Most research on the hydrogen embrittlement of steel dealt with the interaction of hydrogen with the metal bulk microstructural features, whereas the first contact with hydrogen-containing ...environments occurs at the metal surface. Steel (when un-polarized) is always covered with an oxide layer, varying in composition and thickness. The impact of the oxide layer on the hydrogen transport is, however, not fully understood. This study focused on the effect of controlled pre-formed thermal oxide layers at the exit side on the hydrogen transport through the surface of SEA 1010 steel, considering two distinct thermally produced oxide types as test cases. Results demonstrated that thermal oxides can greatly limit hydrogen diffusion, with bilayers (hematite/magnetite) having a greater effect compared to magnetite layers. Increased oxide thickness resulted also in greater limiting diffusion. The main objective of this manuscript is to provide experimental evidence concerning the effect of oxide layers on the hydrogen transport through steel. Model thermal oxide layers were used to emphasize the importance of considering the surface characteristics when investigating hydrogen transport through metallic components.
Dual-phase low-alloy steels combine a soft ferrite phase with a hard martensite phase to create desirable properties in terms of strength and ductility. Nickel additions to dual-phase low-alloy ...steels can increase the yield strength further and lower the transformation temperatures, allowing for microstructure refining. Determining the correct intercritical annealing temperature as a function of nickel content is paramount, as it defines the microstructure ratio between ferrite and martensite. Likewise, quantifying the influence of nickel on the intercritical temperature and its synergistic effect with the microstructure ratio on mechanical properties is vital to designing dual-phase steels suitable for corrosive oil and gas services as well as hydrogen transport and storage applications. In this work, we used a microstructural design to develop intercritical annealing heat treatments to obtain dual-phase ferritic-martensitic low-alloy steels. The intercritical annealing and tempering temperatures and times were targeted to achieve three different martensite volume fractions as a function of nickel content, with a nominal content varying between 0, 1, and 3-wt% Ni. Mechanical properties were characterized using tensile testing and microhardness measurements. Additionally, the microstructure was studied using scanning electron microscopy coupled with electron backscatter diffraction analysis. Tensile strength increased with increasing martensite ratio and nickel content, with a further grain refinement effect found in the 3-wt% Ni steel. The optimal heat treatment parameters for oil and gas and hydrogen transport applications are discussed.
In the current study, ferritic steels containing NbC or NbN precipitates were investigated. The materials were subjected to various heat treatments, giving rise to different precipitate size ...distributions as determined by transmission electron microscopy. Both NbC and NbN precipitates act as hydrogen traps. The steels were hydrogen charged both electrochemically and/or from the gaseous hydrogen source, followed by multiple thermal desorption spectroscopy (TDS) measurements. Electrochemical charging gave rise to a low-temperature peak 323 K to 523 K (50 °C to 250 °C), originating from the hydrogen trapped near grain boundaries, with activation energy ranging between 24 and 33 kJ/mol, and at small NbC (39 to 48 kJ/mol) or NbN precipitates (23 to 24 kJ/mol). Gaseous charging caused a high-temperature TDS peak 723 K to 923 K (450 °C to 650 °C), which was attributed to the presence of incoherent precipitates. The activation energy for NbC precipitates, charged in a hydrogen atmosphere, ranged between 63 and 68 kJ/mol and between 100 and 143 kJ/mol for NbN precipitates.
Hydrogen can degrade the mechanical properties of steel components, which is commonly referred to as "hydrogen embrittlement" (HE). Quantifying the effect of HE on the structural integrity of ...components and structures remains challenging. The authors investigated an X70 pipeline steel through uncharged and hydrogen-charged (notched) tensile tests. This paper presents a combination of experimental results and numerical simulations using a micro-mechanics-inspired damage model. Four specimen geometries and three hydrogen concentrations (including uncharged) were targeted, which allowed for the construction of a fracture locus that depended on the stress triaxiality and hydrogen concentration. The multi-physical finite element model includes hydrogen diffusion and damage on the basis of the complete Gurson model. Hydrogen-Assisted degradation was implemented through an acceleration of the void nucleation process, as supported by experimental observations. The damage parameters were determined through inverse analysis, and the numerical results were in good agreement with the experimental data. The presented model couples micro-mechanical with macro-mechanical results and makes it possible to evaluate the damage evolution during hydrogen-charged mechanical tests. In particular, the well-known ductility loss due to hydrogen was captured well in the form of embrittlement indices for the different geometries and hydrogen concentrations. The limitations of the damage model regarding the stress state are discussed in this paper.
The potential use of carbon steel in CO2-saturated brine is studied for its potential use in heat exchangers in geothermal applications. A dedicated setup, including a double-pipe heat exchanger, is ...developed to study the relation between corrosion and the thermohydraulic behavior inside heat exchangers. Hot brine flows inside the inner carbon steel tube, thus corroding the inner surface of this tube. The thermohydraulic behavior of the heat exchanger, i.e., the pressure drop over the pipe and the heat transfer rate through the pipe, are continuously monitored. On the other hand, weight-loss experiments and microscopic analyses are performed on samples that are periodically removed from the setup. The corrosion rate is studied as a function of temperature, i.e., the entrance vs. the exit of the heat-exchanging section, and flow. Therefore, an experiment with static brine and a uniform temperature is used as a reference. The corrosion rate is generally higher in dynamic compared to static conditions. Furthermore, the corrosion rate increases with increasing temperature in dynamic conditions, whereas it decreases with increasing temperature in static conditions. These observations might be explained by the different corrosion products that formed. The corrosion products have no significant effect on the pressure drop over the pipe, but clear fluctuations in the heat transfer coefficient are observed. The origin of these fluctuations should be further studied before the observed heat transfer coefficient can be used as a measure for corrosion.
•Calculation of all hydrogen trapping parameters made possible in all cases.•Assessing of the contribution given by the oxide layer at the exit side for each single experiment.•Still further ...investigation on deep traps needed.
The Devanathan-Stachurski-cell is the most commonly used electrochemical technique for the investigation of the hydrogen trapping properties of a material. In this set-up, a flux of hydrogen atoms diffuses through a metal membrane under study. The atoms are subjected to the heterogeneities naturally present in the lattice of the metal, acting as traps. This matter influences the diffusion of hydrogen, which is strongly microstructure related. In this paper, we apply a numerical model to different iron-alloys to determine the hydrogen trapping parameters characterizing the materials. We highlight the complexity of the hydrogen trapping mechanism which needs to be further investigated.
The influence of the austenite (γ) phase fraction on the hydrogen embrittlement of duplex stainless steel is investigated. Heat treatments are performed to create two duplex stainless steel ...specimens, containing 50% and 44% of austenite, respectively. Mechanical testing with and without hydrogen charging reveals that significant embrittlement occurs regardless of the austenite fraction. A higher austenite fraction results in a reduced ductility loss under the presence of hydrogen. Samples with a higher ferrite fraction are embrittled more due to their higher hydrogen diffusivity. In-situ tensile tests, interrupted at the ultimate tensile strength, show hydrogen-assisted cracks on the specimen surface both in austenite and ferrite and across the α/γ interface.
Damage in bearings is closely associated with the presence of microstructural alterations, known as white etching areas (WEAs) and white etching cracks (WECs). One of the main reasons for the ...creation of these microstructural alterations is the presence of defects in the material, such as non-metallic inclusions. Manganese sulfides and aluminum oxides are widely reported in the literature as the most common types of non-metallic inclusions found in bearing steels. This study classifies 280 non-metallic inclusions in an investigated bearing steel according to several criteria: bonded/debonded with the matrix, size, shape, orientation angle, depth below the raceway surface, and chemical composition. Contrary to the findings in the literature, this investigation reports that the chemical composition of the inclusion (MnS + Al2O3) is of secondary importance when considering factors for damage initiation. The orientation of the microstructural alterations is observed to coincide with the high-stress regions, indicating a relation between the formation of butterfly wings and the white etching crack. In our investigation, butterfly wings typically exhibit a 45-degree pattern originating from the non-metallic inclusions. Conversely, the white etching crack starts from the non-metallic inclusion at a shallower angle in correspondence to the raceway. This can be attributed to the stress state, which corresponds to a region where extensive white etching cracks are formed. In conclusion, the microstructural observations demonstrate that the state of non-metallic inclusion—i.e., whether they are bonded or not to the steel matrix—plays an essential role in initiating rolling contact fatigue damage.
Surface corrosion product formation is one of the important factors affecting the corrosion rate and hydrogen uptake in a H2S environment. However, it is still unclear how the base material ...composition will affect the corrosion products that are generated, and consequently their impact on the corrosion rate. In this paper, corrosion product formation and the impact of the Mo content of the base material on the composition of the corrosion products and hydrogen absorption in a sour environment was investigated. The corrosion layer was composed of a double layered mackinawite (FeS1−x) structure, which was enriched with molybdenum and chromium. The layers were formed via two different mechanisms, i.e., the inner layer was created via a general oxide film formation corrosion mechanism, whereas the upper layer was formed by a precipitation mechanism. The presence of this double corrosion layer had a large influence on the amount of diffusible hydrogen in the materials. This amount decreased as a function of contact time with the H2S saturated solution, while the corrosion rate of the materials shows no significant reduction. Therefore, the corrosion products are assumed to act as a physical barrier against hydrogen uptake. Mo addition caused a decrease in the maximal amount of diffusible hydrogen.
This work evaluates the impact of different organic acids on the corrosion sensitivity and stress-corrosion cracking (SCC) of NiCrMoV steam turbine steel. For all organic acids, potentiodynamic ...measurements shows linear relationships between corrosion rate and hydrogen proton concentration between pH 2.4 and 3.9. For solutions with the same pH, i.e., similar conductivity, the corrosion rate differs depending on the type of organic acid. The anodic dissolution in formic acid is the highest, followed by acetic, propanoic and nonanoic acid. The acid dissociation reaction is identified as the rate determining step in the corrosion process. Nonanoic acid, alternatively, clearly acts as a corrosion inhibitor. In situ four-point constant-extension tests in formic acid, acetic acid and nonanoic acid, at a pH value of 3.4 were performed to evaluate their impact on the SSC sensitivity. The general degradation followed the trend of the corrosion rate, although the synergetic effect of corrosion and stress resulted in a higher degradation depth. Though nonanoic acid induced little visible corrosion, still stress-corrosion cracks were still detected. It was shown that solutions of different organic acids with the same pH do not have the same influence on stress-induced degradation.
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