Sulphates sulphidation kinetics

This is a simplified treatment but it serves to illustrate the electrochemical nature of rusting and the essential parts played by moisture and oxygen. The kinetics of the process are influenced by a number of factors, which will be discussed later. Although the presence of oxygen is usually essential, severe corrosion may occur under anaerobic conditions in the presence of sulphate-reducing bacteria Desulphovibrio desulphuricans) which are present in soils and water. The anodic reaction is the same, i.e. the formation of ferrous ions. The cathodic reaction is complex but it results in the reduction of inorganic sulphates to sulphides and the eventual formation of rust and ferrous sulphide (FeS). 

Few kinetic studies of the decompositions of these salts are available and, of those which have been investigated, the sulphites have received greatest attention. In general, sulphites disproportionate [803] in an inert atmosphere to sulphides and sulphates, the latter decomposing at a higher temperature. 

A kinetic study of the oxidation of sulphite ions to sulphate ions by gaseous oxygen in melts of lithium and potassium chlorides at temperatures between 414 and 504 °C has been carried out, and a possible reaction mechanism proposed.14 The interaction of oxygen at 20 °C with hydrogen sulphide presorbed at 55 °C and vice versa has been investigated.15... 

Respiratory sulphate reduction ideally takes place when all other electron acceptors are exhausted, but significant overlap may occur between the zones of microbial Fe(III) reduction and sulphate reduction due to kinetic constraints, as discussed before. Sulphate concentrations typically decrease to zero within the upper sediment layer (Fig. 1). In freshwater sediments, reduced S formed mainly by reduction of pore-water sulphate, is predominantly present as inorganic S in the form of AVS. Although pyrite is the most stable sulphide mineral, its formation in permanently submerged freshwater sediments is subject to controversy (Rickard et al., 1995). Because, contrary to marine sediments (S-dominated), there is an excess of Fe liberation over HS production in freshwater sediments (Fe-dominated), FeC03 as well as FeS may control pore-water Fe concentrations in the anoxic sediment layer. 

Some isotope fractionations, notably those in biological systems, are primarily controlled by kinetic effects. For example, the bacterial reduction of seawater sulphate to sulphide proceeds 2.2 % faster for the light isotope than for S. -/For the reactions... 

Ohmoto H. and Lasaga A.C., 1982, Kinetics of reactions between aqueous sulphates and sulphides in hydrothennal systems. Geochim. Cosmochim. Acta 46, 1727-1745,... 

Some thermodynamic functions at saturation of several metal sulphates in H2SO4 and D28O4 have been calculated from solubility data. The kinetics of dissolutionof iron(ii) sulphide in aqueous sulphuric acid have been measured/ The behaviour of various solutes in chlorosulphuric acid has been investigated conductometrically. A potentiometric study of the system (32) in sulfolane has enabled the dissociation constant of the reaction (33) to be determined. The rate of decomposition of trifluoromethanesulphonic acid in water and its reactivity towards various metals have been investigated. ... 

Sulphidation reactions follow a similar series of kinetic phenomena as has been observed for oxidation. Unfortunately, few studies have been made of the basic kinetic phenomena involved in sulphidation reactions at high temperature. Similarly, the volatile species in sulphate and carbonate systems are important in terms of evaporation/condensation phenomena involving these compounds on alloy or ceramic surfaces. Perhaps the best example of this behaviour is the rapid degradation of protective scales on many alloys, termed hot corrosion , which occurs when Na2S04 or other salt condenses on the alloy. 

F. Nami, The Kinetics of Zinc Sulphide Leaching by Oxygen, Sulphur Dioxide and Ferrous Sulphate , Ph.D. Thesis. Columbia University, New York, NY, U.S.A., 1985. 

As with other procedures for the production of a gas sample for isotopic analysis, caution is required to ensure complete conversion at each stage of the preparation to obviate kinetic fractionation. Sulphides may be oxidized directly to SO2 for isotopic analysis [113] and recently methods have been developed for the thermal decomposition of sulphates [114,115]. An alternative approach is the initial conversion of either sulphate or sulphide to hydrogen sulphide [116, 117] from which the sulphur is precipitated as silver sulphide. This latter sulphide is easily oxidised in a reproducible manner to provide pure SO2 for isotopic analysis. 

To understand the accelerated oxidation of metals and alloys under sulphate salt coating, wide ranges of corrosion kinetics and morphologies have been observed by Shi (1993) under controlled laboratory conditions, and he has proposed several kinds of mechanisms. The Na2S04-Na20 eutectic melt was reported to be responsible for the formation of the abundant sulphide in the scale and was believed to play an important role in the present form of accelerated corrosion. Moreover Fe203 can dissolve into the eutectic melt according to the reaction ... 

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