A model for the corrosion of steel subjected to synthetic produced water containing sulfate, chloride and hydrogen sulfide

A model was developed for the prediction of corrosion rates associated with steel subjected to synthetic produced water. The corrosive species included in the model, identified through water analysis conducted in the field, are sulfate, chloride and hydrogen sulfide. The effect on corrosion of these species was examined through polarization experimentation using a three electrode glass corrosion cell and potentiostat. Samples of carbon steel, used in sub-sea pipeline systems, were used at the working electrode and the experiments were carried out at similar physicochemical conditions observed in pipeline systems in the field. The model was based on heterogeneous reactions at the metal surface, with electrochemical parameters determined through experimentation employed in the model to describe the anodic and cathodic processes involved in the corrosion of steel. The model consists of a system of equations with Butler–Volmer kinetics describing the charge transfer and the Nernst diffusion model the mass transfer processes occurring in the corrosion system. The solution is based on a charge balance between the reduction and the oxidation processes which occur at the steel surface. Current density convergence criteria were used in the model to solve the system of equations for corrosion potential, surface species concentration and component current densities. The corrosion rate is determined as the rate of oxidation of iron at the surface and model results have been validated using experimental data. The model demonstrates a reasonable qualitative match with corrosion data collected in the potential region close to the corrosion potential in general, with good qualitative match in the anodic region near the corrosion potential. Some deviation occurs between model and experimental values where overpotentials become large but the model is shown to respond well to changes in input parameter values and predicts the corrosion potential and corrosion rate for each system within experimental variability and the accepted standards of accuracy.     

Water security and rural household food security: empirical evidence from the Mzinyathi district in South Africa

This paper aimed to investigate the determinants of water security in an irrigation scheme, and how this water security level subsequently affects the farmer’s household food security level. Water security refers to access by the irrigating households to sufficient and reliable water to meet the agricultural needs and their ability to assert the water rights against other parties. A random sample of 185 irrigating households was interviewed in Tugela Ferry Irrigation Scheme in Mzinyathi District, South Africa. Data were analysed using principal component analysis and ordinary least squares. The empirical results indicated that factors such as farmer’s age, off-farm income, farmer association membership, use of pumps, location on the upper-end of the canal and training increase household water security. Conversely, factors such as occurrence of conflicts and location at the tail-end of the canal were found to decrease household water security. This study highlights the importance of strengthening farmer organisational capacity and local institutions for enhancing the water security status of farmers in smallholder irrigation schemes. The results also indicated that perceived water security has a positive impact on household food consumption per adult equivalent. Therefore, for better impact on household food security, the study recommends that priority should be placed in ensuring household water security, not just investing in the physical irrigation scheme and irrigation participation. The human and social dimensions need to receive priority. Training farmers in collective water governance and water conservation techniques to improve water-use efficiency as well as introducing motorised pumps would take irrigators a long way in enhancing their water security.     

Thermal Degradation Behavior and Product Speciation in Model Poly(dimethylsiloxane) Networks

The thermal degradation behavior of a series of well defined poly(dimethylsiloxane) (PDMS) model networks has been studied using a combination analytical thermal analysis techniques and multivariate statistical analysis in order to probe the influence of network architecture on degradation chemistry. The aim of this research has been to determine the effect differing network architectures: mono and bimodality, a range of crosslink density, inter-chain molar mass and percentage of free chain ends on the mechanisms of PDMS thermal degradation. A series of model PDMS networks have been formulated using of tin catalyzed condensation cure chemistry and a range of linear precursors to yield a matrix of model network systems. The thermal degradation chemistry of these model networks have been characterized in relation to their structure by means of pyrolysis gas chromatography mass spectrometry (Py-GCMS), thermal gravimetric analysis (TGA) and multivariate statistical analysis. The results clearly demonstrate that the structural architecture of (chemically similar) PDMS networks has a significant impact on the mechanisms of PDMS thermal degradation. Notability, with decreasing inter-crosslink chain length, larger cyclic siloxane species (>D5) become more abundant degradation products and that there is a relationship between inter-chain molar mass, degree of crosslinking and the thermal stability on the mechanisms of degradation. This work effectively demonstrates that quantifiable relationships exist between basic network architectures and the distributions of degradation derived species in PDMS networks.     

Book reviews

Urban Policy Evaluation: Challenge and Change. R. Hambleton & H. Thomas (Eds). Liverpool, Paul Chapman, 1995, 258 pp., £17.95 ISBN 1–85396–271–6

Environmental Strategy and Sustainable Development. R. Welford. Routledge, London, 1995, 217 pp, £12.99, ISBN 0–415–10552–8

Urban Agriculture in Zimbabwe. B. Mbiba. Ashgate, Aldershot, UK and Brookfield, VT, 1995, £35.00, ISBN 185628 857 9

Housing Women. Rose Gilroy & Roberta Woods (Eds.). London, Routledge, 1994, 304 pp., £14.99, ISBN 0–415–09463–1

Census User's Handbook. S. Openshaw (Ed.). Longmans, 1995, 454 pp., £19.95, ISBN 0–470234–814

Reconstituting Rurality. J. Murdoch & T. Marsden. London, UCL Press, 1994, 272 pp., £40.00, ISBN 1–85728–041–5

Development and Administration of Prague. M. Barlow, P. Dostal, & M. Hampl (Eds). Amsterdam, Instituut voor Sociale Geographie, Universiteit van Amsterdam, 1994, 170 pp., 35DG, ISBN 9–06993–086‐Z

Local Development, Restructuring, Locality and Local Initiative in Portugal. S. Syrett. Aldershot, Avebury, 1995, 371 pp., £40.00, ISBN 1 85628 484 0     

CO2, CO, and Hg emissions from the Truman Shepherd and Ruth Mullins coal fires, eastern Kentucky, USA

Carbon dioxide (CO₂), carbon monoxide (CO), and mercury (Hg) emissions were quantified for two eastern Kentucky coal-seam fires, the Truman Shepherd fire in Floyd County and the Ruth Mullins fire in Perry County. This study is one of the first to estimate gas emissions from coal fires using field measurements at gas vents. The Truman Shepherd fire emissions are nearly 1400 t CO₂/yr and 16 kg Hg/yr resulting from a coal combustion rate of 450-550 t/yr. The sum of CO₂ emissions from seven vents at the Ruth Mullins fire is 726 ± 72 t/yr, suggesting that the fire is consuming about 250-280 t coal/yr. Total Ruth Mullins fire CO and Hg emissions are estimated at 21 ± 1.8 t/yr and > 840 ± 170 g/yr, respectively. The CO₂ emissions are environmentally significant, but low compared to coal-fired power plants; for example, 3.9 × 10⁶ t CO₂/yr for a 514-MW boiler in Kentucky. Using simple calculations, CO₂ and Hg emissions from coal-fires in the U.S. are estimated at 1.4 × 10⁷-2.9 × 10⁸ t/yr and 0.58-11.5 t/yr, respectively. This initial work indicates that coal fires may be an important source of CO₂, CO, Hg and other atmospheric constituents.     

Hydrogen Storage in Partially Exfoliated Magnesium Diboride Multilayers

Metal boride nanostructures have shown significant promise for hydrogen storage applications. However, the synthesis of nanoscale metal boride particles is challenging because of their high surface energy, strong inter- and intraplanar bonding, and difficult-to-control surface termination. Here, it is demonstrated that mechanochemical exfoliation of magnesium diboride in zirconia produces 3–4 nm ultrathin MgB₂ nanosheets (multilayers) in high yield. High-pressure hydrogenation of these multilayers at 70 MPa and 330 °C followed by dehydrogenation at 390 °C reveals a hydrogen capacity of 5.1 wt%, which is ≈50 times larger than the capacity of bulk MgB₂ under the same conditions. This enhancement is attributed to the creation of defective sites by ball-milling and incomplete Mg surface coverage in MgB₂ multilayers, which disrupts the stable boron–boron ring structure. The density functional theory calculations indicate that the balance of Mg on the MgB₂ nanosheet surface changes as the material hydrogenates, as it is energetically favorable to trade a small number of Mg vacancies in Mg(BH₄)₂ for greater Mg coverage on the MgB₂ surface. The exfoliation and creation of ultrathin layers is a promising new direction for 2D metal boride/borohydride research with the potential to achieve high-capacity reversible hydrogen storage at more moderate pressures and temperatures.