with those observed in samples tested without hydrogen. Evidence for hydrogen-enhanced plastic flow localization and shear softening on the sub-micron scale was observed very near the final fracture surface (<2 μm) in the tensile samples. The quasi-brittle fracture surfaces were found to be covered with nanoscale dimples 5–20 nm wide and 1–5 nm deep. Based on analyses of conjugate fracture surfaces, most of the nanodimples appear to be “valley-on-valley” type, rather than “mound-on-valley” type, indicating nanovoid nucleation and growth in the plastically flowing medium prior to ultimate failure. Based on these observations, an alternative scenario of plasticity-generated, hydrogen-stabilized vacancy damage accumulation and nanovoid coalescence as the failure pathway for hydrogen embrittlement is proposed.     

Effects of temperature and the incorporation of W on the oxidation of ZrB2 ceramics

The effects of tungsten additions and temperature on the oxidation behavior of nominally pure ZrB₂ and ZrB₂ containing 4, 6 or 8 mol% of W after oxidation at temperatures ranging from 800 to 1600 °C were investigated. For pure ZrB₂, the protective liquid/glassy layer covering the surface as a result of oxidation was evaporated above 1500 °C. For (Zr,W)B₂ specimens, the liquid/glassy layer was present after exposure up to 1600 °C. The higher stability of the liquid/glassy phase in the W-containing compositions was attributed to the presence of tungsten in the liquid/glassy phase, resulting in improved oxidation resistance for ZrB₂ samples containing W.     

Liquid metal embrittlement of aluminium by segregation of trace element gallium

The effect of small amounts of gallium on liquid metal embrittlement of model binary AlGa alloys containing 50–1000 ppm² Ga is studied. Ga segregation did not occur by annealing in the temperature range 300–600 °C because of the high solid solution solubility of Ga in aluminium. Alkaline etching caused significant enrichment of Ga at the surface by dealloying. Diffusion of Ga from the surface into the grain boundaries caused liquid metal embrittlement of samples containing at least 250 ppm Ga. Segregated Ga dissolved back into aluminium by annealing for 1 h at 600 °C after etching, eliminating the grain boundary embrittlement.     

Grain-boundary engineering markedly reduces susceptibility to intergranular hydrogen embrittlement in metallic materials

The feasibility of using “grain-boundary engineering” techniques to reduce the susceptibility of a metallic material to intergranular embrittlement in the presence of hydrogen is examined. Using thermomechanical processing, the fraction of “special” grain boundaries was increased from 46% to 75% (by length) in commercially pure nickel samples. In the presence of hydrogen concentrations between 1200 and 3400 appm, the high special fraction microstructure showed almost double the tensile ductility; also, the proportion of intergranular fracture was significantly lower and the J_c fracture toughness values were some 20–30% higher in comparison with the low special fraction microstructure. We attribute the reduction in the severity of hydrogen-induced intergranular embrittlement to the higher fraction of special grain boundaries, where the degree of hydrogen segregation at these boundaries is reduced.     

Corrosion and hydrogen absorption of commercially pure zirconium in acid fluoride solutions

The corrosion and hydrogen absorption of commercially pure zirconium have been investigated in acidulated phosphate fluoride (APF) solutions. Upon immersion in 2.0% APF solution of pH 5.0 at 25 °C, a granular corrosion product (Na₃ZrF₇) deposits over the entire side surface of the specimen, thereby inhibiting further corrosion. In 0.2% APF solution, marked corrosion is observed from the early stage of immersion; no deposition of the corrosion product is observed by scanning electron microscopy. A substantial amount of hydrogen absorption is confirmed in both APF solutions by hydrogen thermal desorption analysis. The amount of absorbed hydrogen of the specimen immersed in the 2.0% APF solution is smaller than that in the 0.2% APF solution in the early stage of immersion. The hydrogen absorption behavior is not always consistent with the corrosion behavior. Hydrogen thermal desorption occurs in the temperature range of 300–700 °C for the specimen without the corrosion product. Under the same immersion conditions, the amount of absorbed hydrogen in commercially pure zirconium is smaller than that in commercially pure titanium as reported previously. The present results suggest that commercially pure zirconium, compared with commercially pure titanium, is highly resistant to hydrogen absorption, although corrosion occurs in fluoride solutions.     

Hydrogen-assisted failure in a twinning-induced plasticity steel studied under in situ hydrogen charging by electron channeling contrast imaging

We investigated the hydrogen embrittlement of a Fe–18Mn–1.2%C (wt.%) twinning-induced plasticity steel, focusing on the influence of deformation twins on hydrogen-assisted cracking. A tensile test under ongoing hydrogen charging was performed at low strain rate (1.7 × 10^?6 s^?1) to observe hydrogen-assisted cracking and crack propagation. Hydrogen-stimulated cracks and deformation twins were observed by electron channeling contrast imaging. We made the surprising observation that hydrogen-assisted cracking was initiated both at grain boundaries and also at deformation twins. Also, crack propagation occurred along both types of interfaces. Deformation twins were shown to assist intergranular cracking and crack propagation. The stress concentration at the tip of the deformation twins is suggested to play an important role in the hydrogen embrittlement of the Fe–Mn–C twining-induced plasticity steel.     

Role of microstructure, composition and hardness in resisting hydrogen embrittlement of fastener grade steels

The degree of hydrogen embrittlement for several fastener grade steels has been determined. While microstructural alteration resulted in some improvement in resistance to hydrogen embrittlement, the overriding factor contributing to susceptibility of the steel was strength. The degree of susceptibility of the microstructures to hydrogen embrittlement, ranked in increasing order, is as follows: fine pearlite, bainite, tempered martensite. The effects of alloying were also assessed by comparing results from different fastener grade steels with similar microstructures. In most cases, the alloy chemistry had little effect, presumably due to trap saturation associated with this testing technique.     

Moisture-induced embrittlement mechanism for (Ni,Fe)Ti alloys

Recent experimental studies showed that the ductility of NiTi is not affected by moisture, while addition of iron beyond 9 a/o in NiTi leads to moisture-induced embrittlement. To explore the nature of this embrittlement, we studied the chemical interaction between water vapor and (Ni,Fe)Ti(110) surfaces with 5 a/o and 10 a/o Fe. Temperature-programmed desorption and X-ray photoelectron spectroscopy show that decomposition of water to produce atomic hydrogen occurs on both surfaces. Activation energy for surface diffusion was calculated by density functional theory, showing that addition of Fe decreases H surface diffusivity, in agreement with experiment. Together with the observation that addition of 9 a/o Fe increases the strength of NiTi, this indicates that moisture-induced embrittlement in higher strength NiTi alloys is not due to faster H surface diffusion, but lower critical hydrogen concentration required for embrittlement.     

Effect of microstructure on the sulphide stress cracking susceptibility of a high strength pipeline steel

The sulphide stress cracking (SSC) susceptibility of a newly developed high strength microalloyed steel with three different microstructures has been evaluated using the slow strain rate testing (SSRT) technique. Studies were complemented with potentiodynamic polarization curves and hydrogen permeation measurements. Material included a C–Mn steel having Ni, Cu, and Mo as main microalloying elements with three microstructures: martensitic, ferritic and ferritic + bainitic. Testing temperatures included 25, 50, 70 and 90 °C. Detailed SEM observations of the microstructure and fracture surfaces were done to identify possible degradation mechanisms. The results showed that in all cases, the corrosion rate, number of hydrogen atoms at the surface and the percentage reduction in area increased with temperature. The steel with a martensitic microstructure had the highest SSC susceptibility at all temperatures, whereas the ferritic steels were susceptible only at 25 °C, and the most likely mechanism is hydrogen embrittlement assisted by anodic dissolution.     

Precipitation in ductile Fe–18Al–5Cr alloys with additions of Mo,W and C and effects on high-temperature strength

The strength of a Fe–Al-based alloy containing small additions of Mo, W and C has been determined from room temperature up to 800 °C, and the strain rate dependence of strength examined. Strength of the as-cast material is maintained at above 600 °C, but it is lost at higher temperatures, especially at slow strain rates. This behaviour is largely explained by solute hardening effects, with no sign of any precipitates forming. After a solution treatment, annealing material at 800 °C leads to the appearance of Fe–Mo–W carbides which provide better strength under conditions of high temperature and slow strain rate. The possibilities for improving high-temperature strength and creep behaviour by the formation of carbide or intermetallic precipitates are discussed.     

Austenitic steels of different composition in liquid and gaseous hydrogen

The influence of liquid as well as gaseous hydrogen and temperature (?253 to 100 °C) on proof and ultimate strength as well as elongation after fracture and reduction of area at fracture for AISI 304, 304L, TP304L, 304LN, TP316NG, 316LN, 321 and 347 was investigated by constant extension rate tests. The effect of temperature on hydrogen embrittlement could be demonstrated. With respect to the dependence on alloy composition seen at 22 °C, it is concluded that the main alloy element to look at, if hydrogen embrittlement has to be considered, is nickel accompanied by carbon and nitrogen. The critical region of nickel content seems to be 10.5–11 wt.%.     

Effect of hydrogen on the properties and fracture characteristics of TRIP 800 steels

The paper describes effect of hydrogen on the properties and fracture characteristics of two variants of TRIP 800 C–Mn–Si steels. The effect of hydrogen was studied by means of tensile tests on specimens previously charged by hydrogen. Hydrogen provoked embrittlement in both variants but only for very high hydrogen content. Hydrogen embrittlement manifested itself mainly by a loss of plasticity. Both steel variants were able to absorb a large amount of hydrogen, up to 50 ppm. Concerning fractographic characteristics, steels containing higher hydrogen content displayed transgranular cleavage fracture. In exceptional cases, an irreversible embrittlement was revealed initiating on non-metallic inclusions.     

First approach for thermodynamic modelling of the high temperature oxidation behaviour of ternary γ′-strengthened Co–Al–W superalloys

In the present work, thermodynamic modelling of the high temperature oxidation behaviour of a γ′-strengthened Co-base superalloy is presented. The ternary Co–9Al–9W alloy (values in at%) was isothermally oxidised for 500 h at 800 and 900 °C in air. Results reveal that the calculated oxide layer sequence (Thermo-Calc, TCNI6) is in good agreement with the formed oxide scales on the alloy surface. Furthermore, prediction of the influence of oxygen partial pressure on Al₂O₃ formation is presented. The modelling results indicate pathways for alloy development or possible pre-oxidation surface treatments for improved oxidation resistance of the material.     

Effects of ZrB2 and SiC dual addition on the oxidation resistance of graphite at 1600–2000 °C

The effects of ZrB₂ and ZrB₂ + SiC additions on the oxidation kinetics of graphite at 1600–2000 °C in air were investigated. The ZrB₂ + SiC dual addition improves the oxidation resistance of graphite more effectively than the ZrB₂ single addition, because the oxide scale formed on C–ZrB₂–SiC is denser and thinner due to the existence of glassy SiO₂. As the oxidation temperature increases, the oxidation rate of C–ZrB₂–SiC gradually increases and oxide scales with layered microstructures form on its surface due to the greatly enhanced active oxidation of SiC at higher temperatures.     

Fatigue crack growth of two pipeline steels in a pressurized hydrogen environment

Fatigue crack growth tests were conducted on two pipeline steel alloys, API 5L X52 and X100. Baseline tests were conducted in air, and those results were compared with tests conducted in pressurized hydrogen gas. All tests were run at (load ratio) R = 0.5 and a frequency of 1 Hz, except for one test on X100, run at 0.1 Hz. Tests were conducted at hydrogen pressures of 1.7 MPa, 7 MPa, 21 MPa, and 48 MPa. Fatigue crack growth rates for both X100 and X52 were significantly higher in a pressurized hydrogen environment than in air. This enhanced growth rate appears to correlate to pressure for X100 but may not for X52.