Holistic model for atmospheric corrosion Part 7 – Cleaning of salt from metal surfaces

Abstract

This is the seventh paper outlining a holistic model of atmospheric corrosion. Previous papers have outlined how airborne salinity can be estimated at any given location, how salt deposition onto a surface can be modelled and what forms moisture can take on a plate surface. The present paper develops a model for rain induced cleaning of deposited aerosols from metal surfaces. The individual phenomena of splash and wash off are modelled, and then the effect of these processes on salt retention is discussed. The results of the models are compared with data from field exposures and laboratory tests. Simplified rules to allow the process to be incorporated into the holistic model framework are proposed.     

Control of corrosion of carbon steel in crude oil distillation units

Abstract

Experiments were conducted to investigate the behaviour of different concentrations of 3–amino–1,2,4–triazole (ATR), 2–amino-thiazole (ATH), and 2,6–diamino-pyridine (DAP) as corrosion inhibitors for carbon steel exposed to a kerosene-water mixture containing 3ppm HCl and 800ppmH₂S/day at 55–60°C and pH 6–6·5 with different exposure times, using ammonia as a neutralising agent. Screening tests over,a period of 50 h showed that the presence of 10ppm of the compounds (ATR, ATH, and DAP) with ammonia as a neutralising agent in the absence and in the presence of 10ppm octylamine gave good inhibitor efficiencies (～90%). After 400 h of exposure the inhibitor efficiencies ranged between 50–75% depending on the material added to the test solution. XPS studies of the layer formed on the carbon steel surface indicate the formation of Fe(II) and Fe(III) complexes with the organic compounds used.     

Holistic model for atmospheric corrosion Part 4 – Geographic information system for predicting airborne salinity

Abstract

This paper is the fourth in a series of five papers addressing the factors controlling and predicting marine aerosol concentrations on land. It outlines the procedures used to develop a geographic information system (GIS) model of airborne salinity across Australia. The basic mathematical formulae have been presented previously, and this paper outlines how these formulae are applied in a GIS system. While the basic formulae are based on models of the processes controlling the formation and distribution of marine aerosols, some constants have been determined empirically. These constants are established by regression analysis using data derived and presented previously from a long range study of atmospheric salinity levels in a transect across the Australian State of South Australia. The model was then tested with databases of airborne salinity measurements throughout Australia and was shown to possess a high level of accuracy. It was then used to predict the salinity across Australia, with a salinity map of Australia being produced. A number of features of the map are discussed. Lastly, the model is used to illustrate the role of coastal forms in controlling salinity in adjacent regions.     

Review of potable water biofilms in engineered systems

Abstract

Microbial biofilm formation on pipe walls has been shown to cause serious problems in potable water systems, including corrosion and the provision of sites for pathogenic bacterial development. The potential health threat associated with biofilms in potable water systems is discussed. An emerging problem is developing within the water sector with respect to the virulence of opportunistic bacterial pathogens which are, as well as being part of the normal planktonic heterotrophic bacterial population, readily isolated as sessile populations removed from pipes transporting potable water. Essentially only the bacteria in biofilms of potable water systems are multiplying and, owing to sloughing as a result of water shear, these constitute one of the main causes of deterioration of the microbiological quality of potable water systems, which could constitute a public health issue.     

Thermodynamic calculation and reference electrode calibration for high temperature molten salts

Abstract

Molten salts are important reaction media for chemical and electrochemical processing and have recently attracted attention for their potential in reprocessing and partitioning spent nuclear fuels. Electrochemical measurements are a convenient tool for exploring thermodynamic and kinetic properties of molten salts, but inconsistency in acquired data may arise from the use of inaccurate reference electrodes and differences in thermodynamic calculations. A thermodynamic approach to the calculation of half cell potentials for reactions in molten salts is proposed. As examples, chlorine/chloride and lithium ion/lithium half cell potentials in LiCl–KCl eutectic are thermodynamically analysed. The Ag/AgCl reference electrode is discussed as an example of a high temperature reference electrode. A technique involving in situ transient reduction of constitutive metal ions for the calibration of high temperature reference electrodes is developed which may enable the consistency of acquired data using different reference electrodes in a variety of molten salts. The thermodynamic approach and calibration technique may be extended to ionic liquid and other media at high and low temperatures.     

Experimental studies of salts removal from metal surfaces by wind and rain

Abstract

This paper outlines an experimental programme aimed at determining the mechanisms of salt removal from metallic surfaces by wind and rain. The paper provides data and process understanding that will be integrated into a holistic model of atmospheric corrosion. Experiments are reported in which fine salt crystals were deposited either wet (as fine droplets of seawater) or dry onto a surface. The surfaces were then placed in a wind tunnel and salt loss was determined as a function of wind speed. In another experiment, the motion of simulated raindrops falling onto metal plates was determined. Finally, in a third series of experiments, video footage of simulated rain on metal surfaces was studied in order to determine both the subsequent motion of rain and its cleaning efficiency. The efficiency of surface cleaning by wind and rain, as determined by these experiments, was then related to Australian climatic conditions. A future paper will present mathematical models of the processes defined in this paper.     

Reduction of titanium and other metal oxides using electrodeoxidation

Abstract

Many reactive and refractory metals are currently produced industrially by reducing their compounds, including oxides, using a more reactive metal. In some cases, where there is substantial oxygen solubility in the metal, the oxygen is first removed by carbochlorination followed by reduction. Titanium and zirconium are made by reduction of the volatile tetrachlorides by magnesium. The processes consist essentially of two reduction steps: reducing magnesium chloride to magnesium metal and then reduction of the metal compound; this makes the overall reduction process relatively expensive. Electrodeoxidation is very simple in that the oxide to be reduced is rendered cathodic in molten alkaline earth chloride. By applying a voltage below the decomposition potential of the salt, it has been found that ionisation of oxygen is the dominant cathode reaction, rather than alkaline earth metal deposition. In the laboratory, this technique has been applied to reduce a large number of metal oxides to the metals, including titanium, zirconium, chromium, niobium, tantalum, uranium and nickel. Furthermore, when mixed oxides are used as the cathode, alloys or intermetallic compounds of uniform composition are obtained. This may offer advantages over conventional technology for those alloys that are difficult to prepare at present, owing to differences in either density or vapour pressure.     

Statistical analysis of metastable pitting events on carbon steel

Abstract

The metastable pitting behaviour of galvanically coupled and potentiostatically controlled A516–70 carbon steel in a chloride containing solution has been statistically analysed. It is shown that the pitting activity, indicated by the pit initiation rate, depends on immersion time, applied potential, and chloride concentration. There is a strong correlation among the pitting events at a high pitting activity, where the metastable pitting events follow non-Poissonian behaviour and a previous pit has an influence on subsequent pitting events. When the pitting activity decreases, the correlation decreases and approaches the Poisson distribution.