Internal polymeric coating materials for preventing pipeline hydrogen embrittlement and a theoretical model of hydrogen diffusion through coated steel

This work develops a theoretical analysis of the coating permeability necessary for use as internal coatings of transmission pipelines to prevent hydrogen embrittlement. Internal coating materials suitable to be applied in situ on existing steel pipelines are also evaluated. Twelve different commercially available coatings; crosslinked poly (vinyl alcohol) (PVA), poly (vinyl chloride) and bisphenol A diglycidyl ether (DGEBA)/polyetheramine (D-400) epoxy coatings prepared in-house were tested. Films fabricated from two commercial epoxies had hydrogen permeability of 0.40 Barrer and 0.35 Barrer respectively, which show potential as coating materials. A hydrogen permeability of 0.0084 Barrer was achieved with a crosslinked poly (vinyl alcohol) coating, indicating that this material shows the highest potential of all coatings tested. Unsteady-state hydrogen diffusion through coated steel was then modeled to evaluate the effect of the coating film in reducing hydrogen embrittlement. The result shows that with a 2 mm PVA coating, hydrogen permeation inside the coating will take seven years to reach equilibrium and the final hydrogen concentration on the steel surface will be 44% lower than that without a coating. Greater protection can be provided if coatings can be developed with lower hydrogen permeability.     

Study on the hydrogen barrier performance of the SiOC coating

SiOC coatings were prepared on X70 pipeline steel substrate by a simple dipping method at low temperatures, and their performance of hindering hydrogen penetration was studied through electrochemical hydrogen permeation experiment. The sample thermal-treated at 120 °C achieved a low diffusion coefficient of hydrogen of 8.20 × 10^?9 cm² s^?1, which was nearly three orders of magnitude lower than 3.58 × 10^?6 cm² s^?1 for the X70 steel. This was due to that the amorphous coating did not provide a stable hydrogen diffusion channel, thus limiting hydrogen diffusion. Density functional theory (DFT) calculation further proved that hydrogen moleculars were difficult to be adsorbed at different sites on the surface of the coating.     

Hydrogen permeation behavior of X70 pipeline steel simultaneously affected by tensile stress and sulfate-reducing bacteria

The hydrogen permeation behavior of submarine pipelines buried in anoxic sea mud and protected by cathodic potential is affected by both sulfate-reducing bacteria (SRB) and tensile stress. In this study, the individual and simultaneous effects of SRB and tensile stress on hydrogen permeation parameters were investigated using an electrochemical hydrogen permeation method together with mechanical tensile tests. Cathodic potentiodynamic polarization and surface morphology investigations were also conducted. Both elastic and plastic stresses were considered. Results showed that SRB enhanced the sub-surface hydrogen concentration significantly but had little influence on the diffusion coefficient. Elastic stress had a minimal effect on the hydrogen permeation behavior of X70 steel. Plastic stress reduced the diffusion coefficient and increased the sub-surface hydrogen concentration. The lattice trap produced by plastic deformation was responsible for the impact of plastic stress on hydrogen permeation. SRB and plastic stress not only enhanced the sub-surface hydrogen concentration independently, but also had synergistic effects accelerating the hydrogen accumulation on a steel surface.     

Hydrogen interaction characteristic of nanoscale oxide films grown on iron–nickel based stainless steel by selective thermal oxidation

Hydrogen isotopes, the reaction ingredients in the nuclear fusion plant, can easily permeate through the stainless steel (SS) substrate, leading to the so-called hydrogen degradation. Generally, a widely accepted way to reduce the hydrogen permeation is to prepare a barrier coating on the substrate. Nevertheless, the coated layer has the inherent problem of incompatibility with the heterogeneous base materials. In this work, in-situ selective oxidation was used to explore the optimal oxides with the improved hydrogen resistance. Two types of layers thermally formed at 450 °C and 750 °C, respectively, were selected to investigate their hydrogen interaction characteristics. Comprehensive analyses, including Raman spectra, XPS, EIS and AES, indicate that the oxide formed at 450 °C is a better candidate of hydrogen permeation barriers, probably due to the formation of protective layers of chromia and FeCr₂O₄, while the oxides obtained at 750 °C, though exhibiting a much more stable phase, can rarely reduce hydrogen diffusion through the shortcuts of defects. This finding provides a potential new way to prepare a hydrogen permeation barrier.     

Influence of machining parameters on surface roughness and susceptibility to hydrogen embrittlement of austenitic stainless steels

In the present work, an investigation on the susceptibility to hydrogen embrittlement of AISI 304 and 310 austenitic stainless steels was performed. The hydrogen embrittlement process leads to degradation of mechanical properties and can be accelerated by the presence of surface defects combined with elevated surface hardness. Tensile test specimens of the selected materials were machined by turning with different cutting parameters in order to create variations in surface finish conditions. The samples thus prepared were submitted to tensile tests before and after hydrogen permeation by cathodic charging. Regarding the AISI 304 steel, it was possible to notice that the presence of strain-induced martensite on the material surface led to severe hydrogen embrittlement. In the case of the AISI 310 steel, due to its higher nickel amount, no martensite formation could be detected, and this steel was found to be less susceptible to embrittlement in the tested conditions.     

Preparation and hydrogen penetration performance of TiO2/TiCx composite coatings

Titanium carbide is a good candidate for tritium permeation barrier in a fusion reactor. However, its oxidation susceptibility and the mismatch between the ceramic coating and substrate are still a challenge. In this study, a promising candidate as a hydrogen permeation barrier, comprising a titanium-based ceramic TiO₂/TiC_x composite coating, was proposed. The preparation process of this TiO₂/TiC_x composite coating involves two steps of carbon ion implantation and oxidation under ultra-low oxygen partial pressure. According to the results, the optimal oxidation temperature for TiO₂ coating is 550 °C, with the increase of the oxidation temperature, the particles on the surface of the oxide layer become coarse and loosely arranged, and the protective performance of the oxide layer is greatly reduced. The hydrogen barrier permeation behavior of the composite coating in a fusion reactor was simulated via hydrogen plasma discharge environment, the results show that the hydrogen barrier permeation performance of the composite is significantly better than that of a single TiO₂ coating. In addition, the coatings treated with hydrogen plasma showed a certain self-repairing performance through the diffusion growth of the TiC_x layer. These findings illustrate a novel method for preparing composite coatings to restrain hydrogen permeation, providing insight into the development of hydrogen permeation barrier materials.     

Investigation of the hydrogen induced cracking behaviour of API 5L X65 pipeline steel

The effect of microstructural features on the hydrogen induced cracking (HIC) susceptibility of two API 5L X65 pipeline steels were investigated by cathodic charging, hydrogen permeation and hydrogen microprint experiments. Microstructural evaluation after hydrogen charging revealed cracks at the mid-thickness (segregation zone) of both plates. However, more severe cracks were observed in the plate with higher dislocation density and residual stresses. The plate with lower plastic strain and more {111}-oriented grains had less severe cracks. Inclusions found along the crack path, comprising of Si-enriched oxides and carbides contributed to the initiation and propagation of cracks. The variation of the trapping behaviour and hydrogen diffusion through the plates were examined. The results confirmed that a higher ratio of reversible to irreversible traps contributes to increasing HIC severity in steels. Additionally, hydrogen transport through the steels was most prominent along the grain boundaries, indicating the importance of grain boundary character to HIC.     

Effects of hydrogen-charging on the susceptibility of X100 pipeline steel to hydrogen-induced cracking

In this work, the hydrogen-induced cracking (HIC) behavior of X100 pipeline steel was investigated by a combination of tensile test, electrochemical hydrogen permeation measurement and surface characterization techniques. The effect of inclusions in the steel on the crack initiation was analyzed. Results demonstrated that the amount of hydrogen-charging into the X100 steel specimen increases with the charging time and charging current density. Hydrogen-charging will enhance the susceptibility of the steel to HIC. The cracks initiate primarily at inclusions, such as aluminum oxides, titanium oxides and ferric carbides, in the steel. The diffusivity of hydrogen at room temperature in X100 steel is determined to be 1.04 × 10⁻⁸ cm²/s.     

Influence of silicon on hot-dip aluminizing process and subsequent oxidation for preparing hydrogen/tritium permeation barrier

The development of the International Thermonuclear Experimental Reactor (ITER) requires the production of a material capable of acting as a hydrogen/tritium permeation barrier on low activation steel. It is well known that thin alumina layer can reduce the hydrogen permeation rate by several orders of magnitude. A technology is introduced here to form a ductile Fe/Al intermetallic layer on the steel with an alumina over-layer. This technology, consisting of two main steps, hot-dip aluminizing (HDA) and subsequent oxidation behavior, seems to be a promising coating method to fulfill the required goals. According to the experiments that have been done in pure Al, the coatings were inhomogeneous and too thick. Additionally, a large number of cracks and porous band could be observed. In order to solve these problems, the element silicon was added to the aluminum melt with a nominal composition. The influence of silicon on the aluminizing and following oxidation process was investigated. With the addition of silicon into the aluminum melt, the coating became thinner and more homogeneous. The effort of the silicon on the oxidation behavior was observed as well concerning the suppression of porous band and cracks.     

Microstructural and micro-mechanical characterization during hydrogen charging: An in situ scanning electron microscopy study

Despite the tremendous industrial and scientific interest, hydrogen storage and embrittlement studies still suffer from experimental difficulties in studying diffusible hydrogen effects. As a consequence of the small size and high diffusivity of hydrogen atoms, it is challenging to detect and confirm hydrogen presence in material volumes, let alone investigate corresponding effects on microstructure or damage evolution. To address this need, we developed a novel in situ hydrogen-charging setup which can be applied to high vacuum-based systems such as scanning electron microscopes, to enable high-resolution microstructural analysis during electrochemical hydrogen permeation. In this setup, a hydrogen source is isolated from the objective sample surface in order to avoid the contamination problems from the source and enable analyses of the clean surface during hydrogen charging. Moreover, simultaneous microstructural observation and mechanical testing can be performed during hydrogen charging, by using the developed setup with a miniaturized mechanical tester compatible with high vacuum systems. Here, we demonstrate the capabilities of this experimental approach by carrying out three separate investigations on a duplex stainless steel, a α/β titanium alloy and a ferritic stainless steel. These case studies reveal interesting insights regarding hydrogen effects in these materials.     

Influence of microstructure on the hydrogen permeation of alumina coatings

In this work, the influence of microstructure on the hydrogen permeation property of alumina coatings was investigated. The different microstructures were obtained by annealing coatings at 700 °C or 900 °C. The permeation measurements showed that the 700 °C annealed coating exhibited lower hydrogen permeability than the 900 °C annealed coating. The 700 °C annealed coating was amorphous alumina. While, the 900 °C annealed coating had the spinel MnCr₂O₄ phase and γ-alumina. This spinel MnCr₂O₄ formed a network on the coating surface compared with the fine and smooth surface of the 700 °C annealed coating. The MnCr₂O₄ network in the 900 °C annealed coating formed short-cut for hydrogen diffusion, and thus resulted in high permeability. Furthermore, apparent delamination of coating was illustrated on the 900 °C annealed coating after the permeation test, and this was another reason for the high permeability of coating.     

Controlling chromium vaporization from interconnects with nickel coatings in solid oxide devices

Vaporization of Cr-rich volatile species from interconnect materials is a major source of degradation that limits the lifetime of planar solid oxide devices (solid oxide fuel cells and solid oxide electrolysis cells) with metallic interconnects. Some metallic coatings (Ni, Co, and Cu) may significantly reduce the Cr release from interconnects and slow down the oxide scale growth on the steel substrate. To shed additional light upon the mechanisms of such protection and find a suitable coating material for ferritic stainless steel materials widely used for interconnects, we used a combination of first-principles calculations, thermodynamics, and diffusion modeling to investigate which factors determine the quality of the Ni metallic coatings. We found that Cr migration in Ni coatings is determined by a delicate combination of the nickel oxidation, Cr diffusion, and phase transformation processes. Although the formation of Cr₂O₃ is more exothermic than that of NiO, the kinetic rate of the chromia formation in the coating layer and its surface is significantly reduced by the low mobility of Cr in nickel oxide and in NiCr₂O₄ spinel. These results are in a good agreement with diffusion modeling for Cr diffusion through the Ni coating layer on the ferritic 441 steel substrate and available experimental data.     

Time and frequency domain analysis of hydrogen permeation across PdCu metallic membranes for hydrogen purification

Hydrogen permeation is a process used in the industry for purification purposes. Palladium alloys (PdAg and PdCu) are commonly used as membrane material. In this communication, we report on the kinetics of hydrogen permeation across Pd_0.47Cu_0.53 metallic membranes which can be used in catalytic crackers of biofuels. The permeation mechanism is a multi-step process including surface chemisorption of molecular hydrogen (upstream side of the membrane), hydrogen diffusion across bulk regions, hydrogen recombination (downstream side of the membrane) and evolution. The role of different operating parameters (temperature, surface state, sample microstructure) is analyzed and discussed using both time and frequency domain experiments. Experimental pneumato-chemical impedance diagrams show that there is no significant rate-limitation at surfaces, except at low temperatures close to room temperature. Diffusion-controlled transport of hydrogen across the membrane is rate-determining. However, the value of the hydrogen diffusion coefficient does not rise exponentially with operating temperature in the 40–400 °C temperature range under investigation, as expected for a thermally activated diffusion process. At temperatures as low as 300 °C, new rate-limitations appear. They can be attributed to recrystallization and/or phase transformation processes induced by temperature and the presence of hydrogen.     

Strengthening mechanism and hydrogen-induced crack resistance of AISI 316L stainless steel subjected to laser peening at different power densities

Microstructural response of AISI 316L stainless steel to laser peening (LP) was examined by means of optical microscopy (OM) and transmission electron microscopy (TEM) in order to analyze the effects of LP on hydrogen-induced cracking (HIC) resistance. Depth profiles of near-surface microhardness and surface compressive residual stress (CRS) of LP treated specimens were presented respectively. Slow strain rate tensile tests were performed on the hydrogen-charged samples and their corresponding stress-strain curves as well as fracture morphologies were finally investigated in detail. The results demonstrated that LP induced a grain refinement effect on the treated surface while a maximum refining rate of 56.18% was achieved at the laser power density of 10 GW/cm². The near-surface microhardness also exhibited an attenuation trend with the increasing depth. The surface CRS positively correlated with power density before it reached a threshold value. A special U-shaped dislocation tangle band was observed in the LP treated specimen which served as hydrogen trapping sites. The LP treated samples exhibited better toughness after hydrogen charging from both macro mechanical properties and micro fracture morphologies. LP-induced grain refinement and CRS are believed to be the main contributing factors towards inhibiting the diffusion of hydrogen atoms which ultimately leads to the reduction of the hydrogen embrittlement sensitivity of AISI 316L stainless steel.     

Increase in the local yield stress near surface of austenitic stainless steel due to invasion by hydrogen

In order to determine the effect of hydrogen on the local yield stress near the surface of austenitic stainless steel, an indentation test combined with inverse problem analysis was employed. For austenitic stainless steel, the indentation test is an effective method since the hydrogen is distributed near to the surface because of its high solubility and low diffusion coefficient. Although uniaxial tensile tests can also provide useful data, greater variations in the mechanical properties due to the presence of hydrogen can be detected through indentation tests. In this study, Secondary Ion Mass Spectrometry (SIMS) was used to measure hydrogen depth profiles in order to establish the relationships between the hydrogen absorption depth and the effects due to hydrogen evaluated using the indentation test. The results showed that the yield stress doubled due to hydrogen absorption and then reverted to its initial state due to hydrogen desorption at room temperature. Also, hardening due to the presence of hydrogen, which was determined using an indentation test, was found to be dependent on the relationship between the plastic deformation depth and the hydrogen absorption depth.     

Methodology for predicting spray quenching of thick-walled metal alloy tubes

This paper explores the parametric influences of spray quenching for thick-walled metal alloy tubes. Using the point-source depiction of a spray, an analytical model is derived to determine the shape and size of the spray impact zone, as well as the distribution of volumetric flux across the same zone. This distribution is incorporated into heat transfer correlations for all spray boiling regimes to generate a complete boiling curve for every location across the impact zone. By setting boundary conditions for both the sprayed and unsprayed portions of the tube surface, a heat diffusion model is constructed for a unit cell of the tube for both aluminum alloy and steel. This model is used to construct spray quench curves for every point along the sprayed surface and within the wall. Increasing nozzle pressure drop or decreasing orifice-to-surface distance are shown to increase the magnitude of volumetric flux, which hastens the onset of the rapid cooling stages of the quench as well as improves overall cooling effectiveness. The sprayed surface is characterized by fast thermal response to the spray, while regions within the wall display more gradual response due to heat diffusion delays. With their superior thermal diffusivity, aluminum alloy tubes transmit the cooling effect through the wall faster than steel tubes. For steel, the cooling effect is more concentrated near the sprayed surface, causing the sprayed surface to cool much faster and locations within the wall much slower than for aluminum alloy. The predictive approach presented in this paper facilitates the determination of surface temperature gradients in the quenched part to guard against stress concentration. Also, when combined with metallurgical transformation models for the alloy, it may be possible to predict material properties such as hardness and strength.     

Possibility of false interpretations of hydrogen measurements in ferritic steel after an electrochemical cathodic charging process

In an electrochemical charging process applied before measuring the level of hydrogen in ferritic steels, two different side effects, a decrease in diffusible hydrogen content from steel and the detachment of 2nd phase particles from the steel surface, can occur, even during cathodic charging with a high charging current. Neglecting these side effects-associated with the surface chemistry during the electrochemical charging process can result in a false interpretation of the hydrogen diffusion and trapping behaviors in steel, producing conflicting data. A mechanistic reason for the side effects is provided based on the experimental results.     

Aluminizing and oxidation treatments on the porous stainless steel substrate for preparation of H2-permeable composite palladium membranes

Composite palladium membranes based on porous stainless steel (PSS) substrate are idea hydrogen separators and purifiers for hydrogen energy systems, and the surface modification of the PSS is of key importance. In this work, the macroporous PSS tubes were aluminized through pack cementation at 850 °C in argon, followed by an oxidation with air at 600 °C. Palladium membranes were prepared by electroless plating. Their permeation performances were tested, and the hydrogen permeation kinetics was discussed. The substrate materials and the palladium membranes were characterized by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD). An Al₂O₃-enriched surface layer with small pore size was created through aluminizing and oxidation treatments, which greatly improves the membrane integrity. The intermetallic diffusion between the palladium membranes and the PSS substrate material was not observed after a heat-treatment at 500 °C under hydrogen for 200 h. However, the aluminizing and oxidation treatments still need to be further optimized in order to improve the membrane permeability and selectivity, and particularly, the high diffusion resistance of the substrate materials greatly limited the hydrogen flux.     

Combined impact of elastic stress,prestrain and electrochemical charging on the hydrogen-induced cracking of high-strength steel

The elastic stress, prestrain and electrochemical hydrogen charging were controlled separately using a stress ring to investigate their roles in the initiation of hydrogen-induced cracks. The brittle features of hydrogen charging-induced damages, i.e., a mixture of quasi-cleavage and intergranular cracks, on the fracture surface were confirmed for a high-strength steel, made possible by applying degassing and tension-to-fracture to the hydrogen-charged specimens. The hydrogen charging-induced cracks eliminated the ductility of material, leading to premature fracture before the yield point in subsequent tensile tests. The strong dependence of hydrogen-induced cracking sensitivity on hydrogen concentration and hydrogen charging time was observed. X-ray microtomography and tensile tests were also utilized to investigate the effect of inclusions on crack formation. This study contributes to the understanding of the combined effects of residual stress and hydrogen on the cracking of deformed steel plates.     

Effect of plastic deformation at room temperature on hydrogen diffusion of S30408

Understanding the influence of plastic deformation on diffusion is critical for hydrogen embrittlement (HE) study. In this work, thermal desorption spectroscope (TDS), slow strain rate test (SSRT), feritscope, transmission electron microscope (TEM) and TDS model were used to study the relation between plastic deformation and hydrogen diffusion, aiming at unambiguously elucidating the effect of plastic deformation on hydrogen diffusion of austenitic stainless steel, S30408. An effective method was developed to deduce apparent hydrogen diffusion coefficient of austenitic stainless steel in this paper. Results indicate apparent hydrogen diffusion coefficient decreases firstly and then increases with increasing plastic deformation at room temperature. Hydrogen diffusion effected by plastic deformation is a complicated process which is suggested to be divided into two processes controlled by dislocation and strain-induced martensite, respectively, and the transition point is about 20% strain demonstrated by experiments in this case.