Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

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Corrosion is a subject of interest to interdisciplinary research communities, combining fields of materials science, chemistry, physics, metallurgy and chemical engineering. In order to understand mechanisms of corrosion and the function of corrosion inhibitors, the reactions at the interfaces between the corrosive electrolyte and a steel surface, particularly at the initial stages of the corrosion process, need to be described. Naturally, these reactions are strongly affected by the nature and properties of the steel surfaces. It is however seen that the majority of recent corrosion and corrosion-inhibition investigations are limited to electrochemical testing, with ex situ analysis of the treated steels (post-exposure analysis). The characterization of materials and their surface properties, such as texture and morphology, are not being considered in most studies. Similarly, in situ investigations of the initial stages of the corrosion reactions using advanced surface characterization techniques are scarce. In this review, attention is brought to the importance of surface features of carbon steels, such as texture and surface energy, along with defects dislocation related to mechanical processing of carbon steels. This work is extended to a critical review of surface analytical techniques used for characterization of carbon steels in corrosive media with particular focus on examining steel surfaces treated with corrosion inhibitors. Further, emerging surface analysis techniques and their applicability to analyse carbon steels in corrosive media are discussed. The importance of surface properties is commonly addressed by surface scientists as well as researchers in other chemistry fields such as nanotechnology, fuel cells, and catalysis. This article is expected to appeal to a broad scientific community, including but not limited to corrosion scientists, material chemists, analytical chemists, metal physicists, corrosion and materials engineers.

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Carbon steel corrosion: a review of key surface properties and characterization methods

I .. Rates of ionic mass transfer at nickel electrodes rotating about their axes In the center of stationary electrodes were studied using the ferri-ferrocyanide oouple In alkaline solutions. A general mass transfer correlation was found to apply equally well to the dissolution rates of rotating solids and to the rates of ionlo mass transfer at rotating electrodes. This correlation takes into account the physioal properties of the system as well as geometric and hydro dynamic factors. The correlation enables the prediction of limiting currents and concentration polarisation at rotating electrodes under a wide range of conditions. The nature of the polarisation involved in the reduotion of Fe(CNfe~» and the oxidation of Fe(CN#* was also Investigated. The polarisation was found to depend strongly on the presence of "electrode poisons." With freshly prepared solutions, under exclusion of light and with eathodlcally treated nickel electrodes relatively small ehesdcal polarisations were determined. For rotational speeds not exceeding Reynolds number 11,000, the chemical polarisation was found to be negligicle in comparison with the concentration polarisation* Under such conditions the ferro-ferrioyanide oouple can be conveniently used to obtain mass transfer rates for various hydrodynamic conditions, or, conversely, to verify the validity of mass transfer equations by a comparison of experimental .and calculated values of limiting currents and concentration polarisation. The rotating electrode model was found to be most suitable for studying the nature of eleotrolytio polarisation phenomena on aocount of the uniformity of the current distribution as well as of the hydrodynamio diffusion laver at the electrode surfsee.

Mass transfer in electrode processes. 3. ionic mass transfer and concentration polarization at rotating electrodes

Corrosion is a major cause of structural deterioration in marine and offshore structures. It affects the life of process equipment and pipelines, and can result in structural failure, leakage, product loss, environmental pollution and the loss of life. Pitting corrosion is regarded as one of the most hazardous forms of corrosion for marine and offshore structures. The total loss of the structure might be very small, but local rate of attack can be very large and can lead to early catastrophic failure. Pitting corrosion is a localized accelerated dissolution of metal that occurs as a result of a breakdown in the protective passive film on the metal surface. It has been studied for many years; however, the structural failure due to pit characteristics is still not fully understood. Accurate pit depth measurements, precise strength assessment techniques, risk analysis due to pitting, and the mathematical relationship of the environmental factors that causes pitting failure are also factors, which need further understanding. Hence this paper focuses on these issues. It reviews and analyses the current understanding of the pitting corrosion mechanism and investigates all possible factors that can cause pitting corrosion. Furthermore, different techniques employed by scientists and researchers to identify and model the pitting corrosion are also reviewed and analysed. Future work should involve an in-depth scientific study of the corrosion mechanism and an engineering predictive model is recommended in order to assess failure, and thereby attempt to increase the remaining life of offshore assets.

Modelling of pitting corrosion in marine and offshore steel structures – A technical review

Evolution of dissolution processes at the interface of carbon steel corroding in a CO2 environment studied by EIS

The internal corrosion of mild steel in the presence of hydrogen sulfide (H2S) represents a significant challenge in oil production and natural gas treatment facilities, but the underlying mechanisms involved in H2S corrosion are still not fully understood. This lack of knowledge makes the prediction, prevention, and/or control of aqueous H2S corrosion of mild steel much more difficult. In the present study, H2S corrosion mechanisms were experimentally investigated in short-term corrosion tests (lasting 1 h to 2 h), conducted in a 1 wt% sodium chloride (NaCl) solution at different pH (pH 2 to pH 5), at different temperatures (30°C to 80°C), under various H2S/N2 gaseous concentration ratios (0 to 10%[v]) and flow rates, using a X65 mild steel rotating cylinder electrode. Corrosion rates were measured by linear polarization resistance (LPR). Corrosion mechanisms were investigated by using potentiodynamic sweeps and by comparison with electrochemical modeling. LPR results showed that corrosion rates increase...

Electrochemical Study and Modeling of H2S Corrosion of Mild Steel

Expanding the basic knowledge required for improved understanding of hydrogen sulfide (H2S) corrosion of mild steel is needed. When it comes to even the most basic chemical descriptors of aqueous H2S systems such as H2S solubility, water chemistry, polymorphism of iron sulfide, the formation or dissolution of iron sulfide scale, and its protectiveness, many open questions persist and more investigation is required. Inconsistencies in prevailing expressions for H2S solubility constant (KH2S), the first dissociation constant (Ka,1), and the second dissociation constant (Ka,2) are reviewed here. By comparing with experimental data, the best prediction model for H2S solubility and dissociation in an H2S–H2O system was identified. Occurrence of iron sulfide polymorphs was experimentally investigated and it was found that, in short-term exposures, mackinawite formed at 25°C, while greigite and pyrite were detected at 60°C. The solubility limits for these iron sulfides were determined. Simplified Pourbaix diagra...

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Thermodynamic Study of Hydrogen Sulfide Corrosion of Mild Steel

Formation of iron oxide and iron sulfide at high temperature and their effects on corrosion

http://www.icmt.ohio.edu/documents/journals2011/Spontaneous%20passivation%20observations%20during%20scale%20formation%20on%20mild%20steel_JHan.pdf

Bruce Brown

Papers

Evolution of dissolution processes at the interface of carbon steel corroding in a CO2 environment studied by EIS

Electrochemical Study and Modeling of H2S Corrosion of Mild Steel

Thermodynamic Study of Hydrogen Sulfide Corrosion of Mild Steel

Formation of iron oxide and iron sulfide at high temperature and their effects on corrosion

Spontaneous passivation observations during scale formation on mild steel in CO2 brines