Sulphates sulphidation environments

Low-carbon and chromium-nickel steels, certain copper, nickel and aluminium alloys (which are all widely used in marine and offshore engineering) are liable to exhibit stress-corrosion cracking whilst in service in specific environments, where combinations of perhaps relatively modest stress levels in material exposed to environments which are wet, damp or humid, and in the presence of certain gases or ions such as oxygen, chlorides, nitrates, hydroxides, chromates, nitrates, sulphides, sulphates, etc. 

General corrosion damage was the cause of failure of an A1 alloy welded pipe assembly in an aircraft bowser which was attacked by a deicing-fluid — water mixture at small weld defects . Selective attack has been reported in welded cupro-nickel subjected to estuarine and seawater environments . It was the consequence of the combination of alloy element segregation in the weld metal and the action of sulphate reducing bacteria (SRB). Sulphide-coated Cu-enriched areas were cathodic relative to the adjacent Ni-rich areas where, in the latter, the sulphides were being continuously removed by the turbulence. Sulphite ions seemed to act as a mild inhibitor. 

Composition of the liquid environment The ionic composition, arising from dissolved salts and gases, has a considerable influence on the performance of inhibitors. In near-neutral aqueous systems the presence of certain ions tends to oppose the action of inhibitors. Chlorides and sulphates are the most common examples of these aggressive ions, but other ions, e.g. halides, sulphides, nitrates, etc. exert similar effects. The concentration of inhibitor required for protection will depend on the concentrations of these aggressive ions. Laboratory tests " have given some quantitative relationships... 

Five continuous ambient air monitoring stations are indicated in Figure 6. At one minute intervals data on these parameters are transmitted by radio telemetry or land line to a minicomputer in the Operations Laboratory. Data manipulation is performed by the minicomputer and all data are transmitted to a large computer in Edmonton on a daily basis. In addition to these five stations, the monitoring network contains 40 static ("Candle") stations. The location of these stations are also given in Figure 6. The static stations have been in operation since May, 1977, one year before start-up, and provide monthly information on total sulphation and hydrogen sulphide. All data collected from the network are summarized in a prescribed format and submitted to Alberta Environment on a monthly and yearly basis. 

Bacterial reduction of sulfate in an anaerobic environment with large isotope fractionation between the residual sulphate and the sulphide is the source of sulphur and sulphide in many deposits. The recognition that many sulphur... 

Figure 6.3 shows a mass balance model for sulphur speciations. Once other more thermodynamically amenable electron acceptors have been depleted, sulphate is used by sulphate reducing bacteria, such as Desulfovibrio sp. This occurs until the sulphate concentration falls to a point where it is outcompeted for the organic substrate by methanogenic bacteria. The sulphide produced by sulphate reduction is then removed or recycled by different processes depending on the environment of deposition. 

Another major increase in atmospheric 02 may have occurred towards the end of the Proterozoic (Knoll et al. 1986 Derry et al. 1992 Des Marais et al. 1992), providing the opportunity for more complex, multicellular eukaryotes to evolve. Further evidence for such an increase is provided by the isotopic record of sedimentary marine sulphides (Canfield 1998 see Box 1.13). From c.2.3 to between 1.05 and 0.64 Ga the sulphur isotopic fractionation was <4%o with respect to marine sulphate, suggesting low sulphate levels, consistent with limited oxygenation of the atmosphere and surface waters, sufficient to allow enough oxidation of sulphide to sulphate for the use of sulphate-reducing bacteria in deeper anoxic environments. Enough sulphide may have been produced by these bacteria to remove dissolved iron(II) as pyrite. So for a large part of the Proterozoic bottom waters remained anoxic and sulphidic... 

Sulphur mineralisation Sulphur is ubiquitous in minerals at hydrothermal vents (e.g. native sulphur, metal sulphides, sulphates), and the formation of many of these minerals is potentially influenced by the associated microbial activity at vents. For example, microbial filamentous sulphur formation has recently been proposed as a widespread activity in mixed H2S-02 environments and may be a quantitatively important microbial process in hydrothermal vent... 

Reduction environment is described by total absence of O. It includes zones sulphid, where reduction of sulphates begins, and methane, where sulphate-reduction is replaced by methane formation. Significance and distribution of these zones largely depend on the content of organic matter and ground water salinity. 

Lead dross from heating lead ore (principally lead sulphide) in an oxidizing environment is high in lead sulphate. In fact, this process is one of the methods used commercially to manufacture this acid. 

If there is no access to air, there is usually no significant corrosion on ferrous materials when pH > 5. However, in some environments, heavy localized corrosion may occur due to bacteria that thrive in near neutral environment (pH = 5-8) at temperatures of 10-40°C in the absence of air (anaerobic bacteria). These bacteria promote reduction of present sulphate to sulphide. Hence, they are usually called sulphate-reducing bacteria (SRB). The classical mechanism comprises the following reactions at/on the cathodes [6.16] ... 

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