Annotation of the results obtained in 2020
The main purpose of the research in 2020 was the theoretical and experimental investigation of the phenomenon of skin effect due to distribution and transport of hydrogen during the interaction of metal with corrosive environment. The skin effect was discovered at the previous stage of the project. There are no data about this phenomenon in the literature, so the phenomenon has been modeled by means of several approaches that were verified by comparison with experimental data. An uneven distribution of hydrogen inside metal samples due to saturation was explained by means of the diffusion of hydrogen in stress and strain fields obtained within the micropolar theory. Hydrogen accumulates within the area in the vicinity of the border determined by the additional material parameters of order of the grain size that reflect the «size effect». Thus, the skin effect can be explained by means of models accounting for translational degrees of freedom that cause additional stress and couple stress in the vicinity of the border and affect the diffusion. The coupled problem of determining the cylindrical sample stress-strain state during hydrogen diffusion was numerically solved with the finite volume method used. The constants used in the coupled problem were taken from the obtained experimental data. The solution was checked for compliance with the experiment. The terms of the local balance equation for the diffusion component and their influence on the diffusion process were investigated. For the modified diffusion coefficient, it was shown that its value nonlinearly decreases when moving from the sample boundary to the center, and the difference in the numerical value differs by orders of magnitude. Over time, this difference does not become smooth, and the value of the diffusion coefficient decreases. The obtained result agrees with the experimentally observed thin boundary layer saturated with hydrogen, and the resulting model can be used for its theoretical description at the initial stages of hydrogen diffusion into the steel. Hydrogen transport inside metal samples and degassing have been modeled by means of a multichannel diffusion. The finite element method was used to obtain a numerical solution. It was found that the skin effect fundamentally changes the transient process when the diffusion flux of hydrogen through flat samples is established, as well as the thermal desorption spectra of hydrogen for flat and cylindrical samples. This transient process and thermal desorption spectra are generally recognized sources of data on the diffusion coefficients and binding energies of hydrogen. Thus, the observed discrepancies are fundamental and can lead to a wrong determination of specific hydrogen diffusion constants. The errors can be of one or two decimal orders and require additional substantiation and research. Experimental study was carried out to determine the parameters of the models and to verify them. The process of multiple saturation-degassing of samples made of nickel alloy 718 and process of gigacycle fatigue have been investigated. Specimens cut from pipes made of aluminum alloy AMg-6 and nanostructured samples have been examined. All these experiments were carried out to clarify the mechanisms of the appearance of the skin effect and its influence on the metals and nanomaterials properties. It has been established that the skin effect leads to stable irreversible changes in the microstructure of materials. It does not always directly affect the mechanical properties of the samples, which can be completely restored after removing hydrogen by degassing (known as reversible hydrogen embrittlement). At the same time, saturation with hydrogen in a corrosive environment changes the way of the interaction of the metal with this corrosive environment. Such changes can lead to a significant acceleration of the degradation of mechanical properties during further operation of the metal in a corrosive environment. This acceleration is difficult to observe. It is important that the high-temperature degassing of metal, which is widely used to vanish a negative influence of hydrogen, somewhat delays, but does not stop the destruction process. The obtained results are important for technical diagnostics, since at the moment the so-called "surface" hydrogen is not recorded during the experiments and is not taken into account in the measurements. Its separate accounting will make it possible to estimate the quality of the metal more precisely and eliminate the possibility of preparing a "special set" of samples for inspection.

 

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Annotation of the results obtained in 2018
The first year investigations on the project were dedicated to the development of experimental methods for studying hydrogen transport in structural materials and basic models for describing the interaction of structural materials with hydrogen during their operation in a corrosive environment. These experimental methods and models are planned to become a keystone for computational and experimental hydrogen diagnostics of the structural materials properties degradation in aggressive corrosive environments. In order to perform this, it is necessary to conduct a wide range of theoretical and experimental studies, establish cause-and-effect relationships, and verify the methods on a considerable amount of experimental data. Currently the standards of the American Society of ASME Engineers are widely applied. They regulate the testing of new materials in corrosive environments, including hydrogen cracking. However, there is a great variety in the description of the mechanisms of the hydrogen seepage and its influence on the mechanical properties of materials. All possible interaction models from quantum mechanical to purely empirical are used. Such variety does not allow one to describe the interaction of hydrogen with metals adequately and unambiguously in specific technologically important cases, for example, in the operation of metals in a corrosive environment. In addition, there is a large variation in the experimental data. Hydrogen is the most mysterious component of metal alloys, the only one for which there are no international comparisons of measurement results in certified laboratories, and standard calibration samples from different manufacturers may differ in certified and measured values several times. In this regard, during the investigations closer attention was paid to the metrological reliability of all measurement results and their traceability to primary standards. AB-1 certified and verified mass spectrometry analyzer of hydrogen, working according to the methods of vacuum heating and vacuum melting, was used as a device for measuring hydrogen concentrations. Extensive studies have been carried out using both standard calibration samples and samples made from various alloys of iron, nickel, zirconium, titanium, saturated in different ways in corrosive environments according to ASME standards. The most important experimental result obtained on all alloys is the effect of a thin surface layer on hydrogen seepage. It has been established that, regardless of the saturation method, at real exposure times of the order of 100 hours in hydrogen-containing environments, only a thin, of the order of 100 μm thick, surface metal layer is strongly saturated with hydrogen. Slight changes in concentration occur inside the samples, but the difference between the concentration of hydrogen in the surface layer and the internal regions of the metal reaches two decimal orders of magnitude. The duration of exposure in a corrosive environment affects the thickness of the layer and the quantitative indicators of cracking of the internal regions of the sample but practically does not affect the ratio of hydrogen concentrations inside the sample and in the surface layer. External mechanical load neither changes the situation. Hydrogen enrichment under load for the duration of 100 hours leads to an increased concentration of hydrogen only in the surface layer and on the crack edges inside the metal. In the metal itself, the concentration of hydrogen remains background, even in the case when the sample is completely destroyed as a result of hydrogen enrichment under mechanical load. The obtained experimental data are consistent with data published in scientific journals. In one of the works, an equalization of the surface layer and the inner part concentrations in the steel samples was observed after 500 hours of cathode hydrogen enrichment, herewith the entire sample consisted of through cracks by the end of the process. It should be noticed that in various studies of the effect of hydrogen on the metal properties the time of cathode hydrogen enrichment from 4 to 30 hours is used. Due to our findings, there are several issues investigated in the course of the project: • Existing models of hydrogen fragility do not allow to describe the effect of a strong influence on the strength and ductility of the surface layer with a thickness of 100 μm with an average sample size of 20 mm. • The existing generally accepted models of hydrogen embrittlement do not describe the growth of main cracks inside the sample during its hydrogen saturation in a corrosive environment if there is practically no hydrogen inside the metal. The mechanisms of localized plasticity and the decohesion do not work. • It is necessary to provide additional verifications and validations of the identity of the mechanisms of artificial testing and natural operational hydrogen enrichment of metals in corrosive environments. In the first phase of the project, three main approaches to solving these problems were developed: Hydrogen seepage modeling was carried out together with the modeling of the stress-strain state of the material within the framework of multi-continuum models of rational mechanics. This approach allows us to describe the joint distribution of hydrogen and cracks, by taking into account the mutual influence of material deformations and hydrogen transport. Within the micropolar continuum approach, the distributed couple stress on the external boundary of the sample produces additional tensile strains in a thin surface layer, that in turn, initiate hydrogen absorption from the environment. This approach made it possible for the first time to describe the effect of a “rim” formation. Such the "rim" is often observed along the edge of the hydrogen-enriched samples fracture but has not been discussed in the scientific literature. The diffusion of hydrogen from a thin layer of the hydrogenated samples surface was measured and examined separately, which made it possible to obtain new experimental results. In particular, it was found that air storage of hydrogenated samples does not equalize the hydrogen concentrations throughout the sample, but almost completely "weather" hydrogen into the environment, which very often leads to a recovery of the mechanical characteristics of the samples after about two months of exposure. Such recovery is a well-known fact. It is considered as an example of reversible hydrogen fragility. The new data is that the thin surface layer of the sample is saturated with hydrogen and then restored. The inner part is not actually involved in the process of hydrogen redistribution. We believe that this is the reason for the discrepancy between the various results, which appears in many publications on the effect of hydrogen concentration on the properties of materials. Usually, the graphs are constructed depending not on hydrogen concentrations, but on the time of hydrogen absorption or the magnitude of the cathode current at cathode saturation. The additional investigations were made during the implementation of the above approaches, which made it possible to confirm the reliability of the constructed models and their advantages. A number of model problems have been solved. Programs for numerical simulation of the non-uniform concentration redistribution have been developed. Programs for analyzing discrete thermo-desorption spectra have been created, and experimental data processing has been done on their base. Comparisons of measurement results using various measurement techniques have been carried out. All the auxiliary results obtained allow us to state that the observed effect of the high hydrogen concentration in a thin near-surface layer is significant. The developed models allow describing and predicting that surface layer occurrence for some alloys, for example, aluminum and iron alloys. At the second stage of the project, further investigation of the phenomenon of hydrogen seepage during the operation of metals in corrosive environments based on both artificially and naturally hydrogenated samples is planned. Modelling of changes in the structure and strength of metals that accompany this seepage will be provided based on the experimental results. Link to the website dedicated to the project: https://www.researchgate.net/project/Saturation-of-solids-with-hydrogen-in-an-aggressive-environment

 

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Annotation of the results obtained in 2019
Occurrence of a thin hydrogen-containing surface layer observed during artificial hydrogenation of steel specimens carried out in the first stage of the project was studied using the experimental methods realized for model specimens and metal specimens taken from real structures operating in a corrosive environment. Comparison of the obtained results with the data provided by other researchers and obtained for various metals was carried out. The analysis of data of the world scientific literature as well as our data showed that the surface effect is universal and observed for any artificial hydrogen saturation of specimens. A certain exception is the saturation of specimens under pressure in a gaseous hydrogen atmosphere. Nevertheless, even in this case, in the absence of high temperatures, the duration of more than 100 hours of continuous saturation is necessary in order to ensure a uniform distribution of hydrogen concentrations in the specimens. At the same time in case of other standard saturation methods, this takes more than 500 hours. Significantly shorter times were used in 99% of works published over the past 150 years on the effect of hydrogen on the properties of metals when saturated with hydrogen. This casts doubt on some of the recognized scientific results. We would like to note that data on the average hydrogen concentration in specimens after saturation are absent in many researches. Moreover, the mechanical and physical properties of metals were reported as being dependent on the time of exposure to a corrosive solution, charge, or on the time of saturation with hydrogen. There are only less than a dozen works in which such an approach is called into question. In particular, they describe a strong nonlinear dependence of the average hydrogen concentration on the total charge transferred in the electrolyte during cathodic hydrogen distillation. At the same time, the diffusion nature of the saturation of metals with atomic hydrogen after surface sorption of hydrogen molecules from the external environment and their dissociation into hydrogen atoms is generally recognized. Therefore, experimental data on the inhomogeneous distribution of hydrogen concentrations over the depth of the metal in most articles are related to small diffusion coefficients. These coefficients provide a characteristic propagation time of the hydrogen concentration front in metal specimens of the order of several hundred hours. Results provided in our articles and a dozen other published articles do not allow us to consider this approach correct. In particular, in the standard hydrogen cracking test carried out according to NACE Standard TM0284-2003 metal cracking inside the specimens occurs without increasing the average hydrogen concentration. The surface layer of the metal is saturated to a concentration of about 100 ppm in 20 hours followed by cracking of the inner regions of the specimens, while the redistribution of hydrogen does not take place. Extraction of the specimen from the saline solution into the air environment at room temperature leads to a quite rapid removal of hydrogen from the surface layer. Much faster, hydrogen is extracted at room temperature in vacuum, which also casts doubt on the classical diffusion nature of the hydrogen transport in the metal. Thus, when studying interaction with corrosive media and electrochemical saturation with hydrogen, we face non-classical diffusion or transport of hydrogen from the external environment, which is not described by existing models, since they involve surface sorption of hydrogen molecules, an almost uniform distribution of hydrogen traps over the metal volume and diffusion hydrogen between surface and traps through various channels. The thickness of the surface layer is assumed to be equal to 10-100 nm. In turn, we observe a strong correlation between relatively low concentrations of hydrogen and the stress-strain state of the specimens, the thickness of the surface layer of the order of 100 μm and the non-hydrogen nature of the formation of cracks inside metals. The research and mathematical modeling of these phenomena were realized in the current stage of the project. A modified local balance equation of the diffusion component was obtained within the framework of linear nonequilibrium thermodynamics. This equation takes into account the mutual influence of diffusion and the stress-strain state in general terms, as well as the dependence of the diffusion process on temperature, gas concentration, and other thermomechanical loads. Using the finite volume method, a numerical solution of the coupled boundary problem for the stress-strain state of a cylindrical sample under uniaxial tension and the distribution of gas concentration was obtained. It was shown that, within the framework of the proposed model, the diffusion process quickly comes to the stationary mode, and the distribution profile of the hydrogen concentration has nonlinear character, demonstrating a significant decrease in the hydrogen concentration with increasing distance from the boundary to the center of the sample. The non-uniform distribution of hydrogen concentration over the sample is a consequence of internal stresses arising in the material during diffusion. The non-uniform distribution of hydrogen inside metals was also explained by means of the micropolar theory of continua which takes into account both force and couple actions and introduces rotational degrees of freedom of the body particles along with translational ones. As a result, a region of additional displacements of body particles increasing the intergranular space was obtained. This region is comparable with the size of the structural heterogeneity of the material, manifested in the presence of a surface layer containing an excess of hydrogen. Thus, application of the micropolar theory made it possible to relate the inhomogeneous distribution of hydrogen with the structural inhomogeneity of the saturated material. A link to the webpage dedicated to the project: https://www.researchgate.net/project/Saturation-of-solids-with-hydrogen-in-an-aggressive-environment

 

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