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Academic literature on the topic 'Stepwise mechanism'

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Published: 5 June 2025

Last updated: 24 June 2025

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Journal articles on the topic "Stepwise mechanism"

Yuzenkova, Yulia, Aleksandra Bochkareva, Vasisht R. Tadigotla, et al. "Stepwise mechanism for transcription fidelity." BMC Biology 8, no. 1 (2010): 54. http://dx.doi.org/10.1186/1741-7007-8-54.

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YU, LINGJUAN, DACHENG FENG, MAOXIA HE, RUI LI та ZHENGTING CAI. "THEORETICAL STUDY ON HYDROLYSIS MECHANISM OF β-PHOSPHOLACTAMS". Journal of Theoretical and Computational Chemistry 05, spec01 (2006): 421–31. http://dx.doi.org/10.1142/s0219633606002362.

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The neutral hydrolysis mechanisms of a simple β-phospholactam with and without water-assisted reaction have been studied by using quantum chemical method at HF/6-31G**, MP2/6-31G** and B3LYP/6-31G** levels, respectively. The reaction can proceed by two different mechanisms: concerted and stepwise. There are two pathways in stepwise, i.e. pathway a and b, and the energy barriers of them are close. The energy barriers of water-assisted hydrolysis of β-phospholactam are obviously lower than those of no-water-assisted hydrolysis system. The energy barriers of stepwise mechanism are much lower than those of the concerted pathway in both cases. The solvent effects have been considered by means of a polarizable continuum model. The hydrolysis mechanism of β-phospholactam with that of the β-lactam and β-sultam was compared.

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Islam, Zahidul, Timothy S. Strutzenberg, Ilya Gurevic, and Amnon Kohen. "Concerted versus Stepwise Mechanism in Thymidylate Synthase." Journal of the American Chemical Society 136, no. 28 (2014): 9850–53. http://dx.doi.org/10.1021/ja504341g.

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Slepetz, Brad, and Miklos Kertesz. "Divacancies in diamond: a stepwise formation mechanism." Phys. Chem. Chem. Phys. 16, no. 4 (2014): 1515–21. http://dx.doi.org/10.1039/c3cp53384k.

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Cheng, Yi-Han, Yu-Cheng Zhu, Wei Kang, Xin-Zheng Li, and Wei Fang. "Determination of concerted or stepwise mechanism of hydrogen tunneling from isotope effects: Departure between experiment and theory." Journal of Chemical Physics 156, no. 12 (2022): 124304. http://dx.doi.org/10.1063/5.0085010.

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Isotope substitution is an important experimental technique that offers deep insight into reaction mechanisms, as the measured kinetic isotope effects (KIEs) can be directly compared with theory. For multiple proton transfer processes, there are two types of mechanisms: stepwise transfer and concerted transfer. The Bell–Limbach model provides a simple theory to determine whether the proton transfer mechanism is stepwise or concerted from KIEs. Recent scanning tunneling microscopy (STM) experiments have studied the proton switching process in water tetramers on NaCl(001). Theoretical studies predict that this process occurs via a concerted mechanism; however, the experimental KIEs resemble the Bell–Limbach model for stepwise tunneling, raising questions on the underlying mechanism or the validity of the model. We study this system using ab initio instanton theory, and in addition to thermal rates, we also considered microcanonical rates, as well as tunneling splittings. The instanton theory predicts a concerted mechanism, and the KIEs for tunneling rates (both thermal and microcanonical) upon deuteration are consistent with the Bell–Limbach model for concerted tunneling but could not explain the experiments. For tunneling splittings, partial and full deuteration change the size of it in a similar fashion to how they change the rates. We further examined the Bell–Limbach model in another system, porphycene, which has both stepwise and concerted tunneling pathways. The KIEs predicted by instanton theory are again consistent with the Bell–Limbach model. This study highlights differences between KIEs in stepwise and concerted tunneling and the discrepancy between theory and recent STM experiments. New theory/experiments are desired to settle this problem.

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Thakur, Rahul. "Elucidation the Synthesis of an Antibiotic Medicine Chloramphenicol Palmitate with its Stepwise Mechanism." International Journal of Science and Research (IJSR) 10, no. 12 (2021): 365–76. http://dx.doi.org/10.21275/mr211206175033.

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Ehrlich, Joel I., Chi-Ching Hwang, Paul F. Cook, and John S. Blanchard. "Evidence for a Stepwise Mechanism of OMP Decarboxylase." Journal of the American Chemical Society 121, no. 29 (1999): 6966–67. http://dx.doi.org/10.1021/ja990737s.

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Purohit, Prasad, Ananya Mitra, and Anthony Auerbach. "A stepwise mechanism for acetylcholine receptor channel gating." Nature 446, no. 7138 (2007): 930–33. http://dx.doi.org/10.1038/nature05721.

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Siadati, Seyyed Amir. "Beyond the Alternatives that Switch the Mechanism of the 1,3-Dipolar CyCloadditions from Concerted to Stepwise or Vice Versa: A Literature Review." Progress in Reaction Kinetics and Mechanism 41, no. 4 (2016): 331–44. http://dx.doi.org/10.3184/146867816x14719552202168.

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For several decades, the concerted or stepwise character of the mechanism of the 1,3-dipolar cycloaddition reaction has been one of the most debated issues in the field of organic chemistry. The significance of this problem is due to the fact that in a catalyst-free 1,3-dipolar cycloaddition, when the mechanism switches from concerted to stepwise, the stereospecificity is lost and thus unwanted stereoisomers may emerge. The first proposals about the mechanism of the 1,3-dipolar reaction were due to Huisgen (concerted model) and subsequently by Firestone (two-step diradical channel) in the 1960s. After a decade of debate, most researchers accepted the concerted model for the reaction, but during these years, researchers reported some examples of the stepwise mechanism for catalyst-free 1,3-dipolar cycloadditions. This review attempts to find a number of factors that could influence the reaction channels and switch the mechanism from concerted to stepwise, or vice versa.

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Huang, Ming, and Darrin M. York. "Linear free energy relationships in RNA transesterification: theoretical models to aid experimental interpretations." Phys. Chem. Chem. Phys. 16, no. 30 (2014): 15846–55. http://dx.doi.org/10.1039/c4cp01050g.

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Dissertations / Theses on the topic "Stepwise mechanism"

Abdel, Latif Marwa K. "Electrochemical oxidation of aliphatic carboxylates: Kinetics, thermodynamics, and evidence for a shift from a concerted to a stepwise mechanism in the presence of water." Diss., Virginia Tech, 2016. http://hdl.handle.net/10919/82522.

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The mechanism and the oxidation potential of the dissociative single electron transfer for tetra-n-butylammonium acetate has been investigated via conventional (cyclic voltammetry) and convolution voltammetry. The oxidation potential for tetra-n-butylammonium acetate was determined to be 0.60 ± 0.10 (vs. Ag/ (0.1 M) AgNO3) in anhydrous acetonitrile. The results also indicated the mechanism of oxidation was concerted dissociative electron transfer (cDET), rather than stepwise as was previously reported. To further investigate the mechanism, a series of aliphatic and aromatic tetra-n butylammonium carboxylates were synthesized and investigated via convolution and conventional methods under anhydrous conditions (propionate, pivalate, phenyl acetate, and benzoate). The reported results showed high reproducibility and consistency with a concerted dissociative electron transfer for aliphatic carboxylates with a systematic shift in the oxidation potentials (0.60 ± 0.09 V for acetate, 0.47 ± 0.05 V for propionate, and 0.40 ± 0.05 V for pivalate) within the series which is expected trend based on radical stabilization energies of the alkyl groups on the aliphatic carboxylates. Hydrogen bonding was investigated as a possible source for the discrepancy between our results and the reported mechanism of the dissociative electron transfer. Because of the extreme hygroscopic nature of carboxylate salts, it was hypothesized that the presence of small amounts of water might alter the reaction mechanism. Deionized water and deuterium oxide additions to anhydrous acetonitrile were performed to test this hypothesis. The mechanism was noted to shift towards a stepwise mechanism as water was added. In addition, the derived oxidation potentials became more positive with increasing concentrations of water. Several explanations are presented with regards to water effects on the shift in the electron transfer mechanism. Indirect electrolysis (homogeneous redox catalysis) was also employed as an alternative and independent approach to quantify the oxidation potentials of carboxylates. A series of substituted ferrocenes were investigated as mediators for the oxidation of tetra-n-butylammonium acetate. Preliminary data showed redox catalysis was feasible for these systems. Further analyses of the electrochemical results suggested a follow-up chemical step (addition to mediator) that competes with the redox catalysis mechanism. As predicted from theoretical working curves, a plateau region in the ip/ipd plots (where no meaningful kinetic information could be obtained) was observed. Products mixture analyses verified the consumption of the mediator upon electrolysis, but no further information with regards to the nature of the mechanism was deduced. In a related study the effects of hydrogen bonding and ions on the reactivity of neutral free radicals were examined by laser flash photolysis. The rate of the β-scission of the cumyloxyl radical is influenced by cations (Li+ > Mg2+ ≈ Na+ > nBu4N+) due to stabilizing ion-dipole interactions in the transition state of the developing carbonyl group. Experimental findings are in a good agreement with theoretical work suggesting metal ion complexation can cause radical clocks to run fast with a more significant effect if there is an increase in dipole moment going from the reactant to the transition state. Ph. D.

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Akiyama, Shuji. "Stepwise Folding Mechanisms of Globular Proteins." 京都大学 (Kyoto University), 2002. http://hdl.handle.net/2433/149787.

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Ness, Stephanie L. "Evidence for a stepwise mechanism in the cycloreversion of rhenium diolates." Thesis, 1999. http://hdl.handle.net/1957/33581.

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Haregot, Michaele, and 哈里克. "To Facilitate Computer Network Learning with Collaborative Virtualization-Based Mechanism and Instant Stepwise Guidance." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/4n8da8.

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碩士 國立中央大學 網路學習科技研究所 105 Teaching computer networks (both wired and wireless networks) is one of the core topics in computer science curricula for undergraduate students. Recently, a virtualized computer network lab has been used for teaching students to configure network devices, such as routers and switches, to practice in real network configuration scenarios without purchasing network hardware. The virtualized computer network lab provides several educational benefits such as, cost efficiency, less management, and interactive environment. The first experiment of this study proposed a web-based Hybrid Virtualization Laboratory (HVLab), which integrates network emulators and virtualization technology. Moreover, HVlab requires a collaborative mechanism to facilitate discussion among the experimental students. Meanwhile, the HVLab can support multiple network configuration scenarios (network emulators) for students to practice. In this experiment, the students were divided into an experimental and a control group. The experimental group students perform their lab assignment using HVLab collaboratively with the help of chatting window and web terminal, whereas the control group completed using with web terminal without collaboration. After the experiment, the statistical results showed that the post-test scores of the experimental group were higher than the control group. In addition, when experimental students become more familiar with HVLab with collaborative activities, they outperform the control groups in an in-class and homework assignment. Furthermore, the experimental students perceived that were easy to use the HVLab with collaborative activities and useful for accomplishing assignment and homework. Most of the students also expressed their feeling that they were highly motivated to use HVLab with collaborative activities as a learning tool in the future. Finally, the observation and questionnaire with experimental group showed that collaboration in HVLab was potentially helpful during the experiment. The second experiment also proposed an instant stepwise guidance (ISG) as a proposed mechanism; which aimed to provide step-by-step guidance for students to practice wireless network configuration. The students were divided into an experimental and control groups. The experimental students were used ISG to accomplish their lab assignment, whereas the control group students used network simulator. This experiment investigates the effect of the proposed mechanism for students learning achievement, learning behavior, cognitive load and face-to-face help. After the experiment, the statistical result showed that the experimental group outperformed the control groups in most of the laboratory activities and post-test. Furthermore, this study found the students that used the proposed mechanism had less cognitive load and face-to-face help than control students. Moreover, the experimental group perceived that the proposed mechanism was easy to use and useful to practice wireless network configuration. Most students also expressed feeling that they were motivated to use the proposed mechanism as a learning tool in near future. Keyword: Keyword: Hybrid Virtualization, web-based virtualization laboratories, teaching computer network, Simulation based laboratory, instant stepwise guidance, teaching wireless network.

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Tu, Ying-Ting, and 涂穎定. "Non-Dominated Sorting Genetic Algorithm II – Sequential Clustering Classification with Stepwise Dimensionality Reduction Mechanism." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/uax283.

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碩士 國立臺灣科技大學 工業管理系 107 This research aims to improve the performance of the Non-Dominated Sorting Genetic Algorithm – Sequential Clustering Classification (NSGAII-SCC) by integrating a dimensionality reduction technique which is the Principal Component Analysis (PCA). The NSGAII-SCC separates a dataset into two subsets. Next it applies a clustering method to generate labels from a sub-dataset which contains performance related features. Then, the labels generated are used as prediction targets in a classification model to classify data in the second sub-dataset. The sequential clustering and classification (SCC) mechanism is integrated with a feature selection mechanism which is the genetic algorithm (GA). Through the NSGA-II, the compactness of clustering and the prediction accuracy of classification will be simultaneously optimized. Instead of finding the best combination of clustering and classification methods like most studies in the past, this research aims to improve the NSGAII-SCC algorithm performance by integrating a dimensionality reduction technique. The experiment result shows that the proposed method performed better than the original NSGAII-SCC in terms of the solution quality.

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Books on the topic "Stepwise mechanism"

Moloney, Mark G. How to solve organic reaction mechanisms: A stepwise approach. John Wiley & Sons, Inc., 2015.

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Ness, Stephanie L. Evidence for a stepwise mechanism in the cycloreversion of rhenium diolates. 1999.

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Moloney, Mark G. How to Solve Organic Reaction Mechanisms: A Stepwise Approach. Wiley & Sons, Incorporated, John, 2015.

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Moloney, Mark G. How to Solve Organic Reaction Mechanisms: A Stepwise Approach. Wiley & Sons, Incorporated, John, 2015.

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Reaction mechanisms at a glance: A stepwise approach to problem-solving in organic chemistry. Blackwell Science, 2000.

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Moloney, Mark G. Reaction Mechanisms at a Glance: A Stepwise Approach to Problem-Solving in Organic Chemistry. Blackwell Publishing Limited, 1999.

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Book chapters on the topic "Stepwise mechanism"

Pollack, Gerald H., Henk L. M. Granzier, Alicia Mattiazzi, et al. "Stepwise shortening and the mechanism of contraction." In Developments in Cardiovascular Medicine. Springer Netherlands, 1987. http://dx.doi.org/10.1007/978-94-009-3311-8_2.

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Yamamoto, Hiroshi, Naoki Wakamiya, and Masayuki Murata. "An Inter-networking Mechanism Using Stepwise Synchronization for Wireless Sensor Networks." In Lecture Notes of the Institute for Computer Sciences, Social Informatics and Telecommunications Engineering. Springer Berlin Heidelberg, 2012. http://dx.doi.org/10.1007/978-3-642-32615-8_28.

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Tiedemann, Rochelle L., Hope E. Eden, Zhijun Huang, Keith D. Robertson, and Scott B. Rothbart. "Distinguishing Active Versus Passive Using Illumina MethylationEPIC BeadChip Microarrays." In Methods in Molecular Biology. Springer US, 2021. http://dx.doi.org/10.1007/978-1-0716-1294-1_7.

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AbstractThe 5-carbon positions on cytosine nucleotides preceding guanines in genomic DNA (CpG) are common targets for DNA methylation (5mC). DNA methylation removal can occur through both active and passive mechanisms. Ten-eleven translocation enzymes (TETs) oxidize 5mC in a stepwise manner to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC). 5mC can also be removed passively through sequential cell divisions in the absence of DNA methylation maintenance. In this chapter, we describe approaches that couple TET-assisted bisulfite (TAB) and oxidative bisulfite (OxBS) conversion to the Illumina MethylationEPIC BeadChIP (EPIC array) and show how these technologies can be used to distinguish active versus passive DNA demethylation. We also describe integrative bioinformatics pipelines to facilitate this analysis.

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Kosta, Tomislav, Claron J. Ridge, Marcel M. Hatter, and Jesus O. Mares. "Characterization of the Effect of Cold Joints and Functional Grading in Stepwise Constructed Tensile Dog Bones with Embedded Digital Image Correlation." In Challenges in Mechanics of Biological Systems and Materials, Thermomechanics and Infrared Imaging, Time Dependent Materials and Residual Stress, Volume 2. Springer Nature Switzerland, 2024. http://dx.doi.org/10.1007/978-3-031-50470-9_14.

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Maskill, Howard. "Elimination reactions to give alkenes." In Mechanisms of Organic Reactions. Oxford University Press, 1996. http://dx.doi.org/10.1093/hesc/9780198558224.003.0003.

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This chapter focuses on elimination reactions to give alkenes. There are two principal alkene-forming reactions from alkyl halides which closely parallel SN2 and SN1 mechanisms. One is bimolecular and concerted, the other is stepwise with an initial unimolecular step. Both reactions overall involve a base and the elimination of HX from a molecule where X is either halogen or another nucleofuge bonded through a hetero-atom to the α-carbon. The chapter then looks at two other mechanisms for the elimination of HX. One is stepwise and involves a base, the other is a thermally induced single-step unimolecular mechanism which does not involve a base. Finally, the chapter considers briefly the mechanism of a nucleophile-induced reaction which yields alkenes though not by the elimination of HX, but X2.

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Wang, Jing, and Xiang Yi. "A Hybrid Detection Approach for Carbon Emission Intensity Reduction Mechanism Under Environmental Regulations." In Advances in Transdisciplinary Engineering. IOS Press, 2023. http://dx.doi.org/10.3233/atde230302.

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In this paper, an action mechanism of the carbon emission intensity (CE) reduction relevant to the level of industrial agglomeration (IA), scientific and technological progress (TECH), foreign direct investment (F), and environmental regulation (ER) is studied by applying a hybrid framework driven by the K-means clustering algorithm and the stepwise multiple linear regression (SMLR)-based models. To concisely clarify the relationship between the objected variable and impact factors, three echelons of urban agglomerations are summarized in this study from the different industry characteristics of the whole 21 cities in Guangdong, which exhibits a strong growth of the secondary industry in China. Stepwise multiple linear regression (SMLR) analysis is proposed to reveal that strict environmental regulations for the urban agglomeration with low overall development level and weak industrial foundation are more likely to stimulate the “innovation compensation” effect. For the urban agglomeration in the growth stage of industrial economic development or distributed in the developed region, strengthening the level of IA and highlighting the core guiding role of environmental regulation seems to make more sense to reduce the carbon emission intensity and promote the regional industrial green upgrading.

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Hou, Jing, Amy Z. Zeng, and Lindu Zhao. "Revenue Sharing in a Two-Stage Supply Chain with Linear Stepwise Unit Inventory Holding Cost." In Supply Chain Optimization, Management and Integration. IGI Global, 2011. http://dx.doi.org/10.4018/978-1-60960-135-5.ch018.

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In this chapter we focus on examining the coordination mechanisms for a two-stage supply chain comprising one supplier and one retailer. We consider such a channel relationship that the transaction quantity between the two members is sensitive to the supplier’s inventory level and that the supplier’s unit inventory holding cost has a linear stepwise structure. We devise a coordinated revenue-sharing contract with bargaining so that each party’s respective profit is better than that resulted from the simple sequential optimization mechanism. The key contract parameters, namely the supplier’s inventory level and the retailer’s revenue-sharing fraction, are obtained and analyzed. Numerical illustrations of the contracts are given and shed lights on how the supply chain should coordinate in order to gain better performance.

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Kirby, A. J. "Substitutions at saturated centres." In Stereoelectronic Effects. Oxford University Press, 1996. http://dx.doi.org/10.1093/hesc/9780198558934.003.0005.

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This chapter focuses on substitutions at saturated centres. Substitution, or displacement, reactions are the commonest of all organic reactions, and substitutions at sp3-carbon were the first to be studied in depth. We know that two, conceptually quite different, mechanisms may be involved. The reaction can be concerted, with the new bond forming as the old bond breaks, or stepwise, with separate bond-breaking and bond-making steps. It can be a cation, as in the SN1 mechanism for nucleophilic substitution; an anion, in which case the reaction is electrophilic substitution; or a radical. The chapter then looks at concerted nucleophilic substitution; stereoelectronic barriers to intramolecular alkyl-group transfer; and the stabilization of the SN2 transition state by delocalization.

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Ostroumov, Sergey, Leonidas Tsiopoulos, Marina Waldén, and Juha Plosila. "Hierarchical Agent-Based Monitoring Systems for Dynamic Reconfiguration in NoC Platforms." In Advances in Systems Analysis, Software Engineering, and High Performance Computing. IGI Global, 2014. http://dx.doi.org/10.4018/978-1-4666-6034-2.ch013.

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A Network-On-Chip is a paradigm that tackles limitations of traditional bus-based interconnects. It allows complex applications that demand many resources to be deployed on many-core platforms effectively. To satisfy requirements on dependability, however, a NoC platform requires dynamic monitoring and reconfiguration mechanisms. In this chapter, the authors propose an agent-based management system that monitors the state of the platform and applies various reconfiguration techniques. These techniques aim at enabling uninterruptable execution of applications satisfying dependability requirements. The authors develop the proposed system within Event-B that provides a means for stepwise and correct-by-construction specification supported by mathematical proofs. Furthermore, the authors show the mechanism of decomposition of Event-B specifications such that a well-structured and hierarchical agent-based management system is derived.

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"Pericyclic Reactions." In Reactivity and Mechanism in Organic Chemistry. The Royal Society of Chemistry, 2022. http://dx.doi.org/10.1039/9781837670970-00101.

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Chapter 3 is dedicated to pericyclic reactions and selected rearrangement reactions, and to reactions involving transient intermediates, such as radicals, diradicals or carbocations. The class of electrocyclic ring-opening reactions is first used to illustrate the application of FMO theory arguments and the classification of reactions as orbital symmetry allowed or forbidden. After a short section on biradical intermediates in cyclization reactions of enediynes and eneyne-allenes, we next analyze cycloaddition reactions with the goal of classifying reactions as concerted or stepwise, as synchronous or asynchronous, and as antarafacial or suprafacial. Aside from (4+2) Diels–Alder reactions, this also includes a larger selection of (3+2) 1,3-dipolar cycloaddition reactions with dipoles of either the allyl or the propargyl type, and a short note on cheletropic reactions. That the chemistry of carbenes extends far beyond the limits of this latter reaction type is illustrated with reactions catalyzed by N-heterocyclic carbenes. The next large section is then dedicated to sigmatropic rearrangement reactions, where we use FMO theory to understand the bond-making/bond-breaking events in rearrangements of the [3,3]- and the [1,2]-variants. This also includes a comparison with closely related reactions, such as the Hock rearrangement and the Baeyer–Villiger oxidation. The stereochemical requirements of [1,5]- and [1,7]-hydrogen transfer reactions are examined next, followed by those for Wittig rearrangements of the [2,3]- and [1,2]-types. The chapter concludes with a short note on ene reactions.

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Conference papers on the topic "Stepwise mechanism"

Yao, Yanxia, Yuteng Huang, Ke Chen, Tianqi Yu, Zhangchi Ying, and Qiang Yang. "Fuzzy Logic Based Mechanism for Stepwise Carbon Trading in Park-Level Integrated Energy Systems." In 2024 China International Conference on Electricity Distribution (CICED). IEEE, 2024. http://dx.doi.org/10.1109/ciced63421.2024.10753800.

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Haukås-Eide, Odd, Bjørn-Andreas Hugaas, and Espen Heier. "Effect of Plastic Deformation on Hydrogen Induced Cracking (HISC) in Duplex Stainless Steel." In CORROSION 2007. NACE International, 2007. https://doi.org/10.5006/c2007-07494.

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Abstract The oil and gas industry has the last few years experienced several major failures in subsea components made of Duplex stainless steel due to Hydrogen Induced Stress Cracking (HISC) with hydrogen introduced from the cathodic protection system. The main objective of the work presented in this paper has been to gain a better understanding of the HISC fracture mechanism. A test method allowing examination of the deformation behaviour in the individual phases during stepwise loading has been developed, and further the deformation in the individual phases have been documented. Both as delivered and hydrogen pre-charged samples revealed that the plastic deformation starts in the austenitic phase (γ) while the plastic deformation along slip lines in the ferrite appears to be initiated from dislocation pile ups at the austenite/ferrite phase boundaries. For specimens charged with Hydrogen, HISC took place in the ferrite, most likely initiated as a result of dislocation pile ups in adjacent austenite phases.

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Hoffmeister, Hans. "Environmentally Induced Failure of a Nickel Based Ball Valve Plating and Modeling the Corrosion Effects on Pure Nickel of H2S, Chlorides, pH and Pressure." In CORROSION 2009. NACE International, 2009. https://doi.org/10.5006/c2009-09351.

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Abstract A failure of a nickel alloy plating of a sour gas ball valve revealed sulfide-and chloride induced local pitting corrosion. With the background of the prevailing field operating conditions, modeling of local corrosion was carried out for pure nickel by coupling of phase precipitation and polarization behavior. The effects of bulk composition on ion migration to the diffusion layer, its sulfide and chloride precipitations affecting anodic polarization are integrated in a time stepwise procedure. As a main parameter the reduction in bulk pH accelerates local corrosion as well as local acidification in pits. The effects of bulk chloride and hydrogen sulfide contents on corrosion rates varies with the respective ranges of those constituents similar to the behavior of pure iron in earlier work. Depending on the hydrogen sulfide to chloride bulk ratio the local environment may either acidify to low pH levels induced by protecting sulfide precipitations leading to reduced corrosion rates or stay at higher pH with precipitation of non protecting chlorides involving higher corrosion rates. Reduced total pressures within the relevant ranges increase the corrosion rates due to acceleration of the cathodic reaction. Closer understanding is provided of the corrosion mechanism based on thermodynamic heterogeneous equilibrium considerations applied to the small reaction volume of the thin diffusion layer.

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Mahmoud, S. E., C. W. Petersen, and R. J. Franco. "Overview of Hydrogen-Induced Cracking (HIC) of Pressure Vessels in Upstream Operations." In CORROSION 1991. NACE International, 1991. https://doi.org/10.5006/c1991-91010.

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Abstract As a result of the catastrophic failure of an amine absorber vessel in a refinery in 1984 [1], the industry became more aware of a potential failure mechanism for pressure vessels, and many inspections of pressure vessels operating in wet, sour service were performed. Vessels inspected in the downstream during the past five years showed that about 25 percent of all vessels inspected contained cracks [2]. The cracks discovered during these inspections, aside from fabrication defects, were identified as stepwise cracks (SWC), blisters, hydrogen-induced cracks (HIC), and sulfide-stress cracks (SSC). After the 1984 refinery accident, Exxon launched, and continues to conduct, large-scale inspection programs in the downstream. These inspection programs soon extended to pressure vessels operating in wet, sour service in the upstream. One upstream production facility vessel was inspected in late 1987 and was found to contain HIC. Work immediately began on establishing guidelines for upstream operations, and recommendations for preventing HIC in upstream operations were outlined and published in early 1989. In 1990, upstream inspections covered more than 200 vessels in wet, sour service (&gt;50 wppm H2S) and approximately 21 percent of these vessels were found to contain HIC cracks. This paper summarizes Exxon's upstream inspection techniques, inspection findings, and repair options for pressure vessels. The challenges facing the upstream industry regarding cracking are discussed in this paper along with discussions on HIC-resistant steels and weld repair of vessels in sour service.

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Hoffmeister, Hans. "Modelling the Effects of Crevice Geometry and Oxygen on Crevice Corrosion of Pure Chromium at Constant pH and Various Chloride Contents." In CORROSION 2002. NACE International, 2002. https://doi.org/10.5006/c2002-02194.

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Abstract In a recent paper a crevice corrosion model for pure chromium was presented aiming at basic understanding of the local corrosion mechanism in an already deoxygenated crevice with a given geometry. The time stepwise calculation of chromium dissolution was based on arbitrarily selected levels of initial corrosion potential and slopes of the cathodic polarization curve at the open surface in contact with the bulk liquid. As a basic feature, the anodic polarization slopes inside the crevice were controlled by the mass of precipitated chromium hydroxide which was thermodynamically calculated from the resulting crevice solution composition using the tentative ternary phase diagram water - chromium – chromium hydroxide – chromium chloride. The present paper applies additional oxygen diffusion calculation steps and demonstrates the effects of various crevice widths and bulk oxygen contents diffusing into the crevices at pH= 6 and chloride contents of 7 and 13 %. The calculations are based on the assumption of chemical equilibrium conditions as well as on rather short electromigration times of the chromium controlled chloride ions complementary to the OH- ions resulting from oxygen diffusion and reduction during the individual time steps. It is shown that increasing bulk oxygen levels and crevice widths are retarding the breakdown times of the passivating chromium hydroxide, while, under the given conditions, increasing chloride contents mainly reduce the crevice corrosion incubation times. As a specific result it is also demonstrated that after chromium hydroxide break down, the times for consecutive repassivation by restart of the chromium hydroxide precipitation inside an active crevice are reduced at increasing bulk oxygen levels.

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Nyborg, Rolf. "Initiation and Growth of Mesa Corrosion Attack During CO2 Corrosion of Carbon Steel." In CORROSION 1998. NACE International, 1998. https://doi.org/10.5006/c1998-98048.

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Abstract The initiation and development of mesa corrosion attack during CO2 corrosion of carbon steel has been studied in flow loop experiments performed at 80 °C and pH 5.8. Video recordings of growing mesa attacks have been performed in a test section with a glass window in the corrosion loop. These observations have shown that the mesa attack can grow both laterally and in depth below a lid of original corrosion film before the film is tom away stepwise by the flow. Possible mechanisms for initiation of mesa corrosion attack are discussed based on the observations from the video recordings. Mesa attacks can result from several small local attacks growing together into one large mesa attack.

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Olsen, Jim Stian, Bård Nyhus, Vigdis Olden, and Roy Johnsen. "Hydrogen Induced Stress Cracking of Ni-Alloy 625 (UNS N06625)." In CORROSION 2021. AMPP, 2021. https://doi.org/10.5006/c2021-16616.

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ABSTRACT Limited evidence has been published which indicate that UNS N06625 (Alloy 625) is susceptible to hydrogen embrittlement. In this study we present new industry learning about how the performance of Alloy 625 is affected by hydrogen. Three different Ni-alloy 625 batches from different suppliers, have been examined to evaluate the susceptibility to hydrogen induced stress cracking, and to evaluate if inherent differences have an impact on the material behavior in a hydrogen environment. A rigorous test program was established which included metallurgical and mechanical characterization, in air and hydrogen, of smooth and notched specimens. The mechanical characterization was performed using a stepwise constant load testing (SCLT) procedure to determine the fracture stress and effect of time dependent parameters. All specimens were electrochemically pre-charged for 7 days at elevated temperature before the SCLT was conducted with in-situ electrochemical charging. The results exhibit clear evidence of hydrogen embrittlement, as the fracture surfaces were dominated by intergranular and transgranular features. Two materials fractured at net section stresses similar to actual yield strength or below, while one material exhibited significantly higher resistance to hydrogen embrittlement. The results further indicate that the manufacturing method may have impact on hydrogen embrittlement resistance, and that microstructural features may affect fracture mechanisms in a hydrogen environment.

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Crolet, Jean-Louis, and Michel R. Bonis. "Revisiting Hydrogen in Steel, Part I : Theoretical Aspects of Charging, Stress Cracking and Permeation." In CORROSION 2001. NACE International, 2001. https://doi.org/10.5006/c2001-01067.

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Abstract This article first of all summarizes recent changes in the understanding of hydrogen charging and hydrogen stress cracking (HSC) mechanisms : In a completely unexpected manner, hydrogen charging is found to coexist with significant degassing of the metal towards the corrosive medium. The surface reaction 2 Hads → H2 ➚ is thus extremely rapid, even in sour media. It is therefore in no way inhibited, contrary to former belief.Charging probably involves a cathodic reaction in which a proton is transferred directly from the electrolyte into the metal, without requiring the intermediate formation of an adsorbed hydrogen atom. Charging and degassing can therefore coexist. Charging is thus itself a faradaic process and is consequently extremely powerful.Damage due to so-called ‘‘internal” hydrogen (blistering, hydrogen-induced cracking, stepwise cracking) always involves bulk metallurgical processes related to the concentration of dissolved H.In contrast, damage due to so-called “external” hydrogen (HSC, SSC) involves surface processes related only to the charging rate. The forced injection of protons locally expands the crystal lattice, thus inducing high surface stresses that add to the working stresses and cause surface crack initiation.Finally, the coexistence of charging and degassing explains why the severity of the medium varies with PH2S in a different way depending on whether “external” or “internal” hydrogen is concerned. The consequences on permeation measurements and their interpretation are then discussed. In particular, the coexistence of charging and degassing radically modifies permeation conditions, with the need to separate thick membranes, for which the concentration c0 beneath the entry face is constant and the diffusion flux inversely proportional to the diffusion distance d, and thin membranes, for which the diffusion flux is constant and the concentration c* beneath the entry face proportional to d. All these theoretical predictions have the advantage that they can be verified experimentally, and Part II of the present study shows that this is indeed the case.

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He, Yi, Yonghui Sun, Wei Zhou, Pengpeng Wu, Li Chen, and Yunfan Meng. "Robust Optimal Economic Dispatch of Microgrid with Stepwise Demand Response Mechanism." In 2023 IEEE/IAS Industrial and Commercial Power System Asia (I&CPS Asia). IEEE, 2023. http://dx.doi.org/10.1109/icpsasia58343.2023.10294797.

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Mirth, John A. "The Application of Geometric Constraint Programming to the Design of Motion Generating Six-Bar Linkages." In ASME 2012 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference. American Society of Mechanical Engineers, 2012. http://dx.doi.org/10.1115/detc2012-70176.

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This paper looks at the application of Geometric Constraint Programming (GCP) to the synthesis of six-bar planar linkages. GCP is a synthesis method that relies on the built-in geometric capabilities of commercial solid-modeling programs to produce linkage designs while operating in the “sketch” mode for these programs. GCP provides the user with the opportunity to create mechanisms in their entirety at multiple design positions. The complexity of analyzing potential defects (such as circuit or branch defects) within a six-bar mechanism poses significant challenges to the user who might try to design such a mechanism in a single step. The methods presented in this paper apply GCP in a stepwise manner to create six-bar linkages that are less likely to suffer from defects than if they were created in a single step. Stepwise approaches are presented for six-bar mechanisms designed to solve a problem involving rigid-body guidance (motion generation). The linkages considered include the Stephenson I, II, and III chains, as well as the Watt I six-bar. The Watt II six-bar is not included since this mechanism’s application to rigid-body guidance can be handled by GCP methods previously developed for four-bar linkages.

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Reports on the topic "Stepwise mechanism"

Gupta, Anshu, and Christopher W. Macosko. Modeling Strategy for Networks with Both Stepwise and Chainwise Mechanisms: Amine-Epoxy Chemistry with Etherification. Defense Technical Information Center, 1989. http://dx.doi.org/10.21236/ada209889.

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